Hydrogen Production Using Catalytic Supercritical · PDF fileSupercritical Water Gasification...

Hydrogen Production Using Catalytic

Supercritical Water Gasification of

Lignocellulosic Biomass

by

Pooya Azadi Manzour

A thesis submitted in conformity with the requirements

for the degree of Doctor of Philosophy

Graduate Department of Chemical Engineering and Applied Chemistry

University of Toronto

© Copyright by Pooya Azadi Manzour 2012

ii

Hydrogen Production Using Catalytic Supercritical Water

Gasification of Lignocellulosic Biomass

Pooya Azadi Manzour

Doctor of Philosophy

Department of Chemical Engineering and Applied Chemistry

University of Toronto

2012

Abstract

Catalytic supercritical water gasification (SCWG) is a promising technology for

hydrogen and methane production from wet organic feedstocks at relatively low

temperatures (e.g. <500 oC). However, in order to make this process technically and

economically viable, solid catalyst with enhanced activity and improved hydrogen

selectivity should be developed. In this study, different aspects of catalytic SCWG have

been investigated. The performance of several supported-nickel catalysts were examined

to identify catalysts that lead to high carbon conversion and high hydrogen yields under

near-critical conditions (i.e. near 374 oC). Moreover, for the first time, the effects of

several parameters which dominated the activity of the supported nickel catalysts have

been systematically investigated. Among the several different catalyst supports evaluated

at 5% nickel loading, α-Al2O3, carbon nanotube (CNT), and MgO supports resulted in

iii

highest carbon conversions, while SiO2, Y2O3, hydrotalcite, yttria-stabilized zirconia

(YSZ), and TiO2 showed modest activities. Comparing the XRD patterns for the support

materials before and after the exposure to supercritical water, α-Al2O3, YSZ, and TiO2

were found to be hydrothermally stable among the metal oxide supports. Using the same

amount of nickel on α-Al2O3, the methane yield decreased by increasing the nickel to

support ratio whereas the carbon conversion was only slightly affected. At a given nickel

to support ratio, a threefold increase in methane yield was observed by increasing the

temperature from 350 to 410 oC. The catalytic activity also increased by the addition

small quantity of potassium. The activity of Ni/γ-Al2O3 catalyst varied based on the

affinity of the catalyst to form nickel aluminate spinel. This is also the first report on the

role of oxidative pretreatment of the carbon nanotubes by nitric acid on the performance

of these catalysts for the supercritical water gasification process. Using different

lignocellulosic feeds, it was found that the gasification of glucose, fructose, cellulose,

xylan and pulp resulted in comparable gas yields (± 10%) after 60 min, whereas alkali

lignin was substantially harder to gasify. Interestingly, gasification yield of bark, which

had a high lignin content, was comparable to those of cellulose. In summary, the Ni/α-

Al2O3 catalyst had a higher hydrogen selectivity and comparable catalytic activity to the

best commercially available catalysts for SCWG of carbohydrates.

iv

Acknowledgements

I would like to express my deepest gratitude to Professor Ramin Farnood for his

invaluable supports and wise guidance over the past few years. I am thankful for his

patience and confidence in my work.

I would also like to thank the members of my PhD committee, Professor Mims, Professor

Saville, Professor Acosta, Professor Kirk and Professor Jia for their valuable suggestions

and advises. I sincerely thank Professor Nejat Veziroglu from the University of Miami

for kind serving as my external appraiser.

My appreciation goes to Professor James Dumesic at the University of Wisconsin-

Madison, Professor Otomo and Professor Oshima and Dr. Hatano at the University of

Tokyo, and Professor Khodadadi and Professor Mortazavi at the University of Tehran for

the opportunity that they gave me to work in their group.

I would also like to extend my gratitude to Professor Ning Yan and Professor C. Q. Jia

for kindly providing me with some samples and letting me use their equipments.

As well, thanks are given to my friends who helped me with my project: Hooman, Elie,

Faraz, Frida, Sami, Clement, Kashif, Emanuel, Emmanuel, Isabella and Coralie.

v

Table of Contents

1- INTRODUCTION......................................................................................... 1

1.1 BACKGROUND ................................................................................................. 1

1.2 OBJECTIVES AND HYPOTHESES ....................................................................... 2

1.3 STRUCTURE OF THE THESIS ............................................................................. 3

1.4 CONTRIBUTION AND SIGNIFICANCE ................................................................. 5

1.5 PUBLICATIONS ................................................................................................ 5

2- REVIEW OF HETEROGENEOUS CATALYSTS FOR

SUPERCRITICAL WATER GASIFICATION........................................ 7

2.1 INTRODUCTION ............................................................................................... 7

2.2 CURRENT STATUS OF CATALYTIC SCWG .................................................... 16

2.3 PERFORMANCES OF SOLID CATALYSTS ......................................................... 19

2.4 CONCLUDING REMARKS ............................................................................... 39

2.5 REFERENCES ................................................................................................. 39

3. SCREENING OF NICKEL CATALYSTS FOR SELECTIVE

HYDROGEN PRODUCTION USING SUPERCRITICAL WATER

GASIFICATION OF GLUCOSE ............................................................. 52

3.1 INTRODUCTION ............................................................................................. 53

3.2 EXPERIMENTAL ............................................................................................. 56

3.3 RESULTS AND DISCUSSION ............................................................................ 61

3.4 CONCLUSIONS ............................................................................................... 86

vi

3.5 REFERENCES ................................................................................................. 87

4- CARBON-NANOTUBE SUPPORTED NICKEL CATALYST FOR

SCWG ......................................................................................................... 89

4.1 INTRODUCTION ............................................................................................. 90

4.2 EXPERIMENTAL METHODS ............................................................................. 92

4.3 RESULTS AND DISCUSSIONS .......................................................................... 94

4.4 ACTIVITY OF NI/MWCNT CATALYST FOR SCWG OF GLUCOSE ................ 112

4.5 CONCLUSION ............................................................................................... 116

4.6 REFERENCES ............................................................................................... 117

5- CATALYTIC SCWG OF VARIOUS LIGNOCELLULOSIC

MATERIALS ........................................................................................... 118

5.1 INTRODUCTION ........................................................................................... 119

5.2 MATERIALS AND METHODS ........................................................................ 122

5.3 RESULTS ..................................................................................................... 128

5.4 CONCLUDING REMARKS ............................................................................. 135

5.5 REFERENCES ............................................................................................... 136

6- SUMMARY AND FUTURE WORK ...................................................... 138

6.2 COMPARISON OF ACTIVE METALS .............................................................. 141

6.3 CONCLUDING REMARKS ............................................................................. 142

6.4 RECOMMENDATION FOR FUTURE WORK ...................................................... 144

vii

APPENDIX A: .................................. COMPLEMENTARY LITERATURE REVIEW

.................................................................................................................... 145

APPENDIX B: ........................................................ EQUILIBRIUM CALCULATIONS

.................................................................................................................... 155

viii

List of Tables

TABLE 2.1 SELECTED RESULTS OF CATALYTIC SCWG USING ACTIVATED

CARBON AS CATALYST. ...................................................................... 22

TABLE 2.2 SELECTED RESULTS OF CATALYTIC SCWG USING RANEY

(SKELETAL) NICKEL CATALYST. ..................................................... 27

TABLE 2.3 SELECTED RESULTS OF CATALYTIC SCWG USING

SUPPORTED NICKEL CATALYSTS. .................................................. 31

TABLE 2.4 PERFORMANCE OF SUPPORTED CATALYSTS FOR

GASIFICATION OF 2 WT% GLUCOSE SOLUTION. 380O

C, 15MIN,

GLUCOSE 0.2G, CATALYST 1G, 5 WT% NI, WATER 9.8G. ........... 35

TABLE 2.5 SELECTED RESULTS OF CATALYTIC SCWG USING

SUPPORTED RUTHENIUM CATALYSTS. ......................................... 36

TABLE 3.1 CHARACTERISTICS OF THE SUPPORTS USED IN THIS STUDY.

...................................................................................................................... 60

TABLE 3.2 THE EQUILIBRIUM GAS COMPOSITION CALCULATED BASED

ON SCWG OF 2 WT% GLUCOSE AND WATER DENSITY OF 200

KG/M3 USING ASPEN PLUS SOFTWARE. ......................................... 62

TABLE 3.3 PERFORMANCE OF ALUMINA-SUPPORTED CATALYSTS (380

OC, 15MIN, 2 WT% GLUCOSE, 1G CATALYST). .............................. 69

ix

TABLE 3.4 PERFORMANCE OF OTHER SUPPORTED CATALYSTS (380 O

C,

15MIN, 2 WT% GLUCOSE, 1 G CATALYST). .................................... 76

TABLE 3.5 EFFECTS OF THE ADDITION OF PROMOTERS TO NI/Α-AL2O3

CATALYSTS ON GASIFICATION OF 2 WT% GLUCOSE

SOLUTION. (380 O

C, 15MIN, 0.05G NI). ............................................... 85

TABLE 4.1 ELEMENTAL COMPOSITION OF NICKEL DECORATED MWCNT

OBTAINED BY XPS NITRIC ACID CONCENTRATION. ................ 95

TABLE 4.2 WEIGHT PERCENT OF THE REMOVED FUNCTIONAL GROUPS

OBTAINED FROM TGA IN NITROGEN. ............................................ 99

TABLE 4.3 GAS FORMATION FROM THE CATALYST SUPPORT (WITH NO

GLUCOSE). 380 O

C, 30MIN, 0.5 G CATALYST, 9.8 G WATER.

YIELD AND CGR ARE CALCULATED WITH 0.2 G GLUCOSE AS

A HYPOTHETICAL FEED. .................................................................. 114

TABLE 5.1 PHYSICAL CHARACTERISTICS OF THE CATALYSTS USED IN

THIS STUDY. .......................................................................................... 125

TABLE 5.2 THERMODYNAMIC EQUILIBRIUM OF 2 WT% FEEDS AT 380 O

C

AND 230 BAR. ......................................................................................... 129

TABLE 5.3 GAS YIELD AND CGR OBTAINED FROM CATALYST-FREE

SCWG OF 2 WT% FEEDS AT 380 O

C IN 15MIN BATCH

EXPERIMENTS. ..................................................................................... 130

x

TABLE A.1 REACTIONS RATES OF GLUCOSE DECOMPOSITION [S-1

] WITH

RESPECT TO FIGURE 2.1 .................................................................... 146

TABLE A.2 REACTION RATE AND ORDER FOR GLUCOSE

DECOMPOSITION IN SWC [3]. .......................................................... 147

TABLE A.3 CORRESPONDING ACTIVATION ENERGY AND ARRHENIUS

FACTOR, A, FOR REACTIONS PRESENTED IN FIGURE 2.3 [4]. 148

TABLE A.4 REACTION RATES [S-1

] CORRESPONDING TO FIGURE 2.5 [12].

.................................................................................................................... 150

TABLE A.5 REACTION RATES AND PRODUCT’S MASS FRACTION BASED

ON FIGURE 2.7 [14]. .............................................................................. 152

xi

Table of Figures

FIGURE 2.1 .... PROCESS FLOW DIAGRAM OF SCWG AND ITS SUBSEQUENT

APPLICATIONS. ........................................................................................ 9

FIGURE 2.2 ....... VARIATION OF ENTHALPY OF WATER AS A FUNCTION OF

TEMPERATURE. ..................................................................................... 16

FIGURE 2.3 .......................... CATALYST DESIGN TRIANGLE INTRODUCED BY

RICHARDSON [45] (ADAPTED FROM [46]). ...................................... 18

FIGURE 3.1 REACTION PATHWAYS FOR PRODUCTION OF HYDROGEN BY

REACTIONS OF OXYGENATED CARBOHYDRATES WITH

WATER ADAPTED FROM [17]. (* REPRESENTS A SURFACE

METAL SITE) ........................................................................................... 54

FIGURE 3.2 ..... SCHEMATIC DIAGRAM OF THE EXPERIMENTAL SET-UP: 1)

MOLTEN SALT BATH, 2) REACTOR, 3) ELECTRICAL HEATER,

4) THERMOCOUPLE, 5) PID TEMPERATURE CONTROLLER, 6)

FIRST VALVE, 7) LOW-PRESSURE GAUGE, 8) SECOND VALVE.

...................................................................................................................... 61

FIGURE 3.3 ..... HYDROGEN AND METHANE YIELDS AND SELECTIVITY VS.

CARBON CONVERSION USING 5% NI/ALUMINA CATALYSTS,

Α-AL2O3 (),Γ-AL2O3 (), LA2O3-Γ-AL2O3 (), AND

EQUILIBRIUM VALUES (X). (380 O

C, 15MIN, 2 WT% GLUCOSE,

1G CATALYST). ....................................................................................... 70

xii

FIGURE 3.4 HYDROGEN YIELD AND SELECTIVITY VS. SUPPORT SURFACE

AREA USING 5% NI/ALUMINA CATALYSTS, Α-AL2O3 (),Γ-

AL2O3 (), LA2O3-Γ-AL2O3 (). (380 O

C, 15MIN, 2 WT% GLUCOSE,

1G CATALYST). ....................................................................................... 71

FIGURE 3.5 .. XRD PATTERNS OF 5% NI ON A) Α-AL2O3 REDUCED AT 500 O

C

B) Γ-AL2O3 (A-6) REDUCED AT 500 O

C, C) Γ-AL2O3 (A-6)

REDUCED AT 800 O

C, AND D) Γ-AL2O3 (A-8) REDUCED AT 700 O

C.

THE VERTICAL DASHED LINES REPRESENT NICKEL PEAKS. 72

FIGURE 3.6 . XRD PATTERNS OF DIFFERENT TYPES OF ALUMINA BEFORE

(BOTTOM) AND AFTER (TOP) EXPOSURE TO THE

SUPERCRITICAL WATER AT 380 O

C FOR 1H. ................................ 73

FIGURE 3.7 ..................... HYDROGEN YIELD AND SELECTIVITY VS. CARBON

CONVERSION (CGR) USING 5% NI/SUPPORT CATALYSTS, (380

OC, 15MIN, 2 WT% GLUCOSE, 1 G CATALYST). MOLECULAR

SIEVE (), YSZ (), HYDROTALCITE (), SILICA (), TITANIA

(), YTTRIA (), MAGNESIA (), AND CNT (). .......................... 77

FIGURE 3.8 ..... XRD PATTERNS OF MGO, Y2O3, TIO2, HYDROTALCITE, CEO2

AND YSZ BEFORE (BOTTOM) AND AFTER (TOP) EXPOSURE TO

THE SUPERCRITICAL WATER AT 380 O

C FOR 1H. ....................... 78

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FIGURE 3.9 .. CARBON GASIFICATION RATIO AND METHANE YIELD VS. NI

LOADING (ON Α-AL2O3 (A-4)) AT 410 O

C (), 380 O

C (), AND 350

OC (), (15MIN, 2 WT% GLUCOSE, 0.05 G NI). ................................. 81

FIGURE 3.10 ... HYDROGEN SELECTIVITY VS. NI LOADING (ON Α-AL2O3 (A-

4)) AT 350 O

C, 380 O

C, AND 410 O

C, (15MIN, 2 WT% GLUCOSE, 0.05

G NI). ........................................................................................................... 82

FIGURE 3.11 ..... EFFECTS OF CALCINATION TEMPERATURE (FOR 2H) AND

TIME (AT 350 O

C) ON PRODUCT YIELDS FOR SCWG USING 5%

WT NI/Α-AL2O3 (A-4) CATALYST, (380 O

C, 15MIN, 2 WT%

GLUCOSE, 1 G CATALYST). CARBON MONOXIDE YIELD

REMAINED LESS THAN 0.3 MMOL/G (NOT SHOWN). .................. 83

FIGURE 3.12 ......... EFFECTS OF REDUCTION TEMPERATURE (FOR 2H) AND

TIME (AT 500 O

C) ON THE GAS YIELDS USING 5% NI/Α-AL2O3

CATALYST, (380 O

C, 15MIN, 2 WT% GLUCOSE, 1 G CATALYST).

CARBON MONOXIDE YIELD REMAINED LESS THAN 0.3

MMOL/G (NOT SHOWN). ...................................................................... 83

FIGURE 3.13 ............ HYDROGEN YIELD, METHANE YIELD AND HYDROGEN

SELECTIVITY VS. CARBON CONVERSION USING PROMOTED

NI/Α-AL2O3 CATALYSTS. NO PROMOTER (), SN (), NA (), CS

(), AND K (), (380 O

C, 15 MINUTES, 1 G CATALYST). ............... 86

xiv

FIGURE 4.1 ................................................ TGA OF FRESH AND FMWCNT IN AIR.

...................................................................................................................... 96

FIGURE 4.2 .......................... ZETA POTENTIAL OF FMWCNT VS. NITRIC ACID

CONCENTRATION.................................................................................. 97

FIGURE 4.3 ... CONCENTRATION OF CARBOXYL GROUPS VS. TREATMENT

TIME. .......................................................................................................... 98

FIGURE 4.4 ........ TEM MICROGRAPHS OF 20% NI/FMWCNT, 5H OXIDATIVE

TREATMENT WITH NITRIC ACID 4M (LEFT) AND 16M (RIGHT)

.................................................................................................................... 100

FIGURE 4.5 ................................ NICKEL CRYSTALLITE SIZE VS. NITRIC ACID

CONCENTRATION FOR 20% NI/FMWCT. ...................................... 100

FIGURE 4.6 ...................... TEM MICROGRAPHS OF 20% NI/MWCNT, WITH NO

TREATMENT (LEFT) AND WITH 24H TREATMENT IN 10M

NITRIC ACID. ......................................................................................... 101

FIGURE 4.7 .. NICKEL CRYSTALLITE SIZE VS. TREATMENT TIME FOR 20%

NI/FMWCNT. .......................................................................................... 102

FIGURE 4.8 .................. EFFECT OF OXIDATIVE TREATMENT ON CRACKING

TEMPERATURE OF 20%NI/MWCT WITH AND WITHOUT ACID

TREATMENT. ......................................................................................... 103

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FIGURE 4.9 .. NICKEL CRYSTALLITE SIZE VS. TREATMENT TYPE FOR 20%

NI/FMWCNT. .......................................................................................... 104

FIGURE 4.10 .... XRD PATTERNS OF DECORATED CARBON NANOTUBES AT

DIFFERENT NICKEL LOADINGS. .................................................... 105

FIGURE 4.11 ..... NICKEL CRYSTALLITE SIZE AND DISPERSION VS. NICKEL

LOADING. TREATMENT TIME: 5 HOURS). ................................... 106

FIGURE 4.12 TEM MICROGRAPHS OF NI/FMWCNT AT DIFFERENT METAL

LOADINGS, A) 5% B) 20% C) 50% D) POROUS NICKEL

CRYSTALS FORMED ON 50% NI/FMWCNT. ................................. 107

FIGURE 4.13 ............... DERIVATE IF WEIGHT LOSS VS. TEMPERATURE FOR

DIFFERENT NICKEL LOADINGS. .................................................... 108

FIGURE 4.14 ....... ZETA POTENTIAL OF NI/FMWCNT VS. NICKEL LOADING.

.................................................................................................................... 109

FIGURE 4.15 .... NICKEL CRYSTALLITE SIZE FOR DIFFERENT PRECURSOR

SOLVENTS. ............................................................................................. 110

FIGURE 4.16 ........................... NICKEL CRYSTALLITE SIZE VS. CALCINATION

TEMPERATURE FOR 20% NI/FMWCNT. ........................................ 111

FIGURE 4.17 TEMPERATURE PROGRAMMED REDUCTION OF NI/FMWCNT

AND NIO POWDER AT HEATING RATE OF 10O

C/MIN. .............. 112

xvi

FIGURE 4.18 ..... CARBON CONVERSION AND GAS YIELDS VS. NITRIC ACID

CONCENTRATION. 380 O

C, 30MIN, 0.5 G CATALYST, 9.8 G

WATER, 0.2 G GLUCOSE, MWCNT PRETREATMENT TIME=5 H.

.................................................................................................................... 115

FIGURE 4.19 ..................... CARBON CONVERSION AND GAS YIELDS VS. ACID

TREATMENT TIME. 380 O

C, 30MIN, 0.5 G CATALYST, 9.8 G

WATER, 0.2 G GLUCOSE. NITRIC ACID CONCENTRATION=10

M. ............................................................................................................... 116

FIGURE 5.1 ..... SCHEMATIC DIAGRAM OF THE EXPERIMENTAL SET-UP: 1)

MOLTEN SALT BATH, 2) REACTOR, 3) ELECTRICAL HEATER,

4) THERMOCOUPLE, 5) PID TEMPERATURE CONTROLLER, 6)

FIRST VALVE, 7) LOW-PRESSURE GAUGE, 8) SECOND VALVE.

.................................................................................................................... 125

FIGURE 5.2 SEM MICROGRAPH OF THE CATALYSTS USED IN THIS WORK

. A) RANEY NICKEL, B) NI/Α-AL2O3, C) RU/C, D) RU/Γ-AL2O3 ... 126

FIGURE 5.3 ......................... PORE SIZE DISTRIBUTION OF Α-AL2O3 (TOP) AND

HYDROTALCITE (BOTTOM). THE UNIT OF THE Y-AXIS IS

CC/Å/G ..................................................................................................... 127

FIGURE 5.4 .. SCWG OF LIGNOCELLULOSIC FEEDS USING RANEY NICKEL

CATALYST. 380 O

C, 2 WT% FEED, 5, 15, 30 AND 60MIN (BARS

FROM LEFT TO RIGHT RESPECTIVELY), 120 MG NI. ............... 132

xvii

FIGURE 5.5 .... SCWG OF LIGNOCELLULOSIC FEEDS USING NI/Α-AL2O3. 380

OC, 2 WT% FEED, 5, 15, 30 AND 60MIN (BARS FROM LEFT TO

RIGHT RESPECTIVELY), 120 MG NI. .............................................. 132

FIGURE 5.6 .................................. SCWG OF LIGNOCELLULOSIC FEEDS USING

NI/HYDROTALCITE. 380 O

C, 2 WT% FEED, 5, 15, 30 AND 60MIN

(BARS FROM LEFT TO RIGHT RESPECTIVELY), 120 MG NI. .. 133

FIGURE 5.7 ...... SCWG OF LIGNOCELLULOSIC FEEDS USING RU/C. 380 O

C, 2

WT% FEED, 5, 15, 30 AND 60 MIN (BARS FROM LEFT TO RIGHT

RESPECTIVELY), 6 MG RU. ............................................................... 134

FIGURE 5.8 ... SCWG OF LIGNOCELLULOSIC FEEDS USING RU/Γ-AL2O3. 380

OC, 2 WT% FEED, 5, 15, 30 AND 60MIN (BARS FROM LEFT TO

RIGHT RESPECTIVELY), 6 MG RU. ................................................. 135

FIGURE 6.1 ..... THE REACTION PATHWAY FOR THE CATALYTIC SCWG OF

BIOMASS. ................................................................................................ 139

FIGURE 6.2 ...... HYDROGEN SELECTIVITY VS. CARBON CONVERSION FOR

NICKEL-CATALYZED SCWG OF GLUCOSE. DATA: TIME

DEPENDENT FROM CHAPTER 5, NON ALUMINA, PROMOTERS,

GAMMA ALUMINA AND ACTIVATION FROM CHAPTER 3, CNT

FROM CHAPTER 4. ............................................................................... 140


NI/Α-AL2O3 CATALYZED SCWG OF GLUCOSE WITH

xviii

DIFFERENT METAL LOADINGS (G NI/G SUPPORT). THE

DASHED LINE REPRESENTS THE TREND IN FIGURE 6.2. ....... 141


RUTHENIUM-CATALYZED SCWG OF GLUCOSE. THE DASHED

LINE REPRESENTS THE TREND IN FIGURE 6.2. ......................... 142

FIGURE A.1 .. PROPOSED REACTION PATHWAY FOR DECOMPOSITION OF

GLUCOSE [1]. ......................................................................................... 146

FIGURE A.2 ......... MORE DETAILED REACTION PATHWAY OF GLUCOSE [2]

.................................................................................................................... 147

FIGURE A.3 ........ DECOMPOSITION OF GLUCOSE IN SUBCRITICAL WATER

PROPOSED BY QI [4]. ........................................................................... 148

FIGURE A.5 REACTION PATHWAY FOR DECOMPOSITION OF CELLULOSE

AND STARCH IN SCW [12]. ................................................................. 150

FIGURE A.6 . DECOMPOSITION OF GLUCOSE IN SCW ACCORDING TO [13].

