Catalytic conversion of syngas to ethanol and higher ...1048937/FULLTEXT02.pdf · Catalytic...

Catalytic conversion of

syngas to ethanol and higher

alcohols over Rh and Cu

based catalysts

Luis Gagarin Lopez Nina

Doctoral Thesis in Chemical Engineering

KTH Royal Institute of Technology

School of Chemical Science and Engineering

Department of Chemical Engineering and Technology

Stockholm, Sweden 2017

Catalytic conversion of syngas to ethanol and higher alcohols over Rh

and Cu based catalysts

LUIS GAGARIN LOPEZ NINA

TRITA-CHE Report 2017:2

ISSN 1654-1081

ISBN 978-91-7729-206-7

Akademisk avhandling som med tillstånd av Kungliga Tekniska

Högskolan i Stockholm framlägges till offentlig granskning för avläggande

av teknologie doktorsexamen fredagen den 27 januari 2017 kl. 10:00 i sal

Q2, KTH, Osquldas väg 10, Våning 2, Stockholm.

Fakultetsopponent: Prof. Mario Montes Ramírez, Department of Applied

Chemistry, University of the Basque Country, San Sebastián, Spain.

© Luis Gagarin Lopez Nina 2017

Tryck: Universitetsservice US-AB

“To Mom, Hemilse, Mati and Family”

i

Abstract

The thermochemical process converts almost any kind of biomass

to a desired final product, i.e. gaseous or liquid transportation fuels

and chemicals. The transportation fuels obtained in this way are

renewable biofuels, which are alternatives to fossil fuels. During

the last few years, thermochemical plants for the production of

bioethanol have been launched and another is under construction.

A total of about 290 million liters of ethanol are expected to be

processed per year, mostly using municipal solid waste.

Considerable efforts have been made in order to find a more

selective catalyst for the conversion of biomass-derived syngas to

ethanol.

The thesis is the summary of five publications. The first two

publications (Papers I and II) review the state of the art of ethanol

and higher alcohols production from biomass, as well as the

current status of synthetic fuels production by other processes

such as the Fischer-Tropsch synthesis. Paper III analyses the

catalytic performance of a mesoporous Rh/MCM-41 (MCM-41 is a

hexagonal mesoporous silica) in the synthesis of ethanol which is

compared to a typical Rh/SiO2 catalyst. Exhaustive catalytic testing

including the addition of water vapor and modifying the hydrogen

partial pressure in the syngas feed-stream which, in addition to the

catalyst characterization (XRD, BET, XPS, chemisorption, TEM

and TPR) before and after the catalytic testing, have allowed

concluding that some water vapor can be concentrated in the pores

of the Rh/MCM-41 catalyst. The concentration of water-vapor

promotes the occurrence of the water gas shift reaction, which in

turn induces some secondary reactions that change the product

distribution, as compared to results obtained from the typical

Rh/SiO2 catalyst. These results have been verified in a wide range

of syngas conversion levels (1-68 %) and for different catalyst

activation procedures (catalyst reduction at 200 °C, 500 °C and no-

reduction) as shown in Paper IV. Finally, similar insights about the

ii

use of mesoporous catalyst have been found over a Cu/MCM-41

catalyst, shown in Paper V. Also in Paper V, the effect of metal

promoters (Fe and K) has been studied; a noticeable increase of

ethanol reaction rate was found over Cu-Fe-K/MCM-41 catalyst as

compared to Cu/MCM-41.

Keywords: thermochemical process, ethanol, higher alcohols,

mesoporous catalysts, rhodium, copper, metal promoters.

iii

Sammanfattning

Den termokemiska förgasningsprocessen konverterar nästan alla

typer av biomassa till en önskad slutprodukt, det vill säga

gasformiga eller flytande drivmedel och kemikalier. De drivmedel

som erhållits på detta sätt är förnyelsebara biobränslen, som är

alternativ till fossila bränslen. Under de senaste åren har

termokemiska anläggningar för produktion av bioetanol lanserats

och andra är under uppbyggnad. Totalt kan cirka 290 miljoner

liter etanol per år förväntas bli producerade via den termokemiska

processen, mestadels från fast kommunalt avfall. Stora

ansträngningar har gjorts för att hitta en selektiv katalysator för

den önskade produkten, d.v.s. etanol.

Avhandlingen är en sammanfattning av fem publikationer. De två

första publikationerna (I och II) sammanfattar aktuella

forskningsläget kring etanol och högre alkoholer producerade från

biomassa, liksom nuvarande status för syntetiska bränslen

producerade via andra processer såsom Fischer-Tropsch-syntes.

Publikation III analyserar den katalytiska prestandan hos en

mesoporös Rh/MCM-41-katalysator (MCM-41: hexagonal

mesoporös kiseldioxid) vid syntes av etanol och jämfört den med

konventionell Rh/SiO2-katalysator. Från omfattande katalytiska

tester inklusive effekten av vattenångtillsats och vätepartialtrycket

i syntesgasen samt katalysatorkarakterisering (XRD, BET, XPS,

kemisorption, TEM och TPR) före och efter den katalytiska

reaktionen, kan slutsatsen dras att en del vattenånga kan

koncentreras i porerna hos en Rh/MCM-41-katalysator.

Koncentrationen av vattenånga främjar förekomsten av vatten-gas

skiftreaktionen, som i sin tur framkallar sekundära reaktioner som

ändrar produktfördelningen jämfört med resultaten från den

konventionella Rh/SiO2-katalysatorn. Dessa resultat har bekräftats

för ett brett spektrum av syntesgasomsättningar (1-68 %) och vid

olika förfaranden för katalysatoraktivering (reduktion vid 200 °C,

500 °C eller ingen aktivering), som visas i Publikation IV. Slutligen

iv

har liknande slutsatser om användningen av mesoporösa

katalysatorer erhållits över en Cu/MCM-41-katalysator, vilket visas

i publikation V. I Publikationen V, har effekten av

metallpromotorerna (K och Fe) studerats, där de mest intressanta

resultaten från Cu-Fe-K/MCM-41-katalysator var den märkbara

ökningen av reaktionshastigheten till etanol i jämförelse med

Cu/MCM-41-katalysator.

Nyckelord: termokemisk process, etanol, högre alkoholer,

mesoporösa katalysatorer, rodium, koppar, metalliska promotorer.

v

List of appended papers

The work presented in this doctoral thesis is based on the following

publications. The papers are appended at the end of the thesis and

referred to in the text using Roman numerals (I – V).

I. R. Suárez, L. Lopez, J. Barrientos, F. Pardo, M. Boutonnet,

S. Järås

Catalytic conversion of biomass-derived synthesis gas to

fuels

Catalysis Book Series, Royal Society of Chemistry, 27 (2015)

62-143.

II. J.A. Velasco, L. Lopez, M. Velasquez, M. Boutonnet, S.

Cabrera, S. Järås

Gas to liquids: A technology for natural gas

industrialization in Bolivia

Journal of Natural Gas Science and Engineering, 2 (2010)

222-228.

III. L. Lopez, J. Velasco, V. Montes, A. Marinas, S. Cabrera, M.

Boutonnet, S. Järås

Synthesis of ethanol from syngas over mesoporous

Rh/MCM-41 catalyst; effect of water on the product

selectivity

Catalysts, 5 (2015) 1737-1755.

IV. L. Lopez, Jorge Velasco, Saul Cabrera, Magali Boutonnet,

Sven Järås

Effect of syngas conversion and catalyst reduction

temperature in the synthesis of ethanol: concentration of

water vapor in mesoporous Rh/MCM-41 catalyst

Catalysis Communications, 69 (2015) 183-187.

vi

V. L. Lopez, V. Montes, H. Kušar, S. Cabrera, M. Boutonnet,

S. Järås

Syngas conversion to ethanol over a mesoporous

Cu/MCM-41 catalyst: effect of K and Fe promoters

Applied Catalysis A: General, 526 (2016) 77-83.

Contribution to appended papers

I. I have contributed to all sections by discussion with the all

the authors. The following sections were written in

cooperation with R. Suárez: Abstract, Introduction and

Conclusions. I wrote the section Ethanol and mixed alcohols.

II. J. Velasco is the main author of this paper and I had

contributed to the discussion and conclusion sections,

especially the situation in Bolivia.

III. I am the main author of this paper. J. Velasco helped in TPR

experiments and V. Montes performed the TEM and helped

with interpretation of data.

IV. I am the main author of this paper. J. Velasco helped with

the analysis of data.

V. I am the main author of this paper. V. Montes performed the

TEM and helped with interpretation of data.

vii

Other publications and conference presentations

Peer-reviewed publications

J.A. Velasco, L. Lopez, S. Cabrera, M. Boutonnet, S. Järås

Synthesis gas production for GTL applications:

thermodynamic equilibrium approach and potential for

carbon formation in a catalytic partial oxidation pre-reformer

Journal of Natural Gas Science and Engineering, 20 (2014) 175-

183.

J.A. Velasco, C. Fernandez, L. Lopez, S. Cabrera, M.

Boutonnet, S. Järås

Catalytic partial oxidation of methane over nickel and

ruthenium based catalysts under low O2/CH4 ratios and with

addition of steam

Fuel, 153 (2015) 192-201.

L. Lopez, L. Quispe, P. Crespo, M. Litter, S. Cabrera.

Structural and morphological comparison of TiO2 obtained

from novel “atrane route” and conventional route using the

sol-gel processes

(Comparación estructural y morfológica de TiO2 sintetizados

por la Ruta de los Atranos y la ruta convencional en el método

sol-gel)

Bolivian Journal of Chemistry, Vol. 28 no. 2 (2011) 106-112.

L. Quispe, L. Lopez, V. Teixeira, S. Cabrera

Novel titanium oxides doped (Yb, Nd, La and Li) for the

photocatalytic reduction of hexavalent chrome

(Nuevos Óxidos de Titanio Dopados (Yb, Nd, La y Li) para la

Reducción Fotocatalítica de Cromo Hexavalente)


viii

L. Quispe, M.C. Arteaga, E. Cárdenas, L. Lopez, C. Santelices,

E. Palenque, S. Cabrera.

Elimination of Cyanide by means of combined UV/ H2O2/TiO2

system

(Eliminación de Cianuro mediante sistema combinado

UV/H2O2/TiO2)


M. Alacama, L. Lopez, J. Velasco, F. Gracia, W. Yapu, S.

Cabrera

Ni-Cu/MCM-41 catalyst synthetized by the atrane route for the

etanol reforming

(Catalizadores Ni-Cu/MCM-41 sintetizados por la ruta de los

atranos para el reformado de etanol)


Book chapters

S. Cabrera, L. Lopez (editors)

Science and Technology in Bolivia: state of the art of industrial

processes for Hydrocarbons and Petrochemistry

(Ciencia y Tecnología en Bolivia: estado del arte de procesos

industriales para el sector de hidrocarburos y petroquímico)

Book, Natural Gas Institute, Higher University of San Andres,

La Paz-Bolivia, (2016) 300.

L. Lopez (author)

Ethanol and Higher Alcohols: state of the art

(Evaluación de la tecnología disponible para la producción de

etanol y alcoholes superiores)

Book chapter, Natural Gas Institute, Higher University of San

Andres, La Paz-Bolivia, (2016) 237-289.

ix

Conference contributions

L. Lopez, M. Boutonnet, S. Cabrera, S. Järås

Production of ethanol from synthesis gas

Oral presentation, Swedish Catalysis Society, Perstorp, Sweden,

2009.

L. Lopez, L. Quispe, P. Crespo, M. Boutonnet, S. Järås, M. Litter, S.

Cabrera

Effect of precursor in structural-morphological properties of TiO2

obtained by sol gel method; new titanatrane precursor

Poster presentation, European Catalysis Congress IX, Salamanca,

Spain, 2009.

L. Lopez, J. Velasco, S. Cabrera, M. Boutonnet, S. Järås

Rhodium catalysts promoted by Lithium on Si/Zr mesoporous

support for the conversion of syngas to oxygenates

Poster presentation, 16th International Zeolite Conference and 7th

International Mesoporous Materials Symposium, Maiori, Italy,

2010.


Syngas conversion into C1-C2 hydrocarbons and oxygenates over

a novel Rh-Li/Zr-MCM-41

Poster presentation, 14th Nordic Symposium on Catalysis,

Helsingør, Denmark, 2010.


Syngas conversion into ethanol and short hydrocarbons on

Rh/MCM-41: Catalyst Characterization After Reaction

Poster Presentation, 22nd North American Catalysis Meeting,

Detroit-USA, 2011.


Support stabilization for Rhodium catalyst in synthesis gas

conversion: Rh/Zr-MCM-41

x

Poster Presentation, European Catalysis Congress IX, Glasgow,

Scotland, 2011.

L. Lopez, S. Cabrera

Conversion of Bolivian natural gas to national interest products

(Conversión de gas natural boliviano a productos de interés

Nacional)

Oral presentation, VII Bolivian Congress on Chemistry, La Paz,

Bolivia, 2012.


Syngas to ethanol over Rh/MCM-41 catalyst

Poster presentation, 2nd Summer School on Catalysis for

Sustainability, Rolduc Abbey, The Netherlands, 2013.

xi

Table of Contents CHAPTER 1: Introduction…………………………………………………………………………. 1 1.1 Setting the scene……………………………………………………………………………………………. 1

1.2 Scope of the work.…………………………………………………………………………………………. 2

CHAPTER 2: Thermochemical processing of biomass: fuels for the transportation sector……………………………………………………………………………….

3

2.1 Biomass pre-treatment and gasification…………………………………………………………. 5

2.2 Syngas cleaning and conditioning…………………………………………………………………… 8

2.3 Catalytic synthesis of fuels……………………………………………………………………………… 9

2.3.1 Synthetic Natural Gas……………………………………………………………………………….. 11

2.3.2 Gasoline and Diesel…………………………………………………………………………………… 11

2.3.3 Di-Methyl Ether………………………………………………………………………………………… 12

2.3.4 Ethanol and Higher Alcohols…………………………………………………………………….. 13

CHAPTER 3: Ethanol and Higher Alcohols Synthesis…………………………………. 15 3.1 Alcohols as motor fuels………………………………………………………………………………….. 15

3.2 Industrial process development……………………………………………………………………… 16

3.2.1 Process configurations for direct synthesis of alcohols……………………………… 16

3.2.2 Process configurations for multi-step synthesis of ethanol……………………….. 18

3.2.3 Current projects for the thermochemical production of ethanol………………. 19

3.2.4 Preliminary considerations for ethanol production from biomass: the case

of Bolivia…………………………………………………………………………………………………………….

21

CHAPTER 4: Catalytic conversion of synthesis gas to ethanol and higher

alcohols……………………………………………………………………………………………………

23 4.1 Thermodynamic considerations……………………………………………………………………… 23

4.2 Types of catalysts…………………………………………………………………………………………… 24

4.2.1 Rh-based catalysts……………………………………………………………………………………. 25

4.2.2 Cu-based catalysts……………………………………………………………………………………. 25

4.2.3 Role of the catalyst’s support……………………………………………………………………. 25

4.2.3.1 Ordered Mesoporous Silica (MCM-41): the “atrane route”………………….. 26

4.3 Reaction mechanisms…………………………………………………………………………………….. 30

CHAPTER 5: Experimental methods…………………………………………………………. 33 5.1 Catalyst preparation………………………………………………………………………………………. 33

5.1.1 Mesoporous Silica MCM-41……………………………………………………………………... 33

5.1.2 Metal impregnation…………………………………………………………………………………. 34

5.2 Catalyst characterization……………………………………………………………………………….. 34

5.2.1 N2-Physisorption………………………………………………………………………………………. 34

5.2.2 X-Ray Diffraction (XRD)…………………………………………………………………………….. 35

5.2.3 H2-Chemisorption…………………………………………………………………………………….. 35

5.2.4 Transmission Electron Microscopy (TEM)…………………………………………………. 35

xii

5.2.5 Temperature Programmed Reduction (TPR)……………………………………………… 35

5.2.6 X-Ray Photoelectron Spectroscopy (XPS)………………………………………………….. 35

5.2.7 Thermogravimetric analysis (TGA)……………………………………………………………. 36

5.3 Catalytic testing……………………………………………………………………………………………… 36

5.3.1 Reactor set-up and procedures………………………………………………………………… 36

5.3.2 Product analysis and data treatment………………………………………………………… 38

CHAPTER 6: Catalytic performance of a typical Rh/SiO2 catalyst and a

mesoporous Rh/MCM-41 catalyst……………………………………………………………

41 6.1 Product distribution and selectivity to C2-oxygenated compounds..………………. 41

6.1.1 Effect of syngas conversion………………………………………………………………………. 43

6.1.2 Effect of the catalyst reduction temperature……………………………………………. 44

6.1.3 Effect of hydrogen partial pressure………………………………………………………….. 44

6.1.4 Effect of water addition……………………………………………………………………………. 45

6.2 Catalyst characterization before and after catalytic testing……………………………. 47

6.2.1 Specific surface area and pore size distribution………………………………………… 47

6.2.2 Surface composition: Rh0 and Rh

3+……………………………………………………………. 49

6.2.3 Porous ordering by XRD and TEM……………………………………………………………… 49

6.2.4 Metal support interactions……………………………………………………………………….. 51

6.3 Interpretation of the catalytic performance of Rh/MCM-41 compared to

Rh/SiO2………………………………………………………………………………………………………………….

