Heterogeneous Catalysis for Today’s ChallengesSynthesis, Characterization and Applications

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RSC Green Chemistry

Editor-in-Chief:Professor James Clark, Department of Chemistry, University of York, UK

Series Editors:Professor George A. Kraus, Department of Chemistry, Iowa State University,Ames, Iowa, USAProfessor Andrzej Stankiewicz, Delft University of Technology, The NetherlandsProfessor Peter Siedl, Federal University of Rio de Janeiro, BrazilProfessor Yuan Kou, Peking University, China

Titles in the Series:1: The Future of Glycerol: New Uses of a Versatile Raw Material2: Alternative Solvents for Green Chemistry3: Eco-Friendly Synthesis of Fine Chemicals4: Sustainable Solutions for Modern Economies5: Chemical Reactions and Processes under Flow Conditions6: Radical Reactions in Aqueous Media7: Aqueous Microwave Chemistry8: The Future of Glycerol: 2nd Edition9: Transportation Biofuels: Novel Pathways for the Production of Ethanol,

Biogas and Biodiesel10: Alternatives to Conventional Food Processing11: Green Trends in Insect Control12: A Handbook of Applied Biopolymer Technology: Synthesis, Degradation

and Applications13: Challenges in Green Analytical Chemistry14: Advanced Oil Crop Biorefineries15: Enantioselective Homogeneous Supported Catalysis16: Natural Polymers Volume 1: Composites17: Natural Polymers Volume 2: Nanocomposites18: Integrated Forest Biorefineries19: Sustainable Preparation of Metal Nanoparticles: Methods and

Applications20: Alternative Solvents for Green Chemistry: 2nd Edition21: Natural Product Extraction: Principles and Applications22: Element Recovery and Sustainability23: Green Materials for Sustainable Water Remediation and Treatment24: The Economic Utilisation of Food Co-Products25: Biomass for Sustainable Applications: Pollution Remediation and Energy26: From C–H to C–C Bonds: Cross-Dehydrogenative-Coupling27: Renewable Resources for Biorefineries28: Transition Metal Catalysis in Aerobic Alcohol Oxidation29: Green Materials from Plant Oils

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30: Polyhydroxyalkanoates (PHAs) Based Blends, Composites andNanocomposites

31: Ball Milling Towards Green Synthesis: Applications, Projects, Challenges32: Porous Carbon Materials from Sustainable Precursors33: Heterogeneous Catalysis for Today’s Challenges: Synthesis, Characterization

and Applications

How to obtain future titles on publication:A standing order plan is available for this series. A standing order will bringdelivery of each new volume immediately on publication.

For further information please contact:Book Sales Department, Royal Society of Chemistry, Thomas Graham House,Science Park, Milton Road, Cambridge, CB4 0WF, UKTelephone: þ44 (0)1223 420066, Fax: þ44 (0)1223 420247Email: booksales@rsc.orgVisit our website at www.rsc.org/books

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Heterogeneous Catalysis forToday’s ChallengesSynthesis, Characterization andApplications

Edited by

Brian TrewynColorado School of Mines, USAEmail: btrewyn@mines.edu

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RSC Green Chemistry No. 33

Print ISBN: 978-1-84973-627-5PDF eISBN: 978-1-84973-749-4ISSN: 1757-7039

A catalogue record for this book is available from the British Library

r The Royal Society of Chemistry 2015

All rights reserved

Apart from fair dealing for the purposes of research for non-commercial purposes or forprivate study, criticism or review, as permitted under the Copyright, Designs and PatentsAct 1988 and the Copyright and Related Rights Regulations 2003, this publication may notbe reproduced, stored or transmitted, in any form or by any means, without the priorpermission in writing of The Royal Society of Chemistry or the copyright owner, or in thecase of reproduction in accordance with the terms of licences issued by the CopyrightLicensing Agency in the UK, or in accordance with the terms of the licences issued by theappropriate Reproduction Rights Organization outside the UK. Enquiries concerningreproduction outside the terms stated here should be sent to The Royal Society ofChemistry at the address printed on this page.

The RSC is not responsible for individual opinions expressed in this work.

The authors have sought to locate owners of all reproduced material not in their ownpossession and trust that no copyrights have been inadvertently infringed.

Published by The Royal Society of Chemistry,Thomas Graham House, Science Park, Milton Road,Cambridge CB4 0WF, UK

Registered Charity Number 207890

For further information see our web site at www.rsc.orgPrinted in the United Kingdom by CPI Group (UK) Ltd, Croydon, CR0 4YY, UK

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Dedication

This book is dedicated to Dr Victor S.-Y.Lin, who was a professor of chemistry atIowa State University from 1999 until heunexpectedly passed away in 2010. Victorreceived a bachelor’s degree in chemistryfrom National Chung Hsing Universityin Taichung, Taiwan and earned a PhDin chemistry from the University ofPennsylvania in 1996, working under thedirection of Professor Michael Therein.Following graduate studies, he was awar-ded a Skaggs postdoctoral fellow at theScripps Research Institute in La Jolla, CAwith Professor Reza Ghidari. Victor be-came a member of the chemistry faculty atISU in 1999. He joined the Department of

Energy—Ames Laboratory in 2001 and became program director for itsChemical and Biological Sciences Program in 2007. Victor was also directorof the Center for Catalysis at the ISU Institute for Physical Research andTechnology.

His colleagues and students remember Victor as a highly creative chemistwith a seemingly endless supply of energy; I often see him in my mind’s eyeunder the poster of Einstein stating how imagination is more important thanknowledge. Victor is best known for his seminal contributions to the synthesesand applications of mesoporous silica nanoparticles (MSNs), a term he coinedto describe nanometre-sized mesoporous silica materials with a well-definedand controllable morphology. He not only developed reliable synthetic

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protocols for MSNs but also demonstrated the applications of this interestingclass of nanomaterials in including heterogeneous catalysis, renewable energy,biosensing, and nanomedicine. He is still frequently cited and the techniqueswe developed together are still visible in the current literature today.

Victor was profoundly interested in focusing research time and effort onexploring heterogeneous catalysis, specifically those supported on MSNs.He developed novel methods to effectively control the pore environment totune catalytic properties. He also developed a bifunctional mesoporouscalcium-silicate mixed-oxide heterogeneous catalyst for the cooperativeand economic conversion of bio-based high-free-fatty-acid feedstocks intobiodiesel, and founded a startup company, Catilin, to bring this technologyto the market. Shortly after his death, Catilin merged with a multinationalcatalyst company, Albemarle.

One characteristic that comes to mind when I think about Victor was hisgenerosity with his time and knowledge. He always answered the door to hisoffice with a smile on his face and was sincerely happy to see you. He played therole of an important father figure for those of us working in his research group.Along with his astounding intelligence, Victor had a very clever sense of humorand knew exactly when a mood needed lightening with a joke. He allowed andoften encouraged us to pursue our own ideas in the research laboratory,offering both financial and intellectual support to them. This exemplaryattitude extended outside our laboratory, frequently inviting other students togroup meetings and into the lab, openly discussing research ideas and goals.His positive attitude rubbed off on the people around him, often turning a grayday sunny; even when every reaction failed, Victor would find the silver liningand you would leave his office feeling much better than when you arrived. Ifyou ever had the opportunity to meet Victor and spend even a little bit of timewith him, I am sure you understand what I mean – his smile was contagious.

Victor made a number of significant accomplishments in his briefscientific career, holding an impressive number of professional honors. Inaddition to his appointment as the John D. Corbett Professor in Chemistrythat he received shortly prior to his passing, Victor was recognized for hisoutstanding research contributions with a National Science FoundationCAREER Award, the LAS Award for Early Achievements in Research, anOutstanding Technology Development Award from the Federal LaboratoryConsortium, and the ISU Award for Mid-Career Achievement in Research. Hewas also very proud to serve on the Editorial Advisory Board of AdvancedFunctional Materials.

I will never forget the time I spent with Victor, first as a graduate studentin his research group (one of his first) and later as a research scientistassisting him in running his group. Today I have my own research group inthe chemistry department at the Colorado School of Mines and hope to be afraction as successful as he was in the short time he spent on earth.I frequently find myself thinking ‘‘What would Victor do in this situation?’’,thankful to have known such a great man.

Brian Trewyn

viii Dedication

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Contents

Chapter 1 Synthesis of Multi-functionalized MesoporousSilica Nanoparticles for Cellulosic BiomassConversion 1Kevin C.-W. Wu

1.1 Introduction 11.1.1 Background of Cellulosic Biomass

Conversion 11.1.2 Cellulosic Conversion in Ionic Liquid

Systems 21.1.3 Enzyme-assisted Cellulose Conversion 31.1.4 Production of 5-Hydroxymethylfurfural from

Cellulosic Conversion 41.1.5 Mesoporous Catalysts from Cellulosic

Conversion 51.2 Cellulase-immobilized Mesoporous Silica

Nanocatalysts for Efficient Cellulose-to-glucoseConversion 61.2.1 Optimization of Reaction Conditions 61.2.2 Characterization of Mesoporous Silica

Nanomaterials 81.2.3 Cellulase Immobilization 101.2.4 Cellulose Hydrolysis by using

Cellulase-immobilized MSN 11

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1.3 Conversion and Kinetics Study of Fructose-to-5-Hydroxymethylfurfural (HMF) using Sulfonic andIonic Liquid Groups Bi-functionalized MSNs asRecyclable Solid Catalysts in DMSO Systems 131.3.1 Synthetic Process for Bi-functionalized MSN 131.3.2 Characterization of Mesoporous Silica

Nanomaterials 151.3.3 Fructose-to-HMF Conversion using

Bi-functionalized MSN Catalysts 151.3.4 Kinetic Study 16

1.4 Acid–Base Bi-functionalized, Large-pored MSNs forCooperative Catalysis of One-pot Cellulose-to-HMFConversion 191.4.1 Functionalization of MSNs with Acid and

Base Groups 191.4.2 Conversion of Cellulose, Cellobiose, Glucose,

and Fructose using Bi-functionalized MSNs 191.4.3 Characterization of the Bi-functionalized

MSNs 201.4.4 Cellulosic Conversion by using

LPMSN-based Catalysts 211.5 Conclusions 23References 24

Chapter 2 Mesoporous Silica Supported Single-site Catalysis 28Pranaw Kunal and Brian G. Trewyn

2.1 Introduction 282.2 Synthesis and Structural Aspects of Mesoporous

Silica 292.2.1 Functionalization Techniques for

Mesoporous Silica 322.3 Single-site Heterogeneous Catalysts 33

2.3.1 Examples of Single-site Catalysts 342.3.2 Surface Distribution of Immobilized Species 54

2.4 Conclusion 56References 57

Chapter 3 Supported Metal Catalysts for Green Reactions 61K. Hara, H. Kobayashi, T. Komanoya, S.-J. Huang, M. Pruskiand A. Fukuoka

3.1 Introduction 61

x Contents

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3.2 Recent Developments in Supported Metal Catalystsfor Bioindustry 623.2.1 Conversion of Biomass to Chemicals and

Fuels 623.2.2 Catalytic Conversion of Cellulose 633.2.3 Hydrolytic Hydrogenation of Cellulose by

Supported Metal Catalysts 643.2.4 Hydrolytic Hydrogenation of Hemicellulose

by Supported Metal Catalysts 653.2.5 Catalytic Conversion of Cellulose to Ethylene

Glycol and Propylene Glycol 653.2.6 Hydrolysis of Cellulose to Glucose 663.2.7 Valorization of Lignin by Supported

Metal Catalysts 673.2.8 Direct Formation of Syngas or Pure

Hydrogen from Biomass 683.3 Mechanistic Aspects in Preferential Oxidation of

Carbon Monoxide under Excess Hydrogen(PROX Reaction) 683.3.1 Preferential Oxidation of Carbon Monoxide

in Excess Hydrogen (PROX Reaction) 683.3.2 PROX Reaction by Pt Catalysts Supported on

Mesoporous Silica 693.4 Surface-selective Functionalization of Mesoporous

Silica 703.4.1 Novel Types of Functionalized Support

Materials 703.4.2 Surface-selective Modification of

Mesoporous Silica 703.5 Conclusions 72References 72

Chapter 4 Zeolites in the 21st Century 77Wieslaw J. Roth, David Kubicka and Jirı Cejka

4.1 Introduction 774.2 History of Zeolites 784.3 Conventional Zeolites 78

4.3.1 Structures 794.3.2 Synthesis 814.3.3 Role of Organic Structure-directing Agents 834.3.4 Role of Inorganic Species 85

Contents xi

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4.4 From 2D to 3D Zeolites and Vice Versa 864.5 Adsorption 884.6 Catalysis 894.7 Summary 94Acknowledgments 95References 95

Chapter 5 Enzyme Immobilization on Mesoporous Silica Supports 100Cheng-Yu Lai and Daniela R. Radu

5.1 Introduction – Biocatalysis and Porous SilicaMaterials 100

5.2 Types of Porous Silica Support Utilized in EnzymeImmobilization 1015.2.1 Introduction 1015.2.2 Enzyme Immobilization/Encapsulation in

Hexagonally Ordered Porous Silica Materials 1035.2.3 Enzyme Immobilization/Encapsulation in

Hierarchically Ordered Mesoporous SilicaMaterials 106

5.3 Enzyme Immobilization Strategies in Porous Silica 1075.3.1 Introduction 1075.3.2 Non-covalent Binding of Enzymes on Porous

Silica Supports – Adsorption 1095.3.3 Covalent Immobilization of Enzyme onto

Porous Silica Supports 1105.4 Characterization of Catalytic Activity for Enzyme

Immobilized in Porous Silica 1125.4.1 Introduction 1125.4.2 Determination of Enzyme Concentration in

Porous Silica 1125.4.3 Enzymatic Activity 114

5.5 Conclusions 115References 115

Chapter 6 Heterogeneous Catalysts for Biodiesel Production 117Daniela R. Radu and George A. Kraus

6.1 Introduction 1176.2 Previous Work using Mesoporous Materials 119

6.2.1 Structure–Activity Studies of MesoporousSulfonic Acids 119

6.2.2 Catalysis of Organic Reactions 1196.2.3 Lin Group Contributions 120

xii Contents

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6.3 Industrial Partnership 1266.4 Conclusions 128References 128

Subject Index 131

Contents xiii

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CHAPTER 1

Synthesis of Multi-functionalizedMesoporous SilicaNanoparticles for CellulosicBiomass Conversion

KEVIN C.-W. WU

Department of Chemical Engineering, National Taiwan University, No. 1,Sec. 4, Roosevelt Road, Taipei 10617, TaiwanEmail: kevinwu@ntu.edu.tw

1.1 Introduction

1.1.1 Background of Cellulosic Biomass Conversion

The usage of fossil fuels causes serious problems like the energy crisis andglobal warming. In order to solve these problems, so far much attention hasbeen paid to the development of renewable energies such as solar or windenergy. Biofuel produced from biomass is one of the potential alternatives.First-generation biofuels (i.e. biodiesel) produced from corn and soybean oilhave proved that biomass-to-biofuel conversion is possible; however, the useof edible agriculture as a source will cause other problems such as fooddeficiency.1 Therefore, second-generation biofuels generated from non-edible lignocellulosic biomass have attracted more attention recently.

Lignocellulosic (or so-called ‘wood-based’) biomass consists of threemajor components: cellulose (41%), hemicellulose (28%), and lignin (27%).2

Generally, cellulose and hemicellulose can be used to produce bioethanol,

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and lignin offers a broad spectrum of conversion (thermal cracking, fastpyrolysis, and complete gasification) to achieve valuable chemicals andtransportation fuels.3 So far, a great deal of effort has been put toward thedegradation of cellulose with enzymes,4 mineral acids,5 bases,6 and super-critical water.7 The enzymatic hydrolysis of cellulose is effective, but thesystem is sensitive to contaminants originating from other biomass com-ponents. Furthermore, pre-treatment of cellulose (e.g., ammonia or steamtreatments in a high-pressure process or mechanical milling) is typicallyrequired to increase the accessible area of cellulose for a reasonable rate ofenzymatic hydrolysis.8 Mineral acids have been extensively investigated tocatalyze hydrolysis at a variety of acid concentrations and temperatures.A rather high temperature (180–230 1C) has been used in order to obtain anacceptable rate of cellulose hydrolysis. Furthermore, degradation of theresulting glucose becomes an issue at such high temperatures.9

1.1.2 Cellulosic Conversion in Ionic Liquid Systems

Recently, ionic liquids (ILs) have attracted a lot of attention and have beenutilized as solvents for the degradation of the lignocellulosic biomass.10–14

The importance of ionic liquids in cellulose dissolution has been em-phasized in several reviews.15–17 ILs are a kind of novel green solvent. Theyare organic salts with relatively low melting points. In other words, ILsusually appear as crystals under normal conditions; however, they can bemelted and dissociated into two ionic parts at relatively high temperatures(usually less than 100 1C). In contrast to other crystalline salts (e.g. NaCl),the attractive characteristic of ILs is that they can transform into aliquid phase.

The utilization of ILs for the dissolution of lignocellulose started in early2000. Numerous papers have been published on controlling the viscosity andpolarity of ionic liquids by varying their ionic structures.16,17 The main focusof these papers was the solubility of the synthesized ILs toward differentcarbohydrates such as glucose, sucrose, amylose, cellulose, and so on. In2002, Rogers et al. reported that cellulose could be dissolved in ILs at100 1C.10 The solubility of cellulose in ionic liquids results from its anions.It can disrupt the hydrogen bonds between polysaccharide chains ofcellulose and then dissolve it.18 This discovery started a new pathway to dealwith cellulose at low temperatures and ambient pressure.

In 2007, Zhang and his co-workers discovered that CrCl2 in [EMIM]Cl(1-ethyl-3-methylimidazolium chloride, an imidazolium-type ionic liquid)can efficiently catalyze the glucose-to-HMF conversion.19 HMF is apromising platform chemical because it can further transform to a widelyused biofuel called 2,5-dimethylfuran (DMF)20 and other useful materials.21

Since then, many have worked on the production of HMF from cellulose orglucose in ionic liquid systems. Binder and Raines combined HCl, CrCl2 orCrCl3, DMA/LiCl and [EMIM]Cl to convert cellulose to HMF;13 Zhang andhis co-workers used CrCl2/CuCl2 as catalysts in [EMIM]Cl;14 Han and his

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co-workers also discovered SnCl4 in [EMIM]BF4 can convert glucose to HMFwith a high yield;22 Riisager and his co-workers discussed HMF producedfrom lanthanide-containing ionic liquid systems;23 Bell and Chidambaramdiscovered that 12-molybdophosphoric acid in [EMIM]Cl/acetonitrile or[BMIM]Cl/acetonitrile can selectively convert glucose to HMF.24 Althoughthere has been much research focused on the addition of various kinds ofcatalysts in ionic liquid systems, very few papers discussed the effects ofreaction conditions (such as dissolution temperatures and times of ILs, re-action temperatures and times, and the amounts of water) on the conversionefficiency in ionic liquids without additional catalysts.25 In fact, in the above-mentioned papers, HMF could still be produced when using ILs only (noother additives), although the yields were very low. This indicates that ILs inthese systems serve not only as solvents but also as catalysts. We suggest thatthe low HMF yield was because the reaction conditions for HMF productionin these cases were not optimized. For example, Zhao et al. has shown thatthe yield of HMF converted from fructose was greatly affected by the reactiontemperature in an [EMIM]Cl only system.14 Very recently, Binder and Rainesdiscussed the sequence and timing of the addition of water into the cellu-losic conversion and showed that an optimal sequence and timing stronglyaffected the conversion efficiency.26

1.1.3 Enzyme-assisted Cellulose Conversion

In recent decades, cellulase was broadly studied for the hydrolysis of cellu-lose.27–31 Cellulase is a mixture of enzymes containing three main com-ponents: (1) endo-1,4-beta-D-glucanase (EG) which randomly cleaves thecellulose chain to lower the crystallinity; (2) cellobiohydrolase (CBH) whichdegrades cellulose by releasing cellobiose units; (3) beta-glucosidase whichhydrolyzes cellobiose and other oligomers to get glucose units. To date, thereaction conditions and the hydrolytic processes of hydrolyzing cellulose byusing free cellulase have been optimized with a glucose yield as high as70%.29 However, one critical problem when using cellulase as a catalyst isthe easy deactivation of cellulase by environmental factors (e.g., tempera-ture), which greatly hinder its practical use in industry.32,33 In order toovercome such difficulties, the immobilization of cellulase onto solidmaterials is a feasible way to enhance its stability.

Many research papers have demonstrated that immobilizing cellulaseonto organic and inorganic materials could improve the stability andreusability of cellulase without reducing its catalytic ability.34–41 Among thehost materials, mesoporous silica materials have gained much attentionbecause of their large specific surface area, high mechanical strength, andtunable surface functionality.34,35 Recently, Sakaguchi et al. studied the en-capsulation of cellulase by using mesoporous silica SBA-15 with various poresizes as hosts.36 They found that the enzymatic activity of cellulase stronglydepended on the pore size of SBA-15. The best performance of cellulasecould be obtained when using SBA-15 with pore diameter around 8.9 nm.

Synthesis of Multi-functionalized Mesoporous Silica Nanoparticles 3

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However, the structure of SBA-15 is 2D hexagonal with length of several mm,which would inhibit the adsorption of cellulase into the inner surface of theSBA-15 and result in a low adsorption amount. Lu et al. studied the effect ofsurface functionalities of a mesoporous silica FDU-12 (pore size is around25.4 nm) on the immobilization of cellulase.35 They functionalized FDU-12with phenyl, thiol, amino and vinyl groups. The results showed that theelectrostatic and hydrophobic interactions between cellulase and functio-nalized FDU-12 play significant roles on the activity and stability of immo-bilized cellulase. Amine-functionalized FDU-12 adsorbed the largest amountof cellulase but exhibited the lowest activity. They explained this was due tothe interaction between amine groups of FDU-12 and the carboxyl groups ofcatalytic site of cellulase which thereby inhibited the activity of cellulase. Incontrast, vinyl-functionalized FDU-12 not only maintained the activity ofcellulase up to 80% but also temporal enzyme stability owing to the existenceof hydrophobic groups. Despite these pioneering studies, none of them hasstudied different immobilization methods (i.e., physical adsorption andchemical binding) on the efficiency of cellulase, and cellulosic hydrolysis byimmobilized cellulase has never been reported yet.

1.1.4 Production of 5-Hydroxymethylfurfural from CellulosicConversion

5-Hydroxymethylfurfural (HMF), converted from lignocellulosic biomass, isconsidered one of the ‘‘top value-added chemicals’’; this results from itsutilization as a building-block platform between biomass and promisingchemical intermediates, such as 2,5-furandicarboxylic acid (FDCA),42 2,5-dimethylfuran (DMF),43 5-ethoxymethylfurfural (EMF),44 and ethyl levulinate(EL),45 which have been studied extensively in recent years and demonstratethe significance of HMF.

HMF has been successfully generated from fructose, glucose, and cellu-lose using various kinds of reaction systems with homogeneous or hetero-geneous catalysts.46–48 The mechanism of cellulose-to-HMF conversion isstill unclear, but the conversion can be divided into several reactions. First,cellulose is usually pre-treated by alkaline, acid, or certain ionic solutions todestroy its rigid framework. The pre-treated cellulose then goes through thedepolymerization process in an acidic system in order to break the 1,4-b-glycosidic bonds of cellulose and produce glucose. Subsequently, glucoseconverts to fructose via isomerization, which is a so-called Lobry de Bruyn–Alberda van Ekenstein transformation.49 Finally, the dehydration of fructosegenerates HMF. The mechanism of fructose-to-HMF conversion has beendiscussed in numerous studies.50–52 The micro-kinetic model for this three-stage water-removed process53 has been constructed to determine an ap-parent activation energy.54 In addition, according to computational results,both the estimated equilibrium constant and activation energy can begreatly influenced by reaction conditions, including temperature, solvent,and catalysts.55

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Different solvents have been used in the fructose-to-HMF conversion be-cause of the contrast between water-soluble reactants (e.g., fructose andglucose) and organic-solvent-soluble products (e.g., HMF). The careful se-lection of solvents can promote the preferential reaction and enhanceproduct yield. The Dumesic group studied the effects of solvents on thedehydration of fructose in biphasic systems, and demonstrated the catalyticability of dimethylsulfoxide (DMSO), which is able to suppress the undesiredside reactions effectively.55 Recently, ILs have been widely used as bothcatalysts and solvents for producing HMF from lignocelluloses because oftheir comparatively higher catalytic activity and adjustable composition.56

However, despite the excellent activity and recyclability of ILs, their potentialis restricted to laboratory-scale experiments due to high costs. Therefore, alow-price solvent with the desired properties (e.g., high boiling point and lowviscosity) such as DMSO can have more potential in industrial applications.

In recent years, several groups have reported the production of HMF fromfructose in DMSO-based reaction systems via homogeneous and hetero-geneous catalysts, including acids, salts, and metal ions.57 The Dumesicgroup has investigated the catalytic capabilities of various homogeneousmineral acids.58 Recently, Wang et al. used carbon-based p-toluenesulfonicacid (TsOH) at 130 1C for 1.5 h resulting in a 91.2% yield of HMF.59 Althoughthese pioneering studies showed high yields of HMF, harsher reactionconditions are always needed in such homogeneous catalytic systems. Fromeconomic and sustainable viewpoints, scientists have turned to hetero-geneous solid catalysts and mild reaction conditions. For example, theSidhpuria group immobilized ILs onto silica particles as an efficient het-erogeneous catalyst for fructose-to-HMF conversion with a yield of 63% in aDMSO system.60

1.1.5 Mesoporous Catalysts from Cellulosic Conversion

Mesoporous silica nanoparticles (MSNs) have attracted a great deal of at-tention in the field of catalysis because of their high surface area and con-trollable pore size. In addition, abundant SiOH groups on the surface ofMSNs provide the possibility of further functionalization with other organicgroups.61 For example, the Lin group has used a co-condensation method tofunctionalize MSNs with a general acid (i.e., a ureidopropyl (UDP) group) anda base (i.e., a 3-[2-(2-aminoethylamino)ethylamino]-propyl (AEP) group) as acooperative acid–base catalyst for aldol, Henry, and cyanosilylation re-actions.62 We also have used a grafting method to functionalize MSNs withseveral metal–histidine complexes for H2O2-assisted tooth bleaching. However,the conventional MSNs synthesized from the cationic surfactant cetyl-trimethylammonium bromide (CTAB) exhibit a pore size of around 2 nm. Forseveral catalytic reactions involving large molecules (e.g., proteins or cellulose),this pore size is too small to allow the reactants to diffuse into the mesopores,thus losing the advantage of high surface area inside the MSNs. Therefore, thesynthesis of MSNs with pore sizes large than 10 nm is highly desirable.

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The MSNs with large pore sizes can be synthesized through two ap-proaches: (1) using high-molecular-weight surfactants as templates; and (2)adding hydrophobic additives as swelling agents. For example, the Zhaogroup succeeded in synthesising mesoporous silica with an ultra-large poresize of approximately 37.0 nm by using a high-molecular-weight surfactant(poly(ethylene oxide)-b-poly(methyl methacrylate); PEO-b-PMMA).63 Thesame group has also reported the addition of 1,3,5-trimethylbenzene (TMB)as a swelling agent to synthesize mesoporous silica with a large pore of 25.4nm (as denoted as FDU-12).63,64 The Lu group has functionalized FDU-12materials with phenyl, thiol, amino, and vinyl groups and studied the effectof these functional groups on the immobilization efficacy of an enzyme (i.e.,cellulase).65 Despite these pioneering studies, researchers have not yetutilized large-pored MSNs with various functional groups for cellulosicconversion in ionic liquid systems. In general, cellulosic conversion involvesthree main reactions: (1) cellobiose-to-glucose depolymerization, (2) glucose-to-fructose isomerization, (3) fructose-to-HMF dehydration. These reactionsneed acid, base, and acid catalysts, respectively, as illustrated in Scheme 1.1.Consequently, to synthesize large-pored MSNs with both acid and basefunctionalities as a new cooperative solid catalyst would be helpful forone-pot cellulose-to-HMF conversion.

1.2 Cellulase-immobilized Mesoporous SilicaNanocatalysts for Efficient Cellulose-to-glucoseConversion

1.2.1 Optimization of Reaction Conditions

Optimal reaction conditions with respect to temperature, the amount ofcatalyst and the reaction time are crucial to maximising the final yield of

Scheme 1.1 An illustration showing the production of 5-HMF converted fromcellulose through a series of reactions.

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cellulosic hydrolysis when using cellulase as a catalyst. Therefore, we firstoptimized the reaction temperatures, the amount of free cellulase and thereaction time. As shown in Figure 1.1a, 15 mg of cellulose was hydrolyzedusing free cellulase (50 Unit, 1 Unit indicates the amount of enzyme that cancatalyze 1�10�6 mole substrate in one minute) as the catalyst at differenttemperatures for 24 h; the maximum yield of glucose was obtained at 50 1C.By repeating the experiment, we found that 50 1C is also the most stableoperating condition for cellulose. Thus, we chose 50 1C as the suitabletemperature for cellulase-assisted cellulose hydrolysis. From the economicpoint of view, an optimal amount of cellulase means the minimum amount

(a)

(b)

(c)

Figure 1.1 Optimization of reaction conditions for cellulase-assisted cellulose-to-glucose conversion: (a) reaction temperature; (b) cellulase amount;(c) reaction time.

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of cellulase while keeping the maximum yield of glucose. Various amountsof cellulase, ranging from 1 mg to 23 mg, were used for the hydrolysis ofcellulose (15 mg) at 50 1C for 24 h. Based on the results in Figure 1.1b, wefound that the optimal amount of celluase was 25 Unit (i.e., 4.5 mg). Morecellulase than 25 Unit did not increase the yield of glucose at the currentoperation coniditions. After obtaining the optimal reaction temperature andthe amount of cellulase, the optimal reaction time was also examined. 15 mgof cellulose was hydrolyzed using free cellulase (25 Unit) at 50 1C for variousreaction times (i.e., ranging from 3 to 48 hours). According to the resultsshown in Figure 1.1c, in order to reach 90% glucose yield, cellulose has to behydrolyzed for at least 24 h although 80% glucose yield could be obtained in12 h. For consistency, here we chose 24 hours as the optimal reaction time.

1.2.2 Characterization of Mesoporous Silica Nanomaterials

The morphology and porous properties of the synthesized MSNs with twodifferent pore sizes (NB these are referred to as large-pore MSNs (LPMSNs)and small-pore MSNs (SPMSNs) hereafter) are characterized with scanningelectron microscopy (SEM) and nitrogen adsorption–desorption isotherms.The SEM images in Figure 1.2a and b show the uniform and sphericalmorphology for both LPMSNs and SPMSNs with diameters around 600 and150 nm, respectively. In Figure 1.2c and d, LPMSNs and SPMSNs exhibit typeIII and type IV nitrogen adsorption–desorption isotherms, respectively. Thetype III isotherm of LPMSNs exhibits prominent adsorption at high relativepressures (P/P0), which is indicative of adsorption in macropores. In con-trast, the type IV isotherm of SPMSNs has been widely shown to occur in atypical mesoporous material with a two-dimensional hexagonal structure.The Brunauer–Emmett–Teller (BET) specific surface areas for LPMSNs andSPMSNs are 262.6 and 820.1 m2 g�1, respectively. In addition, the pore-sizedistribution calculated from the Barrett–Joyner–Halenda (BJH) methodclearly shows that LPMSNs exhibit a broad pore size around 20–40 nm whileSPMSNs exhibit a narrow pore size around 2–5 nm, as depicted in Figure 1.2eand f, respectively. The structural properties of LPMSNs and SPMSNs aresummarized in Table 1.1.

In addition to pore size and surface area, the surface functionality of theMSN also affects the immobilization of cellulase. We qualitatively andquantitatively study the functional groups on the SPMSNs and LPMSNs by29Si and 13C solid-state nuclear magnetic resonance (NMR). Because thesynthetic methods for SPMSNs and LPMSNs are different, the organicfunctional groups and their amounts on the surface of the prepared ma-terials are different. As shown in the 29Si NMR spectra (Figure 1.3a), LPMSNsexhibited Q3, Q4, T3 and T2 bonds, indicating that there are Si–O–H andSi–O–C bonds in the material. Because we added 3-aminopropyltrimethoxy-silane (APTMS) during the synthesis of LPMSN, the Si–O–C bonds should bedue to the presence of APTMS. On the other hand, SPMSN exhibited only Q3

and Q4 peaks that represent the presence of Si–O–Si and Si–O–H bonds, as

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Table 1.1 Summary of structural properties and functionalities of SPMSNs, LPMSNsand TESP-SA-functionalized LPMSNs.a

SampleParticlesize/nm

Specificsurfacearea/m2 g�1

Poresize/nm

–R grafted/mmol g�1

–OH residues/mmol g�1

SPMSN ca. 150 820.1 2–5 N.D. 9.89LPMSN ca. 600 262.6 20–40 1.06 6.12TESP-SA

LPMSNca. 600 N.D. N.D. 1.49 4.37

aN.D.¼not done.

(a) (b)

(d)(c)

(e) (f)

Figure 1.2 Characterization of LPMSNs and SPMSNs. SEM, nitrogen adsorption–desorption isotherms and pore-size distribution, respectively, for (a, cand e) LPMSNs and (b, d and f) SPMSNs.

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shown in Figure 1.3b. These two materials were used as hosts for physicaladsorption of cellulase. For chemically binding cellulase, we further func-tionalize LPMSN with carboxyl groups by reacting LPMSN with an organo-silane 3-triethoxysilylpropyl succinic acid anhydride (TESP-SA). Based onthe 13C NMR spectra of LPMSN and TESP-SA-functionalized LPMSN inFigure 1.3c and d, respectively, we could conclude that the TESP-SA wassuccessfully grafted onto the surface of LPMSN. The TESP-SA-functionalizedLPMSN should exhibit two functional groups, i.e., the amino group fromAPTMS and the carboxyl group from TESP-SA, on its surfaces. The amountsof the functional groups on the SPMSN, LPMSN, and TESP-SA-functionalizedLPMSN were calculated and summarized in Table 1.1.

1.2.3 Cellulase Immobilization

Cellulase was immobilized into SPMSN and LPMSN by physical adsorptionand into TESP-SA-functionalized LPMSN by chemical binding. The amountsof cellulase immobilized into these materials were quantitatively measuredby UV-Vis spectroscopy. As shown in Table 1.2, on the basis of the sameamount of host materials (i.e., 50 mg), the amounts of the immobilizedcellulase are 14.6 mg, 23.4 mg, and 19.2 mg for SPMSN, LPMSN, andTESP-SA-functionalized LPMSN, respectively. The highest amount ofimmobilized cellulase was found in the case of cellulase-adsorbed LPMSN,which was 1.6 times the amount in cellulase-adsorbed SPMSN. Although theSPMSN exhibits a higher surface area than LPMSN, the small pore size of

(a) (b)

(c) (d)

Figure 1.3 29Si solid-state NMR spectra of (a) LPMSN and (b) SPMSN. 13C solid-stateNMR spectra of (c) LPMSN and (d) TESP-SA-functionalized LPMSN.

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SPMSN (i.e., 2–5 nm) made the diffusion of cellulase into the pore difficultdue to the large size of cellulase (around 8 nm). Therefore, a pore size largerthan 8 nm in MSNs is essential for the immobilization of cellulase. Inaddition, although the amount of cellulase chemically bonded with TESP-SA-functionalized LPMSN was less than that of cellulase-adsorbed LPMSN, theamount was still larger than 4.5 mg which had been considered to be theminimum amount for maximum glucose production under optimal reactionconditions.