.................................................................................................................... 151

FIGURE A.7 ....... REACTION PATHWAYS AND PRODUCT DISTRIBUTION OF

GLUCOSE GASIFICATION IN QUARTZ CAPILLARIES [14]. A, B,

C, D, E, AND F, REPRESENT THE MASS FRACTION OF EACH

PRODUCT. ............................................................................................... 152

xix

Nomenclature

A Specific surface area of catalyst [m2/g]

AS Aluminum silicate

BC Biocrude oil

BET Brunauer-Emmett-Teller method

CGR Carbon gasification ratio

CNT Carbon nanotube

d Metal dispersion [%]

EG Ethylene glycol

FMWCNT Functionalized multiwalled carbon nanotube

HA Hmuic acid

HGR Hydrogen gasification ratio

HMF Hydroxymethylfurfural

HS Hydrogen selectivity

LMC Lignin model compound

MA Microalgae

MWCNT Multiwalled carbon nanotube

P Pressure [bar]

PEG Polyethylene glycol

PS Peanut shell

SEM Scanning electron microscopy

SCW Supercritical water

SCWG Supercritical water gasification

SFS Sunflower stalk

SLW Synthetic liquefied wood

xx

T Temperature [oC]

TEM Transmission electron microscopy

TGA Thermogravimetry analysis

TCD Thermal conductivity detector

TiO2-AR Mixed anatase and rutile titania

TiO2-R Rutile titania

TPR Temperature programmed reduction

WGS Water-gas shift reaction

XRD X-ray diffraction

XPS X-ray photoelectron spectroscopy

YSZ Yttria-stabilized zirconia

1

1- Introduction

1.1 Background

Substantial efforts have been devoted to decrease our dependence on fossil fuels, and

much of these efforts heavily rely on the development of new and improved catalytic

processes. Among various catalytic processes, hydrogen production from wet biomass

and organic compounds in sub- and supercritical water (SCW) has gained significant

attention over the past two decades (in this document, the terms “supercritical water” and

“hydrothermal” are used interchangeably). In this process, catalysts are employed to

enhance the gas formation rate at moderate temperatures (e.g. <450 oC). Catalysts can be

also utilized to shift the product distribution towards a more desirable compound (e.g.,

hydrogen). The effectiveness of various types of heterogeneous catalysts, mainly based

on nickel and ruthenium metals, have been demonstrated for hydrothermal gasification of

organic compounds. Catalyst formulation along with the operating conditions such as

temperature and feed concentration can significantly affect the conversion and selectivity

of the process. In spite of major advancements over the past decades, there are still

important challenges that need to be addressed to make catalytic SCWG technically and

economically viable for hydrogen production. Poor hydrogen selectivity, catalyst

instability, tar and char formation, heat recovery and precipitation of inorganic matters

are among the most important issues that are yet to be addressed. The low hydrogen

selectivity is caused by the high activity of nickel and ruthenium to open the C-O bonds,

which in turn results in the formation of alkanes. Also, most solid catalysts suffer from

instability in SCW due to sintering, metal oxidation and support phase transformation and

2

hydrolysis. Furthermore, the high reactivity of oxygenated compounds in SCW, specially

the carbohydrates, results in char formation through bimolecular condensation reactions.

Review of literature indicated that the physical and chemical properties of nickel catalyst

have significant impacts on the performance of these catalysts for SCWG of biomass. It

is also known that the catalytic properties depend upon the support materials, addition of

promoters and catalyst preparation. Therefore, this study was focused on the development

of solid nickel catalysts for SCWG of biomass using industrial catalyst supports such as

metal oxides, but other catalyst supports such as carbon nanotubes which are more

suitable for fundamental studies have been also considered. Study of other challenges

such as heat recovery and ash precipitation fall beyond the scope of this work and will

not be covered in this document.

1.2 Objectives and Hypotheses

The overall objective of this project is to identify/develop solid catalysts to improve the

conversion of supercritical water gasification of lignocellulosic biomass at mild reaction

conditions (i.e. <400 oC), and particularly increasing the hydrogen yield by decreasing

the alkane selectivity. The specific objectives of this project are:

1- To identify/develop catalysts with high catalytic activity and high hydrogen

selectivity for the gasification of biomass model compounds in supercritical

water.

2- To study the relation between the physical and chemical characteristics of nickel

catalysts with their performance for SCWG reactions.

3

3- To evaluate the performance of the promising catalysts as identified in the first

part for the SCWG of different types of feedstocks, particularly the lignocellulosic

materials.

The hypotheses of this work were as follows:

1- The catalytic activity and hydrogen selectivity of nickel catalysts can be tailored

by selecting the catalyst support, tuning the metal loading, and optimizing the

catalyst preparation conditions.

2- The catalytic activity and hydrogen selectivity of nickel catalysts can be improved

by the addition of alkali promoters and tin.

3- Multiwalled carbon nanotube-supported nickel catalysts are active for SCWG of

biomass and its catalytic activity and hydrogen selectivity can be modified by

oxidative pretreatment of the support.

4- Due to the presence of strong linkages between the phenolic monomers, and

thereby the low hydrolysis yield, (extracted) lignin is substantially more difficult

to decompose compared to the carbohydrates.

1.3 Structure of the Thesis

A critical review of the current literature on the catalytic supercritical water gasification

(SCWG) of biomass is given in the next chapter. In this review, performance and

durability of commercially available and laboratory-made catalysts including supported

and skeletal metal catalysts, activated carbon, metal wires and other innovative catalysts

for the purpose of hydrothermal hydrogen production from biomass are discussed. Our

review suggests that it may be possible to improve the performance of supported nickel

4

catalysts under near critical conditions by the proper catalyst design. Hence, there is a

need for a more comprehensive screening of nickel catalysts, aiming to address the

following issues: first , by conducting the experiments under similar operating conditions,

one can better understand the effects of catalyst support on the activity and selectivity

through comparison of the results, and second, by considering a wider range of support

materials, there will be a possibility to come across new catalyst formulations which

overperform the existing ones in terms of activity, selectivity and stability. Chapter 3

presents the results of the catalyst screening study for identifying useful nickel catalysts

for the gasification of glucose in supercritical water. Among the 44 supports studied, α-

Al2O3 and carbon nanotube were identified as most promising catalysts and were

subsequently subjected to a more in depth investigation. The effects of metal loading,

catalyst preparation conditions and promoters on the performance of Ni/α-Al2O3 are also

discussed in Chapter 3. The effects of various parameters on the structure of Ni/carbon

nanotube catalyst (CNT), identified as one of the most active catalysts in the previous

chapter, is discussed in Chapter 4. The results of the SCWG of glucose using Ni/CNT is

also included in the same chapter. To further examine the performance of Ni/α-Al2O3 for

hydrogen production from real lignocelllosic biomass, catalytic supercritical water

gasification of different feedstocks; including wood pulp, lignin, and bark, are examined

and compared to those of selected commercially available catalysts in Chapter 5. Finally,

the concluding remarks and recommendations for the future work are discussed in

Chapter 6. Also, a complementary literature review on some other aspects of the SCWG

is provided in Appendix A.

5

1.4 Contribution and significance

The contribution of this study to the field of catalytic supercritical water gasification can

be summarized as follows:

1- The major findings of catalytic SCWG of biomass in the past two decades are

critically reviewed and remarks on the future direction were given.

2- A comprehensive set of nickel catalysts were synthesized and ranked with regard to

their activity and hydrogen selectivity for the SCWG of biomass model compound.

3- The relationship between the catalyst properties and the performance within the

context of SCWG was discussed in details.

4- The effect of oxidative pretreatment of carbon nanotubes (CNT) on the dispersion and

other characteristics of nickel-decorated CNT catalysts were studied, and the subsequent

effect on the catalytic activity of such materials for SCWG were evaluated.

5- Being identified in the previous section as the most promising catalyst, the activity of

Ni/ α-Al2O3 catalyst for gasification of different lignocellulosic materials was examined.

It was found that this catalyst have a high activity and selectivity for the SCWG of

carbohydrates.

1.5 Publications

Each chapter of this document is based on a journal paper as listed below.

6

Chapter 2: P. Azadi, R. Farnood, Review of Heterogeneous Catalysts for Sub and

Supercritical Water Gasification of Biomass and Wastes, International

Journal of Hydrogen Energy, 2011, 36, 9529-9541

Chapter 3: P. Azadi, E. Afif, F. Azadi, R. Farnood, Catalyst Screening for Selective

Hydrogen Production Using Supercritical Water Gasification of Biomass,

Green Chemistry. DOI:10.1039/C2GC16378K.

Chapter 4: P. Azadi, R. Farnood, E. Meier. Preparation of Multiwalled Carbon Nanotube-

Supported Nickel Catalysts Using Incipient Wetness Method. Journal of

Physical Chemistry A, 2010; 114:3962-3968.

Chapter 5: P. Azadi, S. Khan, F. Strobel, F. Azadi, R. Farnood, Hydrogen Production

from Cellulose, Lignin, Bark and Model Carbohydrates in Supercritical Water

using Nickel and Ruthenium Catalysts, Applied Catalysis B: Environmental,

117, 330-338.

In addition, the following papers have been published or submitted for publication from

my PhD work.

P. Azadi, R. Carrasquillo-Flores, Y.J. Pagán-Torres, E.I. Gürbüz, R. Farnood, J.

Dumesic, Catalytic Conversion of Biomass Using Solvents Derived from Lignin,

Green Chemistry, 2012, DOI: 10.1039/C2GC35203F

P. Azadi, J. Otomo, H. Hatano, Y. Oshima, R. Farnood, Interactions of Supported

Nickel and Nickel Oxide Catalysts with Methane and Steam at High

Temperatures, Journal of Chemical Engineering Science, 2011, 66, 41964202.

7

P. Azadi, Clement Vuillardot, R. Farnood, Estimation of Heating Time and

Length in Supercritical Water Reactors, Journal of Supercritical Fluids, 2010, 55,

1038-1045.

P. Azadi, J. Otomo, H. Hatano, Y. Oshima, R. Farnood, Hydrogen production by

catalytic near-critical water gasification and steam reforming of glucose,

International Journal of Hydrogen Energy, 2010, 35, 3406-3414

P. Azadi, K. M. Syed, R. Farnood, Catalytic Gasification of Biomass Model

Compound in Near-Critical Water. Journal of Applied Catalysis A, 2009, 358, 65-

72.

P. Azadi, A. Khodadadi, Y. Mortazavi, R. Farnood, Hydrothermal Gasification of

Glucose using Raney Nickel and Organometallic Catalysts. Journal of Fuel

processing technology, 2009, 90, 145-151.

2- Review of Heterogeneous Catalysts for Supercritical Water

Gasification

* Based on: P. Azadi, R. Farnood, (2011), Review of Heterogeneous Catalysts for Sub and Supercritical

Water Gasification of Biomass and Wastes, International Journal of Hydrogen Energy, 36, 9529-9541

2.1 Introduction

Gasification is an effective thermochemical route to convert carbon-containing feeds into

carbon monoxide, hydrogen, carbon dioxide and methane. The resulting gas (i.e.

collectively known as syngas) can be combusted to produce heat or it can be further

processed to produce more hydrogen by water-gas shift reaction, methanol, and synthetic

liquid fuels using Fischer-Tropsch process. Currently, hydrogen is mainly produced by

8

steam reforming of natural gas and naphtha due to their lower costs compared to biomass.

However, it is anticipated that biomass as feedstock for hydrogen production will have a

more significant contribution in near future because of the increasing demand for natural

gas and the undesirable environmental impacts associated with the excessive use of fossil

fuels. Since biomass gasification is theoretically carbon neutral, it is expected to play a

crucial role in future energy. In conventional gasification techniques, a controlled amount

of oxygen and/or steam are injected into the gasifier to partially oxidize or reform the

carbon, resulting in the formation of hydrogen, carbon monoxide and carbon dioxide.

Partial oxidation releases energy that is used to keep the temperature of the gasifier at the

desired value. Also, injection of steam into the reactor promotes hydrogen production by

the water gas shift reaction. One important drawback of this process is the need for

preprocessing of the feed to reduce its water content. As an alternative, researchers have

studied the gasification processes in sub and supercritical water to address this issue.

Supercritical water (SCW) can be used as both reaction media and reactant

simultaneously. Hydrogen production is a promising application of the catalytic SCWG

process. In this process, biomass is decomposed to form hydrogen, methane, carbon

dioxide, carbon monoxide, and small amounts of higher hydrocarbons [1-12]. The unique

properties of SCW such as harsh critical temperature and pressure along with its low

dielectric constant make SCW an attractive solvent with tunable dissolving power. Many

permanent gases and most organic compounds are soluble in SWC; therefore, the mass

transfer barrier between different phases does not exist in reactions occurring in SCW. It

should be noted that depending on the temperature and pressure, the density of water near

9

its critical point varies between 100 to 600 kg/m3. These properties render SCWG as a

promising technology for gasification of dilute wet feeds.

Figure 2.1 demonstrates the process flow diagram of SCWG of wet feeds along with its

subsequent applications.

Figure 2.1 Process flow diagram of SCWG and its subsequent applications.

2.1.1 Feedstocks

Performance of catalytic SCWG is generally influenced by feedstock type and

concentration, catalyst type and loading, and the contact time between the catalyst and

the feed. Two different types of feedstocks are typically utilized in the laboratories to

evaluate the effectiveness of the SCWG process: real biomass and model compounds.

10

Investigations on real biomass feeds provide practical information on the performance of

gasifier at different operating conditions while model compounds are used for sake of

more fundamental studies, aiming at representing the actual gasification conditions.

A variety of real biomasses including lignocellulosic biomass from different sources,

sewage sludge, chicken manure, food wastes, algae, and fermentation residue have been

successfully gasified in SCW. However, as lignocellulosic biomass is the most abundant

type of biomass on earth, gasification of such materials has been the primary focus of

many investigations. There are three constituents in lignocellulosic biomass: cellulose,

hemicellulose and lignin. Cellulose is a linear biopolymer with both crystalline and

amorphous structures made of glucose monomers. Hemicellulose is an amorphous

polymer with different sugar monomers such as xylose and glucose. Lignin is a highly

cross-linked copolymer mostly made of three types of phenylpropane monomers which

are methoxylated to different degrees: para-coumaryl alcohol, coniferyl alcohol and

sinapyl alcohol.

During the SCWG, cellulose, which is the major component of woody biomass, is

initially hydrolyzed at temperatures above 200oC, resulting in formation of glucose,

fructose and sugar oligomers [1]. This step is known to be fast and it is associated with

high yields. Subsequently, furfural, phenols, acids and other intermediates are formed

from soluble carbohydrates, which further undergo decomposition and reforming to

generate hydrogen, carbon dioxide, carbon monoxide and methane. However, at low

temperature hydrothermal gasification, specially in absence of catalysts, a significant

amount of char is produced due to condensation reactions.

11

Gasification of model compounds can be studied both as a single component and as

mixtures of various model molecules to determine possible interactions among different

reactants during the gasification process. Examples of single reactant are glucose,

cellulose, xylan, ethylene glycol, glycerol and various functional types such as alkanes,

alcohols, phenolics, organic acids to name a few. It has been found that due to the very

fast hydrolysis of the cellulose and the consequent formation of glucose and sugar

oligomers, gasification of glucose and cellulose practically lead to identical gas yields

[13]. Moreover, mixtures of cellulose/xylan, cellulose/lignin and xylan/lignin have been

considered for gasification [14]. Fundamental studies by these model compounds

indicated that there is a strong deviation from rule of mixture while gasifying a mixture

of lignin with cellulose and xylan [14, 15]. However, mixtures of cellulose and xylan at

any ratio exhibit a very predictable gasification efficiency that perfectly matches with the

expectations from the rule of mixtures in terms of gas composition and yield [14, 15].

Similar deviation from the rule of mixture has been observed when humic acid is mixed

with other model compounds representing activated sludge [16]. Despite the existence of

some similarities between gasification of real biomass and pure model compounds

obtained by utilization of fresh catalysts, gasification of these two types of feedstocks

may result in a significant difference in the rate of catalyst deactivation. This is partly due

to the existence of sulfur and other inorganic compounds (e.g. ash) that react with the

active metal and form a dramatically less active surface, or physically block the catalyst

pores and reduce the number of accessible sites [17].

12

2.1.2 Types of reactors

During the past years, different laboratory reactor systems have been utilized to study the

catalytic gasification in supercritical water medium, including batch, fixed bed, CSTR,

and quartz capillary reactors. Also, few studies reported application of fluidized beds in

SCWG [18, 19]. During the course of these studies, researchers found that tar formation

is accelerated when the reactants are kept at moderate temperatures for a prolonged time.

Although tarry materials undergo further decomposition to form gases at high

temperatures, it is suggested that their contribution to char formation is significant [13,

20]. Therefore, in order to avoid char formation or to minimize this effect, the heating

rate should be as high as possible. Typical heating rate values in literature range from 1 to

500oC/min. Recently, a computational fluid dynamics model has been developed to

estimate the heating rate of tubular SCW reactors [21].

Batch reactors are essentially autoclaves with a typical volume between few milliliters to

1 liter. Since the vapor pressure of water is significantly higher than the partial pressure

of the produced gases at the reaction conditions, the pressure of the batch reactors cannot

be easily controlled and it is mainly governed by the reaction temperature and the fraction

of reactor initially filled with water. Furthermore, since reactions also take place during

the warm-up time, the heating rate may affect the product distribution. Therefore,

autoclaves heated by external electrical heaters would not be appropriate systems for

investigating catalysis in SCWG process. Nonetheless, such systems have been widely

used for SCWG experiments and there is a large body of literature that overlooked this

issue by the inaccurate reporting of the batch reaction time. Utilization of molten salt bath

13

with a high thermal mass and superior heat transfer properties, sand bath and injection of

concentrated feed to a previously heated reactor via high pressure pumps are among the

possible solutions to this issue.

It is suggested that capillary quartz tubes may be a safe and cheap high-pressure batch

reactor with a high heating rates [22-29]. Although the final gasification products can be

evaluated both visually and quantitatively, these capillary tubes does not seem to be

suitable for testing catalyst powders due to their small inner diameter which leads to non-

uniform distribution of catalyst along the reactor. As a result, diffusion in the small

capillary tubes could become the rate limiting step and affect the product yields. Overall,

these capillary tubes are quite useful in providing additional information in terms of

visual observation and gaseous products for catalyst-free tests.

Continuous flow reactors have been also used in SCW experiments. However, due to

practical difficulties with pumping precise amounts of slurries into high-pressure

reactors, except for a few studies, their application was mostly limited to soluble organic

feeds. Running the flow reactors at high heating rates is not a serious issue as there are a

variety of techniques to fulfill this task, among them are utilization of a preheater [30],

multiple zone furnaces [31], mixing of a concentrated feed with hot water right before the

catalyst bed [32], and using a swirl generator at the entrance of the reactor [33].

2.1.3 The role of catalysts in SCWG

Gasification of organics in SCW without catalyst has been studied thoroughly. A part of

data on this topic has been reported in papers dealing with catalytic SCWG for evaluating

14

the catalyst effectiveness. Catalyst-free SCWG usually results in a higher CO

concentration due to intrinsically low rate of water-gas shift reaction. Also, very high

temperatures are required to achieve acceptable conversions. A comprehensive overview

of the SCWG without catalyst is provided in [1].

In terms of reaction temperature, three ranges have been considered for SCWG process

[2]: high temperature supercritical water, low temperature supercritical water and

subcritical water gasification. The reaction mechanism changes from free radical to ionic

by decreasing the temperature [3, 34]. A typical temperature range for high temperature

supercritical water gasification is between 550 to 700oC. Thanks to high reaction rates,

complete gasification is achievable in the absence of catalysts or by utilization of carbon

(e.g., coconut shell activated carbon) or alkali salts to inhibit tar formation [2, 35, 36]. It

was shown that a solution containing 11 wt% glucose can be completely gasified at

700oC using a flow reactor [37].

For low temperature (374 to 550oC) supercritical water gasification, transition metal

catalysts, such as nickel and ruthenium, are usually employed to overcome the energy

barriers at lower reaction temperatures [2, 3, 38, 39]. Although complete gasification of

cellulose and lignin has been reported for this range of temperature, catalyst deactivation

is still problematic and needs further improvements [39].

In subcritical region, only highly active metal catalysts such as Raney nickel and

platinum at low space velocities are useful for gasifying oxygenated organic compounds

[40].

15

Gasification in SCW is overall an endothermic reaction. Since SCWG is operated at very

high water content and given the considerably high specific heat of water, it is of great

interest to reduce the reaction temperature as much as possible. Also, it is crucial to

recover the thermal energy of the reactor’s effluent to heat up the feed. Figure 2.2 shows

the change in the enthalpy of water as a function of temperature. It is worth mentioning

that running the reactor in the temperature range at which a sudden increase in enthalpy

of water occurs may dramatically decrease the efficiency of the heat recovery and thus,

should be avoided [1, 32].

In this paper, different aspects of SCWG with heterogeneous catalysts are critically

reviewed. Without underestimating the possible roles of homogeneous catalysts for

conversion of biomass into gases in SCW, this review paper solely focuses on catalytic

SCWG catalyzed by solid catalysts. The heterogeneous catalysts are categorized in three

major groups: activated carbon, metal, and oxide catalysts. Metal catalysts are further

divided to unsupported (e.g., metal wires and skeletal catalysts) and supported catalysts.

Major findings regarding activity, selectivity, stability and the rationale for use of such

catalysts are presented.

16

Figure 2.2 Variation of enthalpy of water as a function of temperature.

2.2 Current Status of Catalytic SCWG

Being the subject of research for more than 20 years, SCWG has been now established as

one of the most promising routes for converting wet biomass to gaseous fuels. Three

different scenarios for SCWG are practically possible, each of which offers its own

advantages and drawbacks: a) high temperature SCWG either in absence of catalysts or

with activated carbon as the catalyst; b), gasification in the presence of homogeneous

alkali catalysts and c) gasification at milder conditions with the aid of active metal

catalysts.

During the past two decades, over 100 journal papers have been published regarding

different aspects of heterogeneous catalysis in the SCWG process. These studies include

both commercial as well as catalysts tailored for gasification in SCW.

17

Majority of commercially available catalysts applied in SCWG process are industrially

practiced for hydrogenation (e.g., Raney nickel) and steam reforming of methane and

naphtha (e.g., Ni/Al2O3). In addition to this, supported noble metal catalysts (e.g., Ru/C,

Ru/Al2O3 and Pt/Al2O3) prepared by catalyst manufacturers are widely used for their high

activity and ease of use as there is often no need for reduction of the active metal prior to

the experiment. Also, dispersion of the noble metals in commercial catalysts are rather

high (e.g., 30-50%), providing a greater number of catalytic sites per mass of active metal

and thus, lowering the price of the catalyst. Modification of some industrial nickel

catalysts have been carried out aiming at increasing the life time of the catalyst (e.g.,

addition of Ru to BASF reforming catalyst [38]) or improving the selectivity (e.g., Sn

incorporated onto the surface of Raney nickel [41-44]).

So far, extensive efforts have been made by researchers to demonstrate the potential

application of SCWG process to convert various types of biomass into hydrogen,

methane and syngas. A great deal of investigation has been focused on the effects of

operating conditions and reactor design on gasification efficiency. Although many

researchers made use of heterogeneous catalysts to accelerate the gasification rate, few

studies were solely dedicated to design of novel catalysts tailored for SCWG, and provide

fundamental insight that leads to further advancements in the field. This was partly due to

the fact that the available catalysts in the market or laboratory-made supported catalysts

with simplest formulations (such as nickel on alumina or magnesia) were sufficiently

active to make a remarkable difference in the gas yields obtained from catalyzed and

uncatalyzed experiments and prove the potential advantageous of a catalytic process.

18

However, the key to further advance the effectiveness of heterogeneous catalysts for

SCWG is to develop a better fundamental understanding of the relationships between

catalyst formulation and structure to its performance. Particularly, formulating bimetallic,

alkali promoted and sulfur resistant catalysts would be very beneficial for improving

selectivity and lifetime of the catalyst and consequently, making SCWG process an

economically viable process.

Successful design of catalysts depends on careful consideration of its catalytic,

chemicophysical and mechanical properties. This concept has been introduced by

Richardson [45] as the catalyst design triangle (Figure 2.3).

Figure 2.3 Catalyst design triangle introduced by Richardson [45] (adapted from

[46]).

Review of open literature on catalytic SCWG indicates that among the above factors,

only catalytic activity has been well studied. On the other hand, very limited studies were

focused on enhancing the selectivity and stability of the heterogeneous catalysts for

19

SCWG. More detailed fundamental studies in future should be undertaken to clarify the

relationship between chemicophysical and mechanical properties of catalysts to their

performance.