52

6.3.1 Water vapor concentration in mesoporous Rh/MCM-41 catalyst……………… 52

6.3.2 Higher ethanol selectivity as consequence of the water vapor

concentration in mesoporous Rh/MCM-41 catalyst……………………………………………

54

CHAPTER 7: Effect of Fe and K promoters on Cu/MCM-41 catalyst..………… 57 7.1 Catalysts characterization: XRD,TGA, N2 physisorption, TEM, XPS and TPR..…… 57

7.2 Catalytic testing: product selectivity trends……………………………………………………. 60

7.2.1 Effect of K as promoter…………………………………………………………………………….. 62

7.2.2 Effect of Fe as promoter…………………………………………………………………………… 62

7.2.3 Effect of K and Fe addition……………………………………………………………………….. 62

7.3 Selectivities to alcohols and other oxygenated compounds……………………………. 63

7.4 Anderson–Schulz–Flory (ASF) distribution……………………………………………………… 64

CHAPTER 8: Conclusions………………………………………………………………………….. 67 8.1 Main results of this work………………………………………………………………………………… 67

8.2 Final remarks and future work……………………………………………………………………….. 69

Nomenclature…………………………………………………………………………………………. 71

Acknowledgements…………………………………………………………………………………. 73

References……………………………………………………………………………………………… 76

Appendices: Papers I-V

1

CHAPTER 1

Introduction

1.1 Setting the scene

Concerns about climate change, energy resources and energy

efficiency are discussed both locally and internationally context.

Although attempts to reduce the use of highly polluting energy

resources such as fossil oil and coal have been made during the last

decades, they still are represent the major contributors to the

world energy supply: 70.7 % in 1973 and 59.9 % in 2014 [1]. Less

contaminating resources such as natural gas and renewable energy

like biofuels and wastes, are the other major contributors to the

energy world supply: 26.5 % in 1973 and 31.5 % in 2014. Increase

in the use of renewable biofuels and waste from 10.3 % in 2014 to

about 15 % by 2040 is estimated [1]. On the other hand, the world

population is estimated to growth from 7.34 billion in 2015 to 9.15

billion by 2040 [2], which means an increase in the energy

demand. Moreover, by 2040 CO2 emissions are forecast to increase

by 13 %, in comparison with year 2014 [1].

Biofuels are produced from renewable organic material (obtained

directly from plants, or indirectly from agricultural, commercial,

domestic, and/or industrial wastes). An interesting route for

biofuel production is thermochemical processing in which almost

any kind of organic material or biomass is converted to a desired

fuel. In this process, biomass is converted to an intermediary

mixture of gases known as “synthesis gas” or “syngas”, consisting

mainly of CO, CO2 and H2. The syngas is then catalytically

converted to produce i) conventional fuels such as diesel and

gasoline via the Fischer-Tropsch Synthesis, but also ii) alternative

fuels such as di-methyl-ether (DME), ethanol and higher alcohols

using similar catalytic processes.

2

1.2 Scope of the work

The objective of the thesis was to study the influence of operating

conditions as well as catalyst properties and composition on the

catalytic performance during the catalytic conversion of syngas to

ethanol and higher alcohols. The latter, within the context of

generation of renewable fuels for the transportation sector,

especially the thermochemical process using biomass-derived

syngas. In particular the work was focused on; (I) The effect of

reaction conditions (temperature, pressure, space velocity and

syngas H2/CO ratio) and the catalyst activation procedure

(different reduction temperatures) on the conversion of syngas to

ethanol; (II) The catalytic performance and characterization of

novel mesoporous catalysts using a hexagonal mesoporous silica

(i.e. MCM-41) as catalyst support and Rh as the active metal for the

synthesis of ethanol; (III) The effect of Fe and K promoters on the

catalytic behavior of mesoporous Cu/MCM-41 catalyst for the

synthesis of ethanol and higher alcohols.

The work included in this thesis was mostly conducted at the

Department of Chemical Engineering and Technology at KTH

Royal Institute of Technology, Stockholm, Sweden. Experimental

work was also performed at the Natural Gas Institute at the Higher

University of San Andres, La Paz, Bolivia.

3

CHAPTER 2

Thermochemical processing of biomass: fuels for

the transportation sector

Biomass is biological material derived from living, or recently

living organisms. This material is used to produce energy; in fact,

this represents around 10 % of the world‟s primary energy supply

[3]. The four main types of biomass for energy purposes are: i)

forest products (e.g. fuelwood, residues and processing, and

municipal solid waste), ii) energy crops, iii) agricultural residues

(harvesting residue and processing residue) and iv) animal manure

[4]. It is estimated that world energy supply from biomass could

increase from 56 EJ in 2010 to 97-147 EJ by 2030 [4]. From this,

about 38-45 % would originate from agricultural residues and

waste. The remaining supply is shared between energy crops (26-

34 %) and forest products (28-30 %).

The composition of biomass depends on its origin. Proximate

analysis of various sources of biomass, Table 2.1. [5], indicates that

biomass is mostly composed of volatile matter (material that is

released from biomass when it is heated up to approx. 400-500

°C), followed by moisture and fixed carbon. Ash content varies

considerably, being low in woody biomass (mean of 2.7 wt.%) and

high in animal manure (mean of 28.8 wt.%). On a dry and ash-free

basis, biomass is composed by >40 wt.% carbon, 20 wt.% oxygen,

4 wt.% hydrogen and minor amounts of other elements. For

manufacturing purposes, the major elements of biomass (carbon,

oxygen and hydrogen) are chemically converted into a desired fuel,

such as ethanol. However, some minor elements, such as sulfur

and chlorine, should be eliminated since they represent potential

sources of poisoning during chemical conversion.

4

Table 2.1. Chemical composition of various types of biomass and some solid fossil fuels. Adapted from Vassilev et al. [5],

Copyright (2010), with permission from Elsevier.

SOURCE Range

Proximate analysis (wt.%) Ultimate analysis (wt.%)

(dry and ash-free basis)

Volatile matter

Fixed carbon

Moisture Ash

C O H N S Cl

WOOD AND WOODY BIOMASS

Minimum 30.4 6.5 4.7 0.1 48.7 32.0 5.4 0.1 0.01 0.01 Mean 62.9 15.1 19.3 2.7 52.1 41.2 6.2 0.4 0.08 0.02 Maximum 79.7 24.1 62.9 8.4 57.0 45.3 10.2 0.7 0.42 0.05

HERBACEOUS AND AGRICULTURAL BIOMASS


ANIMAL BIOMASS*


CONTAMINATED BIOMASS**


OTHER TYPES OF BIOMASS Wheat straw 67.2 16.3 10.1 6.4 49.4 43.6 6.1 0.7 0.17 0.61 Straw 64.3 13.8 12.4 9.5 48.8 44.5 5.6 1.0 0.13 0.54 Rice husks 56.1 17.2 10.6 16.1 49.3 43.7 6.1 0.8 0.08 0.12 Soya husks 69.6 19.0 6.3 5.1 45.4 46.9 6.7 0.9 0.10 - Sugar cane bagasse 76.6 11.1 10.4 1.9 49.8 43.9 6.0 0.2 0.06 0.03 Marine macroalgae 45.1 23.1 10.7 21.1 43.2 45.8 6.2 2.2 2.60 3.34

SOLID FOSSIL FUELS Peat 57.8 24.3 14.6 3.3 56.3 36.2 5.8 1.5 0.2 0.04 Lignite 32.8 25.7 10.5 31.0 64.0 23.7 5.5 1.0 5.8 0.01 Coal 30.8 43.9 5.5 19.8 78.2 13.6 5.2 1.3 1.7 0.03

* Based on chicken litter and meat-bone meal. ** Furniture waste, waste paper, sewage sludge and others.

5

One route for biomass manufacturing is the thermochemical

process [6]. This process can be divided in three sections; i)

pretreatment and gasification, ii) cleaning and conditioning and,

iii) catalytic synthesis and purification, as schematically shown in

Figure 2.1. Thus, a variety of synthetic fuels can be obtained from

biomass, either traditional fuels such as Gasoline, Diesel and

Natural Gas or alternative fuels such as Di-Methyl Ether (DME),

Ethanol and Higher Alcohols.

Figure 2.1: Fuels obtained from the thermochemical processing of biomass.

Author’s own elaboration.

2.1 Biomass pre-treatment and gasification

Raw biomass must be pretreated and converted into a suitable

form for subsequent gasification. This includes:

− Size reduction, smaller particles have larger surface area

per unit of mass which facilitates higher rates of heat

Renewable Feedstock

(Municipal solid wastes, forest and agricultural biomass, other).

ii) Cleaning and

Conditioning

i) Pretreatment and

Gasification

iii) Catalytic Synthesis and

Purification

- Synthetic Natural Gas

- Gasoline and Diesel (via Fischer-Tropsch

Technology or Methanol to Hydrocarbons)

- Di-Methyl Ether

- Ethanol and Higher Alcohols

6

transfer and gasification. This could be further enhanced

with porous particles. The smaller particles also favor the

yield to CO and H2, while the yield to CO2 is diminished.

− Drying, biomass with low moisture (<10 %) is desired for

the gasification process. Drying is an energy-consuming

process and for that reason several dryer configurations

have been used to dry biomass (e.g. bin dryer, band

conveyor dryer, rotary cascade dryer) [7].

After the pretreatment step, the biomass is thermally processed via

gasification. The gasification takes place at high temperature (600-

1 000 °C) in the presence of an oxidizing agent (air, steam, oxygen

or a combination of them). During this process, the large

polymeric molecules are decomposed into lighter ones (e.g. CO,

H2, CH4 and short hydrocarbons); ash, char, tar and minor

contaminants are also obtained. Char and tar are the result of

incomplete conversion of biomass. The overall reaction involved in

the biomass gasification can be represented by the following

equation (2.1)

( ) (2.1.)

The product composition, obtained from the gasification, depends

on various parameters, such as:

- Type of gasifier. Various kinds of gasifiers have been

developed. The most used are fixed-bed and fluidized bed

[8]. Fixed-bed gasifiers can be either updraft

(countercurrent) or downdraft (cocurrent). In the updraft

gasifier, the biomass is introduced from the top and moves

downwards while the gasifying agent is introduced at the

bottom. The product gas moves upwards and the

temperature decreases to 500 °C in the gasifier exit and, as

a consequence, the resulting products contain large

amounts of tar. In the downdraft gasifier, biomass and

7

product gas move downdraft and thus the exit temperature

is high (800 °C), so most of the tar is consumed. On the

other hand, fluidized gasifiers have better heat and mass

transfer which increases the reaction rates and conversion

efficiencies. Typically, silica or alumina is used as fluidizing

medium. Solid catalysts can also be used to increase

conversion efficiency and reduce tar formation. For that,

the catalyst must be stable at high temperatures and

resistant to poisoning.

- Biomass flow rate, type and properties. Overfeeding of

biomass can lead to plugging and reduce conversion

efficiency, while starve-feeding results in lower yield.

Moreover, the biomass type (Table 2.1) affects the product

composition.

- Air and steam flow rate. Air flow rate directly affects the

degree of combustion and the gasification temperature,

which in turn affect the biomass conversion and the

product composition. Supplying steam increases the partial

pressure of H2O and, thus water gas shift (WGS) and

methane steam reforming (MSR) reactions are favored. As

a consequence, the resulting syngas contains more

hydrogen.

- Gasification temperature profile. The gasification

temperature is one of the most important factors for

controlling the product composition and properties. At

temperatures of 750-800 °C, the WGS and MSR reactions

dominate favoring the H2 production. At 850-900 °C, the

MSR and Boudouard reactions dominate, resulting in

higher CO content and, as consequence, a decrease in tar

formation and an increase of gas yield are observed [5].

8

2.2 Syngas cleaning and conditioning

The gasification product needs further processing in order to

achieve an adequate synthesis gas composition. Thus,

contaminants such as particulates, tars, alkalis, nitrogen and,

sulfur should to be eliminated. The type of contaminant and the

degree of purification depend on the subsequent application; for

the production of fuels a low level of contaminants is typically

needed while for power generation relatively high amounts of

contaminants can be tolerated.

- Particulate removal. Particulates consist of unconverted

biomass products such as ash and char. Depending on their

particle sizes, they can be removed by using i) cyclones in

the case of large particles (i.e. >5 m), ii) porous solid

filters for a wide range of particle sizes (0.5-100 m) or iii)

electrostatic separation.

- Alkali removal. Alkali compounds (CaO, K2O, P2O5, MgO,

N2O) are vaporized at high temperatures during

gasification, but they condense and form particles in the

posterior equipment. Specific alkali removal devices are

under research and development; for example, a bauxite

filter which can remove most of the Na and K compounds,

at temperatures of around 650-725 °C [9].

- Nitrogen compounds. Most of the nitrogen content of the

biomass is converted to ammonia during gasification,

which can be further removed by wet scrubbing (at low

temperatures) or destroyed at higher temperatures using

either dolomites, or Ni-based or Fe-based catalysts.

- Sulfur compounds. Most of the sulfur content of the

biomass is converted to H2S and SO2 during gasification,

which can be then removed by adsorption on limestone,

dolomite or CaO (at low temperatures) or calcined at high

temperatures.

9

- Tar removal. Tar consists of compounds that are not

decomposed and remain in the product gas. They are mixed

oxygenates, phenolic ethers, alkyl phenolics, heterocyclic

ethers, short and large poly-aromatic hydrocarbons. Two

approaches are applied for tar removing [10]; i) optimizing

the gasification operation conditions together the use of

catalysts (so called in-bed catalysts) and ii) a secondary

reactor for tar destruction and achievement an acceptable

level in the product gas.

2.3 Catalytic synthesis of fuels

Traditional fuels such as gasoline and diesel as well as alternative

fuels such as ethanol, higher alcohols and DME, are produced from

the catalytic conversion of syngas. Syngas can be obtained from

renewable sources like biomass or from non-renewable sources

like natural gas or coal. Various projects have been identified for

biomass conversion to final fuels, as is summarized in Table 2.2.

The preferred route to produce diesel is the Fisher-Tropsch (FT)

Synthesis. An important aspect is the syngas composition obtained

from biomass gasification and from natural gas processing,

respectively. The syngas composition derived from biomass or

natural gas yields a H2/CO equal to 0.5-1.0 [11] and 1.6-4.0 [12],

respectively. Thus, additional treatment of the biomass-derived

syngas is required for a process that needs higher H2/CO1, e.g.

Fischer-Tropsch Synthesis, Synthetic Natural Gas and ethanol and

higher alcohols (as will be shown in the following sections).

10

Table 2.2. Status on facilities for the conversion of biomass (via gasification) to transportation fuels [13].

Company/ Institution

Location Technology

level Startup Status Raw material Fuel

Cool Planet Alexandria LA

(USA) Demonstration - Commercial

2016 Under

construction Forest residues

Gasoline-type products (30 000 t/y)

Go Green Fuels Ltd. Swindom

(UK) Demonstration - Commercial

2018 Under

construction Organic residues and

waste streams Synthetic Natural gas

(1 500 t/y)

Goteborg Energi AB Gothenborg (Sweden)

Demonstration plant

2014 Operational Lignocellulosic crops Synthetic Natural gas

(11 200 t/y)

Karlsruhe Institute of Technology

Karlsruhe (Germany)

Demonstration plant

2014 Operational Lignocellulosic crops - DME (608 t/y)

- Gasoline-type fuels (360 t/y)

Consortium Groen Gas 2.0

Alkmaar, (Netherlands)

Demonstration plant

2018 Planned Lignocellulosic crops Synthetic Natural gas

(6 500 t/y)

TUBITAK Gebze

(Turkey) Pilot plant 2013 Operational Forest residues

Fischer-Tropsch liquids (250 t/y)

GTI Gas Technology Institute

Des Plaines IL (USA)

Pilot plant 2012 Operational Lignocellulosic crops - Fischer-Tropsch liquids

- Gasoline-type fuels

BioTfuel-consortium Dunkirk (France)

Pilot Plant 2012 Operational Lignocellulosic crops - Fischer-Tropsch liquids

(60 t/y) - Jet fuel

Greasoline GmbH Oberhausen (Germany)

Pilot plant 2011 Operational Oil crops, oils and fats Diesel-type hydrocarbons

(2 t/y)

11

2.3.1 Synthetic Natural Gas

The simplest reaction that converts syngas into a fuel is the

methanation reaction, equation (2.2.), which uses a Ni-based

catalyst. This reaction is carried out at high temperatures of 200-

700 °C, resulting in high reaction rates. Also, the heat of reaction

can be used to produce high quality steam and thus a quite energy

efficient process can be achieved.