Another factor affecting the immobilization of cellulase is surface charge.The surface charges of cellulase and SPMSN at pH¼ 4.8 are both negative(�6.7 and �14.8 mV, respectively). Therefore, the electrostatic interactionbetween SPMSN and cellulase was negligible. The surface charge of LPMSNis around zero (þ1.0 mV) because of the existence of both Si–OH and Si–NH2

groups. Therefore, in addition to the larger pore size of LPMSN, the in-creased adsorption amount in the case of cellulase-adsorbed LPMSN alsoresulted from the electrostatic interaction between cellulase and Si–NH2

groups of LPMSN. It is worth noting that the surface charge of TESP-SA-functionalized LPMSN was negative (�40.5 mV) owing to the existence ofcarboxylic acid groups in TESP-SA. Therefore the immobilized cellulaseamount in this case was less than that of cellulase-adsorbed LPMSN. How-ever, we have confirmed that the immobilized cellulases here were covalentlylinked with TESP-SA-functionalized LPMSN by 13C NMR, and such chem-ically linked cellulases could avoid the problem of cellulase detachment,resulting in excellent stability (see discussion below).

1.2.4 Cellulose Hydrolysis by using Cellulase-immobilizedMSN

As we have previously examined, 4.5 mg of free cellulase was enough toconvert 15 mg of cellulose to glucose with a high yield of around 85% at50 1C for 24 hours. However, although the amounts of cellulase immobilized

Table 1.2 Summary of cellulase loading amount and yields of glucose for freecellulase, cellulase-adsorbed SPMSN, cellulase-adsorbed LPMSN andcellulase-linked TESP-SA-functionalized LPMSN.

Sample Cellulase/mg

Glucoseyield (%)before

Glucoseyield (%)after

Percentagelost (%)

Free cellulase 4.5 85.86 53.39 37.78(24.89)a

Cellulase-adsorbedSPMSN

14.6 33.3 4.59 83.30(46.14)a (24.89)a

Cellulase-adsorbedLPMSN

23.4 77.89 10.64 86.56(42.71)a

Cellulase-linkedTESP-SA LPMSN

19.2 83.79 82.15 4.61

aYield of cellobiose.

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in the MSN materials are all larger than 4.5 mg, their glucose yields were allless than 85% at the same optimal reaction conditions. As shown inFigure 1.4 and Table 1.2, the glucose yields for cellulase-adsorbed SPMSN,cellulase-adsorbed LPMSN, and cellulase-linked TESP-SA LPMSN were33.30%, 77.89%, and 83.79%, respectively. It is clearly seen that the cellulasechemically bonded with TESP-SA functionalized LPMSN exhibited almostthe same activity with free cellulase. For the case of cellulase-adsorbedSPMSN, the glucose yield was the smallest, which might be due to the for-mation of a byproduct (i.e., cellobiose with yield of 46.14%). Because thecellulase-adsorbed LPMSN catalyst still exhibited a respectable glucose yieldof 77.89%, we suggest that the low glucose yield for cellulase-adsorbedSPMSN was due to the small pore size. When the pore size of MSN was small,not only could cellulase not be adsorbed in the pores but also the pre-treatedcellulose could not diffuse into the pores. This result again proved the sig-nificance of suitable pore size when using MSN materials as hosts.

One may be concerned that the chemically linked cellulase will lose itsactivity due to the change of its conformation via covalent binding. The Lugroup has utilized amino-group-functionalized mesoporous silica materialsto immobilize cellulase and found that the activity of the immobilized cel-lulase decreased although the amount of immobilized material was large, ascompared to free cellulase.35 They suggested that the amino group of thematerials would bind to the catalytic domains of the cellulase, therebyreducing its catalytic ability. In contrast to their result, our data indicatedthat the activity of chemically linked cellulase was similar with that of freecellulase. We conclude that the carboxylic-group-functionalized LPMSN usedin this study would bind with the cellulose binding domains (not catalytic

Figure 1.4 Yields of glucose for free cellulase, cellulase-adsorbed SPMSN, cellulase-adsorbed LPMSN, and cellulase-linked TESP-SA-functionalized LPMSNof LPMSN and TESP-SA-functionalized LPMSN. ‘‘Glucose_before’’ indi-cates that the catalysts were used immediately after preparation.‘‘Glucose_after’’ indicates that the catalysts were used after aging atroom temperature for 23 days.

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domains) of the cellulase, thereby retaining the activity of cellulase. In fact,several papers have also reported the preservation/enhancement of cellulo-lytic activity by connecting the cellulose binding domains (CBD) of cellulasewith scaffolds.66

One of the advantages of immobilizing cellulase within porous materialsis to increase the stability of cellulase. To examine the stability of theimmobilized cellulase, this study tests different cellulase-immobilized ma-terials, including free cellulase, cellulase-adsorbed SPMSN, cellulase-adsorbed LPMSN and cellulase-linked TESP-SA-functionalized LPMSN bystoring these catalysts at room temperature (usually cellulase should bestored at 4 1C). After 23 days, these materials were used to hydrolyzecellulose, and the results are refered as ‘‘After_storage’’ in contrast to‘‘Before_storage’’, which involved catalysts before stability experiments.Figure 1.4 and Table 1.2 show that after 23 days the glucose yields for freecellulase, cellulase-adsorbed SPMSN, cellulase-adsorbed LPMSN, and cellu-lase-linked TESP-SA-functionalized LPMSN all decreased, yielding 53.39%,4.59%, 10.64%, and 82.15%, respectively. The corresponding percentage lossof glucose yields for free cellulase and cellulase-immobilized catalysts were37.78%, 83.30%, 86.56% and 4.61%, respectively.

The stability experiments in this study reveal three important findings. (i)The SPMSN and LPMSN could not protect cellulase when the cellulase wasmerely immobilized by physical adsorption. The large percentage loss (over80%) of glucose yield in these two cases indicated that cellulase easily de-tached from MSN at room temperature, resulting in deactivation. (ii) Inaddition to glucose, cellobiose was formed in the cases of free cellulase andcellulase-adsorbed SPMSN and LPMSN. This result indicates that the activityof cellobiohydrolase, an enzyme in cellulase that hydrolyzes disaccharidesinto individual monosaccharides, decreased after storage at room tem-perature for 23 days. (iii) The cellulase chemically linked to TESP-SA-func-tionalized LPMSN exhibited the best stability. This indicates that thechemical bonding between cellulase and TESP-SA-functionalized LPMSNdecreased its hydrolytic activity while preserving the catalytic specificitytoward cellulose-to-glucose conversion, which is the best way to immobilizecellulase with a stable efficiency.

1.3 Conversion and Kinetics Study of Fructose-to-5-Hydroxymethylfurfural (HMF) using Sulfonic andIonic Liquid Groups Bi-functionalized MSNs asRecyclable Solid Catalysts in DMSO Systems

1.3.1 Synthetic Process for Bi-functionalized MSN

The synthetic process of the bi-functionalized MSN is shown in Scheme 1.2and is described as follows: Brij-97 was used as the template and was firstdissolved in 180 g of deionized water. Then, APTMS and DOP were added to

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the Brij-97 solution with stirring at room temperature. After stirring for30 min, organosilanes (i.e., MPTMS and CPTES) were added to the reactionsystem along with TEOS, and the whole system, with a composition (inmolar ratios) of water: Brij-97 : TEOS : MPTMS : CPTES¼ 433 : 0.293 : 1 : 0.009 :0.009, was prepared and kept stirred for 24 h at room temperature. Themixture was subsequently heated at 100 1C overnight. Finally, the precipi-tated solid was collected by filtration and washed sequentially with waterand methanol. It is worth noting that the template can be extracted by thiswashing step. To further convert the thiol group of (MPþCP)-MSN to asulfonic group, the (MPþCP)-MSN was oxidized in an H2O2 solution with acomposition of (MPþCP)-MSN : H2O : MeOH : H2O2¼ 0.5 g : 10 ml : 10 ml :10 ml. After reaction at room temperature overnight, the obtained sample(i.e., (HSO3þCP)-MSN) was washed and dried in vacuum. To further func-tionalize the (HSO3þCP)-MSN with ionic liquid, the (HSO3þCP)-MSNsample and solid imidazole were degassed for 3 h before the addition ofanhydrous benzene and chlorobutane with the molar ratio of imidazole :chlorobutane¼ 1 : 2. After one day of reflux, the product (i.e., (HSO3þ ILs)-MSN) was collected through filtration and washed with anhydrous benzene.Finally, the (HSO3þ ILs)-MSN sample was immersed in CrCl2 solution, andthe whole mixture was kept stirred overnight. CrCl2 can be physically

Scheme 1.2 Synthetic process for preparing the bi-functionalized MSN.

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absorbed on the surface of (HSO3þ IL)-MSN. The final product([HSO3þ (ILs/CrCl2)]-MSN) was collected by centrifugation.

1.3.2 Characterization of Mesoporous Silica Nanomaterials

The morphology and porous properties of the series of synthesized bi-functionalized MSN were characterized with SEM and nitrogen adsorption–desorption isotherms. The SEM image shows an uniform and sphericalmorphology for bi-functionalized MSN with particle size of sub-microns. Inaddition, the bi-functionalized MSNs exhibit a type IV nitrogen adsorption–desorption isotherm, indicating multilayer adsorption by capillary conden-sation. Moreover, the results of BET specific surface area and pore-sizedistribution calculated from the BJH method are listed in Table 1.3 togetherwith other structural properties of bi-functionalized MSNs.

Next, we qualitatively and quantitatively investigated the functionalgroups on the bi-functionalized MSN using 29Si and 13C solid-state NMR.The 13C NMR spectrum contains ten identified signals, and these resultsevidenced the successful grafting of the organosilane MPTMS and ILs on theMSN. Additionally, we further quantified the amounts of each functionalgroup on the MSN by 29Si NMR. There were Qn and Tn peaks, which repre-sents the relative amount of silica framework and their covalent bondingwith organosilanes. The amounts of functional groups are summarized inTable 1.3. In addition, results of element analysis indicated the presence ofthe sulfonic acid (S, around 2.3%) and ionic liquid (N, around 1.7%).

1.3.3 Fructose-to-HMF Conversion using Bi-functionalizedMSN Catalysts

In order to demonstrate the effect of bi-functionalized MSN on the fructose-to-HMF conversion, the reaction was executed without catalysts (blanksample), with MSN and bi-functionalized MSN. The result including theefficacy of fructose conversion, HMF yield, and selectivity is depicted inFigure 1.5a. The cases labeled ‘blank’ (no catalyst) and ‘MSN’ (with non-functionalized MSN as the catalyst) showed low conversion (around 25%)and almost no HMF yield, indicating that the dehydration of fructose togenerate HMF was difficult to achieve in such reaction conditions (i.e.,DMSO solvent, 90 1C, 3 h). In contrast, with the same reaction conditions,

Table 1.3 Summary of characterization of bi-functionalized MSNs.

Physical properties Functionalization

Specificsurfacearea/m2 g�1

Poresize/nm

Particlesize/nm

Numberof Si–OH/mmol g�1

Functionalgroup/mmolg�1

ElementalanalysisN% C% H% S%

98.0 4.4 400 6.12 1.06 1.7 16.0 27.6 2.3

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the reaction with the presence of the bi-functionalized MSN surprisinglyexhibited enhanced fructose conversion (almost 100%) and HMF yields ashigh as 72.5%, as shown in Figure 1.5a. These results clearly demonstratethe effectiveness of the bi-functionalized MSN on the catalytic production ofHMF, which was due to the contributions of the functional groups ofR–HSO3 acid and [EMIM]Cl/CrCl2 ionic liquid. It has been reported that thedehydration of fructose can be promoted with the assistance of differenthomogeneous acids and metal chlorides.61 Here we report a successfulfunctionalization of both sulfonic acid and ionic liquid/metal chloride ontothe surface of MSN materials as a new method to provide efficient hetero-geneous catalysts. In addition to the sulfonic acid groups, chloride ions canact as an effective catalyst due to their nucleophilicity, and the acidic C-2proton of the imidazolium part of the ionic liquid also promotes the de-hydration of fructose.52 Furthermore, in contrast to other solid nanoparticles,MSN exhibits a high surface area and large mesopores, which should enhancethe efficiency greatly, owing to the increased number of reaction sites.

Reusability of the [HSO3þ (ILs/CrCl2)]-MSN was further studied over fivecycles. As shown in Figure 1.5b, it is seen that the conversions of fructosewere maintained at almost 98% until the fifth run. Additionally, no sig-nificant loss of HMF yield was observed. It means that the grafted functionalgroups (i.e. sulfonic acid (HSO3) and ILs) did not leach during the compli-cated reiterating process and further hold their original activity withoutobvious decay. Therefore, the recyclability of the synthesized bi-functiona-lized material has been exactly confirmed.

1.3.4 Kinetic Study

We further study the kinetics of the fructose-to-HMF conversion and com-pare the rate constants, reaction orders, and activation energies for thesystems with and without bi-functionalized MSN catalysts. First, we

(a) (b)

Figure 1.5 (a) Results of fructose-to-HMF conversion in different systems.Blank: without any catalyst. MSN: with non-functionalized MSN.[HSO3þ (ILs/CrCl2)]-MSN: with bi-functionalized MSN. (b) Recyclabilityof [HSO3þ (ILs/CrCl2)]-MSN in fructose-to-HMF conversion.

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conducted two reaction systems (i.e., with and without catalyst) at the samereaction temperature (i.e., 90 1C) but for different reaction time periods.We observed the variation of HMF yields with reaction time and the data isshown as Figure 1.6a. The reaction rates of both cases are initially higher buttend to be almost constant after a time-span, indicating that the reactantshave already run out, resulting in no more HMF being produced. However,adding our synthesized catalysts, [HSO3þ (ILs/CrCl2)]-MSN, obviously ac-celerates the generation of HMF to some extent. Three hours later, there isabout 73.4% of HMF, which is nearly three times larger than the percentagein blank (25.3%). That is to say, our material has an effect on the catalyticfructose conversion.

For the purpose of identifying the relationships between kinetic par-ameters (such as rate constants, reaction orders, and activation energies)and the addition of the bi-functionalized catalyst, we further comprehen-sively constructed the kinetics profiles at different temperatures for systemswith and without [HSO3þ (ILs/CrCl2)]-MSN (Figure 1.6a). We supposed thatthe ultimate product of fructose dehydration is HMF only, i.e. we did nottake other by-products into account. Besides, we assumed the degradation ofHMF would not occur under our designed condition (90 1C, 0–6 h). Thishypothesis can be supported by the test results of HMF decomposition, asshown in Figure 1.6b. In this experiment, HMF is considered as the reactant,placed in an identical environment as before. From the data shown, we cansee that the apparent decay of HMF (15.18%) could be noticed only undersufficient reaction temperature and time (140 1C, 6 h), which is very far fromour practical operating conditions. That is to say, in our milder conditions(90 1C, 3 h), the decomposition of HMF is negligible and this outcomebolsters the previous model we have set up.

In Figure 1.7, we analyzed the kinetics profiles in order to systematicallyunderstand the shifts of each kinetics parameter caused by the catalysts.Referring to the previous published research,54 we assumed that the

(a) (b)

Figure 1.6 (a) Effect of reaction time and catalyst on the yield of HMF. (b) Thekinetics profiles of HMF decomposition in DMSO at different tempera-tures. Diamonds: 120 1C; squares: 140 1C.

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transformation of fructose is a 1st order process and that the reaction ratecould be expressed as follows:

d½HMF�dt

¼ k½fructose� ¼ d½fructose�dt

d½HMF�dt

¼ k½fructose� ¼ � d½fructose�dt

where [ ] means the molar concentration of each chemical and k is the rateconstant for fructose conversion at a certain temperature. Next, we trans-formed this equation into a numerical form and made the [fructose] in termsof conversion X, i.e. [fructose]t¼ [fructose]t0 (1�X). After the subsequentintegral calculation, the original equation will become:

� ln(1�X)¼ ktþC

where t is the reaction time and C is an arbitrary constant. Therefore, weplotted a figure with �ln(1�X) as the y-axis and t as the x-axis, fitting thedata linearly, and evaluated reaction constants from the slopes. As shown inFigure 1.7, there is an obvious increase of k in the presence of bi-functio-nalized MSNs, confirming their ability to promote this reaction. Next, wecalculated the activation energy (Ea) of each system from rate constants weobtained by the Arrhenius equation. The Ea values of systems with andwithout catalysts are 67.5 and 80.05 kJ mol�1, respectively. This fact indi-cated that our addition of [HSO3þ (ILs/CrCl2)]-MSN has altered the reactionroute to a certain degree, consequently lowering the activation energy andleading to a higher reaction rate.

(a) (b)

Figure 1.7 The kinetics profiles of fructose-to-HMF conversion (fitted by 1storder assumption) (a) without catalysts and (b) with [HSO3þ (ILs/CrCl2)]-MSN.

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1.4 Acid–Base Bi-functionalized, Large-pored MSNsfor Cooperative Catalysis of One-pot Cellulose-to-HMF Conversion

1.4.1 Functionalization of MSNs with Acid and Base Groups

To functionalize LPMSN with acid and base groups, the organosilane, i.e.,MPTMS and APTMS, was grafted onto the surface of LPMSN. Typically, 1 g ofLPMSN in a two-necked round-bottom flask was degassed in vacuum at110 1C for 3 h. After that, dried toluene (40 mL) was injected into the flaskunder nitrogen atmosphere, followed by injecting organosilanes. Theamount of organosilane used was 1.3 times the amount of silanol groupon the LPMSN that was previously calculated by solid-state NMR (i.e.,6 mmol g�1). Then, the mixture was heated and refluxed at 110 1C for 24 h.Finally, the acid- and/or base-functionalized LPMSN was collected byfiltration, washed with toluene several times in order to remove theresidual reactant, and dried in vacuum. The resulting samples were calledLPMSN-NH2 and LPMSN-SH. Then, the LPMSN-SH was oxided to becomeLPMSN-SO3H by modifying a published procedure.67 Typically, 0.5 gof LPMSN-SH was added to the mixture of hydrogen peroxide (10 mL),deionized water (10 mL), and methanol (10 mL). The mixture was stirredat room temperature for 12 h. After that, the resulting precipitate was col-lected by filtration, washed with deionized water several times and dried invacuum. The resulting sample was named LPMSN-SO3H. LPMSN-SO3Hand LPMSN-NH2 are used as acid and base solid catalysts, respectively,in this study. For preparation of bi-functionalized LPMSN exhibitingboth acid and base groups (denoted as LPMSN-Both), APTMS was graftedonto the pre-synthesized LPMSN-SO3H using the same grafting processdescribed above.

1.4.2 Conversion of Cellulose, Cellobiose, Glucose, andFructose using Bi-functionalized MSNs

The cellulosic conversion includes pre-treatment and reaction. For pre-treatment, cellulose (15 mg) was added into [EMIM]Cl (150 mL), and thewhole mixture was heated at 120 1C for 0.5 h with stirring for dissolution ofcellulose. For reaction, LPMSN-based catalysts (4 mg) and water (16.67 mL)were added to the cellulose/[EMIM]Cl solution while keeping heatingat 120 1C for another 3 h. All reactions were repeated three times andthe average yields of products were obtained. After the optimization of thereaction conditions beforehand, the amount of catalyst was determined as4 mg. The conversion process for cellobiose, glucose and fructose was thesame as that for cellulose except for exclusion of the pre-treatment step.When using glucose and fructose as reactants, water did not be added intothe reaction systems.

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1.4.3 Characterization of the Bi-functionalized MSNs

Before functionalizing LPMSN with other functional groups, we qualitativelyand quantitatively investigated the amounts of hydroxyl group in the LPMSNusing 29Si and 13C solid-state NMR. As shown in the 29Si NMR spectra,LPMSN exhibited Q3, Q4, T3 and T2 bonds, indicating that there are Si–O–Hand Si–O–C bonds in the material. Since 3-aminopropyltrimethoxysilane(APTMS) was added during the synthesis of LPMSN, the Si–O–C bondsshould be due to the presence of APTMS. For functionalization of LPMSNwith acid (SO3H) or base (NH2) groups, the as-synthesized LPMSN was fur-ther reacted with an organosilane 3-(mercaptopropyl)trimethoxysilane(MPTMS) or APTMS, respectively. As shown in the 13C NMR spectra ofLPMSN-SO3H (Figure 1.8a), three peaks at 11, 18 and 54 ppm correspond tothe carbons of the Si–CH2–CH2–CH2–SO3H from left to right, respectively,indicating the appearance of the acid functionality. On the other hand, thereare three distinct peaks at approximately 11, 22 and 42 ppm representing thecarbons of the Si–CH2–CH2–CH2–NH2 from left to right, respectively,68 asdepicted in the 13C NMR spectra of LPMSN-NH2, proving the existence of thebase functionality (Figure 1.8b). The peak of around 71 ppm indicates theexistence of Brij97 residue.

Since the as-synthesized LPMSNs also exhibit base functionality, wequantified the amounts of functional groups for all four samples (i.e.,LPMSN, LPMSN-SO3H, LPMSN-NH2 and LPMSN-Both) by 29Si NMR in orderto distinguish the degree of different functionality. After deconvolution of29Si NMR peaks, we calculated the amounts of hydroxyl group and functionalgroup on the surface of each sample. As summarized in Table 1.4, on thebasis of the functionality of LPMSN (i.e., 1.06 mmol g�1), LPMSN-Both ex-hibited the highest amount of functional groups (i.e., 2.32 mmol g�1), in-dicating the successful addition of both acid and base groups. AlthoughLPMSN contains base groups (from APTMS during synthesis), the amount ofits functional group is less than LPMSN-NH2 that was further grafted withAPTMS. In addition, the surface areas and pore sizes of LPMSN-SO3H,

(a) (b)

Figure 1.8 13C solid-state NMR spectra of (a) LPMSN-NH2 and (b) LPMSN-SO3H.

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LPMSN-NH2 and LPMSN-Both decreased as compared to those of LPMSN,indicating a pore filling effect upon functionalization (Table 1.4).69

The acidity of each LPMSN catalyst was estimated using following process.Samples were added into properly chosen indicator solution, and the colorchange of the solution was observed. As summarized in Table 1.4, the se-quence of acidity (i.e., lower pKa value) is LPMSN-SO3H4LPMSN4LPMSN-Both4LPMSN-NH2, indicating that LPMSN-SO3H is the strongest acidcatalyst and LPMSN-NH2 is the strongest base catalyst. Although LPMSN alsocontains NH2 groups, the silanol groups on the surface of LPMSN wouldprovide acidity, giving a weak acid property in total.

1.4.4 Cellulosic Conversion by using LPMSN-based Catalysts

Fructose (15 mg) and LPMSN-based catalysts (4 mg) were added into[EMIM]Cl (150 mL), and the whole mixture was heated at 120 1C for 3 h. Asshown in Figure 1.9a, the yields of HMF converted from fructose with thepresence of four LPMSN-based catalysts were similar with each other (inthe range around 66–70%). It has been considered that an acid catalyst isnecessary for the dehydration of fructose to produce HMF. However, wesuggested that fructose could be easily converted into 5-HMF in the ILssystem with a high temperature (120 1C) because such conditions (ILs andhigh temperature) favor dehydration. Therefore, there is no differencebetween all LPMSN-based catalysts. Other groups have also reported similarfindings.

The yields of HMF converted from glucose with the presence of fourLPMSN-based catalysts are shown in Figure 1.9b. It can be clearly seen thatthe cases with LPMSN-Both and LPMSN-NH2 catalysts exhibited the highestyield around 13%, in contrast to that of LPMSN-SO3H (ca. 10%) and LPMSN(ca. 7%). The conversion of glucose to HMF involves two steps: isomerizationof glucose to fructose and dehydration of fructose to HMF. In general, basecatalysts are helpful for isomerization of glucose to fructose.51 From theresult of fructose-to-HMF conversion, we have confirmed that four differentLPMSN catalysts had a similar effect on the dehydration of fructose to HMF;

Table 1.4 Summary of porous properties, acidity, and surface functionality ofLPMSN-based catalysts.

Samples

Specificsurfacearea/m2 g�1

Poresize/nm Acidity/pKa

Functionalgroup/mmol g�1

Hydroxygroup/mmol g�1

LPMSN 233.2 31.4 2.0–4.8 1.06 6.12LPMSN-

NH2

166.3 32.2 9.3–15 1.67 5.28

LPMSN-SO3H

170.0 28.1 0.8–2.0 1.35 5.14

LPMSN-Both

63.0 26.7 7.7–9.3 2.32 2.51

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in other words, HMF can be easily converted from fructose in our reactionsystem. Therefore, the high HMF yields for the cases using LPMSN-Both andLPMSN-NH2 indicate that base catalysts indeed promote the production offructose converted from glucose, and the glucose-to-fructose conversion canbe regarded as the rate-determining step in the glucose-to-HMF conversion.

To stimulate the structure of pre-hydrolyzed cellulose, we used cellobioseas the reactant and studied its conversion with the presence of LPMSN-basedcatalysts. The yields of glucose and HMF converted from cellobiose areshown in Figure 1.9c. It can be seen that the LPMSN-SO3H exhibited thehighest yields of both glucose and HMF (25.6 and 18.9%, respectively). Thecellobiose-to-HMF conversion contains three steps: hydrolysis of cellobiose-to-glucose, isomerization of glucose-to-fructose and dehydration of fructoseto HMF. Because acid catalysts can facilitate the first step, we suggest that itis the reason why the highest yield of glucose appeared in the case ofLPMSN-SO3H. In addition, it can also be proposed that the cellobiose-to-glucose is the rate-determining step of the three reactions. Therefore, cata-lysts with stronger acidity (i.e., LPMSN-SO3H and LPMSN) would favor theproduction of glucose that was then converted to HMF, resulting high yieldsof glucose and HMF.

We further directly used pre-hydrolyzed cellulose as the reactant andperformed the cellulose-to-HMF conversion in ionic liquid system with the

(a)

(c)

(b)

(d)

Figure 1.9 Yields of products converted from different reactants with the presenceof four LPMSN-based catalysts. (a) Fructose-to-HMF, (b) glucose-to-HMF,(c) cellobiose-to-HMF, and (d) cellulose-to-HMF conversions.

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presentence of four LPMSN-based catalysts. As shown in Figure 1.9d, thehigh yields of glucose and HMF were found in the cases of LPMSN-SO3H(35.8 and 19.2%, respectively) and LPMSN-Both (36.3 and 14.7%, respect-ively). It is predictable that LPMSN-SO3H showed the highest efficiencytoward cellulosic conversion because it exhibits the strongest acidity that canfacilitate the hydrolysis of cellulose. In fact, several groups have also syn-thesized SO3H-functionalized ILs for effective cellulosic conversion.70 How-ever, it is surprising to us that the LPMSN-Both also provided high yields ofglucose and HMF, even its acidity is less than LPMSN-SO3H and LPMSN.

Since the cellulosic conversion involves a series of complicated reactionsthat need different acid and base catalysts in each step, we propose that theenhanced efficacy of LPMSN-Both could be attributed to the cooperativecatalysis of both acid and base functional groups in the LPMSN-Both. Inorder to prove our hypothesis, we used a mixture of LPMSN-SO3H andLPMSN-NH2 (1 : 1 in weight ratio) as the catalyst for the cellulose-to-HMFconversion at the same reaction conditions. The reaction efficacy of themixed catalyst was compared with that of LPMSN-Both. As shown inFigure 1.10, the yields of cellobiose, glucose, and HMF for the case of mixedcatalyst were all similar to those for the LPMSN-Both case, indicating thatthe acid-and-base mixed catalyst exhibited the same efficacy as acid- andbase-containing catalysts. The results above indeed prove that the unusualcatalytic enhancement is a strong indication of the existence of cooperationbetween the acid (SO3H) and base (NH2) groups in our LPMSN-Both system.

1.5 ConclusionsIn this chapter, we have demonstrated the successful synthesis of multi-functionalized MSNs as effective, reliable, and re-usable solid catalysts for

Figure 1.10 Yields of cellobiose, glucose, and 5-HMF converted from cellulose usinga LPMSN-NH2 and LPMSN-SO3 mixed catalyst (Mixed) and a LPMSN-Both catalyst.

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cellulosic biomass conversion. In the enzyme-assisted catalytic system, weoptimized the reaction conditions for cellulase-immobilized solid catalystsin cellulosic hydrolysis. For the first time, carboxyl-group-functionalizedMSNs with large pore size of 40 nm were synthesized and used to chemicallylink cellulase. The proposed cellulase-assisted biocatalyst exhibits a highcellulose-to-glucose conversion efficiency (over 80%) with outstanding sta-bility. In the chemical-assisted catalytic system, we demonstrated thesynthesis of MSNs with both acid and ionic liquid groups. Such bi-functionalized MSN solid catalysts have enhanced the production of HMFfrom fructose dehydration in mild conditions using DMSO as a solvent. Thekinetics study has indicated that our bi-functionalized MSN could acceleratefructose dehydration by reducing the activation energy required. In addition,we also demonstrated the synthesis of large-pored mesoporous silicananoparticles (LPMSN) and functionalization of LPMSN with acid, base, andboth acid and base groups. The functionalized LPMSN-based catalysts haveshowed enhanced catalytic efficacy toward cellulosic conversion includingfructose-to-HMF dehydration, glucose-to-fructose isomerization, and cello-biose-to-glucose hydrolysis. The bi-functionalized LPMSN enhanced theyields of glucose and HMF directed converted from cellulose, indicating thecooperative catalytic ability. We envisage that the multi-functionalizedLPMSN materials could serve as new selective catalysts for other importantreactions.

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67. C. Gill, B. Price and C. Jones, J. Catal., 2007, 251, 145.68. H. M. Kao, C. H. Liao, A. Palani and Y. C. Liao, Microporous Mesoporous

Mater, 2008, 113, 212.69. K. T. S. Ikeda, Y. H. Ng, Y. Ikoma, T. Sakata, H. Mori, T. Harada and

M. Matsumura, Chem. Mater., 2007, 19, 4335.70. A. S. Amarasekara and O. S. Owereh, Catal. Commun., 2010, 11,

1072.

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CHAPTER 2

Mesoporous Silica SupportedSingle-site Catalysisy

PRANAW KUNAL AND BRIAN G. TREWYN*

Department of Chemistry and Geochemistry, Colorado School of Mines,Golden, CO 80401, USA*Email: btrewyn@mines.edu

2.1 IntroductionRapid increases in the cost of feedstock chemicals, precious metals, andrecent environmental concerns about chemical waste have caused a shiftfrom homogeneous catalysis to heterogeneous catalysis where stability andrecyclability are clear advantages. The times of utilizing stoichiometricreagents for organic chemical transformations to convert starting reagents toproducts are long gone. Today, optimized catalysts are critical to maximizeproduct yield and minimize the thermal and capital expenses that contributeto increased chemical costs. Many highly efficient and selective catalystshave been developed in the last three decades establishing the field ofcatalysis. Among these, homogeneous catalysts, mainly organometallic,enjoyed a substantial growth and were used with diverse reactive reagents toproduce a large number of fine chemicals. The significant drawbacks ofhomogeneous catalysis include challenging molecular synthesis, difficulty inseparating and recycling, and poor thermal stability to name a few. Theadvantages of homogeneous catalysis are the feasibility of followingreactions via spectroscopic methods to gain an understanding of the real

yElectronic supplementary information (ESI) available. See DOI: 10.1039/9781849737494.

RSC Green Chemistry No. 33Heterogeneous Catalysis for Today’s Challenges: Synthesis, Characterization and ApplicationsEdited by Brian Trewynr The Royal Society of Chemistry 2015Published by the Royal Society of Chemistry, www.rsc.org

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active species, transition states, and mechanistic details, good productselectivity, and single-phase reaction conditions. The sheer volume andcapacity common to the petroleum and chemical industries make utilizingrationally designed homogeneous catalysts prohibitively expensive; i.e.,continuous flow reactors, which are often the reactors used in high-capacitysynthesis and cracking systems, are not compatible with homogeneouscatalysts.

Heterogeneous catalysis has been a major factor in the development ofsustainable processes in fuel and fine chemical syntheses. The advantages ofheterogeneous catalysis over homogeneous include easier separation,recyclability, decreased level of metal contamination in the products andselectivity based on substrate and structural parameters. One importantexample of heterogeneous catalysts is inorganic mesoporous materials.These materials have been extensively researched for the past two decades assupports for a variety of different active species. The research in the field ofmesoporous silica materials has been ever-growing since the discovery of theMCM family of mesoporous materials by the Mobil Corporation.1,2 A seriesof MCM-type mesoporous silica materials with tunable pore sizes and poremorphology were developed by varying surfactants as structure-directingtemplates. For example, MCM-41-type materials consisting of hexagonalchannels and MCM-48-type materials with cubic pores were synthesized.Having a high surface area, being easily controllable, and having relativelyuniform and narrow pore-size distributions, these materials were effectivelyutilized for various applications such as drug delivery, enzyme encapsu-lation, catalysis, sensors, and nanoelectronics.3–10 Over the last decade othermesoporous silica materials like SBA-, MSU-, FSM-, and MSN-10-typematerials have been developed and have been used for variety ofapplications as well.11–14

2.2 Synthesis and Structural Aspects of MesoporousSilica

Figure 2.1 shows the schematic formation of mesoporous silica in steps. Theacidic or basic medium used for the synthesis of these materials first leadsto the formation of self-assembled micelles (hexagonal array shown inFigure 2.1), followed by interaction with a silica source during hydrolysis and

Figure 2.1 The formation mechanism of mesoporous silica materials.

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condensation which leads to an inorganic mesostructured solid–surfactantcomposite. The final step involves surfactant removal either by calcinationor acid extraction resulting in the formation of inorganic mesoporous silicamaterials.

The synthesis of mesostructured materials can be controlled by varyingthe organic–inorganic interactions and cooperative assembly of the silicaspecies and surfactants. The final mesostructures are dependent on thesurfactant liquid-crystal phases or silica–surfactant liquid-crystal-likephases. Often the critical micelle concentration (CMC) values for a givensurfactant need to be between 0 and 20 mg L�1 in order to achieve orderedmesostructures.15 CMC values between 20 and 300 mg L�1 can be stra-tegically reduced to give ordered mesostructures. Surfactants with large CMCvalues usually result in cubic mesostructures and it is difficult to formordered mesostructures if the CMC values are above 300 mg L�1. The finalmesostructures formed from ionic surfactants are explained by usingpacking parameter values (g value). The g value can be calculated usingg¼ V/(a0l), where V is the total volume of the hydrophobic chains (tail region)and cosolvent (organic molecules) between the chains, a0 is the effectivehydrophilic head group area at the aqueous–micelle surface, and l is thesurfactant tail length. Cubic (Pm3n) and 3D hexagonal (P63/mmc) meso-structures have g valueso1/3, 2D hexagonal (p6mm) and cubic bicontinuous(Ia3d) have g values between 1/3 and 1/2 and lamellar with gE1/2 – 1.