It is known that the electronic structure of a metal or alloy is solely responsible for its

catalytic activity with respect to any given reaction [47]. Recent advancements in

computer-based catalyst design, particularly in density functional theory (DFT), has now

made it possible to predict the reactivity of catalytic surfaces and reactants as a tool to

improve catalytic performance and for the design of better catalysts. One is now able to

find more active, more selective and perhaps cheaper catalysts for SCWG by applying

these methods to transition metals and their alloys. From economic perspectives, it is of

particular value that the formulations of the new catalysts for SCWG be preferably based

on earth-abundant elements.

2.3 Performances of Solid Catalysts

Review of literature indicates that three types of heterogeneous catalysts have been used

for accelerating the reactions associated with the gasification of organics in SCW:

activated carbon, transition metals, and oxides. In this section, we review the major

findings in the field of catalytic SCWG and present the achievements in gasification of

biomass in SCW.

Selected results from literature are tabulated and captured in terms of carbon conversion

and gas yields. Carbon gasification ratio (CGR) is defined as the ratio of carbon in the

final gas products over initial amount in the feed to the reactor. Yields of hydrogen and

methane, the two most useful products of SCWG, are reported per unit mass of the

20

organic feed. As for the continuous reactors this is represented by weight hourly space

velocity (WHSV) which is reported as the mass flow rate of the organic matter passed per

unit mass of the catalyst.

2.3.1- Activated carbon

Carbons obtained from natural sources such as trees, plants, shells, coal and wood can be

treated in high-temperature inert gas, carbon dioxide and/or steam to tailor its properties

for being used as a catalyst support or as a catalyst by itself. The physical and chemical

properties of carbons are not only affected by the treatment conditions but also by the

source of carbon. Treatment of carbon at a moderate temperature and an active

atmosphere results in the production of activated carbon of ultra-high surface area. On the

contrary, treatment at higher temperatures an inert atmosphere produces low-surface area

graphite. The pore size and surface area of the activated carbons typically varies between

0.5-1nm and 800-1500 m2/g, respectively. Due to their superior stability in reducing

environments, negligible effects on the reaction, and high degree of metal dispersion,

carbon supported metal catalysts are widely used in industry for hydrogenation reactions

in production of fine chemicals.

Activated carbons from different sources are found to be catalytically active for high

temperature SCWG reactions (Table 2.1). The idea of using activated carbon as a catalyst

was possibly originated from their previously proven performance for tar cracking in

downdraft gasifiers. Despite the fact that activated carbon is the best known

21

heterogeneous catalyst for high temperature (e.g. > 600oC) SCW applications in terms of

cost, the exact mechanism of its action as a catalyst in the context of SCWG is still

unknown. Since the activity of the carbon catalyst was not a strong function of its total

surface (i.e. sum of outer and pore surfaces), it was concluded that only the external

surface of carbon particles takes part in gasification [33, 48]. Furthermore, the presence

of different minerals in the activated carbons from various sources did not alter the

gasification rate [33]. Also, it has been reported that the rate of gasification of activated

carbon in SCW at 600oC and 650

oC are about 2.7x10

-6 and 7.2x10

-6 s

-1, respectively [30].

Accordingly, at 650oC, about half of the initial mass of carbon catalyst would be gasified

in about 27 h. The hydrogen to carbon dioxide ratio in the produced gas from activated

carbon in SCW was roughly two and the small amount of methane formed in such

reactions likely had a pyrolytic origin [30].

Using carbon in low temperature SCW cannot enhance the rate of gasification [49]. Thus,

useful data on activity of carbon for catalyzing SCWG reactions are limited to high

temperature SCWG region. However, due to high conversion of gasification at such high

temperature SCW even in absence of catalyst (typically above 80%), comparison

between the results obtained from uncatalyzed and carbon-catalyzed experiments is

challenging. Nevertheless, the gas mixtures obtained from carbon-catalyzed SCWG in

continuous reactors contain significantly smaller amounts of carbon monoxide. Overall,

more fundamental research should be conducted to clearly explain the role of carbon in

SCWG reactions, particularly for tar cracking and water-gas shift reactions, and to better

demonstrate its usefulness in practical applications.

22

Table 2.1 Selected results of catalytic SCWG using activated carbon as catalyst. T

emp.

Rea

ctor

*

Fee

d

Conc.

Cat

alyst

Fee

d /

Cat

alyst

Tim

e

WH

SV

CG

R

H2

CH

4

Ref

.

[oC] - - wt% - g/g min h

-1 % mmol/g mmol/g -

400 B Lignin 3.3 Charcoal 0.7 60 - 8 0.3 0.8 50

500 C Glucose 18 Coconut

shell AC - - 14 51 2.5 1.3 33

600 C Glucose 22 Coal AC - - 20 97 8.2 5.7 33

600 C Glucose 22 Coconut

shell AC - - 22 100 12.4 6.8 33

600 C Glucose 22 Spruce

wood

charcoal - - 13 99 21.4 7.5 33

600 C Sewage

sludge 3

Coconut

shell AC - - 0.5 11 0.6 33

650 C Corn

starch 10

Coconut

shell AC - - 3.8 100 31 9.2 51

650 C Glucose 11 Activated

charcoal - - 12 92 8.9 7.2 52

710 C Potato

starch 12

Coconut

shell AC - - 89 15.5 10.5 48

* B: batch, C: continuous

Separation and recovery of activated carbon catalyst is an important practical

consideration when catalyst is suspended in the reactor. This concept has been

23

successfully illustrated in a pilot scale SCWG plant used for gasification of poultry

manure [53].

2.3.2- Transition metals

There is a wide range of metal catalysts with adjustable physicochemical properties that,

in contrast to activated carbon catalyst, could be tailored to meet the requirements of the

catalytic process of interest. In SCWG process, metal catalysts have been employed in

both supported and unsupported forms.

2.3.2.1- Unsupported catalysts

Within the context of catalytic SCWG, unsupported metal catalysts can be categorized in

two groups: powders and wires with low specific surface area and skeletal structures (i.e.

Raney catalysts). Applications of both forms in SCWG are discussed in details below.

Powders and wires

Few researchers have used metals and metal oxides in forms of powder or wire as a

catalyst for SCWG process. There are two motivations behind using such materials in

laboratory experiments: 1) demonstrating the inherent ability of different metals for

catalyzing SCWG reactions (mostly in case of metal wires), and 2) to examine the

possibility of using these unsupported metal particles as the actual catalyst in real large

scale gasification process. It should be noted that, in most cases, the specific surface areas

of these two forms of metallic materials are very small (i.e. <<1 m2/g). Indeed, metal

powders and wires have an extremely limited number of catalytically active sites. Nickel

[39, 54], nickel oxide [55], inconel [29], ruthenium [24], ruthenium oxide [55, 56], and

24

platinum [57] have been tested for catalyzing gasification of organics in supercritical

water. Pt-black, which has an appreciable surface area, showed considerable turnover

frequency for gasification of ethylene glycol at low temperatures [57]. However, given

the high price and low dispersion of platinum or other noble metals in powder form,

application of such materials has been limited to fundamental laboratory studies. Except

Pt-black, other metal powders have exhibited insignificant activity at low temperatures.

For instance, complete gasification of cellulose was achieved at 450o

C in the presence of

RuO2 powder whereas no improvement over uncatalyzed experiment was observed for

the same reactions at 350oC [55]. Moreover, lignin and lignin-containing mixtures

inhibited the catalytic effects of RuO2 [56]. It has been hypothesized that the flexible

structure of lignin’s aliphatic chains captured the RuO2 catalyst and diminished its

catalytic effects. However, more detailed investigations on this issue are needed to

validate this hypothesis.

Metal wires have been used to act as catalyst in capillary quartz reactors. In such reactors,

the distance between the reactants and the metallic surface (i.e. catalyst) is small and it is

always less than the tube diameter. Nevertheless, there are some concerns regarding the

application of metal wires to catalytic supercritical water gasification reactions. Firstly,

metal wires are unable to provide a reasonable surface area per mass of the feed in the

reactor. Secondly, catalysts cannot be reduced in situ prior to the experiment to ensure its

surface reactivity. Furthermore, since the diameters of these wires (e.g. 0.25 mm) are

typically comparable to the inner diameter of the capillary reactors (e.g. 1-2 mm), both

free volume and open cross section of the capillary tubes may change upon use of wires,

25

especially if multiple wires are employed. Overall, quartz capillary tubes seem to be quite

suitable for study of unanalyzed SCWG reactions due to elimination of wall effects, but

their utility for heterogeneous catalysis is inherently limited.

Raney (skeletal) catalysts

Raney catalysts are prepared by leaching out aluminum from a metal-aluminum alloy,

resulting in formation of the target metal with a spongy structure. The remaining material

typically contains a few percent of aluminum [58]. The low initial cost of the raw

materials per unit mass of metal used for making skeletal nickel results in lowest the cost

per unit mass of active catalyst [59]. Among various skeletal catalysts, Raney nickel is

found to be most active in SCWG [60]. Despite the fact that many different Ni-Al phases

can be treated with a solvent to leach away aluminum, the ratio between nickel and

aluminum in the initial alloy plays an important role on the activity of such catalysts.

Ni2Al3 (59% Ni) and NiAl3 (42% Ni) are the two most commonly used proportions for

synthesis of Raney nickel catalysts. Dissolving of NiAl3 in an alkali solution occurs more

effectively than Ni2Al3. In the early stages of development of Raney nickel catalysts, the

Ni-Al alloys were used to treat with excess amounts of sodium hydroxide at relatively

high temperatures (~120 oC) at prolonged times (~7h). It should be noted that reaction of

aluminum with sodium hydroxide is extremely exothermic and digestion of aluminum at

high temperatures and long treatment times would result in formation of alumina hydrate

(i.e., Al(OH)3) through hydrolysis of sodium aluminate. In order to address this issue,

several methods have been suggested by the researchers, among them are the gradual

26

addition of Ni-Al alloy to the caustic alkali and the addition of Ni-Al alloy to the solvent

at a low temperatures (e.g. -20 oC). For more detailed information on the preparation of

Raney catalysts refer to [58].

The specific surface area of fresh Raney nickel ranges from 50 to 100 m2/g. However,

there are some evidences that Raney nickel may undergo aging even at room temperature

and partially lose its active area and the stored hydrogen and slowly forms nickel oxide

on the surface [61, 62]. Therefore, it is suggested that skeletal nickel catalysts should be

used within a year after synthesis.

Raney nickel is often available in the slurry form in degassed water and it is often used

without any pretreatment (such as reduction). It should be also noted that depending on

the preparation method, Raney nickel may contain a considerable amount of stored

hydrogen in its structure. This amount can be as much as one order of magnitude greater

than the amount of hydrogen that is chemisorbed on the catalyst surface [59]. When

Raney nickel catalyst is subjected to hydrothermal environment, it may release the stored

hydrogen. Consequently, at high catalyst to feed ratio in batch experiments, the release of

stored hydrogen along with the partial oxidation of nickel by water may lead to the

formation of considerable amounts of hydrogen in the gas phase, leading to erroneous

results that are difficult to interpret and sometimes misleading. The X-ray photoelectron

spectroscopy (XPS) analysis as well as CO and CH4 formation in SCW in absence of

organic feed indicated that the surface of fresh commercial Raney nickel catalyst may

also contain carbon [24, 42].

27

Table 2.2 Selected results of catalytic SCWG using Raney (skeletal) nickel

catalyst.

Tem

p.

Rea

ctor

*

Fee

d

Conc.

Fee

d /

Cat

alyst

Pro

mote

r

Hea

ting

tim

e

tim

e

WH

SV

CG

R

H2

CH

4

Ref

.

oC - - wt % g/g min min h

-1 % mmol/g mmol/g

225 C Sorbitol 5 Sn 0.27 59 20.2 2.9 41

225 C Glycerol 5 Sn 0.54 81 46 3.2 41

265 C Sorbitol 5 Sn 0.54 75 22.6 3.6 41

265 C Glycerol 5 Sn 0.54 99 51.3 5.8 41

350 B Cresol 10 45 90 93 0.7 39.8 39

350 B Glucose 1 2 1 15 70 6.8 4.5 63

350 B Glucose 6 3 1 15 43 5.4 1.2 63

380 B Glucose 6 7 1 15 45 7 5.6 60

380 B Glucose 6 7 Mo 1 15 43 7.2 5.2 60

380 B Sludge 3 0.5 1 15 68 11.8 11.7 17

380 B Glycerol 3 1.3 1 15 87 27.1 15.4 16

380 B Glycine 3 1.3 1 15 51 16.7 5.1 16

380 B HAa 3 1.3 1 15 19 9.2 2.0 16

400 C SLW b 20 5 ~3 ~6 64

400 B Sawdust 10 2 5 24 46 1.2 7.4 65

400 B Sawdust 10 2 6 92 100 3.3 20.1 65

450 B PS c 10 2 Fe 30 20 - 22 16 66

450 B PS c 10 2 Mo 30 20 94 17 13 67

500 B Glucose 5 10 475 60 45 8.3 4 20

500 B Glucose 5 10 158 60 38 9 2.2 20

500 B SFS d 6 10 158 60 8 3.2 68

500 B Corncob 6 10 158 60 3.5 3.7 68

650 B Methane 15 26% 66% 69

750 C Coal 2 15 60% 10% 70

* B: batch, C: continuous a Humic acid

b synthetic liquefied wood

c peanut shell

d sunflower stalk

Selected results obtained from SCWG of organic maters with Raney nickel catalyst are

listed in Table 2.2. These results indicate that carbon conversion and hydrogen yield

strongly depend on the nature of the feed, feed to catalyst ratio and the contact time

between the feed and the catalyst. Raney nickel has been applied to a variety of feeds at

a wide range of operating conditions in SCWG process. In all cases, authors reported a

28

significant improvement in conversion upon utilization of this catalyst. Furthermore, it

should be highlighted that whenever Raney nickel and other catalysts were used in the

same reaction, Raney catalysts resulted in one of the highest conversions compared to

other catalysts. Fresh skeletal nickel catalyst has a considerable capability in cleaving C-

O bonds. Therefore, if used for gasification of oxygenated compounds, it consumes a

portion of the produced hydrogen and results in a methane-rich gas mixture. This is

particularly more pronounced at lower operating temperatures and higher feed

concentrations which both favor high methane concentrations at equilibrium. If hydrogen

production is the target of gasification, three approaches can be utilized to address this

concern and increase the hydrogen selectivity. Firstly, reaction time (or equivalently

weight hourly space velocity) can be optimized to achieve maximum amount of hydrogen

before substantial methanation occurs. Consequently, the maximum hydrogen yield may

not translate to complete carbon gasification and its exact value depends on the feed type,

concentration as well as the reaction temperature. However, there is always a risk

associated with this strategy: partial carbon gasification could result in faster catalyst

deactivation and/or reactor clogging due to formation of tarry materials over time. The

second strategy to address the low hydrogen selectivity of Raney nickel catalyst is to

modify its surface chemistry to retain its high C-C breaking activity but to retard C-O

breaking ability. It has been shown that surface modification of Raney nickel with small

quantities of tin can fulfill this requirement and significantly enhance the hydrogen to

methane ratio in the products [41-44]. Other than tin, the addition of other promoters such

as Mo and Fe may also alter the activity and selectivity of Raney nickel, but the effects of

these promoters are far less pronounced. The third strategy to increase the hydrogen

29

selectivity, which is only applicable to the supported catalysts, is utilization or

modification of the supports. For instance, it has been shown that addition of ceria to

alumina support can enhance the selectivity of the catalyst for SCWG reactions [71].

Also, it should be noted that methanation reaction is more sensitive to metal dispersion,

and as a result, increasing the metal loading on the catalyst may improve the hydrogen

selectivity.

2.3.2.2- Supported catalysts

Supported nickel catalysts

A wide variety of supported nickel catalysts have been used for catalyzing SCWG (Table

2.3). In the absence of an organic feed, nickel is found to react with SCW to form nickel

oxide and hydrogen. Nickel oxide hardly has any catalytic activity for the reactions

involved in SWG. Given that, a minimum feed concentration is needed to keep the nickel

surface reduced. A large number of papers on the activity of nickel catalysts in SCW

have been published where stability of the support in SCW was overlooked. In batch

experiments, the rates of gasification reactions and deactivation of a catalyst with an

unstable support (i.e., due to hydrolysis, phase change, etc.) may be comparable.

Therefore, even for a typical batch experiment (e.g. ~30 min), instability of catalyst

supports may significantly affect the carbon conversion and product distribution. Due to

the high turnover frequency associated with nickel (mostly edges and steps [47]) for

cleaving C-O bonds, high carbon conversions in batch reactors are always associated

with low hydrogen selectivity whereas higher hydrogen selectivity may be obtained in a

30

continuous reactor with the same catalyst at the same carbon conversion. Until now, no

useful support or promoter has been found to be able to significantly improve the

hydrogen selectivity of supported nickel catalysts. Our recent study showed that using the

same total weight of nickel, hydrogen selectivity improves by increasing the nickel

loading on an alumina support. Typically, if a nickel/support catalyst is found to be active

for gasifying a certain type of biomass or a model compound under hydrothermal

condition, the same catalyst will be effective for the decomposition of other organic feeds

under similar operating conditions. One major exemption from this rule is gasification of

lignin, which is highly cross-linked biopolymers, as well as the humic substances.

31

Table 2.3 Selected results of catalytic SCWG using supported nickel catalysts.

Tem

p.

Rea

ctor

*

Support

Ni

Fee

d

Conc.

Fee

d /

Cat

alyst

Tim

e

WH

SV

CG

R

H2

CH

4

Ref

.

oC - - % - wt % g/g min h


210 C SiO2 19 EG a 10 12 1.6 1.1 72

350 B G1-80BASF Phenol 10 120 88 1.9 31.6 38

350 B -Al2O3 48 Cresol 10 0.7 100 89 2.8 34.8 73

350 B -Al2O3 48 Ethanol 10 0.7 80 100 1.3 32.8 73

350 B YSZ Cresol 10 60 0.2 39

350 B Graphite Cresol 10 60 ~ 0 39

350 B SiO2-Al2O3 62 Cresol 10 100 54 1.5 21 39

350 B Kieselguhr 50 Cresol 10 100 38 1.8 15 39

350 B MgO-Al2O3 25 Cresol 10 80 24 10.1 7.7 39

350 B -Al2O3 25 Cresol 10 100 5 4.3 1.3 39

350 C -Al2O3 48 Cresol 2 99 1 40 74

350 C BASF Cresol 2 97 0.7 34.5 74

350 C SiO2-Al2O3 62 Cresol 2 69 1.9 28.9 74

350 B Ni5132-Engel. Cellulose 2.5 20 74 11 7 15

350 B Ni5132-Engel. Xylan 2.5 20 69 7.5 8.5 15

350 B Ni5132-Engel. Lignin 2.5 20 9 1 0.5 15

350 B SiO2-Al2O3 50 Cellulose 14 2.5 70 14 6.8 86

350 B AS b Cellulose 14 2.5 ~30 58 8.6 6.7 75

350 B SiO2 Cellulose 14 2.5 ~30 46 6.6 4.5 75

350 B MgO Cellulose 14 2.5 ~30 76 8 12.8 75

350 C C 46 Phenol 0.26 0.18 100 6.3 35 76

390 C ZrO2 15 PEG d 50 37 3 77

400 C Ni5256-Engel. Glucose 0.4 ~60 15 1 78

400 B MgO 20 Lignin 5.5 1 120 15 5 2.5 79

400 B Kieselguhr Cellulose 33 5 60 67 3.5 10.6 80



400 B Ni5132-Engel. Lignin 0.8 20 17 1 3 14

400 B C 5 Lignin 3.3 0.7 60 19 0.7 2.6 49

400 B -Al2O3 20 Glucose 9 5 20 33 10.5 2.5 71

400 B CeO2- -Al2O3 20 Glucose 9 5 20 35 12.7 2.1 71

420 B -Al2O3 Wood 1 23 4.2 1.7 81

500 B CNT d 58 Cellulose 9 1.6 15 30 8 3.4 82

500 B SiO2-Al2O3 65 Glucose 4.5 6.3 30 78 6.2 4.1 83

550 C 32 Decane 7.5 50 26.7 26 84

600 C ZrO2 10 BC e ~2.5 0.2 96 39.2 ~ 8 85

600 C TiO2 10 BC e ~2.5 0.2 74 21.6 ~6.1 85

650 C C 16 Glucose 11 98 13.6 6.2 52

* B: batch, C: continuous a ethylene glycol b

aluminum silicate c polyethylene glycol d

carbon nanotube e Biocrude oil obtained from switchgrass

32

Many research groups have studied the performance of supported nickel catalysts for

SCWG. However, since the gas yield strongly depends on reactor design, feed

concentration and operating conditions (e.g. temperature, feed to catalyst ratio, water

density and reaction time), direct comparison between these results is rather challenging,

if not impossible. Critical review of the literature on the SCWG of biomass with

supported nickel catalysts reveals that no clear correlation between support’s properties

and gasification yield is yet established. Instability of the support and its impact to the

activity of the supported catalysts adds to the complexity of this issue.

Our recent results in batch experiments (to be published elsewhere) showed that nickel

dispersion, which can be implicitly correlated to metal loading and support’s surface area,

mostly affects the methane formation rate, whereas its impact on the carbon gasification

efficiency is less pronounced. This is consistent with the results presented in [86] that

there is no clear evidence that carrier’s surface area can directly affect the carbon

conversion. However, the suggestion that only nickel particles deposited on the external

surface of the catalysts contributes to the gasification [86] is questionable. In fact, it has

been reported that ruthenium/activated carbon catalyst, in which the active metal is

dispersed into the fine pores of activated carbon (see table 2.5), is quite effective for the

gasification of various biomass compounds. Furthermore, our experiments with pelletized

and powder Ni/Al2O3 catalysts produced the same amount of gas with similar

composition (15min, 380 oC and 2 wt% glucose). This result suggests that gasification

reactions, at least at the condition mentioned above, are not diffusion limited.

33

Catalyst supports with strong acidic properties such as zeolites result in the dehydration

of the organics in aqueous phase, which subsequently consumes a portion of the

generated hydrogen to hydrogenate the carbon double bond. The existence of carbon

double bond would also lead to C-C bond formation, and as a result, lower gasification

efficiency will be achieved. These two pathways eventually lead to poor CGR and

hydrogen selectivity.

In order to better understand the effects of catalyst carrier on the catalytic gasification of

organics in SCW, we have recently carried out a high throughput catalyst screening.

Table 2.4 lists some of the results obtained from the gasification of 2 wt % glucose

solution using 1 g of supported catalysts containing 5 wt% reduced nickel. This study

was conducted using a 50 mL stainless steel batch reactor that was heated by immersing

into a molten salt bath at 380oC. For more detailed information about the experimental

setup and analytical methods, please refer to [17].

According to Table 2.4, Ni/α-Al2O3 was found to be, at least as active as Ni/γ-Al2O3

under the conditions tested, clearly in contrast to previously published data [39, 81]. We

also found that catalyst particle size affected neither the carbon gasification efficiency nor

gas composition. The complete set of results and more detailed experimental methods

will be published elsewhere.

Although nickel crystallites may sinter under hydrothermal conditions, the long term

activities of supported nickel catalysts are closely related to the stability of the carrier

under the reaction conditions. Stable supports in SCW are found to be carbon (e.g.

activated carbon [39, 52], carbon nanotube [82, 87]), α-Al2O3, rutile TiO2, and

34

monoclinic ZrO2 [39]; whereas silica, alumina (except α-Al2O3), MgO, Cubic ZrO2,

silica-alumina, aluminosilicate, and most zeolites are unstable in SCW. γ-Al2O3 is

reported to transform into α-Al2O3 [88] or boehmite (AlO(OH)) [39]. Using XRD

analysis, we confirmed the phase change from γ-Al2O3 to boehmite by hydrothermal

treatment of γ-Al2O3 at 380oC and 250bar for 60 minutes. Our studies indicated that

although α-Al2O3 does not undergo any phase change in SCW, it becomes more

crystalline upon exposure to SCW even for few minutes. Also, there is an inconsistency

in the literature regarding stability of high surface area TiO2 (i.e., anatase phase) in SCW

[38, 88]. A 15 wt% Ni/ZrO2 has been shown to retain its activity for at least 85h in SCW

at 390oC [77]. Stabilities of other supports such as YSZ have not been yet evaluated in

the literature.

It has been reported that doping of the supported nickel catalysts with Ru, Cu and Ag

[38] can improve the catalyst lifetime by suppressing the hydrothermal crystallite growth.

