eprints.kfupm.edu.sa · iv acknowledgement all praise and glory be to allah for his limitless help...

iii

DEDICATE TO

MY LOVING FATHER, MOTHER, BROTHERS AND WIFE

iv

ACKNOWLEDGEMENT

All praise and glory be to Allah for his limitless help and guidance. peace pleasing

of Allah is upon his prophet Mohammed.

I would like to express my profound gratitude and appreciation to my advisor Dr.

Than Thun Maung and co-advisor Dr. Mohammed Abdulmajeed Al-Daous, for their

guidance and patience through this dissertation, their continuous support and

encouragements can never be forgotten.

I am also, grateful to the other member of my dissertation committee, Dr.

Mohammed Morsy, Dr. Mohammed Wzeer, and Dr. Shkeel Ahmed for their help,

suggestions and encouragements.

I would like to express my thanks to Dr. Abdullah Al-Hamdan the chairman of

Chemistry Department for his limitless help and support during the course of studies.

I would like also, to thank Dr. Bassam El-Ali the advisor of graduate studies in

Chemistry Department for his guidance, help and encouragements.

I wish to thank Dr. Nemr Mehana for his help and support.

I wish to express my thanks to all faculty members and staff of Chemistry

department for their support.

My sincere appreciation is to all of my friends, not only for the moral support but

also for their concern and encouragements in the course of my studies.

v

I would like to acknowledge KFUPM and King Abdulaziz City for Science and

Technology (KACST) for the support provided through the Science and Technology Unit

at King Fahd University of Petroleum and Minerals (KFUPM) for funding the work

through project No. 08-PET88-4 as part of the National Science, Technology and

Innovation Plan.

Finely, my profound gratitude an appreciation is to my family, my father, brothers,

relatives and friends in Yemen for their affection and concern.

vi

TABLE OF CONTENTS

DEDICATE ..................................................................................................... ii

ACKNOWLEDGEMENT ............................................................................. iv

LIST OF TABLES ......................................................................................... ix

LIST OF FIGURES ......................................................................................... x

DISSERTATION ABSTRACT ..................................................................xvii

CHAPTER 1 .................................................................................................... 1

INTRODUCTION ........................................................................................... 1

1.1 SUPPORT ACID-BASE CHARACTER ........................................................................................... 1

1.2 ISOPROPANOL CONVERSION ...................................................................................................... 3

1.3 ACETYLENE HYDROGENATION ................................................................................................. 3

1.4 THE STUDY OBJECTIVES ............................................................................................................ 6

CHAPTER 2 .................................................................................................... 7

LITRATURE REVIEW .................................................................................. 7

2.1 CATALYSIS AND CATALYSTS ..................................................................................................... 7

2.2 BRIEF REVIEW ON Au CATALYSIS ............................................................................................. 9

2.3 GOLD AS A WORKING CATALYST ........................................................................................... 10 2.3.1 THE PREPARATION METHOD ................................................................................. 11

2.3.2 PARTICLE SIZE EFFECT ........................................................................................... 13

2.3.3 THE ROLE OF THE SUPPORT/ADDITIVES ............................................................ 14

2.3.4 THE NATURE OF THE Au ACTIVE SITET .............................................................. 16

2.4 ISOPROPANOL CONVERSION .................................................................................................... 18

2.5 ACETYLENE HYDROGENATION ............................................................................................... 27 2.5.1 HYDROGEB ADSORPTION ....................................................................................... 27

2.5.2 ACETYLENE ADSORPTION AND HYDROGENATION ........................................ 28

2.5.3 FACTORS AFFECTING THEW ACETYLENE HYDROGENATION ..................... 31

CHAPTER 3 .................................................................................................. 35

SAMPLE PREPARATTION AND EXPERIMENTAL METHODS .......... 35

3.1 OUTLINE............................... .......................................................................................................... 35

3.2 SUPPORTS............................. .......................................................................................................... 36 3.2.1 GAMA-ALUMINA ....................................................................................................... 36