(2.2.)

The effects on the methanation reaction of the low H2/C ratio and

the inherent impurities of the biomass-derived syngas are

currently under investigation. Especially, novel configurations for

biomass gasifiers such as i) absorption-enhanced gasification/

reforming, ii) internally circulating fluidized beds and others types

of reactors are being investigated, in order to generate a hydrogen-

rich syngas [11]. A commercial project (the GoBiGas project) in

Göteborg (Sweden) is being developed which aims at the

production of both methane equivalent to 20 MW and gas

equivalent to 80-100 MW. Also, it is expected that the gas

production will reach 1 TWh by 2020, which represents about 30

% of the gas consumption in Gothenburg city or the fuel needed for

100 000 cars [14].

2.3.2 Gasoline and Diesel

The Fischer-Tropsch Synthesis (FTS) refers to the conversion of

syngas to a wide range of liquid and gaseous products following a

polymerization reaction, equation 2.3. Gasoline and diesel can be

obtained after upgrading and purification steps. If gasoline is the

desired product, the FTS synthesis is conducted using Fe-based

catalysts at high temperature (HTFT), i.e. 300-350 °C. If diesel (or

middle-distillate products) is desired, the FTS synthesis is

12

performed using Co-based catalysts at low temperature (LTFT), i.e.

200-240 °C.

(2.3.)

Considerable efforts have been made by several institutions in

order to scale-up the processes for gasoline and diesel generation,

as can be noted from Table 2.2, and a commercial plant is expected

to be launched soon. On the other hand, FT Synthesis is already

commercialized using natural gas (so-called Gas to Liquids, GTL)

and coal (Coal to Liquids, CTL), as raw materials [12]. The

production volume of these commercial plants, both situated in

South Africa, varies between 2 500 bpd (e.g. Sasolburg plant) to

165 000 bpd (e.g. Secunda plant). This technology becomes

important for diesel production, especially in regions where no

heavy oil is available.

2.3.3 Di-Methyl Ether

DME is an alternative fuel for compression-ignition engines

because of its high cetane number (55-60), low emissions

(particulates, SOx and NOx) and low engine noise. DME can also be

blended with diesel thus increasing the viscosity, density and

heating value. The synthesis involves three reactions; i) methanol

synthesis, ii) methanol dehydration and iii) water gas shift

reaction, which can be summarized by equation (2.4.). DME can be

obtained by direct conversion of syngas using a bifunctional

catalyst (methanol synthesis and methanol dehydration). It can

also be in two consecutive steps; methanol synthesis using

traditional Cu-Zn/Al3O2 catalyst and methanol dehydration using

an acid catalyst (-alumina or HZSM-5).

(2.4.)

13

In 2014, a demonstration plant was installed in Germany by

academia and industry jointly. The plant uses lignocellulosic crops

as main raw material, which is processed via fast pyrolysis and

gasification to produce syngas. The final products are DME (608

tons/year) and gasoline-type fuels (360 tons/year). With the

results from the demonstration plant, the next step for

commercializing can be planned. However, so far no news has

been release about the DME production at commercial scale.

2.3.4 Ethanol and Higher Alcohols

Currently, ethanol grade fuel is being obtained from biochemical

processes (so-called first-generation bioethanol) using sugar,

starch and oil seed-based feedstocks, which are also food

resources. However, this bioethanol involves some controversial

issues: global food supply; limited GHG (greenhouse gases)

emissions reduction; and environmental impacts, such as

competition for water supplies or impact of land use change [15].

Thus, the sustainability of first-generation bioethanol is being

questioned, second-generation bioethanol is emerging, since it is

based on non-food resources (e.g. energy crops, forest and

agricultural residues, municipal waste, etc.) [16, 17]. Second-

generation biofuels can be produced by means of either a

biochemical or a thermochemical route. The main drawback of the

biochemical alternative is the low conversion achieved, since lignin

contained in the biomass cannot be processed and it represents 25-

30 wt.% of the biomass. Hydrolysis of cellulose and hemicellulose

is also difficult and there are some recalcitrant materials that must

necessarily be thermochemically handled [18].

The production of ethanol, and also higher alcohols, via the

thermochemical route will be covered in the following chapters,

since it is the main concern of this thesis.

15

CHAPTER 3

Ethanol and Higher Alcohols Synthesis

3.1 Alcohols as motor fuels

Table 3.1 summarizes the basic properties of alcohols as compared

to other types of fuels. Alcohols make excellent motor fuels with

high octane values and allow high compression ratios and thus

increase efficiency with improved power output per cylinder.

Alcohols can run at lean air-fuel ratios with better energy efficiency

than gasoline. However, alcohols have very low cetane numbers

and cannot be used in compression-ignition diesel-type engines,

unless gas temperatures are high at the time of injection. Some

companies of heavy-duty engines have developed several types of

systems to assist autoignition of directly injected alcohols [19]. The

blending of additives to the alcohol fuels has turned out to be

effective in improving the cetane number. Another way is to add

small amount of diesel in order to improve the ignition of the

alcohols.

Table 3.1. Basic properties of fuels (From Paper I). Adapted from [11] with

permission from The Royal Society of Chemistry.

SNG Methanol Ethanol

Mixed

alcohols

(Octamix)

DME FT

Gasoline

FT

Diesel

Formula CH4 CH3OH C2H5OH C1-8OH CH3OCH3 C6-C12 C12-C20

MW (g/mol) 16.80 32.04 46.07 41.68 46.07 100-105 200

Density (kg/L) 0.41-

0.5a

0.792 0.789 0.807 0.661 0.745b

0.77-

0.78

Energy content

(MJ/L) 24.9

a 15.4-15-6 21.3 18 18.2-19.3 32.4b

33.1-

34.3

Blending RON >127 133 130 118-123 n/a 95 n/a

Cetane number n/a 5 5 n/a 55-60 n/a 70-80

a Values corresponding to liquefied natural gas (LNG).

b Values corresponding to conventional gasoline.

16

The most commercialized alcohol used as fuel is ethanol. Vehicles

known as flexible fuel vehicles (FFV) can run either with pure

ethanol, pure gasoline or a mixture of both. For example, pure

ethanol vehicles are largely commercialized in Brazil. A mixture of

gasoline and ethanol (abbreviated as EX, where X indicate the

percentage of ethanol e.g. E85 has 85 % of ethanol) is

commercialized in the USA and Europe. Today, most vehicles are

able to run with at least 10 % ethanol (E10) blended into the

gasoline.

3.2 Industrial process development

3.2.1 Process configurations for direct synthesis of

alcohols

Various process configurations have been patented for the direct

synthesis of alcohols [11]. These processes involve a single or a few

main reactor units in series for the conversion of syngas to

alcohols. The produced alcohols are, mostly, in the range of C2-

4OH. It should be mentioned that a mixture of alcohols is allowed

to be blended with gasoline in many regions around the world. For

example, in Europe the limit of alcohol content varies between 3 %

and 15 % (volume), and the range of alcohols from methanol to

octanol (alcohols with boiling point lower than 210°C). Due to the

toxicity of methanol, it should preferably be the lowest amount in

the alcohol mixture ( 3 %).

17

Figure 3.1. Basic flow diagram for the synthesis of alcohols (C2–4OH) [20].

The flow diagram of a process from Haldor Topsøe for alcohol

synthesis is shown in Figure 3.1 [20]. This process uses a main

reactor unit to convert syngas to alcohols and a secondary

hydrogenation unit to convert other types of oxygenated by-

products (e.g. ketones, esters, and/or aldehydes) to the

corresponding alcohol. The resulting products are sent to a gas–

liquid separator, from which the gas fraction (composed mostly of

methane and unreacted syngas) is sent to a methanation reactor

unit to produce synthetic natural gas and the liquid fraction is sent

to a stripper unit. In the latter unit, short alcohols (mostly

methanol) are stripped from the liquid fraction using a stream of

Admixing H2 rich gas

Stripper syngas

Alcohol-synthesis gas

Alcohol

Synthesis Reactor

Crude

alcohol

Hydrogenation

Reactor

Hydrogenated

crude alcohol

Gas-Liquid

Separator

Methanator

SNG

Destillation

Section

Lower Alcohols

SYNGAS

Higher Alcohols

18

fresh syngas and recycled to the main reactor unit. The latter is

done in order to promote the carbon chain growth to higher

alcohols, via alcohol homologation or alcohols coupling. The main

reactor unit operates at 20 bar and 270-400 °C and uses a Cu-

ZnO/Al2O3 based catalyst. The hydrogenation reactor unit operates

at similar pressure but in a temperature range of 100-220 °C using

a typical noble metal-based catalyst (e.g. Pt and/or Pd) or a Cu–

ZnO/Al2O3-based catalyst.

3.2.2 Process configurations for multi-step synthesis of

ethanol

The latest patented process configurations for the synthesis of

ethanol use various reactor units, this in order to maximize the

overall ethanol yield [11]. Various projects regarding multi-step

ethanol synthesis have been developed around the world by private

and state institutions. One of the companies that has patented

various process configurations is Enerkem, which in 2014

launched the first commercial plant for ethanol production from

municipal solid waste (MSW). Although the actual process

configuration for this plant is not known, it is possibly based on a

multistep configuration like that shown in Figure 3.2. In a

gasification island the MSW is converted to syngas, which after

conditioning and cleaning enters to the methanol island. Methanol

synthesis is a mature technology in the petrochemical sector, and

pure methanol can be obtained. The produced methanol is mixed

with carbon monoxide in a carbonylation and esterification reactor

unit, where methyl acetate is produced. Then, methyl acetate is

hydrogenated to methanol and ethanol, from which ethanol is

purified.

19

Figure 3.2. Block diagram for the multistep synthesis of ethanol by Enerkem [21].

3.2.3 Current projects for the thermochemical production

of ethanol

The thermochemical route for ethanol production is under

development at several companies. The current status of some of

these projects is shown in Table 3.2. With the operation of the

Edmonton plant in 2014 and the other projects to be in operation

soon, a relatively mature technology for the thermochemical

production of ethanol has been developed.

20

Table 3.2. Current projects for thermochemical production of ethanol.

COMPANY LABORATORY PILOT PLANT DEMOSTRATION PLANT COMMERCIAL PLANT

2002 – 2008 2009 – 2013 2014 –

Enerkem Alberta

Biofuels LP [22]

Sherbrooke University

(Canada).

Evaluation of:

- Pure compound feeding.

- Selectivity/ conversion.

- Recycling.

Sherbrooke plant

(Canada).

Evaluation of:

- Simulation of processes.

- PFD diagram.

- PID diagram.

- Process integration and

recycling.

Westbury facilities (Canada).

Evaluation of:

- Verify the results from the pilot

plant.

- Process integration and recycling.

- Preliminary cost estimation.

- Catalyst stability.

- Continuos operation 24h/day.

Biorefinería de Edmonton (Canada),

2014.

Evaluation of:

- Final equipment design and operation

conditions.

- Cost estimation.

- Production of 151 million liters of

ethanol / year.

Vanerco( Enerkem & Greenfield Ethanol) [13]

- Gasification and synthesis of fuels. Varennes (Canada), 2016.

- Under construction. - Ethanol production (30 000 tons /year).

Tembec Chemical Group [13]

- Demonstration plant: Gasification and synthesis of fuels - Ethanol (13 000 tons/ year)

Coskata [23] - Hybrid process:

gasification and

biochemical.

- Pilot plant in

Warrenville, Illinois (USA).

Demonstration plant in Madison,

Pennsylvania (USA).

- Ethanol production (120 tons/

year)

- Under operation for 21 months.

Ineos [24] * Hybrid process:

gasification and

biochemical.

* Pilot plant in operation

for 9 years.

21

3.2.4 Preliminary considerations for ethanol production

from biomass: the case of Bolivia

In this section Bolivia is presented as a particular example of

ethanol production from biomass. Bolivia has large forest

resources, which constitute an important source of biomass. A

preliminary estimation of biomass in the form of woody material,

animal manure and bagasse has been reported to be about 408

million tons [25]. On the other hand, about 1.9 million of tons of

waste are generated every year in all Bolivia‟s municipalities [26].

In spite of this, few projects have been implemented in order to

process the biomass. One of the few projects, was located in the

city of Montero which produced 600 000 liters/year of ethanol and

was commercialized in the European market (Germany, Spain and

Italy) [27]. Presumably, the production is based on first-generation

technology using residues of sugar cane as raw material.

A preliminary economic evaluation of the thermochemical

production of ethanol from biomass in Bolivia, has been made

[28]. A relevant variable in the production economy is the cost of

biomass, which in Bolivia is lower than for other regions; about 40

$US/ton in Bolivia compared to 60 $US/ton in Europe [18]. This

constitutes an attractive aspect of ethanol production in Bolivia. A

sensitivity analysis was also made, one of the most important

production variables was found to be the plant capacity; a

production greater than 200x103 ton of ethanol/year is desired to

reach a favorable economy of scale. A lower effect is estimated

from changes in the capital investment. The latter results are in

agreement with other studies made in other locations [18, 29, 30].

In Bolivia there is no specific regulation regarding the use of

alcohols as motor fuel, but a regulation for oxygenates compounds

indicates a maximum of 2.7 wt.% in a mixture with gasoline [31].

Also, the local market for transportation fuel is reduced; for

22

example, assuming 10 vt.% of ethanol blended with gasoline to be

sold in Bolivia this would require a production of about 102x103

ton/ year which does not take advantage of the economy of scale

[28]. Then, a larger percentage (>10 vt.%) of ethanol blended with

gasoline may be appropriate to justify ethanol production. On the

other hand, ethanol could be produced for the external market. In

the Latin American region most countries have a neutral balance

between production and consumption, with the exception of

Mexico which imports around 100 ktons/year of ethanol. Larger

markets such as the USA, Canada and Europe could be

investigated as potential markets.

23

CHAPTER 4

Catalytic conversion of synthesis gas to ethanol and

higher alcohols

4.1. Thermodynamic considerations

A thermodynamic evaluation of the basic reactions involved in the

synthesis of ethanol and higher alcohols from syngas is important

in order to know the limits of the reactions. If only alcohols, in the

range of C1OH to C8OH, are considered to be formed from syngas

(Figure 4.1. a) two comments arise: high syngas conversion is

expected at low temperature (<350 °C) and C5-8OH are the most

stable alcohols. When CO2 is allowed to be formed, via the water

gas shift reaction [32], together with the alcohols: a similar

composition profile is expected as shown in Figure 4.1. b, where

CO2 is an important product at intermediate temperatures (250-

550 °C). When CH4 is expected to be formed, via the methanation

reaction [32], the product profile change drastically (Figure 4.1. c);

almost no alcohols are formed and CH4 is favored up to around

520 °C. Finally, when both CO2 and CH4 are allowed to be formed,

shown in Figure 4.1. d; in this case, almost no alcohols are

expected to be formed in the whole range of temperatures.

Regarding the catalyst design for the synthesis of ethanol, some

ideas can be extracted from the analyzed profiles (Figure 4.1.): i)

the catalyst should have low reactivity to both methanation and

WGS reactions and ii) the catalyst should not promote the

formation of larger compounds (i.e. low activity for

polymerization).

24

Figure 4.1. Chemical equilibrium composition in syngas conversion to mixed

alcohols (C1–8OH) as function of the temperature (a) and the effect of: WGSR (b), methanation (c) and WGSR and methanation (d). Calculation conditions: 10 bar, H2/CO=2, initial mass flow for CO=28 g/h and for H2=4 g/h. Method based on the

minimization of Gibbs free energy using a PSRK equation of state (From Paper I). Adapted from [11] with permission from The Royal Society of

Chemistry.