Catalyst regeneration (Ni/MgO) after SCWG has been also attempted [89]. It was found

that MgO interacts with SCW to form Mg(OH)2, and the initial composition can be

retrieved upon calcination at 750oC. However, the reduction of regenerated catalyst

resulted in lower active surface and hence, less catalytic activity.

35

Table 2.4 Performance of supported catalysts for gasification of 2 wt% glucose

solution. 380oC, 15min, glucose 0.2g, catalyst 1g, 5 wt% Ni, water 9.8g.

Support BET

[m2/g]

Particle

size

[µm]

CGR

%

H2

[mmol/g]

CH4

[mmol/g]

Catalyst-free - - 22 2.7 0.1

α-Al2O3 powder 10 1 84 18.3 9.5

α-Al2O3 powder 8 118 73 21.3 5.6

γ-Al2O3 spheres 225 4000 65 19.0 4.1

γ-Al2O3 powder 225 100 68 18.6 5.0

TiO2 49 59 16.7 3.5

MgO 14 71 26.6 6.2

YSZ 144 0.1 51 11.5 2.4

Supported ruthenium catalysts

Ruthenium is found to be very active for reactions involved in SCWG. Ruthenium as

well as other noble metal catalysts usually have a higher metal dispersion compared to

nickel catalysts, partly because their lower metal loadings on the support (typically below

5%), limited surface mobility and hence better resistance against sintering due to their

high melting points, and finally, their milder reduction temperatures. Table 2.5

summarizes the results obtained from ruthenium-catalyzed SCWG of different organic

materials.

36

Table 2.5 Selected results of catalytic SCWG using supported ruthenium

catalysts.

Tem

p.

Rea

cto

r *

Support

Ru

Fee

d

Conc.

Fee

d /

Cat

alyst

Tim

e

WH

SV

CG

R

H2

CH

4

Ref

.

oC - - % - wt % g/g min h


250 B TiO2-AR a 3 Phenol 10 120 61 1.1 18.1 38

350 B TiO2-R b 3 Phenol 10 100 76 1 28.7 38

350 B TiO2-AR 3 Phenol 10 100 94 0.2 33.4 38

350 B C 8 Phenol 10 122 88 0.6 32.8 38

350 B -Al2O3 5 Cresol 10 90 89 0.6 34.8 39

350 B -Al2O3 5 Cresol 10 120 44 2.2 15.8 39

350 B -Al2O3 5 Cresol 10 120 ~ 0 0.6 0 39

350 B ZrO2 5 Cresol 10 90 29 2.9 10.7 39

350 C Al2O3 5 Cresol 2 100 0.4 13 74

380 B -Al2O3 5 Glucose 6 3.5 15 47 5.5 5.2 60

380 B C 5 Glucose 6 3.5 15 42 4 5.1 60

360 C C 5 Glucose 2 1.2 82 3.2 8.8 32

400 C C 5 Glucose 2 1.2 98 18.5 11 32

400 B C 2 Algae 5 0.5 60 45 2.7 4 94

400 B ZrO2 2 Algae 10 0.7 63 25 2.4 2.6 94

400 B TiO2 2 Cellulose 5 0.3 15 74 2.7 13.4 95

400 B -Al2O3 5 LMC c 17 4 15 15 1.3 6.6 96

400 B C 5 LMC c 17 4 15 5 0.6 2.2 96

400 C C 2 SLW d 20 1.6 100 ~ 0 25.5 64

400 B -Al2O3 5 Lignin 3.3 120 75 2.4 30 88

400 B TiO2 2 Lignin 3.3 0.3 180 97 2.6 28.5 92

400 B C 5 Lignin 3.3 0.7 60 80 2.4 23.6 91

450 B C 5 Lignin 3.3 0.7 60 100 4.1 27.7 49

450 C C 5 Glucose 2 1.2 100 33.4 6.5 32

500 B C 5 Cellulose 10 2.5 20 100 17 12 97

550 C ZrO2 1 Glycerol 5 20 6.3 2.1 98

600 C TiO2-R b 3 Glucose 10 30

e 85 21 8 28

600 C TiO2-R b 3 Glycerol 10 30

e 100 22.3 12 28

600 B TiO2 2 MA f 7 2 72 10 6 29

600 B TiO2-R b 3 Glucose 5 1 100 22.8 9.7 27

600 B TiO2-R b 3 Glucose 17 1 100 4.4 16.1 27

600 C ZrO2 2 BC j ~2.5 0.2 67 ~0 11.1 85

600 C TiO2 2 BC j ~2.5 0.2 78 26.0 6.5 85

700 C -Al2O3 5 Glycerol 5 2.5 98 55.4 5.8 99

800 C -Al2O3 5 Glycerol 5 2.5 93 70.6 3.7 99

* B: batch, C: continuous a mixture of anatase and rutile

b rutile

c lignin model compound (alkylphenol)

d synthetic liquefied wood e

30 Nm3/ (h. m

3cat)

f microalgae j

Biocrude oil obtained from

switchgrass

37

The most frequently used supported ruthenium catalysts for SCWG are Ru/C, Ru/TiO2

and Ru/Al2O3. Carbon is widely used as a catalyst support without solid acid-base

properties; however, it is difficult to create and retain high dispersions of metals on the

surface of carbon due to the lack of metal-support interaction. In general, there are two

approaches to anchor an active metal onto the surface of carbon: fixing the active metal

to the defects (e.g., steps in a basal plane of graphite) as well as attachment of metals to

the previously created oxygen functional groups [59]. Wide varieties of different

functional groups may exist on the surface of carbon and the performance of carbon as

both catalyst and support is highly related to the types and concentrations of these

functional groups at the surface. The most abundant hetro-atoms found on the surface of

carbon are hydrogen and oxygen. It has been also suggested that the chemical vapor

deposition (CVD) techniques can be used to apply carbon on the uncovered surface of a

conventional oxide-supported catalyst to suppress its acid-base properties.

There are few of studies focused on the stability of supported ruthenium catalysts [38, 49,

64, 72, 74, 88, 90-93]. Ruthenium is more resistant than nickel against oxidation as well

as hydrothermal sintering. Ruthenium supported on rutile titania [38] and carbon [64] are

confirmed to have a good long term stability. However, presence of sulfur-containing

compounds, even at very low concentrations, dramatically deactivates the catalyst by

successive adsorption and/or solid state reaction on the metal surface [64, 90-92]. One

possible solution to address this concern is to remove sulfur in a hydrothermal salt

separator before passing the feed over the catalytic bed [94]. The salt separator should be

operated at near-critical water conditions to take the advantageous of the low solubilities

38

of the salts. In the context of steam reforming of natural gas, the sulfur content of the feed

should be normally reduced to less than 0.01 ppm, which is well within the capabilities of

the existing sulfur removal processes. In order to develop a viable biomass SCWG

process, more detailed studies on both salt separator unit and development of sulfur-

resistant catalysts should be conducted.

Other precious metals such as platinum [44, 57, 72, 100] and palladium [63, 72, 100,

101] have been also found to be active for catalyzing the decomposition reactions in

SCW.

2.3.3- Oxides

Few oxides such as CaO [102], ZrO2 [103], CeO2 [31] and RuO2 [55, 56, 104] have been

also employed for catalyzing the SCWG. CaO is known to capture the produced carbon

dioxide (and forms carbonate); hence, it resulted in an increase in the hydrogen

concentration [102]. Zirconia was found to be useful in increasing the hydrogen yield

obtained from SCWG of glucose [103]. Using ceria as a catalyst did not significantly

increase the carbon gasification efficiency [31].

Red mud, a byproduct of Bayer process for aluminum production from bauxite, has been

also used as a potential catalyst for SCWG [68]. Red mud contains large amounts of iron

oxides (typically 30-60%) and smaller quantities of other oxides (such as CaO and NaO).

Red mud can facilitate the rate of hydrogen production in SCWG likely through

accelerating the water-gas shift reaction [68]. However, further experiments with pure

Fe2O3 and other oxides that present in red mud (e.g. CaO and NaO), particularly with

39

biomass model compounds, are needed to identify the active element(s) and the exact

mechanism that leads to the higher hydrogen production yields.

2.4 Concluding Remarks

In this paper, a comprehensive review of the recent advancements in heterogeneous

catalysis for supercritical water gasification has been provided. Performance and

durability of commercially available and laboratory-made catalysts including supported

and skeletal metal catalysts, activated carbon, metal wires and other innovative catalysts

for the purpose of hydrothermal hydrogen production from biomass are discussed.

Experimental data presented here covered a wide range of reaction temperature and types

of catalysts and was presented on a common basis in terms of carbon conversion and

hydrogen and methane yields. This information can be served as a useful tool for the

selection and design of new catalysts and SCWG processes. Despite recent advances,

there is a need to better understand the relationship between chemical and physical

properties of solid catalysts to their performances in catalyzing different reactions

involved in SCWG.

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52

3. Screening of Nickel Catalysts for Selective Hydrogen Production

Using Supercritical Water Gasification of Glucose

* Based on P. Azadi, E. Afif, F. Azadi, R. Farnood, Catalyst Screening for Selective Hydrogen Production

Using Supercritical Water Gasification of Biomass, Journal of Green Chemistry, DOI:

10.1039/c2gc16378k.

Abstract

In this chapter, we report the activity and the selectivity of several heterogeneous nickel

catalysts for the supercritical water gasification (SCWG) of biomass. The effects of

catalyst support on the carbon conversion and hydrogen selectivity were demonstrated

using 44 different materials, covering a wide range of chemical and physical properties.

At 5% nickel loading, α-Al2O3, carbon nanotube (CNT), and MgO supports resulted in

high carbon conversions, while SiO2, Y2O3, hydrotalcite, yttria-stabilized zirconia (YSZ),

and TiO2 showed modest activities. Utilization of different γ-Al2O3 supports resulted in a

wide range of catalytic activities from almost inactive to highly active. Other catalyst

carriers such as zeolites, molecular sieves, CeO2, and ZrO2 had an insignificant activity at

the conditions tested (i.e. 380 oC, 2 wt% feed). Aside from the catalytic activity, the

stable metal oxide supports at the experimental conditions of this work, as identified by

XRD, were α-Al2O3, Boehmite, YSZ, and TiO2. Given the high hydrogen yield and

carbon conversion as well as its superior stability in supercritical water, α-Al2O3 was

chosen for a more elaborate investigation. It was found that using the same amount of

nickel, the methane yield significantly decreased by increasing the nickel to support ratio

whereas the carbon conversion was only slightly affected. At a given nickel to support

ratio, a threefold increase in methane yield was observed by increasing the temperature

from 350 to 410 oC. The catalyst activation conditions (e.g., calcination and reduction)

had a small impact on its catalytic performance. The catalyst activity increased with the

53

addition of alkali promoters (i.e., K, Na, Cs) and decreased with the addition of tin. The

highest catalytic activity was obtained with the addition of 0.5% potassium. In summary,

nickel loading and alkali promoters improved the hydrogen selectivity and the carbon

conversion of the Ni/α-Al2O3 catalyst.

3.1 Introduction

Catalytic hydrogen production is among the most important industrial processes for the

production of high quality fuels and foods. Hydrogen is used in ammonia and methanol

syntheses, fuel cells, hydrotreating and hydrogenation processes. Currently, hydrogen is

mainly produced by steam reforming of natural gas and naphtha due to their low costs.

However, it is anticipated that utilization of biomass as a feedstock for hydrogen

production will gain a more significant share in the near future due to the increasing

concerns over the adverse environmental impacts of the fossil fuels. Since lignocellulosic

materials are the most abundant biomass species, it is highly desirable to produce

renewable hydrogen from such resources.

Supercritical water gasification (SCWG) is a promising technology for the conversion of

wet biomass into a gas mixture composed of hydrogen, methane, carbon dioxide and

carbon monoxide without the need for drying the feedstock. The gasification of organics

in supercritical water typically proceeds at lower temperatures (e.g. < 600 oC) compared

to the conventional gasification processes (e.g. >800 oC). Considering the amount and the

high specific heat of water as well as the high costs associated with the construction and

operation of high pressure/high temperature equipments, it is highly desirable to operate

54

the supercritical water gasifiers at milder reaction conditions. To achieve this goal, solid

catalysts are employed to enhance the gasification rates and obtain a high conversion at

temperatures below 500 oC and more preferably, below 400

oC [1-14]. Nickel and

ruthenium have been found to be active for catalyzing the SCWG reactions which require

C-C bond cleavage at such low temperature. For a review of the recent advances in

catalytic SCWG please refer to [15, 16].

Figure 3.1 Reaction pathways for production of hydrogen by reactions of

oxygenated carbohydrates with water adapted from [17]. (* represents a surface

metal site)

The reaction pathway for the nickel-catalyzed supercritical water gasification of

cellulosic biomass is depicted in Figure 3.1. According to this figure, the polysaccharides

are initially hydrolyzed to sugar monomers and small oligomers, followed by chemical

55

transformation to other intermediates such as alcohols and acids. Decomposition of these

chemical intermediates on the surface of a metal catalyst through C-C bond cleavage

results in the formation of syngas. The syngas is further upgraded to hydrogen and carbon

dioxide by water-gas shift reaction on the metal. On the other hand, if the catalyst has a

high activity for the cleavage of C-O bonds, the produced hydrogen will be used for

hydrogenation of the adsorbed species and results in the formation of methane and other

alkanes. From a thermodynamic standpoint, reforming of the oxygenated compounds

(e.g. biomass) at lower temperatures and higher pressures favors greater alkane yields and

thus, results in poorer hydrogen selectivity. Consequently, water near its critical point

(374 oC and 221 bar) is a highly effective medium for the formation of alkanes, especially

methane. Fortunately, methane formation in SCWG of biomass does not have a pyrolytic

origin and it is almost solely formed through the catalytic cleavage of C-O bonds in the

presence of hydrogen. Due to this reason, at low conversions where the H2, CO and CO2

partial pressures are still low, the methane yield is negligible. However, when the

reaction proceeds further, the partial pressures of these gases increase and as a result of

that, the methanation rate increases. In spite of the high activity of nickel for C-C bond

scission and the water-gas shift reaction, the undesirable high activity of this metal for C-

O bond cleavage makes hydrogen production in SCW challenging. Dumesic et al.

showed that the addition of tin to Raney nickel can considerably suppress the ability of

the catalyst for cleaving the C-O bonds while retaining its original activity for the

cleavage of the C-C bonds [18-21].

56

Although there is a large body of literature on the catalytic supercritical water

gasification of biomass [15], little is still known about the effects of catalyst support and

structure on its catalytic performance and there is a need for a systematic study to address

this issue. Hence, in this study, we systematically examined the performance of several

supported nickel catalysts for SCWG of glucose in terms of carbon conversion and

hydrogen selectivity. The reaction conditions (i.e., 380 oC, 15min, 2 wt% feed, 1 g

catalyst) resulted in a wide range of carbon conversions and hydrogen selectivities,

allowing for a fair distinction between the catalyst performance in terms of the activity

and selectivity. The catalyst supports included various materials covering a wide range of

chemical and physical characteristics. Among the catalysts tested, Ni/α-alumina catalyst

was selected for a more detailed investigation. The effects of nickel to carrier ratio,

reaction temperature, catalyst activation conditions and promoters were studied.

3.2 Experimental

The catalysts used in this study were prepared by impregnation of the supports with a

nickel nitrate hexahydrate (Sigma-Aldrich, Canada) precursor using incipient wetness

method. The characteristics of these supports are given in Table 3.1. A-1 to A-4, A-12,

Ca-1, HT-1, Mg-1, Mg-2, MS-1, MS-2, Si-1, Si-2, Yt-1, YZ-1, Ze-4, and Zr-1 were

obtained from Sigma Aldrich, Canada. A-5 to A-11, SA-1 to SA-4, Si-3, Si-4, Ze-2, and

Ze-3 were obtained from Grace Davision, USA. Ca-2 was obtained from Cheap Tubes

Inc. (Brattleboro, USA), Ti-1 to Ti-4 were obtained from Degussa AG (Germany), and

Ze-1 was obtained from United Catalyst Inc. (USA). In some experiment, as indicated by

“*” in Tables 3.3 & 3.4, the support pellets were crushed to fine powder prior to the

57

impregnation. The catalytic activity of all catalyst support materials prior to the

deposition of nickel nanoparticles were found to be negligible. Following the

impregnation of the support materials with the desired amount of nickel nitrate solution,

catalysts were dried at 110 oC and calcined at 350

oC for 2 h (except for Ca-1 and Ca-2).

Then, catalysts were reduced in flowing hydrogen (50 ml/min STP, 30% H2, 70% N2) for

2 h. The appropriate reduction temperature for each catalyst was identified in preliminary

experiments and it ranged from 400 to 950 oC, depending on the severity of interaction

between nickel oxide and the catalyst carrier. The desired amount of potassium nitrate,

sodium nitrate, cesium carbonate and tri-n-butyltin acetate (Sigma Aldrich) were

dissolved in water and added together with the nickel nitrate to synthesize the promoted

catalysts.

A schematic of the experimental setup is shown in Figure 3.2. A 50 mL stainless steel

batch reactor was used in this study to evaluate the activity of supported nickel catalysts

for supercritical water gasification of biomass. The reactor was used several times prior

to the experiments to ensure a minimal catalytic effect of the walls. The heating medium

was a molten salt bath containing sodium nitrate, potassium nitrate, and sodium nitrite by

which an initial heat-up rate of greater than 300oC/min was achieved. Using the explained

reaction system, 90% of the temperature change occured within the first 100s of the

experiments and the temperature reached its final value in about 3min. One should refer

to [7] for typical variation of the reactor temperature and pressure using the salt bath

systems and to [4] for the heat transfer issues from molten salts to SCW reactors.

Generally, the pressure inside the reactor closely matched with the corresponding water

58

vapor pressure at any given time during the heat-up. The temperature of the salt bath was

measured using a Type K thermocouple (Omega Engineering, Canada) and was

maintained at the desired value using a temperature controller (Hanyoung Electronic Co.

Ltd., Korea). In all experiments, 0.2 g of glucose (Sigma-Aldrich, Canada) was added to

9.8 g of deionized water to obtain a 2 wt% solution. This solution along with the desired

amount of catalyst was then introduced into the reactor and immersed in the molten salt

bath. After 15min, the reactor was taken out from the bath and immediately quenched in

a cold water bath. Then, the pressure in the reactor was determined using a digital

pressure gauge (Cecomp Electronics, USA) and was used to calculate the gas yield using

the ideal gas law. The gaseous product was then collected in a gas bag for composition

analysis. In all experiments, the amount of nickel metal was set at 50 mg regardless of the

catalyst formulation. In the support screening section, 1g of 5 wt% Ni catalyst was used

to gasify the feed at 380 °C. The corresponding pressure in the reactor at 380 oC was

estimated to be approximately 230 bar. A gas chromatograph (Hewlett-Packard 5890

series) equipped with a thermal conductivity detector and argon as the carrier gas was

used to determine the product gas composition. The data reproducibility was confirmed

to be within ± 5% by performing at least one replicate run for each data point.

The BET surface areas of the catalyst supports were obtained using Quantachrome

Autosorb catalyst characterization system (Quantachrome, USA). The equilibrium gas

compositions were calculated using Aspen Plus software (AspenTech, Burlington, USA).

The results are reported in terms of gas yields (mmol/g glucose), carbon gasification ratio

(CGR), hydrogen gasification ratio (HGR) and hydrogen selectivity (HS). The CGR and

59

HGR are defined as the ratio of carbon and hydrogen in the gas products to the carbon

and hydrogen in the initial feed, respectively. We note that the maximum amount of

hydrogen that can be generated from glucose is 66.6 mmol/g that is twice the amount of

hydrogen initially presented in the glucose molecules; therefore, the HGR as defined here

can reach a maximum value of 200%. Hydrogen selectivity is defined as the number of

moles of H2 to the number of moles of hydrogen in methane. The CGR, HGR and

hydrogen selectivity have been calculated throughout this study as follow:

100/

42

feedgCmmol

YYYCGR

COCHCO

(3.1)

100/

2

2

42

feedgHmmol

YYHGR

CHH

(3.2)

4

2

22

CH

H

Y

YySelectivitH

(3.3)

.

60

Table 3.1 Characteristics of the supports used in this study.

Entry Support BET

[m2/g]

Particle size

[µm]

Pore vol.

[cc/g] Comment

A-1 γ-Al2O3 200 * 100 Acid, pH 5

A-2 γ-Al2O3 200 * 100 Basic, pH 9

A-3 γ-Al2O3 200 * 100 Neutral, pH 7

A-6 γ-Al2O3 147 * 4100 1.2 Spheres

A-7 γ-Al2O3 225 * 4100 1.1 Spheres

A-8 γ-Al2O3 170 * 15 0.7 Powder

A-9 γ-Al2O3 210 * 1900 1.1 Minispheres

A-10 La2O3-γ-Al2O3 200 * 78 0.8 4% Lanthana

A-11 γ-Al2O3 165 * 70 0.5 Microspheres

A-4 α- Al2O3 8.2 118 0.12 Corundum

A-12 α- Al2O3 0.6 * 10

A-13 α- Al2O3 5.5 * 35 Powder

A-14 α- Al2O3 10 * 1 Powder

A-15 α- Al2O3 7.5 110 0.05 Crystalline

A-5 AlO(OH) 312 * 15 0.9 Pseudo-boehmite

Ca-1 Activated carbon 600 * 6500

Ca-2 Carbon nanotube 495 20 1.1 Multiwalled, OD<8nm

Ce-1 CeO2 3 5

Ce-2 CeO2 39 0.02 Nano powder

HT-1 Hydrotalcite 12 1 0.06 Synthetic

Mg-1 MgO 14 40 0.13

Mg-2 MgO 0.5 70

MS-1 Molecular sieve 338 4 Organophilic

MS-2 Aluminosilicate MCM41 800 1.0 3% Al, 5nm pore

Si-1 SiO2 500 * 0.01 Nano powder

Si-2 Silica gel 357 13 0.7 Pore diameter 6nm

Si-3 SiO2 40 * 50 1.0 Large pore size

Si-4 SiO2 687 * 2000 0.4 Granules

SA-1 Silica-alumina 424 * 74 1.1 13% Al2O3 powder

SA-2 Silica-alumina 327 * 59 0.6 SLA pyridine catalyst




Ti-1 TiO2 49 0.9 7710

Ti-2 TiO2 57 PF2

Ti-3 TiO2 50 1600 0.4 7711, 75% anatase

Ti-4 TiO2 50 PF25

YT-1 Y2O3 7 6

YZ-1 Yttria-stabilized zirconia 144 0.1 YSZ, 8% Yttria

Ze-1 Zeolite pentasil 298 2500 Extrudate

Ze-2 Zeolite H-ZSM5 414 * 14 0.2

Ze-3 Zeolite USY 760 * 5 0.3

Ze-4 Zeolite Y 748

ZR-1 ZrO2 7 5 *Reported by the manufacturer.

61

Figure 3.2 Schematic diagram of the experimental set-up: 1) molten salt bath, 2)

reactor, 3) electrical heater, 4) thermocouple, 5) PID temperature controller, 6) first

valve, 7) low-pressure gauge, 8) second valve.

3.3 Results and Discussion

In the first part of this section, the calculated equilibrium gas composition is given. Then,

the effects of the supports on the catalytic activity are discussed. In addition to that, the

effects of the nickel to support ratio, the reaction temperature, the catalyst activation

conditions and the addition of promoters have been studied for one of the promising

alumina catalysts found in the course of catalyst screening.

Thermodynamic considerations

Table 3.2 shows the equilibrium gas composition calculated at 350, 380, and 410 oC (and

at their corresponding pressures). It is known that the hydrogen mole fraction at

equilibrium is positively correlated with temperature and inversely correlated with the

pressure. Since the pressure inside a closed vessel (e.g., batch reactor) dramatically

62

changes with temperature near the critical point of water, the hydrogen mole fraction at

equilibrium decreases with increasing temperature at the conditions studied in this work.

However, the variation of the gas yields with temperature within the investigated

temperature range was rather small. The calculation of the gas composition at equilibrium

revealed that only about 6 mmol of hydrogen would be formed per gram of glucose, and

a larger fraction of the hydrogen will be consumed to hydrogenate carbon dioxide. The

corresponding equilibrium hydrogen selectivity and hydrogen gasification ratio (HGR) at

the conditions relevant to this work was found to be approximately 0.2 and 108%,

respectively.

Table 3.2 The equilibrium gas composition calculated based on SCWG of 2

wt% glucose and water density of 200 kg/m3 using Aspen Plus software.