3.2.2 MAGNESIA-ALUMINA MIXTURE ........................................................................... 36

3.2.3 GOLD SUPPORTED CATALYSTS ............................................................................ 38

vii

3.2.3.1 Using Aluminum Iso-propoxide Precursor (Al(OCH2CH2CH3)3).. ........................ 38

3.2.3.2 Using Boehmite Precursor ( AlO(OH).2H2O) ........................................................ 40

1%Au/Al2O3 catalyst ...................................................................................................... 40

1%Au/15%MgO-Al2O3 and Other Gold Loadings ......................................................... 40

3.3 CHARACTERIZATION OF SUPPORTS AND CATALYSTS ...................................................... 41 3.3.1 ELEMENTAL ANALYSIS .......................................................................................... 41

3.3.2 X-RAY POWDER DIFFERACTION, XRD ................................................................ 41

3.3.3 TRANSMISSION ELECTRON MICROSCOPY (TEM) ............................................. 45

3.3.4 BET TOTAL SURFACE AREA ................................................................................... 45

3.3.5 TEMPERATURE PROGRAMMED REDUCTION MEASUREMENTS (TPR) ........ 49

3.3.6 TEMPERATURE PROGRAMMED OXIDATION (TPO) .......................................... 49

3.3.7 TEMPERATURE PROGRAMMED DESORPTION (TPD) ....................................... 50

3.3.8 CO2-FT-IR ANALYSIS ................................................................................................ 52

3.4 CATALYTIC REACTION.................... ................................................................................... ........52 3.4.1 ISOPROPANOL CONVERSION ................................................................................. 52

3.4.2 ACETYLENE HYDROGENATION ............................................................................ 54

CHAPTER 4 .................................................................................................. 56

CATALYSTS' CHARACTERIZATION AND ISOPROPANOL

CONVERSION ............................................................................................. 56

4.2 GOLD LOADING (ICP-MS), XRD, BET SURFACE AREA AND TEM RESULT.......................56 4.2.1 INDUCTIVE COUPLED PLASMA MASS SPECTROSCOPY ANALYSIS (ICP-MS)

……………………………………………………………………………………………….56

4.2.2 X-RAY DIFFRACTION ANALYSIS .......................................................................... 57

4.2.3 BET SURFACE AREA ANALYSIS ............................................................................ 59

4.2.4 TRANSMISSION ELECTRON MICROSCOPYT (TEM) .......................................... 63

4.3 CO2 TEMPERATURE PROGRAMMED DESORPTION (TPD) .................................................. 65

4.4 NH2 TEMPERATURE PROGRAMMED DESORPTION (TPD) ................................................... 70

4.5 HYDROGEN TEMPERATURE PROGRAMMED REDDUCTION (TPR) ................................... 73

4.6 CO2-FT-IR SPECTROSCOPY MEASUREMENTS. ...................................................................... 76

4.7 CATALYSTS' ACID-BASE FVALUATION: ISOPROPANOL CONVERSION .......................... 79 4.7.1 INTRODUCTION ......................................................................................................... 79

4.7.2 ISOPROPANOLL CATALYTIC CONVERSION ....................................................... 80

4.7.3 The γ-Al2O3 and 1%Au/γ-Al2O3 CATALYSTS ............................................................ 84

4.7.4 The 4%MgO-Al2O3 and 1%Au/4%MgO-Al2O3 CATALYSTS .................................. 101

4.7.5 The 8%MgO-Al2O3 and 1%Au/8%MgO-Al2O3 CATALYSTS .................................. 115

viii

CHAPTER 5 ................................................................................................ 131

CATALYSTS' CHARACRERIZATION AND ACETYLENE

HYDROGENATION .................................................................................. 131

5.1 INTRODUCTYION............... ........................................................................................................ 131

5.2 GOLD LOADING (ICP-MS), XRD, BET SURFACE AREA AND TEM RESULTS.................. 132 5.2.1 INDUCRTIVE COUPLED PLASMA ANALYSIS (ICP-MS)................................... 132