The hydrophilic/hydrophobic volume ratios (VH/VL) are suggested for theformation of different mesophases involving non-ionic surfactants. Blockcopolymers such as F108, F98, F127 and Brij 700 with high VH/VL ratiosusually direct the synthesis of cage-type cubic mesoporous materials withhigh topological curvature. Medium hydrophilic/hydrophobic volume ratioblock copolymers such as P123, B50-1500 form mesostructures with mediumcurvature like 2D hexagonal or 3D bicontinuous cubic (Ia3d). Tailoring thepore size of ordered mesoporous materials is one of the most remarkablefeatures.16 Several methods have been used to control the pore sizes ofmesoporous silica as illustrated in Table 2.1.15

Hydrophobic groups in the surfactants play a large part in controlling thepore size of the mesoporous materials. Larger pore sizes are observed whenthe length of alkyl chains of cationic quaternary surfactants is increased; forexample, when the surfactant chain length is increased from C8 to C22, thepore diameter derived from the Barrett–Joyner–Halenda (BJH) methodincreases from 1.6 nm to 4.2 nm.17 The pore size of MCM-48 mesostructurescan be tuned by adjusting the carbon chain length in a cationic geminisurfactant increased from 1.6 nm to 3.8 nm.18 Mesoporous silicas syn-thesized using block copolymers have larger pore sizes as compared to lowmolecular weight surfactant systems. Pore sizes for conventional PEO-PPO-PEO triblock copolymers are increased with growing molecular weights ofthe hydrophobic blocks rather than those of the copolymers.19 Diblockcopolymers often lead to larger pore sizes than triblock copolymers ofsimilar molecular weight because the PPO chains of the triblock copolymers

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tend to bend during aggregation. Pore-size tuning of MCM-41 materials canalso be achieved by using a mixture of two surfactants with alkyl chainlengths ranging from C8 to C22. The range of pore sizes from the resultingmaterials obtained is between the pore sizes of the two surfactants whenused individually. Pore sizes increase upon adjusting the fraction of thesurfactant with the larger chain length.17 A good sense of the suitability of aswelling agent while using a particular surfactant can be made by con-sidering the structure of the surfactant. For instance P123 has B70 wt%of hydrophobic PPO domains that can accommodate the hydrophobicswelling agent whereas F127 has only B30 wt% of hydrophobic PPO.16 Largeorganic hydrocarbons such as 1,3,5-trimethylbenzene (TMB), dodecane,triisopropylbenzene (TIPB), tertiary amines, and polypropylene glycol can actas micelle swelling agents/expanders to increase the pore size.20–23

The challenges of using the micelle expanders include the tendency toreduce the mesoporous structure order or even change the structure type: forexample, changing SBA-15 to mesocellular foam.20–25 If the swelling action istoo strong, well-defined large pore structures are not formed and no no-ticeable pore enlargement occurs if the swelling action is weak.26–28 Only alimited solubilization of swelling agents in the pore-forming micelles leadsto the retention of highly ordered mesoporous structures with a significantincrease in pore size.16 For optimum swelling action, Pluronic P123 shouldbe used with swelling agents less soluble in pluronic surfactants like TIPB orcyclohexane; similarly Pluronic F127 should be used with swelling agentsmore soluble in pluronic surfactants like xylene, ethylbenzene, andtoluene.28–32 For ordered mesostructures, the addition of TMB increases thepore size to 13 nm for SBA-15-type materials and to 6 nm for MCM-41-typematerials. Trimethylbenzene increases the pore size to 40 nm in acidictriblock copolymer systems and to 10 nm in basic CTAB surfactant systems.The addition of sodium dioctyl sulfosuccinate (AOT) and TMB leads tohighly ordered 2D hexagonal mesoporous silicates with 11 nm pores.33

Table 2.1 Pore sizes of ordered mesoporous structures obtained using differentmethods.

Pore size/nm Method

2–5 Surfactants with different chain lengthsQuaternary cationic salts and neutral organoamines

4–7 Long-chain quaternary cationic salt surfactantsHigh-temperature hydrothermal treatments

5–8 Charged surfactants with the addition of organic swelling agentssuch as TMB and mid-chain amines

2–8 Non-ionic surfactants4–20 Triblock copolymer surfactants4–11 Secondary synthesis, for example water–amine post-synthesis10–27 High-molecular-weight block copolymers such as PI-b-PEO,

PIB-b-PEO and PS-b-PEO triblock copolymers with the addition ofswelling agents such as TMB and inorganic salts

Low-temperature synthesis

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The pore diameter also expanded when other substances were incorporatedthat can be solubilized in the micelle core. For example, super critical (sc)carbon dioxide was used as a swelling agent for tuning the pore size duringsilicate hydrolysis for hexagonal mesoporous silica.34 It was also discoveredthat incorporation of a specific concentration of metallic nanoparticles inthe system where P123 was used as the surfactant and tetraethyl ortho-silicate (TEOS) as the silica source could also expand the mesochannels.35

Additionally, the pore size can be tailored by changing the initial synthesistemperature and hydrothermal treatment conditions.27,36 Carrying out thesynthesis at a low temperature (B25 1C) showed that the combination ofpluronic block copolymers with the appropriate swelling agents formmesoporous silica with an unusually large pore size.37 Lowering the initialtemperature from 25–40 1C to 15 1C in the synthesis of FDU-12 templated byF127 in the presence of TMB led to a doubling of the pore diameter to27 nm.36 Hydrothermal temperature can significantly affect pore size; thepore sizes of SBA-15 can be altered from 4.6 nm to 10 nm and from 9.5 nm to11.4 nm by increasing the hydrothermal temperature from 70 to 130 1C andby increasing the time from 6 h to 4 days.38–41 Similar trends were seen formesoporous silicates with body centered cubic Im3m mesostructures and forcubic bicontinuous Ia3d mesostructures when using F127 and the triblockcopolymer P123 as a template with butanol as a co-solute, respectively.42,43

Increasing the hydrothermal treatment conditions from 45 1C and 24 h to100 1C and 48 h resulted in SBA-16 with larger mesopores, thinner porewalls, and reduced intrawall micropores. The pore sizes of mesoporous silicawith Ia3d symmetry could be tuned from 4 to 10 nm by increasing thehydrothermal temperature from 65 to 130 1C.42

2.2.1 Functionalization Techniques for Mesoporous Silica

There are two common techniques used to functionalize the surfaces ofmesoporous silica with organic moieties for supporting single-site catalyticgroups. The first approach, post-synthesis grafting, is the more popularmethod due to fewer synthetic variables that go into the preparation.As is shown in Scheme 2.1b, either a modified homogeneous catalyst ora metal-free organic ligand is covalently immobilized on the surface ofpresynthesized mesoporous support through a silylation reaction, typicallyin a moisture-free environment. The lack of moisture is critical to avoid self-condensation of organosilanes. While the porous structure stays intact, theplacement of the ligands/catalysts may not be uniform, but focused on theexternal surface and near the pore openings. This phenomenon is typicallydependent on the pore size and the freedom of mass transport into thepores. The reactivity of the surface silanol groups is a diffusion-dependentcharacteristic and the kinetically most accessible regions are on the exteriorand pore openings.

The other approach, in situ co-condensation, is a direct synthesis methodin which the organic functional group (organoalkoxysilane) is introduced to

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an aqueous solution (acidic or basic) of template-forming surfactant alongwith the silanol precursor (i.e. tetraethylorthosilicate, TEOS), illustrated inScheme 2.1a. The condensation of both the organoalkoxysilane and TEOSoccurs simultaneously (co-condensation) leading to uniform distributionof the organic ligand on the surfaces of the mesoporous materials. Thereare some limitations to this method: firstly, only water soluble organicfunctional groups can be used and they need to tolerate pH extremes sinceeither acids or bases are catalysts for the silicate hydrolysis; secondly, theincorporation of bulky organic functional groups is not always successfulbecause the bulky groups interfere with silica condensation; finally,the amount of functional group incorporated in the mesoporous materialthis way cannot exceed 25% surface coverage without having a negativeeffect on the structure integrity. Some ingenious recent publications willbe discussed within this chapter that report chemical methods to measurethe spatial independence of ligands using the in situ co-condensationmethod.

2.3 Single-site Heterogeneous CatalystsThe types of supported catalysts can be divided into two major categories:single-site heterogeneous catalysis (SSHC) and multi-site heterogeneouscatalysis (MSHC). While these are general categories, they are suitable forthis review which focuses on SSHC. Multi-site heterogeneous catalysis, alsocalled connected-site, is defined as closely packed atoms of reactive metal,metal oxides, alloys and, rarely, halides. Spatial independence is not acharacteristic of this type of catalyst and strong interactions occur betweenactive sites, making kinetic and thermodynamic analyses very challenging.In contrast, SSHC consists of spatially independent active sites and lacksinteractions between active sites, making the interactions between eachsupported active site and reactant equivalent. This review will discuss SSHCsupported on various mesoporous materials. We will focus, in detail, on

(a)

(b)

Scheme 2.1 Schematic illustration of the two most common surface functionalizationtechniques for mesoporous materials: (a) the in situ co-condensationmethod, and (b) the post-synthesis grafting method.

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several groundbreaking studies that have been recently published in thepeer-reviewed literature. Since the first report by Mazzei et al.44 to immo-bilize rhodium cationic complexes on clays for carrying out asymmetrichydrogenation of substituted acrylic acid, a large volume of work has beenpublished and shared.45 Two important structural features of SSHC arespatial separation and consistency in the structures of the active sites. Thisensures constant energetic interaction between each active site and thereactant, thereby minimizing additional variables that frequently complicatecatalytic studies. In many conditions, these catalysts are superior to MSHCwhere significant energetic interactions between the active sites lead toadditional phenomena, i.e. oscillatory and chaotic behavior, making thechemistry tedious to interpret and comprehend even for simple chemicalprocesses. While characterizing these catalysts remains challenging, com-putational and kinetic measurements of SSHC are less problematic thanthose of close-packed heterogeneous catalysts.45–47 One of the remarkablefeatures offered by these catalysts is spatial restriction in the pores ofmesoporous materials which has been utilized for asymmetric catalysis.48

This review of single-site heterogeneous catalysts will discuss both metal-coordinated and organic-ligand-tethered mesoporous silica nanoparticles asadvanced catalysts.

2.3.1 Examples of Single-site Catalysts

Huang et al. supported two mutually incompatible groups; basic primaryamines and sulfonic acids on the same support by taking advantage ofspatial separation between internal and external surfaces of the mesoporoussilica nanoparticles (MSNs).49 The two surfaces of MSNs were functionalizedindependently through synthetic design. The internal surface was decoratedby adding the desired silane in situ along with the silica monomer and theexternal surface was decorated by reacting the desired silane post-synthetically prior to removal of the surfactant from the MSNs. Scheme 2.2below shows the synthesis of one of the acid–base catalysts, which wasprepared by co-condensing the silica monomer, TEOS, and 3-mercaptopro-pyltrimethoxysilane (MPTMS) in the presence of the pore-templatingsurfactant, cetyltrimethylammonium bromide (CTAB), under basic reactionconditions followed by post-synthetic grafting with 3-aminopropyl-trimethoxysilane (APTMS). The presence of CTAB in the mesoporous chan-nels directs the APTMS to the external surface and the concentration ofsulfonic acid was kept equal to the amine group.

Another MSN-based catalyst was synthesized where APTMS was co-condensed and MPTMS was grafted using similar methods to those shownin Scheme 2.2 to form APMSN-SA. The mesoporous structure was main-tained on these synthesized materials with type IV isotherms, high surfaceareas and pore volumes, and narrow pore-size distributions. The successfulincorporation of organic functional groups was observed when the productwas characterized using powder X-ray diffraction (XRD), nitrogen sorption,

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and 13C and 29Si solid-state NMR spectroscopy. Organic functional grouploading was quantified via elemental analysis. The activity for a one-potreaction sequence involving acid-catalyzed hydrolysis of an acetal followedby a subsequent base-catalyzed Henry reaction was tested as shown inTable 2.2.

(CH2)3NH2

H2N(H2C)3

(CH2)3NH2

H2N(H2C)3

1) CTAB,NaOH,H2O

2) APTMOS, toluene

(CH3O)3Si(CH2)3SH(STMOS)

+Si(OCH2CH3)4

(TEOS)

SAMSN-AP

3) H2O2,CH3COOH4) CTAB removal5) 150 oC, 0.1 mmHg

Scheme 2.2 Synthesis of bifunctional MSN having sulfonic acid groups on theinternal surface and organic amine groups on the external surface.

Table 2.2 Cascade reaction consisting of acid-catalyzed hydrolysis and base-catalyzed Henry reaction. Reaction conditions: catalyst, 100 mg(1.5 mmol); H2O, 1.5 mmol; CH3NO2, 1 ml; 80 1C; 24 h.

O2N

OCH3

OCH3

O2N

CHO

O2N

NO2Acid

H2O

Base

CH3NO2A B C

Entry Catalysta Bb (%) Cb (%) Conv. of Ab (%)

1 SAMSN-AP 2.3 97.7 1002 APMSN-SA 1.9 98.1 1003 SAMSN/APMSN 4.5 95.5 1004 SAMSN 100 0 1005 APMSN 0 0 06 SAMSN-AP/AP 0 0 07 SAMSN-AP/PTSA 100 0 1008 APMSN-SA/AP 0 0 09 APMSN-SA/PTSA 100 0 10010 MSN 0 0 0aAP: 1-aminopropane, PTSA: p-toluenesulfonic acid.bConversions and yield determined using gas chromatography (GC).

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Both bifunctional materials, SAMSN-AP and APMSN-SA, gave 100% con-version of A and a very high yield of the desired product C by catalyzing thereaction in a cascade fashion. The catalytic activity for the same reaction wassimilar for physically mixed APMSN and SAMSN (see entry 3 in Table 2.2).However, when a single functionality was incorporated on the MSN surface(either acid or base) there was a lack of measured product C formation.SAMSN-AP or APMSN-SA when combined with the homogeneous analoguesof sulfonic acid (PTSA) and propylamine (AP) gave no product C either, sincethe free acid and base molecules neutralized the corresponding hetero-geneous base or acid. These catalysts were recyclable up to five times with anegligible decrease in catalytic activity. The kinetics of the catalysts werestudied as well: incorporating both acidic and basic groups introduced bythe co-condensation method showed better reactivity (TOF, turnoverfrequency) than the analogous catalysts where grafting was used to tether thesame groups onto the external surface of the MSNs (Figure 2.2a). Theseresults indicate that diffusion limitations during the catalytic cycle might beminimized and the dispersion or surface coverage of the catalytic sites mightplay a role in the activity. To investigate this, a series of catalytic reactions wererun using MSN-based catalysts with five different concentrations of amines(Figure 2.2b). The catalytic activity by measuring TOF decreased steeply as thesurface coverage of the catalysts increased, showing a direct relationshipbetween the catalysts activity and the catalyst coverage on the MSN surface.

Rothenberg et al. also used MSN to support two antagonistic functionalgroups:50 they reported the synthesis of MSN-supported catalysts withsite-isolated amine and phosphotungstic acid groups. Amine groups wereattached to SBA-15 using the grafting method and the resulting material wasthen treated with a methanolic solution of the phosphotungstic acid(H3PW12O40) which led to a bifunctional (acid–base) catalyst as shown inScheme 2.3.

This bifunctional catalyst demonstrated high conversion and yield forthe acid-catalyzed deacetalization of benzaldehyde dimethylacetal followed

Figure 2.2 (a) Turnover frequency of the acid and base catalysts located on eitherthe internal or external surfaces of SAMSN-AP and APMSN-SA. (b) Fittedcurve of base activity versus base concentration on the APMSN surface.

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by the base-catalyzed Henry reaction between 4-benzaldehyde and nitro-methane, and base-catalyzed aldol condensation of 4-benzaldehyde andmalononitrile as shown in Scheme 2.4.

The catalyst activity for the tandem reaction was governed by the tunableratio of aminopropyl (AP) groups and phosphotungstic acid (HPW) groupson the MSN support. When the AP/HPW ratio on MSN was tuned to 2 : 1, halfof the total amine on the MSN surface was used for immobilizing polyacidswhereas a ratio of 1 : 1 stopped the reaction sequence at step 1 with theformation of benzaldehyde as all the AP groups of the MSNs were used forpolyacid immobilization, leading to only acidic sites being present on theMSN catalyst surface.

Asefa et al. developed MSN-based bifunctional, highly efficient catalystsfor the Henry reaction.51–53 The catalysts were prepared by post-syntheticgrafting of 3-APTMS on MCM-41-type mesoporous silica materials atelevated temperature in ethanol and toluene and were labeled as AP-E1,AP-T1, and AP-T2 as described in Scheme 2.5.

The organoamine loading was the highest for AP-T2 (4.3 mmol g�1)followed by AP-T1 (4.1 mmol g�1), and AP-E1 had the lowest loading of1.3 mmol g�1. Catalytic studies for the Henry reaction showed that AP-E1was a superior catalyst to AP-T1 and AP-T2. Taking into consideration thefact that AP-E1 had the lowest density of catalytically active organoaminegroups of the three catalysts synthesized, it was thought that surface silanolgroups activated the carbonyl group of benzaldehyde to facilitate thenitroaldol reaction as shown in Scheme 2.6. Grafting additional organicgroups led to significantly reduced catalytic efficiency indicating theimportance of well-isolated active sites and silanol groups for improved

S SB SAB

Scheme 2.3 Synthesis of bifunctional catalyst by grafting APTMS to SBA-15 followedby controllable immobilization of H3PW12O40 using the –NH2 group ofimmobilized APTMS.

CHO

NO2

SAB,H2O-2CH3OH

OCH3

OCH3 NC

1.SAB,CH3NO2

2.SAB,CH2CN2

-H2O

-H2O

CN

Scheme 2.4 One-pot tandem deacetalization followed by (1) a Henry reaction and(2) an aldol reaction.

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catalytic efficiency. Improved catalytic efficiency was demonstrated onsamples with a higher surface area which contributed to lower ligandloading. The authors also demonstrated that the nature of the solvent usedfor grafting played a major role in the spatial distribution of the organoa-mines on the MSN surface: spatial distribution in ethanol occurred due tothe competition for the aminopropylsilane between ethanol which is a proticpolar solvent and hydrophilic surface silanol groups, whereas for toluene, anon-polar aprotic solvent, aggregation of aminopropylsilanes occurred andled to their preferential interaction with the silanol groups.

Bass and Katz used a thermolytic imprinting method to approachsub-nanometre patterning of mixed organic functional groups within

AP-E1

AP-T1,AP-T2

Scheme 2.5 Reaction scheme for post-synthesis grafting aminopropyl groups inethanol at 78 1C (AP-E1) and in toluene at 78 1C (AP-T1) and in refluxingtoluene at 112 1C (AP-T2).

SiO

O O

SiO

OO

Si SiO

O O

OSi

O O

O

SiO O

OSi

HO O

H2N

SiO

O

T3T2

HO

H

HO

NH3+

H

OH

HO

CH2NO2CH2NO2

SiHO

OO

Si SiO

O

O Si

+

O O O O

O

SiO

SiHOO O

SiO O

SiO

O SiO

OSiO

H2NH2NH3N

+

T2 T2 T3 T3

O

HHO

CH2NO2

AP-E1 AP-T1,AP-T2(a) (b)

H3N

Scheme 2.6 Proposed reaction mechanism illustrating the enhanced efficiency of(a) AP-E1 in the Henry reaction relative to (b) AP-T1 and AP-T2. Thepresence of a significant number of spatially isolated silanol groups inAP-E1 led to the activation of the carbonyl group of benzaldehyde fornucleophilic attack.

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discrete active sites on the MSN surface.54 The imprint 1 containing axanthate-protected thiol with the ethoxysilane group, which facilitatesgrafting on to the MSN surface, was used. Also imprinting was achieved formixed functional groups; thermally labile carbamate, and xanthate groupswere simultaneously incorporated in the molecules 2, 3, and 4. Molecule1 led to a single thiol group after deprotection by thermal treatment whereas2 led to two thiol groups and one amine group and 3 and 4 both led to onethiol and amine group each (Figure 2.3).

Solid-state UV-Vis (xanthate absorption seen at 280 nm) and 13C CP/MASNMR spectroscopy was used to characterize the molecules tethered on MSNbefore and after the deprotection steps. 29Si CP/MAS NMR was used todetermine the degree of imprint condensation with T3/T2/T1 ratios of 25/55/20 thus implying multiple point connectivity. The quantification of amineand thiol groups was done by titration with perchloric acid and Ellman’sreagent and ratio of the two groups were found to be close to 1 : 1 for mixedmaterials 3-S and 4-S and 2 : 1 for 2-S. The Ellman’s reagent selectivelyconverted thiol groups to a nitrobenzoic acid moiety as shown inScheme 2.7; this acidic species showed an absorbance band at 332 nm insolid-state diffuse-reflectance (DR) UV-Vis absorption. The local organizationof functional groups on the imprinted materials was measured usingo-phthaldehyde as a selective probe for thiol–amine pairs. Fluorescentisoindole species were formed due to the reaction of o-phthaldehyde with onethiol and one amine group, this species was confirmed by observing itsabsorbance peak at 330 nm in solid-state DR UV-Vis spectra as well as afluorescence emission peak at 410 nm. The absolute amount of thiol–aminepairing was estimated by comparing the solid-state DR UV-Vis absorptionspectra of chromophore 7 with 6 (Scheme 2.7). The highest value of pairing wasobserved for materials imprinted with thiol–amine bifunctional moiety 4 ascompared to the materials prepared by grafting a 1 : 1 molar ratio of 3-MPTESand 3-APTES and by grafting a 1 : 1 molar ratio of the corresponding xanthateand carbamate silanes and thermally deprotecting them after grafting.

The reaction of o-phthaldehyde with thiol–amine pairs was also used tocharacterize the site isolation of imprinted sites. Carbamate imprint 2 was

(EtO)3Si

(EtO)3Si

(EtO)3Si

S

S

O O

S

S

O

O

NH

O

S

S Si(OEt)3

S

S

O O

O

NH

S

S

O O

O

NH

1 3

45

O

O

NH

Si(OEt)3

Si(OEt)3

Si(OEt)3

Si(OEt)3

Si(OEt)3

2

Figure 2.3 Molecular imprints used by Bass and Katz for templating sub-nanometreimprints in MSN. Grafting was used to tether these organic moieties tothe MSN surface followed by thermal deprotection of xanthate group tothiol and carbamate group to amine resulting in materials termed 1-S,2-S, 3-S, and 4-S.

Mesoporous Silica Supported Single-site Catalysis 39

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grafted onto the surface of two differently functionalized MSNs, 8 and 9, butwith the same surface distribution and active sites; the uniformity betweenthe two surfaces was achieved by the synthesis of 9 directly by thermaldeprotection of 8. An increase in thiol–amine pairing was seen in material 10over 11 which has contributed to a greater tendency of carbamate silaneprecursor 2 to condense on the MSN surface next to xanthate silaneprecursor 1 as compared to the condensation of 2 next to tetheredmercaptopropyl groups (formed after thermal deprotection of 1 on the sur-face) as illustrated in Scheme 2.8. In contrast, the condensation tendencyshown by xanthate silane 1 next to carbamate silane 2 and primary amine(formed by thermal deprotection of 2 on the surface) was the same. Thiol–amine pairing was also different based on the sequence of grafting, materialwith 1 grafted first followed by 2 showed different pairing as compared tothe case when 2 was grafted before 1. Such dependences indicated a non-random distribution of imprints on the surface possibly driven by kineticphenomena or specific interactions between the surface species.

Cooperative heterogeneous catalysis is a remarkable concept; supportingdifferent functionalities on a heterogeneous support which may worktogether in a cooperative way either to alter the reaction characteristics or toassist in performing several steps of a reaction sequence. One of the re-actions where this concept has been applied is the synthesis of bisphenols.Bisphenol A (BPA) and bisphenol Z (BPZ) are important feedstocks in the

Si

SS

Si

SiOO OH

OHO OO

O OH

SS NO2

NH2

O2N

HOOCCOOH

3-S

S

HOOC

O2N

S

COOHO2N

SH

Si

NH2

SiOO OH

OHO O

Si SiOO OH

OHO O

O

O

NS

6 7

(a) (b)

Scheme 2.7 (a) Derivatization of thiol groups on 3-S using Ellman’s reagentyielded nitrobenzoic acid 6 bound to the surface. (b) Derivatizationof thiol–amine pair by using o-phthaldehyde yielded fluorescentchromaphore 7.

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OS

S

Si

OS

S

Si

OS

S

Si

HS

Si

SH

Si

HS

Si

NH O

OO

SS

Si

OS

S

Si

OS

S

Si

OO

NH

Si

OO

NH

Si

OO

NH

Si

HS

Si

SH

Si

HS

Si

OO

NH

Si

OO

NH

Si

OO

NH

Si

(EtO)3Si

(EtO)3Si

NH O

O

SH

Si

SH

Si

HS

SiSi Si

NH2

Si

N2,250oC

HS

Si

SH

Si

HS

Si

NH2

NH2

NH2

NH2NH2

Si

Si

Si

8

9

10

11

N2, 250 oC

N2, 250 oC

Scheme 2.8 Interactions during condensation between carbamate 2 and immobilized xanthate 1 and mercaptopropyl species onhomologous surfaces 8 and 9 influenced the distribution of species on the final surfaces 10 and 11.

Mesoporous

SilicaSupported

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plastics industry especially due to their use as monomers in polycarbonatematerials and epoxy resins. They are synthesized using an acid-catalyzedcondensation reaction between a ketone and a phenol leading to a p, p0

isomer (desired product) and a o, p0 isomer (byproduct). A schematicrepresentation for the formation of bisphenols is shown in Scheme 2.9.

The addition of thiols as cocatalysts has been shown to improve thekinetics of the reaction for the formation of bisphenols as well as theselectivity of the p, p0 isomers.55,56 Davis et al. reported a novel route forthe synthesis of acid/thiol-paired heterogeneous catalysts by designing anorganosilane precursor which could be tethered to the support at two pointsusing the grafting method and could be cleaved later to deprotect thecatalytic sites.57 The bis-silane precursor having a disulfide group and anaryl sulfonate ester group separated by two methylene groups was syn-thesized using the procedure outlined in Scheme 2.10.

To demonstrate that bis-silane 3 could be incorporated into MSN to forman organic-inorganic hybrid material, the authors used SBA-15 as the start-ing material due to its high surface area, large pore diameter, and frameworkrigidity and grafted the bis-silane (SBA-g3). A deliberate low loadingof B0.2 mmol g�1 was achieved to ensure that the effect of pairing could beobserved. The XRD pattern of SBA-g3 showed three characteristic peaks ofhexagonally ordered mesopores. Complete nitrogen sorption results areshown in Table 2.3 but, in particular, the surface area reduced to 230 m2 g�1

from 860 m2 g�1 and the pore size reduced to 5.8 nm from 6.3 nm aftergrafting the large bis-silane. Such a large decrease in the surface area couldnot be explained solely by the presence of organic surface groups, it waslikely that some pore openings were blocked by organic species makingthem inaccessible to the adsorbent. The tethered organic fragment wasconfirmed using 13C CP/MAS NMR spectroscopy, the 13C CP/MAS spectrumof the grafted molecule matched well with the solution phase spectrum ofbis-silane 3 confirming the presence of intact bis-silane 3 after grafting. Thepresence of T1, T2, and T3 sites from 29Si CP/MAS NMR analysis correspondto the bis-silane 3 indicating its attachment to the surface at one, two, andthree points, respectively. Of these, T2 sites were the largest suggesting

O

O

OH

OH

+

+

+

+

OHHO HO

OH

OH

HOOHHO

Bisphenol A(p, p' isomer) o, p' isomer

Bisphenol Z(p, p' isomer)

o, p' isomer

Scheme 2.9 Syntheses of bisphenol A (top) and bisphenol Z (bottom) utilizingphenol and either acetone or cyclohexanone as starting materials.

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that silanes are covalently bound to the surface at two points. Theconfirmation that both the trialkoxysilane groups of 3 are attached to thesurface was deduced by the absence of T0 peaks, which would have appearedat �41 ppm. Thermogravimetric analysis (TGA) in air was used for thequantitative determination of the organic content of SBA-g3. After theintroduction of bis-silane 3 to the silica surface, deprotection was carried outin one step using aqueous tris(2-carboxyethyl)phosphine hydrochloride(TCEP �HCl) leading to SBA-AT-p (Scheme 2.11).

The success of the deprotection step was confirmed using 13C CP/MASNMR spectroscopy. A comparison of 29Si CP/MAS NMR spectrum of SBA-AT-pwith SBA-g3 showed an increase in T3 signal due to further condensation ofalkoxysilane moieties in the aqueous condition used for deprotection, but T2

still remained the most intense peak. The surface area of SBA-AT-p increasedto 430 m2 g�1 from 230 m2 g�1 for SBA-g3 probably due to the removal of

SS +

SH

OH

SS

OH

1

SH

SiOEt

OEtEtO

S

SiEtO OEt

OEt

S

OH

SiMeO

OMeOMe

+

S

SiEtO

OEtOEt

S

O S OOCl

Si

MeOOMeOMe

SO

O

23

Bis-silane

Scheme 2.10 Synthesis of disulfide silane 2 and bis-silane 3 acid/thiol functionalgroups used in the synthesis of MSN catalysts used for the formationof selective bisphenols.

Table 2.3 Catalyst characterization data.

Material SBETa/m2 g�1 Dp

b/nm H1 c/mmol g�1 SHd/mmol g�1 SH/H1

SBA-15 860 6.3SBA-g3 230 5.8SBA-A 0.20SBA-AT-p 430 6.0 0.21 0.19 0.90SBA-AT-r 0.13 0.12 0.92SBA-T 0.32aSpecific surface area, calculated using the BET method.bAverage pore diameter, calculated from adsorption isotherm using the BJH method.cAcid loading, measured by ion exchange/titration.dThiol loading, measured by reaction with Ellman’s reagent.

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mercaptoethanol and also unblocking of some blocked pore openings. SBA-AT-p was characterized for acid content and thiol content and the values areshown in Table 2.3. Notably, the acid/thiol ratio was close to 1 in support ofthis co-catalyst functionalization method.

For catalytic studies, catalysts containing (i) only thiol (SBA-T), (ii) onlyarylsulfonic acid (SBA-A), and (iii) randomly distributed arylsulfonic acid andthiol groups (SBA-AT-r) were also prepared as illustrated in Scheme 2.12.

The catalytic activity of various materials was tested for the synthesis ofbisphenols A and Z. For bisphenol A, the catalysts having both the thiol andacid groups in close proximity on the same silica support for both organized(SBA-AT-p) as well as randomly distributed (SBA-AT-r) exhibited similar ac-tivity but a remarkable advantage of the paired catalyst over the randomlydistributed catalyst was the selectivity (isomer ratio of 19.3 versus 15.2).SBA-T was inactive for the catalysis of bisphenol A synthesis as there was noacid group present, SBA-A showed some activity but low selectivity due toabsence of thiol sites, a physical mixture of SBA-A and SBA-T demonstratedlow activity and selectivity as the acid and thiol groups are unable to interactexcept at the outer catalytic sites whereas PTSA was able to interact withsurface bound thiols by entering the pores. For bisphenol Z, a similar trendwas seen but the selectivity using SBA-T-p was more than three times higherthan the selectivity observed for SBA-AT-r. These data suggests that catalyticcooperativity for the synthesis of bisphenol Z was more dependent onacid/thiol distance than for bisphenol A.

The thermolytic molecular precursor (TMP) method is a unique andefficient approach to attain single-site catalysts developed by Tilley et al. Thismethod was first used to synthesize non-mesostructured homogeneousmixed elemental oxides, for example ZrO2 � 4SiO2 was prepared by thermallydecomposing Zr[OSi(OtBu)3] with the elimination of alkene and water asshown in the following equation:58

Zr½OSiðOtBuÞ3�4 �!D

ZrO2 � 4SiO2 þH2C¼CMe2 þ 6H2O

A further development was made when mesostructured homogeneousmixed elemental oxides were synthesized using a mixture of the molecularprecursors and toluene solutions of various block polyalkyne oxide co-polymers. This was an important step forward as the synthetic solvent usedhere was non-polar (toluene) which inhibited metal aggregation unlike

Si

SS

O

EtOO

O

SO

O

SiO O

OMe

TCEP.HClH2O

Si

HO

EtOO

O

SO

O

SiO O

OMe

SH

SBA-g3 SBA-AT-p

Scheme 2.11 SBA-AT-p synthesized by deprotection from grafted disulfide/sulfonateester intermediate SBA-g3.

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aqueous solvents.59 Tilley et al. also covalently attached well-defined oxygen-rich organometallic molecular precursors onto the mesoporous silicasupport using post-synthetic grafting; this approach not only controlled thestructure of the catalytic site on the molecular level but also led to anexcellent spectroscopic model for the catalytic active sites.60–65 Tilley andRioux et al. recently reported Pd(II) centers supported on mesoporous silicausing the TMP method,66 where two novel tris(t-butoxy)siloxy palladium(II)complexes of the form (4,4 0-di-tert-butyl-2,20-bipyridyl)Pd-[OSi(OtBu)3](R)(where R¼OSi(OtBu)3 for complex 1 and R¼CH3 for complex 2) weresynthesized as shown in Scheme 2.13.

The structures of the complexes 1 and 2 were characterized using 1H, 13C,29Si NMR spectroscopy, and X-ray analysis: the latter showed that bothstructures were found to have slightly distorted cis square planar geometryaround the Pd center. The thermal decomposition behaviors of the com-plexes were studied using TGA and differential scanning calorimetry (DSC).

Si

SH

OMeOMeMeO

SBA-15Toluene

Si

SH

OO

HO

MPTMSSBA-T

SiOMe

OMeMeO

S OOO

SBA-15Toluene

Toluene

SiO

OHO

SBA-g4

S OOO

4

TCEP.HCl

SiO

OHO

SBA-A

S OOOH

Si OMeOMeMeO

SS

SiOMe

OMeMeO

S OOO

42

+

SBA-15

SiO

OHO

S OOO

SS

SiO

OHO

TCEP.HCl

SiO

OHO

S OOOH

HS

H2O

H2O

SiOO

HO

SBA-g2,4 SBA-AT-r

Scheme 2.12 Synthesis of SBA-T, SBA-A, and SBA-AT-r. Top: grafting 3-mercapto-propyltrimethoxysilane (MPTMS) onto SBA-15 generated SBA-T, con-taining only thiol sites. Middle: grafting sulfonate ester precursor 4generated intermediate SBA-g4, which was hydrolyzed to SBA-A, con-taining only acid sites. Bottom: grafting both disulfide 2 and sulfonateester 4 generates intermediate SBA-g2, 4 which was deprotected toform SBA-AT-r, containing randomly distributed acid and thiol sites.

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Under inert atmosphere, at about 195 1C, complex 1 condensed to form(tBu2-bpy)Pd � SiO2. Continued temperature increases led to the formation ofPd � SiO2, suggesting that ligand loss occurred beyond 195 1C. These resultswere confirmed by decomposition of 1 under vacuum at 250 1C for 2 hand identification of volatile elimination products such as HOSi(OtBu)3,t-butanol, isobutene, and water (1.1, 0.3, 0.2, and 0.2 equiv., respectively)using 1H NMR and the residual black decomposition product was identifiedas Pd0 using powder XRD. Under an oxygen atmosphere, decomposition of 1was fast and exothermic caused by Pd-catalyzed combustion of organicbyproducts leading to PdO formation. Under inert atmosphere, compound 2decomposed at 195 1C leading directly to Pd black formation. Decompositionof 2 under vacuum at 250 1C led to 0.4 equiv. of HOSi(OtBu)3 along with smallamounts (o0.1 equiv.) of t-butanol, isobutene, water, and methane with theformation of Pd0. Under O2 atmosphere; the decomposition behavior of 2 wasthe same as that of 1, resulting in the formation of PdO.