T

[oC]

P

[bar]

H2

[mmol/g]

CO

[mmol/g]

CH4

[mmol/g]

CO2

[mmol/g]

Hydrogen

selectivity

HGR

[%]

350 165 6.7 0.005 14.9 18.4 0.22 110

380 230 5.5 0.008 15.2 18.1 0.18 108

410 320 4.7 0.012 15.4 17.9 0.15 107

Alumina-supported catalysts

Table 3.3 demonstrates the gas yields and the carbon conversion of glucose using the

alumina-supported catalysts at 380 oC. For comparison, results from the SCWG of

glucose under the catalyst-free condition are also given in this table. The CGR obtained

from these Ni/Al2O3 catalysts ranged from 21% (the same as catalyst-free) to 84%,

implying that the chemistry of the support material (Al2O3 in this case) was not solely

responsible for catalyst activity, and other factors significantly influenced the catalytic

63

performance as well. It should be noted that the variation in the catalytic activity among

the Ni/α-Al2O3 was less pronounced (typically CGR > 70%) compared to that of Ni/γ-

Al2O3 (i.e. 21%<CGR<79%). Figure 3.3 shows the variation of hydrogen yield and

selectivity with carbon conversion using alumina-supported catalysts. It was observed

that up to carbon conversions of about 60-70%, the hydrogen yield increased, after which

it leveled off. The initial increase in hydrogen yield, however, appeared to be due to the

increase in conversion, such that the hydrogen selectivity remained constant at low

conversions. At higher conversions (i.e. CGR > 60%), hydrogen selectivity showed a

clear declining trend with CGR. This observation can be justified considering the partial

pressures of the hydrogen and carbon dioxide (and carbon monoxide to a lesser extent).

At low carbon conversions, the partial pressure of the hydrogen and carbon dioxide are

low and hence, the rates of the formation of these two compounds (from the remaining

feed and the intermediates) are much higher than their consumption rate by methanation

reactions. At moderate conversions, hydrogen formation and consumption rates are

comparable and therefore the hydrogen yield remains almost constant. At higher

conversions, hydrogen is formed at a lower rate due to the progressive consumption of

the feed and intermediates, while the rate of hydrogen consumption is significantly higher

due to the higher partial pressures of hydrogen and carbon dioxide. As a result, at high

carbon conversions, the net rate of change in hydrogen yield is expected to decrease. It is

worthwhile to mention that the amount of hydrogen at the highest carbon conversion was

almost threefold higher than the equilibrium hydrogen yield (see Table 3.2), indicating

that the gas phase composition was far from equilibrium.

64

Given that all catalysts in Table 3.3 (except A-15) have the same chemical formula and

were prepared using the same procedure, the large differences among the performance of

these catalysts could be attributed to one or more of the following reasons: i) nickel

dispersion, ii) severity of the nickel-support interaction, iii) support stability, iv) mass

transfer issues, and v) support’s acidity. Here, we discuss the possibility of how and to

what extent each of these parameters could affect the catalytic performance in SCWG.

i) Nickel dispersion

As a general rule, the activity of a catalyst is proportional to the number of the available

active sites. At a given metal loading, the number of active sites in a catalyst is a function

of the metal dispersion, which is defined as the fraction of the total atoms of the active

element that is exposed at the surface. The dispersion is, in turn, inversely proportional to

the crystallite size. In the case of commercial Ni/Al2O3 catalysts, the metal dispersion is

typically below 15%, corresponding to an average crystallite size of about 6.5 nm (and

larger). The main factors that govern the dispersion of Ni on alumina include the support

surface area, the metal loading, and the activation conditions. Since the metal loading and

the activation conditions were identical for all catalysts shown in Table 3.3, the support

surface area was the only parameter which could possibly change the metal dispersion.

Using Scherrer equation, it was revealed that the size of NiO (before reduction, XRD not

shown here) and Ni (XRD shown in Figure 3.5) on α-Al2O3 were approximately 20 nm

and 32 nm, respectively. These crystallite sizes correspond to 5% and 3% dispersion,

respectively. These values are consistent with the previously reported data for the

dispersion of nickel on α-Al2O3 at low metal loadings [22]. Therefore, considering the

65

metal dispersion in α-Al2O3 and γ-Al2O3-supported catalysts, no clear relationship was

observed between the metal dispersion and the activity of the catalyst for the gasification

of glucose in SCW. However, Ni dispersion effects might have been masked by other

factors (discussed below) that had more pronounced impacts on the activity of the

alumina-supported catalysts. Figure 3.4 demonstrates the hydrogen yield and selectivity

as a function of support surface area. Similar to the carbon conversion (CGR), no

correlation between hydrogen yield and selectivity and the support surface area was

observed.

ii) Nickel-support interaction

The physicochemical properties of Ni/alumina are affected by the thermal treatment

process prior and after the metal deposition. The amorphous transition alumina phases

(i.e. γ, δ, θ) undergo phase transformation to form crystalline α phase upon calcination at

high temperatures (e.g. 1300 oC). This phase change causes a significant

dehydroxylation, loss of surface area and pore volume and an increase in the average

pore diameter. Three distinguishable species of nickel oxide can present upon calcination

of a nickel salt deposited on alumina [23]: bulk NiO (reducible at ~ 400 oC), nickel oxide

with strong interaction with support (reducible at ~ 400-800 oC), and surface and bulk

nickel aluminate spinel (hardly reducible at > 800 oC). Nickel aluminate (NiAl2O4) is

formed through strong bond between Ni2+

and the divalent vacancy in transition alumina.

The formation of nickel aluminate on α-Al2O3 only occurs at high calcination

temperatures (e.g. > 600 oC), whereas nickel aluminate is the most dominant species

found in unreduced NiO/γ-Al2O3 catalysts regardless of the calcination temperature [22,

66

24], particularly at low metal loadings where the surface cation vacancies of the γ-Al2O3

are yet to be fully occupied by the Ni2+

ions. Also, it has been shown that the reducibility

of NiO/Al2O3 is typically facilitated by increasing the metal loadings and decreasing the

calcination temperature [22]. It has been also reported that the addition of a few percent

La2O3 to γ-Al2O3 had little influence on the nickel-support interaction [25]. In summary,

at low metal loadings (e.g. 5 wt%), NiO and NiAl2O4 are the more dominant species in α-

Al2O3 and γ-Al2O3-supported catalysts, respectively. Therefore, the greater catalytic

activity of Ni/α-Al2O3 compared to that of Ni/γ-Al2O3 can be partly attributed to the

better reducibility of Ni2+

deposited on α-Al2O3 and consequently, the higher extent of

reduction (EOR). It is worth mentioning that reduction of 5% Ni/ γ-Al2O3 (A-8) catalyst

at 500, 700 and 950 oC failed to activate the catalyst for the reaction of interest. The XRD

pattern of this catalyst (Figure 3.5) exhibited broad peaks associated with nickel alumina

spinel, implying the formation of well-dispersed spinel phase at the surface and perhaps

in the bulk of the γ-Al2O3 support.

iii) Support stability

Chemical degradation and/or phase transformation of the catalyst support in SCW may

diminish the catalytic activity even during a short batch experiment. Figure 3.6 shows the

XRD of patterns of α-Al2O3 (A-4), γ-Al2O3 (A-6), La2O3- γ-Al2O3 (A-10), and AlO(OH)

(A-5) supports before and after exposure to SCW at 380 oC for 1h. Thanks to its high

surface area and superior thermal stability over a wide range of temperatures, γ-Al2O3 is

the most widely used industrial catalyst support. However, as previously reported in the

literature, γ-Al2O3 undergoes phase transition to boehmite upon exposure to SCW [10].

67

The phase transformation of γ-Al2O3 resulted in a significant loss of the mesopores and

thus, the surface area [26]. This would, in turn, lead to the occlusion of the catalytically

active crystallites (i.e. nickel) and cause a fast catalyst deactivation. In contrast, α-Al2O3

was found to be fairly stable under similar conditions. We note that the BET surface area

of α-Al2O3 also remained unchanged after exposure to SCW. Exposure of the high-

surface area pseudo-boehmite to SCW for 1h increased the crystallinity of the material

but did not change its chemistry. Consequently, one can expect that crystalline pseudo-

boehmite could potentially be a good candidate as a catalyst carrier in SCW.

Unfortunately, the Ni/pseudo-boehmite catalyst exhibited negligible activity for SCWG

of glucose (Table 3.3). Addition of small quantities of lanthanum oxide did not improve

the hydrothermal stability of γ-Al2O3. Considering the above discussions, instability of γ-

Al2O3 as well as the negligible activity of the boehmite-supported nickel catalysts can be

partly responsible for the higher activity of Ni/ α-Al2O3 compared to that of γ-Al2O3.

Therefore, except for Ni/α -Al2O3, no practical use is foreseen for other alumina

supported catalysts in SCWG process. It should be also emphasized that the extremely

small solubility of α-Al2O3 in supercritical water [27] along with its superior mechanical

strength [26] render α-Al2O3 an appropriate candidate for catalysis in SCW. We

experimentally confirmed that the BET surface area of α-Al2O3 remained unchanged after

the exposure to SCW.

iv) Mass transfer issues

Under similar operating conditions, the rate of mass transfer towards the active sites of a

catalyst depends on the morphological and textural characteristics of the catalyst

68

particulate (pellet, extrudate, granule, sphere, powder). The SCWG of glucose with finely

crushed spheres (A-6*, A-7

*, and A-9

*) revealed that, expectedly, the inter-particle pores

had no measureable impact on the catalytic activity. Moreover, considering the data

presented in Tables 3.1 and 3.3 for original and crushed catalyst, it becomes apparent that

there was no relationship between the catalyst particle size and activity. This implies that

the mass transfer in the mesopores was not the rate determining factor under the

experimental conditions used.

Therefore, based on these evidences and also considering the high diffusion coefficients

in supercritical state [28, 29], the chemical reaction on the catalytic active site was most

likely the rate determining step under the conditions used in this study.

v) Support acidity

In SCW, sugars undergo dehydration on the catalyst acidic sites. Utilization of

bifunctional catalysts, with active hydrogenation metal and strong acidic sites, results in

successive dehydration/hydrogenation and therefore, produces higher alkanes. The acid

sites also promote the polymerization reactions, which in turn, lead to the formation of

tarry materials. Among the various alumina phases, α-Al2O3 is the least acidic one [26].

Therefore, the higher gasification efficiency obtained by Ni/α-Al2O3 can be partly due to

the weaker acidic strength of α-Al2O3 compared to that of γ-Al2O3.

69

Table 3.3 Performance of alumina-supported catalysts (380 oC, 15min, 2 wt%

glucose, 1g catalyst).

Support Ni

[%]

H2

[mmol/g]

CO

[mmol/g]

CH4

[mmol/g]

CO2

[mmol/g]

CGR

[%]

HGR

[%]

No

catalyst - 2.7 1.0 0.1 6.5 22 9

A-1 5 20.5 0.3 8.1 17.9 79 110

A-2 5 22.3 0.3 5.0 18.6 72 97

A-3 5 8.8 0.6 1.6 10.9 39 36

A-6 5 20.8 0.5 4.1 17.1 65 87

A-6* 5 16.2 0.2 4.9 15.7 62 78

A-7 5 19.0 0.5 4.1 17.1 65 81

A-7* 5 18.6 0.4 5.0 17.3 68 85

A-8 5 2.5 1.1 0.0 6.1 21 7

A-9 5 19.5 0.5 3.6 17.1 63 80

A-9* 5 18.8 0.4 4.8 17.3 67 85

A-10 5 11.6 0.5 2.1 12.4 45 47

A-11 5 14.5 0.5 2.3 13.1 48 57

A-4 5 24.3 0.3 4.6 17.4 67 101

A-12 5 16.8 0.5 2.9 13.8 51 68

A-13 5 17.9 0.4 7.4 16.8 73 98

A-14 5 18.3 0.4 9.5 18.0 84 112

A-15 5 17.6 0.2 9.9 17.8 84 112

A-5 5 1.4 1.3 0.0 6.4 23 4

* Catalyst pellets were crushed to fine powders before impregnation.

70

Figure 3.3 Hydrogen and methane yields and selectivity vs. carbon conversion

using 5% Ni/alumina catalysts, α-Al2O3 (),γ-Al2O3 (), La2O3-γ-Al2O3 (), and

equilibrium values (X). (380 oC, 15min, 2 wt% glucose, 1g catalyst).

0

5

10

15

20

25

30

30 40 50 60 70 80 90 100

H2 yie

ld [

mm

ol/

g]

CGR %

0

5

10

15

20

30 40 50 60 70 80 90 100

CH

4 yie

ld [

mm

ol/

g]

CGR %

0

1

2

3

30 40 50 60 70 80 90 100

H2 se

lect

ivit

y

CGR %

71

Figure 3.4 Hydrogen yield and selectivity vs. support surface area using 5%

Ni/alumina catalysts, α-Al2O3 (),γ-Al2O3 (), La2O3-γ-Al2O3 (). (380 oC, 15min,

2 wt% glucose, 1g catalyst).

0

5

10

15

20

25

30

0 50 100 150 200 250 300

H2 yie

ld [

mm

ol/

g]

Surface area [m2/g]

0

1

2

3

0 50 100 150 200 250 300

H2 se

lect

ivit

y

Surface area [m2/g]

72

Figure 3.5 XRD patterns of 5% Ni on a) α-Al2O3 reduced at 500 oC b) γ-Al2O3

(A-6) reduced at 500 oC, c) γ-Al2O3 (A-6) reduced at 800

oC, and d) γ-Al2O3 (A-8)

reduced at 700 oC. The vertical dashed lines represent nickel peaks.

0

200

400

600

800

1000

30 40 50 60 70

Position [2 Theta]

c

d

a

b

73

Figure 3.6 XRD patterns of different types of alumina before (bottom) and after

(top) exposure to the supercritical water at 380 oC for 1h.

In summary, the following factors were proposed to influence the catalytic activity of

alumina-supported nickel catalysts for SCWG in this study: a) reducibility and the form

of nickel oxide prior to the reduction (NiO vs NiAl2O4), b) support stability, and c)

support acidity.

74

Other catalysts

The gas yields and carbon conversion for the SCWG of glucose using other catalyst

support materials are shown in Table 3.4 and the corresponding hydrogen yields and

selectivities versus carbon conversion are plotted in Figure 3.7. The relative activity for

these catalysts reduced depending on the support material in the following orders: carbon

nanotube (CNT) > MgO > Y2O3, TiO2, SiO2 > Hydrotalcite, YSZ > CeO2, activated

carbon, ZrO2, molecular sieve, silica-alumina and zeolites. The result obtained from

gasification of glucose in the presence of carbon nanotube (and activated carbon) should

be interpreted with caution when using a batch reactor as the catalyst support may be also

partially gasified. Zeolite pentasil (Ze-1) showed an exceptionally high hydrogen yield

(~30 mmol/g). Similarly, MgO and Y2O3 performed better than alumina supports

(hydrogen yield: ~ 24-26 mmol/g). Hydrotalcite (HT) also demonstrated a extraordinarily

high hydrogen selectivity of ~6.5 but with a modest CGR of 54%. The XRD spectra of

the studied catalyst supports before and after 1h exposure to SCW at 380 oC are given in

Figure 3.8. Based on these plots, TiO2 and YSZ were the only stable supports in such

conditions and other supports materials; including MgO, hydrotalcite, Y2O3, and CeO2

underwent chemical reaction with water and/or phase transformation and thereby, were

not useful for the process of interest. The high hydrogen selectivity obtained from

Ni/hydrotalcite may be attributed to the decomposition of the support. Despite YSZ was

chemically stable under the SCW conditions, its specific surface area decreased from 144

to 103 m2/g upon 3h exposure to SCW at 380

oC. Also the Silica-containing supports

such as SiO2, molecular sieves, silica alumina and zeolites are unstable under

75

hydrothermal conditions. For instance, the BET surface area of molecular sieve and

MCM41 changed from 338 and 800 m2/g to 10 and 13 m

2/g after 3h exposure to SCW at

380 oC, respectively. As discussed in the previous section for the Ni/Al2O3 catalysts, the

acidic supports (e.g. zeolites, silica-alumina) led to very poor gasification yields. Once

again, no clear relationship was observed between the support surface area and particle

size with the catalyst activity for SCWG of glucose.

76

Table 3.4 Performance of other supported catalysts (380 oC, 15min, 2 wt%

glucose, 1 g catalyst).

Support Ni

[%]

H2

[mmol/g]

CO

[mmol/g]

CH4

[mmol/g]

CO2

[mmol/g]

CGR

[%]

HGR

[%]

No catalyst - 2.7 1.0 0.1 6.5 22 9

Ca-1 5 3.1 0.7 0.1 8.3 27 10

Ca-2 5 17.3 0.4 9.8 18.1 85 111

Ce-1 5 6.0 1.0 0.1 7.0 24 19

Ce-2 5 14.1 0.7 0.7 10.7 36 46

HT-1 5 24.5 0.15 1.9 15.9 54 85

Mg-1 5 26.6 0.3 6.2 17.3 71 117

Mg-2 5 26.9 0.6 4.6 18.9 72 108

MS-1 5 13.0 0.7 2.6 10.9 43 55

MS-2 5 10.0 1.8 1.2 4.7 20 37

SA-1 5 3.4 2.5 0.2 4.8 22 11

SA-2 5 4.5 1.9 0.04 4.5 19 14

SA-3 5 2.5 2.4 0.07 4.1 20 8

SA-4 5 3.4 2.4 0.4 4.7 22 13

SA-5 5 2.6 2.7 0.1 4.3 21 8

Si-1 5 20.5 0.2 3.7 14.4 53 84

Si-2 5 13.6 1.2 4.5 13.6 58 68

Si-3 5 16.8 1.0 3.7 13.7 55 73

Si-4 5 4.3 1.2 1.0 6.3 25 19

Si-4* 5 5.2 1.2 1.4 6.8 28 24

Ti-1 5 16.7 0.6 3.5 15.6 59 71

Ti-2 5 15.6 0.4 1.8 13.1 46 58

Ti-3 5 14.6 0.3 2.8 14.4 53 61

Ti-4 5 12.8 0.3 2.1 11.0 40 51

YT-1 5 26.5 0.3 7.3 12.0 59 123

YZ-1 5 11.5 0.6 2.4 14.0 51 49

Ze-1 5 30.7 0.2 2.7 7.2 30 108

Ze-2 5 4.9 3.8 0.3 5.6 29 16

Ze-3 5 6.5 1.8 0.3 6.0 24 21

Ze-4 5 0.5 1.4 0.14 7.0 26 2

ZR-1 5 2.9 1.1 0.1 7.3 26 9

* Catalyst pellets were crushed to fine powders before impregnation.

77

Figure 3.7 Hydrogen yield and selectivity vs. carbon conversion (CGR) using 5%

Ni/support catalysts, (380 oC, 15min, 2 wt% glucose, 1 g catalyst). Molecular sieve

(), YSZ (), Hydrotalcite (), Silica (), Titania (), Yttria (), Magnesia (),

and CNT ().

0

5

10

15

20

25

30

40 50 60 70 80 90 100

H2 yie

ld [

mm

ol/

g]

CGR %

0

2

4

6

40 50 60 70 80 90

H2 se

lect

ivit

y

CGR %

78

Figure 3.8 XRD patterns of MgO, Y2O3, TiO2, hydrotalcite, CeO2 and YSZ

before (bottom) and after (top) exposure to the supercritical water at 380 oC for 1h.

Optimization of Ni/α-Al2O3 Catalyst

79

Based on the above findings, among the active catalysts, Ni/α-Al2O3 and Ni/CNT had

high CGR values (~ 85%) and high hydrogen yields (~ 17-24 mmol/g) while Ni/MgO

exhibited an excellent hydrogen yield (~26 mmol/g) and a CGR of about 72%.

Considering its low cost and high stability, α-Al2O3 was chosen as support for further

study to enhance catalyst performance by optimizing synthesis conditions and by

introducing promoters.

Effects of nickel loading at different reaction temperatures

Figure 3.9 shows the relationship between the nickel loading and the catalyst activity for

SCWG of glucose at 350, 380 and 410 oC. It should be noted that regardless of the level

of nickel loading on the support, the total nickel weight was kept constant in the

experiments (i.e. 0.05g). It can be seen that carbon conversion slightly decreased (with a

slope of approximately -0.5 %CGR / %Ni with a R2 value of 0.7-0.8) while the methane

yield substantially dropped by increasing the nickel to the support ratio. As discussed

earlier, nickel loading affects the nickel crystalline size and the nickel-support

interactions. Therefore, it may be hypothesized that under the experimental conditions of

this work, the methane formation, which is mostly a secondary reaction, was more

affected by the nickel crystallite size and the extent of Ni-support interaction than the

primary reactions such as C-C bond rupture. For Raney nickel-catalyzed SCWG it has

been reported that the methane formation rate considerably decreased over time (due to

substantial sintering and hence, crystallite growth) while the carbon conversion did not

drop at the same pace [3]. Moreover, according to Figure 3.9, the methanation rate was

80

heavily dependent on the reaction temperature. A threefold increase in methane yield was

observed upon increase of temperature from 350 to 410 oC at any level of the nickel

loading which in turn, dramatically decreased the hydrogen selectivity (Figure 3.10).

81

Figure 3.9 Carbon gasification ratio and methane yield vs. Ni loading (on α-

Al2O3 (A-4)) at 410 oC (), 380

oC (), and 350

oC (), (15min, 2 wt% glucose, 0.05

g Ni).

0

20

40

60

80

100

0 5 10 15 20

CG

R

%Ni/alumina

0

2

4

6

8

10

12

0 5 10 15 20

CH

4 [

mm

ol/

g]

%Ni/alumina

82

Figure 3.10 Hydrogen selectivity vs. Ni loading (on α-Al2O3 (A-4)) at 350 oC, 380

oC, and 410

oC, (15min, 2 wt% glucose, 0.05 g Ni).

Effects of catalyst activation

Figure 3.11 & 3.12 demonstrate the effects of calcination and reduction on the activity of

the catalyst, respectively. Based on the results presented in Figure 3.11, the calcination

temperature and duration had a relatively small impact on the performance of the catalyst

for SCWG. In fact, the highest methane yield was obtained with the catalyst that was

prepared without calcination. Furthermore, the rate of methane formation decreased with

increasing reduction temperature. This observation may be justified due to the possible

decrease in the metal dispersion caused by sintering of the crystallites at higher reduction

temperatures.

0

1

2

3

4

5 10 15 20

H2 se

lect

ivit

y

Nickel [wt%]

350 380 410

83

Figure 3.11 Effects of calcination temperature (for 2h) and time (at 350 oC) on

product yields for SCWG using 5% wt Ni/α-Al2O3 (A-4) catalyst, (380 oC, 15min, 2

wt% glucose, 1 g catalyst). Carbon monoxide yield remained less than 0.3 mmol/g

(not shown).

Figure 3.12 Effects of reduction temperature (for 2h) and time (at 500 oC) on the

gas yields using 5% Ni/α-Al2O3 catalyst, (380 oC, 15min, 2 wt% glucose, 1 g

catalyst). Carbon monoxide yield remained less than 0.3 mmol/g (not shown).

0

10

20

30

40

50

280 350 500 0 120 400

Yie

ld [

mm

ol/

g]

Temperature [oC]

CO2 CH4 H2

Time [min]

0

10

20

30

40

50

500 600 700 30 120 240

Yie

ld [

mm

ol/

g]

Temperature [oC]

CO2 CH4 H2

Time [min]

84

Effects of promoters

Due to the proven usefulness of alkali metals for facilitating the SCWG reactions [30-32],

these metals have been added to the Ni/α-Al2O3 catalyst as promoters. Additionally,

when added to the surface of Raney nickel, tin has been found to dramatically decrease

the methane formation rate and improve the hydrogen selectivity [18-21]. Hence, the

addition of tin as a promoter was also considered to potentially enhance the hydrogen

selectivity of Ni/α-Al2O3.

The corresponding results obtained from SCWG of glucose using promoted α-Al2O3 are

presented in Table 3.5. For comparison, performance of the same catalyst without a

promoter is also given in the same table. It can be seen that addition of tin while

enhanced hydrogen selectivity also considerably decreased the activity of the Ni/α-Al2O3

catalyst for gasification of glucose. On the other hand, the addition of small quantities of

alkali metals improved the CGR and methane yield but decreases the hydrogen

selectivity. A near complete carbon conversion was achieved upon the addition of 0.5%

K to 10% Ni/α- Al2O3. Figure 3.13 illustrated the hydrogen and methane yields and the

corresponding hydrogen selectivity using alkali and tin-promoted nickel catalysts. It was

found that the hydrogen yield remained almost constant while the methane yield

increased upon doping of the catalyst with alkali metals, particularly with potassium. In

all cases, the hydrogen selectivity was inversely correlated with the carbon conversion.