5.2.2 X-RAY DIFFRACTION ANALYSIS (XRD) ............................................................ 133

5.2.3 BET SURFACE AREA ANALYSIS .......................................................................... 136

5.2.4 TRANSMISSION ELECTRON MICROSCOPY (TEM) ........................................... 140

5.3 NH3-TEMPERATURE PROGRAMMED DESORPTION (TPD) ................................................. 142

5.4 ACETYLENE CATALYTIC HYDROGENATION ...................................................................... 145 5.4.1 EFFECTS OF THE H2/C2H2 RATIO ON ACETYLENE HYDROGENATION ....... 145

5.4.2 THE EFFECT OF REACTION TEMPERATURE ON ACETYLENE

HYDROGENATION ........................................................................................................... 149

5.4.3 ACETYLENE HYDROGENATION IN THE PRESENCE OF ETHYLENE ........... 160

5.4.4 CATALYTIC ACTIVITY OF Au-BASED CATALYSTS IN C2H2

HYDROGENATION WITH TIME ON STREAM, CATALYSTS' DEACTIVATION. .... 163


HYDROGENATION WITH TIME ON STREAM IN THE PRESENCE OF WATER IN

STREAM, DEACTIVATION EVALUATION. .................................................................. 173

5.4.6 TEMPERATURE PROGRAMMED OXIDATIONT (TPO)...................................... 175

5.4.7 PROPPOSED MECHANISM OF C2H2 HYDROGENATION................................... 178

CHAPTER 6 ................................................................................................ 180

CONCLUSION AND RECOMMENDATIONS ....................................... 180

6.1 CONCLUSION......................... ...................................................................................................... 180

6.1 RECOMMENDATIONS.............. .................................................................................................. 181

APPENDIX ................................................................................................. 183

References ................................................................................................... 190

VITA .........................................................................................................................................210

ix

LIST OF TABLES

Table Page

4.1 Textural properties and elemental analysis of the catalysts.

62

4.2 CO2 and NH3 temperature programmed desorption (TPD) analysis

results of calcined catalysts.

67

4.3 Temperature programmed reduction analysis (TPR) results of

gold-based catalysts

75

4.4 First order reaction kinetic parameters for isopropanol conversion

over γ-Al2O3 and 1%Au/Al2O3 catalysts

90


over 4%MgO-Al2O3 and 1%Au/4%MgO-Al2O3 catalysts

104


over and over 8%MgO-Al2O3 1%Au/8%MgO-Al2O3 catalysts

118

5.1 Textural properties and elemental analysis of calcined catalysts

137

5.2 NH3 –temperature programmed desorption (TPD) analysis results

of calcined catalysts.

143

5.3 CO2-Temperaure programmed oxidation (TPO) analysis results of

spent catalysts

176

x

LIST OF FIGURES

Figure Page

2.1 Formation of C3= and C3one on MgO and A/MgO catalysts via

base-catalyzed E1cB mechanisms. L: weak Lewis acid site (A+ or

Mg2+

cations); B: medium-strength Brönsted base sites (combined

oxygen in A–O or Mg–O pairs); B: high-strength Brönsted base

sites (isolated O2−

)

26

3.1 The XRD pattern for Au-based catalyst

44

3.2 Nitrogen adsorption/desorption isotherm

47

3.3 The pore width distribution of gold supported catalyst

48

3.4 NH3-TPD profile

51

4.1 XRD patterns of calcined (a) γ-Al2O3, (b) 4%MgO-Al2O3, (c)

8%MgO-Al2O2, (d) 1%Au/ γ-Al2O3, (e) 1%Au/4%MgO-Al2O3,

(f) 1%Au/8%MgO-Al2O3

58

4.2 Nitrogen adsorption/desorption isotherms of calcined (a) γ-Al2O3,

(b) 4%MgO-Al2O3, (c) 8%MgO-Al2O2, (d) 1%Au/ γ-Al2O3, (e)

1%Au/4%MgO-Al2O3, (f) 1%Au/8%MgO-Al2O3

60

4.3 NLDFT-Pore size distribution for calcined (a) γ-Al2O3, (b)

4%MgO-Al2O3, (c) 8%MgO-Al2O2, (d) 1%Au/ γ-Al2O3, (e)

1%Au/4%MgO-Al2O3, (f) 1%Au/8%MgO-Al2O3

61

4.4 TEM micrographs and the corresponding particle size histograms

of hydrogen pre-treated (a) 1%Au/8%MgO-Al2O3, (b)

1%Au/4%MgO-Al2O3, and (c) 1%Au/–Al2O3.