Precursors 1 and 2 were supported on a SBA-15 silica surface using thegrafting method in benzene under N2 atmosphere resulting in materialslabeled Pd(1)SBA15 and Pd(2)SBA15. The wt% of Pd was measured to be1.89 wt% and 1.90 wt% for Pd(1)SBA15 and Pd(2)SBA15, respectively, whichcorresponded to 0.16 Pd nm�2 for each. Low concentrations of 1 and 2 wereused to ensure a high spatial dispersity of Pd centers on the silica surface.Scheme 2.14 shows the synthesis of these SSHC.

The grafting reaction of complexes 1 and 2 with SBA was monitored usingsolution 1H NMR spectroscopy. Protonolysis of the Pd–OSi bond by thesurface silanol groups resulted in elimination of HOSi(OtBu)3. A reactionbetween excess 1 and SBA-15 in benzene-d6 produced 0.94 equiv. ofHOSi(OtBu)3 with a maximum Pd loading of 5.0 wt% and, for complex 2,0.97 equiv. of silanol was observed per equiv. of 2 reacted with 5.1 wt% Pdloading. Nitrogen sorption analysis of Pd(1)SBA15 and Pd(2)SBA15 showedtype IV isotherms which indicates that mesoporosity was maintained aftergrafting. The narrow pore-size distribution of SBA-15 was preserved upon

N

NPd

Cl

Cl

N

NPd

CH3

I

+

+

2KOSi(OtBu)3

2HOSi(OtBu)3

+ Ag2O

N

NPd

OSi(OtBu)3

OSi(OtBu)3

OSi(OtBu)3

N

NPd

CH3

(1)

(2)

CH2Cl2, 25 oC,16 h

PhMe, 25 oC,3 d (dark)

-2KCl

-AgI, -AgOH

Scheme 2.13 Synthesis of supported organometallic complexes 1 and 2 utilized byTilley and Rioux et al.66 for catalysis.

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grafting and the grafted materials exhibited reduced surface areas andpore volumes as compared to SBA-15. The properties of the two catalysts andSBA-15 are shown in Table 2.4.

FTIR spectroscopy showed several analogous peaks contributed to thetBu2-bpy ligand structure for both the Pd(1)SBA15 and Pd(2)SBA15 materialssuggesting intactness of the (tBu2bpy)Pd core after grafting. DR UV-Visanalysis for the surface structure (Figure 2.4) showed strong high energy

N

N

Pd R

OSi(OtBu)3

R = OSi(OtBu)3 or CH3

25 oC, 16 h+ SBA-15

N

N

Pd R

O

+ HOSi(OtBu)3

OHOHOH

Scheme 2.14 Grafting of 1 (R¼OSi(OtBu)3 and 2 (R¼CH3) onto the surface ofSBA-15.

Table 2.4 Nitrogen porosity and palladium loading data for the PdSBA15 materials.

MaterialPd content (wt%)[ICP-OES]a

Pd coverage/nm2

SBETb/

m2 g�1rP

c/nm

Vpore, avgd/

cm3 g�1

SBA15 666 2.8 0.685Pd(1)SBA15 1.89 0.16 458 2.8 0.500Pd(2)SBA15 1.90 0.16 467 2.8 0.504aAverage value of triplicates with relative standard deviations o4% in all cases.bSpecific surface area using the BET method.cAverage pore diameter.dAverage pore volume.

(A) (A) (B)

(B)

Figure 2.4 (1) DR UV-Vis spectra for (A) complex 1 and Pd(1)SBA15, and (B) complex2 and Pd(2)SBA15. (2) XANES (A) and extended EXAFS (B) region of PdK-edge for complex 1 and Pd(1)SBA15 at room temperature.

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absorption below 320 nm and weak, broad, low-energy absorption centeredat 350 nm assigned to a p-p* transition for coordinated tBu2bpy and chargetransfer between Pd(II) and the tBu2bpy. The spectra showed similar maximapositions and shape of absorption bands for 1, 2, Pd(1)SBA15, andPd(2)SBA15, which suggests a small electronic effect on the surface structureeven when the ligand moiety was changed from –OSi(OtBu)3 to –CH3 as wellas suggesting that the coordination environment around the metal centersremained the same after grafting. X-ray adsorption spectroscopy (XAS) ofprecursor 1 and Pd(1)SBA15 examined in the solid state provided furtherstructural information. Both the XANES and EXAFS regions of the Pd centersshowed similarity (Figure 2.4); the XANES region showed that the Pd centersin 1 are divalent pre- and post-grafting. An extended region of the spectrumindicated the number and identity of the immediate neighbors, so the oxi-dation state of the Pd remained þ2 and the coordination number remainsB4 for both 1 and Pd(1)SBA15.

The stability of silica-supported Pd centers was analyzed by TGA. A similartrend was observed for onset temperatures for the precursor 1 andPd(1)SBA15, and 2 and Pd(2)SBA15; a gradual weight loss suggests that thesupported Pd centers were more thermally stable than the correspondingmolecular precursors. In situ FTIR spectroscopy with heating under inertatmosphere also indicated that MSN-supported Pd centers were stable attemperatures near the decomposition temperature of the precursors. Thenature of the supported metal center was further analyzed by XAS for de-composition at lower temperatures. EXAFS spectra for Pd(1)SBA15 suggestedthat with the increase in temperature, Pd–X bonds (X¼C, O, and N)dissociated and Pd–Pd bonds formed which led to Pd aggregation. Afterexposing Pd(1)SBA15 to 200 1C under He, 92% of the Pd was in the þ2oxidation state whereas at 300 1C, 60% of the Pd remained as Pd21.

A comparison of MSN-supported Pd centers and molecular precursors wasmade by testing them for catalytic hydrogenation of 1-phenyl-1-propyne to(Z)-1-phenyl-1-propene. MSN-supported Pd centers exhibited better select-ivity and stability but slower kinetics. The improved selectivity of Pd(1)SBA15and Pd(2)SBA15 was attributed to the increased stability of the supportedmetal centers against particle aggregation as well as the preservation of thesupported Pd centers. Also, no leaching of Pd was observed during thereaction.

Stack et al. reported the incorporation of Fe(II) centers on a heterogeneoussystem in a site-isolated fashion for enhanced selectivity and catalytic ac-tivity for olefin epoxidation reactions.67 They used site isolation for creatingmetal coordination environments with labile exogenous ligands on the MSNsupport; these are otherwise not accessible in homogeneous medium. Usinga metal-templating (metal-exchange) approach on mesoporous silica, Stacket al. created a derivatized Fe(II)-bis(1,10-phenanthroline) which containedadditional labile ligands. The formation of stabilized Fe(II)-bisphen, whichis only transiently stable in homogeneous solutions, was supported byspectroscopic and catalytic oxidative reactivity studies. Thiol derivatives of

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phenanthroline 1 and 2 were used for elemental tagging for ligand identi-fication via ICP analysis (Figure 2.5).

Trialkoxysilyl groups enabled immobilization to mesoporous silica forligand 1. Ligand 1 was tethered on SBA-15-type mesoporous silica using twomethods: metal templating and random ligand grafting. The materials werenamed T, CuIIT, FeIIT, G, CuIIG and FeIIG where T is the material prepared bymetal-templating method (discussed below) and G stands for materialsprepared by random ligand grafting. Procedures shown in Schemes 2.15 and2.16 outline the preparation steps of different materials studied.

1. Metal templating. This was achieved using the following steps and isshown in Scheme 2.15.(a) Formation of thermodynamically stable [Cu12]1.(b) Covalent attachment to mesoporous silica using post-synthetic

grafting method.(c) Removal of Cu atom template using EDTA.(d) Metalation of immobilized metal templated 1 with Fe(OTf)2 to form

the final catalyst.2. Random ligand grafting. It involves two steps as shown in

Scheme 2.16.(a) Covalent attachment of ligand 1 on SBA-15.(b) Metalation of immobilized grafted ligand.

An equal loading of ligand 1 was achieved by both the random ligandgrafting and metal-templating methods. As shown in Table 2.5, the templatematerial proved better in that it achieved a 2 : 1 ratio of ligand : metal ascalculated by ICP analysis, thus ensuring the presence of active catalyticspecies in the desired form. The ligand : metal ratio was 2.9 : 1 for FeII-coordinated grafted materials, implying the formation of either a highly stableFeII-(tris-1) complex or loss of loosely associated FeII ions upon washing.

N N

O

O

S

Si(OEt)3

N N

O

O

S

1 2

Figure 2.5 Phenanthroline derivatives used by Stack et al.67

NN

NN

Cu

NN

NN

FeFeII

CuI

OTfOTf

N N

O

O

S

Si(OEt)3

[Cu(CH3CN)4]PF6

CH3CN

1

Cu12+ SBA-15,70 oC

Overnight

Scheme 2.15 Metal-templating method used by Stack et al.67

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The copper coordination environments in CuIIT, CuIIG, and [Cu22](OTf)2

was studied using X-band EPR spectroscopy studies.68 The spectrum of[Cu22](OTf)2 is more similar to CuIIT than to CuIIG indicating that thegeometry and coordination environments around CuII center of [Cu22](OTf)2

more closely matches the templated material. The catalytic reactivity studiesfor epoxidation reaction of olefin while using peracetic acid as the oxidantshowed that mesoporous silica supported with templated FeII-(bis-1) speciesshowed no noticeable induction period and was found to be the bestcatalyst. It showed the highest TOF, selectivity and yield of all the catalyststested (Table 2.6).

More recently, Stack et al. reported a much more robust and superiorcatalytic system by using MSN-supported [MnII(Phen)2]21 as the epoxidationcatalyst and peracetic acid (PAA) as the oxidant.69 The purpose of this studywas to overcome the main drawbacks frequently found in homogeneous

Table 2.5 Concentration of metal and ligand in the materials synthesized by Stacket al.67

MaterialLoading of ligand1/mmol g�1] (� 0.01)

Metal loading/mmolg�1 (� 0.002) Ratio of ligand : metal

T 0.11 0.001 —CuIIT 0.11 0.052 2.1 : 1FeIIT 0.11 0.055 2.0 : 1G 0.10 0.00 —CuIIG 0.10 0.055 1.7 : 1FeIIG 0.10 0.035 2.9 : 1

N N

N N

O

O

S

Si(OEt)3

Fe(OTf)2

MeOH,Overnight

1

SBA-15, 70 oCCH3CN, Overnight

NN

FeN N

NN

OTfOTf

NN NN

N N Fe(OTf)2

MeOH,Overnightt

NN

NN

Scheme 2.16 Random ligand grafting method used by Stack et al.67

Table 2.6 Results of epoxidation reactions catalyzed by FeIIT, FeIIG, and[Fe23](OTf)2.

SubstrateEpoxide yield [%] (selectivity [%])FeIIT FeIIG [Fe23](OTf)2

Vinylcyclohexane 85 (90) 60 (65) 60 (65)1-Octene 80 (95) 75 (95) 65 (80)Allyl acetate 25 (70) 20 (45) 15 (40)2-Cyclohexen-1-one 15 (85) 0 (0) o5 (o5)

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catalysts i.e. the requirement of 2 equiv. of PAA (the oxidant), degradation ofthe electron-rich olefins in the catalytic reaction condition and formation ofpartially isomerized trans-epoxide products from cis-olefins. A comparisonof catalytic results was made between the metal-templating method, graftingmethod and the homogeneous Mn complex. Scheme 2.17 illustrates astepwise procedure for the two methods.

Calculations were completed to estimate the maximum ligand loading,which would allow for site isolation for the materials having a surface area of600 m2 g�1 using the following equation:

distance to nearest neighborðAÞ ¼2 surface area

A2

g

! !ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

ligand loadingmolecules

g

� �� p

� �s :

Knowing that 20 Å is twice the distance between the silicon atom andcenter of two coordinating N atoms of the ligand in an extended alkyl chainconformation, a ligand loading of greater than 0.30 mmol g�1 was calculatedto be unfavorable for site isolation.

Metal templating with ligand loadings of 0.3, 0.11 and 0.025 mmol g�1 ledto a ratio of 2 : 1 of S (from ligand) : metal whereas for grafted ligandmaterials the ratio varied randomly. The coordination environments ofCuIIG and CuIIT complexes were analyzed using X-band EPR spectroscopywhich showed that coordination environments of templated materials(CuIIT) were more similar to the organometallic complex in the

Cu

CuCu MnMn

SBA-15

Random grafting

[Mn(CF3SO3)2] Mn

MnMn

Covalentattachment

Dematalation

Grafted MaterialG

Templated MaterialT

MnIIG

MnIIT

N NOO

SSi(OEt)3

1

Scheme 2.17 Schematic representation of the covalent attachment of Ligand 1 toSBA-15 silica via random grafting and metal-templating methods toform MnIIG and MnIIT, respectively.

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homogeneous state than the grafted materials (CuIIG). For catalytic activityexamination, epoxidation of vinyl cyclohexane was used as a test reactionand all the catalysts and controls were tested for it (Table 2.7).

The most reactive and selective catalyst was MnIIT. The reactivity and se-lectivity of MnIIT did not vary with ligand loading. A lower epoxide selectivityfor MnIIG was observed and it was similar to the homogeneous catalyst when1 equiv. of 2 or 1,10-phenanthroline (phen) for 1 equiv. of MnII was used forthe catalyst synthesis. The kinetics while using similar MnIIT loading wasmuch faster (approx. 5 times) than MnIIG for epoxidation of 1-octene. Resultsindicate that both catalysts, MnIIT and MnIIG, have a broad substrate scopefor the epoxidation reaction: a high yield was demonstrated for trisubstitutedelectron-rich olefins for both whereas [MnII(2)2]21 completely oxidized tri-substituted olefins. Also, MnIIT led to no isomerization of cis-olefins and avery efficient formation of epoxides a, b-unsaturated ketones and esters.

With efficient heat control of the reaction, MnIIT could be recycled fivetimes without any significant change in the percent yield, the amount ofoxidant needed and the Mn content of the catalysts. However, in the randomgrafting sample, MnIIG showed that the ligand content attached to the solidsupport remained the same but metal leaching occurred which led to a lossof reactivity.

Stack et al. reported the synthesis of mesoporous silica functionalizedwith azide groups with various loadings, which could be easily controlledby varying the ratio of 3-azidopropyltriethoxysilane (3-AzPTES) addedduring the synthesis using the co-condensation method.70,71 The idea ofusing the co-condensation method was to avoid clustering of the ligandsites. The hybrid azidopropyl mesoporous silica were labeled SBA-15-N3-x(x is the mole percentage of the 3-AzPTES used during the synthesis; thesum of the total mole percentage with TEOS is 100). Four strategicallyimportant alkyne-terminated organic compounds were attached using

Table 2.7 Epoxidation reactivity of MnII catalysts with vinylcyclohexane (loadingvalues are for ligands).

O

MnPAA

Catalyst loading/mmol g�1 Yield (� 3%) Selectivity (� 3%)

[MnII(Phen)2]21 95 72[MnII(Phen)1]21 72 72[MnII(2)2]21 80 85[MnII(2)1]21 73 73MnIIT (0.30) 97 97MnIIT (0.11) 98 98MnIIT (0.025) 98 98MnIIG (0.30) 80 84MnIIG (0.11) 83 83MnIIG (0.025) 72 72

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copper-catalyzed azide alkyne cycloaddition (CuAAC) which could provideinsight into the packing of molecules on the surface and variation innearest neighbors as the surface loadings varied. Functionalized materialswere also tested for catalytic activities and the information about surfacecoverage and site isolation were used to further explore the reactionmechanism. Acidic media synthesis using non-ionic surfactant as thetemplate, TEOS as the silica source and 3-AzPTES as the functionalizingagent was used to synthesize the materials which were further modifiedvia CuAAC using excess (2–20 equiv.) ethynylferrocene, 1-ethynylpyrene,ethynylTPA and Fe-ethynylTPPCl to form SBA-15-R-x materials as shownbelow in Scheme 2.18.

X-ray diffraction and nitrogen sorption analysis were used to determinethe ordered mesoporous structures of all the materials synthesized. Add-itionally, CP-MAS 13C solid-state NMR was used to confirm the organicfunctionality in SBA-15-N3-8 and SBA-15-R-x materials. The quantification ofimmobilized functional groups was done by ICP analyses and UV-Visspectroscopic analyses of digested samples.

This study was significant mainly because it reported and discussedmethods for determination of ligand distribution and ligand densityutilizing numerous MSN surface functionalizations.

1. SBA-15-pyrene-x monomer/excimer fluorescence. The fluorescence ofsurface attached pyrene is different depending on if the species is ina monomeric or excimeric form: monomeric pyrene is observed at400 nm and the excimer has a fluorescence maximum of 480 nm. Atlow pyrene loading (xr0.2 mmol g�1), fluorescence was primarily ob-served at 400 nm mainly due to the existence of monomeric pyrenespecies. Higher pyrene loading led to greater intermolecular pyrene

Si(OEt)3

N3

(x %)Si(OEt)4+

(100-x %)

N3 N

RClick

NN

R

Fe N

N

N2

N

N N

NFe Cl

Ph

PhPh

R = Fc Pyrene TPA FeTPP

SBA-15-N3-x SBA-15-N3-x

Scheme 2.18 Synthesis of surface-modified mesoporous silicas, SBA-15-R-x, re-ported by Stack et al.67

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interaction and an increase in excimer fluorescence. A direct depend-ence of increased loading with increased excimer detected was ob-served (Figure 2.6).

2. SBA-15-TPA-x dioxygen adduct monomer/dimer of CuTPA complexes.Oxygenation of [CuI(TPA)]1 in homogeneous solutions generateduniquely colored species depending on the dioxygen adduct formed.The monomeric superoxo complex was green whereas the dicoppercomplex, formed by rapid oxidation of [CuI(TPA)]1, was purple. CuTPAcomplexes supported on SBA-15 were treated with dioxygen and theresults were compared with that of the analogous homogeneous com-plexes. Similar results were observed for supported CuTPA complexesand the site-isolated loadings on heterogeneous support led to theformation of stable reactive monomeric species; these monomericspecies were highly unstable in the homogeneous state. The variouslyloaded SBA-15-[CuI(TPA)]1-x showed different results. Materials withhigh loading turned purple due to densely packed complexes leadingto the formation of trans-peroxo complexes whereas materials withlow loading, where adequate site isolation was possible, showed theformation of green copper superoxide complexes.

2.3.2 Surface Distribution of Immobilized Species

Three idealized surface distributions of functionalized species on the surfaceof MSN particles were proposed in this study:

(a) Clustered distribution. In this situation, dense condensation offunctionalized monomers occurred during MSN synthesis. Since no

Figure 2.6 Fluorescence spectra of (a) SBA-15-pyrene-x with different loadings.A figure showing color changes upon oxygenation of SBA-15-TPA-4 andSBA-15-TPA-0.5 with [CuI(MeCN)](SbF6) and the characteristic colors ofintermediates is available as ESI.y

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excimer fluorescence at low loadings was observed, this arrangementwas found to be less likely (Figure 2.7a).

(b) Uniform distribution. This arrangement would minimize the objectoverlap and maximize the interspecies distance of pyrene attached tomesoporous silica. Excimer fluorescence in this situation would beexpected only when the loading exceeds the overlapping limit(B0.2 mmol g�1). However, significant excimer fluorescence wasobserved for SBA-15-pyrene materials with 0.10 mmol g�1 and loading0.19 mmol g�1 implying that uniform distribution also was not likely(Figure 2.7b).

(c) Random distribution. The nearest-neighbor method gave an estimateof theoretical random distribution using the Poisson distributionfunction as a function of surface loading.72,73

Probability density function: f(r)¼ 2pdr exp(�dpr2)

Cumulative distribution function: F(r)¼ 1� exp(�dpr2)

Mean distance: r¼ 1=ð2ffiffiffidpÞ

where r is 30 Å, which was the distance between the centroids of twoobjects as 15 Å was the length of each surface immobilized pyreneentity, and d is the average surface loading. The percentage of siteisolation was calculated by the following equation:

% site isolation¼ 100�exp(�dpr2).

The nearest-neighbor method was used to obtain the trend betweenthe percent of pyrene present in monomeric form and the loading ofpyrene molecules functionalized on the MSN surface. The theoreticaltrend of randomly distributed objects matched very well with theexperimentally observed trend of decreasing ratio of Imonomer/Iexcimer

with increasing surface loading of the pyrene molecules. This ledto the conclusion that pyrene and, hence, the organoazides

(a) (b) (c)

Figure 2.7 Graphical representations of theoretical surface distributions: (a) clus-tered, (b) uniform, and (c) random.

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supported on mesoporous silica, were randomly distributed as shownin Figure 2.7c.

Two different types of catalytic reactions were tested using thesematerials.

1. Epoxidation of 1-octene. The SBA-15-TPA-x materials withMn(CF3SO3)2 were evaluated as catalysts for the epoxidation of1-octene. A 10-fold excess of ligand with respect to Mn concentrationgave the maximum percentage yield. All SBA-15-TPA-x materials weretested as catalysts for epoxidation at 0.1 mol% MnII salt and 1 mol%ligand to see the effect of ligand loading on the catalytic activity. Thepercentage yields of epoxides decreased with increasing surfaceloading. SBA-15-TPA-0.5 was the best catalyst leading to B85%epoxide conversion in 2 min. Both pyrene fluorescence and Cu(TPA)dioxygen adduct formation indicated the site isolation of a majority ofspecies on SBA-15-R-0.5. An insight into the reaction mechanismwas provided by comparing yields obtained with SBA-15-TPA-0.5 andTPA-Tz-tBu ligands under identical concentrations which ruled outthe existence of Mn(TPA) dimers in the catalytic cycle; similar ligandturnover numbers shown by SBA-15-TPA-0.5 and its homogeneousanalogue TPA-Tz-tBu also implied that at this surface loading and insolution, Mn(TPA) dimerization was not the major cause of liganddeactivation.

2. Carbene insertion. Homogeneous FeTPPCl catalyzing carbene in-sertion yielded greater than 95% conversion and the heterogeneousSBA-15-FeTPP system provided similar high yields; however, the re-action duration was twice as long. Carbene insertion involved mildreaction conditions, occurred at a slower rate and used the highlystable FeTPP as a catalyst. Hence, under these conditions, the effectsof metal lability, ligand decomposition and site accessibility wereminimal and no significant dependence between surface loading andreactivity was observed.

2.4 ConclusionIn this review, we outlined the common strategies of surface functionali-zation of mesoporous silica materials and summarized recent develop-ments in single-site heterogeneous catalysis. Single-site heterogeneouscatalysis is defined as using supported active catalysts that are independentand have no measurable interaction with one another. We focused on alimited number of recent publications that have been central to the de-velopment and understanding of the physical characteristics of thesecatalysts. Herein, we described publications that used clever moleculardesigns to control the amount of space between the supported active

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species and novel methods to measure the ligand–catalyst interaction. Wereviewed publications that reported catalyst analysis utilizing new X-rayadsorption methods that give additional physical characterization data onsupported SSHC which previously was only possible with molecular catalystspecies. We envision that further development in SSHC will lead to cata-lysts with increased stability and recyclability which will be active for im-portant chemical reactions including the oxidation of methane and finechemical syntheses.

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70. J. Nakazawa, B. J. Smith and T. D. P. Stack, J. Am. Chem. Soc., 2012,134(5), 2750–2759.

71. J. Nakazawa and T. D. P. Stack, J. Am. Chem. Soc., 2008, 130(44), 14360–14361.

72. M. A. Cousins and K. Durose, Thin Solid Films, 2000, 361–362, 253–257.73. D. Simberloff, Ecology, 1979, 60.

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CHAPTER 3

Supported Metal Catalystsfor Green Reactions

K. HARA,a H. KOBAYASHI,a T. KOMANOYA,b S.-J. HUANG,c

M. PRUSKId,e AND A. FUKUOKA*a

a Catalysis Research Center, Hokkaido University, Kita 21 Nishi 10, Kita-ku,Sapporo, Hokkaido 001-0021, Japan; b Graduate School of Science,Hokkaido University, Kita 12 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-0012,Japan; c State Key Laboratory of Catalysis, Dalian Institute of ChemicalPhysics, Chinese Academy of Sciences, Dalian 116023, China; d U.S. DOEAmes Laboratory, Ames, Iowa 50011, USA; e Department of Chemistry,Iowa State University, Ames, Iowa 50011, USA*Email: fukuoka@cat.hokudai.ac.jp

3.1 IntroductionSupported metal catalysts have played numerous significant roles in thechemical industry. The current global environmental and resource problemsare motivating further research and development of supported metal cata-lysts. The typical general requirements expected for ideal catalysts nowadaysinclude high activity, selectivity, durability, recyclability, compatibility withpractical reaction conditions, broad applicability as well as cost, safety andenvironmental benignness. Regardless of the presence of such multiple re-quirements, a number of supported metal catalysts were recently developedand applied to a wide range of reactions.

The recent developments in supported metal catalysts were significantlyenhanced with the aid of newly established methodologies in novel ma-terials synthesis as well as those in spectroscopy, microscopy and model

RSC Green Chemistry No. 33Heterogeneous Catalysis for Today’s Challenges: Synthesis, Characterization and ApplicationsEdited by Brian Trewynr The Royal Society of Chemistry 2015Published by the Royal Society of Chemistry, www.rsc.org

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systems. One of the biggest contributions to the synthesis of novel materialswas brought about by the establishment of versatile and general proceduresto prepare metal nanoparticles.1 Another contribution came from the ap-pearance of various new support materials which were introduced by findingnovel zeolitic and/or mesoporous materials or by the functionalization ofconventional support materials.

In light of such developments in the related methodologies, supportedmetal catalysts recently accomplished remarkable progress in meeting themultiple requirements mentioned above. Many efforts were dedicated tolower metal loading by introducing novel catalyst structures presentingunique metal–support interactions or by using naturally more abundantmetals. Another noteworthy recent area of progress in supported metalcatalysts is the extended scope of their applications. These are not only incatalytic reactions for conventional petrochemical processes, but numbersof new applications of supported metal catalysts were also reported inother fields such as fuel cell-related catalysis, photocatalysis, fine organicsynthesis and bioindustry.

This chapter focuses on the following selected topics from recent advancesin supported metal catalysts:

1) Recent developments in supported metal catalysts for bioindustry;2) Mechanistic aspects in preferential oxidation of carbon monoxide in

excess hydrogen (PROX reaction); and3) Surface-selective functionalization of mesoporous silica.

Topic 1 reviews recent progress in supported metal catalysts for the directand indirect conversion of biomass to chemicals and fuels. As one of therecent advances in biomass conversion with supported metal catalysts is theproduction of hydrogen, the next important process for fuel cell applicationsis the purification of the hydrogen produced, which can be conductedthrough catalytic preferential oxidation of carbon monoxide in excesshydrogen (PROX reaction). Topic 2 thus focuses on the recent mechanisticfindings in a PROX reaction catalyzed by Pt nanoparticles on mesoporoussilica. Topic 3 deals with surface-selective functionalization of mesoporoussilica as a fundamental example in tailor-made functionalization of con-ventional support materials.

3.2 Recent Developments in Supported MetalCatalysts for Bioindustry

3.2.1 Conversion of Biomass to Chemicals and Fuels

One of the most notable recent developments in supported metal catalysts istheir extended application towards bioindustry. The techniques and know-ledge obtained through the previous development of supported metal cata-lysts were applied to the direct and indirect conversion of biomass to

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chemicals and fuels. The newly established methodologies for catalystpreparation also brought significant progress in these areas.

The current largest biomass conversion is bioethanol production, whichwas initially derived from food crops such as corn, wheat, sugar cane andsugar beet. After confronting a serious competition between food and fuelover utilization of the edible biomass, non-food biomass became the nexttarget to be converted to fuels and chemicals. Among non-food biomass,lignocellulose, the main components of wood and grass, has been attractingsignificant attention as a promising carbon resource because of itshuge abundance in nature. Lignocellulose consists of cellulose (40–50%),hemicellulose (20–40%) and lignin (20–30%).

3.2.2 Catalytic Conversion of Cellulose

Cellulose is a polymer of glucose linked by b-1,4-glycosidic bonds, whichshows different stereochemistry from that of starch (amylose), linked bya-1,4-glycosidic bonds. Cellulose molecules have linear structures fixed byintra-molecular hydrogen bonds, and they are closely packed by inter-molecular hydrogen bonds. These hydrogen bonds of cellulose bring its highchemical stability and insolubility in water. Therefore, the conversion ofcellulose into its monomers or other useful chemicals remained a challenge.

Figure 3.1 shows an overview of the typical initial pathways to chemicalsand fuels starting from cellulose. The hydrolysis of cellulose to its monomer,glucose, has been extensively investigated. Although sulfuric acid has beencommonly applied in this reaction,2,3 this process involves critical problemssuch as the corrosive property of sulfuric acid and neutralization require-ment for product separation. As another frequently studied method,

Glucose

HO OHO

OHOH

OH

Cellulose

Sorbitol

HOOH

OH

OH

OH

OH

OHO

O

5-Hydroxymethylfurfural

CO, CH4, H2

HOOH

OH

OH

OH

OH

OGluconic acid

OHHO

OH

OHEthyleneglycol

Propyleneglycol

HOO

OH

OH

OH

OH

Hydrolysis

Gassification

Hydrogenation

OOH

OH

HO OHSorbitan

Dehydration

Oxidation& dehydrationIsomerizationHydrogenolysis

Figure 3.1 Typical initial pathways of cellulose conversion into chemicals and fuelsby supported metal catalysts.

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cellulase enzymes can be utilized, where cellulose selectively converts toglucose under ambient conditions.4 However, the high cost of the enzymelimits the utilization of this strategy. Application of sub- and super-criticalwater to the hydrolysis of cellulose5 was also investigated, resulting indecreased yields and selectivity of glucose due to further degradation ofglucose by high reaction temperatures. Supported metal catalysts thus havethe potential to overcome these problems.6,7

3.2.3 Hydrolytic Hydrogenation of Cellulose by SupportedMetal Catalysts

One pioneering study on the conversion of cellulose with supported metalcatalysts can be found by going back to the 1950’s, when Balandin et al.obtained sorbitol and sorbitan from cellulose using supported Ru catalystsin the presence of mineral acids under 7 MPa H2.8 In this methodology, thein situ hydrogenation of glucose to the sugar alcohols prevents the de-composition of relatively unstable glucose.

The next challenge was to find solid catalysts which function in the ab-sence of soluble mineral acids. After three decades, in 1989, a related patentwas filed by Jacobs, where conversion of starch to sorbitol using Ru/USY wasreported.9 This catalyst consists of two catalytic functionalities: USY as asolid acid to hydrolyze soluble starch and Ru for reduction of glucose tosorbitol.

The first conversion of cellulose to sugar alcohols using only a solid catalystwas accomplished by Fukuoka et al. in 2006, when sorbitol and mannitol wereobtained from cellulose by Pt/g-Al2O3 under 5 MPa H2.10 Supported Ru cata-lysts also converted cellulose effectively to these sugar alcohols. The solidcatalysts were easily separated from the products by simple filtration.

In addition to catalytic activity, the durability of a catalyst is also an im-portant factor. The catalytic activity of Pt/g-Al2O3 was not retained in thereuse experiments, which is due to the crystalline phase transformation inAl2O3 from parent g-Al2O3 to boehmite [AlO(OH)] during the reaction.11 Thisphase change caused the destruction of its pore structures and the burial ofPt particles. The screening of water-tolerant catalyst supports such as TiO2,ZrO2 and carbon showed that the Pt catalyst supported on commerciallyavailable carbon black (BP2000, Cabot) converts cellulose into the sugar al-cohols without loss of its activity in at least three reuse experiments. Detailedkinetic studies for hydrolytic hydrogenation of cellulose over Pt/BP2000 re-vealed a two-step reaction mechanism: the first slow hydrolysis of cellulose toglucose via oligosaccharides followed by the fast hydrogenation reaction ofglucose to sorbitol. The Ru catalysts supported on other carbon supports,including activated carbon (AC) and carbon nanotube (CNT), were also ap-plied in the cellulose conversion to exhibit good activities and reusability.12,13

Notable practical advantages were found in cellulose hydrolytic hydro-genation using Ru/AC catalysts.14 Even under a hydrogen pressure as low

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as 0.8 MPa, the formation of sugar alcohols was realized efficiently,while most of the other supported metal catalysts require much higherpressure, typically more than 2 MPa. Furthermore, in the presence of2-propanol as a reducing reagent, the Ru/AC catalysts can be utilized forcellulose hydrolytic hydrogenation even without H2 pressure application.A detailed characterization of the Ru/AC catalyst suggested that the Ruspecies on AC were in a form of RuO2 � 2H2O with a diameter of 1–2 nmbefore the reaction.15

The metal particle-size dependence on the catalytic performance wassystematically studied in hydrolytic hydrogenation of cellobiose, a modelmolecule for cellulose, by using a series of Ru/CNT catalysts, which indicatesthat the optimal Ru particle size for the CNT support is 9 nm.16 The smallerRu particles (2.4 nm) were found to afford a fast reaction rate; however, theyalso prompt side reactions to lower the selectivity. In contrast, the larger Ruparticles (12 nm) have less catalytic activity, which also results in a lowselectivity due to decomposition of intermediate glucose.

The recent global trend to seek for alternative metals to previouslydeveloped noble metals brought about another phase of development insupported metal catalysts. For example, Ni2P/AC catalyst was applied in thehydrolytic hydrogenation of cellulose, which demonstrated its high catalyticactivity, although low durability in the reuse experiments showed its draw-back.17 A high durability was found with the reshaped Ni crystal catalysts onthe carbon nanofibers formed on Al2O3 (Ni/CNF).18

3.2.4 Hydrolytic Hydrogenation of Hemicellulose bySupported Metal Catalysts

Depolymerization of hemicellulose proceeds more rapidly than that of cel-lulose, yielding C5 sugars which are useful as sweeteners and potentialprecursors to ethylene glycol and propylene glycol, although being morereactive than glucose. After a patent claiming the hemicellulose conversionvia a three-step process including hydrolytic hydrogenation over RANEYs

Ni,19 one-pot hydrolytic hydrogenation of hemicellulose was recently ac-complished by using a Ru/AC catalyst under 5 MPa H2, where arabitol wasobtained in a high yield from beet fiber.20 However, the catalyst durability inthe reactions using real biomass still remains as a problematic issue.Pre-treatment to remove the deactivating components from real biomass isthus necessary at the moment. Another recent application is the conversionof bleached birch kraft pulp into sugar alcohols by Pt/MCM-48 catalyst.21

3.2.5 Catalytic Conversion of Cellulose to Ethylene Glycoland Propylene Glycol

The conversion of cellulose catalyzed by Ni–W2C/AC and Ni–W/SBA-15 wasreported to produce ethylene glycol by Zhang et al.22,23 A modified catalyst

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without using Ni was later developed by high dispersion of tungsten carbideon three-dimensional interconnected mesoporous carbon support.24

This modified catalyst can maintain its catalytic activity over three runsand the slightly reduced activity later observed can be recovered in part byH2 reduction. An application with real biomass was demonstrated by usingNi–W2C/AC to efficiently convert milled woody biomass to ethylene glycol,propylene glycol, and 1,2-butanediol together with the formation of guaiacylpropane, syringyl propane, and their hydroxylated analogs, the latter half ofwhich are derived from the lignin part.