4.2. Types of catalysts

Several catalysts have been proposed and tested for the synthesis

of ethanol and higher alcohols [11, 32, 33]. They can roughly be

classified as Rh-based, Cu-based, Co-based and Mo-based

catalysts. Among them, the Rh-based catalysts present the highest

selectivity to ethanol. The rest of the catalysts have similar

selectivity to ethanol, but higher activity (conversion of syngas per

catalyst volume). In the present work, the attention was focused on

the study of Rh and Cu based catalysts.

0

3

6

9

12

15

18

200 300 400 500 600 700 800

Mass r

ate

(g/h

)

T ( C)

CO

H2

H2O

CH4

C1-8OH

c)

0

3

6

9

12

15

18

200 300 400 500 600 700 800

Mass r

ate

(g/h

)

T ( C)

CH4 CO2 CO

H2

H2O

C1-8OH

d)

0

4

8

12

16

20

24

200 250 300 350 400 450 500

Mass r

ate

(g/h

)

T ( C)

CO

H2

H2O

C5-8OH

C1-4OH

a)

0

4

8

12

16

20

24

28

200 300 400 500 600 700

Mass r

ate

(g/h

)

T ( C)

H2O

CO2

C5-8OH

C1-4OH

CO

H2

b)

25

4.2.1. Rh-based catalysts

Several metal promoters have been tested with Rh in order to

increase both the selectivity and activity to ethanol. Among the

most promising metal promoters are Fe, La, V, Zr, Ce, and Mn [11].

Typically, they are combined in multi-promoted Rh-based

catalysts, for example, the Rh-La-V/SiO2 catalyst is able to increase

the ethanol selectivity from 16.7 % to 39 %, as compared to

unpromoted Rh/SiO2 [34]. Rh-Mn/SiO2 catalyst has shown a high

syngas conversion (42 %), but low ethanol selectivity (<9 %) [35].

Rh-Ce-Zr/SiO2 catalyst has showed both a high ethanol selectivity

and a high syngas conversion, that is 35 % and 27 %, respectively

[36]. Similar results were reported using a Rh-CeO2/TiO2 catalyst

showing a selectivity to ethanol of 33 % and a syngas conversion of

32 % [37].

4.2.2. Cu-based catalysts

Previous work has shown that Cu-based catalysts are less selective

for ethanol formation, and can produce a mixture of alcohols at

high syngas conversion. In this case, ethanol could be extracted

from the produced alcohols. Metal promoters such as Fe, Co, Ni,

La, Mn or Pd are active for increasing the yield to alcohols [11]. For

example, a catalyst containing Cu-Pd-Fe-Co metals, has shown a

syngas conversion as high as 84 % and an alcohol selectivity of 37

% with the following alcohol distribution: 26 % methanol, 38 %

ethanol, 27 % propanol and 9 % butanol [38].

4.2.3. Role of the catalyst’s support

The main role of the catalyst support is to increase and maintain

the dispersion of the metal active sites. The catalyst support should

be thermally stable and not suffer alterations during the catalytic

testing. Ideally, the catalyst support should not interact or affect

chemically the course of a reaction. However, it is know that a

26

catalyst support can affect the product distribution by different

mechanisms, such as increasing the acidity (e.g. aluminas or

zeolites), basicity (e.g. MgO or TiO2) or generating strong metal-

support interactions. Thus, it is desirable to have an inert catalyst

with no effect on the active sites of a catalyst. Among the typical

catalyst supports, silica (SiO2) is one of the most inert supports and

is widely used in different catalytic processes. In our work, we have

chosen a typical SiO2 and a novel mesoporous silica (MCM-41,

described in the following section) as catalyst supports.

4.2.3.1. Ordered Mesoporous Silica (MCM-41):

The “Atrane Route”

Ordered mesoporous silica was reported in the early of 1990 [39].

At that time, various kinds of mesoporous materials were originally

reported by Mobil Corporation scientists such as MCM-41 (Mobil

Composition of Matter No. 41), which exhibits 1D-cylindrical

hexagonal pores with amorphous walls; other related phases were

cubic (MCM-48) and lamellar (MCM-50). These structures are

known as the M41S family. Afterward, modified and new

mesoporous materials have been obtained by both academy and

industry [40]. Moreover, ordered mesoporous materials are being

applied with promising results in different fields [40-42];

chemistry, engineering, catalysis, health and others.

Generally, ordered mesoporous materials are obtained via the sol-

gel processes, with hydrolysis and condensation as main reactions

[43]. The novelty of the Mobil Corporation scientists was in the use

of assemblies of surfactant molecules as framework templates [44].

Surfactants are large organic molecules, such as

cetyltrimethylammonium bromide (C16H33)N(CH3)3Br (CTAB),

which have a hydrophilic head and a long hydrophobic tail of

variable length. In aqueous solutions, these species can form

different forms of micelles (spherical, cylindrical or higher–order)

depending on the solution conditions (pH, temperature, others). In

27

case of MCM-41, the surfactant (CTAB) packs together to form

cylindrical micelles.

Further Beck et al. [45] proposed two mechanisms for the

formation of MCM-41; „„liquid-crystal templating‟‟ (LCT) and self-

assembly (Figure 4.2). In the first mechanism, the structure is

defined by the organization of surfactant molecules into LC phases

and this serves as templates for the building of the MCM-41

structure. The inorganic silicate species would occupy the space

between a preformed hexagonal lyotropic crystal phase, and are

deposited on the micellar rods of the LC phase as shown in Figure

4.2, pathway 1. However, since the LC structures are very sensitive

to the characteristics of the solution, the same authors proposed

that the addition of inorganics could mediate the ordering of the

surfactants into specific mesophases (pathway 2). In both cases,

the inorganic species interact electrostatically with the charged

surfactant head groups and condense into a hexagonal array of

surfactant micellar rods embedded in an inorganic matrix.

Figure 4.2. MCM-41 formation via possible mechanistic pathways: (1) liquid

crystal phase initiated and (2) silicate anion initiated. Reprinted with permission from Beck et al. [45], Copyright (1992) American Chemical Society.

Ordered mesoporous materials have some remarkable properties

[46] such as: i) well-defined pore structure, ii) high thermal

stability, iii) large pore volume, and iv) high specific surface area.

Since these properties are desirable for a catalyst support, it has

28

been used in different catalytic reactions with interesting results

[41].

The “Atrane Route”

The hydrolysis and condensation of transition metals are generally

so fast that their control is difficult. This is due to the high

reactivity of the metal precursor either in the form of inorganic salt

or metal alkoxides [43]. In order to reduce the reactivity of the

metal precursor, an intermediate with low tendency to hydrolyse

and condense is required.

The atrane compounds are formed by two bridgehead atoms linked

by three three-atom moieties, with the general chemical formula of

EN(YCH2CH2)3R [47], where: E=metal; Y=”an element like

oxygen” and; R=an organic substituent or water molecule. A

schematic of an atrane compound (silatrane) is presented in Figure

4.3. This particular structure of atrane compounds makes it

possible to regulate the hydrolysis rate and thus the packing with

surfactant molecules is improved.

29

Figure 4.3. Schematic drawing of silica precursors for the synthesis of MCM-41:

a) silatrane like compound (R= organic substituent or water molecule) and, b) tetraethyl orthosilicate (TEOS). In the right part in both a) and b), the surface total

charge density (calculated by the Chem3D software [48]) is shown.

The synthesis of mesoporous materials using atrane compounds as

metal precursors basically involves five steps:

1. Preparation of atrane compound by transesterification of

a metal alkoxide with triethanolamine (TEA):

( ) ( ) ( ) ( )( ) (4.1)

Where M is metal (Si, Al, Ti, etc.). Various forms of atrane-like

compounds may be present in aqueous solution. For example, in

basic aqueous solution the following compounds have been found

for silatrane: SiNa(TEA)2H2+, Si(TEA)2H2, Si2Na(TEA)3H+,

Si3Na(TEA)4+ [49].

a)

b)

30

2. Surfactant (e.g. CTAB) addition to the mixture of atrane

(e.g. silatrane) and TEA. TEA acts as a co-solvent which

enhances the solubility of the CTAB.

3. Hydrolysis and condensation by addition of water to the

mixture of silatrane + CTAB + TEA. Cabrera et al. [49]

have studied the stability of the atrane compounds in

aqueous media; in the case of the siliatrane, the main

specie has been characterized as [Si(TEA)2(H3O)H2]+.

4. Aging in order to favor condensation reactions and thus

consolidate the meso-structures at long range

(gelification).

5. Elimination of the surfactant and TEA by thermal

treatment (calcination), leaving a solid mesoporous

structure (e.g. MCM-41).

Thus, silica mesoporous materials can be obtained via the atrane

route and most importantly, mesoporous material containing

several metals can be formed, corresponding to the following

formula: Mx-MCM-41 [49]. The latter is successfully achieved

because of the low reactivity of the atrane complexes towards

hydrolysis and condensation.

4.3 Reaction mechanisms

The reaction mechanism for ethanol and higher alcohols formation

is still under investigation. Regarding the Rh-based catalysts, some

mechanistic steps have been generally accepted [33, 50, 51],

schematically shown in Figure 4.4. The rate-limiting step of the

overall syngas conversion is the hydrogenation of surface CO* to

formyl species (HCO*) which is further hydrogenated to CH3O*. At

this point, this methoxy specie may either dissociate to CH3* and

O* or may be hydrogenated to form methanol (CH3OH). Methane

(CH4) is formed from the hydrogenation of CH3*. Ethanol

(CH3CH2OH) is formed from the CO insertion to CH3* and

subsequent hydrogenation steps.

31

Figure 4.4. Schematic mechanism for ethanol synthesis and main byproducts

formation (methanol and methane) over Rh-based catalysts, elaborated from

references [33, 50, 51].

Similar reaction pathways have been discussed for the formation of

ethanol and higher alcohols over Cu-based catalysts [11, 52, 53].

The higher alcohols (C2+-OH) are formed from consecutive C-C

coupling between formyl species and methyl species (CHx*) and/or

CO insertion. The higher alcohols distribution follows the

Anderson-Shultz-Flory (ASF) model. The ASF model is defined by

equation (4.2) [54]:

( ) (4.2)

(

)

( )

( ) (4.3)

where Sn is the selectivity to molecules with n carbon atoms and

is the chain growth propagation probability (independent of n).

The is obtained by least-squares linear regression of the

logarithmic form of the ASF equation (4.3). The is directly

related to the rate of chain propagation and inversely to the rate of

chain termination.

CO+1/2H2

CO*+H* HCO*+2H* CH3O*

dissociation

insertion

non-dissociation

CH3*+O*

CH3CO*

CH3OH

+ H*

+ H* CH4

+ 3H* CH3CH

2OH

+ CO*

33

CHAPTER 5

Experimental methods

5.1. Catalyst preparation

5.1.1. Mesoporous Silica MCM-41

The “Atrane Route” was used to prepare mesoporous silica MCM-

41 [49]. This method uses cetyltrimethylammonium bromide

(CTAB) as a structural directing agent, and 2,2′,2″-nitriletriethanol

(triethanolamine, TEA) as a complexing polyalcohol which

regulate the hydrolysis rate. The synthetic procedure is described

elsewhere [49, 55, 56]. The main steps for the preparation of

MCM-41 are shown in Figure 5.1. The atomic ratios are

4TEA:1Si:1Na:0.1CTAB:90H2O.

Figure 5.1. Main steps for the preparation of MCM-41 via the “Atrane Route”.

Elaborated from references [49, 55, 56].

Tetraethyl orthosilicate

(TEOS)Triethanolamine

(TEA)

- 130 ºC

- NaOH

SILATRANE

- Surfactant CTAB

- 60 ºC

- H2O

- Aging

(48h at 20ºC)

- Calcination

(6h at 550ºC)

MCM-41

- Washing/filtration

34

A commercial hexagonal MCM-41 was purchased from Sigma

Aldrich which was indistinctly used with the MCM-41 obtained

from the “Atrane Route”.

5.1.2. Metal impregnation

Rh/MCM-41 and Rh/SiO2 catalysts

The Rh/MCM-41 and Rh/SiO2 catalysts were prepared by

successive incipient wetness impregnation, using an aqueous

solution of RhCl3·nH2O. After impregnation, the catalysts were

dried at 120 °C for 6 h and then calcined at 500 °C for 5 h. The

total metal loading was 3 wt.% Rh for both the Rh/MCM-41 and

the Rh/SiO2 catalysts.

Cu-promoters/MCM-41 catalysts

Copper (from the precursor Cu(NO3)2·3H2O ) was incorporated

into MCM-41 by aqueous wetness impregnation using an excess of

water under constant stirring for 3 h. After impregnation, the solid

was dried at 120 °C for 3 h and then calcined at 500 °C for5 h to

obtain the Cu/MCM-41 catalyst. Then, potassium (from the

precursor K2CO3) was added to the Cu/MCM-41 catalyst following

the same procedure as for Cu addition and using the same thermal

treatment as described above. Iron (from the precursor

Fe(NO3)3·9H2O) was added to the Cu-K/MCM-41 catalyst also

following the same procedure as for the Cu addition.

5.2. Catalyst characterization

5.2.1. N2-Physisorption

N2-physisorption analyses were carried out using a Micromeritics

ASAP 2000 instrument and the Brunauer-Emmett-Teller (BET)

method was used to calculate the surface area and the Barrett-

Joyner-Halenda (BJH) method was used to calculate the pore size

and pore volume from the desorption isotherm.

35

5.2.2. X-Ray Diffraction (XRD)

Powder X-ray diffraction was performed using a Siemens D5000

instrument with Cu K-alpha radiation (2θ = 10°–90°, step size =

0.04°) equipped with a Ni filter and operated at 40 kV and 30 mA.

5.2.3. H2-Chemisorption

H2-chemisorption was carried out on a Micromeritics ASAP 2020

instrument. Prior to the analysis, the catalyst sample was reduced

with hydrogen. Repeated analyses were made in order to

discriminate between the amounts of hydrogen adsorbed via

physisorption and chemisorption.

5.2.4. Transmission Electron Microscopy (TEM)

TEM images were obtained using a JEOL JEM 1400 microscope.

Samples were mounted on 3 mm holey carbon copper grids. The

particle size and distribution were estimated after examination of

more than 100 metal particles.

5.2.5. Temperature Programmed Reduction (TPR)

TPR analysis was carried out in a Micromeritics Autochem 2910

instrument, a reducing gas mixture (5 % H2 in Ar) at a flow of 50

mL/min passed through the catalyst sample while the temperature

was increased by 5 °C/min up to 900 °C.

5.2.6. X-Ray Photoelectron Spectroscopy (XPS)

For XPS analysis, the data were recorded on 4 mm × 4 mm pellets

of 0.5 mm thickness, outgassed to a pressure below 2×10−8 torr at

150 °C in the instrument pre-chamber. The main chamber of a

Leybold-Heraeus LHS10 spectrometer is equipped with an EA-

200MCD hemispherical electron analyzer and a dual X-ray source

using AlKα (hv = 1486.6eV) at 120 W, 30 mA, with C(1s) as energy

reference (284.6 eV).

36

5.2.7. Thermogravimetric analysis (TGA)

TGA was carried out using a SETARAM (TG-DTA-DSC 1600 °C).

The sample was heated up to 400 °C at a rate of 5 °C/min in a

continuous flow of He. The differential thermo-gravimetric (DTG)

curves were estimates from the TGA data.

5.3. Catalytic testing

5.3.1. Reactor set-up and procedures

The catalytic testing was carried out in a stainless-steel, down-flow

fixed bed reactor. The internal diameter of the reactor was 8.3 mm,

into which about 300 mg of catalyst was charged with a particle

size between 160 and 250 μm. Prior to the testing, the catalyst was

reduced with H2. After reduction, the reactor was cooled to 280 °C

and pressurized with syngas. The gas hourly space velocity (GHSV)

was varied between 3 000 and 19 000 mLsyngas/gcath. Premixed

syngas bottles with a H2/CO ratio of 2:1 and 1:1 were used. Water

addition was regulated by means of a Gilson 307 pump, and joined

to the syngas feed stream. In all the experiments, the axial

temperature gradient through the catalyst bed (measured by a

mobile thermocouple introduced in a thermowell inside the

catalyst bed) was always less than 1 °C. The catalytic set-up has

been described elsewhere by Andersson et al. [57], from which a

schematic drawing is shown in Figure 5.2.