64

4.5 CO2-TPD profiles of nitrogen pre-treated catalyst (a) -Al2O3, (b)

4%MgO- Al2O3, (c) 8%MgO- Al2O3, (d) 1%Au/-Al2O3, (e)

1%Au/4%MgO-Al2O3, and (f) 1%Au/8%MgO-Al2O3

68

4.6 CO2-TPD profiles of hydrogen pre-treated catalyst (a) -Al2O3,

(b) 4%MgO-Al2O3, (c) 8%MgO- Al2O3, (d) 1%Au/-Al2O3, (e)

1%Au/4%MgO-Al2O3, and (f) 1%Au/8%MgO-Al2O3

69

xi

4.7 NH3-TPD profiles of hydrogen pre-treated catalyst (a) -Al2O3, b)

4%MgO- Al2O3, c) 8%MgO- Al2O3, d) 1%Au/ -Al2O3, e)

1%Au/4%MgO-Al2O3 f) 1%Au/8%MgO-Al2O3

72

4.8 H2-TPR profiles of (a) 1%Au/-Al2O3, (b) 1%Au/4%MgO-Al2O3,

and (c) 1%Au/8%MgO-Al2O3.

74

4.9 CO2-FT-IR profiles of (a) 1%Au/-Al2O3, (b) 1%Au/4%MgO-

Al2O3, and (c) 1%Au/8%MgO-Al2O3 at temperature of (i) 323,

(ii) 373, (iii)423, (iv) 473 and (v) 523 K.

78

4.10 Variation of rate with concentration for simple unimolecualr

process.

82

4.11 The rate of isopropanol decomposition as a function of time and

reaction temperatures: ●-498 k, ∎-473 k, ▲-448 k; and

isopropanol vapor pressure of 0.021 atm , over 1%Au/γ-Al2O3

(dashed-line) and over γ-Al2O3 (solid-line).

91

4.12 . The rate of isopropanol decomposition as a function of time

and reaction temperatures: ●-498 k, ∎-473 k, ▲-448 k; and

isopropanol vapor pressure of 0.030 atm, over 1%Au/γ-Al2O3


92

4.13 The rate of isopropanol decomposition as a function time and

reaction temperatures: ●-498 k, ∎-473 k, ▲- 448 k; and

isopropanol vapor pressure of 0.042 atm, over 1%Au/γ-Al2O3


93

4.14 Propylene formation as a function of time and reaction

temperatures: ●-498 k, ∎-473 k, ▲-448 k; and isopropanol vapor

pressure of 0.021 atm, over 1%Au/γ-Al2O3 (dashed-line) and over

γ-Al2O3 (solid-line)

94

4.15 Acetone formation as a function of time and reaction




95





96

xii


temperatures: ●-498 k, ∎-473 k, ▲-448 k, and isopropanol vapor



97





98





99

4.20 First order kinetics of isopropanol decomposition at ●-498 k, ∎-

473 k, ⧫-448 k, over 1%Au/γ-Al2O3 (dashed-line) and over γ-

Al2O3 (solid-line).

100



isopropanol vapor pressure of 0.021 atm, over 1%Au/4%MgO-

Al2O3 (dashed-line) and over 4%MgO-Al2O3 (solid-line).

105





106





107



pressure of 0.021 atm, over 1%Au/4%MgO-Al2O3 (dashed-line)

and over 4%MgO-Al2O3 (solid-line)

108




and over 4%MgO-Al2O3 (solid-line).

109

xiii





110

4.27 acetone formation as a function of time and reaction




111





112





113


498 k, ⧫-473 k; over 1%Au/4%MgO- Al2O3 (dashed-line) and

over 4%MgO-Al2O3 (solid-line).