Several pathways from cellulose to propylene glycol using supported metalcatalysts were also reported recently. In addition to a Pt/HZSM-5 catalyst,25

Ni/ZnO was found to be a good catalyst,26 where the C3 unit seems to beformed via a retro-aldol reaction of fructose generated by isomerization ofglucose. A different catalyst system using a combination of WO3/Al2O3 andactive carbon to produce ethylene glycol and propylene glycol was estab-lished by Liu et al., who proposed a reaction mechanism in which struc-turally stable crystalline WO3 promotes both the hydrolysis of cellulose toglucose and the selective cleavage of the C–C bonds in sugar molecules,while active carbon isomerizes glucose to fructose by its basicity.27 Ni–W2C/AC was utilized for the selective production of propylene glycol from realbiomass, Jerusalem artichoke tuber, which contains inulin as its carbo-hydrate storage.28 The propylene glycol formation here is reasonable be-cause inulin predominantly consists of linked fructose units terminating ina glucose unit.

3.2.6 Hydrolysis of Cellulose to Glucose

The selective and efficient hydrolysis of cellulose to glucose is a challengingreaction because of the relatively unstable chemical structure of glucose inspite of its high potential as a precursor to a number of chemicals(Figure 3.1). In 2008, hydrolysis of cellulose to oligosaccharides and glucoseusing a sulfonated carbon prepared from cellulose and fuming sulfuric acidwas demonstrated by Hara et al.29 It was reported that this catalyst isreusable up to 25 times without loss of activity. They mentioned that theimportant feature of this catalyst is the combination of sulfonic, phenolicand carboxylic groups on the small graphene sheets, which might give highactivity and durability. Sulfonic acid catalysts prepared from a commercialactive carbon and a silica/carbon nanocomposite were also effective.30,31

In addition, a mesoporous carbon CMK-332 treated with concentratedsulfuric acid produced a high yield of glucose.33 The high catalytic per-formance observed here might be due to the facilitated interaction betweenthe surface acid sites and the substrate within the mesoporous structure.

Supported metal catalysts have also been investigated for cellulose hy-drolysis. For example, 10 wt% Ru/CMK-3 catalyzed the hydrolysis of celluloseto glucose in 31% yield.34 Ru/CMK-3 catalyst showed a higher catalyticactivity than the corresponding catalysts of Rh, Ir, Pd, Pt and Au. In addition,

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the Ru/CMK-3 catalyst kept its high catalytic activity in at least five successivereused experiments and gave no leaching of the Ru species. The Ru specieson Ru/CMK-3 catalyst before the reaction was revealed to be RuO2 � 2H2O,which is produced by the reduction and re-oxidation of supported RuCl3.The high valence of Ru species might be the origin of high catalytic activity(Figure 3.2). Regarding the role of the carbon support, CMK-3 without Rugave a 16% yield of glucose and a 22% yield of oligosaccharides in the hy-drolysis reaction, which indicates that the carbon support itself promotesthe hydrolysis as do the Ru species.

As described above, a mesoporous carbon CMK-3 can hydrolyze cellu-lose,34 which suggests that the weak acid sites such as carboxylic (pKa¼ 4)and phenolic groups (pKa¼ 10) on the carbons are also effective for thereaction. Katz et al. recently reported that silanol groups (pKa¼ 7) on silicacan catalyze the hydrolytic reaction by attaching cellulose chains onto asilica surface to induce strained conformations.35,36

3.2.7 Valorization of Lignin by Supported Metal Catalysts

Lignin is one of the major components of abundant biomass resource andits uniqueness as a chemical resource lies in the presence of aromaticfunctionalties. Using such a promising and unique biomass resource viavalorization over heterogeneous catalysts has been investigated for morethan half a century. The hydrogenation of hardwood lignin was tested with acopper–chromium oxide catalyst in 1938 to produce propylcyclohexanolsand methanol.37 After catalyst improvements over the following three dec-ades, significant studies on lignin hydrodeoxygenation started by using Co–Mo and Ni–Mo type catalysts, which were originally developed for catalyticremoval of sulfur and nitrogen in conventional petro-based processes.Weckhuysen et al. recently reported a systematic review on the catalyticvalorization of lignin, which covers from the initial studies just mentioned tononconventional catalysts developed in the last decade.38 Examples of therecently reported catalysts include Ni–W/SiO2–Al2O3 catalyst for ligninhydrocracking,39 Ru/C and Pd/C catalysts for full hydrogenation of the aro-matic rings of model compounds and lignin,40–42 Ni–Cu/ZrO2 for selectivehydrodeoxygenation of a model aromatic compound with retention of itsaromaticity,43 Pt/Al2O3 for lignin depolymerization process44 and Ni/MgO

GlucoseCellulose OligomersCMK-3

RuO2 2H2O

HOO

OHOOC O

Ru

OH2

O OO

OORuRu

OH

HOH2

OH2

O

O

OH2

RuHO

HO

OH2

OH2

H H

Surface functional groups

Ru

Figure 3.2 Main roles and possible active sites of Ru/CMK-3 catalyst for thehydrolysis of cellulose.

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catalyst for lignin gasification.45 Oxidative conversion of lignin or its modelcompounds has been also investigated by using supported metal catalystssuch as Pd/Al2O3 for production of aldehydes from extracted lignin46 andPt/TiO2 for photocatalytic lignin degradation.47

3.2.8 Direct Formation of Syngas or Pure Hydrogen fromBiomass

The direct formation of syngas (CO and H2) has been investigated by usingsupported metal catalysts. Supported Ni catalysts were utilized for thedegradation of cellulose at ca. 900 K in the presence of water vapor toobtain CO, H2, CH4, and CO2 with small amounts of C2 compounds.48

However, catalyst deactivation due to carbon deposition was observed. Thecatalyst durability was improved by using Rh/CeO2/SiO2 catalyst, whichafforded complete conversion of cellulose to gaseous C1 products and H2 inthe presence of air and steam even at a lower reaction temperature of773 K.49 The optimization of the reactors is also important in producingsyngas from biomass, which was shown in an example using a dual-bedreactor for gasification of actual lignocellulosic biomass.50 Supportedmetal catalysts have also been developed for the direct formation of purehydrogen from biomass. In order to utilize the obtained hydrogen forpolymer electrolyte fuel cells (PEFCs), it is necessary to restrict CO for-mation to less than 10 ppm. One accomplishment was brought about by aNi/TiO2 catalyst in the presence of a stoichiometric amount of NaOH,where cellulose was directly converted to pure H2 with CO and CO2 con-centrations of less than 30 ppm.51

3.3 Mechanistic Aspects in Preferential Oxidationof Carbon Monoxide under Excess Hydrogen(PROX Reaction)

3.3.1 Preferential Oxidation of Carbon Monoxide in ExcessHydrogen (PROX Reaction)

As just described in the previous section, one of the recent advances inbiomass conversion with supported metal catalysts is the production of H2.In order to utilize the produced H2 for PEFCs,52 the next important process ispurification of the H2 by removing small amount of CO, which is poisonousfor Pt anode PEFCs even at low concentration levels (o10 ppm). The elim-ination of such a small amount of CO can be conducted through the pref-erential oxidation of CO in excess H2 (PROX reaction) by using various typesof supported metal catalysts.53 Accompanying the extensive research anddevelopment seeking active catalysts for the PROX reaction, a mechanisticunderstanding of the catalytic reaction has also been developed. It has beenproven that the properties of support materials have significant effects on

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catalytic performances.54 However, unprecedented support effects were re-cently found in the PROX reaction catalyzed by Pt catalysts supported onmesoporous silicas, which are generally regarded as ‘‘inert’’ materials. Thischapter focuses on this promotional effect of mesoporous silica as one of therecently found mechanistic aspects in the PROX reaction.

3.3.2 PROX Reaction by Pt Catalysts Supported onMesoporous Silica

Mesoporous silicas have been widely used as catalyst support materialssince their discovery in the 1990’s55–58 due to their high specific surfacearea and well-defined pore geometry. The amorphous nature of the wallstructure leads to the general understanding that mesoporous silicas act asinert materials in catalytic cycles. In contrast to this understanding, onesurprising catalytic effect was found in PROX reaction using Pt catalystssupported on mesoporous silica. The platinum catalysts supported onmesoporous FSM-16 or MCM-41 showed approximately 100% CO con-version over a wide range of reaction temperatures (298–423 K), while theplatinum catalysts supported on amorphous silica showed sluggishactivity.59 Isotope tracer experiments indicated that the surface silanolgroups on the Pt/FSM-16 catalyst can oxidize the adsorbed CO on Pt in theabsence of O2. The successive work revealed that the difference in the porediameter of support FSM silica has a significant effect on the catalytic per-formances of the Pt catalysts.60 In particular, Pt/FSM-22 catalyst, having 4 nmpore diameter, exhibited the highest CO conversion (entry 3 in Table 3.1),whereas the Pt/FSM-10 catalyst, having 1.8 nm pore diameter, resulted ina lower conversion (entry 1). Such a pore-size effect might be due tothe different micro-reactive environment at the Pt–mesoporous silica interface.

In order to achieve an image of the reactive interface of the Pt/mesoporoussilica, two model Pt/MCM-41 catalysts were employed in the mechanismstudy.61 Starting from the same pristine as-synthesized materials, two typesof MCM-41 supports were prepared by using different procedures at thesurfactant removal steps: one-step calcination for MCM-41-A and two-step

Table 3.1 Catalytic performancesa of Pt nanoparticles supported on mesoporoussilicas.

Entry CatalystPorediameter/nm

Conversion (%)at 298 KCO O2

1 1 wt% Pt/FSM-10 1.8 42 252 1 wt% Pt/FSM-16 2.7 95 563 1 wt% Pt/FSM-22 4.0 100 1004 1 wt% Pt/FSM-22 7.0 96 555 5 wt% Pt/MCM41-A 2.9 100 1006 5 wt% Pt/MCM41-B 2.8 10 5aReaction conditions: 0.20 g catalyst, flow rate 40 ml min�1, CO 1%, O2 1%, N2 5%, H2 93%.

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extraction–calcination for MCM-41-B, respectively. The resulting twoPt/MCM-41 catalysts displayed strong similarities in Pt morphology, particle-size distribution, electronic states, support architecture, and pore-sizedistribution, and thus isolate the interferences from these apparent physico-chemical parameters in the mechanism study. These two model Pt catalystsshowed a dramatic difference in catalytic activity: ca. 100% CO conversionwith Pt/MCM-41-A versus 10% with Pt/MCM-41-B at 298 K (entry 5 vs. 6).Based on the isotope tracer experiments, it is revealed that the surfacesilanol groups at the interface of Pt/MCM-41-A ignite the CO oxidation.These active interface silanols were regenerated in situ by the dissociation–combination of O2 and H2, which in turn sustains the entire catalytic cyclefor the PROX reaction. The absence or shortage of interface silanols onPt/MCM-41-B not only impeded the CO oxidation in the initial stage but alsohindered the activation of O2 (Figure 3.3). This image of the micro-reactiveenvironment also proved that the catalytically relevant silanol groups areonly a small portion of the total number of silanol groups.

3.4 Surface-selective Functionalization ofMesoporous Silica

3.4.1 Novel Types of Functionalized Support Materials

As mentioned in the Introduction, the recent developments in supportedmetal catalysts were significantly enhanced with the aid of newly establishedmethodologies in materials synthesis, which includes the appearance ofvarious new support materials. Recent advances in inorganic synthesisintroduced numbers of novel zeolitic and/or mesoporous materials asavailable catalyst components.62–66 In contrast, many types of purely organicmaterials, such as functionalized polymers and dendrimers, were also de-veloped as promising support materials for the preparation of metal catalysts.Another approach to the synthesis of functional materials is a combination ofinorganic and organic materials synthesis, which produces hybrid materialsconsisting of an organic functionality and an inorganic structure framework.The most commonly used inorganic component here is mesoporous silica,which is focused on in the following last part of this chapter.

3.4.2 Surface-selective Modification of Mesoporous Silica

Numerous examples of surface functionalization on mesoporous silicasfor the preparation of supported metal catalysts have been reported.67

Pt

Si

OH

Si

OH

Si

OH

SiO

Si

Pt

SiO

SiSiO

Si Si

OH

(a) (b)

Figure 3.3 Models of the interface on Pt catalyst supported on mesoporous silica:(a) active Pt/MCM-41-A and (b) inactive Pt/MCM-41-B for PROX reaction.

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The functionalization of mesoporous silica with an organic functionality canbe done by treating parent mesoporous silica with modification reagents.However, special attention should be paid if precise differentiation isnecessary between the exposed external surface and the internal poresurface. For example, whether deposition of active metal species is realizedon the external surface or on the internal pore surface can make a significantdifference in the catalytic property in some cases.68–70

One solution for such spatial control of functionalization is thepreparation of targeted functional mesoporous silicas by co-condensationusing the corresponding functional silica precursors. This method realizesthe homogeneous incorporation of functional groups predominantly insidethe pores.71 Lin et al. successfully demonstrated linear alkyne polymer-ization within the mesopore by depositing an active Cu catalyst on theinternal surface of a functionalized mesoporous silica prepared by theco-condensation method.72

An alternative method, called the post-synthesis method, generally allowsmore versatile functionalization. This method utilizes selective grafting onthe external surface of as-synthesized mesoporous silica, followed by theremoval of the structure-directing agent and subsequent functionalization ofthe internal pore.73,74 Such an approach has been used to prepare severalsupported metal structures with definite spatial control.75–78 However,finding the optimum conditions for the synthesis of such complex materialsis not always straightforward.

Recently, detailed structural and quantitative analyses of the selectivesurface silylation of MCM-41 were conducted by using the silylating reagentsshown in Table 3.2.79 The selective silylation of the external surface wastested by the reaction of as-synthesized MCM-41 and one of the silylatingreagents, followed by extraction of the surfactant (Figure 3.4). The pore-size

Table 3.2 External surface modification of mesoporous silica MCM-41 withdifferent silylating reagents.

Silylating reagentDFT porediametera/nm

Silylationdensityb/mmol g�1

TMSOTf 4.6 0.31 (� 0.05)

TMCS 4.6 0.06 (� 0.01)

BSA 4.6 0.09 (� 0.01)

MeSi(OEt)3 4.2 1.04 (� 0.10)

aDerived from DFT analysis of nitrogen adsorption measurement.bDerived from solid-state 1H NMR analysis by using hexamethylbenzene as an external reference.

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distributions after the silylation shown in Table 3.2 were determined bynitrogen adsorption experiments using density functional theory (DFT).Clearly, MeSi(OEt)3, the most commonly used silylating reagent, resulted ina narrower pore size, indicating that part of the internal surface was alsosilylated.80–83 In contrast, the pore distributions with the other reagentstested in this study showed exclusive modification on the external surfaces.Solid-state NMR techniques84–86 gave precise and quantitative evaluation ofthe silylation efficiencies, which indicates that TMSOTf gave the highestloading among the three surface-selective silylating reagents. In addition,the solid-state NMR measurements were found to be powerful methods todetect the presence of unwanted organic impurities which can be in-corporated during the functionalization of mesoporous silica.79

3.5 ConclusionsIncreasing demand from society has been promoting continuous advancesin supported metal catalysts. Such developments include extensive appli-cations in bioindustry as well as the utilization of novel catalyst componentsto exhibit uniquely required catalytic performances. These accomplishmentswere achieved with the aid of advances in related areas of research such asmaterials synthesis, spectroscopy, microscopy and model systems. However,many challenges remain unsolved by the catalyst systems developed so far.Apart from the conventional methodologies, a completely new design ofcatalyst might be necessary for the further development of supported metalcatalysts in the near future.

References1. N. Semagina and L. Kiwi-Minsker, Catal. Rev.: Sci. Eng., 2009, 51, 147.2. R. Rinaldi and F. Schuth, ChemSusChem, 2009, 2, 1096.3. W. L. Faith, Ind. Eng. Chem., 1945, 37, 9.4. Y.-H. P. Zhang and L. R. Lynd, Biotechnol. Bioeng., 2004, 88, 797.

Figure 3.4 External surface modification on mesoporous silica.

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CHAPTER 4

Zeolites in the 21st Century

WIESLAW J. ROTH,a DAVID KUBICKAb AND JIRI CEJKA*a

a J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of theCzech Republic, v.v.i., Dolejskova 3, CZ-182 23, Prague 8, Czech Republic;b Research Institute of Inorganic Chemistry – UniCRE-RENTECH,Chempark Litvınov, 436 70, Litvınov, Czech Republic*Email: jiri.cejka@jh-inst.cas.cz

4.1 IntroductionZeolites are of great interest because their ordered microporous structurescombined with strong acid activity, capacity for selective sorption, thermaland chemical resistance, and other beneficial qualities proved very useful forpractical applications with significant commercial impact.1–4 The ability ofzeolites to discriminate molecules based on size and shape expandedthe concept of molecular sieving and in more detail the so-called shapeselectivity. Zeolites have been used commercially as selective catalysts, e.g.for many hydrocarbon conversions in refineries and chemical industry,selective sorbents and ion exchangers.5,6 Zeolites have a frameworkmolecular structure constructed as an extended network of corner sharingTO4 tetrahedra, with T¼Si, optionally substituted with heteroatoms such asAl, which imparts strong acid character, as well as B, Fe and others.The network is 4-connected with maximum framework density below ca.19–21 T atoms per 1 nm3.7 As elaborated below, zeolites were first recognizedas aluminosilicate minerals about 250 years ago but since about the 1940’sthey have been extensively studied and developed as diverse syntheticmaterials including aluminophosphates and other compositions. Recently,novel classes of zeolites, such as mesoporous single crystals, hierarchic

RSC Green Chemistry No. 33Heterogeneous Catalysis for Today’s Challenges: Synthesis, Characterization and ApplicationsEdited by Brian Trewynr The Royal Society of Chemistry 2015Published by the Royal Society of Chemistry, www.rsc.org

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materials and two-dimensional zeolites extensively enlarged the area ofzeolitic materials.

4.2 History of ZeolitesThe history of zeolites dates back to the middle of the 18th century whenSwedish mineralogist Cronsted described an aluminosilicate mineral, nowbelieved to be stilbite, which released water upon heating.8 This materialwas called a zeolite based on the Greek ‘zeo’ (boiling) and ‘lithos’ (stone). Indue course, several dozens of zeolite minerals were identified and describedin the literature.9,10 Even recently, a few new zeolite minerals were identified.Remarkably, one of them, mutinaite, is isostructural with the synthetic,highly profitable and versatile zeolite MFI, while gottardite and tschernikitehave the topology of previously synthesized zeolites NU-87 (NES) and beta(BEA*). In the 19th century some useful properties of zeolites were recog-nized including the reversible adsorption of water and gases, and ion ex-change.11 Around the 1940’s, further interest in the extraordinary propertiesof zeolites was initiated by Barrer resulting in the first successful discoveriesof useful synthetic zeolites. The last 60 years experienced a fast increase inthe number of zeolites synthesized as well as in findings of some new zeoliteminerals. At present, we recognize over 200 different structural types ofzeolites (or zeotypes defined as non-Si-based compositions, e.g. alumino-phosphates) and more than 60 natural zeolites.7 The number of syntheticzeolites is steadily increasing each year. The ensuing advances include bothnew compositions and structures with novel pores dimensions. Whilenatural zeolites possess medium- or large-pore systems with low Si/Al ratios(usually below 5), some of their synthetic analogs were prepared as moresiliceous materials having extra-large pore channels. The effort in zeolitesinspired the discovery and extensive study of related novel classes likemesoporous materials,12,13 zeolite lamellar solids,14,15 and metal–organicframeworks (MOFs).16 There is also a significant and growing interest inzeolites as structured products like membranes.

4.3 Conventional ZeolitesZeolites attracted particular attention because of their frequently shownsuperiority in comparison with other functional solids, such as clays,amorphous or organic materials, in terms of high activity, stability, orderedstructure and/or other properties considered useful.17 The conventionalzeolites are recognized as those with an extended 4-connected periodicframework in three-dimensions (3D). There was an implicit assumption oftheir direct formation in 3D and structure immutability, i.e. the impossibilityof post-synthesis modification except for partial or complete degradation. Infact, a partial degradation (dealumination) of zeolite FAU is used atthe industrial level to prepare the most important zeolite catalyst for oilcracking. Recently, desilication of high-silica zeolites has been extensively

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investigated aiming at the formation of mesopores and the increasedaccessibility of active sites through the removal of silicon atoms and part ofthe framework.

As nature and human ingenuity usually find ways to expand beyond rigiddefinitions, new materials are continually added to the area of interest underthe zeolite umbrella. For this reason we will present the conventionalzeolites according to the above formal definition and later on separatelydiscuss selected ‘non-conventional’ ones as an integral part of thecontinually expanding zeolite field.

4.3.1 Structures

The molecular structure of zeolites, also referred to as topology, is anintrinsic property identifying and defining different zeolite types anddetermining their usefulness because of the pore size(s), shape andconnectivity (one-dimensional – 1D or higher dimensionality) of channelsfor the diffusion of guest molecules. The composition, especially the Si/Alratio, influences activity and often is limited by the framework type itself.Lowenstein’s rule forbids neighboring Al atoms, hence the atomic Si/Al ratiocannot fall below 1. In the opposite direction, purely siliceous forms ofdifferent structural types of zeolites were reported, although even they mostlikely contain Al inclusions at ppm or ppb levels. In general, a higher Alcontent translates into greater overall acidity but sometimes it is offset bylowered stability, in the extreme resulting in degradation and reducedpractical usefulness.

The number of possible 4-connected networks is theoretically infinite andhence the respective number of possible zeolites in terms of structures isunlimited. As mentioned above, more than 200 unique topologies have beenformally recognized by the IZA structural commission and assigned a 3-lettercode, the so-called Framework Type Code (FTC). The naming convention,adopting the designation used by an inventor, underscores the lack ofsystematic classification of these topologies. The structures define a specificunit cell and hence manifest themselves in a characteristic, indexable X-raydiffraction pattern. In line with the presumed immutability of theconventional extended 3D zeolites, it is expected that as-synthesized andactivated, i.e. calcined, products show analogous peak positions in theirX-ray diffraction patterns. As will be highlighted later, the observeddifferences in X-ray diffraction patterns of as-synthesized and calcinedzeolites led in some cases to the identification of layered zeolite precursorsand new insights into the synthesis mechanisms.

Zeolite structure is most often described in terms of pore sizes andconnectivity/dimensionality as it reflects the practical potential. The usefulmeasure of pore size is in terms of the number ‘n’ of T atoms in thecircumference of the channel, defined as the ‘n-ring’. In catalysis, the largepore, i.e. 12-ring, and medium-pore, 10-ring, zeolites have been by fardominant and useful. This is exemplified by the two most commonly used

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and profitable zeolites: FAU, 3D (channel connectivity) 12-ring and MFI, 3D10-ring. The others, such as BEA* (3D, 12-ring), FER (2D, 10-ring� 8-ring),MOR (2D, 12-ring� 8-ring), MWW (2D, 2� 10-ring with supercages),reinforce the dominance of the large and medium-pore zeolites in synthesisand applications. The attempts to expand zeolites above 12-ring resulted insuccessful synthesis of several 14-ring frameworks or even larger pores but todate no promising practical benefits have been identified for them. This ismainly connected with the high cost of structure-directing agents (SDAs)used for the synthesis, the low concentration of acid centers and the possiblelower stability of these extra-large-pore zeolites. At the lower pore size end,the small, i.e. 8-ring, zeolites initially demonstrated a great value for sorptionand ion-exchange applications embodied by zeolite A, LTA, while apparentlyhaving little benefit for catalysis. This could be attributed to generaldiffusion problems in pores of that size but in selected non-trivialapplications the value of an 8-ring zeolites proved exceptional, e.g. methanol-to-olefin (MTO) over CHA zeolite/zeotype and related zeolite types. The mostindustrially relevant zeolite structures are provided in Figure 4.1.

The frameworks, which are networks of T atoms bridged by oxygen atoms,contain various combinations of rings. The smallest ones, i.e. 3-ring, are very

Figure 4.1 Schematic structures of the most industrially relevant zeolites MFI,BEA*, MOR, and FAU, figures used from the website of the InternationalZeolite Association.

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rare (although the number of zeolites with this secondary structural unitincreased in the last years) while 4, 5 and 6 dominate and through differentarrangements circumscribe the bigger n-rings. A useful concept dis-tinguishes secondary building units (SBUs), some of which were believed tobe involved in the assembly of zeolite structures during synthesis. This isnow regarded as an unlikely event in most situations since structurebuilding by smaller entities, mainly by addition of monomers, is consideredthe predominant process. One should not rule out the possibility of aframework assembly by bigger/extended units and in fact such a process hasbeen recently identified in our laboratory (vide supra).

The above general outline of concepts and issues associated with zeolitemolecular structure ought to be followed by some discussion of specific casesor classes. However, we feel that the subject is continuously addressed in di-verse reviews and the reader will be better served by being referred to thesepublications.18–20 Instead, we will focus here on selected aluminosilicatestructures, which we find intriguing and worth mentioning for some particularreasons (vide infra) i.e. TUN, IMF (see Section 4.3.3 below), MSE and MCM-71.

MCM-71 is notable as one of the most recent new zeolites obtained byan inorganic route, without a template. It is a high-Al material with one-dimensional 10-ring channels having elliptical cross-sections. Its structure isinteresting as being predicted by Breck and complementing the series hedesigned.17,21

MSE is the first 12� 10� 10-ring zeolite. Its first synthesis was attributedto a specially designed dipositive rigid template.22 Since this first synthesisother less elaborate and more cost effective SDA’s have been identified toafford this structure. The intersecting 12- and 10-ring channels can beviewed as combining the pore features and high Al concentration of the mostcatalytically valuable zeolites to date, i.e. FAU and MFI. Pore characteristicsof zeolites used commercially, mainly in catalysis, are given in Table 4.1(based on ref. 23).

4.3.2 Synthesis

While structures represent the most significant conceptual side of zeolites,the synthesis is the most important practical one. Only a fraction of thepossible frameworks have been synthesized before now. The synthesis hasplayed a role well beyond the traditional supply of high quality materials fortesting and practical use. It has been the driver of innovation and source ofbreakthroughs both technical and fundamental. The discovery of zeoliteLTA, synthetic FAU, MFI, MWW and mesoporous molecular sieves, to namea few, are examples of landmarks resulting in expanded effort and newdirections in zeolite science and technology.1

All zeolite syntheses are carried out by essentially one type of process,mostly batch-wise at all scale levels from small laboratory to commercialone.19,24,25 The synthesis entails a hydrothermal reaction of a mixturecomprising sources of silica and alumina, a mineralizer, in most cases

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soluble hydroxide and, optionally, organic SDA(s) promoting formation of aparticular structural type, known or unknown (the latter if trying a newsynthesis mixture combination). Different insoluble crystalline products areobtained, including mixtures, depending on the composition and synthesisconditions. The most critical quantitative compositional variables are theratios Si : Al : OH : water. The nature of the cation has a dominant effect inmost cases on the structure of materials produced and the kinetics of thesynthesis. Less critical factors like the nature of the raw materials andcertain additives may also influence the outcome of preparation, but ingeneral their effect can be moderated. The examples relevant to the lattermay be substituting an expensive source with a more economical one.

The products obtained by synthesis must be identified because even inwell-known systems unexpected reactions may take place for variousreasons. In most cases, X-ray powder diffraction is the first and adequatetool for that purpose. Additional initial characterization may be obtained bySEM (crystal size and habit) and elemental composition with the Si/Al ratioas the primary descriptor. The crystalline product can be rarely used ‘as is’and must undergo activation, especially for catalytic applications. A typicalactivation process entails a series of calcination and ion exchanges toremove organic templates and cations with simultaneous introduction ofprotons generating acidity and open pores. Some degree of dealuminationmay occur during this process. It depends on the properties of particularcrystals and is generally considered undesirable although interactionbetween undamaged Brønsted acid sites (framework preserved) with extra-framework Al due to defects is believed to enhance acid activity. Regardingactivation, there are in most cases additional steps required for larger-scalecommercial catalytic applications.23 Zeolite crystals are combined with a

Table 4.1 Pore properties of synthetic zeolites used commercially.

Zeolite IZA codeaChanneldimensionality

Poreopening Pore dimensions2/nm

A LTA 3D 12 0.41� 0.41Y FAU 3D 12 0.74� 0.74ZSM-5 MFI 3D 10� 10 0.53� 0.56; 0.51� 0.55Beta BEA* 3D 12 0.66� 0.67; 0.56� 0.56Mordenite MOR 2D 12� 8 0.65� 0.7; 0.26� 0.57Ferrierite FER 2D 10� 8 0.42� 0.54; 0.35� 0.48MCM-22, UZM-8 MWW 2D 2� 10 0.55� 0.40; 0.41� 0.51SAPO-11 AEL 1D 10 0.40� 0.65SSZ-13, SAPO-34 CHA 3D 8 0.38� 0.38EU-1 EUO 1D 10 0.41� 0.54; side pocketsL LTL 1D 12 0.71� 0.71ZSM-48 *MRE 1D 10 0.53� 0.563

ZSM-23 MTT 1D 10 0.45� 0.52ZSM-12 MTW 1D 12 0.56� 0.60Rho RHO 3D 8 0.36� 0.36Theta-1, ZSM-22 TON 1D 10 0.46� 0.57aBold: zeolites with known 2D layered precursor.

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binder, very often with alumina, and formulated into larger particles. Thisprocess is usually omitted in laboratories as it requires larger quantitiesof material and specialized equipment. This is often not consideredin publications comparing the performance of laboratory samples with ‘com-mercial catalysts’ while the differences may be quite dramatic. As discussed inref. 26, the extrusion of platelet zeolite crystals, MCM-22, with alumina af-forded an entire range of special crystal orientations with many fracturedfragments edge-on. On the other hand, unformulated crystals have a tendencyto agglomerate and usually expose flat surfaces when imaged in microscopy.

It must be appreciated that zeolite synthesis/formation occurs spon-taneously and without human intervention when appropriate compositionand conditions are established. Arguably, the process is driven by someself-controlling mechanism but, alas, our knowledge and understanding ofit is practically non-existent. This has not been an obstacle to impressiveexpansion in both fundamental understanding and applications as well asdiversity of structures. In the long run, a greater reliance on the fundamentalknowledge of zeolite formation, especially quantitative, may be necessary tobetter understand opportunities as well as limitations of zeolite synthesis.

This lack of understanding is surprising after many decades of develop-ment especially since some basic tenets are quite clear – zeolite formationbegins with the nucleation of viable nuclei, i.e. ones that can growspontaneously. Then, as established empirically in many cases, the growthfollows the McCabe law,27 i.e. individual crystal size increases with aconstant linear rate until availability of nutrients becomes the rate limitingfactor. Initial attempts to elucidate zeolite formation focused on rational-izing the long-range order,24 which some assumed could not arise from theattachment of small, e.g. silicate, aluminate fragments. In fact, the oppositeis the prevailing opinion right now. The nature of the nucleus has not beenresolved and is rarely considered.24,28 Some detailed proposals have oftenbeen structure-specific and not necessarily applicable to other structures.Ideally, one would want to relate the kinetics of crystallization tocomposition, which might enable predictions including new species.

4.3.3 Role of Organic Structure-directing Agents

The addition of organic polar molecules, mainly amine-based, has beenfound to modify properties of the synthesis gel and induce crystallization ofnew zeolite frameworks or novel crystal forms in terms of size and/or habit.The early examples of this new strategy include tetraalkylammoniumcations: tetramethyl, tetraethyl, and tetrapropyl, which produced lower Alzeolite A, zeolite beta and ZSM-5, respectively. Many more organic com-pounds have been tried and a plethora of novel structures were discoveredincluding ones with pores larger than 12-ring, which were the largest withnon-organic preparations. Initially the role of the organic additives wasrationalized as templating i.e. organizing inorganic species around thetemplate in an early stage of the zeolite synthesis. An alternative name, i.e.

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structure-directing agent (SDA), is also commonly used. In general, it is notpossible to predict either the structure or what size and dimensionalityof pores a particular template may produce. The chief strategy uses atrial-and-error approach together with experience in the design and selectionof SDAs. The area has been extensively reviewed from the standpoint ofvarious templates and the structures they produce.29 Here, we will presentselected examples indicating some influence extending beyond simple porefilling and occlusion.

The formation of the same structure by different templates is such a case,indicating more than a pore-filling effect. These SDAs often also templatedifferent structures. Initially, the zeolite MFI was frequently invoked ashaving many templates but it can be also produced from a purely inorganicsystem. Recently, the structure of MWW zeolite, which is represented byMCM-22, MCM-49, EMM-10 and considered isostructural with SSZ-25, ITQ-1and other materials, was found to be formed with structurally diversetemplates.30 They include cyclic amines, especially hexamethyleneimine,diquaternary penta- and hexamethonium ions, asymmetric diquaternary,sparteinium and trimethyladamantamonium ions. The same templatesproduce other frameworks upon changing Si/Al, basicity or another syn-thesis parameter. There is also the possibility of first crystallizing one zeoliteand then recrystallizing into another (MCM-22 precursor to TNU-9).31

As an alternative to the discovery of new zeolites using complicatedorganoammonium SDAs, a very effective approach has been designed at UOPreferring to charge-density matching.32 This design has several conceptualcomponents: mixtures of smaller templates and a relatively high Al contentto enable organics coming close together; and low content of alkali cationsto reduce competition with organic cations. Initially, an apparently homo-geneous mixture of silica, alumina and some of the SDAs is prepared with apositive-charge deficiency. Then another template is added and the hydro-thermal synthesis is carried out to crystallize a zeolite phase. This techniqueafforded some new frameworks and compositions exemplified by UZM-4(BPH) and UZM-5 (UFI). On the other hand, it had a great impact onidentifying simple organic templates for the synthesis of zeolites that werepreviously obtained with quite complex and not readily available SDAs.The best illustration is provided by the synthesis of zeolite ZSM-18.33 Itrequired a trisquaternary ammonium compound, 2,3,4,5,6,7,8,9-octahydro-2,2,5,5,8,8-hexamethyl-1H-benzo[1,2-c:3,4-c0:5,6-c00]tripyrrolium (Scheme 4.1),and was presented as exemplifying true templating, i.e. an exact matchbetween the channel system of the zeolite and the size and shape of the SDA.The equivalent structure, UZM-22 was obtained with choline as SDA and Sr21

and Li1 as additional essential components in the synthesis mixture,thus complicating the ZSM-18 formation rationale.34 More recently, thealready-mentioned zeolite MCM-68 was claimed to be replicated with a muchmore convenient template: dimethyldipropylammonium hydroxide.35

Some new medium-pore zeolites were obtained recently with aflexible linear diquaternary alkylammonium cation composed of two

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N-methylpyrrolidinium groups connected by a tetra- or pentamethylene chain.They include TUN and IMF and a high-silica analog of stilbite (STI) designatedTNU-10 with a Si/Al ratio of about 7 : 1.36 Their structure and properties aretypically compared to MFI as the most prominent and, in catalysis, the bestperforming medium-pore zeolite. The projection of TUN down the b-axis issimilar to that of MFI but the channel connectivity in the third direction israther complex. Zeolite IMF has formally a 3D 10-ring channel system but itsconnectivity is complicated. It is considered to be effectively a 2D pore systemwith restricted diffusion. The properties connected with shape-selectivecharacteristics are quite different from those of MFI.