37

Figure 5.2. Schematic overview of the gas feed, high pressure reactor and

analysis system connections. Main parts are from left to right: gas bottles, mass flow controllers, hot box preheating (and post heating), high-temperature reactor, pressure controller, and heated line 1, GC1. Reprinted from Andersson at al. [57],

Copyright (2012), with permission from Elsevier.

Preliminary estimations were carried out to ascertain transport

effects on the mesoporous catalyst (Rh/MCM-41). The results

indicated that, at the experimental conditions used in this study,

intraparticle diffusion limitation might begin to occur at high

syngas conversion (>70 % in terms of CO conversion), according to

the Weisz-Prater‟s criterion [58]. An additional experiment was

carried out at 230 °C, 20 bar and 40 000 mLsyngas/gcat h (H2/CO =

2/1) using a catalyst sample with a fixed number of active sites (3

wt.% Rh and a metal dispersion of H/Rh = 22 %, Paper III). A

fraction of this sample was diluted with pure MCM-41 support, at a

ratio of 1:3, resulting in a catalyst with nominal composition 0.75

wt.% Rh and a metal dispersion of H/Rh = 22 %. The results

indicate a minor variation (<10 %) between the TOFs of the non-

diluted catalyst and the diluted catalyst, suggesting that the

38

reaction operates in the kinetic regime according to the Koros-

Nowak criterion [58].

5.3.2. Product analysis and data treatment

An on-line GC, Agilent 7890A, was used to quantify the reaction

products. A detailed description of the GC configuration and the

analytical procedure has been published previously by Andersson

et al. [57]. N2 was added to the syngas mixture and used as internal

standard to quantify CH4, CO and CO2 in a thermal conductivity

detector. The hydrocarbon and oxygenated compounds were

analyzed in a flame ionization detector using the internal

normalization of corrected peak areas method. In all the

experiments the carbon balance was between 99.1-100 %.

The expressions used to evaluate the results are the following [57]:

- Syngas conversion (in terms of CO conversion):

( )

(

) (5.1)

- Selectivity to CO2:

( )

( )

( ) (

)

(5.2)

Because CH4 is detected by both FID and TCD detectors, but CO2

only by the TCD detector, the CH4 in the FID is equivalent to the

CH4 in the TCD without CO2 (CO2-free).

- Selectivity to CH4 (CO2-free) by FID:

( )

∑ ( )

(5.3)

39

Where Rij is the corrected response factor (relative to methane or a

hydrocarbon compound). The selectivity to the other organic

compounds measured by FID is calculated using the above

equation replacing CH4 by an organic compound (SCorg, FID).

- Selectivity to CH4 (CO2-free) by TCD:

( )

(

)

( )

⁄

(

)

(

)

⁄

(5.4)

- Material Balance:

The total flow (in terms of carbon moles) that enters the reactor

should be equal to the total flow that leaves the reactor, assuming

no carbon accumulation inside the reactor. This relation is given

below and measures the carbon material balance (MB) for each

catalytic testing.

( )

(5.5)

41

CHAPTER 6

Catalytic performance of a typical Rh/SiO2 catalyst

and a mesoporous Rh/MCM-41 catalyst

6.1. Product distribution and selectivity to C2-oxygenated

compounds

An examination study of the catalytic performance of the

mesoporous Rh/MCM-41 catalyst shows a different product

distribution as compared to a typical Rh/SiO2 catalyst, Figure 6.1.

In addition to the selectivity to each compound is being different,

the total selectivity to the main groups of compounds, i.e.

hydrocarbons (methane and higher hydrocarbons) and oxygenated

compounds (methanol, carbon dioxide, ethanol, acetaldehyde,

acetic acid, methyl acetate and ethyl acetate), is also different.

Thus, for the mesoporous Rh/MCM-41 catalyst the total selectivity

to hydrocarbon and oxygenated compounds is 43.4 % and 56.6 %,

respectively. For the Rh/SiO2 catalyst the total selectivity to

hydrocarbon compounds (58.2 %) is higher than the selectivity to

oxygenated compounds (41.8 %). The higher selectivity to

hydrocarbons than to oxygenated compounds is characteristic for

the Rh/SiO2 catalyst [59] and, for that reason the addition of metal

promoters was investigated in order to increase the selectivity

towards oxygenated compounds [32, 33].

42

Figure 6.1. Product selectivities obtained from the Rh/SiO2 and Rh/MCM-41

catalysts. Higher hydrocarbons: ethane, propane and butane. Acetate compounds: methyl acetate and ethyl acetate. Reac. Cond.: 280 °C, 20 bar, GHSV=12000 mlsyngas/gcat h for Rh/SiO2 and GHSV=3000 mlsyngas/gcat h for

Rh/MCM-41 (From Paper III).

An interesting result was obtained regarding the selectivity to the

C2-oxygenated compounds (ethanol, acetaldehyde and acetic acid),

where the selectivity to ethanol for the mesoporous Rh/MCM-41

catalyst is three times that of the typical Rh/SiO2 catalyst, that is

24 % and 8 %, respectively. Still, the total selectivity to C2-

oxygenated compounds has a similar value for both catalysts; 34 %

for Rh/SiO2 and 32 % for Rh/MCM-41. This result suggests a

different catalytic performance of the mesoporous Rh/MCM-41

catalyst. In order to verify this observation, additional experiments

were conducted changing the reaction conditions for the

Rh/MCM-41 catalyst: i) syngas conversion, ii) pre-reduction

treatments, iii) water addition and iv) hydrogen partial pressure.

0

3

6

9

12

15

18

21

24

27

30

33

36

39

42

45

Methane Higherhydrocarbons

Carbondioxide

Methanol Ethanol Acetaldehyde Acetic acid Acetatecompounds

Sele

ctiv

ity

(%)

Rh/SiO2 catalyst

Rh/MCM-41 catalyst

Rh/SiO2

Rh/MCM-41

43

6.1.1. Effect of syngas conversion

It is known that the product distribution depends, in some degree,

on the syngas conversion [32, 33]. Thus, both Rh/MCM-41 and

Rh/SiO2 catalysts have been tested in a large range of syngas

conversion, as can be seen in Figure 6.2. The range of syngas

conversion has been obtained by varying the reaction conditions

(temperature, pressure and gas hourly space velocity). At low

syngas conversions, the selectivity to oxygenated compounds for

Rh/MCM-41 is higher than for Rh/SiO2, in agreement with

previous results. At high syngas conversion, the hydrocarbon

compounds dominate, in agreement with thermodynamic

equilibrium at high temperature (see section 4.1). The most

notable difference in the product distribution obtained from

Rh/MCM-41 and Rh/SiO2 catalysts, is the selectivity to CO2.

Almost zero selectivity to CO2 is found over the Rh/SiO2 catalyst

while for the Rh/MCM-41 catalyst the CO2 selectivity varies from

16 % (at low syngas conversion) up to 57 % (at high syngas

conversion). It is known that the formation of CO2 is

thermodynamically possible and favored at high temperatures (see

section 4.1).

Figure 6.2. Effect of the catalyst reduction temperature on the product selectivity

over Rh/MCM-41 and Rh/SiO2 catalysts. Catalysts pre-reduced at 200 °C (solid lines) and nonreduced catalysts (segmented lines) (From Paper IV). Reprinted

from Lopez at al. [60], Copyright (2015), with permission from Elsevier.

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70

Sele

ctiv

ity

(%)

syngas conversion (%)

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50

Sele

ctiv

ity

(%)

Syngas conversion (%)

Hydrocarbons CO2 Oxygenates

Rh/MCM-41 Rh/SiO2

44

6.1.2. Effect of the catalyst reduction temperature

As can be seen in Figure 6.2, different product distributions are

found as the catalyst reduction temperatures are changed, in both

Rh/MCM-41 and Rh/SiO2 catalysts. This again evidences the

different reactivity of the mesoporous Rh/MCM-41 catalyst

compared to the typical Rh/SiO2 catalyst. Regarding the selectivity

to the C2-oxygenated compounds, the ethanol selectivity is much

higher in the mesoporous Rh/MCM-41 catalyst than in the typical

Rh/SiO2 catalyst, independently of the syngas conversion and the

catalyst reduction temperature, Table 6.1.

Table 6.1. Distribution of C2-oxygenates for Rh/MCM-41 and Rh/SiO2 catalysts as

function of the catalyst reduction temperature and syngas conversion (From Paper IV). Reprinted from Lopez at al. [60], Copyright (2015), with permission

from Elsevier.

Catalyst Reduction

Temperature

Range of syngas

conversion (%)

C2-oxygenate ratio

Ethanol /

Acetaldehyde

Ethanol /

Acetic Acid

Rh/MCM-41

Not reduced 1.5 - 68% 6 – 17 1 – 12

200°C 0.8 - 58% 5 – 11 10 – 27

500°C 0.6 - 64% 7 – 13 4 – 13

Rh/SiO2

Not reduced 0.9 - 33% 0.7 – 2 1 – 2

200°C 5.9 - 45% 0.6 – 2 1 – 22

500°C 4.4 - 28% 0.5 – 1 1 – 4

6.1.3. Effect of hydrogen partial pressure

The partial pressures of the syngas feed stream (PH2 and PCO) can

modify the product distribution [32, 33]. Thus, the evaluation of

the syngas composition (H2/CO) is important in order to elucidate

the reactivity of the mesoporous Rh/MCM-41 catalyst. In Figure

6.3, the product distribution obtained at low syngas ratio

(H2/CO=1) is compared to the results obtained at higher syngas

ratio (H2/CO=2). The lowering of the syngas ratio decreases the

selectivity to methane and methanol, while in contrast, the

selectivities to higher hydrocarbons, carbon dioxide, and acetate

increases. The latter indicates that compounds with high H/C

45

ratios (like methane or methanol) are favored at high syngas ratios

(high hydrogen partial pressure), possibly by increasing the

occurrence of hydrogenation reactions.

Among the C2-oxygenated compounds, the selectivity to the more

oxidized compound (i.e. acetic acid) is decreased at high syngas

ratio (i.e. high hydrogen partial pressure). In the Rh/MCM-41

catalyst ethanol and acetaldehyde are favored by increasing the

syngas ratio.


catalysts. Reac. Cond. (syngas H2/CO=1): 280 °C, 20 bar, GHSV=6000 mlsyngas/gcat h for Rh/SiO2 and GHSV=3000 mlsyngas/gcat h for Rh/MCM-41. Reac. Cond. for the experiments with syngas H2/CO=2 as indicated in Figure 6.1 (From

Paper III).

6.1.4. Effect of water addition

Typically, Rh-based catalysts do not form CO2 as a main product

[32, 33]. However, the mesoporous Rh/MCM-41 has shown a

high selectivity to CO2 as reported in section 6.1.1. An additional

0

3

6

9

12

15

18

21

24

27

30

33

36

39

42

45

48


Carbondioxide


Sele

ctiv

ity

(%)

Rh/SiO2, syngas ratio: 2

Rh/SiO2, syngas ratio: 1

Rh/MCM-41 catalyst

Rh/MCM-41, syngas ratio: 1

Rh/SiO2 (H2/CO=2)

Rh/SiO2 (H2/CO=1)

Rh/MCM-41 (H2/CO=2)

Rh/MCM-41 (H2/CO=1)

46

reaction taking place during many catalytic processes, is the

water gas shift reaction (WGS) which consumes CO and H2O to

generate CO2 and H2 [32, 33]. If the WGS reaction would occur

over the Rh active sites in the mesoporous Rh/MCM-41 catalyst,

it should also occur in the Rh/SiO2 if water is added to the syngas

feed stream. Figure 6.4 shows the obtained product distributions

found when water is added to the syngas feed stream in both

catalysts. It can be observed that CO2 is produced for the typical

Rh/SiO2 catalyst by water addition. Interestingly, the selectivity

to methanol is also increased by water addition. The latter may be

related to the conversion of CO2 to methanol, as known in

literature [61].


catalysts. Reac. Cond. (with addition of water): 280 °C, 20 bar, GHSV=6000 mlsyngas/gcat h (H2:CO:H2O=2:1:2.7) for Rh/ SiO2 and GHSV=3000 mlsyngas/gcat h (H2:CO:H2O=2:1:1.5) for Rh/MCM-41. Reac. Cond. for the experiments without

addition of water as indicated in Figure 6.1 (From Paper III).

0

10

20

30

40

50

60

70

80

90


Carbondioxide


Sele

ctiv

ity

(%)

Rh/SiO2, feed stream: syngas

Rh/SiO2, feed stream: syngas + water

Rh/MCM-41 catalyst

Rh/MCM-41, feed stream: syngas + water

Rh/SiO2 (only syngas)

Rh/SiO2 (syngas+water)

Rh/MCM-41 (only syngas)

Rh/MCM-41 (syngas+water)

47

6.2. Catalyst characterization before and after catalytic testing

Extensive catalyst characterization has been performed for both

the mesoporous Rh/MCM-41 and the typical Rh/SiO2 catalysts in

order to understand the catalytic results. Particular attention was

given to the characterization after the catalytic testing.

6.2.1. Specific surface area and pore size distribution

The surface area of the mesoporous Rh/MCM-41 catalyst is almost

four times the surface area of the typical Rh/SiO2 catalyst. The

high surface area of the mesoporous catalysts is due to the small

1D-porous structure. In the case of Rh/MCM-41 the average pore

size is 2.5 nm. In contrast, the typical Rh/SiO2 catalyst does not

present any structural ordering. When both catalysts were tested

without water addition to the syngas feed stream, their surface

areas were not really affected. The surface area decreased by 9.0 %

in the Rh/MCM-41 catalyst and by 3.7 % in the Rh/SiO2 catalyst

(Table 6.2). More significant changes in the surface area values

were observed when the catalytic tests were carried out with water

addition in the syngas feed stream. In this case, the surface area

values decreased by 19.3 % for the Rh/MCM-41 catalyst and by

10.3 % for the Rh/SiO2 catalyst. Partial structure degradation may

occur in the Rh/MCM-41 catalyst in presence of an excess of water;

possibly the hydration of the siloxane structure and siloxane

hydrolysis-hydroxylation [62]. This could result in the collapse of

some 1D-channels into a single one.

48

Table 6.2 . N2-physisorption and XPS analyses of the Rh/SiO2 and Rh/MCM-41 catalysts, before and after the catalytic testing (From Paper III).

Catalyst Condition Surface

area (m2/g)

Change in

surface area

Pore volume

(cm3/g)

Rh0 Rh

3+

Rh/Si % B.E. (eV) % B.E. (eV)

Rh/SiO2 Pure support 238 - 0.87 - - - - -

Before catalytic testing 236 0.5 %* 0.90 86.5 307.3 13.4 309.7 0.0057

After catalytic testing

(280 °C, 20 bar, 12000

ml/gh):

only syngas

228 3.7 %** 0.90 82.4 307.3 17.6 309.7 0.0064


(280 °C, 20 bar, 6000

ml/gh):

syngas and water

212 10.3 %** 0.88 100 307.2 - - 0.0079

Rh/MCM-41 Pure support 970 - 1.08 - - - - -

Before catalytic testing 961 0.9 %* 1.15 80.6 307.1 19.4 308.6 0.0079


(280 °C, 20 bar, 3000

ml/gh):

only syngas

875 9.0 %** 1.04 90.8 307.4 9.2 309.3 0.0087


(280 °C, 20bar, 3000

ml/gh):

syngas and water

776 19.3 %** 0.94 100 307.1 - - 0.0100

* Compared to the pure support. ** Compared to the catalyst before catalytic testing.

49

6.2.2. Surface composition: Rh0 and Rh3+

The catalytic surface of the Rh/MCM-41 and Rh/SiO2 catalysts

consists of metallic Rh (Rh0) and tri-valent Rh (Rh3+). In both

catalysts, before and after the catalytic testing the proportion of

Rh0 to Rh3+ was kept in the same range as can be seen in Table 6.2.

Moreover, after the catalytic testing the increased Rh/Si atomic

ratio indicates that Rh species migrate to the outer surface in both

catalysts.

6.2.3. Porous ordering by XRD and TEM

The long-range periodic structure of the mesoporous Rh/MCM-41

catalyst is evidenced by its characteristic reflections (100), (110),

and (200) in the XRD diffractogram at low angles (Figure 6.5),

which corresponds to a hexagonal structure [39]. Both Rh/MCM-

41 and Rh/SiO2 catalysts are amorphous materials with no

segregation of rhodium oxide species. After the catalytic testing

(with no addition of water to the syngas feed stream) no changes in

the surface area values were observed in either catalysts. However,

when the catalytic testing was done in presence of excess water, a

peak at around 2θ=35–40, is observed. This peak can be

attributable to the growth of Rh0 clusters in accordance with the

XPS results (section 6.2.2.).