114



isopropanol vapor pressure of 0.021atm, over 1%Au/8%MgO-


119





120





121





122

xiv





123





124





125





126





127


498 k, ⧫-473 k; over 1%Au/8%MgO- Al2O3 (dashed-line) and

over 8%MgO-Al2O3 (solid-line).

128

4.41 Arrhenius Plots for ●-8%MgO-Al2O3 based catalysts, ∎- -Al2O3

based catalysts, ⧫-4%MgO-Al2O3 based catalysts; with Au

(dashed-line) and without Au (solid-line).

129

5.1 XRD patterns of calcined (a) γ-Al2O3, (b) 1%Au/% γ-Al2O3, (c)

1%Au/15%%MgO-Al2O2, (d) 2%Au/15%MGO- -Al2O3, (e)

2%Au/15%MgO-Al2O3.

135

5.2 Nitrogen adsorption/desorption isotherms of calcined (a)

1%Au/Al2O3, (b) 1%/15%MgO-Al2O3, (c) 2%Au/15%MgO-

Al2O2, (d) 4%Au/15%MgO-Al2O3. 138

5.3 Pore size distribution by BJH of adsorption branch of calcined

1%Au/Al2O3, 1%/15%MgO-Al2O3, 2%Au/15%MgO-Al2O2, and

4%Au/15%MgO-Al2O3. 139

xv

5.4 TEM micrographs of (a) , (b) 1%Au/15%MgO-Al2O3 ;(c) , (d)

4%Au/15%MgO-Al2O3

141

5.5 NH3-TPD profiles of hydrogen pre-treated catalyst; 1%Au/Al2O3,

1%Au/15%MgO- Al2O3, 2%Au/ 15%MgO- Al2O3, 4%Au/

15%MgO-Al2O3. 144

5.6 Acetylene conversion over 1%Au/Al2O3, 1%Au/15%MgO-Al2O3,

2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 at different

H2/C2H2 ratios and temperature of 523 K.

146

5.7 Ethylene selectivity over 1%Au/Al2O3, 1%Au/15%MgO-Al2O3,

2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 at different

H2/C2H2 ratios and at temperature of 523 K.

148

5.8 Acetylene conversion over 1%Au/Al2O3, 1%Au/15%MgO-

Al2O3, 2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3

catalysts at different reaction temperature.

152

5.9 Ethylene selectivity over 1%Au/Al2O3, 1%Au/15%MgO-Al2O3,

2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 catalysts at

different reaction temperature.

154

5.10 The selectivity for methane over 1%Au/Al2O3, 1%Au/15%MgO-


catalysts at different reaction temperatures.

157

5.11 The selectivity for ethane over 1%Au/Al2O3, 1%Au/15%MgO-


catalysts at different reaction temperatures.

158

5.12 The selectivity for propylene over 1%Au/Al2O3,

1%Au/15%MgO-Al2O3, 2%Au/15%MgO-Al2O3 and

4%Au/15%MgO-Al2O3 catalysts at different reaction

temperatures.

159

5.13 C2H2 conversion versus temperature during selective catalytic

hydrogenation of acetylene (A) in the presence and absence of

C2H4 (E). Reaction condition 200 mg catalyst, 47 ml/min feed and

C2H2/H2/C2H4 = 1/10/1

162

5.14 C2H2 conversion on stream over 1%Au/Al2O3, 1%Au/15%MgO-


catalysts. Reaction condition 0.2g catalyst, 47 ml/min feed and

C2H2/H2 = 1/10

165

xvi

5.15 Aging of the catalyst reactant mixture (R H2/C2H2 = 7) at 210 ◦C

for 4 h. [170]

166

5.16 Ethylene selectivity on stream over 1%Au/Al2O3,


4%Au/15%MgO-Al2O3 catalysts. Reaction condition 0.20g

catalyst, 47 ml/min feed and C2H2/H2 = 1/10

168

5.17 The selectivity on stream for methane over 1%Au/Al2O3,


4%Au/15%MgO-Al2O3 catalysts.