It is a bit of a paradox that in spite of the enormous success of SDA use inthe discovery of new zeolites and crystals, the preferred synthesis approachwould ultimately be to reduce procedures to non-organic mixtures. The cost,handling and environmental issues associated with the use of SDAs presenta significant burden in larger-scale implementation. The achievement ofnon-organic syntheses may be difficult to envision in every case but none-theless some promising examples may be mentioned. Okubo’s groupshowed that some zeolites made originally with an SDA can be synthesizedwithout organic templates.37 The working hypothesis of the zeolite synthesiswithout the presence of SDA is based on the utilization of some commoncomposite building units in different zeolite structural types. A noteworthyexample of successful template-free synthesis is the recent report onhexagonal faujasite, i.e. EMT.38

4.3.4 Role of Inorganic Species

Inorganic cations accompany the greatest part of zeolite preparations,mainly in the form of added hydroxides as mineralizers. Even in the absenceof addition on purpose, the presence of inorganic cations is to be expected asan adventitious impurity, just as with Al in the case of silica-based prepar-ations. The nature of the inorganic cation is not irrelevant and may inducealternative structures too, for example, Na, which is the most common alkaliused. The presence of inorganic cations is ultimately critical for the suc-cessful preparation of some zeolites (e.g. UZM-2232), but in other casesshould be reduced to a minimum. For example, sodium cations usually limitthe incorporation of Ti in the synthesis of TS-1.39 Sometimes the directsynthesis of proton forms of zeolites, through preparation with organic

Scheme 4.1

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cations only, can be preferred to avoid the necessary ion exchange of sodiumforms to protonic ones followed by the treatment of wastewater solution.

Entirely different effects have been observed with inorganic species thatare supposed to be part of the framework. There is always the possibility ofisomorphic substitution, like B, Ga, and Fe for Al, or Ge for Si. The firstgroup of atoms carries a þ3 charge and generates a negative charge of theframework, introducing acidic properties to zeolites.17,40 The second andparticularly fruitful area resulting in numerous novel, usually extra- or large-pore, structures arose from the synthesis of Ge-based zeolites.18,19 Asdiscussed in ref. 19 there are practical problems with Ge-based materials,one of them being cost and availability of Ge, which is almost prohibitiveunless a really high-value application can be found. Nonetheless, theGe-zeolites set a new standard/frontier for more traditional compositions toadvance. The chemistry of germanosilicates in combination with appropri-ate SDAs offers new possibilities to synthesize novel zeolites. Another issuewith the Ge-zeolites is lower thermal and hydrothermal stability oftenresulting in significant degradation upon standard treatments or activation.19

However, this apparent adversity has been turned into an opportunitythat resulted not only in novel-type materials but also the demonstration ofa new process: a 3D to 2D transformation, which will be discussed in thefollowing section.

4.4 From 2D to 3D Zeolites and Vice Versa2D solids are recognized as lattices with strong bonding in two crystallo-graphic directions and a much weaker one in the third with the capabilityof easily severing these connections. These solids are considered to becomposed of rigid layers with an interlayer region that can be spatially andcompositionally modified by intercalation and pillaring, and ultimatelydelaminated into individual lamellae.41,42

Zeolites, as lattices defined by their 3D connectivity, seemed outside the2D concept but nonetheless their crystal formation was considered to entailgrowth through layers.43,44 No doubt the appearance of zeolite structures insome projections as compact layers with much less dense Si–O–Si bridgesreinforced this notion. Zeolite MCM-22 reported in the early 1990’sdemonstrated the formation of its 3D framework via a definitely layeredintermediate designated the layered MCM-22 precursor, MCM-22P, whichwas the final product from the hydrothermal synthesis.45,46 Its conventionalpost-synthesis processing including calcination produced a complete 3Dframework material MCM-22/MWW. The layered nature was further con-firmed and exploited in practice by intercalation with surfactants resultingin a swollen MCM-22. It was an inorganic–organic layered composite com-prising MWW monolayers, 2.5 nm thick, separated with surfactant bi-layers,2.5–3 nm thick. As the first case of exfoliated layered zeolite it was trans-formed by treatment with TEOS (tetraethylorthosilicate), hydrolysis andcalcination into pillared material, designated MCM-36. The latter had layers

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permanently separated by ca. 2.5 nm and combined micro- and meso-poresof size 3� 0.5 nm with strong acid activity typical of zeolites. Later on, theswollen product was subjected to sonication affording eventually, aftercalcination, a delaminated zeolite designated ITQ-2.47 Another delaminatedMWW zeolite was obtained earlier by direct synthesis and designatedMCM-56 but exhibited a standard Brunauer–Emmett–Teller (BET) surfacearea of 400–500 m2 g�1 compared with the ITQ-2 BET surface area of around800 m2 g�1. The difference was attributed to house-of-cards vs. deck-of cardsarchitecture for ITQ-2 and MCM-56.48 While special, MCM-22 proved not tobe unique and other frameworks were found to exhibit the 3D–2D lamellarduality and were found to form by two pathways (at least) – direct 3D andindirect 2D. The number of known lamellar zeolites is so far limited to about10, but it is believed to be possible that all 3D frameworks can have alamellar counterpart.49,50 This is reinforced by the recent remarkableinvention of layered zeolite synthesis by the design of appropriate SDAs.51

The authors used long-chain surfactant molecules with one end templatingthe appropriate zeolite structure while the long tail prevented the initiationof a new layer in close proximity. Based on this principle any zeolite structuremay be viewed as possible to make via such an inorganic–organic composite.

The framework MWW continues to afford novel structures resulting fromvarious packings of the layers and bonding arrangements directly or throughadditional linkers. Particularly noteworthy are the so-called interlamellarexpanded zeolites (IEZ), which are simply the layered precursors stabilized intheir expanded form (i.e. not condensing into the standard 3D frameworkupon calcination) by appropriate treatment, like silylation. The interlayerpore openings are effectively enlarged by two Si–O units.52 New types ofmaterials, sometimes unexpected, are constantly added to the layered zeolitefamilies. These developments have been expressed as a formal 2D concept ofthe variability of zeolite structures with one dimension being the variousframeworks and the second their different architectures arising from layerpackings.53

The area of 2D zeolites may have a great potential for practical use butin the realm of applications these new materials have to compete withestablished zeolites not only in performance but also cost and ease ofimplementation. Claims of improved performance are quite abundant,49,50,54

including examples of those under commercial conditions, like MCM-56 inliquid alkylation. The actual industrial use is sometimes hard to confirm asproprietary but it is possible that the mentioned use of UZM-8,25 an MWWzeolite with apparently disordered layer structure, may be one of the firstexamples of commercialized 2D zeolites.

Another new development in the area of 2D zeolites has been the trans-formation of an existing frameworks structure into a lamellar precursor (of athen unknown zeolite) by selective chemical degradation.55 It was contingenton particular features of the initial structure, i.e. layers supported by cubicD4R entities composed mainly of readily hydrolyzable Ge. The product,prepared by a top-down approach, was designated IPC-1P (P¼precursor)

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and was shown to undergo transformations typical for layered precursors:swelling, pillaring and IEZ stabilization.55

4.5 AdsorptionZeolites have a leading role in industrial selective adsorption and separationprocesses. Their benefits rely in particular on their large specific surface areaand easy modifiability through ion-exchange and molecular-sieving prop-erties. Consequently, zeolites have found application in many industries,particularly refining, petrochemical and chemical industries. Adsorption onzeolites can follow several different pathways including ion exchange, shapeselectivity and, most importantly, equilibrium-selective adsorption.56 Fromanother point of view, zeolite-based adsorption applications can be dividedinto impurity removal and separation processes.

The main application of zeolite ion-exchange adsorption is as additivesin laundry powders/detergents using sodium forms of zeolite A or P.57,58

Zeolites act as water softeners replacing sodium tripolyphosphate, whichwas previously used but proved to be an environmentally unacceptablecomponent. By ion exchange, the calcium and magnesium cations, whichare responsible for water ‘‘hardness’’ hindering washing efficiency andleading to formation of carbonates,59 are removed. The ion-exchangeprinciple is also used in environmental remediation applications, such asradioactive or heavy metal removal from soil and ground water. The shape-selective adsorption principle is the key to separation of n-alkanes fromi-alkanes using 5A zeolites (CaA60). Due to the suitable pore dimensions of5A, n-alkanes readily enter the pore system, while the branched isomers aretoo bulky and cannot enter the pores to be adsorbed.

Most of the zeolite adsorption applications are based on differences in theequilibrium selectivity of various sorbates with the adsorbent.56 Concerningthe contaminant removal, water (as moisture) is the primary concern inmany industrial applications. The activated, i.e. dehydrated, zeolites A or Xare typically used as the chosen desiccants. Generally, water removal isoperated as a pressure swing adsorption (PSA), i.e. water is removed atelevated pressure in one adsorbent bed, while in the other bed the pressureis decreased to regenerate the adsorbent. Very low residual humidity(o1 ppm) can be achieved for a wide range of gases (CH4, N2, O2, H2, CO2,SO2, H2S, NH3, HCl).61,62 Other gaseous impurities that can be removed byusing zeolites A, X or mordenite include CO2, SO2, H2S, HCl and NH3.Table 4.2 lists characteristic applications of zeolites in adsorption andseparation processes.

Zeolites are commonly used in separations of permanent gases andhydrocarbons. The industrial separation of permanent gases uses PSA pro-cesses and the most important ones include air separation to produce N2

and O2 (CaX, LiX, LiCaX63,64) and H2 separation from refinery off-gases (NaX,NaA).65 The most important process, besides separation of n-alkanes fromi-alkanes due to the shape selectivity of zeolites, is isolation of p-xylene from

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the mixture of xylenes and ethylbenzene using cationic forms of zeolites Xand Y (e.g. process Parext).66,67 Other examples of industrial separationsinclude separation of olefins from paraffins (process Olext, CaX) or fructoseseparation from glucose (process Sarext, Ca Y68).

4.6 CatalysisZeolites are now indispensable in many industrial catalytic processes,particularly in the petroleum refining and petrochemical industries wheretheir application facilitated significant innovations. They have acquired thisposition thanks to their unique properties including a tunable acid–basiccharacter, shape selectivity, ion exchangeability and large specific surfacearea. Among more than 200 zeolite structures that have been discovered andsynthesized, about 20 are utilized as commercial catalysts.25 In this group,zeolites Y (FAU), ZSM-5 (MFI), beta (BEA*), mordenite (MOR) and MCM-22(MWW) are most extensively used. From the industrial catalysis point ofview, the most important zeolites and the processes they are applied in aresummarized in Table 4.3.

Fluid catalytic cracking (FCC) is the single most important catalyticprocess using zeolites. It is consuming more zeolite volume than allother processes altogether.5 The FCC together with hydrocracking is the keyprocess in a modern refinery for the conversion of vacuum distillates, i.e.heavy oil fractions, into light products, mainly gasoline and diesel fractions.The catalyst used in FCC has a very complex composition but the develop-ment of zeolite Y, in particular its steam pretreatment affording ultrastable

Table 4.2 Separation and adsorption processes using zeolites.

Zeolite (IZA code) Industrial application

A (LTA) O2/N2 separationH2 separation from off-gasesCO2 removalNatural gas dryingDrying of industrial gasesn-/iso-Paraffin separation – Sorbex (Molex)

Mordenite (MOR) O2/N2 separationRemoval of SO2Removal of HClTrapping Hg vaporsAcid gases drying

X, Y (FAU) O2/N2 separationH2 separation from off-gasesCO2 removalCO2/CH4 separationNatural gas dryingXylenes separation – Sorbex (Parex)Olefin separation – Sorbex (Olex)Removal of dioxins

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zeolite USY, has made it so that rare earth (RE)-exchanged USY is thekey component of FCC catalysts. A fresh FCC catalyst contains typically20–40 wt% of USY, the other components being matrix (includes catalyticallyactive binder consisting of alumina and amorphous aluminosilicate, clay)that is responsible for pre-cracking of large feed molecules which couldnot enter into USY pores, and various promoters. The role of promoters isthree-fold: protection of USY structure from vanadium, environmental pro-tection (SOx, NOx and CO abatement) and product selectivity adjustment.The last one may be aimed at increasing the yield of propylene, a desired

Table 4.3 Industrial catalytic processes using zeolites and zeotypes.

Zeolite or zeotype (IZA code) Catalytic process

Beta (BEA*) Benzene alkylationAcylationBaeyer–Villiger reaction

Erionite (ERI) Selectoforming

Ferrierite (FER) n-Butene skeletal isomerization

L (LTL) Alkane aromatization

MCM-22 (MWW) Benzene alkylation

Mordenite (MOR) Light alkanes hydroisomerizationDewaxing (cracking)Aromatics alkylation and transalkylationOlefin oligomerization

SAPO-11 (AEL) Dewaxing (long-chain alkane hydroisomerization)

SAPO-34 (CHA) Methanol to olefins

Y (FAU) Fluid catalytic crackingHydrocrackingAromatics alkylation and transalkylationOlefin/paraffin alkylationNOx reductionAcylation

ZSM-5, TS-1, Silicalite (MFI) Fluid catalytic crackingDewaxing (cracking)Methanol to gasoline/olefinsOlefin cracking and oligomerizationBenzene alkylationXylene isomerizationToluene disproportionation and alkylationAromatizationNOx reductionAmmoxidationBeckmann rearrangement

ZSM-12 (MTW) Aromatics alkylation

ZSM-22, Theta-1 (TON) Dewaxing (long-chain alkane hydroisomerization)Olefin skeletal isomerization

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petrochemical raw material. FCC has now become the second mostimportant source of propylene supplying about one third of the world pro-pylene production. This has been enabled by the use of zeolite MFI, typicallyin amounts equaling 0.5–3 wt% of the total FCC catalyst loading.

At the core of the FCC catalyst, there is the zeolite FAU characterized by itsunit cell size (a0) being a measure of framework aluminium content anddetermining the acid site density and strength, a key property of the zeolitefor FCC.69,70 Catalyst activity as well as product yields, coke formation andproduct quality (e.g. gasoline octane number) depend directly on the unitcell dimension. The maximum activity for gas oil cracking was observedwhen all framework Al atoms in the faujasite structure were isolated, i.e. at aframework Si/Al ratio of 5–8 (a0 ¼ 2.436–2.440 nm). So far, zeolite USY with aframework Si/Al ratio 45 was not synthesized by direct synthesis and thedealumination of the parent zeolite Y with Si/Al of about 2.8 is the onlyalternative to prepare the optimum catalyst. The dealumination helps notonly to adjust the Si/Al ratio to the optimum value range, but also affects thecatalyst performance by creating secondary mesopores and forming extra-framework Al species (EFAL).71 Moreover, the dealumination of FAU zeoliteby steaming affords the ultrastable Y zeolite (USY) that has substantiallybetter hydrothermal stability than the parent zeolite FAU. This is essential,particularly for the catalyst regeneration by burning off coke at temperaturesabove 700 1C in the presence of steam.69 A further improvement of thezeolite FAU stability is achieved by introducing rare earth (RE) metals, suchas lanthanum, by ion exchange. Currently, the limited availability of thesemetals has led to an increased demand for low RE metals catalysts.

Besides the optimized strength and density of acid sites, shape selectivityplays an important role in FCC catalysts that are designed accordingly. Toprevent deactivation of the active sites in zeolite Y channels that areresponsible for the optimum selectivity to light products, the catalystconsists of an active matrix where pre-cracking takes place and where heavymetals such as vanadium and nickel are deposited. Consequently, instead ofblocking zeolite Y pores by large feed molecules the pre-cracked moleculesenter the channel system and are further converted to gasoline and a rangeof gaseous hydrocarbons. With the current shift in the demand for FCCproducts, the yield of propylene is being maximized by adding a ZSM-5additive allowing only a small range of gasoline molecules into its pores;these are further cracked to yield gaseous products, mainly propylene and C4

olefins. Moreover, the 10-ring system of ZSM-5 efficiently suppresses theformation of carbonaceous deposits as the condensation intermediateformation is sterically prohibited. To achieve sufficient stability of ZSM-5, itis typically treated with phosphoric acid and steaming at 800 1C to createaluminium–phosphate complexes in the phosphorus-stabilized ZSM-5.69

Zeolite USY is at the heart of the hydrocracking process, which is anotherimportant industrial process aimed at upgrading heavy fractions from crudeoil distillation. Unlike in FCC, this catalytic cracking is combined with deephydrotreating and hydrogenation. As a result the products are sulfur- and

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nitrogen-free and have a significantly reduced aromatic content. Theadvantage of hydrocracking over FCC in bottom-of-the-barrel processing isthe superior quality of the products, particularly where middle distillatesquality is concerned. The type of catalyst depends on the feedstock andthe technology solution; nonetheless the catalyst is bifunctional, thehydrogenation function being provided by metal sulfides, such as NiWsulfides, or by Pt and Pd. The choice depends obviously on sulfur concen-tration in the effluent from the hydrotreating stage. The necessary acidityis delivered either from amorphous silica–alumina or USY. The acidityand mesoporous character is adjusted again by the ultra-stabilizationprocedure. The presence of mesopores ensures facile molecular trafficleading to suppressed secondary cracking reactions and hence to betterselectivity to middle distillates (kerosene, diesel). An optimized 5-stepultra-stabilization procedure consisting of three ion-exchange and twosteam-calcination steps provides a highly ultrastable zeolite Y (VUSY) with aunit cell size of 2.432 nm (Si/Al¼ 9.6).69 Other zeolites and mesoporousmolecular sieves have been studied for hydrocracking (MOR, LTL, omega,BEA* and MCM-41);70 however, their commercial application has not beenconfirmed.

In addition to processes focused on enhancing the quantity of lightproducts (gasoline and middle distillates), zeolites are extensively used inrefining processes with the objective of improving the fuel properties of thelight fractions. Typical examples are catalytic dewaxing and light-paraffinisomerization.69,70 Catalytic dewaxing is a process used either for middledistillates or lube oils which aims to remove n-alkanes having high meltingpoints. Two catalytic approaches are possible: selective cracking of n-alkanesinto smaller molecules, and catalytic isomerization to iso-alkanes exhibitingbetter flow properties at cold conditions due to lower melting points. Toachieve this, bifunctional catalysts are applied, the hydrogenation/dehydrogenation component being typically platinum and the isomerizationor cracking function being provided by Brønsted acidity of zeolites. Theisomerization option should be preferred as it ensures higher yields ofdesired products. An important feature of the zeolites to be used is theirshape-selective properties which ensure that only n-alkanes can enter thezeolite channel system to be isomerized or cracked, while multi-branchedalkanes that are more reactive than n-alkanes are denied access to activeBrønsted acid sites and over-cracking is hence avoided. The followingzeolites fulfill the criteria on shape selectivity and acidity: MFI, TON, MTTand zeotype AEL. An excellent example of shape selectivity is the comparisonbetween ZSM-22 and ZSM-23. The very small difference in their pore open-ings (TON: 0.46 � 0.57 nm, MTT: 0.45 � 0.52 nm) results in large differ-ences in diffusivity (1–2 orders of magnitude larger diffusion coefficient formethylnonanes in TON than in MTT) and a larger Henry coefficient in TONthan in MTT (about two orders of magnitude). As a consequence, MTTshows lower branching in n-octadecane isomerization (dewaxing).72–75

An important aspect is also the concentration of active sites on the

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external surface; the lower it is, the higher the selectivity of the isomerizationto monobranched alkanes.

The main objective of light paraffin (C5, C6) isomerization is to maximize theoctane number of the light naphtha cut. In contrast to dewaxing, a maximumdegree of branching is desirable. The key parameter is the strength of acidsites, as stronger acidity allows operation at lower reaction temperatureswhich is beneficial from the thermodynamics point of view. Zeolites, typicallyPt/MOR, have to compete with other catalytic systems, such as chlorinated-alumina- or sulfated-zirconia-supported platinum catalysts which can beapplied at temperatures o200 1C. The operating window for Pt/MOR is around250 1C.5 Pt/MOR has the maximum strength of Brønsted acid sites providingthe maximum isomerization activity and has a Si/Al ratio of about 10. Thedealumination of MOR decreases diffusional limitations and coking rate. Theoverall strength of Brønsted acid sites can be further increased by an intro-duction of controlled amounts of extra-framework Al by steaming and acidleaching, which interact with Brønsted acid sites.76 While the activity of thezeolite-based catalyst is low, resulting in a lower gain in research octanenumber (ca. 10 for Pt/MOR vs. ca. 14 for Pt/chlorinated alumina), it out-performs the non-zeolite catalysts in terms of sensitivity to contaminants suchas water and small amounts of sulfur and regenerability.5 The lower activity ofPt/MOR can be addressed by integrated separation of n- and iso-paraffinsusing, e.g. CaA as discussed above. The process is called the ‘‘total iso-merization process’’.77 Light paraffins, generally C6 and C7, are alternativelyalso commercially converted over a modified bifunctional zeolite L to affordaromatics by dehydrocyclization. Typically, Pt/K-LTL or Pt/KBa-LTL zeolites areused in processes, such as Platforming (UOP) or AROMAX (Chevron Philips).78

Unlike in isomerization, it is essential to remove any sulfur and acidity.79

Apart from the cracking process, zeolites are indispensable catalystsfor the petrochemical transformation of aromatics, in particular for theisomerization, disproportionation and transalkylation of alkyl aromatics(C7–C9) and for the alkylation of benzene. The primary objective of the firstgroup of processes is to maximize the production of p-xylene and benzene,while alkylation is targeted for the production of ethylbenzene and cumene.The choice of catalyst for isomerization of C8 aromatics depends on thetreatment of ethylbenzene. If it is to be isomerized to xylenes (and ultimatelyto p-xylene) then Pt/MOR or Pt/EU-1 (EUO) are applied; if selective deal-kylation of ethylbenzene is the aim then dealuminated Pt/MFI is used.5,6 Theselectivity over Pt/MOR is generally improved by avoiding any mesoporosityand by partial ion exchange by Na or Ca cations to decrease Brønsted acidsites density.80 The decreased density of Brønsted acid sites limits the extentof disproportionation reactions leading to undesired by-products, such astrimethylbenzenes. When Pt/MFI is used as a xylene isomerization catalyst,the formation of trimethylbenzenes is suppressed due to its transition-stateshape selectivity. The product shape selectivity of MFI to p-xylene, originat-ing from a higher diffusivity of p-xylene in comparison with m- and o-xylenefrom inside zeolite pores, can be further improved by using larger zeolite

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crystals, deactivating active sites on the external surface of these crystals andreducing the size of pore windows, e.g. by coke or inorganic speciesdepositions.4,6

The shape selectivity of MFI in favor of the para-isomer is the key catalystfeature used in toluene disproportionation to afford a high yield ofp-xylene.77 The intrinsic selectivity of MFI is further enhanced by suitablecatalyst design as discussed above. Toluene can be alternatively utilized asthe feedstock for transalkylation with heavier alkylaromatics (C9 and C10).Several processes were developed (Tatoray, PX-Plus, PX-Plus-XP by UOP);however, catalyst details were not disclosed.65

The most important processes of aromatics alkylation are the syntheses ofethylbenzene (an intermediate of styrene production) and cumene (an inter-mediate of phenol production).4,81,82 Two principal technology solutions weredeveloped for ethylbenzene production: a vapor-phase process using MFIcatalyst (Mobil Badger process) and liquid-phase processes using MWW orBEA* (EBMAX, Exxon Mobil and Polimeri Europa, respectively). The mainadvantages of the liquid-phase process include a lower excess of benzeneneeded and lower reaction temperatures.5 Nearly exclusive by-products arediethylbenzenes that can be either recycled to the feed (gas-phase processes)or fed into a second (transalkylation) reactor with benzene where they areconverted to ethylbenzene over the same catalyst as in the alkylation reactor.The production of cumene by alkylation of benzene with propene is analogousto ethylbenzene production. Several processes using different zeolitesare applied commercially: dealuminated mordenite (Dow), beta (Eni) andMCM-22 (ExxonMobil).79,81–83 Moreover, the dealuminated MOR (3-DMM) ishighly active and the alkylation can be operated at low temperatures, whichis beneficial for minimizing the formation of n-propylbenzene, a highlyundesirable by-product.5

Another important class of industrial technologies using zeolites or zeo-types is related to methanol as the cornerstone of C1 chemistry, which is analternative to the Fischer–Tropsch synthesis (FTS) starting directly fromsynthesis gas. Both FTS and methanol conversion process allow the util-ization of other carbon containing feedstocks (natural gas, coal or biomass)for the production of products obtained currently as a result of petroleumprocessing. The main conversion processes include methanol-to-gasoline(MTG), methanol-to-olefin (MTO) and methanol-to-propylene (MTP). ZSM-5is the catalyst of choice for MTG84 and MTP77 processes, while SAPO-34is used in the MTO process as its structure is optimal for the selectiveformation of ethylene and propylene.84

4.7 SummaryZeolites may be viewed as embodying the ultimate best of natural and syn-thetic inorganic compounds through a fundamental elegance and diversityof structures combined with exceptional practical usefulness while beingenvironmentally benign and eventually relatively easy to manufacture. Over

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the years their properties and selective uses have been perfected to suchlevels that they are hard to beat, including by other new zeolites. They pro-vided inspiration for innovation leading to related materials like orderedmesoporous materials, metal–organic frameworks and recently expandinginto the area originally thought to be the contradiction of zeolites i.e. 2D orlayered zeolites, which now appears to be an integral part of the family. Fewother areas of science seem to prosper so well in both improvements of theknown and advancements of the new and unknown.

AcknowledgementsThis work was supported by the Czech Science Foundation Grant No. P106/12/G015 (Centre of Excellence).

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CHAPTER 5

Enzyme Immobilization onMesoporous Silica Supports

CHENG-YU LAI* AND DANIELA R. RADU

Delaware State University, 1200 N. DuPont Highway, Dover, DE 19901, USA*Email: cylai@desu.edu

5.1 Introduction – Biocatalysis and Porous SilicaMaterials

The application of porous silica in biocatalysis involves enzyme (biocatalyst)immobilization on solid supports, which integrates enzymatic catalysis withheterogeneous catalysis.

Biocatalysis could be defined as the use of biological entities with catalyticproperties (biocatalysts) for industrial synthetic chemistry. Biocatalysts in-clude: enzymes, whole-cell catalysts, catalytic antibodies and nucleic-acid-based enzymes (ribozymes and DNAzymes). Enzymes are the predominantcategory of biocatalysts due to their historical use in biochemical processeswhich drove comprehensive studies directed to their fundamental under-standing. They are highly effective and versatile biological catalysts, anddisplay high chemo-, stereo- and regioselectivity while operating under ambi-ent conditions (physiological temperature and pH, atmospheric pressure).When enzymes are used as catalysts, no activation or protection/deprotectionof functional groups – which are typically required in organic synthesis – arenecessary. Furthermore, synthetic catalysts often demand harsh chemicalconditions and multi-step processes, leading to excessive energy consumptionand the generation of much waste. In contrast, due to their operation mostly in

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water as solvent, enzymes generate less waste. Due to their high selectivity,shorter synthetic routes are achieved through enzymatic catalysis in com-parison to traditional organic synthetic routes. Enzymatic processes can nowbe carried out in organic solvents1 as well as aqueous environments, so thatnon-polar organic compounds as well as water-soluble compounds can bemodified selectively and efficiently. As the use of biocatalysis for industrialchemical synthesis becomes easier, several chemical companies have begun toincrease significantly the number and sophistication of the biocatalytic pro-cesses used in their synthesis operations2 and, thus, the use of enzyme bio-catalysis for industrial synthetic chemistry is on the verge of significant growth.

However, the use of enzymes in their native form is often hampered byseveral limitations such as high costs, low operational stability and difficultiesin recovery and reuse. Fortunately, there are many techniques available thatpermit enzyme performance to be improved, involving areas of science thathave undergone impressive developments in recent years: microbiology, pro-tein engineering, chemistry of proteins, etc. Additionally, enzyme immobil-ization, albeit considered old-fashioned, is a very powerful tool to improvealmost all enzyme properties, if properly designed: e.g., stability, activity, spe-cificity and selectivity, and reduction of inhibition. Moreover, product con-tamination with enzymes can be minimized or completely avoided, which isparticularly critical for applications in the pharmaceutical and food industries.Immobilization techniques have been recently revamped as new developmentswithin material science have highlighted the potential of a variety of organicand inorganic materials for use as supports for enzyme immobilization.

Many catalytic supports such as resins, polymers, electropolymerizedfilms, sol–gel materials, inorganic solids such as solid and porous silica etc.,have been explored with regard to enzyme immobilization and com-prehensive reviews have been published.3–8

Here we outline recent advances in the area of mesoporous silica involved inenzyme immobilization development. Mesoporous silica supports provide aset of the most attractive features toward overcoming enzyme stability draw-backs both in biotechnology and biocatalysis applications.9 Mesoporous silicamaterials are structurally robust, chemically stable over a broad pH and tem-perature range and benefit from flexible synthetic conditions that enable tai-loring of their properties for a plethora of host–guest chemistry applications.

The large surface area and pores with an adjustable pore size, typically inthe size range of 2 to 40 nm, make them suitable for accommodating largebiomolecules, including enzymes.

5.2 Types of Porous Silica Support Utilized in EnzymeImmobilization

5.2.1 Introduction

The immobilization of enzymes on a solid support is a methodology de-signed to overcome enzymes’ limitations, such as low long-term stability

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and poor reusability. Their immobilization within the pores of poroussilica permits the full dispersion of enzyme molecules without thepossibility of interacting with any external interface. Thus, the immobil-ization provides a gate-keeping effect by stabilizing the enzyme againstinteraction with molecules from the enzymatic extract, and preventingaggregation, autolysis or proteolysis by proteases from the reaction extract.Moreover, the immobilized enzyme molecules are not in contact with anyexternal hydrophobic interface, such as air bubbles originated by supplyingthe required gases or promoted by the strong stirring which necessary tocontrol pH in industrial catalytic processes. Gas bubbles are known toinduce enzyme inactivation of soluble proteins.10–12 Pore-immobilizedenzymes, however, are protected from gas bubbles which cannot reach intopores and thus, cannot induce enzyme inactivation, as illustrated inFigure 5.1.13

Since the reporting of MCM-41 mesoporous silica materials in 1992 byBeck et al.,14 the family of porous silica materials has experienced con-tinuous growth as a myriad of templates – from surfactants (ionic and non-ionic) to nanoparticles – have been used in a continuous quest to control andoptimize particle morphology and pore-size distribution toward applicationsthat demand properties like pore uniformity, easy pore access, selectivefunctionalization, large-molecule accommodation, and controlled particlesize and morphology. In biocatalysis, porous silica offers an ideal platformdue to the flexibility in tailoring both the particle size and pore-size distri-bution for each enzyme of interest.

The ideal support in each application is process- and enzyme-dependent. Figure 5.2 presents a useful flow chart that helps in supportselection.15

Figure 5.1 Porous supports vs. solid supports for enzyme encapsulation – the poroussupport protects enzyme from interacting with inactivation promoters(illustrated: gas bubbles).Reproduced with permission from ref. 13.

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Two major groups of porous silica have been studied for enzyme immo-bilization applications, classified based on porosity order: (i) hexagonallyordered porous silica materials (Section 5.2.2), and, (ii) hierarchicallyordered porous silica materials (Section 5.2.3). Details of both categories ofenzyme supports are presented.

5.2.2 Enzyme Immobilization/Encapsulation in HexagonallyOrdered Porous Silica Materials

Templating surfactant materials that are capable of forming cylindricalmicelles, which further organize in hexagonal arrays, are at the foundationof forming mesoporous silica with hexagonal arrays of pores (uponproviding a silica source). Most representative members of the hexagonallyordered porous silica materials group are MCM-, SBA- and FSM-typematerials, as indicated in Table 5.1.6

Figure 5.2 Systematic approach for selection of mesoporous particles (MPs), theimmobilization method, and study of enzyme–MPs system.Reproduced with permission from ref. 15.

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Table 5.1 A summary of characteristic properties of various mesoporous silica materials employed for encapsulating enzymes. Reprintedwith permission from ref. 6.

Mesoporous materials Silica source Template Description Pore diameter/nm

MCM-41 TEOS, sodium silicate CnTMA1 (n¼ 12–18) 20 hex.channels

2–10

MCM-48 TEOS, silicate sodium CTAB,C16H33(CH3)2N(CH2)(C6H5)

Bicontinuous 2–4

FSM-16 Polysilicate kanemite Gemini Cm-12-m 20 hex.channels

B4CnTMA1 (n¼ 12–18)

SBA-1 TEOS CnH2n11 N(C2H5)3X 30 cubicmesostructure

2–3(n¼ 12–18), 18B4-3-1Cn–s�1 (n¼ 12–18)

SBA-15 TEOS, sodium silicate Pm, Pas, P6s, 850–1500(B01oE016),Brij97(C1aHl5E01ol

20 hex.channels

5–30

SBA-16 TEOS, TMOS F127, F108, or F98 Spherical cages 5–30MCF TEOS F127 (EOul6PO70EO106) with

TMBCellular foam 10–50

HMS TEOS CmH2m11NH2 (m¼ 8–22) Disorderedmesostructure

2–10

MSU-X TEOS, TMOS CmEOn (m¼ 11–15) Disorderedmesostructure

2–15

IBN-X TEOS C8PhEOn, EO13P030EO13 Nanoparticle 5–20F108, F127, P65, P123 with

FC-4 and TMBPMOs (RO3Si-R0-Si(OR)3 CTAB, OTAB, CPS, P123,

F127, Brij 56, Brij 7620 or 30 hex. 2–20

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MCM-41, FSM-16 and SBA-15 have 2D hexagonal channels and could bedeveloped into particles – typically spherical – with controlled particle-sizedistribution. Their pore size is directed by: (i) the templating agents utilizedin the synthesis of the materials: cationic surfactants for MCM-41-type ma-terials, and non-ionic, block-copolymer type for SBA-15,14,16 and (ii) the silicasource: tetraethyl orthosilicate (TEOS), sodium silicate and polysilicate.Thus MCM-41-type materials feature an average pore size of 2–10 nm,FSM-16 B4 nm, whereas SBA-15 could achieve much larger pores, typically5–30 nm.16 The larger range for MCM-41 is due to the demonstration of poreexpansion, utilizing pore-enhancement molecules.17

Synthetic conditions could be optimized toward tailoring the particlemorphology of mesoporous materials. Lin et al. demonstrated the firstmesoporous silica nanosphere (MSN),18 functionalized with amine groups,opening new avenues in mesopores: silica-based heterogeneous catalysis bycontrolling the size of nanoreactors inside MSNs, which led to a dramaticimprovement in control of reaction kinetics.

A large group of enzymes have been reported as immobilized into orderedmesoporous silica.19 The pore size is critical for enzyme packing and,consequently, the accessibility to the active sites (Figure 5.3).20 As aconsequence, MCM-41, MSN and FSM-16 structures are restricted to theimmobilization of enzymes with relatively small size (with diameters belowpore size diameter).