50

Figure 6.5. Small and wide angle XRD patterns of pure SiO2 and MCM-41

supports, impregnated with Rh A, after catalytic testing B, and after catalytic

testing with addition of water C (From Paper III).

In the case of the mesoporous Rh/MCM-41 catalyst, Rh species are

dispersed within the 1D-channel pores as observed by TEM (Figure

6.6). The average Rh particle size for the Rh/MCM-41 catalyst and

the Rh/SiO2 catalyst is around 3 and 4 nm, respectively. These

results are in agreement with the particle size estimated from H2-

chemisorption, which gives an average particle size of 3 nm for

both catalysts (metal dispersion H/Rh=22 %) assuming an

icosahedral particle shape [63].

1 2 3 4 5 6 7 8 9 10

2 (degrees)

(100)

(200)

(110)

MCM-41

SiO2

Rh/MCM-41 B

Rh/MCM-41 A

A

Rh/MCM-41 C

15 30 45 60 75 90

2 (degrees)

Rh/MCM-41

Rh/SiO2

51

Figure 6.6. TEM images of Rh/SiO2 and Rh/MCM-41 catalysts; reduced A,

after catalytic testing B, and after catalytic testing with addition of water C (From Paper III).

6.2.4. Metal support interactions

According to previous work, metal support interactions can occur

in both mesoporous and other types of catalysts [41, 64]. In our

case, the broad signal found in the TPR profile from the

Rh/MCM-41 catalyst, Figure 6.7, suggests some metal support

interaction. In contrast, the typical Rh/SiO2 catalyst shows a well-

defined signal in its TPR profile, suggesting a minor or negligible

metal support interactions.

52

Figure 6.7. Temperature-programmed reduction profiles of the Rh/SiO2 and

Rh/MCM-41 catalysts (From Paper III).

6.3. Interpretation of the catalytic performance of Rh/MCM-41

compared to Rh/SiO2

The novel mesoporous Rh/MCM-41 catalyst has shown a different

reactivity in the conversion of syngas, as compared to typical a

Rh/SiO2 catalyst. In this section, an explanation for the different

product selectivity obtained from the Rh/MCM-41 catalyst is

proposed. This explanation has been based on; i) additional

catalytic experiments (effect of hydrogen partial pressure and

addition of water to the syngas feed stream), and ii) the exhaustive

characterization before and after the catalytic testing.

6.3.1. Water vapor concentration in mesoporous

Rh/MCM-41 catalyst

The following aspects must to be considered in order to interpret

the product distribution obtained in the mesoporous Rh/MCM-41

catalyst:

30 60 90 120 150 180 210 240

H2

-C

on

sum

pti

on

Temperature (°C)

G

C

Rh/SiO2

Rh/MCM-41

53

Some metal-support interactions seem to be present in the

Rh/MCM-41 catalyst, but not in the Rh/SiO2 catalyst (as

suggested from TPR analysis). However, the Rh surface

composition (in terms of Rh0 and Rh3+) is similar in both

catalysts (as indicated from XPS analysis), which could

result in similar reactivity for both catalysts. Thus, a minor

or negligible effect of the metal-support interactions could

be related to the different product selectivity obtained in

the Rh/MCM-41 catalyst.

In both catalysts, the Rh particle size is kept around the

same value (3-4 nm); as obtained from H2-chemisorption

and confirmed by TEM, thus no effect of particle size on the

product distribution should be expect.

The 1D-channels with small pore diameter of the

mesoporous MCM-41 seem to be retained in the Rh/MCM-

41 catalyst (as suggested from N2-physisorption, XRD and

TEM analyses), which is an important and different

characteristic as compared to the typical Rh/SiO2 catalyst.

This aspect may possibly be mainly responsible the

different product distribution found for the Rh/MCM-41

catalyst. The proposed role of the 1D-channels of Rh/MCM-

41 in the product distribution is further discussed in the

following paragraphs.

During the preparation of mesoporous catalysts some metal-

support interactions are favored by the concentration of water

vapor in the mesopores [41, 65]. Consequently, for the reactions

involved in the catalytic conversion of syngas producing water as a

main product [32, 33], reactions (6.1-6.4), some water vapor can

be concentrated in the mesopores of the Rh/MCM-41 catalyst.

Ethanol generation (6.1)

Methanation (6.2)

54

Hydrocarbons formation (6.3)

Methanol synthesis

(6.4)

This environment rich in water vapor in the mesopores of

Rh/MCM-41 may induce some reactions, such as the water-gas-

shift-reaction (WGSR).

Water gas shift reaction (6.5)

WGSR generates extra carbon dioxide and hydrogen (reaction 6.5).

In agreement, the selectivity to CO2 is much higher in Rh/MCM-41

than in Rh/SiO2, independently of the syngas conversion and

catalyst reduction temperature (section 6.1.1.). Furthermore, when

water is added to the syngas feed stream the selectivity to CO2 is

notably increased in both catalysts; from almost zero to 20 % in

Rh/SiO2 and90 % in Rh/MCM-41.

It is generally accepted that methanol synthesis from syngas take

places via the hydrogenation of CO2 [61]. So, if CO2 is generated in

Rh/MCM-41, methanol can be part of the products, which is true

(Figure 6.1). In agreement, methanol is also produced by the

Rh/SiO2 catalyst when water is added to the syngas feed stream.

6.3.2. Higher ethanol selectivity as consequence of the

water vapor concentration in mesoporous Rh/MCM-41

catalyst

According to the proposed WGSR occurrence in the mesoporous

Rh/MCM-41 catalyst, more H2 can be produced. Which, in

addition to the H2 from the syngas feed stream may induce some

hydrogenation reactions. It may explain the distribution between

the C2-oxygenated compounds, where a higher selectivity to the

55

more hydrogenated compounds, i.e. ethanol and acetaldehyde, is

found in the Rh/MCM-41 catalyst (92 %) than in the Rh/SiO2

catalyst (75 %). The hydrogenation reactions in Rh/MCM-41 may

convert acetic acid or a C2-oxygenated intermediary, perhaps an

acetyl intermediate [35], to acetaldehyde and ethanol. Moreover,

the selectivity to ethanol is higher than the selectivity to

acetaldehyde (Figure 6.1 and Table 6.1). The latter may occur via

the hydrogenation of acetaldehyde to ethanol, as suggested in the

literature [66]. A schematic representation of the hydrogenation

steps is shown in Figure 6.8.

Figure 6.8. Schematic representation of the hydrogenation of the C2-oxygenated

compounds. Author’s own elaboration.

The hydrogen partial pressure affects the distribution between the

C2-oxygenated compounds. The selectivities to ethanol and

acetaldehyde have been favored at high hydrogen partial pressure

(varying H2/CO2), while the selectivity to acetic acid is increased

at low hydrogen partial pressure as shown in Figure 6.3 for the

Rh/MCM-41 catalyst. Indeed, the selectivity between the C2-

oxygenated compounds over the Rh/MCM-41 catalyst can be

related to the degree of oxidation: ethanol > acetaldehyde > acetic

acid.

Regarding the selectivity to hydrocarbons and acetate compounds

(Figure 6.1), they may be understood by exploring the reaction

mechanism proposed by Bowker [50], where acetate compounds

are formed by the reaction of CO2 with CHx* species. Following this

reaction mechanism, the additional CO2 formed via WGSR would

consume much more CHx* species in the Rh/MCM-41 catalyst than

in the Rh/SiO2 catalyst. This means that a lower number of CHx*

OH

H

H

H

H

H

CH3

O

H

CH3

O

OH

H2 H2+ +

Acetaldehyde EthanolAcetic acid

56

species would be available for conversion into hydrocarbon

compounds and more acetate compounds should be expected from

the Rh/MCM-41 catalyst. In fact, lower selectivity to hydrocarbon

and higher selectivity to acetate compounds are found over

Rh/MCM-41 as compared to Rh/SiO2 (Figure 6.1).

57

CHAPTER 7

Effect of Fe and K promoters on Cu/MCM-41 catalyst

In this chapter, a mesoporous Cu/MCM-41 catalyst has been

studied for the synthesis of ethanol. In addition to the

mesoporosity of the catalyst the effect of metal promoters such as

Fe and K, have also been studied.

7.1. Catalyst characterization: XRD, TGA, N2 physisorption,

TEM, XPS and TPR

CuO is present in all catalysts; Cu/MCM-41, Cu-Fe/MCM-41, Cu-

K/MCM-41 and Cu-Fe-K/MCM-41 (XRD pattern in Fig. 1., Paper

V). The Fe and K promoters have not been detected by XRD,

indicating low metal segregation of these promoters. Minor mass

loss in the all catalysts (1.5-2.5 wt.% up to 400 °C) has been

ascribed to dehydroxylation reactions (TGA profiles in Fig. 6.,

Paper V), which indicates that the catalysts are thermally stables.

The Cu/MCM-41 catalyst keeps the mesoporous structure of the

MCM-41 support (as suggested from the N2 physisorption

isotherms in Figure 7.1. and TEM analysis in Paper V). However,

in presence of the Fe or K promoters, the mesoporosity of the

support disappears due to collapsing of the pores, which results in

the reduction of the surface area as can be seen in Table 7.1.

Further degradation of the pores occurs when both Fe and K are

impregnated simultaneously and, as a consequence there is a

further decrease of the surface area to 72 m2/g.

58

Figure 7.1. BJH pore size distribution and (inset) N2-isotherm of the prepared catalysts (From Paper V). Reprinted from Lopez at al. [67], Copyright (2016), with

permission from Elsevier.

For all the catalysts, the surface composition consists of Cu0 and

Cu+1 sites as indicated by XPS analysis (Table 7.1). By adding the

metal promoters, the surface atomic ratios are changed (Table 7.1).

When Fe is added to the Cu/MCM-41 catalyst, the ratio Si/Cu is

increased, which indicates that either Fe atoms cover some Cu

particles or some Cu atoms migrate from the outside to the core of

the catalyst particle. While when K is added the Si/Cu ratio is kept

low, suggesting a better Cu dispersion. The latter may be related to

the literature suggesting that K stabilizes some Cu sites,

presumably Cu+ species [68, 69].

0.0

0.3

0.6

0.9

1.2

1.5

1.8

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

dV

/dlo

gD (

cm3/g

nm

)

Pore diameter (nm)

0

100

200

300

400

500

0.0 0.2 0.4 0.6 0.8 1.0V

olu

me

ad

sorb

ed

(cm

3 /g)

P/Po

MCM-41 Cu/MCM-41 Cu-K/MCM-41 Cu-Fe/MCM-41 Cu-Fe-K/MCM-41

59

Table 7.1. Physico-chemical properties of the prepared catalysts (From Paper V). Reprinted from Lopez at al. [67], Copyright

(2016), with permission from Elsevier.

Catalyst

BET

surface

area

(m2/g)

Pore

volume

(cm3/g)

Average

pore

size

(nm)

Average metal

particle size by

TEM (nm)

Metal loading (%) by AAS

Atomic ratios by XPS

Cu K Fe

Si/Cu Cu/K Cu/Fe Fe/K

MCM-41 710 0.49 2.9 - - - - - - - -

Cu/MCM-41 375 0.24 3.2 4.8 28.89 - - 2.8 - - -

Cu-K/MCM-41 208 0.26 5.1** 5.0* 29.79 1.53 - 2.9 13.0 - -

Cu-Fe/MCM-41 200 0.32 6.6** 5.0* 26.82 - 2.28 7.0 - 4.3 -

Cu-Fe-K/MCM-41 72 0.27 14.8** 6.1* 29.30 1.65 2.07 2.9 11.7 5.3 2.2

* Average of all the metals. ** Average of the interparticle space.

60

The potential metal-support interactions in the Cu/MCM-41

catalyst are partially eliminated or weakened by the presence of K,

as deduced from the reduction temperature shift from 245 °C to

210 °C observed in the TPR prolife of this catalyst (Figure 7.2.). For

Fe (Cu-Fe/MCM-41 and Cu-Fe-K/MCM-41) the resulting catalysts

show a new peak between 335 °C and 350 °C: that can be attributed

to the reduction of Fe2O3→Fe3O4 [70]. For these catalysts, CuO is

reduced at higher temperatures than in the Cu/MCM-41 and Cu-

K/MCM-41 catalysts, which indicates some electronic effects

induced by Fe.

Figure 7.2. TPR profiles of the prepared catalysts (From Paper V). Reprinted


7.2. Catalytic testing: product selectivity trends

The mesoporosity of the Cu/MCM-41 catalyst could induce the

increased concentration of water vapor as occured in the

Rh/MCM-41 catalyst (Chapter 6). This phenomenon is supported

by the results obtained from the product distribution. This

distribution shows that the selectivity to oxygenated compounds is

higher than the selectivity to hydrocarbon compounds. The

opposite selectivity trend for hydrocarbon and oxygenated

130 170 210 250 290 330 370 410 450

Temperature (°C)

Cu-K/MCM-41Cu-Fe/MCM-41

Cu-Fe-K/MCM-41

Cu/MCM-41

61

compounds is found in typical Cu-based catalysts [71, 72]. The

concentration of water vapor can promote the occurrence of the

WGSR which in turn promotes secondary reactions such as

hydrogenation reactions (as explained in Chapter 6 in the case of

Rh-based catalysts).

Figure 7.3. Product selectivity trends obtained from the prepared catalysts. Reac.

Cond.: syngas H2/CO ratio: 2/1, 300 °C, 20 bar and a GHSV of 1500 mlsyngas/gcat h for the Cu/MCM-41 and Cu-K/MCM-41 catalysts and 30000 mlsyngas/gcat h for the Cu-Fe/MCM-41 and Cu-Fe-K/MCM-41 catalysts (From Paper V). Reprinted


Due to the pores collapsing of the Cu/MCM-41 catalyst when K and

Fe are added (Figure 7.1., Table 7.1.), minor or negligible water

vapor concentration may be expected. For future work, another

method of catalyst preparation could be used in order to keep the

structure of mesoporous MCM-41. It is known that a narrow metal

particle size distribution (2 nm) can be obtained by novel

techniques such as the so-called microemulsion [73]. This

technique could be used to introduce the metal promoters inside

the mesoporous (2.9 nm) of MCM-41.

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Sele

ctiv

ity

and

co

nve

rsio

n (

%)

Time on stream (h)

Cu-Fe-K/MCM-41

0 5 10 15 20 0 5 10 15 0 5 10 15 0 5 10 15 20

Cu/MCM-41 Cu-K/MCM-41 Cu-Fe/MCM-41

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

Sele

ctiv

ity

and

co

nve

rsio

n (

%)

Time on stream (h)0 5 10 15 0 5 10 0 5 10 0 5 10 15 20 25

Cu Cu-K Cu-Fe Cu-Fe-K

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

Sele

ctiv

ity

and

co

nve

rsio

n (

%)

Time on stream (h)0 5 10 15 0 5 10 0 5 10 0 5 10 15 20 25


0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

Sele

ctiv

ity

and

co

nve

rsio

n (

%)

Time on stream (h)0 5 10 15 0 5 10 0 5 10 0 5 10 15 20 25


Hydrocarbons

Oxygenates

Conversion

62

7.2.1. Effect of K as promoter

The addition of K to the Cu/MCM-41 catalyst results in a notable

increase of the selectivity to oxygenated compounds, with a value

equal to 92 % for all the products (Figure 7.3). The effect of the K

promoter can be related to two main actions: i) stabilizing the

active sites, presumably the Cu+ species [68, 69], which can be

related to the lower reduction temperature of the Cu-K/MCM-41

catalyst compared to the Cu/MCM-41 one (Figure 7.2) and ii)

blocking some acid sites thus decreasing the selectivity to

hydrocarbons [74]. Moreover, some metal-support interactions are

present in the Cu/MCM-41 catalyst, which are weakened or

eliminated by the presence of K (Figure 7.2). This suggests that the

metal-support interactions have a minor effect on the formation of

oxygenated compounds.

7.2.2. Effect of Fe as promoter

As opposed to the action of the K promoter, the addition of Fe to

Cu/MCM-41 catalyst promotes the selectivity towards hydrocarbon

compounds (Figure 7.3). The effect of Fe can be related to its

action in the Fischer-Tropsch synthesis. There the formation of

hydrocarbons is believed to occur via CO dissociation to produce

C* (or *CH) radicals, which react to form C-C bonds and are

subsequently hydrogenated [11].