85

Table 3.5 Effects of the addition of promoters to Ni/α-Al2O3 catalysts on

gasification of 2 wt% glucose solution. (380 oC, 15min, 0.05g Ni).

Ni

%

Promoter H2

[mmol/g]

CO

[mmol/g]

CH4

[mmol/g]

CO2

[mmol/g]

CGR

[%]

HGR

[%] Element [%]

No catalyst 2.7 1.0 0.1 6.5 22 9

5 - - 24.3 0.3 4.6 17.4 67 101

5 Sn 0.50 9.8 0.3 1.6 11.4 40 39

5 Sn 1.00 11.1 0.4 1.9 12.2 43 45

5 Cs 0.50 21.7 0.3 6.7 18.7 77 105

5 Cs 1.00 22.1 0.4 6.2 18.1 74 103

5 Na 0.25 20.3 0.3 5.7 17.4 70 95

5 Na 0.50 21.3 0.3 5.7 17.6 71 98

5 Na 1.00 21.5 0.2 5.2 16.8 67 96

5 K 0.25 22.7 0.3 6.1 18.9 76 105

5 K 0.50 21.2 0.4 6.8 19.6 80 104

5 K 1.00 20.6 0.3 5.9 17.6 71 97

10 K 0.25 21.2 0.3 9.4 20.7 92 120

10 K 0.50 21.3 0.3 10.5 22.6 99 127

86

Figure 3.13 Hydrogen yield, methane yield and hydrogen selectivity vs. carbon

conversion using promoted Ni/α-Al2O3 catalysts. No promoter (), Sn (), Na (),

Cs (), and K (), (380 oC, 15 minutes, 1 g catalyst).

3.4 Conclusions

The performance of supported nickel catalysts for the gasification of a biomass model

compound in supercritical water was studied. The support materials covered a wide range

0

5

10

15

20

25

35 45 55 65 75 85 95

H2 yie

ld [

mm

ol/

g]

CGR%

No promoter

Sn

Cs

Na

K

0

5

10

15

35 45 55 65 75 85 95

CH

4 yie

ld [

mm

ol/

g]

Carbon conversion %

87

of physical and chemical characteristics, allowing for the determination of the effects of

different parameters (such as chemistry, surface area, and particle size) on the catalytic

performance. Among the tested catalysts, α-Al2O3, carbon nanotube (CNT), and MgO

supports resulted in the highest carbon conversions, while SiO2, Y2O3, hydrotalcite,

yttria-stabilized zirconia (YSZ), and TiO2 showed modest activities and other catalysts

including zeolites showed negligible activities. γ-Al2O3 supports resulted in a wide range

of catalytic activities from almost inactive to highly active. No clear relationship was

found between the catalyst surface area (and particle size) and the catalytic activity.

However, it is possible that other factors; such as acidity, nickel-support interaction and

stability, played a more dominant role under the conditions used in this study. The

hydrogen selectivity significantly increased by increasing the nickel loading on α-Al2O3

catalyst. The maximum hydrogen selectivity was obtained using a 20% Ni/ α-Al2O3 at

380 oC. Addition of alkali promoters enhanced the carbon conversion whereas addition of

tin decreased the catalyst activity.

3.5 References

[1] T. Yoshida, Y. Oshima, Y. Matsumura, Biomass Bioenerg, 2004, 26, 71-78.

[2] T. Yoshida, Y. Matsumura, Ind Eng Chem, 2001, 40, 5469-5474.

[3] E. Afif, P. Azadi, R. Farnood, Appl. Catal B- Environ, 2011, 105, 136-143.

[4] P Azadi, R. Farnood, C. Vuillardot, J Supercrit Fluids, 2011, 55, 1038-1045.

[5] P. Azadi, J. Otomo, H. Hatano, Y. Oshima, R. Farnood, Int J Hydrogen Energy, 2010,

35, 3406-3414.

[6] P.Azadi, K. M. Syed, R. Farnood, Appl Catal A- Gen, 2009, 358, 65–72.

88

[7] P. Azadi, A.A. Khodadadi, Y. Mortazavi, R. Farnood, Fuel Process Technol, 2009,

90, 145–151.

[8] P. Azadi P, R. Farnood, E. Meier, J Phys Chem A, 2010, 114, 3962-3968.

[9] D.C. Elliott, T.R. Hart, G.G. Neuenschwander, Ind Eng Chem, 2006, 45, 3776-3781.

[10] D.C. Elliott, L.J. Sealock, E.G. Baker, Ind Eng Chem 1993, 32, 1542-1548.

[11] T. Minowa, S. Inoue, Renewable Energy, 1999, 16, 1114-1117

[12] P. Azadi, S. Khan, F. Strobel, F. Azadi, R. Farnood, Appl. Catal B- Environ, 2012,

DOI 10.1016/j.apcatb.2012.01.035

[13] T. Minowa, Z. Fang, Catalysis Today, 45, 1998, 411-416

[14] L. Zhang, P. Champagne, C. Xu, Int J Hydrogen Energy, 2011, 36, 9591-9601.

[15] P. Azadi, R. Farnood, Int J Hydrogen Energy, 2011, 36, 9529-9541.

[16] D.C. Elliott, Biofuels, Bioprod Bioref, 2008, 2, 254–265.

[17] R. D. Cortright, R. R. Davda, J. A. Dumesic , Nature, 2002, 418, 964-967

[18] G.W Huber, J.W. Shabaker, J.A. Dumesic, Science, 2003, 300, 2075-2077.

[19] J.W. Shabaker, D.A. Simonetti, R.D. Cortright, J.A. Dumesic, J Catal, 2005, 231,

67–76.

[20] J.W. Shabaker, J.A. Dumesic, Ind Eng Chem Res, 2004, 43, 3105-3112.

[21] J.W. Shabaker, G.W. Huber, J.A. Dumesic, J Catal 2004, 222,180–191.

[22] R. Molina, G. Poncelet, J Catal, 1998, 173, 257-267.

[23] J. G. Seo, M. H. Youn, S. Park, I. K. Song, 2008, 33, 7427-74334.

[24] I. Chen, S. Y. Lin, D. W. Shiue, Ind Eng Chem Res, 1988, 27, 926-929.

[25] L. Zhang, J. Lin, Y. Chen, J. Chem. Soc. Faraday Trans., 1992, 88, 497-502.

89

[26] Bartholomew CH, Farrauto RJ. Fundamentals of industrial catalytic processes. 2nd

edition. NJ, John Wiley and Sons; 2006.

[27] K. H. Becker L. Cemic, K. Langer, Geochimica et Cosmochimica Acta, 1983, 47,

1573-1578.

[28] H. Weingartner and E. U. Franck, Angew. Chem., Int. Ed., 2005, 44, 2672-2692

[29] A. Kruse, E. Dinjus, J. Supercrit. Fluids, 2007, 39, 362-380

[30] H.X. Hao, L.J. Guo, Z. Mao, Z.M. Zhang, X.J. Chen, Int J Hydrogen Energy 2003,

28, 55–64.

[31] J.A. Onwudili, P. T. Williams, Int J Hydrogen Energy, 2009, 34, 5645-5656.

[32] A. Kruse, D. Meier, P. Rimbrecht, M. Schacht, Ind. Eng. Chem. Res., 2000, 39,

4842-4848.

4- Carbon-nanotube Supported Nickel Catalyst for SCWG

Parts of this chapter have been published under: Azadi P, Farnood R, Meier E.

Preparation of Multiwalled Carbon Nanotube-Supported Nickel Catalysts Using Incipient

Wetness Method, Journal of Physical Chemistry A 2010, 114, 3962-3968.

Abstract

http://www.scopus.com.myaccess.library.utoronto.ca/source/sourceInfo.url?sourceId=25870&origin=resultslist

90

In this work, a systematic study on preparation of multiwalled carbon nanotube

(MWCNT)-supported nickel catalyst is pursued. Functional groups are introduced on the

surface of MWCNT using nitric acid, sulfuric acid as well as partial oxidation in air.

Nickel oxide nanoparticles are formed on the surface of functionalized multi-walled

carbon nanotubes (FMWCNT) by incipient wetness impregnation of nickel nitrate,

followed by calcination in air. Effects of acid type and concentration, acid treatment time,

partial oxidation, nickel loading, precursor solvent and calcination temperature on size of

the nickel nanoparticles and homogeneity of the composite material are evaluated.

Characteristics of the Ni/MWCNT catalysts were examined using BET, scanning

transmission electron microscope (STEM), X-ray diffraction (XRD), thermogravimetric

analysis (TGA) in air and nitrogen, temperature programmed reduction (TPR), X-ray

photoelectron spectroscopy (XPS), acid-base titration and zeta potential analyzer. Results

of this work are useful for formulating CNT supported nickel catalysts for a wide range

of different applications such as reforming of hydrocarbons, catalytic hydrothermal

gasification of biomass, and energy storage. Moreover, the results obtained from SCWG

of glucose using Ni/MWCNT are presented in the last section of this chapter.

4.1 Introduction

Since their discovery in 1991, many applications have been suggested for carbon

nanotubes (CNT). Carbon nanotubes offer excellent properties as a catalyst support such

as proper pore sizes, moderate to high specific area, great thermal stability and stability in

acidic or basic environments. Due to the novel properties of these cylindrical carbon

molecules, they can act as catalyst support. However, due to lack of oxygen functional

91

groups on their outer surface and their hydrophobicity, formation of bonds between metal

precursor and CNT is not an easy task compared to the metal oxide supported catalysts

such as alumina. Once synthesized, CNT contain some impurities. Hence, some

pretreatment steps are usually implemented in preparation of CNT-supported catalysts

(like mild acid treatment) for removal of amorphous carbon and the metal nanoparticles

that are used to catalyze the CNT synthesis. Then, a controlled amount of oxygen

functional groups are added to the outer surface by either partial oxidation in air or by

means of a liquid oxidizing agent such as hydrogen peroxide, nitric acid, etc [1].

A few number of techniques [2] have been developed for decoration of carbon nanotubes,

among them are electrochemical methods [3-5], wet impregnation [6] and incipient

wetness [7]. Each method leads to a certain degree of control over nanoparticle size and

metal dispersion. The functionalized CNT readily interact with the metal precursors

which can be reduced either directly to metal in a reducing atmosphere or it can be

calcined to create metal oxides followed by a reduction in hydrogen flow. Regarding the

oxidation of CNT, it is found that acid concentration and treatment time play a significant

role in functionalization of the nanotubes. Also, it has been reported that the carboxylic

groups are the dominant groups added on the CNT during the oxidative treatment by

nitric acid. Furthermore, acid treatment may increase the specific area of the CNT,

particularly for multiwalled carbon nanotubes by oxidizing and dissolving the outer wall

of CNT, as well as breaking the CNT particles.

Although many researchers investigated the activity of CNT supported catalyst for

variety of applications including synthesis of more carbon nanotubes [1], reforming of

92

hydrocarbons [7], hydrogen storage [8] and hydrogenation [9], but there is a lack for a

systematic study on different aspects of particles formation on the surface of carbon

nanotubes.

4.2 Experimental methods

Multiwall carbon nanotube with average length and diameter of 7m and 120 nm

obtained form Sigma-Aldrich. An oxidative pretreatment in boiling nitric or sulfuric acid

is conducted to introduce oxygenated functional groups onto the surface of the nanotubes.

The following conditions were used throughout catalyst preparation unless mentioned

otherwise. The nitric acid concentration and treatment time were 10M and 5h

respectively. In all experiments, 0.2g MWCNT was dispersed in 50ml acid and the

mixture was boiled using a hot plate and reflux system. The functionalized CNTs were

washed with water, centrifuged twice and dried at 110oC overnight. Following this step,

MWCNT is impregnated with nickel precursor, followed by calcination at 350oC for 3 h.

The nickel loadings were controlled by changing the concentration of nickel nitrate. A

Quantachorome catalyst characterization unit has been utilized to study the pore size

distribution of the carbon nanotubes as well as reduction of nickel oxide nanoparticles in

flowing hydrogen at heating rate of 10oC/min. Scanning transmission electron

microscopy (STEM) was performed on a Hitachi HD-2000 STEM microscope.

Thermogravimetric Analysis (TGA) scans were obtained using a TGA Q500 apparatus

from TA instruments. X-ray diffraction (XRD) patterns obtained from a Philips XRD

system at a scanning rate of 0.015° per second. The average size of nickel oxide

nanoparticles were calculated by Scherrer equation and corrected to account for the

93

instrument line broadening obtain from standard LaB6 [10]. The metal dispersion is

defined as the percentage of metal atoms exposed on the surface and it can be calculated

based on crystallite size.

In order to determine the amount of carboxyl groups on the MWCNTs, 20mg of

functionalized carbon nanotubes was dispersed in 50ml of 0.001M NaOH solution by

ultrasonic bath and then titrated with 0.001M HCL solution.

A 50 mL stainless steel batch reactor was used throughout all gasification experiments.

The heating medium was a molten salt bath. In all experiments, 0.2 g of glucose was

added to 9.8 g of pure water to obtain a 2 wt% solution. This solution along with 0.5 g of

catalyst was then introduced into the reactor which was further immersed in the salt bath

at 380 oC. After reacting for 30min, the reactor was immediately cooled to room

temperature by quenching in cold water. Then, the pressure in the reactor was determined

using a digital gauge. The gaseous product was then collected in a gas bag for

composition analysis by GC.

High surface area multiwalled carbon nanotube was obtained from Cheap Tube Inc.

(Vermont, USA). The length, outer and inside diameter of the nanotubes are 20 µm, 8 nm

and 2-5 nm, respectively. The BET surface area of the CNT was measured at 495 m2/g.

The oxidative pretreatment was performed by nitric acid on the MWCNT prior to the

impregnation. Catalysts were prepared by impregnation of the functionalized MWCNT to

the incipient wetness using nickel nitrate (Sigma Aldrich) precursor. After drying at 110

oC overnight, the catalysts were reduced in 20% hydrogen for 2 h at 400

oC.

94

4.3 Results and Discussions

In this section, physical properties of MWCNT are given and different aspects of

functionalization of carbon nanotubes and their impact on surface and structural

properties are discussed. Following this part, effect of different parameters on

impregnation and calcination of functionalized multi-walled carbon nanotubes

(FMWCNT) will be presented.

Physical properties of MWCNT

According to the manufacturer, the average length, diameter and density of the multi-

walled carbon nanotubes are 7 m, 120 nm and 2.1 g/ml, respectively. Based on our

nitrogen adsorption tests, the specific area of these multi-walled carbon nanotubes is

equal to 14.3 m2/g. The total pore volume and average pore diameter are equal to 0.1

ml/g and 27.7 nm, respectively.

Functionalization of MWCNTs

In order to be able to uniformly attach metal nanoparticles on the carbon nanotubes, the

surface of MWCNT should be chemically modified by an oxidizing agent. It is known

that oxidative treatment not only introduces oxygen-containing functional groups such as

carboxyl and hydroxyl onto the outer surface of carbon nanotubes, but also it increases

the specific surface area of the multi-walled carbon nanotubes. The BET surface area of

the MWCNTs used in this study changed from 14.3 m2/g to 19.5 m

2/g after five hours of

treatment in 10M nitric acid. The extent of this surface treatment can be determined by

different analytical methods such as UV spectroscopy, XPS and acid-case titration.

95

Table 4.1 summarizes the elemental composition of the catalysts at different preparation

steps obtained from XPS. As it is expected, the amount of oxygen increases after the acid

treatment. Then, addition of nickel nitrate hexahydrate further increases the oxygen

content of the surface. Finally, nickel nitrate hexahydrate is decomposed to nickel oxide

and as a result, the surface concentration of oxygen decreases.

Table 4.1 Elemental composition of nickel decorated MWCNT obtained by XPS

Nitric acid concentration.

Preparation step Carbon Oxygen Nickel Nitrogen

No treatment 98.5 1.5 0 0

Acid treated 89 11 0 0

Impregnated 75 19 2.8 3.2

Calcined 82 11.6 6.4 0

Figure 4.1 depicts TGA of fresh and FMWCNT in air. Expectedly, due to an increase in

defective sites and addition of oxygen-containing functional groups, the cracking

temperature of nanotubes decreases upon subjecting to an oxidative treatment. This result

is consistent with previous literature.

96

Figure 4.1 TGA of fresh and FMWCNT in air.

It is also found that treatment in boiling nitric acid can decrease the zeta potential of the

carbon nanotubes to more negative values. This effect is more pronouned for treatment in

higher acid concentrations. Figure 4.2 shows the zeta potential after five hours treated

MWCNT at different acid concentrations. Increase in the negative surface charge of

FMWCNT improves its dispersion in water and enhance the impregnation and adsorption

of precursor on its surface.

97

Figure 4.2 Zeta potential of FMWCNT vs. nitric acid concentration.

Acid treatment time

The concentration of carboxyl groups on the surface of FMWCNT is measured by acid-

base titration. Figure 4.3 shows the relationship between acid treatment time and the

concentration of carboxyl groups. Based on this figure, the concentration of carboxyl

groups significantly increases at prolonged treatment times.

-40

-35

-30

-25

-20

-15

-10

0 5 10 15

Zet

a p

ote

nti

al

(mV

)

Acid concentration (M)

98

Figure 4.3 Concentration of carboxyl groups vs. treatment time.

In order to better evaluate the effectiveness of different oxidative treatments, a

thermogravimetric analysis in an inert environment is performed to thermally remove the

functional groups from the surface of FMWCNT. For all samples, temperature is raised

to 800oC at a heating rate of 10

oC/min and kept at 800

oC for 1 hour. Table 4.2 illustrates

the weight percent of removed functional groups by thermal annealing of functionalized

carbon nanotubes.

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25

CO

OH

[m

mo

l/g

]

Treatment time [h]

99

Table 4.2 Weight percent of the removed functional groups obtained from TGA

in nitrogen.

Sample Acid Conc. [M] Treat. Time [h] Percent removed

No treatment - - - 0.15

Acid treated Nitric 10 5 1.23



Acid treated Sulfuric 10 5 2.01

It should be also noted that almost no weight loss is observed below 700oC, therefore the

weight loss corresponds to removal of functional groups rather than the adsorbed species.

From this table, increasing acid concentration has a more dramatic effect on the surface

concentration of functional groups. Also, it is realized that at a same concentration,

sulfuric acid leads to a higher concentration of functional groups than nitric acid does.

Impregnation of functionalized MWCNT

Effect of acid concentration

It has been shown that the amount of oxygen-containing functional groups has a positive

relationship to concentration of the utilized acid. Figure 4.4 demonstrates two TEM

micrographs of carbon nanotubes decorated with nickel on their outer surface. Based on

this image, it is apparent that dispersion of nickel nanoparticles is considerably better

when MWCNT is functionalized in a more concentrated oxidizing agent. Figure 4.5

shows the effect of surface modification using different acid concentrations on the

average nickel crystallite size as determined using XRD. In all cases, nickel loading is

20% by weight and treatment time is set to 5 hours.

100

Figure 4.4 TEM micrographs of 20% Ni/FMWCNT, 5h oxidative treatment with

nitric acid 4M (left) and 16M (right)

Figure 4.5 Nickel crystallite size vs. nitric acid concentration for 20%

Ni/FMWCT.

15

20

25

30

35

0 5 10 15

Siz

e [n

m]

Concentarion [M]

101

Treatment time

Titration of functionalized MWCNT indicated that prolonged treatment times results in a

higher concentration of functional groups. Therefore, it is anticipated that nickel

dispersion increases with the extent of treatment. Figure 4.6 exhibits nickel nanoparticles

on 20% Ni/MWCNT catalysts with and without surface functionalization.

Using Scherrer equation, nickel crystallite size is calculated and results are plotted in

Figure 4.7. The average crystallite size decreases from 30nm to about 15nm upon surface

treatment of nanotubes for 24 hours in 10M nitric acid.

Figure 4.6 TEM micrographs of 20% Ni/MWCNT, with no treatment (left) and

with 24h treatment in 10M nitric acid.

102

Figure 4.7 Nickel crystallite size vs. treatment time for 20% Ni/FMWCNT.

In order to better assess the effect of functionalization, oxidation of 20% Ni/MWCT with

and without surface modification is studied by TGA (Figure 4.8). It is obsereved that

20% Ni/FMWCNT has a single craking temperature at about 550oC, while the weight

derivative curve of the composite made of 20%Ni/MWCNT can be deconvluted into two

separate peaks whose centers are at 570oC and 740

oC. This further confirm that without

functionalization, a significant number of carbon nanotubes have no nickel attached to

their surface while other ones are decorated with nickel nanoparticles. This observations

confirms the nonuniformity of non-chemically modified catalysts as oppose to modified

ones.

10

15

20

25

30

35

0 4 8 12 16 20 24

Siz

e [n

m]

Treatment time [h]

103

Figure 4.8 Effect of oxidative treatment on cracking temperature of

20%Ni/MWCT with and without acid treatment.

Treatment method

Four different oxidizing agents (HNO3, H2SO4, mixture of HNO3 and HCl as well as

partial oxidation in air) are considered for addition of functional groups onto MWCNT.

Partial oxidation is performed in air at 600oC until 20% of the weight is burned. As can

be seen in Figure 4.9, partial oxidation in air cannot improve the dispersion of

nanoparticles. For other treatment methods, at a same treatment condition (concentration

and time), sulfuric acid treated sample has the smallest crystallite size compared to the

other two. This finding suggest that at the same acid molar concentration, sulfuric acid

treatment can lead to a more uniform Ni nanoparticle dispersion.

0

0.2

0.4

0.6

0.8

1

350 450 550 650 750

Wei

gh

t d

eriv

ati

ve

Temp. [0C]

No treatment

5h treatment

104

Figure 4.9 Nickel crystallite size vs. treatment type for 20% Ni/FMWCNT.

Nickel loading

Ni/FMWCNT catalysts with five different nickel loadings are prepared using incipient

wetness. The XRD pattern of these samples are given in Figure 4.10. The first peak at 26o

corresponds to CNT while the other two peaks at 37o and 43

o are NiO(111) and NiO(200)

respectively. However, MWCNT also has a very small peak at around 43o that coincides

with NiO(200).

0

10

20

30

40 S

ize

[nm

] HNO3 + HCl

Partial oxidation

HNO3

H2SO4

105

Figure 4.10 XRD patterns of decorated carbon nanotubes at different nickel

loadings.

The nickel oxide crystallite size and its corresponding dispersion for FMWCNT are

presented in Figure 4.11. It is noted that nickel nanoparticles become larger at higher

loadings and as a result, better dispersions can be only achieved at low nickel

percentages.

106

Figure 4.11 Nickel crystallite size and dispersion vs. nickel loading. Treatment

time: 5 hours).

Figure 4.12 shows TEM of decorated carbon nanotubes with 5%, 20% and 50% nickel.

As it was indicated by XRD, the nickel crystallites become larger at higher nickel

loadings, resulting in a poor dispersion. Furthermore, it is noticed that at high nickel

loadings, nickel crystallites are somewhat porous with a pore diameter of about few

nanometers. This phenomena has been observed for nickel nano-belts and it has been

suggested that it could be an artefect of interaction of effect of TEM electron beam with

nickel crystalls [11,12].

2

3

4

5

6

7

8

10

15

20

25

30

35

40

0 10 20 30 40 50 60

Dis

per

sio

n %

Siz

e [n

m]

Nickel loading %

107

Figure 4.12 TEM micrographs of Ni/FMWCNT at different metal loadings, a) 5%

b) 20% c) 50% d) porous nickel crystals formed on 50% Ni/FMWCNT.

Thermogravimetric analysis of the samples with different nickel loadings (Figure 4.13)

indicated that at a lower nickel loadings (<20%) oxidation of NiO/FMWCNT is non-

uniform at occurs at two distinct temperatures (~600oC and 710

oC). Meanwhile, TEM

images show that nearly all FMWCNT are somewhat covered with Ni nanoparticles

regardless of Ni loading. This observation suggest that non-uniform oxidation of

FMWCNT are likely due to the presence of relatively large uncovered area on the surface

of individual CNT particles. This phenomenon can be different form Figure 4.8 where Ni

nanoparticles were absent from a portion of CNT’s. Also, higher peak for

a b

c d

108

20%Ni/FMWCNT compared to that of 50% Ni/FMWCNT is due to lower amount of

residue (i.e. nickel oxide).

Figure 4.13 Derivate if weight loss vs. temperature for different nickel loadings.