In addition, relatively low enzyme loadings (typicallyo10 wt%) and slowenzyme immobilization rates are observed, in spite of the fact that thesematerials have surface areas as high as B1000 m2 g�1.21–23

More recently, improved enzyme loadings have been reported for SBA-15materials (pore sizes in the range of 5–15 nm.)24 However, the correlationbetween enzyme size and pore size is not a linear one. Takahashi et al.reported on the catalytic behavior of three silica mesoporous materialsFSM-16, MCM-41, and SBA-15 with various pore diameters from 2.7 to9.2 nm when they were used to adsorb horseradish peroxidase (HRP) in asingle immersion method.22 The study of thermal stabilities and enzymaticactivities in an organic solvent revealed that, surprisingly, FSM-16 andMCM-41 showed a larger amount of adsorption of HRP than SBA-15 or silicagel when the pore sizes were larger than the 5 nm. The increased enzymeadsorption capacity was attributed to the surface characteristics of FSM-16and MCM-41 that may be related to the methods used for their synthesis, inwhich cationic alkyltrimethylammonium salts were used as template for thesynthesis, while SBA-15 materials were prepared by using a non-ionicsurfactant. Furthermore, the immobilized HRP on FSM-16 and MCM-41 withpore diameters B5 nm showed the highest enzymatic activity in tolueneand thermal stability in aqueous solution at the temperature of 70 1C.This finding led to the conclusion that surface character and size matchingbetween pore sizes and the molecular diameters of HRP are important inachieving high enzymatic activity in organic solvent and high thermalstability.22

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5.2.3 Enzyme Immobilization/Encapsulation inHierarchically Ordered Mesoporous Silica Materials

Recent studies revealed that mesoporous silica spheres with hierarchicalstructure, which usually implies bimodal mesoporous structure (BMS),having pore ranges in the 2–3 nm and 10–40 nm range, show faster immo-bilization rates and significantly improved enzyme-immobilization capacitycompared to similar particles with smaller mesopores. Caruso and co-workers verified the enzyme immobilization capacity of the BMS spheres bythe color variation of the particles following exposure to enzyme solutions.7

Cytochrome C and catalase solutions show a red and brown–green color,respectively. The white BMS and SBA-15 particles turn deep red after ex-posure to cytochrome C solution, whereas a control experiment utilizingmesoporous silica with a 2 nm pore diameter showed very small colorchange.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Figure 5.3 Schematic illustration of how protein packing varies as a functionof pore diameter in MCM-41 and SBA-15 and enzyme diameter: (a)close-packed, (b) 3d/2 interval, (c) 2d interval, (d) separated single-molecular adsorption, (e) separated double-molecular adsorption, (f)separated triple-molecular adsorption and (g) interdigitated triple-molecular adsorption.Reproduced with permission from ref. 33.

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Bernal et al. showed that hierarchical porous silica, particles or monoliths,synthesized by the polycondensation of sodium silicate in the presence ofcetyltrimethylammonium bromide and ethylacetate at different concen-trations under hydrothermal conditions have been able to immobilizeb-galactosidase from Kluyveromyces lactis by adsorption.25 The enzymeloading capacity (higher than 50 mg (g support)�1) and the retention ability(lixiviation less than 20% after 72 h of catalysis) of these supports areexplained as a function of the hierarchical porosity, mesopore sizes of10–40 nm, macropore sizes of 0.07–20 mm, and the presence of ionizedsilanol groups on the surface. The optimum pH value and temperature forthe maximum activity of the obtained hybrid biocatalyst indicated that thethree-dimensional structure of the enzyme was not significantly affectedduring the immobilization process. The stability under extreme conditionswas improved in comparison with the homogeneous solution of lactase.Furthermore, the porous supports exhibited morphology and porous sta-bility under the immobilization and catalytic processes. These results showthat the obtained materials are good candidates for the immobilization oflarge enzymes.

5.3 Enzyme Immobilization Strategies in PorousSilica

5.3.1 Introduction

General enzyme immobilization methods may be subdivided into threegeneral classes according to the forces involved: physical adsorption, wherehydrogen bonding, and electrostatic and hydrophobic interactions betweensupport and enzyme exist; covalent attachment, where covalent bonds areformed with the enzyme; and cross-linking which leads to self-immobiliza-tion as illustrated in Figure 5.4.26 The advantages and disadvantages of eachstrategy are highlighted in Table 5.2.

(a) (b) (c)

Figure 5.4 General enzyme immobilization techniques.Reproduced with permission from ref. 20.

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Table 5.2 Advantages and disadvantages of the different immobilization methods.

Immobilizationmethod Advantages Disadvantages

Physical adsorption � Cheap, simple and rapid experimentalprocedure

� (Mostly) no functionalization of support isrequired

� No toxic solvents are required� No conformational changes of the enzyme� No destruction of the active site of the enzyme

� Leaching of enzymes from the support during thecatalytic reaction due to changes in reactionconditions (e.g. temperature, pH) or throughmechanical shear forces

Chemical (covalent)binding

� No leaching of enzymes from the support� Tight binding of enzyme to the support� Wide choice of organic linkers is available� Established methods of functionalization/

modification of supports

� Most complicated and expensiveimmobilization method

� Functionalization/modification of support surfaceis necessary

� Use of toxic chemicals (e.g. glutaraldehyde)� Reduction or even loss of catalytic activity resulting

from conformational changes of the enzymeEntrapped CLEAs/

encapsulation� Stabilization of multimeric enzymes� Stabilization towards harsh reaction

conditions (e.g. extreme pH)� High purity of enzyme is not required� No leaching from the support� Different enzymes can be co-immobilized

(tandem-system)� No or minimal conformational changes of

the enzyme� Size of the CLEAs is restricted by the cage size

� Complicated experimental process (more than twosteps are necessary)

� Use of toxic chemicals (e.g. glutaraldehyde)� Decreased diffusion rate of substrates/products due

to reduced pore volume

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This general methodology translates to porous silica in physical ad-sorption, chemical adsorption and encapsulation or self-immobilization,shown in Figure 5.4c. Cross-linked enzyme aggregates (CLEAs) were recentlyexplored as a combined cross-link/encapsulation approach. Specifically,Mateo et al. demonstrated that a mild cross-linking upon enzyme en-capsulation generated CLEAs from several enzymes (penicillin G acylase,hydroxynitrile lyase, alcohol dehydrogenase, and two different nitrilases) byprecipitation and subsequent cross-linking using dextran polyaldehyde in-side the silica pores.27 In most cases, higher immobilization yields wereobtained using the latter cross-linker as compared with the commonly usedglutaraldehyde. Active site titration of penicillin acylase CLEAs showed thatthe higher activity originated from a significantly lower loss in active sitesusing dextran polyaldehyde as a cross-linking agent. It is proposed thatmacromolecular cross-linkers are too large to penetrate the protein activesite and react with catalytically essential amino acid residue.

5.3.2 Non-covalent Binding of Enzymes on Porous SilicaSupports – Adsorption

Non-covalent immobilization (physical adsorption) of an enzyme onto asolid is probably the simplest way of preparing immobilized enzymes. Themethod relies on a non-specific physical interaction between the enzymeprotein and the surface of the matrix, achieved by mixing a concentratedsolution of enzyme with the solid. A major advantage of adsorption as ageneral method of insolubilizing enzymes is that usually no reagents andonly a minimum of activation steps are required. As a result, adsorption ischeap, easily carried out, and tends to be less disruptive to the enzymeprotein than chemical means of attachment. The binding is by hydrogenbonds, multiple salt linkages, and van der Waals forces.

In this respect, the method is similar to actual biological membranes andhas been used to model such systems. A disadvantage is the weakness ofthe adsorptive binding forces; adsorbed enzymes are easily desorbed bytemperature fluctuations and even more readily by changes in substrateconcentration and ionic strength.28

As indicated in Figure 5.2, surface characteristics must be accounted forprior to the immobilization process. In the case of favorable electrostaticenzyme–support interactions, the actual immobilization process consists ofdispersing the porous silica material in an enzyme solution for a determinedperiod of time to allow the enzyme to diffuse in the pores. For materials withantagonistic properties, i.e. hydrophobic silica and hydrophilic enzyme, thepore surface is further functionalized with hydrophilic groups to match theenzyme surface properties.

In addition to the traditional procedure described above, other innovativeapproaches have been developed, mostly to address weak electrostaticenzyme–pore interactions. To prevent enzyme leaching several reports referto physical entrapment, for example by sol–gel coating or polyelectrolyte (PE)

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multilayer entrapment, where a layer of protective encapsulant is added onthe surface of the silica particle to sequester the enzyme that was previouslyadsorbed. Caruso et al. reported the encapsulation of several enzymes(catalase, peroxidase, lysozyme) in mesoporous silica spheres by physicaladsorption, followed by stabilization through deposition of multilayeredpolyelectrolyte shells on to the enzyme-loaded silica and demonstrated thatencapsulation resulted in enhanced enzyme properties.7

5.3.3 Covalent Immobilization of Enzyme onto Porous SilicaSupports

The most intensely studied immobilization technique is the formation ofcovalent bonds between the enzyme and the support matrix. The functionalgroups of proteins suitable for covalent binding under mild conditionsinclude (i) the a-amino groups of the chain end and the epsilon aminogroups of lysine and arginine, (ii) the a-carboxyl group of the chain end andthe b- and g-carboxyl groups of aspartic and glutamic acids, (iii) the phenolring of tyrosine, (iv) the thiol group of cysteine, (v) the hydroxyl groupsof serine and threonine, (vi) the imidazole group of histidine, and (vii)the indole group of tryptophan. Covalent bonding should provide stable,insolubilized enzyme derivatives that do not leach enzyme into thesurrounding solution.

Furthermore, covalent immobilization must consider the positioning ofactive sites in respect to the substrate. The choice of single-point or multi-point enzyme anchoring depends on the enzyme conformation, and enzymestability in the process condition. Multipoint anchoring confers rigidization,which would be correlated with high enzyme stability. A schematic of the twostrategies is illustrated in Figure 5.5.

Figure 5.5 Single vs. multipoint immobilization of enzymes on the pore surface.Reproduced with permission from ref. 13.

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Enzyme stability is tremendously improved by the covalent attachment ofenzymes in mesoporous silica. In a recent study, three SBA-15 functionalizedmaterials, prepared by a typical process and grafted with 3-aminopropyl-trimethoxysilane (ATS), 3-glycidoxypropyltrimethoxysilane (GTS) and with3-aminopropyltrimethoxysilane and glutaraldehyde (GA-ATS), respectively,were used for the immobilization of chloroperoxidase and glucose oxidaseand the resulting biocatalysts were tested in the oxidation of indole. Itwas found that enzymes anchored to the mesoporous host by the organicmoieties can be stored for weeks without losing their activity. Furthermore,the covalently linked enzymes are shown to be less prone to leaching thanthe physically adsorbed enzymes, as tested in a fixed-bed reactor undercontinuous operation conditions.

The activity of the immobilized enzymes inside the pores is often differentfrom that of the free enzymes, and an important challenge is to understandhow the immobilization affects the enzymes in order to design immobil-ization conditions that lead to optimal enzyme activity. A complete under-standing of active enzyme conformation will lead to controlling the type oflinkage (single-point vs. multipoint).

A recent work reported by Bernal et. al. demonstrates this concept.23

Hierarchical meso-macroporous silica (average mesopore diameter: 20 nm)was synthesized and chemically modified to be used as a support for theimmobilization of lipases from Candida antarctica B and Alcaligenes sp. andb-galactosidases from Bacillus circulans and Aspergillus oryzae. The catalyticactivities and thermal stabilities of enzymes immobilized by multipointcovalent attachment in silica derivatized with glyoxyl groups were comparedwith those immobilized in glyoxyl-agarose, assessing the biocatalyst’s per-formance under non-reactive conditions in an aqueous medium. In the caseof A. oryzae, b-galactosidase and Alcaligenes sp. lipase, an additional step ofamination was needed to improve the immobilization yield. The specificactivities of lipases immobilized in glyoxyl-silica were high (232 and62 IU per gram, for C. antarctica B and Alcaligenes sp. respectively); thermalstabilities were higher than those immobilized in glyoxyl-agarose. Althoughin the case of b-galactosidases from B. circulans and A. oryzae, the specificactivities (250 and 310 IU per gram, respectively) were lower than the onesobtained with glyoxyl-agarose, expressed activities were similar to valuespreviously reported. Thermal stabilities of both b-galactosidases immobil-ized in glyoxyl-silica were higher than when glyoxyl-agarose was used as thesupport. Results indicate that hierarchical meso-macroporous silica is aversatile support for the production of robust heterogeneous biocatalysts.

The immobilization of Mucor miehei lipase onto mesoporous silica ma-terials using supports with different pore diameters show the substrate usedin the reaction acting as an enzyme activator. The lipase uses the fattysubstrate as lipophilic interface required for the opening of the active site ofthe enzyme. IR spectroscopy was used to determine the adsorption iso-therms in different pH conditions. The biocatalyst was tested for themethanolysis of colza oil. The production of methyl esters was monitored

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over time by gas chromatography coupled to a mass spectrometer. By using aratio lower than the stoichiometry, the methanol conversion was completeand high transesterification yields could be obtained even in the absence ofnon-polar solvents (i.e. hexane).29

5.4 Characterization of Catalytic Activity for EnzymeImmobilized in Porous Silica

5.4.1 Introduction

A detailed characterization of porous silica materials involves a largespectrum of analyses. Transmission electron microscopy (TEM), scanningelectron microscopy (SEM), physisorption, small angle X-ray scattering(SAXS), and solid-state NMR (13C and 29Si) are used to determine particlemorphology, pore structure, pore size and pore-size distribution, specificsurface area and pore volume of the materials. We will not present a detaileddescription of all the characterization techniques relevant to mesoporoussilica, as they are well-established analytical methods, well-documented forporous silica materials. We will focus on characterization techniques thatmeasure the enzymatic activity post-immobilization and on quantitativedetermination of enzyme loading.

The immobilization parameters are directly related to the activity andstability of the enzyme. Therefore, the percentage of enzyme immobilizedand the enzyme activity remaining after immobilization are stated togetherwith the experimental conditions used for their determination. Enzyme ac-tivities for immobilized enzymes are defined in the same way as for freeenzymes i.e. the katal is the recommended unit. This information is used forcomparing immobilization methods.

The percentage of enzyme immobilized is usually calculated by measuringthe amount of enzyme remaining in the supernatant after immobilizationand subtracting this from the amount originally present. The absolute en-zyme activity remaining on the support after immobilization is more difficultto determine and an apparent activity is usually measured which takes intoaccount mass transfer and diffusional restrictions in the experimentalprocedure. The other critical performance indicator is the stability of theimmobilized enzyme with respect to time, temperature and other storageconditions.

5.4.2 Determination of Enzyme Concentration in PorousSilica

5.4.2.1 UV Absorption

UV-Vis spectrometry is a widely used method for measuring concentrationsof proteins. Proteins have characteristic absorption peaks at 200 nm and280 nm. All peptide bonds absorb UV light at 200 nm whereas the aromatic

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amino acids tryptophan and tyrosine absorb UV light at 280 nm. The ab-sorbance at 200 nm is mainly due to the many peptide bonds in a protein.However, many other substances also absorb light in this area and 200 nm istherefore not a suitable wavelength for estimating the concentration ofproteins. Therefore measurements are normally performed at 280 nm, thusone should be aware of the possible interference by contaminating nucleicacids (and possibly other substances) that absorb strongly at 260 nm.

5.4.2.2 Colorimetric Assays

Colorimetric assays are also used to determine protein concentration.Well-known examples are the Lowry assay,30 the BCA (bicinchoninic acid)assay,31 and the Bradford assay.32 These assays are all based on a color shiftof an extrinsic molecule in the presence of a protein.

A recent review of physicochemical properties of immobilized enzymesapproaches both characterization methods and challenges.33 The authorssummarize methods that can be used to understand how material propertiescan be linked to changes in enzyme activity. Real-time monitoring of theimmobilization process and techniques that demonstrate that the enzymesare located inside the pores are discussed by contrasting them with thecommon practice of indirectly measuring the depletion of the protein con-centration or enzyme activity in the surrounding bulk phase. A new meth-odology, based on pore filling (pore volume fraction occupied by proteins) isproposed as an accurate standard for comparing the amount of immobilizedenzymes at the molecular level. The article introduces methods to detectchanges in enzyme structure upon immobilization and to study the micro-environment inside the pores.

5.4.2.3 Infrared Spectroscopy (IR)

IR can be used to obtain information about enzyme–substrate (ES) com-plexes. In ES complexes there are well-organized binding modes, which arequantifiable using infrared methods. In analyzing infrared data, it is pos-sible to identify binding modes and heterogeneity of ES complexes.

5.4.2.4 Other Techniques

More recently, spherical aberration (Cs) correlation scanning transmissionelectron microscopy (STEM) has elucidated the presence of enzymes insidethe pores. In the current manuscript we report a detailed characterizationbased on spherical aberration (Cs) cor. STEM of enzyme (lipase)-loadedordered mesoporous silica (SBA-12) at an accelerating voltage of 80 kV.The extremely high-resolution images combined with electron energy lossspectroscopy (EELS) analysis have allowed a complete and unambiguousdetection of enzyme presence inside the pores.34

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5.4.3 Enzymatic Activity

The following spectroscopic techniques are the most utilized to determineimmobilized enzyme activity: fluorescence spectroscopy and UV-Visspectroscopy.

5.4.3.1 Fluorescence Spectroscopy

Fluorescence spectroscopy reveals the existence of ES complexes and whatthey are made of. Substrate fluorescence is measured and compared to theproduct fluorescence and the difference is reflecting enzyme activity. How-ever, many impurities found in fluorescent compounds, when exposed tolight, interfere with the spectroscopy, making this technique more sensitivethan other assays.

5.4.3.2 UV-Vis Spectroscopy

The activity of the enzymes immobilized in various materials is typicallyevaluated by UV-Vis spectrometry. The Michaelis–Menten equation is one ofthe simplest and best-known models of enzyme kinetics. The model takesthe form of an equation describing the rate of enzymatic reactions (eqn(5.1)), by relating reaction rate v to [S], substrate concentration:

v¼ d½P�dt¼ Vmax½S�

Km þ ½S�(5:1)

where v is the velocity of the reaction, Vmax is the maximum (theoretical)velocity, and Km is the Michaelis constant, (k�1þ kcat)/k1.

The maximum theoretical velocity, Vmax, is the velocity when the substratebinds to all of the active sites on all the enzymes, when it is totally ‘‘satur-ated’’. This is impossible because there will always be some free enzymeavailable; the reaction to produce product and free enzyme is always goingon. The Michaelis constant Km is numerically equal to the substrateconcentration [S] that produces a velocity v¼ Vmax/2. Vmax can be roughlyestimated from plots of v vs. [S], and then Km can be obtained from the valueof [S] at Vmax/2 on the plot.

Km and Vmax provide very important information about an enzymaticreaction, and are among the very first things that scientists try to determineor verify for an enzyme they are using. One of the reasons that Km is im-portant is that it provides an idea of the affinity, the binding strength, ofthe enzyme for the substrate. With Vmax and the actual molar concentrationof the enzyme, the reaction rate, kcat, can be calculated. This is also calledthe turnover number, the number of substrate molecules transformed toproduct per unit time by a single enzyme molecule under maximal con-ditions. This provides a good measure of the speed and efficiency of anenzyme.

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5.5 ConclusionsPorous silicas are suitable supports for the immobilization of enzymes. Theimmobilization strategy must be chosen based on the enzyme and the tar-geted application. Physical adsorption, encapsulation, covalent binding andrecently cross-linking are the predominantly employed routes that possesscertain advantages and drawbacks. The use of tailor-made silica supportswith optimized particle size and morphology, pore diameter and surfaceproperties as detailed in the flowchart in Figure 5.2 will result in biocatalystswith increased activity, higher stability and reusability. Novel surface-func-tionalization strategies for modification of the support properties are con-stantly being developed. However, until now, industrial applications exploringthe specific features of porous silica supports have not been disclosed.

Combining the knowledge generated from all presented methodologieswill aid in rationally designing biocatalyst based on enzymes immobilized inmesoporous materials.33

References1. A. M. Klibanov, Nature, 2001, 409, 241–246.2. A. Schmid, J. S. Dordick, B. Hauer, A. Kiener, M. Wubbolts and

B. Witholt, Nature, 2001, 409, 258–268.3. R. A. Sheldon, Adv. Synth. Catal., 2007, 349, 1289–1307.4. P. N. Barlett and J. M. Cooper, J. Electroanal. Chem., 1993, 362, 1–12.5. B. Krajewska, Enzyme Microb. Technol., 2004, 35, 126–139.6. C.-H. Lee, T.-S. Lin and C.-Y. Mou, Nano Today, 2009, 4, 165–179.7. Y. Wang and F. Caruso, Chem. Commun., 2004, 1528–1529.8. N. Duran, M. A. Rosa, A. D’Annibale and L. Gianfreda, Enzyme Microb.

Technol., 2002, 31, 907–931.9. M. Hartmann and X. Kostrov, Chem. Soc. Rev., 2013, 42, 6277–6289.

10. A. S. Bommarius and A. Karau, Biotechnol. Prog., 2005, 21, 1663–1672.11. M. Caussette, A. Gaunand, H. Planche and B. Lindet, Enzyme

inactivation by inert gas bubbling, in Progress in Biotechnology,ed. A. Ballesteros, F. J. Plou, J. L. Iborra and P. J. Halling, Elsevier, 1998,vol. 15, pp. 393–398.

12. S. Colombie, A. Gaunand and B. Lindet, J. Mol. Catal. B: Enzym., 2001,11, 559–565.

13. C. Mateo, J. M. Palomo, G. Fernandez-Lorente, J. M. Guisan andR. Fernandez-Lafuente, Enzyme Microb. Technol., 2007, 40, 1451–1463.

14. J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge,K. D. Schmitt, C. T. W. Chu, D. H. Olson and E. W. Sheppard, J. Am.Chem. Soc., 1992, 114, 10834–10843.

15. C. Ispas, I. Sokolov and S. Andreescu, Anal. Bioanal. Chem., 2009, 393,543–554.

16. D. Margolese, J. A. Melero, S. C. Christiansen, B. F. Chmelka andG. D. Stucky, Chem. Mater., 2000, 12, 2448–2459.

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17. A. Sayari, Y. Yang, M. Kruk and M. Jaroniec, J. Phys. Chem. B, 1999, 103,3651–3658.

18. C.-Y. Lai, B. G. Trewyn, D. M. Jeftinija, K. Jeftinija, S. Xu, S. Jeftinija andV. S. Y. Lin, J. Am. Chem. Soc., 2003, 125, 4451–4459.

19. D. Jung, C. Streb and M. Hartmann, Int. J. Mol. Sci., 2010, 11, 762–778.20. M. Miyahara, A. Vinu and K. Ariga, Mater. Sci. Eng., C, 2007, 27, 232–236.21. H. H. P. Yiu, P. A. Wright and N. P. Botting, Microporous Mesoporous

Mater., 2001, 44–45, 763–768.22. H. Takahashi, B. Li, T. Sasaki, C. Miyazaki, T. Kajino and S. Inagaki,

Chem. Mater., 2000, 12, 3301–3305.23. C. Bernal, P. Urrutia, A. Illanes and L. Wilson, New Biotechnol., 2013, 30,

500–506.24. J. Fan, J. Lei, L. Wang, C. Yu, B. Tu and D. Zhao, Chem. Commun., 2003,

2140–2141.25. C. Bernal, L. Sierra and M. Mesa, ChemCatChem, 2011, 3, 1948–1954.26. M. Fernandez-Fernandez, M. A. Sanroman and D. Moldes, Biotechnol.

Adv., 2013, 31(8), 1808–1825.27. C. Mateo, J. M. Palomo, L. M. van Langen, F. van Rantwijk and

R. A. Sheldon, Biotechnol. Bioeng., 2004, 86, 273–276.28. R. A. Messing, Immobilized Enzymes for Industrial Reactors, Academic

Press, 1975.29. J. Jacoby, A. Pasc, C. Carteret, F. Dupire, M. J. Stebe, V. Coupard and

J. L. Blin, Process Biochem., 2013, 48, 831–837.30. O. H. Lowry, N. J. Rosebrough, A. L. Farr and R. J. Randall, J. Biol. Chem.,

1951, 193, 265.31. P. K. Smith, R. I. Krohn, G. T. Hermanson, A. K. Mallia, F. H. Gartner,

M. D. Provenzano, E. K. Fujimoto, N. M. Goeke, B. J. Olson andD. C. Klenk, Anal. Biochem., 1985, 150, 76–85.

32. M. M. Bradford, Anal. Biochem., 1976, 72, 248–254.33. N. Carlsson, H. Gustafsson, C. Thoern, L. Olsson, K. Holmberg and

B. Aakerman, Adv. Colloid Interface Sci., 2014, 205, 339–360.34. A. Mayoral, R. M. Blanco and I. Diaz, J. Mol. Catal. B: Enzym., 2013, 90,

23–25.

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CHAPTER 6

Heterogeneous Catalysts forBiodiesel Production

DANIELA R. RADU*a AND GEORGE A. KRAUSb

a Department of Chemistry, Delaware State University, Dover,DE 19901, USA; b Department of Chemistry, Iowa State University, Ames,Iowa 50011, USA*Email: dradur@desu.edu

6.1 IntroductionBiodiesel is a renewable fuel that can be generated from plant oils,rendered animal fats and industrial waste oils.1 It has evolved into asignificant industry in the midwestern United States and in Europe. In theUnited States biodiesel is prepared primarily from soybean oil; in Europe,biodiesel is generated mostly from rapeseed oil. Biodiesel has a number ofadvantages over diesel fuel. Although the National Biodiesel Boardhas listed a number of benefits, the most significant ones are that biodieselis a renewable fuel that is non-toxic, biodegradable and free of sulfur-containing impurities.2 Biodiesel also has significant lubricity comparedto petrochemical fuels. These qualities make biodiesel especially usefulin places such as national parks, harbors and other environmentallysensitive areas. Biodiesel is commonly sold as a mixture with petroleumfuels: for example, B20 is a mixture of 20% biodiesel and 80%petroleum fuel.

Biodiesel, also known as fatty acid methyl esters (FAME), is one ofthe most promising alternative biofuels and is currently produced by a

RSC Green Chemistry No. 33Heterogeneous Catalysis for Today’s Challenges: Synthesis, Characterization and ApplicationsEdited by Brian Trewynr The Royal Society of Chemistry 2015Published by the Royal Society of Chemistry, www.rsc.org

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base-catalyzed transesterification reaction with triglycerides and methanolas illustrated in Scheme 6.1. Biodiesel is a mixture of compounds becauseit is produced by transesterification of triglycerides found in soybean oil.Triglycerides contain mixtures of esters of different chain lengthsand some chains have alkenes or epoxides. The most common alcoholused in the transesterification of soybean oil is methanol, in large partbecause it is less expensive than other alcohols. For many decades thetransesterification reaction was conducted by heating soybean oil, an ex-cess of methanol and a few percent of a homogeneous catalyst such assodium hydroxide or sodium methoxide for several hours and then neu-tralizing the catalyst and separating the biodiesel from the glycerol.Although catalysts such as sodium methoxide afford excellent yields of thetransesterification product, a methyl ester, the catalyst must be neutralizedwith an acid when the transesterification reaction is complete. Thisneutralization produces a salt which often winds up as an impurity in theglycerol, reducing its value. Additionally, the production of biodieselusing homogeneous catalysts has a large water footprint. For every liter ofbiodiesel produced, almost four liters of water are utilized. Moreover, thefree fatty acids present in feedstocks such as rendered animal fats andindustrial waste oils would neutralize the basic catalyst. Therefore, a pre-treatment step which converts the free fatty acids into esters using an acidcatalyst is required for feedstocks that contain significant amounts of freefatty acids.

When this work began in 1999, the use of heterogeneous catalysts for thegeneration of biodiesel was not yet employed on an industrial scale. Certainacidic materials such as zeolites and ion exchange resins had been reportedto catalyze the transformation.3 They have the advantage that any free fattyacids present in the feedstock would be esterified. However, the rate of thetransesterification using acidic catalysts is much slower than the reactionemploying basic catalysts. Certain metal oxides have been used for thetransesterification reaction.4 However, many metal oxides are somewhatsoluble in methanol. Also, free fatty acid impurities would react with themetal oxides. The ideal catalyst for the transesterification reaction would beeconomical, recyclable, stable to fatty acid impurities, and would eliminatethe costly aqueous work-up step.

R = C14 ~ C24 Hydrocarbon Chain Biodiesel

OO

O

O

C

CNaOH

3 CH3OH

C

R OH

OH

OH

RO

RO

+

OO CH3R C

Scheme 6.1 Base-catalyzed transesterification of triglycerides to biodiesel.

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6.2 Previous Work using Mesoporous MaterialsIn recent years there have been a number of reports of demonstrating theability of heterogeneous catalysts to promote the preparation of biodiesel.The reports have been collated in timely reviews by Basumatary5 and by Maand Hanna.6 Melero and coworkers recently utilized mesoporous arene-sulfonic acids to convert crude palm oil containing approximately 6% freefatty acids into biodiesel.7 Chang and coworkers studied mesoporouscarbon–silica composites and found these sulfonic acid catalysts to beeffective for the production of biodiesel.8 Zuo and coworkers showed thatmesoporous silica functionalized with alkyl sulfonic acids gave good yieldsof biodiesel with soybean oil contaminated with 20% oleic acid.9 Mar andSomsook synthesized a propyl sulfonic acid-functionalized mesoporouscatalyst that could esterify oleic acid and was superior to Amberlyst 15.10

Mesoporous materials have received much attention in the past decadebecause of their ease of formation and their ease of functionalization. Thishas permitted an extensive study of structure–activity relationships amongcatalysts. They have larger pore sizes compared to zeolites, allowing largerorganic molecules to enter.

6.2.1 Structure–Activity Studies of Mesoporous Sulfonic Acids

Shanks and Mbaraka have studied mesoporous silica with different sulfonicacids at different surface concentrations.11 Chen and coworkers have studieddual-functionalized mesoporous silica containing both sulfonic acid groupsand disulfide groups.12 Dhainaut and coworker prepared macroporous–mesoporous SBA silicas via dual templating. They cite rate enhancements forthe transesterification of bulky esters.13 Chen and coworkers utilizedmesoporous sulfonic acids with short channels and high acid capacities.They found that these catalysts were superior to Amberlyst 15 resin.14

Yadav and Sharma used a mesoporous sulfonic acid catalyst treated withlanthanum chloride to convert fructose into 5-hydroxymethylfurfural in goodyield.15 Karimi and coworkers utilized a phenylene-bridged mesoporoussilica catalyst to produce biodiesel.16 They attributed the enhanced yield tothe hydrophobic character of the bridged catalyst. Tang and coworkersproduced mesoporous silicas having both platinum and sulfonic acidgroups. They utilized this catalyst for a novel one-step hydrogenation-esterification of acetic acid and acetaldehyde.17 Karimi and Mirzaei evaluateda number of mesoporous sulfonic acid catalysts and correlated the improvedyields of hydroxymethylfurfural with lower surface hydrophobicity.18

6.2.2 Catalysis of Organic Reactions

Mesoporous silicas functionalized with sulfonic acids have been effective ina number of widely used organic reactions. Bossaert and coworkers haveutilized mesoporous sulfonic acids to catalyze the synthesis of

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monoglycerides.19 Clark and coworkers have used mesoporous sulfonicacids as substitutes for environmentally hazardous Lewis and Bronstedacids.20 Macquarrie and coworkers have used perfluorinated sulfonic acidsimmobilized onto mesoporous silica to catalyze Friedel–Crafts acylation re-actions.21 Shanks and Bootsma have evaluated the hydrolysis of cellobioseusing mesoporous silica catalysts.22 Chen and coworkers studied the syn-thesis of bisphenol A using dual-functionalized catalysts.23 Davis and cow-orkers also synthesized bisphenol A employing a novel thiol/sulfonic acidpaired catalyst.24 Castanheiro and coworkers have evaluated mesoporoussilica sulfonic acids for the successful methoxylation of a-pinene.25

Shi and coworkers studied the transformation of xylose into furfural usingmesoporous SBA-15 catalysts.26 Lopez-Sanz and coworkers used mesoporoussulfonic acids to prepare a library of quinolines.27 Thiel and coworkers usedacid–base bifunctional mesoporous silica nanoparticles to achieve a cleverone-pot deacetalization–aldol reaction.28 Peng and coworkers created acid–base bifunctional mesoporous catalysts by in situ cleavage of a sulfonamidebond. They employed this novel catalyst in several solvent-free Knoevenagelcondensation reactions.29 Agirrezabal-Telleria and coworkers studied thereaction of mesoporous sulfonic acids with xylose as a function of sulfonicacid load and temperature. In their optimized reaction conditions, theyobtained a 82% yield of furfural at 170 1C.30 Hakki and coworkers found thatco-condensation of orthosilicates in the presence of titania afforded asuperior catalyst for the photocatalytic conversion of aromatic nitrocompounds into a variety of quinolines.31 Jun and coworkers utilized amagnetically recyclable mesoporous catalyst for tandem acid–base re-actions.32 Zhang described a mesoporous acid catalyst for the Mukaiyamaaldol reaction in aqueous media.33

6.2.3 Lin Group Contributions

The Lin group had developed mesoporous silica nanomaterials with well-defined geometries. Mesoporous silica nanospheres (MSNs), as they werenamed by Lin’s group in 2001, benefit from a facile synthetic methodologyas well as potential for tailoring their structural properties. The MSN syn-thesis requires the condensation of a silica source around a template. Themost studied templating reagents are cationic surfactants, which rendermaterials with porosities in the range of 2–5 nm and block co-polymers,generating 5–30 nm porosities. The interior of these materials resembles ahoneycomb, with parallel channels running throughout the nanosphericalparticle. The channel diameter could be fine-tuned to accommodate a largevariety of molecules, from simple organic compounds to macromolecules.These interior structures led to the idea of converting the pores in nano-reactors, which engaged the group in a large effort toward altering porefunctionality with catalytic groups. The functional groups in the channelscould be added through either post-synthesis grafting or by in situ additionof an organo-functionalized silane, a method also called co-condensation.

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The difference between the two methods is highlighted in Figures 6.1and 6.2. Figure 6.1 illustrates the post-synthesis grafting method thatoften leads to functional groups on the exterior surface and near the poreopenings of the mesoporous silica particle. In contrast, co-condensationdrives the majority of functional groups inside the pores with some of themwith the undesirable possible location inside the pore walls. When applyingco-condensation, the Lin group obtained materials functionalized with avariety of functional groups, including thiols, sulfonic acids, amines andureas.34

In addition, they discovered that by performing the post-synthesis graftingon materials prior to removing the surfactant templates, certain functional

Figure 6.1 Schematic representation of post-synthesis grafting of organo-silanesonto mesoporous silica surface in respect to the silanol groups existenton the pores surface.

Figure 6.2 Schematic representation of silica functionalization via co-condensationof an organosilane in the presence of a silica source (showed silicic acid).

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groups could be selectively grafted to the outside of the nanoparticles. Anotherinteresting discovery was that multiple functional groups could be simul-taneously added, thus moving toward forming multifunctional materials.

Novel catalysts could be created by combining grafting and co-conden-sation to generate selective functionalization of MSNs, leading to a synergyof catalytic properties.

These durable materials were shown to catalyze several common organicreactions. In addition to organic functional groups, the Lin group also reportedthe incorporation of inorganic materials into the channels. The general struc-ture of the mesoporous nanomaterial catalyst is shown in Figure 6.3 below.

The Lin group used these catalysts for multistep organic reactions.35 Oneof the most significant applications of the catalysts fabricated in Lin’s groupwas biodiesel preparation, for which the group focused on mesoporoussulfonic acids.36

Biodiesel fabrication presents a series of challenges, as shown inScheme 6.2. To bypass the potential saponification reaction, a solid acidcatalyst is needed to transform free fatty acids, typically present in oilfeedstocks in various percentages.