7.2.3. Effect of K and Fe addition

When both K and Fe promoters are added to the Cu/MCM-41

catalyst, the selectivity to hydrocarbons and oxygenated

compounds become similar: 52 % and 48 %, respectively. This

suggests that both kinds of mechanisms could take place: CO

dissociation and CO non-dissociation. In addition, the CO

insertion mechanism could also take place over the Fe promoter,

63

which may produce oxygenated compounds via the so-called enol

route [11].

7.3. Selectivities to alcohols and other oxygenated

compounds

Among the alcohols and oxygenated compounds (acetates,

aldehyde and acetic acid), methanol is the dominant product (>98

%) on both Cu/MCM-41 and Cu-K/MCM-41 catalysts, Figure 7.4.

When Fe is incorporated, the methanol selectivity decreases to 45

% and, at the same time, the selectivity to the higher alcohols is

increased to about 33 %. When both K and Fe promoters are

incorporated, the selectivity to the higher alcohols increased to 49

%. The latter indicates that the combination of these promoters is

favorable for the formation of higher alcohols.

Figure 7.4. Oxygenates and alcohol selectivities obtained from the prepared catalysts. Reac. Cond. as specified in Figure 7.3 (From Paper V). Reprinted from

Lopez at al. [67], Copyright (2016), with permission from Elsevier.

The selectivity to ethanol related to all other alcohol products (C1–

5OH) changes notably in presence of the metal promoters. While

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Sele

ctiv

ity

(%)

Time on stream (h)

Cu/MCM-41

0 5 10 15 20 0 5 10 15 0 5 10 15 0 5 10 15 20

Cu-K/MCM-41 Cu-Fe/MCM-41 Cu-Fe-K/MCM-41

Methanol

C2+-Alcohols

Acetates

Others

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

Sele

ctiv

ity

(%)

Time on stream (h)

0 5 10 15 0 5 10 0 5 10 0 5 10 15 20 25


0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

Sele

ctiv

ity

(%)

Time on stream (h)

0 5 10 15 0 5 10 0 5 10 0 5 10 15 20 25


0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

Sele

ctiv

ity

(%)

Time on stream (h)

0 5 10 15 0 5 10 0 5 10 0 5 10 15 20 25


0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

Sele

ctiv

ity

(%)

Time on stream (h)

0 5 10 15 0 5 10 0 5 10 0 5 10 15 20 25


Methanol

C2+-Alcohols

Acetates

Others

64

ethanol selectivity is 1-2 % on the Cu/MCM-41 and Cu-K/MCM-41

catalysts, this increases to 30 % with the Cu-Fe/MCM-41 catalyst.

By combining Fe and K promoters (Cu-Fe-K/MCM-41 catalyst) the

ethanol selectivity is even boosted up to 41 %.

Besides the fact that the ethanol selectivity is enhanced by action of

the metal promoters, the syngas conversion and reaction yield are

also increased as reported in Table 7.2. The ethanol yield in the

non-promoted Cu/MCM-41 catalyst is 0.3×10−5 Cmol/gcath, which is

enhanced to 165.5×10−5Cmol/ gcath in the Cu-Fe-K/MCM-41

catalyst. For the latter catalyst, the surface atomic composition,

estimated from XPS analysis is Cu0.34Fe0.08K0.08Si1.00. Further

studies may be made in order to modulate the surface composition

of the Cu-Fe-K/MCM-41 catalyst and, thus boost the ethanol yield.

Table 7.2. Space time yield (STY) of alcohols and other oxygenated compounds

over the mesoporous catalysts. Reac. Cond. as indicated in Figure 7 (From Paper V). Reprinted from Lopez at al. [67], Copyright (2016), with permission

from Elsevier.

Catalyst Conv.

(%)

STY (10-5 C mol/ gcat h)

C1OH C2OH C3-C4OH Acetates* Others**

Cu/MCM-41 2.2 27.3 0.3 0.0 0.0 0.2

Cu-K/MCM-41 2.8 42.9 0.8 0.1 0.0 0.0

Cu-Fe/MCM-41 10.0 307.2 159.2 61.4 46.4 101.8

Cu-Fe-K/MCM-41 4.9 145.4 165.5 83.3 53.0 59.4

* Methyl acetate and ethyl acetate. ** Propanal, acetaldehyde and acetic acid.

7.4. Anderson–Schulz–Flory (ASF) distribution

The plotting of product distribution in the form of ASF distribution

(Fig. 9 in Paper V) indicates that the hydrocarbon distribution

from the Cu/MCM-41 catalyst is quite similar to the one from the

Cu-K/MCM-41 catalyst, suggesting that a similar reaction

mechanism occurs in both catalysts. This mechanism may be

related to formation of oxygenated compounds, especially

65

methanol, as observed from the selectivity trends (Figure 7.4).

Thus, this may be initially related to the CO non-dissociation

and/or CO insertion mechanisms. In the same way, the

hydrocarbon distribution obtained from the Cu-Fe/MCM-41 and

Cu-Fe-K/MCM-41 catalysts are similar (Figure 9 in Paper V). This

can be associated to the formation of hydrocarbon compounds

(Figure 7.4) and thus to the CO dissociation mechanism.

Further and detailed studies are needed to clarify the reaction

mechanism over the Cu-Fe-K/MCM-41 catalyst. We believe that a

synergic combination of CO non-dissociation, CO dissociation and

CO insertion takes place which boosts the alcohol formation, in

particular the ethanol formation.

67

CHAPTER 8

Conclusions

8.1. Main results of this work

Large efforts are being carried out in order to diversify fuel sources

in the transportation sector, specially the generation of renewable

biofuels. This aims to reduce the fossil fuel dependency and the

abatement of GHG emissions. First-generation biofuels, obtained

from food-related biomass (e.g. corn, sugar cane or vegetable oils)

have some disadvantages for larger production in terms of land

and water availability. As an alternative source of biomass, in

recent years, forest and agricultural biomass as well as municipal

solid waste and different industrial waste products are being

evaluated for producing the so-called second-generation biofuels.

One route for biomass manufacturing is the thermochemical

process, in which biomass resources are converted into syngas

which is subsequently catalytically converted into liquid and

gaseous fuels. One of the most commercialized biofuels is

bioethanol, which is used as a pure fuel or blended with gasoline.

During the last years, some thermochemical plants for bioethanol

production were launched and another is under construction. A

total of about 290 million liters of ethanol are expected to be

processed per year via the thermochemical route, mostly using

municipal solid waste. Some critical parameters for ethanol

manufacturing are the biomass cost and the plant capacity (volume

production of >200x103 ton of ethanol/year is desired in order to

obtain a favorable economy of scale).

An essential stage in the ethanol production from biomass is the

final catalytic conversion of syngas to ethanol. In this stage,

considerable effort has been made in order to find a catalyst

selective for the desired product, i.e. ethanol. A novel type of

68

catalysts is the mesoporous catalysts based on mesoporous support

which have a 1D-porous ordering with pore diameters of around 2-

10nm. Few studies have been made using this kind of catalysts for

the conversion of syngas to fuels. The first part of the work

presented is focused on the performance evaluation of mesoporous

Rh/MCM-41 catalyst (MCM-41 is a hexagonal mesoporous silica).

Under identical reaction conditions (level of syngas conversion,

metal dispersion, pressure and temperature) the mesoporous

Rh/MCM-41 has shown different product distribution as compared

to typical Rh/SiO2 catalyst. Additional experiments were made in

order to clarify the results obtained; i) water was added to the

syngas feed-stream, ii) the hydrogen partial pressure was modified

by changing the syngas feed-stream ratio (H2/CO), iii) testing for a

wide range of conversion levels (1-68 %) and iv) different catalyst

reduction temperatures (no-reduction, reduction at 200 °C and

reduction at 500 °C). Also, the catalysts were characterized (XRD,

BET, XPS, chemisorption, TEM and TPR) before and after the

catalytic testing. So, analyzing these results we conclude that the

different product selectivities obtained over the mesoporous

Rh/MCM-41 is mainly due to the porous ordering of the catalyst,

in which water vapor can be concentrated and then the water-gas-

shift reaction promoted. Water vapor is the main by-product

generated in most of the catalytic reactions of syngas. The

occurrence of the water gas shift reaction in turn induces side

reactions thus changing the product distribution, as compared to a

typical Rh/SiO2 catalyst which does not have porous ordering. The

extra hydrogen generated from the water-gas-shift reaction could

facilitate the hydrogenation of acetic acid and/or acetaldehyde (or

some C2-oxygenated intermediary) to ethanol and, this could be

related to the higher selectivity to ethanol observed in Rh/MCM-41

(24 %) compared to Rh/SiO2 (8 %).

The second part of the work was focused on the evaluation of the

effect of metal promoters such as Fe and K on the catalytic

69

performance of a Cu/MCM-41 catalyst. Firstly, the non-promoted

Cu/MCM-41 catalyst produces more oxygenated products than

hydrocarbons; this result is the opposite of that obtained from

typical Cu/SiO2 catalyst. The suggested explanation is that some

water vapour could concentrate in the 1D-pores of this catalyst,

Cu/MCM-41, affecting the product distribution. The addition of

both K and Fe to the Cu/MCM-41 catalyst causes a large

deterioration of the pore structure, resulting in a minor or

negligible water vapor concentration in these catalysts. The most

interesting result obtained from the Cu-Fe-K/MCM-41 catalyst,

was the noticeable increase of the reaction rate to ethanol as

compared to the non-promoted Cu/MCM-41 catalyst; in terms of

space time yield 0.3×10−5 carbon-mol/gcath and 165.5×10−5carbon-

mol/gcath, respectively. This result could be ascribed to a synergic

action of both K and Fe promoters, favoring the CO non-

dissociation and CO dissociation mechanism, respectively. From

XPS analysis, the atomic surface composition in this catalyst has

been found equal to Cu0.34Fe0.08K0.08Si1.00. Further studies are

needed in order to boost the yield to ethanol even more by

modulating the surface composition in the Cu-Fe-K/MCM-41

catalyst.

8.2. Final remarks and future work

The work presented in this thesis has contributed to increasing the

knowledge concerning the syngas conversion using mesoporous

catalysts. The findings concerning water vapor concentration in the

pores of mesoporous catalysts have not been extensively reported

in the literature. Nor, more importantly, the effect of the water

vapor concentration over the product distribution. Although

several experimental techniques have been used to evaluate the

catalytic performance of the mesoporous catalysts, further and

more detailed characterization is desirable. For example, in-situ

techniques such as FT-IR spectroscopy can give greater insight

concerning the water vapor concentration in the mesopores of the

70

catalysts. Also, the results obtained in this work can be further

evaluated by theoretical approaches (computational catalysis) and

if possible by a micro-kinetic model. Specially, the occurrence of

the water gas shift reaction in the mesoporous catalysts and its

effect on the product distribution.

Another aspect that deserves further study is the incorporation of

nano-metal particles, with a diameter of about 2nm or smaller,

inside the 1D-pore of mesoporous catalysts, which have a pore

diameter of about 3 nm. Moreover, the incorporation of multi-

metal promoters inside the pores would result in a synergic

combination of; i) CO dissociation activity (e.g. Fe promoter), ii)

CO non-dissociation activity (e.g. Cu and Cu-K promoters) and iii)

water vapor concentration which promotes the water gas shift

reaction. The microemulsion method could be exploited for the

incorporation of multi-metal promoters, allowing the preparation

of monodispersed nanoparticles with controlled size.

71

Nomenclature

1D 1-Dimensional

AAS Atomic Absorption Spectroscopy

ASF Anderson-Schulz-Flory

BE Binding energy

BET Brunauer-Emmett-Teller

C1-8OH Mixture of alcohols from methanol to octanol

CTAB Cetyltrimethylammonium bromide

CTL Carbon to Liquids

DME Di-Methyl Ether

DTG Differential thermogravimetric analysis

E10 A blend of 10 % ethanol and 90 % gasoline by volume

E85 A blend of 85 percent ethanol and 15 % by volume

gasoline

FFV Flexible fuel vehicle

FID Flame ionization detector

FTS Fischer-Tropsch Synthesis

GC Gas chromatograph

GoBiGas Gothenburg Biomass Gasification Project

GTL Gas to Liquids

GHSV Gas hourly space velocity

HTFT High temperature Fischer-Tropsch synthesis

ktons Thousands of tons

LCT Liquid crystal templating

LTFT Low temperature Fischer-Tropsch synthesis

MFC Mass flow controller

MSR Methane steam reforming

MSW Municipal solid waste

PSRK Predictive Soave-Redlich-Kwong (equation of state)

SEM Scanning electron microscopy

SNG Synthetic natural gas

STY Space time yield

TEA Triethanolamine

TCD Thermal conductivity detector

72

TEOS Tetraethyl orthosilicate

TEM Transmission electron microscopy

TGA Thermogravimetric analysis

TPR Temperature programmed reduction

vt.% Percentage in terms of volume

XRD X-ray diffraction

wt.% Percentage in terms of mass

WGSR Water-gas shift reaction

XPS X-Ray Photoelectron Spectroscopy

73

Acknowledgements

First and foremost, I would like to thank my supervisors Associate

Professor Magali Boutonnet and Professor Sven Järås for giving

me the opportunity to carry out doctoral studies at the division of

Chemical Technology. I appreciate their patience, support,

confidence in me and my work and, the possibilities you gave me

to participate in conferences. I want to acknowledge to Dr. Saul

Cabrera for his supervision, support and friendship. Thanks also to

Associate Professor Henrik Kušar for his interest and supervision

during the last years of my studies.

I would also like to acknowledge the Swedish International

Development Cooperation Agency (SIDA) for financial support,

and thus making this work possible.

I would like to thank Professor Lars J. Pettersson for his cordiality

and good humor. I am indebted to Robert Andersson for guidance

with experimental work and friendship. Also, I want to express my

sincere gratitude to Rolando Zanzi for his help and friendship

during all these years. Thanks to Yohannes Kiros for his kindness

and support.

Thanks to all colleagues at Chemical Technology for making the

department a stimulating and friendly workplace. I learned a lot

from the formal discussions at the Catalysis Meetings, as well as

after, in the informal discussions in the coffee room.

The largest thank you goes to Jorge Velasco, Mauricio Ormachea

and Adhemar Araoz for sharing with me the experience of being a

“sandwich” doctoral student. I will remember the discussions that

had did about almost everything (academy, science-technology,

politics, life...) and of course the great moments doing sports; wa-

sáá at the “international football matches”, volleyball and

swimming.

74

I had the opportunity to meet very nice people from different

countries at Chemical Technology. Special thanks to Robert

Andersson, Sara Lögdberg, Mattelo Lualdi, Francesco Regali,

Xanthias Karatzas, Moa Ziethén Granlund, Vera Nemanova, and

Angelica González for good times and fruitful experiences. Thanks

to Fatima Pardo, Rodrigo (Rodri) Suárez, Javier (Barri) Barrientos,

Pouya Haghighi, Francesco Montecchio, Jonas Granestrand,

Alagar Raj Paulraj, Jerry Solis, Sandra Dahlin, Lina Norberg

Samuelsson and Zari Musavi for your kindness and good humor.

Special thanks to Otto von Krusenstierna for being an inspiration

of scientific enthusiasm and charisma at social events. I am also

grateful to Christina Hörnell for correcting and improving the

linguistic quality of this thesis and appended papers.

I would like to thank Oscar Laguna and Mariarosa Brundu who

were source of enthusiasm while doing their research stay at

Chemical Technology. Thanks to Vicente Montes for collaborating

in TEM and XPS analyses and interpretation.

Furthermore, I am grateful to Avelino Corma, Susana Valencia and

Urbano Díaz from the Instituto de Tecnología Química (ITQ) in

Valencia, Spain for receiving me for a short research stay. Special

thanks to Raquel Simancas, Diego Cómbita and Yannick Mathieu

for your support and friendship.

Many thanks to my colleagues at the Natural Gas Institute (IGN-

UMSA) and Chemical Research Institute (IIQ-UMSA) in La Paz,

Bolivia, for their support and friendship. Special thanks to Ronald

Escobar, Cristhian Fernández and Pedro Limachi for their

enthusiasm, good work and friendship. Thanks to all the persons

who made their research project (graduate and postgraduate

scholarship holders) at IGN and IIQ during these years, for sharing

not only work, but also nice social activities.

The deepest thanks go to the memory of mom (mamá Silvia) for all

the love and support. Finally, I would like to thank all my family,

75

especially my wife Hemilse and my son Matias for your patience,

understanding and love.