It is found that the zeta potential of the nickel-decorated carbon nanotubes increases from

-32 to about zero by increasing the nickel content of the samples from 0 to 50% (Figure

4.14). This increase in hydrophobicity of the Ni/FMWCNT may cause clustering of the

catalyst in aqueous solutions.

0

0.2

0.4

0.6

0.8

1

370 470 570 670 770

Wei

gh

t d

eriv

ati

ve

Temp. [0C]

5% Nickel

10% nickel

20% Nickel

50% Nickel

109

Figure 4.14 Zeta potential of Ni/FMWCNT vs. nickel loading.

Due to high hydrophobicity of carbon, solvent polarity adversely affects the size of Ni

nanoparticles. Therefore, it is anticipated that organic solvents result in a better metal

dispersion compared to water-based precursors. To examine this effect, three solvents

(i.e. water, methanol and acetone) with different dielectric constants are used for the

preparation of nickel nitrate precursor (Figure 4.15). The corresponding dielectric

constant of water, methanol and acetone are 80, 33 and 21, respectively. Figure 4.16

presents size of the nickel oxide nanoparticles obtained from Scherrer equation. It

becomes apparent that the nanometal crystallite size significantly decreases by decreasing

the polarity of the solvent.

-35

-30

-25

-20

-15

-10

-5

0

5

0 10 20 30 40 50

Zet

a p

ote

nti

al

(mV

)

Nickel %

110

Figure 4.15 Nickel crystallite size for different precursor solvents.

Calcination and Reduction

Calcination temperature

Drying of impregnated samples forms nickel nitrate crystals on the surface of FMWCNT.

Following this step, calcination of the catalyst at higher temperatures decomposes the

nickel nitrate crystals to nickel oxide with permanent chemical bonds to the functional

groups of the FMWCNT. Figure 4.16 (left) shows the decomposition of Ni(NO3)2.

6H2O in air as measured using TGA. It can be seen that at 225oC water is completely

evaporated and nickel nitrate starts to decompose to nickel oxide at temperatures greater

than 280oC. Therefore, the calcination temperature should be greater than 280

oC. Figure

4.17(right) illustrates the effect of calcination temperature on nickel oxide crystallite size.

It is found that calcination temperature does not affect the crystal size up to 450oC.

5

10

15

20

25

Water Methanol Acetone

Siz

e [n

m]

111

Figure 4.16 Nickel crystallite size vs. calcination temperature for 20%

Ni/FMWCNT.

Finally the calcined samples are reduced in a TPR unit to study the reduction temperature

of nickel oxide nanoparticles.

The TPR results (Figure 4.17) of the Ni/FMWCNT at a heating rate of 10oC/min indicate

that the reduction of nickel oxide starts at about 320oC. However, depending on the

amount of nickel, it might be required to increase the reduction temperature up to 550oC

to obtain a complete reduction. Also, it is realized that due to the interaction between

nickel oxide and carbon support, the peak temperature for a same amount of nickel oxide

(i.e. 10% NiO/FMWCNT and nickel oxide powder) is shifted towards higher

temperatures.

112

Figure 4.17 Temperature programmed reduction of Ni/FMWCNT and NiO

powder at heating rate of 10oC/min.

4.4 Activity of Ni/MWCNT Catalyst for SCWG of Glucose

In this section, results of the hydrothermal gasification of glucose with Ni/MWCNT are

presented. Figure 4.18 illustrates the effects of acid concentration on the catalytic activity

of 7% and 20% Ni/MWCNT for the gasification of glucose in SCW. It was observed that

the carbon conversion and methane yield dramatically dropped upon oxidative

pretreatment of the MWCNT in 5 M nitric acid. However, the activity of the catalysts

increased when the carbon nanotubes were functionalized in a more concentrated acid. In

order to better understand the variation in catalytic activity of Ni/MWCNT, a few number

of experiments without the addition of glucose were carried out. These experiments

(Table 4.3) helped to determine the approximate amount of carbon originally from the

catalyst that ends up in the gas phase upon the exposure to SCW. It was found that the

MWCNT support without nickel is fairly stable in SCW condition (entry 1). The amount

113

of carbon in the gas phase increased by the impregnation of the nickel, and it was further

increased by the oxidative pretreatment of the nanotubes. Therefore, it can be concluded

that functionalization of the carbon nanotube makes it more vulnerable to gasification in

SCW medium. On the other hand, addition of oxygen functional group and defects that

were created during the acid pretreatment increased the metal dispersion which may

change the catalytic activity on Ni/MWCNT through changing the metal crystallite size.

This decrease in the metal nanoparticle size may have a positive or negative effects. The

positive effect can be attributed to the greater number of active sites at higher dispersions

and the negative effect may be due to the loss of catalytic activity of metal crystallite at

extremely small sizes. The effect of acid treatment time was almost identical to the effect

of acid concentration, the catalytic activity dropped to a minimum and then increased.

114

Table 4.3 Gas formation from the catalyst support (with no glucose). 380 oC,

30min, 0.5 g catalyst, 9.8 g water. Yield and CGR are calculated with 0.2 g glucose

as a hypothetical feed.

Entry Ni

[wt %]

Nitric

acid

[M]

Treatment

time

[h]

pf

[bar]

Yield

[mmol/g]

CGR

%

1 0 - - 0.14 1.2 4

2 20 - - 0.27 2.4 8

3 20 5 5 0.54 4.5 14

4 20 10 5 0.41 3.4 10

5 20 16 5 0.81 6.8 20

115

Figure 4.18 Carbon conversion and gas yields vs. nitric acid concentration. 380 oC, 30min, 0.5 g catalyst, 9.8 g water, 0.2 g glucose, MWCNT pretreatment time=5

h.

116

Figure 4.19 Carbon conversion and gas yields vs. acid treatment time. 380

oC,

30min, 0.5 g catalyst, 9.8 g water, 0.2 g glucose. Nitric acid concentration=10 M.

4.5 Conclusion

In this study, different aspects of formation of nickel nanoparticles on multi-walled

carbon nanotubes are discussed. It has been shown that functionalization of MWCNT in a

concentrated oxidizing agent for prolonged times results in a better dispersion of the

nanoparticles. Also, it is found that nickel crystal size increases by increasing nickel

loading. Non-polar solvents such as acetone are found to be more appropriate for

formation of well-dispersed nickel nanoparticles. Calcination temperature does not

change the crystallite size of nickel nanoparticles in temperature range of 280 oC to 450

117

oC. Based on TPR test, the nickel oxide deposited on carbon nanotube can be reduced

between 350 oC to 450

oC.

4.6 References

[1] Iosif Daniel Rosca, Fumio Watari, Motohiro Uo, Tsukasa Akasaka, Oxidation of

multiwalled carbon nanotubes by nitric acid, Carbon, 43, 2005, pg. 3124–3131

[2] Gregory G. Wildgoose, Craig E. Banks, and Richard G. Compton, Metal

Nanoparticles and Related Materials Supported on Carbon Nanotubes: Methods and

Applications, Small, 2(2), 2006, pg. 182–193

[3] Huaping Liu, Guoan Cheng, Ruiting Zheng, Yong Zhao, Changlin Liang, Influence of

synthesis process on preparation and properties of Ni/CNT catalyst, Diamond & Related

Materials, 15, 2006, pg. 15–21

[4] H. Liu, G. Cheng, R. Zheng, Y. Zhao, C. Liang, Influence of acid treatments of

carbon nanotube precursors on Ni/CNT in the synthesis of carbon nanotubes, Journal of

Molecular Catalysis A: Chemical, 230, 2005, pg. 17–22

[5] L.M. Ang, T.S.A. Hor, G.Q. Xu, C.H. Tung, S.P. Zhao, J.L.S. Wang, Decoration of

activated carbon nanotubes with copper and nickel, Carbon, 38, 2000, pg. 363–372

[6] H. M. Yang, P. H. Liao, Preparation and activity of Cu/ZnO CNTs nano-catalyst on

steam reforming of methanol, Applied Catalysis A: General, 317, 2007, pg. 226–233

[7] P.G. Savva, G.G. Olympiou, C.N. Costa a, V.A. Ryzhkov, A.M. Efstathiou,

Hydrogen production by ethylene decomposition over Ni supported on novel carbon

nanotubes and nanofibers, Catalysis Today, 102–103, 2005, pg. 78–84

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[8] H.S. Kim, H. Lee, K. S. Han, J.-H. Kim, M. S. Song, M. S. Park, J. Y. Lee, J. Kang,

Hydrogen Storage in Ni Nanoparticle-Dispersed Multiwalled Carbon Nanotubes, Journal

of Physical Chemistry B, 109,2005, pg. 8983-8986

[9] J. M. Planeix, N. Coustel, B. Coq, V. Bretons, P. S. Kumbhar, R. Dutartre, P.

Geneste, P. Bernier, P. M. Ajayan, Application of Carbon Nanotubes as Supports in

Heterogeneous Catalysis, Journal of American Chemical Society, 116,1994, pg. 7935-

7936

[10] C. H. Bartholomew, R. J. Farrauto, Fundamental of Industrial Catalytic Processes,

2nd Edition, Wiley, New York, 2005

[11] L. Dong, Y. Chu, W. Sun, Controllable Synthesis of Nickel Hydroxide and Porous

Nickel Oxide Nanostructures with Different Morphologies, Chemistry European Journal,

14, 2008, pg. 5064–5072

[12] W. Yue, W. Zhou, Porous crystals of cubic metal oxides templated by cage

containing mesoporous silica, Journal of Materials Chemistry, 17, 2007, pg. 4947–4952

5- Catalytic SCWG of Various Lignocellulosic Materials

* Based on: P. Azadi, S. Khan, F. Strobel, F. Azadi, R. Farnood, Hydrogen Production

from Cellulose, Lignin, Bark and Model Carbohydrates in Supercritical Water using

Nickel and Ruthenium Catalysts, Applied Catalysis B: Environmental, 117, 330-338.

Abstract

In this chapter, the catalytic activity and hydrogen selectivity of Ni/α-Al2O3,

Ni/hydrotalcite, Raney nickel, Ru/C and Ru/γ-Al2O3 catalysts for hydrothermal hydrogen

production from lignocellulosic biomass have been evaluated. The feedstocks included

glucose, cellulose, fructose, xylan, pulp, lignin and bark. The experiments were carried

http://ca.wiley.com/WileyCDA/Section/id-302478.html?query=C.+H.+Bartholomew

http://ca.wiley.com/WileyCDA/Section/id-302478.html?query=Robert+J.+Farrauto

119

out at 380 oC in a batch reactor with 2 wt% feed concentration. It was found that the

gasification of glucose, fructose, cellulose, xylan and pulp resulted in comparable gas

yields (± 10% at 60min), whereas lignin was substantially harder to gasify. Interestingly,

gasification yield of bark which has a high lignin content was comparable to those of

carbohydrates after 60min reaction time. For a given feedstock, catalyst type affected

both the gasification yield and the product distribution. Ni/α-Al2O3 and Ni/hydrotalcite

catalysts were not only highly active for the gasification of carbohydrates, but also had

better hydrogen selectivity when compared to Raney nickel, Ru/C and Ru/γ-Al2O3. In

particular, gasification of bark in the presence of these catalysts resulted in negligible

amounts of alkanes.

5.1 Introduction

Supercritical water gasification (SCWG) is a promising process for the production of

biorenewable hydrogen from biomass. In this process, wet biomass is decomposed to

form hydrogen, carbon dioxide, methane and carbon monoxide. In comparison with the

steam reforming process (e.g., T>650 oC and low pressures), catalytic supercritical water

gasification proceeds at milder temperatures (e.g., T<450 oC) with little or no char

formation [1]. In spite of major advancements over the past decades, there are still

important challenges that need to be addressed to make catalytic SCWG technically and

economically viable for hydrogen production. Poor hydrogen selectivity, catalyst

deactivation, tar and char formation, heat recovery and precipitation of inorganic salts are

among the most important issues that are yet to be addressed.

120

Since SCWG is operated at a high water content (typically 80-90%), and given the

considerably high specific heat of water, it is of great importance to run the reaction at

the lowest possible temperatures. It has been shown that homogeneous alkali and nickel

and ruthenium-based solid catalysts are useful for enhancing the gasification rates at

moderate temperatures [2, 3]. Supercritical water gasification of glucose in presence of

different skeletal catalysts indicated that the inherent activity of the nickel is substantially

higher than that of cobalt and copper [4]. In general, catalyst formulation along with the

operating conditions such as temperature and feed concentration can significantly affect

the conversion and product distribution. More detailed discussion regarding the current

status of the catalytic SCWG can be found in [5-6].

Review of the literature illustrates that both real biomass and model compounds have

been employed to evaluate the effectiveness of the catalytic SCWG process [5-7].

Investigations on real biomass provides practical information on the performance of the

SCW gasifiers at various operating conditions while model compounds may be used in

the laboratory for fundamental studies. The real biomasses considered for gasification in

SCW include lignocellulosic biomass from different sources, sewage sludge [8-10], pulp

and paper wastes [11], chicken manure [12], food wastes [13], and algae [14, 15]. Given

the fact that lignocellulosic biomass is the most abundant type of biomass on the earth, its

conversion and upgrading is expected to play a significant role in the production of future

fuels. Representative model compounds of various types of biomass such as cellulose,

glucose, phenolics, glycerol and alcohols have been also considered in the literature.

Earlier studies have shown that due to the fast hydrolysis of cellulose to glucose and

121

other sugar oligomers, gasification of glucose and cellulose led to identical gas yields

[16]. Osada et al. showed that ruthenium is an effective catalyst for decomposition of

organosolv lignin in SCW [17], but its activity decreased by the addition of sulfur to the

feed [18]. The same behavior was observed when sulfur was added to a Raney-nickel

catalyzed SCWG reaction [9]. Interaction among different reactants during the SCWG

has been also studied using representative mixtures of model compounds as a feedstock.

In the case of cellulose/lignin sulfonate mixtures at low catalyst loadings (0.4 g [Ni

5132P, Engelhard]/g feed), a strong negative deviation from the rule of mixtures in terms

of carbon conversion was observed whereas mixtures of cellulose and xylan at any ratio

were found to exhibit a very predictable gasification efficiency that closely matched the

rule of mixtures [19, 20]. The magnitude of the negative deviation for the cellulose-lignin

sulfonate substantially decreased at higher catalyst to feed ratios [20] and the authors

suggested that the possible mechanism for this deviation the catalyst deactivation by tarry

products formed due to the reaction between cellulose and lignin. Moreover, Yoshida et

al. [20] found that the gasification yield of saw dust was nearly 90% of the predicted

value based on the rule of mixtures.

In general, SCWG of carbohydrates using supported nickel catalysts at temperatures

below 400oC typically results in 1-15 mmol H2/g dry feed [6], corresponding to 1.5-23%

of the maximum stoichiometric value (not considering the thermodynamic equilibrium),

while the SCWG of lignin at the same operating range generate 1-2 mmol/g dry feed [6].

We note that it is not be possible to increase the hydrogen yield simply by extending the

reaction time, as the hydrogen yield decreases due to methanation reaction in the

122

presence of typical Ni and Ru catalysts. The low hydrogen selectivity is caused by the

high activity of nickel and ruthenium to open the C-O bonds, which in turn would result

in the formation of alkanes. Although many studies have been devoted to evaluate the

short term activity of heterogeneous catalysts, limited work is done on enhancing the

hydrogen selectivity and decreasing the alkane selectivity. In one of the few attempts,

Dumesic et al. showed that the hydrogen selectivity of Raney nickel catalyst for aqueous

phase reforming can be significantly improved by adding tin onto the surface of the

Raney nickel catalysts [21-24]. Addition of molybdenum as a promoter to Raney nickel

did not have a pronounced effect on the catalyst performance [4].

In this chapter, we present a systematic study of SCWG of several lignocellulosic

materials with five promising catalysts including three nickel and two ruthenium

catalysts. Commercially available Raney nickel, Ru/activated carbon and Ru/γ-alumina

along with two laboratory-made nickel catalysts supported on α-alumina and hydrotalcite

were considered. The relationship between hydrogen selectivity and carbon conversion

for the gasification of glucose, cellulose, xylan, kraft lignin and birchwood bark is

discussed in details. To the best of our knowledge, this is the first paper concerning

supercritical water gasification of bark. Results of this study provide a better

understanding of the performance of solid catalysts for the gasification of lignocellulosic

biomass in supercritical water in terms of activity and hydrogen selectivity.

5.2 Materials and Methods

A schematic of the experimental setup is shown in Figure 1. A non-stirred 50 mL

stainless steel batch reactor was used in the SCWG experiments. In all experiments, 0.2 g

123

of feedstock was added to 9.8 g of deionized water to obtain a 2 wt% mixture. This

mixture along with the desired amount of catalyst was introduced into the reactor and

immersed in a molten salt bath containing sodium nitrate, sodium nitrite (General

Chemical, USA), and potassium nitrate (Alphachem, USA). The heat transfer from

molten salts to SCW reactors has been discussed in [25].

The metal loadings of Ni and Ru (excluding the catalyst support) were set at 120 mg and

6 mg, respectively. These amounts were chosen in such way that the carbon conversions

of about 60% can be obtained from SCWG of glucose in 15min reaction experiments.

The temperature of the salt bath was measured using a Type K thermocouple (Omega

Engineering, Canada) and was maintained at 380 °C using a PID temperature controller

(Hanyoung, Electronic Co. Ltd., Korea). The corresponding pressure in the reactor at 380

oC was estimated to be 230 bar. Four different reaction times were considered: 5, 15, 30

and 60min. After a predetermined reaction time, the reactor was rapidly cooled by

quenching in cold water and allowed to reach the room temperature. Then, the pressure in

the reactor was determined using a digital pressure gauge (Cecomp Electronics) and was

used to calculate the gas yield using the ideal gas law. The gaseous product was then

collected in a gas bag for composition analysis. The data reproducibility was confirmed

to be within ± 5% by performing at least one replicate run for each data point.

A gas chromatograph (Hewlett-Packard 5890 series) equipped with a thermal

conductivity detector and argon as carrier gas was used to determine the product gas

composition. A 5m general SupelcoSS 60/80 CARBOXEN 1000 column (Sigma-

124

Aldrich, Canada) was used for fractionation of the permanent gases. The oven, injector

and detector in the GC were all set at 140 °C.

The model compounds investigated in our experiments included: glucose (CAS 50-99-7),

microcrystalline cellulose (CAS 9004-34-6), alkali lignin (CAS 8068-05-1) and xylan

from oat spelts (CAS 9014-63-5), all of which were obtained from Sigma-Aldrich,

Canada. Kraft pulp was obtained from the National Institute of Standards and

Technology USA, and bark from birchwood was kindly provided by Professor Ning Yan

from the Department of Forestry at University of Toronto. The elemental analysis (2400

Series II CHNS Analyzer from Perkin Elmer operating at the C-H-N mode) indicated that

the empirical formulas of lignin and bark were CO0.95H0.63N0.002 and CO0.88H0.83N0.01,

respectively.

The commercial catalysts including Raney nickel 4200 (received as aqueous slurry

containing 50% solid content), 5% Ru/C and 5% Ru/γ-Al2O3 as well as α-Al2O3,

hydrotalcite (HT), and nickel nitrate were all obtained from Sigma-Aldrich Canada.

Ni/α-Al2O3 and Ni/HT catalysts were prepared by incipient wetness impregnation with

nickel nitrate solution. The catalyst precursors were dried at 110 oC over night and

calcined in air at 350 oC for 3 h. The catalyst was then reduced in flowing hydrogen for 2

h (50 ml STP/min, 30% H2, 70% N2) and stored in water before being used in the

experiments. The SEM micrographs of the catalysts used in this study are depicted in

Figure 2. These images are obtained using JEOL JSM-840 scanning microscope. The

BET surface area and metal dispersion were obtained using Quantachrome Autosorb

catalyst characterization system (Quantachrome, USA). Figure 3 shows the pore size

125

distributions of α-Al2O3 and hydrotalcite based on nitrogen adsorption. The supports

materials were degassed under vacuum at 300 oC for 3 h prior to nitrogen adsorption.

Based on this figure, a large fraction of the pores in the α-alumina support were greater

than 40 nm whereas hydrotalcite had a bimodal pore size distribution at ~5 and 20 nm.

The characteristics of the utilized catalysts are listed in Table 1.

Table 5.1 Physical characteristics of the catalysts used in this study.

Catalyst Ni or Ru

wt%

Particle size

(µm)

BET

(m2/g)

Metal dispersion *

%

Raney nickel 93 35 78 13.4

Ni/α-Al2O3 5 118 8 4.5

Ni/Hydrotalcite 5 1 12 1.3

Ru/C 5 18 900 41.0

Ru/γ-Al2O3 5 40 90 16.7

* The metal dispersion obtained from [26] for Ru/C and [27] for Ru/γ-Al2O3.

Figure 5.1 Schematic diagram of the experimental set-up: 1) molten salt bath, 2)

reactor, 3) electrical heater, 4) thermocouple, 5) PID temperature controller, 6) first

valve, 7) low-pressure gauge, 8) second valve.

126

The equilibrium gas yields were calculated using Gibbs free energy minimization using

Aspen Plus (AspenTech, Burlington, USA).

Figure 5.2 SEM micrograph of the catalysts used in this work . a) Raney nickel,

b) Ni/α-Al2O3, c) Ru/C, d) Ru/γ-Al2O3

127

Figure 5.3 Pore size distribution of α-Al2O3 (top) and hydrotalcite (bottom). The

unit of the Y-axis is cc/Å/g .

0.0

0.5

1.0

1.5

2.0

10 100 1000

Dv

(r)

x 1

04

Pore width (Å)

0.0

0.5

1.0

1.5

2.0

10 100 1000

Dv

(r)

x 1

04

Pore width (Å)

128

5.3 Results

The effects of catalyst type, feedstock, and the reaction time on the supercritical water

gasification of several lignocellulosic materials have been systematically investigated.

Experimental results are presented in terms of gas yields (mmol gas/ g feed), carbon

gasification ratio (CGR) and hydrogen selectivity. Carbon gasification ratio is defined as

the ratio of carbon in the gas products to the carbon in the initial feed. Hydrogen

selectivity is defined as the number of moles of H2 to the number of moles of hydrogen in

methane and it has been calculated as follow:

Hydrogen selectivity = moles of H2 / (2 × moles of CH4) (5.1)

Given the differences in the quantities of the active elements (i.e. Ni & Ru) and the metal

dispersions, comparison was only made between the activities of catalysts with the same

type of active metal. We also note that the CO yield in all catalytic SCWG experiments

were below 1 mmol/g and therefore not shown in the figures.

5.3.1. Thermodynamic considerations

The theoretical equilibrium concentrations of the gaseous products obtained using Gibbs

free energy minimization at 380 oC and 230 bar for the mixtures of 98 wt% water and 2

wt% feed are given in Table 2. Under these operating conditions, the equilibrium

hydrogen yield for the carbohydrates was limited to about 6mmol/g feed and the methane

yield varied from 15 to 17 mmol/g feed, with cellulose having the highest methane yield

compared to other carbohydrates examined in this work.

129

As methane formation during gasification of biomass in SCW medium does not have a

pyrolytic origin and it is mostly formed through hydrogenation on the catalytically active

metal surface [28], it is possible to minimize the methane yield through careful design of

the reactor and the use of appropriate catalysts. Indeed, this highlights the need for design

of selective catalysts with minimal activities for hydrogenation through cleavage of C-O

bond.

Table 5.2 Thermodynamic equilibrium of 2 wt% feeds at 380 oC and 230 bar.

Feed Formula C/O

ratio

H2

(mmol/g)

CO

(mmol/g)

CH4

(mmol/g)

CO2

(mmol/g)

Hydrogen

selectivity

Glucose C6H12O6 1.00 5.7 0.008 15.2 18.1 0.19

Fructose C6H12O6 1.00 5.7 0.008 15.2 18.1 0.19

Cellulose (C6H10O5)n 1.20 5.8 0.009 17.0 20.0 0.17

Xylan (C5H8O4)n 1.00 5.7 0.008 15.2 18.1 0.19

Lignin CH0.63O0.95N0.002 1.05 4.8 0.009 11.0 24.8 0.22

Bark CH0.83O0.88N0.01 1.14 5.1 0.010 12.8 24.1 0.20

5.3.2. Catalyst-free SCWG

In order to evaluate the effectiveness of the catalysts, the gas yields and carbon

conversion obtained from catalyst-free supercritical water gasification in 15min batch

experiments are given in Table 3. It was found that without the addition of catalysts, the

130

carbon conversions obtained from these compounds were typically between 6%-22%,

depending on the feedstock.