6.2.3.1 Acid Catalyst

To address the first stage of biodiesel fabrication, the Lin group createdperiodically ordered, sulfonic acid-functionalized mesoporous silicas withpores sizes ranging from 20 to 60 Å and high acid-exchange capacities(1–2 mequiv. of H1 (g of SiO2)�1). To do this the Lin group utilized a recentlydeveloped synthetic method which allows the facile incorporation of variousacidic sulfonic groups to the mesoporous structures with the ability to fine-tune the loading of these groups.

As depicted in Scheme 6.3 the sulfonic acids are contained in the chan-nels. The soybean oil and the solvent methanol enter the channel andtransesterification occurs in the channel. Free fatty acids also enter thechannel and are esterified with methanol.

Figure 6.3 TEM image of a mesoporous silica nanosphere showing the hexagonal,parallel array of pores.

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In addition to the intrinsic catalytic ability and the quantity of acidgroups in these catalytic systems, two important factors that can alsoinfluence the overall performance of the proposed heterogeneous solid

Scheme 6.2 Challenges in biodiesel fabrication: presence of free fatty acids.

Scheme 6.3 Synthesis of benzene-sulfonic mesoporous silica catalyst.

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acid catalysts are the sizes of the mesopores and the pore surfaceproperties. These characteristics can have large effects on pore mass transferof reactant, product, and intermediate species. In particular, the functio-nalization of these derivatized mesoporous silica materials with varioussurface-bound groups allows the modulation of the catalytic systems to en-hance substrate selectivity by tuning substrate accessibility and porehydrophobicity.

Thus, to address the above toward increasing selectivity of the transes-terification reactions, various functional groups have been employed inaddition to the sulfonic acid groups. The materials design involved makingthe pore surface hydrophobic. The reasoning behind this step was matchingthe channel hydrophobicity with the long alkyl chains in the triglycerides,and thus, promoting their fast diffusion into the channels; increasing re-action kinetics.

In preliminary investigations, the Lin group tested the catalytic activity ofthe aforementioned mesoporous solid acid catalyst in comparison withtwo commercially available homogeneous catalysts (sulfuric acid, H2SO4,and p-toluene sulfonic acid, p-TSA) and an SBA-type mesoporous silicacatalyst with a propylsulfonic acid functionality (SBA-15-SO3H-P123)also developed in the Lin group. Compared with its homogeneous coun-terpart, p-TSA, it showed similar reaction kinetics, indicating a fast mass-transfer process for the reactants and the products to diffuse in and out ofthe large pores (Figure 6.4).

6.2.3.2 Acid–Base Catalyst

In parallel with fatty acids transformation, transesterification reactionsconvert triglycerides in oil feedstocks in biodiesel. To avoid introducing asecond base catalyst, acid and base groups were introduced simultaneouslyand site-separated on the different surfaces of mesoporous silica nano-particle through co-condensation to functionalize the internal surface andpost-synthesis grafting to functionalize the external surface. As a result ofthis ideal site-isolation, reaction cascades requiring two or more catalysts,which are incompatible in one solution system, could be done by this newinternal and external surface-bifunctionalized particle.

Furthermore, following the same strategy used for sulfonic acid catalysts,the unoccupied pore surface of the superbase and the acid-derivatizedmesoporous silicas were functionalized with propyl, phenyl, or penta-fluorophenyl groups via post-synthesis grafting procedures using propyl,phenyl, and pentafluorophenyl trialkoxysilanes, respectively, to yield a seriesof multi-functionalized mesoporous silica supported ‘‘microreactors’’ for thetransesterification and esterification of various FFA-containing oil feed-stocks. These mixed-functional materials allowed control of the surface re-activity of these materials. There were three organosulfonic acid functionalgroups used in the study. The catalytic activities of the functionalizedmesoporous silicas were compared with several commercial catalysts,

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including homogeneous catalysts (sulfuric acid and p-TSA) and hetero-geneous catalysts such as Nafions.

Following the success of acid catalyst for esterification, the Lin group alsoconducted a preliminary transesterification of purified soybean oil to methylester using a functionalized mesoporous solid catalyst. The resultingnanomaterials were inexpensive to produce, were reproducible, and could bereactivated simply by heating. A 100% conversion of the soybean oil tomethyl ester was accomplished in 20 min at 25 1C with a 5-fold excess ofmethanol. The above studies suggested that a mesoporous sulfonic acid withsuperbase and hydrophobic-group functionality could be employed for thetransformation of crude oil feedstocks in biodiesel from soybean oil effi-ciently. Based on these preliminary results, there is an opportunity to designa series of new solid catalysts with (i) higher amounts of catalytic groups,and (ii) more reactive catalytic functionalities for both the proposed esteri-fication and transesterification of various feedstocks.

Figure 6.4 Schematic representation of the proposed catalytic system for the syn-thesis of methyl soyate and glycerol formation.

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6.3 Industrial PartnershipWe sought an industrial partner to explore the large-scale applications ofthe Lin catalysts. West Central Cooperative is a full-service, farmer-ownedcooperative located in west-central Iowa, less than a one-hour drive fromIowa State University. The corporate headquarters at Ralston, Iowa acts asthe hub for the company’s trade territory that spans ten counties and ex-tends 55 miles in each direction. West Central’s operating divisions in-clude grain, agronomy, feed, soy processing, and administration. WestCentral Soy is the manufacturing division of West Central Cooperative.West Central Soy products are processed in a $30 million manufacturingcomplex in Ralston, Iowa. More than six million bushels of soybeans areprocessed annually at this facility. The product line of West Central Soyincludes: biodiesel, graffiti remover, penetrant and lubricant, methylesters, diesel fuel additive, asphalt release concentrate, fifth wheel grease,and soy-based lubricants.

West Central’s $6 million biodiesel plant is adjacent to its Soy Center. Thebiodiesel plant processes 8 million pounds of the co-op’s soybean oil intomethyl esters each year. West Central’s current methyl ester process involvesheating the soybean oil and introducing alcohol and a catalyst. The resultingreaction separates glycerol from the soybean oil. The catalyst is then removedfrom the methyl ester through water washing and a neutralization process.The end products are biodiesel, glycerol, and fatty acid. The excess water andcatalyst is left with the glycerol byproduct, which is sold at an 80% purity levelto companies that further process it into hundreds of industrial products.

In 2000, methyl-ester-production practices at the West Central Co-operative biodiesel plant include the use of a non-recyclable catalyst(sodium methoxide, 1%). During processing, the catalyst concentrates inthe glycerol phase and must be neutralized with aqueous hydrochloricacid. This adds downstream shipping weight in the form of water,methanol, and sodium chloride (transportation energy costs) and energyrequired to distill the glycerol. They also need to dispose of the non-recyclable catalyst in landfills (at the rate of 10 pounds per 1000 pounds ofmethyl ester produced).

They perceive several environmental benefits of a heterogeneous catalyst:

� less energy expended to produce the (currently) non-renewable catalyst;� less transportation (and its associated emissions) to move the

(currently) non-renewable catalyst from its point of production to theend user;

� less potential of contamination from a spill of (currently) non-renew-able catalyst during transport, handling, and storage;

� minimal water usage due to the elimination of the wash step; and� lower disposal costs associated with the removed catalyst residue since

it will be recyclable and regeneratable.

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Utilizing a heterogeneous solid acid catalyst for the synthesis of biodieselcould also circumvent the catalyst separation problem and at the same timeeliminate the free fatty acids (FFA) in the crude FFA-containing feedstocks.To prevent ionic base-induced saponification during the transesterificationreaction, the West Central process uses sodium methoxide. Therefore, ahighly efficient solid acid catalyst can serve not only as a ‘‘pretreatmentcatalyst’’ to remove FFA’s from the triglycerides, but also as a catalyst for theconversion of the oil to biodiesel.

The synthesis of over 700 g of the acid-functionalized mesoporous solidcatalyst was completed using a pilot-scale test stand configured at the WestCentral facilities. Analysis by the Lin lab confirmed success in both reactivityand structural integrity of the synthesized materials. These catalysts alsoperform as expected with respect to recyclability during multiple bench-scaleconversion reactions. Figure 6.5 shows a representation of the catalyst pro-duced to date.

The economic modeling used to gauge the viability of these catalyst ma-terials was modified with data from the pilot-scale synthesis activities.Knowing the usage rates and costs of the raw materials used in the synthesis,the economic model could be refined with more realistic inputs. The resultsof this ongoing feasibility test still show promise in use of these catalysts asan economically viable alternative to today’s homogeneous catalysttechnology.

Upon completion of the acid-catalyst synthesis reactions, the pilot-scaletest stand was re-configured to utilize the catalyst in larger scale conversionreactions (1 gallon h�1). Much of the same resources from previous testswere able to be reused in the new configuration, with the exception of the1-gallon reaction vessel that was incorporated into the flow scheme(Scheme 6.4).

Figure 6.5 Acid-functionalized mesoporous solid catalyst, synthesized at West Cen-tral.

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6.4 ConclusionsMultifunctional mesoporous silica nanosphere catalysts have been syn-thesized by Lin’s group through simultaneous derivatization with acid-containing aryl radicals and superbase on the surface of mesoporous silica.The derivatization was designed to reach a complete separation of the acidand base groups, which enables the catalyst to perform this otherwise an-tagonistic acid–base function. The benefit of such catalyst is the capability ofconverting both free fatty acids and triglycerides in a one-pot reaction withhigh yield and excellent separation which allows biodiesel fabrication in onestep. This implies a tremendous processing-costs reduction due to elimin-ating expensive intermediate steps. Compared with other solid acid andsolid base catalysts, the MSN catalyst materials show stable and highly ef-ficient catalytic performance in biodiesel production with conversionsreaching 100%. The catalysts are highly efficient, environmentally friendly,inexpensive, and easy to prepare.

References1. R. C. Brown and T. R. Brown, Why are we producing biofuels? 2012.2. National Biodiesel Board: http://www.biodiesel.org/.3. A. Brito, M. E. Borges and N. Otero, Energy & Fuels, 2007, 21, 3280–3283.4. A. A. Refaat, Int. J. Environ. Sci. Technol., 2011, 8(1), 203–221.5. S. Basumatary, J. Chem. Pharm. Res., 2013, 5, 1–7.6. F. Ma and M. A. Hanna, Bioresour. Technol., 1999, 70(1), 1–15.7. J. A. Melero, L. F. Bautista, J. Iglesias, G. Morales, R. Sanchez-Vazquez

and I. Suarez-Marcos, Top. Catal., 2010, 53(11–12), 795–804.

Scheme 6.4

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8. B. Chang, Y. Tian, W. Shi, J. Liu, F. Xi and X. Dong, J. Porous Mater., 2013,20(6), 1423–1431.

9. D. Zuo, J. Lane, D. Culy, M. Schultz, A. Pullar and M. Waxman, Appl.Catal., B, 2013, 129, 342–350.

10. W. W. Mar and E. Somsook, J. Oleo Sci., 2013, 62(6), 435–442, DOI:10.5650/jos.62.435.

11. I. K. Mbaraka and B. H. Shanks, J. Catal., 2006, 244(1), 78–85.12. S.-Y. Chen, T. Yokoi, C.-Y. Tang, L.-Y. Jang, T. Tatsumi, J. C. C. Chan and

S. Cheng, Green Chem., 2011, 13(10), 2920–2930.13. J. Dhainaut, J.-P. Dacquin, A. F. Lee and K. Wilson, Green Chem., 2010,

12(2), 296–303.14. S.-Y. Chen, T. Yokoi, C.-Y. Tang, L.-Y. Jang, T. Tatsumi, J. C. C. Chan and

S. Cheng, Green Chem., 2011, 13(10), 2920–2930.15. G. D. Yadav and R. V. Sharma, Process for converting fructose into

5-(hydroxymethyl)furfural using a mesoporous silica based catalyst im-pregnated with rare earth metals PCT Int. Appl., 2012, WO 2012038969 A1.

16. B. Karimi, H. M. Mirzaei and A. Mobaraki, Catal. Sci. Technol., 2012, 2(4),828–834.

17. Y. Tang, S. Miao, L. Mo, X. Zheng and B. H. Shanks, Top. Catal., 2013,56(18–20), 1804–1813.

18. B. Karimi and H. M. Mirzaei, RSC Adv., 2013, 3(43), 20655–20661.19. W. D. Bossaert, D. E. De Vos, W. M. Van Rhijn, J. Bullen, P. J. Grobet and

P. A. Jacobs, J. Catal., 1999, 182(1), 156–164.20. J. H. Clark, D. J. Macquarrie and K. Wilson, Functionalised mesoporous

materials for green, in Nanoporous Materials II, Proceedings of the Con-ference on Access in Nanoporous Materials chemistry, Studies in SurfaceScience and Catalysis, 2000, vol. 129, pp. 251–264.

21. D. J. Macquarrie, S. J. Tavener and M. A. Harmer, Chem. Commun., 2005,18, 2363–2365.

22. J. A. Bootsma and B. H. Shanks, Appl. Catal., A, 2007, 327(1), 44–51.23. C.-C. Chen, S. Cheng and L.-Y. Jang, Microporous Mesoporous Mater.,

2008, 109(1–3), 258–270.24. E. L. Margelefsky, A. Bendjeriou, R. K. Zeidan, V. Dufaud and

M. E. Davis, J. Am. Chem. Soc., 2008, 130(40), 13442–13449.25. J. E. Castanheiro, L. Guerreiro, I. M. Fonseca, A. M. Ramos and J. Vital,

Mesoporous silica containing sulfonic acid groups as catalysts for thealpha-pinene methoxylation, in Zeolites and Related Materials, Studies inSurface Science and Catalysis, 2008, vol. 174B, pp. 1319–1322.

26. X. Shi, Y. Wu, H. Yi, G. Rui, P. Li, M. Yang and G. Wang, Energies, 2011, 4,669–684.

27. J. Lopez-Sanz, M. E. Perez Mayoral, R. M. Martin Aranda and A. J. LopezPeinado, Process for preparation of quinolines using mesoporous hybridsolids as catalysts Span, 2013, ES 2395109 A1.

28. S. Shylesh, A. Wagner, A. Seifert, S. Ernst and W. R. Thiel, Chem. – Eur. J.,2009, 15(29), 7052–7062.

29. Y. Peng, J. Wang, J. Long and G. Liu, Catal. Commun., 2011, 15(1), 10–14.

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30. I. Agirrezabal-Telleria, J. Requies, M. B. Gueemez and P. L. Arias, Appl.Catal., B, 2012, 115–116, 169–178.

31. A. Hakki, R. Dillert and D. W. Bahnemann, ACS Catal., 2013, 3(4), 565–572.

32. S. W. Jun, M. Shokouhimehr, D. J. Lee, Y. Jang, J. Park and T. Hyeon,Chem. Commun., 2013, 49(71), 7821–7823.

33. F. Zhang, C. Liang, M. Chen, H. Guo, H. Jiang and H. Li, Green Chem.,2013, 15(10), 2865–2871.

34. S. Huh, J. W. Wiench, J.-C. Yoo, M. Pruski and V. S.-Y. Lin, Chem. Mater.,2013, 15(22), 4247–4256.

35. Y. Huang, B. G. Trewyn, H.-T. Chen and V. S.-Y. Lin, New J. Chem., 2008,32(8), 1311–1313.

36. V. S.-Y. Lin, D. R. Radu and H.-T. Chen, Prepr. Symp. – Am. Chem. Soc.,Div. Fuel Chem., 2005, 50(1), 306–307.

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Subject Index

A (LTA – zeolite A), 80, 81, 82acid–base bi-functionalized catalyst

biodiesel production, 124–5large-pore mesoporous silica

nanoparticles, 19–23acid catalyst in biodiesel production,

122–4, 127activity of enzymes (catalytic activity)

immobilized on porous silicasupports, characterization, 112–15

adsorptionenzyme immobilization by,

103, 104, 107, 108, 109–10by zeolites, 88–9

AEL (zeolite SAPO-11), 82, 90Alcaligenes lipase immobilization,

111aldol reaction, 37, 120alkylation, aromatics, 90, 93, 94allyl acetate epoxidation, 50aluminosilicate minerals, zeolites

as, 77, 78, 81amino groups of amino acids and

covalent enzyme immobilization,110

aminopropyl (AP) groups, 37, 383-aminopropyltrimethoxysilane

(APTMS), 8, 10, 13, 19, 20, 34, 37APTMS (3-aminopropyltrimethoxy-

silane), 8, 10, 13, 19, 20, 34, 37arenesulfonic acids in biodiesel

production, 119arginine e-amino group and

covalent enzyme immobilization,110

aromatics alkylation, 90, 93, 94aspartic acid carboxyl groups and

covalent enzyme immobilization,110

Aspergillus oryzae b-galactosidaseimmobilization, 111

Bacillus circulans b-galactosidaseimmobilization, 111

base group-functionalized large-poremesoporous silica nanoparticles,19–23, see also acid–basebi-functionalized catalyst

BEA* (beta zeolite), 78, 80, 82, 89, 90,92, 94

benzene, alkylation, 90, 93, 94benzene-sulfonic mesoporous silica,

123beta zeolite (BEA*), 78, 80, 82, 89, 90,

92, 94bi-functionalized mesoporous silica,

35, 36, 37, 120nanomaterials, 13–24, 35,

36, 37bimodal mesoporous structure

(BMS), 106biocatalysis see enzymesbiofuels, 1

biodiesel, 1, 117–30supported metal catalysts,

62–3bioindustry

fuels see biofuelssupported metal catalysis,

62–6

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biomass conversion (cellulosic/lignocellulosic), 1–27, 62–8

background of, 1–2to chemicals, 62–3enzyme-assisted, 3–4, 6–13, 64to fuels see biofuelsin ionic liquid systems, 2–3multi-functionalized

mesoporous nanoparticlesfor, 1–27

supported metal catalysts,62–8

bisphenols A and Z, 40–2, 44, 120bis-silane, 42–3bis(trimethylsilyl)acetamide (BSA), 71block copolymers, mesoporous silica

synthesis, 30, 31, 32BSA (N,O-bis(trimethylsilyl)acetamide),

71

CaA (5A zeolite), 88Candida antarctica B lipase

immobilization, 111carbamate and xanthate

co-condensation, 39, 40, 41carbene insertion, 56carbon, mesoporous (CMK-3), 66–7carbon monoxide

hydrogen and (syngas), 68preferential oxidation under

excess hydrogen, 68–70carboxyl groups of amino acids and

covalent enzyme immobilization,110

cations, inorganic, zeolite synthesis,85–6

cellobiose conversion to HMF, 19,22, 23

cellulase, 3–4, 6–13, 24, 64immobilization, 10–11

cellulosic biomass see biomasscetyltrimethylammonium bromide

(CTAB), 5, 31, 34CHA (SAPO-34), 82, 90, 94charge, surface, mesoporous silica

nanoparticles, 11

chemical(s), supported metalcatalysts in biomass conversionto, 62–3

chemical (covalent) binding, enzymeimmobilization by, 103, 107, 108,110–12

clustered surface distribution offunctionalized immobilizedspecies, 54–5

CMKJ-3 (mesoporous carbon), 66–7co-catalysis see cooperative

catalysisco-condensation (in situ addition of

organosilane), 32–3, 36, 52, 71,120–1

colorimetric assays of enzymeconcentration in porous silica, 113

connectivity, zeolites, 79, 85, 86cooperative catalysis (co-catalysis),

40–1cellulose-to-HMF conversion,

19–23copolymers, mesoporous silica

synthesis, 30, 31, 32copper (Cu), 49, 50–4cost (economic) modeling in

biodiesel production, 127covalent binding, enzyme

immobilization by, 103, 107, 108,110–12

critical micelle concentration, 30cross-linked enzyme aggregates

(CLEAs), 108, 109CTAB (cetyltrimethylammonium

bromide), 5, 31, 34cumene production, 93, 942-cyclohexen-1-one epoxidation, 50cysteine thiol group and covalent

enzyme immobilization, 110

deacetalization, one-pot, 35, 120density functional theory (DFT),

71, 72dewaxing, catalytic, 92–3dimensionality of zeolites, 78, 79–80,

81, 82, 84, 86–8

132 Subject Index

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dimethylsulfoxide (DMSO), fructose-to-HMF conversion, 5

DMSO (dimethylsulfoxide), fructose-to-HMF conversion, 5

economic modeling in biodieselproduction, 127

electron microscopy of mesoporousmaterials

enzymes, 113–14nanomaterials, 8, 15

Ellman’s reagent, 39EMIMCl (1-ethyl-3-

methylimidazolium chloride),2–3, 16, 19, 21

encapsulation, enzyme, 2, 108, 109cellulase, 3in hexagonally ordered porous

silica materials, 103in hierarchically ordered

porous silica materials,106–7

environmental benefits ofheterogeneous catalysts, 126

enzymes (in biocatalysis), 100–16cellulose conversion, 3–4,

6–13, 64immobilization on porous

silica supports seeimmobilization

epoxidation, 48, 50, 52, 56erionite (ERI), 90ethylbenzene production, 93, 94ethylene glycol, cellulose conversion

to, 65–61-ethyl-3-methylimidazolium

(EMIM) chloride, 2–3, 16, 19, 21excimer fluorescence, 53–4, 55

fatty acid(s), free (FFAs), biodieselproduction and, 118, 119, 122,123, 124, 127, 128

fatty acid methyl esters (biodiesel),1, 117–30

FAU (zeolite Y), 78, 80, 81, 82, 89,90, 91

FDU-12 (mesoporous silica), 4, 6, 32Fe(II), 48ferrierite (FER), 80, 82, 90Fischer–Tropsch synthesis, 945A zeolites (CaA), 88fluid catalytic cracking with zeolites,

89–90fluorescence, excimer, 53–4, 55fluorescence spectroscopy of

enzymes immobilized in poroussilica, 114

4-connected networks, zeolites, 77,78, 79

free fatty acids (FFAs) and biodieselproduction, 118, 119, 122, 123,124, 127, 128

fructoseglucose conversion to, 4, 22to HMF conversion, 4–5, 13–18,

19, 21–2, 22–3FSM silica, 69, 103

FSM-16, 69, 104, 105fuel production from biomass see

biofuelsfunctionalized mesoporous silica

nanoparticles see nanoparticlestechniques of

functionalization, 32–3, 70–2

b-galactosidase immobilization,107, 111

gases, permanent, zeolites inseparation of, 88–9

germanium (Ge)-based zeolites, 86glucose

cellulose conversion to, 6–13supported metal

catalysts, 63–4, 66–7to fructose conversion, 4, 22to HMF conversion, 2, 19,

22–3glutamic acid carboxyl groups and

covalent enzyme immobilization,110

glycerol formation, 125gottardite, 78

Subject Index 133

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graftingpost-synthesis, 32, 34, 36–40,

42, 45, 46–7biodiesel production,

120–2, 124random ligand, 49

hemicellulose, hydrolytichydrogenation, 65

Henry reaction, 35, 37, 38hexagonally ordered porous silica

materials, enzymeimmobilization/encapsulation in,103–6

hierarchically ordered porous silicamaterials, enzymeimmobilization/encapsulation in,106–7

highly ultrastable zeolite Y (VUSY), 92histidine imidazole group and covalent

enzyme immobilization, 110horseradish peroxidase, 105hydrocarbons

zeolite USY in hydrocrackingof, 91

zeolites in separation of, 88–9hydrogen

from biomass, 68preferential oxidation of

carbon monoxide underexcess of, 68–70

hydrogenation, hydrolyticcellulose, 64–5hemicellulose, 65

hydrophilic/hydrophobic volumeratios, mesoporous silicasynthesis, 30

hydroxides, zeolite synthesis, 85hydroxyl (–OH) groups

of mesoporous silicananoparticles, 9, 20, 21

of serine and threonine acidsand covalent enzymeimmobilization, 110

5-hydroxymethylfurfural (HMF), 2–3,4–6, 13–24

IBN-X, 104IMF (zeolite), 85imidazole group of histidine and

covalent enzyme immobilization,110

immobilization (on porous/mesoporous silica supports)

enzyme, 100–16cellulose conversion, 3–4,

6–13characterization of

catalytic activity,112–15

strategies of enzymeimmobilization,107–12

types of support used inenzymeimmobilization, 101–7

in single-site catalysis, 37, 49, 53surface distribution of

functionalizedimmobilized species,54–6

indole group of tryptophan andcovalent enzyme immobilization,110

industry, zeolites applications, 80,90–4, see also bioindustry

adsorption, 88infrared spectroscopy, enzyme

concentration in porous silica, 113inorganic species, zeolites, 85–6International Zeolite Association

(IZA) Structural Commission’s3-letter (framework type) code, 79,82, 89, 90

ion-exchange adsorption, zeolitesin, 88

ionic liquid systems, celluloseconversion in, 2–3

iron(II)/Fe(II), 48ITQ-2 (zeolite), 87IZA Structural Commission’s 3-letter

(framework type) code, 79, 82,89, 90

134 Subject Index

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kinetics, fructose-to-HMFconversion, 13–18, 16–18

L (LTL zeolite), 82, 90, 92, 93large-pore mesoporous silica

nanoparticles (LPMSNs), 8–13acid–base bi-functionalized,

19–23large-pore zeolites, 79–80light paraffin isomerization, 92, 93lignin valorization, 67–8lignocellulosic biomass see biomassLin group, 5, 120

biodiesel production, 29,120–2, 128

lipase immobilization, 111Lobry de Bruyn–Alberda van

Ekenstein transformation, 4LTA (zeolite A), 80, 81, 82LTL (zeolite L), 82, 90, 92, 93lysine e-amino group and covalent

enzyme immobilization, 110

manganese (Mn), 50, 51, 52, 56MCM family of mesoporous

materials, 29, 30, 31, 37, 69–70,71, 81, 102, 104, 105

MCM-22 (MMW), 80, 81, 82, 83,84, 86, 87, 89, 90, 94

MCM-41, 29, 31, 37, 69–70, 71,102, 104, 105

MCM-48, 29, 30, 104MCM-56, 87

medium-pore zeolites, 79–80, 84–5mercaptopropyl groups, 40, 443-(mercaptopropyl)trimethoxysilane

(MPTMS), 14, 15, 19, 20, 34, 45MeSi(OEt)3 (methyltriethoxysilane),

71mesoporous carbon (CMK-3), 66–7mesoporous materials in biodiesel

production, 119–25, 127, 128mesoporous silica, 1–60

functionalized seefunctionalized mesoporoussilica

immobilization on seeimmobilization

single-site catalysis, 28–60examples, 34–54

structural aspects, 29–33pore sizes see pore sizes

synthesis, 29–33metal catalysts, supported, 61–76, see

also organometallic complexes;rare earth metal zeolites

in bioindustry, 62–6metal templating, 48, 49, 51methanol conversion, zeolites, 94methyl ester formation, 111–12, 118,

125, 126methyltriethoxysilane (MeSi(OEt)3),

71MFI (ZSM-5), 78, 80, 81, 82, 83, 84,

85, 89, 90, 91, 92, 93–4micelles, mesoporous silica

synthesis, 29, 30, 31, 32, 103Michaelis constant, enzymes

immobilized in porous silica,114

microporous structure of zeolites,77

mordenite (MOR), 80, 82, 89, 90, 92,93, 95

MPTMS (3-(mercaptopropyl)trimetho-xysilane), 14, 15, 19, 20, 34, 45

MSE (zeolite), 81MSU-X, 104MTT (ZSM-23), 82, 92MTW (ZSM-12), 82, 90Mucor niehei lipase immobilization,

111multi-functionalized mesoporous

silica nanoparticles seenanoparticles

multi-point vs. single-point enzymeanchoring, 110, 111

multi-site heterogeneous catalysis(MSHC), 33, 34

mutinaite, 78MWW (MCM-22) zeolites, 80, 81, 82,

83, 84, 86, 87, 89, 90, 94

Subject Index 135

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nanoparticles/nanomaterials,mesoporous silica (mesoporoussilica nanoparticles), 34–9, 48,54, 120

bi-functional, 13–24, 35, 36, 37biodiesel production, 120–1,

124, 125for cellulosic biomass

conversion, 1–27characterization, 8, 15, 20–1enzyme immobilization, 105Pt catalysts on, 69surface area, 8, 9, 10, 15, 21,

29, 42nitrogen adsorption–desorption

isotherms, mesoporous silicananomaterials, 8, 15

nitrogen porosity for PdSBA15materials, 47

NMR, mesoporous silicananomaterials, 8–10, 15

nuclear magnetic resonance (NMR),mesoporous silica nanomaterials,8–10, 15

1-octene epoxidation, 50, 52, 56olefins, 51, 52

epoxidation, 48, 50one-dimensionality (1D), zeolites,

79, 82one-pot reactions

cellulose-to-HMF conversion,19–24

deacetalization, 35, 120organic reactions in biodiesel

production, 119–20organic structure-directing agents

see structure-directing agentsorganoamines, 37–8organometallic complexes, 45, 46, 51organosilanes, 10, 14, 15, 19, 20, 32

biodiesel production, 120–1in situ addition of (¼ co-

condensation), 32–3, 36, 52,71, 120–1

oxidation of carbon monoxide underexcess hydrogen, preferential, 68–70

palladium (Pd), 45–8paraffins, light, isomerization,

92, 93penicillin acylase cross-linked

enzyme aggregates, 109peracetic acid, 50–1permanent gases, zeolites in

separation of, 88–9phenol ring of tyrosine and covalent

enzyme immobilization, 110phosphotungstic acid (HPW) groups,

36, 37platinum (Pt)

as supported metal catalyst,64–5, 66, 67, 68, 69–70

zeolites, 92, 93PMOs, 104pore openings

mesoporous silica, 32, 42zeolites, 82, 87, 92

pore sizes/dimensionsmesoporous silica, 30, 32, see

also large-pore mesoporoussilica nanoparticles; small-pore mesoporous silicananoparticles

enzyme immobilizationand, 102, 104, 105,107

nanoparticles, andcellulosic biomassconversion, 5–6, 8, 11,12, 21, 24

supported metal catalystsand, 69–72

zeolites and, 79–80, 82, 88porous silica see silicapropylene glycol, cellulose

conversion to, 65–6proton forms of zeolites, 85–6pyrene, 53, 55, 56

random ligand grafting, 49random surface distribution of

functionalized immobilizedspecies, 54–5

rare earth metal zeolites, 91

136 Subject Index

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rigidization in enzyme anchoring, 110ruthenium (Ru) as supported metal

catalyst, 64–5, 66–7

SAPO-11 (AEL), 82, 90SAPO-34 (CHA), 82, 90, 94SBA-1, 104SBA-15 (mesoporous silica), 3–4, 31,

32, 36, 42, 45, 46–7, 49, 52, 53–4,56, 104, 105, 106, 111, 120

SBA-16, 32, 104SBA-A, 43, 44SBA-AT-p, 43, 44SBA-AT-r, 43, 44SBA-g3, 42, 43SBA-T, 43, 44scanning electron microscopy (SEM)

of mesoporous silicananomaterials, 8, 15

scanning transmission electronmicroscopy (STEM) of enzymesimmobilized in porous silicaenzymes, 113–14

self-immobilization, enzyme, 107, 109serine hydroxyl groups and covalent

enzyme immobilization, 110silica

mesoporous see mesoporoussilica

porous, biocatalysis and, 100–1enzyme immobilization

see immobilizationzeolite synthesis from mixture

of alumina and, 81–2silylation, 71–2single-site catalysis, mesoporous

silica-supported see mesoporoussilica

single-point vs. multipoint enzymeanchoring, 110, 111

small-pore mesoporous silicananoparticles (SPMSNs), 8–13

sodium cations, zeolite synthesis,85–6

solvents for lignocellulosic biomassdegradation, 2

fructose-to-HMF conversion, 5

soybean oil, biodiesel production,118, 119, 122, 125, 126

spectrometric/spectroscopicdeterminations with enzymesimmobilized in porous silica

activity, 114concentration, 112–13, 113

structure-directing agents (SDA) inzeolite synthesis

inorganic, 85–6organic, 80, 83–4

sulfonic acids in biodieselproduction, 119–20, 122, 123, 124

sulfonic groups, fructose-to-5-HMFconversion using, 13–18

surface area, mesoporous silicananoparticles, 8, 9, 10, 15, 21,29, 42

surface charge of mesoporous silicananoparticles, 11

surface distribution offunctionalized immobilizedspecies, 54–6

surface functionality of mesoporoussilica materials, 33, 70–1, 70–2

nanoparticles, 21surfactant in mesoporous silica

synthesis, 30–2, 33, 34mesoporous silica

nanoparticles, 6syngas from biomass, 68

TESP-SA (3-triethoxysilylpropylsuccinic acid anhydride), 9–13

thermolytic molecular precursor(TMP) method, 44, 45

theta-1 (ZSM-22; TON), 82, 90thiol–amine pairs, 39–40thiols/thiol groups, 14, 39, 44

as cocatalysts, 42covalent enzyme

immobilization and, 110three-dimensionality (3D), zeolites,

78, 80, 82, 85, 86–8threonine hydroxyl groups and

covalent enzyme immobilization,110

Subject Index 137

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titanium silicalite (TS-1; ZSM-5;MFI), 78, 80, 81, 82, 83, 84, 85, 89,90, 91, 92, 93–4

TMCS (trimethylchlorosilane), 71TMSOTf (trimethylsilyl

trifluoromethanesulfonate),71, 72

toluene disproportionation andalkylation, 94

p-toluenesulfonic acid (TsOH), 5TON (theta-1; ZSM-22), 82, 90transesterification, 124, 125

triglycerides, 118trialkoxysilanes, 43, 49, 1243-triethoxysilylpropyl succinic acid

anhydride (TESP-SA), 9–13triglycerides, transesterification,

118trimethylchlorosilane (TMCS), 71trimethylsilyl

trifluoromethanesulfonate(TMSOTf), 71, 72

tryptophan indole group andcovalent enzyme immobilization,110

TS-1 (ZSM-5; MFI), 78, 80, 81, 82, 83,84, 85, 89, 90, 91, 92, 93–4

tschernikite, 78TUN (zeolite), 85tungsten (W) as supported metal

catalyst, 66two-dimensionality (2D), zeolites, 80,

82, 85, 86–8tyrosine phenol ring and covalent

enzyme immobilization, 110

ultrastable zeolite Y (USY), 89–90,91, 92

ultraviolet–visible spectroscopy seeUV-VIS spectrometric/spectroscopic determinations

uniform surface distribution offunctionalized immobilizedspecies, 54–5

USY (ultrastable Y zeolite), 90, 91, 92UV-VIS spectrometric/spectroscopic

determinations of enzymesimmobilized in porous silica

activity, 114concentration, 112–13

valorization of lignin, 67–8vinylcyclohexane epoxidation, 50VUSY (highly ultrastable zeolite YY),

92

West Central Cooperative, 126, 127

X (zeolite X), 88, 89xanthate and carbamate

co-condensation, 39, 40, 41

Y (FAU; zeolite Y), 78, 80, 81, 82, 89,90, 91

zeolites, 77–99adsorption, 88–9catalysis, 89–94conventional, 78–86

structures, 79–81synthesis, 81–6

dimensionality, 78, 79–80, 81,82, 84, 86–8

history, 78novel classes, 77–8organic structure-directing

agents, 80, 83–4ZSM-5 (MFI; TS-1), 78, 80, 81, 82, 83,

84, 85, 89, 90, 91, 92, 93–4ZSM-12 (MTW), 82, 90ZSM-22 (Theta-1; TON), 82, 90ZSM-23 (MTT), 82, 92

138 Subject Index

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