76

References

[1] Key World Energy Statistics, International Energy Agency, Paris, France, 2016, p. 80. [2] Population Estimates and Projections, The World Bank (http://datatopics.worldbank.org/hnp/popestimates#), Accessed 06/10/2016. [3] Renewable Energy: bioenergy, International Energy Agency, https://www.iea.org/topics/renewables/subtopics/bioenergy/, Accessed 20/09/2016. [4] Global Bioenergy Supply and Demand Projections: A working paper for REmap 2030, International Renewable Energy Agency, 2014, p. 80. [5] S.V. Vassilev, D. Baxter, L.K. Andersen, C.G. Vassileva, An overview of the chemical composition of biomass, Fuel, 89 (2010) 913-933. [6] R.C. Brown, Thermochemical processing of biomass conversion into fuels, chemicals and power, Chichester, West Sussex, Wiley, 2011. [7] K.R. Cummer, R.C. Brown, Ancillary equipment for biomass gasification, Biomass and Bioenergy, 23 (2002) 113-128. [8] R. Warnecke, Gasification of biomass: comparison of fixed bed and fluidized bed gasifier, Biomass and Bioenergy, 18 (2000) 489-497. [9] S.Q. Turn, C.M. Kinoshita, D.M. Ishimura, T.T. Hiraki, J. Zhou, S.M. Masutani, An Experimental Investigation of Alkali Removal from Biomass Producer Gas Using a Fixed Bed of Solid Sorbent, Industrial & Engineering Chemistry Research, 40 (2001) 1960-1967. [10] L. Devi, K.J. Ptasinski, F.J.J.G. Janssen, A review of the primary measures for tar elimination in biomass gasification processes, Biomass and Bioenergy, 24 (2003) 125-140. [11] R. Suarez Paris, L. Lopez, J. Barrientos, F. Pardo, M. Boutonnet, S. Jaras, Chapter 3 Catalytic conversion of biomass-derived synthesis gas to fuels, Catalysis: Volume 27, The Royal Society of Chemistry, 2015, p. 62-143. [12] J.A. Velasco, L. Lopez, M. Velásquez, M. Boutonnet, S. Cabrera, S. Järås, Gas to liquids: A technology for natural gas industrialization in Bolivia, Journal of Natural Gas Science and Engineering, 2 (2010) 222-228.

http://datatopics.worldbank.org/hnp/popestimates

https://www.iea.org/topics/renewables/subtopics/bioenergy/

77

[13] Database on facilities for the production of advanced liquid and gaseous biofuels for transport, BIOENERGY 2020+: International Energy Agency, Accessed 26/09/2016 (http://demoplants.bioenergy2020.eu/). [14] Gothenburg Biomass Gasification Project, GoBiGas, Göteborg Energi (Sweden), Accessed 30/09/2016 (http://www.goteborgenergi.se/English/Projects/GoBiGas__Gothenburg_Biomass_Gasification_Project). [15] R.C. Brown, Thermochemical processing of biomass: conversion into fuels, chemicals and power, Wiley, Chichester, Wiley, 2011. [16] R. Sims, M. Taylor, J. Saddler, W. Mabee, From 1st- and 2nd-Generation Biofuels Technologies: An Overview of Current Industry and RD&D Activities, International Energy Agency Bioenergy, Paris, 2008. [17] R.E.H. Sims, W. Mabee, J.N. Saddler, M. Taylor, An overview of second generation biofuel technologies, Bioresource Technology, 101 (2010) 1570-1580. [18] J. He, W. Zhang, Techno-economic evaluation of thermo-chemical biomass-to-ethanol, Applied Energy, 88 (2011) 1224-1232. [19] M.D. Jackson, C.B. Moyer, Alcohol Fuels, Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons Inc., 2000. [20] C. Wix, Process for synthesis of alcohols, Patent: WO 2013/120548 A1, Haldor Topsoe, 2013. [21] E. Chornet, B. Valsecchi, Y. Avila, B. Nguyen, J.M. Lavoie, Production of ethanol from methanol, Patent US 2009/0221725 A1, Enerkem, Inc., 2009. [22] Enerkem, Introduction to Enerkem, October 2014, p. 1-23. [23] Coskata; Process Overview (http://www.coskata.com/process/index.asp?source=7E352957-657F-44D4-8CEC-3FCA8BBB2D7C), Accessed 21/05/2015. [24] INEOS Bio; Technology Platform (http://www.ineos.com/businesses/ineos-bio/technology/#gasification), Accessed 21/05/2015. [25] G.F.T. Orías, J.A. Lordemann, L.F. Buch, A.B. Lanchez, J.C.G. Salinas, M.F. Fuentes, R.O. Lafuente, R.R. Zurita, E.G. D’Angelo, Perspectivas de la Matriz Energética Boliviana, Fundación Hanns Seidel, 2011. [26] Ley para el manejo y tratamiento de la basura, Ministerio de Medio Ambiente y Agua de Bolivia, Accessed 07/10/2016 (http://www.mmaya.gob.bo/index.php/noticias/0,1434.html).

http://demoplants.bioenergy2020.eu/

http://www.goteborgenergi.se/English/Projects/GoBiGas__Gothenburg_Biomass_Gasification_Project

http://www.goteborgenergi.se/English/Projects/GoBiGas__Gothenburg_Biomass_Gasification_Project

http://www.coskata.com/process/index.asp?source=7E352957-657F-44D4-8CEC-3FCA8BBB2D7C

http://www.coskata.com/process/index.asp?source=7E352957-657F-44D4-8CEC-3FCA8BBB2D7C

http://www.ineos.com/businesses/ineos-bio/technology/#gasification

http://www.mmaya.gob.bo/index.php/noticias/0,1434.html

78

[27] L.F. Vincenti, Estado del Arte y Novedades de la Bioenergía en Bolivia, Organización de las Naciones Unidas para la Alimentación y la Agricultura (FAO), 2011. [28] L. Lopez, Evaluación de la tecnología disponible para la producción de etanol y alcoholes superiores, in Ciencia y Tecnología en Bolivia: Estado del arte de procesos industriales para el sector de hidrocarburos y petroquimico, Editors: S. Cabrera and L. Lopez, Instituto del Gas Natural - Universidad Mayor de San Andrés, La Paz, Bolivia, 2016, p. 52. [29] A. Dutta, M. Talmadge, J. Hensley, M. Worley, D. Dudgeon, D. Barton, P. Groenendijk, D. Ferrari, B. Stears, E. Searcy, C. Wright, J.R. Hess, Techno-economics for conversion of lignocellulosic biomass to ethanol by indirect gasification and mixed alcohol synthesis, Environmental Progress & Sustainable Energy, 31 (2012) 182-190. [30] S. Chovau, D. Degrauwe, B. Van der Bruggen, Critical analysis of techno-economic estimates for the production cost of lignocellulosic bio-ethanol, Renewable and Sustainable Energy Reviews, 26 (2013) 307-321. [31] Reglamento de Calidad de Carburante y Lubricantes, Gaceta Oficial de Bolivia, Decreto Supremo No. 1499, 2013. [32] J.J. Spivey, A. Egbebi, Heterogeneous catalytic synthesis of ethanol from biomass-derived syngas, Chemical Society Reviews, 36 (2007) 1514-1528. [33] V. Subramani, S.K. Gangwal, A review of recent literature to search for an efficient catalytic process for the conversion of syngas to ethanol, Energy & Fuels, 22 (2008) 814-839. [34] N.D. Subramanian, J. Gao, X. Mo, J.G. Goodwin Jr, W. Torres, J.J. Spivey, La and/or V oxide promoted Rh/SiO2 catalysts: Effect of temperature, H2/CO ratio, space velocity, and pressure on ethanol selectivity from syngas, Journal of Catalysis, 272 (2010) 204-209. [35] D. Mei, R. Rousseau, S.M. Kathmann, V.-A. Glezakou, M.H. Engelhard, W. Jiang, C. Wang, M.A. Gerber, J.F. White, D.J. Stevens, Ethanol synthesis from syngas over Rh-based/SiO2 catalysts: A combined experimental and theoretical modeling study, Journal of Catalysis, 271 (2010) 325-342. [36] Y. Liu, K. Murata, M. Inaba, I. Takahara, K. Okabe, Synthesis of ethanol from syngas over Rh/Ce1-xZrxO2 catalysts, Catal. Today, 164 (2011) 308-314.

79

[37] C. Li, J. Liu, W. Gao, Y. Zhao, M. Wei, Ce-Promoted Rh/TiO2 Heterogeneous Catalysts Towards Ethanol Production from Syngas, Catal. Lett., 143 (2013) 1247-1254. [38] X. Yang, X. Zhu, R. Hou, L. Zhou, Y. Su, The promotion effects of Pd on Fe-Cu-Co based catalyst for higher alcohols synthesis, Fuel Process. Technol., 92 (2011) 1876-1880. [39] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism, Nature, 359 (1992) 710-712. [40] G. Ertl, H. Knözinger, J. Weitkamp, Handbook of Heterogeneous Catalysis, VCH, Weinheim, 1997. [41] A. Martínez, G. Prieto, The Application of Zeolites and Periodic Mesoporous Silicas in the Catalytic Conversion of Synthesis Gas, Topics in Catalysis, 52 (2009) 75-90. [42] A. Wingen, F. Kleitz, F. Schüth, Ordered Mesoporous Materials: Preparation and Application in Catalysis, in: M. Baerns (Ed.) Basic Principles in Applied Catalysis, Springer, Berlin, Heidelberg, 2004, p. 281-319. [43] C.J. Brinker, G.W. Scherer, CHAPTER 3 - Hydrolysis and Condensation II: Silicates, Sol-Gel Science, Academic Press, San Diego, 1990, p. 96-233. [44] S. Biz, M.L. Occelli, Synthesis and Characterization of Mesostructured Materials, Catalysis Reviews, 40 (1998) 329-407. [45] 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, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, A new family of mesoporous molecular sieves prepared with liquid crystal templates, Journal of the American Chemical Society, 114 (1992) 10834-10843. [46] X.S. Zhao, G.Q. Lu, G.J. Millar, Advances in Mesoporous Molecular Sieve MCM-41, Industrial & Engineering Chemistry Research, 35 (1996) 2075-2090. [47] J.G. Verkade, Main group atranes: chemical and structural features, Coordination Chemistry Reviews, 137 (1994) 233-295. [48] Perkin Elmer Informatic Inc., Chem3D Professional, 2016. [49] S. Cabrera, J. El Haskouri, C. Guillem, J. Latorre, A. Beltrán-Porter, D. Beltrán-Porter, M.D. Marcos, P. Amorós, Generalised syntheses of

80

ordered mesoporous oxides: The atrane route, Solid State Sci, 2 (2000) 405-420. [50] M. Bowker, On the mechanism of ethanol synthesis on rhodium, Catal. Today, 15 (1992) 77-100. [51] Y.M. Choi, P. Liu, Mechanism of Ethanol Synthesis from Syngas on Rh(111), J. Am. Chem. Soc., 131 (2009) 13054-13061. [52] R. Zhang, X. Sun, B. Wang, Insight into the Preference Mechanism of CHx (x=1-3) and C-C Chain Formation Involved in C2 Oxygenate Formation from Syngas on the Cu(110) Surface, J. Phys. Chem. C, 117 (2013) 6594-6606. [53] M. Gupta, M.L. Smith, J.J. Spivey, Heterogeneous Catalytic Conversion of Dry Syngas to Ethanol and Higher Alcohols on Cu-Based Catalysts, ACS Catal., 1 (2011) 641-656. [54] C.H. Bartholomew, R.J. Farrauto, Fundamentals of Industrial Catalytic Processes, New Jersey, Second ed., Wiley-Interscience, 2006. [55] J. El Haskouri, S. Cabrera, C. Guillem, J. Latorre, A. Beltrán, D. Beltrán, M.D. Marcos, P. Amorós, Atrane Precursors in the One-Pot Surfactant-Assisted Synthesis of High Zirconium Content Porous Silicas, Chemistry of Materials, 14 (2002) 5015-5022. [56] M.S. Cabrera, Química en medios organizados para la obtención de nuevas alúminas, aluminosilicatos y ALPOs mesoporosos con tamaño de poro modulable. PhD thesis, Instituto Universitario de Ciencia de los Materiales, Universitat de Valencia, Valencia, Spain, 1999. [57] R. Andersson, M. Boutonnet, S. Järås, On-line gas chromatographic analysis of higher alcohol synthesis products from syngas, Journal of Chromatography A, 1247 (2012) 134-145. [58] R.J. Madon, M. Boudart, Experimental criterion for the absence of artifacts in the measurement of rates of heterogeneous catalytic reactions, Industrial & Engineering Chemistry Fundamentals, 21 (1982) 438-447. [59] R.P. Underwood, A.T. Bell, Influence of particle size on carbon monoxide hydrogenation over silica- and lanthana-supported rhodium, Applied Catalysis, 34 (1987) 289-310. [60] L. Lopez, J. Velasco, S. Cabrera, M. Boutonnet, S. Järås, Effect of syngas conversion and catalyst reduction temperature in the synthesis of ethanol: concentration of water vapor in mesoporous Rh/MCM-41 catalyst, Catalysis Communications, 69 (2015) 183-187.

81

[61] J.B. Hansen, P.E. Højlund Nielsen, Methanol Synthesis, in: G. Ertl, H. Knözinger, J. Weitkamp (Eds.) Handbook of Heterogeneous Catalysis, VCH, Weinheim, 2008. [62] M.V. Landau, S.P. Varkey, M. Herskowitz, O. Regev, S. Pevzner, T. Sen, Z. Luz, Wetting stability of Si-MCM-41 mesoporous material in neutral, acidic and basic aqueous solutions, Microporous and Mesoporous Materials, 33 (1999) 149-163. [63] A. Borodziński, M. Bonarowska, Relation between Crystallite Size and Dispersion on Supported Metal Catalysts, Langmuir, 13 (1997) 5613-5620. [64] S.D. Robertson, B.D. McNicol, J.H. De Baas, S.C. Kloet, J.W. Jenkins, Determination of reducibility and identification of alloying in copper-nickel-on-silica catalysts by temperature-programmed reduction, Journal of Catalysis, 37 (1975) 424-431. [65] J. Panpranot, J.G. Goodwin Jr, A. Sayari, Synthesis and characteristics of MCM-41 supported CoRu catalysts, Catalysis Today, 77 (2002) 269-284. [66] A. Egbebi, J.J. Spivey, Effect of H2/CO ratio and temperature on methane selectivity in the synthesis of ethanol on Rh-based catalysts, Catalysis Communications, 9 (2008) 2308-2311. [67] L. Lopez, V. Montes, H. Kušar, S. Cabrera, M. Boutonnet, S. Järås, Syngas conversion to ethanol over a mesoporous Cu/MCM-41 catalyst: Effect of K and Fe promoters, Applied Catalysis A: General, 526 (2016) 77-83. [68] G.R. Sheffer, T.S. King, Potassium's promotional effect of unsupported copper catalysts for methanol synthesis, Journal of Catalysis, 115 (1989) 376-387. [69] L.E.Y. Nonneman, V. Ponec, On the problem of the activity in methanol synthesis of supported, unpromoted copper catalysts, Catalysis Letters, 7 (1990) 213-217. [70] S.V. Sirotin, I.F. Moskovskaya, B.V. Romanovsky, Synthetic strategy for Fe-MCM-41 catalyst: a key factor for homogeneous or heterogeneous phenol oxidation, Catalysis Science & Technology, 1 (2011) 971-980. [71] L.E.Y. Nonneman, V. Ponec, Promotion and support effects in syngas reactions, Catalysis Letters, 7 (1990) 197-203.

82

[72] A. Gotti, R. Prins, Basic Metal Oxides as Cocatalysts for Cu/SiO2 Catalysts in the Conversion of Synthesis Gas to Methanol, Journal of Catalysis, 178 (1998) 511-519. [73] M. Boutonnet, S. Lögdberg, E. Elm Svensson, Recent developments in the application of nanoparticles prepared from w/o microemulsions in heterogeneous catalysis, Curr. Opin. Colloid Interface Sci., 13 (2008) 270-286. [74] A.-M. Hilmen, M. Xu, M.J.L. Gines, E. Iglesia, Synthesis of higher alcohols on copper catalysts supported on alkali-promoted basic oxides, Applied Catalysis A: General, 169 (1998) 355-372.

Date post:	10-Jun-2020
Category:	Documents
Upload:	others
View:	18 times
Download:	3 times

Catalytic conversion of syngas to ethanol and higher ...1048937/FULLTEXT02.pdf · Catalytic...

Documents