Table 5.3 Gas yield and CGR obtained from catalyst-free SCWG of 2 wt%

feeds at 380 oC in 15min batch experiments.

Feed H2

(mmol/g)

CO

(mmol/g)

CH4

(mmol/g)

CO2

(mmol/g)

CGR

%

Glucose 2.7 1.0 0.10 6.5 22

Fructose 1.5 0.7 0.02 4.9 17

Cellulose 1.1 1.0 0.10 6.3 20

Xylan 1.1 0.8 0.02 4.8 17

Lignin 1.6 0.1 0.14 1.8 6

Bark 1.3 0.4 0.03 3.2 9

5.3.3. Nickel-Catalyzed SCWG

Figures 5.3-5.5 illustrate the yields of gaseous products obtained from the catalytic

SCWG of different species using Raney nickel, Ni/α-Al2O3 and Ni/hydrotalcite,

respectively. It was observed that the gasification yields and the product distributions of

glucose and fructose were similar to those of cellulose (± 10 % at 60min). However,

SCWG of lignin was more difficult (i.e. lower yield) compared to carbohydrates.

Although hardwood bark typically contains 40-50% lignin, 32-47% carbohydrates and 5-

12% extractives [29], its gasification yield was found to be comparable to that of

carbohydrates and significantly higher than that of lignin. The experimental gas yields

positively deviated from the expected values based on the rule of mixtures. For example a

total gas yield of 70mmol/g feed was obtained for the gasification of bark with Ni/α-

Al2O3 catalyst after 60min whereas the expected value for this experiment based on the

131

rule of mixtures was only 51mmol/g. Furthermore, we note that the decomposition of

bark in the presence of Ni/α-Al2O3 and Ni/hydrotalcite catalysts resulted in low methane

yields even after long reaction times. Yoshida et al. have previously shown that for the

mixture of cellulose and lignin sulfonate, the negative deviation from the expected yields

(obtained from the rule of mixtures) became less pronounced by increasing the catalyst

loading and reported that the gasification yield of sawdust in near critical water and in the

presence of nickel catalyst was nearly 90% of the theoretical value [20]. Similarly,

Waldner and Vogel [30] reported near-complete gasification yield of bark-free fir and

spruce sawdust at 400oC with Raney nickel as catalyst. These observations imply that

naturally occurring lignocellulosic compounds could be entirely gasified in supercritical

water with sufficient amount of catalyst. Hence, the low conversion of lignin and also the

negative deviation of blends of lignin and carbohydrates from the rule of mixtures are

likely caused by the catalyst poisoning due to the sulfur content and/or the condensed

chemical structure of lignin sulfonate (compared to the original protolignin in wood).

In general, in order to interpret the results obtained from the SCWG of a binary mixtures

of compounds A & B the following parameters should be considered: i) the relative

gasification rates of A and B, ii) the rates of bimolecular condensation reactions between

A-A, B-B and A-B, iii) the rate of catalyst deactivation by A, B and the reaction

intermediates, iv) the amount of the produced hydrogen from the easier-to-gasify

compound and v) the initial feed to catalyst ratio.

132

Figure 5.4 SCWG of lignocellulosic feeds using Raney nickel catalyst. 380 oC, 2

wt% feed, 5, 15, 30 and 60min (bars from left to right respectively), 120 mg Ni.

Figure 5.5 SCWG of lignocellulosic feeds using Ni/α-Al2O3. 380 oC, 2 wt% feed,

5, 15, 30 and 60min (bars from left to right respectively), 120 mg Ni.

0

10

20

30

40

50

60

70

80

90

Glucose Fructose Cellulose Pulp Xylan Lignin Bark

Yie

ld [

mm

ol/

g f

eed

]CO2

CH4

H2

0

10

20

30

40

50

60

70

80

90

Glucose Cellulose Xylan Lignin Bark

Yie

ld [

mm

ol/

g f

eed

]

CO2

CH4

H2

133

Figure 5.6 SCWG of lignocellulosic feeds using Ni/Hydrotalcite. 380 oC, 2 wt%

feed, 5, 15, 30 and 60min (bars from left to right respectively), 120 mg Ni.

5.3.4. Ruthenium-Catalyzed SCWG

Ruthenium has been reported to be quite active for reactions involved in SCWG [6].

Figures 5.7 and 5.8 depict the gas yields obtained from the catalytic SCWG of

lignocellulosic materials using Ru/C and Ru/γ-Al2O3 catalysts, respectively. After 60min,

Ru/γ-Al2O3 resulted in a total yield of about 45-60 mmol/g for glucose, fructose,

cellulose, xylan and pulp, whereas Ru/C resulted in a total yield of about 40-55 mmol/g

for the SCWG of the same compounds. Furthermore, it is demonstrated that even at low

catalyst loadings, ruthenium was highly active for the hydrogenation of CO2 and CO, and

consequently, resulted in poor hydrogen selectivity. Similar to nickel catalysts, alkali

lignin had the lowest total gasification yield likely due to catalyst poisoning and/or

condensed chemical structure of lignin. Contrary to nickel-catalyzed reactions, use of

ruthenium as catalyst resulted in a lower gas yield for gasification of bark (~ 40mmol/g)

0

10

20

30

40

50

60

70

80

90

Glucose Cellulose Xylan Lignin Bark

Yie

ld [m

mo

l/g

fee

d]

CO2

CH4

H2

134

compared to cellulose and glucose (~ 50-60mmol/g). However, the gas yields obtained

from bark were still higher than the expected values based on the rule of mixtures

between the carbohydrates and lignin. The difference in the gasification yield of bark

may be due to the difference in the pore structure of catalysts. The pore size of nickel

catalysts used in this study is in the order of tens of nanometers while activated carbon

and γ-Al2O3 are known to have a smaller pore structure. Therefore, the rate of

gasification of larger molecules (e.g. lignin) generated by the hydrothermal degradation

of birchwood bark could be lowered due to the reduced diffusion rate of these molecules

in the smaller pore structure of Ru catalysts, reducing the total yield.

Figure 5.7 SCWG of lignocellulosic feeds using Ru/C. 380 oC, 2 wt% feed, 5, 15,

30 and 60 min (bars from left to right respectively), 6 mg Ru.

0

10

20

30

40

50

60

70


Yie

ld [

mm

ol/

g f

eed

]

CO2

CH4

H2

135

Figure 5.8 SCWG of lignocellulosic feeds using Ru/γ-Al2O3. 380 oC, 2 wt% feed,

5, 15, 30 and 60min (bars from left to right respectively), 6 mg Ru.


In this paper, results of the supercritical water gasification of several lignocellulosic

materials using Ni/α-Al2O3, Ni/hydrotalcite, Raney nickel, Ru/C and Ru/γ-Al2O3

catalysts have been presented. It was found that for a given catalyst, gasification of

carbohydrates (i.e. glucose, fructose, cellulose, xylan and pulp) resulted in comparable

gas yields (± 10 %) after 60min reaction time. However at short reaction times (e.g.,

5min) and in the presence of Ni-based catalysts, cellulose had consistently higher carbon

conversion than glucose and xylan. Although lignin was substantially harder to gasify

than cellulose, gasification yield of bark, which has high lignin content, was comparable

to that of cellulose. Utilization of Raney nickel, Ru/C and Ru/γ-Al2O3 resulted in high

methane yields. In spite of the low surface area of the laboratory-made Ni/α-Al2O3 and

Ni/hydrotalcite catalysts, they exhibited high activities and superior hydrogen

0

10

20

30

40

50

60

70


Yie

ld [

mm

ol/

g f

eed

]CO2

CH4

H2

136

selectivities compared to those of the other catalysts tested in this work. More

specifically, the amount of alkane formed upon supercritical water gasification of bark in

the presence of Ni/α-Al2O3 and Ni/hydrotalcite catalysts was found to be very small (i.e.

< 1.5 mmol/g after 60 minutes). Additional work is ongoing to examine the stability and

possible deactivation of these catalysts under SCW conditions.

Acknowledgements

Financial support from NSERC CGS-D and NSERC-DG is gratefully acknowledged.

Also, the authors would like to thank Professor Ning Yan from University of Toronto for

providing bark samples, Professor C. Q. Jia for providing access to the pore analysis

equipment and Mr. Elie Afif for his help with the experiments.

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6- Summary and Future work

6.1 Comparison of Nickel Catalysts

The proposed reaction pathway for metal-catalyzed supercritical water gasification of

biomass is depicted in Figure 6.1. Based on this figure, methane is a secondary product

which is mainly formed over the metal catalyst through hydrogenation of CO2 and CO.

Therefore, the methane yield should be directly related to the partial pressure of the

primary products (i.e. H2, CO2 and CO) as well as activity of the catalyst for dissociating

hydrogen and cleaving the C-O bonds which will in turn lead to the formation of

139

methane. In order to compare the selectivity of the catalysts, the hydrogen to methane

ratio at a specific carbon conversion should be considered because the carbon conversion

directly represents the partial pressure of the carbon-containing gases (Figure 6.2).

According to this figure, the hydrogen selectivity decreased at higher carbon conversions

as it was expected from the reaction pathway. Except for the higher nickel loading on the

alpha-alumina support (Figure 6.3), the selectivity-conversion data obtained from all

other catalysts followed a certain trend, implying that it the change in the activity of the

catalysts was mainly due to the change in the number of catalytic active sites.

Figure 6.1 The reaction pathway for the catalytic SCWG of biomass.

Ni

CO2 + H2

H2, CO2, CO Catalytic decomposition

Feed

Intermediates

Condensation

reactions Char

CO2 + 4 H2 CH4 + 2 H2O

CO + H2O Ni

140

Figure 6.2 Hydrogen selectivity vs. carbon conversion for nickel-catalyzed

SCWG of glucose. Data: time dependent from chapter 5, non alumina, promoters,

gamma alumina and activation from chapter 3, CNT from chapter 4.

0

1

2

3

4

5

50 60 70 80 90 100

H2

sele

ctiv

ity

CGR %

gamma-alumina alpha-alumina Promoters

CNT non alumina Activation

time dependent Raney

141

Figure 6.3 Hydrogen selectivity vs. carbon conversion for Ni/α-Al2O3 catalyzed

SCWG of glucose with different metal loadings (g Ni/g support). The dashed line

represents the trend in Figure 6.2.

Changing the nickel loading from 5% to 20% increased the hydrogen selectivity from

about 2 to 4. Further experiments with different nickel to support ratio should be

conducted to give a better understanding of the changes in the structure these catalysts by

changing the nickel to support ratio, which in turn resulted in a different selectivity-

conversion trend from the majority of other nickel catalysts.

6.2 Comparison of Active Metals

It was observed that the hydrogen selectivity obtained from ruthenium-catalyzed SCWG

slightly also slightly decreased with carbon conversion (Figure 6.4). Comparison of

selectivity-conversion plot obtained from ruthenium-catalyzed SCWG with that of

nickel-catalyzed results (dashed line in Figure 6.4) revealed that ruthenium was more

0

1

2

3

4

5

50 60 70 80 90 100

H2

sele

ctiv

ity

CGR %

Nickel loading

142

active for the methanation reactions and therefore, it resulted in lower hydrogen

selectivity.

Figure 6.4 Hydrogen selectivity vs. carbon conversion for Ruthenium-catalyzed

SCWG of glucose. The dashed line represents the trend in Figure 6.2.


Considering the objectives of this study as given in Chapter 1, we developed a cheap

catalyst based on nickel and alumina which showed high catalytic activity and hydrogen

selectivity for supercritical water gasification of carbohydrates. Also, we identified

Ni/CNT as an active catalyst for SCWG reactions. Finally, using the promising catalysts,

the SCWG of various lignocellulosic feedstocks were investigated. The conclusions of

this work can be summarized as follows.

0

1

2

3

4

5

50 60 70 80 90 100

H2

sele

ctiv

ity

CGR %

Ruthenium

143

Among the support materials considered as a support for the nickel-catalyzed SCWG of

glucose, α-Al2O3, carbon nanotube (CNT), and MgO resulted in the highest carbon

conversions, while SiO2, Y2O3, hydrotalcite, yttria-stabilized zirconia (YSZ), and TiO2

showed modest activities and other catalysts including zeolites showed negligible

activities. γ-Al2O3 supports resulted in a wide range of catalytic activities from almost

inactive to highly active. No clear relationship was found between the catalyst surface

area (and particle size) and the catalytic activity. The hydrogen selectivity significantly

increased by increasing the nickel loading on α-Al2O3 catalyst. The maximum hydrogen

selectivity was obtained using a 20% Ni/α-Al2O3 at 380 oC. Addition of alkali promoters

enhanced the carbon conversion whereas addition of tin decreased the catalyst activity. In

summary, we developed a cheap catalyst based on nickel and alumina with acceptable

catalytic activity and tunable hydrogen selectivity. Furthermore, different aspects of the

formation of nickel nanoparticles on multi-walled carbon nanotubes are discussed. It has

been shown that functionalization of MWCNT in a concentrated oxidizing agent for

prolonged times results in a better dispersion of the nanoparticles. Also, it is found that

nickel crystal size increases by increasing nickel loading. Non-polar solvents such as

acetone are found to be more appropriate for formation of well-dispersed nickel

nanoparticles. Calcination temperature does not change the crystallite size of nickel

nanoparticles in temperature range of 280 oC to 450

oC. Based on TPR test, the nickel

oxide deposited on carbon nanotube was reducible at temperatures between 350 oC to 450

oC. Finally, results of catalytic gasification of several lignocellulosic materials have been

presented. It was found that for a given catalyst, gasification of glucose, fructose,

cellulose, xylan and pulp resulted in comparable yields (± 10 %) after 60 minutes

144

reaction time. Although alkali lignin was substantially harder to gasify than cellulose,

gasification yield of bark which had high lignin content was comparable to that of

cellulose. Ruthenium catalysts showed high catalytic activity. However, utilization of

Ru/C and Ru/γ-Al2O3 resulted in a relatively high methane concentration in the

gasification products and as a result the hydrogen selectivity was poor. In spite of their

low surface area, the laboratory-made Ni/α-Al2O3 catalyst exhibited good activities and

superior hydrogen selectivity compared to those of commercial catalysts tested in this

work. More specifically, the amount of alkane that was formed by gasification of bark in

that presence of Ni/α-Al2O3 and Ni/hydrotalcite was very small (i.e. < 1.5 mmol/g after

60 minutes).

6.4 Recommendation for Future work

The recommendations for the future work are given below.

Studying the stability of Ni/α-Al2O3 and Ni/MWCNT catalysts in flow reactors.

Studying the deactivation mechanism and regeneration of supported nickel

catalysts for SCWG using continuous reactors.

Studying the effects of pressure (or water density) on the selectivity of the

promising catalysts using a flow reactor.

Synthesis of bimetallic catalysts in order to improve the catalytic activity and

hydrogen selectivity of nickel and ruthenium catalysts.

Performing a more detailed characterization of Ni/MWCNT in order to better

understand the structural/performance relationship.

145

Conducting SCWG of different feedstocks using Ni/MWCNT catalyst.

Purging the batch reactor prior to the experiment to exclude the possible

homogeneous oxidation of the organic feed by air.

Studying the effects of hydrogen pressure on the gasification yield of lignin.

Appendix A: Complementary Literature Review

Kinetics of supercritical water decomposition of organics

As mentioned before, three applications have been proposed for reactions of organic

maters in supercritical water, namely: gasification, oxidation of hazardous substances and

material recovery.

Kinetics of supercritical water oxidation (SCWO) for several organic feeds has been

investigated by various researchers. However, due to utilization of an oxidizing agent

(such as H2O2), the rate constants obtained for SCWO reactions are not applicable to the

target of present work. In addition to this, majority of these studies reported the rate

constants based on the degradation of the feed rather than dealing with the end products

in the gas phase.

Many researchers studied material recovery and liquifaction of sugars in SCW.

Kabemaya et al [1] reported the values of rate constants for decomposition of glucose and

fructose. In the proposed reaction pathway, fructose is formed via isomerization of

glucose and both fructose and the remaining glucose may undergo other decomposition

reactions (Figure 2.1). According to this work and assuming that all reactions are first

146

order with respect to the reactant, the following rate constants are obtained for

decomposition of glucose at 400bar:

Figure A.1 Proposed reaction pathway for decomposition of glucose [1].

Table A.1 Reactions rates of glucose decomposition [s-1

] with respect to figure

2.1

Temp. [C] kgf kg kf

300 0.35 0.15 1

350 1 1 3

400 6.9 8.9 21.4

They further studied the rates of glucose decomposition at 300-400oC and 250-400 bar in

a flow reactor with 0.02-2 seconds residence time [2]. In the course of their work, they

found that fructose, saccharinic acids, erythrose, glyceraldehyde, 1,6-anhydroglucose,

dihydroxyacetone, pyruvaldehyde, and 5-hydroxymethylfurfural are the products of

glucose decomposition. Furthermore, it was realized that pressure has no effect on the

decomposition rate of glucose in subcritical region while it has a negative impact on the

decomposition of glucose in supercritical condition. The following figure illustrates the

proposed reaction pathway for decomposition of glucose. The subsequent reaction rates

kf

Glucose Fructose

Other products

kgf

kg

147

for each reaction were calculated at four different temperatures.

Figure A.2 More detailed reaction pathway of glucose [2]

Matsumura et al. [3] reported decomposition of glucose at 175-400oC and 250bar using a

flow reactor. In this study, the reaction rate and its order was determined based on

consumption of glucose and no further information on composition and concentration of

products are given. The result of this work is summarized in Table A.2.

Table A.2 Reaction rate and order for glucose decomposition in SWC [3].

Temp [oC] k [s

-1] n [-]

175 1.10*10-4

1.13

200 4.10*10-4

0.87

225 1.75*10-3

0.9

250 5.00*10-3

0.77

300 1.20*10-1

0.84

350 2.43*10-1

0.73

400 1.44 0.76

More recently, Qi et al. [4] conducted a series of experiments to identify the reaction

Glucose

Fructose 5-HMF

Acids Anhydroglucose

Erythrose

Glyceraldehyde

148

rates of glucose in subcritical water (180-220oC, 100bar) using a batch reactor. The

following scheme summarizes the reaction pathway that has been suggested in this work.

Figure A.3 Decomposition of glucose in subcritical water proposed by Qi [4].

The values of activation energy’s and rate constants are given in Table A.3.

Table A.3 Corresponding activation energy and Arrhenius factor, A, for

reactions presented in figure 2.3 [4].

Reaction Activation energy [kJ/mol] A

1 108 5.06*109

2 135 4.3*1012

3 89 2.6*106

4 109 3.6*109

5 31 0.031

Kabemaya and his co-workers [5] also looked into decomposition and hydrolysis of

cellobiose in range of 300-400oC, 250-400bar and residence time of 0.04-2s. The

following reaction scheme was proposed for decomposition of cellobiose.

Glucose 5-HMF

Humic acid

1

2 4

3 Levulinic acid

Decomposition

products

5

149

Figure A.4 Cellobiose hydrolysis and decomposition pathway proposed by [5].

In a series of papers Haghighat Khajavi et al. [6-8] investigated the decomposition of

monosaccharadies, sucrose and maltose in subcritical water (180-260oC, 100 bar) in a

flow reactor. Effects of pH, feed concentration and residence time on the reaction rates

were addressed.

Vostrikov et al [9] studied the kinetics of coal conversion in supercritical water medium

using a semi-batch reactor. The operating temperature and pressure and residence times

were 500-750oC, 300 bar and 60-720 s, respectively. The reported activation energy and

Arrhenius factor were 103kJ/mol and 1.3E3.1 (s-1

), respectively.

Sato et al. [10] investigated water-gas-shift reaction in supercritical water (380-400oC,

250-300bar and 0-300s residence time) by means of a flow reactor. The initial carbon

monoxide to water mole ratio was set at 0.03. According to this study, the rate constant

and activation energy of WGS reaction are found to be 10E3.09 and 1.16E5, respectively.

Kinetics of decomposition of microcrystalline cellulose have been determined by Sasaki

et al. [11] under T=290-400oC, 250 bar and t=0.02-13.1 s. In this study, it was proposed

that cellulose was first hydrolyzed into glucose, followed by subsequent glucose

decompositions. Later, Rogalinski et al. [12] reported the hydrolysis rate of cellulose and

Cellobiose

Glucose

Eruthrose

glycolaldehyde

glycosylerythrose

glycosylglycolaldehyde

Hydrolysis

Pyrolysis

Hydrolysis

Hydrolysis

150

starch based on consumption of reactants as well as formation of glucose, followed by

decomposition of glucose. The operating conditions of this study were 230-310oC, 200-

250bar and with residence time of between5 and200s.

Figure A.5 Reaction pathway for decomposition of cellulose and starch in SCW

[12].

The corresponding reactions rates for the above reaction pathway are given in Table A.4.

Table A.4 Reaction rates [s-1

] corresponding to figure 2.5 [12].

Temp. [oC] 1 2 3 4 5 6

210 - 0.0006 - - - -

230 - - - - 1.3*10-3

5.2*10-3

250 0.0025 0.01 2*10-3

6*10-3

5.4*10-3

1.2*10-2

270 0.0214 0.0317 4*10-2

3*10-2

8.1*10-3

1.8*10-2

310 0.108 - 7.5*10-3

3.1*10-1

- -

Kinetics of glucose gasification:

Lee et al. [13] measured the reaction rates of glucose decomposition with respect to total

Cellulose Hydrolysis products 1

2 Starch Hydrolysis products

Glucose 6

Decomposition products

4 Cellulose Glucose 3

5 Starch

151

amount of gas as well as liquid products (Figure 2.6). The operating condition that

considered in this work were 480-750oC, 280bar and t=10-50s.

Figure A.6 Decomposition of glucose in SCW according to [13].

The corresponding activation energies of reaction 1 and 2 are 67.6 and 71kJ/mol

respectively. Also, the reaction rate constants are found to be 10E3.09 and 10E2.95 s-1

.

Finally, Knezevic et al [14] reported the reaction rates of glucose gasification with

respect to total gas, water and acetone soluble compounds (WSS) and insoluble

compounds (WSIS). This work is based on about some of 600 experiments using quartz

capillary reactor. It is suggested that a portion of WSS materials is responsible for the

formation of WSIS, which will either remain in the liquid phase or it may further

decomposes into gas. Figure 2.7 illustrate the complex reaction pathway suggested in this

study, while the mass fraction of different components along with their corresponding

reaction rates at 300 & 350oC are tabulated in Table A.5.

Glucose Soluble

organics

1

2

Gas

152

Figure A.7 Reaction pathways and product distribution of glucose gasification in

quartz capillaries [14]. a, b, c, d, e, and f, represent the mass fraction of each

product.

Table A.5 Reaction rates and product’s mass fraction based on figure 2.7 [14].

T 300oC 350

oC

a 0.1 0.12

b 0.4 0.33

c 0.07 0.12

d 0.13 0.13

e 0.3 0.3

f 0.35 0.35

ka [s-1

] 0.0042 0.108

kb [g-1

L s-1

] 0.000083 0.000083

kc [g-1

L s-1

] 0 0.0000033

kd [s-1

] 0.00025 0.00058

Glucose kg

n=1

a WSS-A gas ka, n=1

(1-f) WSIS-B

kd, n=1 f gas

d WSS-D

e water

kb, n=2 b WSS-B

c WSS-C kc, n=2

WSIS-

A

153

References

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155

Appendix B: Equilibrium Calculations

All the thermodynamic equilibrium yields in this study were calculated using Aspen Plus

simulation software. Due to the low feed concentrations used in this study (e.i. 2 wt%), It

was assumed that the entire amount of carbon initially presented in the feed will present

in the gas phase and no carbon-containing compound will remain as liquid or solid once

the system reached equilibrium. Also, when comparing the experimental yields obtained

in this study, we assumed that the gas composition does not change by cooling down the

reaction products to the ambient temperature as cooling happens in extremely short time

periods and reaction rates dramatically decline at lower temperatures. The elemental

composition of solid feedstocks were used for defining the feed and the software was

allowed to find the composition at which a mixture of carbon dioxide, hydrogen,

methane, carbon monoxide and water had the lowest Gibbs Free Energy. Due to high

accuracy for the prediction of fluid properties near the critical point, the Peng-Robonson

equation of state was employed in the calculations.

P = [RT / (Vm - b)] - [(aα) / (Vm2 + 2bVm – b

2)] (B.1)

where:

a= (0.4572 R2 Tc

2) / Pc (B.2)

b= (0.07779 R Tc) / Pc (B.3)

α= (1+ κ (1 - Tr0.5

))2 (B.4)

κ = 0.3746 + 1.54226 ω – 0.2699 ω2 (B.5)

156

Tr = T/Tc (B.6)

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