170

5.18 The selectivity on stream for ethane over 1%Au/Al2O3,


4%Au/15%MgO-Al2O3 catalysts

171

5.19 The selectivity on stream for propylene over 1%Au/Al2O3,


4%Au/15%MgO-Al2O3 catalysts.

172

5.20 C2H2 conversion on stream over 1%Au/Al2O3, 1%Au/15%MgO-


catalysts in the presence of H2O in stream. Reaction condition

0.20 g catalyst, 47 ml/min feed and C2H2/H2 = 1/10

174

5.21 TPO for spent catalysts after aging under reaction mixture of

H2/C2H2 = 10 for 24 hrs.

177

xvii

DISSERTATION ABSTRACT

NAME: ABDULLAH ABDOH MANDA

TITLE: EFFECTS OF ACIDO-BASIC SUPPORT PROPERTIES ON THE

CATALYTIC HYDROGENATION OF ACETYLENE ON GOLD NANO-

PARTICLES

MAJOR FIELD: CHEMISTRY

DATE OF DEGREE: MAY 2012

Metallic gold nanoparticles supported on γ-Al2O3 and magnesia-alumina mixed

oxide, with different magnesia content have been prepared by sol-gel method and

characterized by different techniques (inductive coupled plasma-mass spectroscopy (ICP-

MS), XRD, BET surface area analysis, transmission electron microscopy (TEM), CO2 and

NH3 temperature programmed desorption (TPD), H2 temperature programmed reduction

(TPR) and FTIR of adsorbed CO2). Such systems were found to produce catalysts with

controllable acidity, varying from catalyst possessing large density of acidic and low

density of basic sites, others with acidic and basic sites of equal strength and density, and

others with large basic and low acid sites densities, respectively. The catalytic assessment

of the generated acidity was carried out using 2-propanol decomposition as a test reaction.

The results obtained indicate that the presence of magnesia and reduced gold

nanopartilces has imparted the catalysts, 1%Au/4%Mg-Al2O3 and 1%Au/8%Mg-Al2O3,

with significant base-catalytic properties.

Acetylene hydrogenation and formation of coke deposits were investigated on a

gold catalyst supported on γ-Al2O3 and gold supported on alumina-magnisia mixed oxide

xviii

with different gold content; 1%Au/γ-Al2O3, 1%Au/15%Mg-Al2O3, 2%Au/15%Mg-Al2O3

and 4%Au/15%Mg-Al2O3. The effect of the H2/C2H2 ratio was studied over a range of

values. The catalytic activity and selectivity towards ethylene and other products were

investigated at different reaction temperatures. Acetylene hydrogenation was investigated

in the presence and absence of ethylene in stream. It is investigated that the adsorption of

the triple bond is preferred over the double bond and during selective catalytic (SCR) of

C2H2 the two hydrocarbons do not compete for the same adsorption sites. The deactivation

of catalysts was studied by temperature programmed oxidation (TPO). Higher content of

coke over 1%Au/Al2O3 catalyst was investigated in contrast to the other catalysts in

which the basisity of magnesia-alumina supports could have played an important role in

inhibiting the acidic carbon formation by possessing smaller number and/or weaker acid

sites.

DOCTOR OF PHILOSPHY IN SCIENCE

KING FAHD UNIVERSITY OF PETROLEUM & MINERALS DHHRAN,

SAUDI ARABIA

xix

ملخص الرسالة

عبدهللا عبده احمد مندع: اإلســــــــــــــــم

تأثير الخواص الحامضية والقاعدية للمدعم على الهدرجة الحفزية لالستيلين على سطح جزيئات : عنوان الرسالة

الذهب النانونية

كيميـــــــــاء: التخصص

2102مايو : تاريخ التخرج

-باستخدام طريقة السول γ-Al2O3 ،4%MgO-Al2O3،8%MgO-Al2O3حضرت الحفازات التالية

وباستخدام % 1باإلضافة إلى الحفازات السابقة حضرت حفازات الذهب المدعم على الحفازات السابقة بنسبة ،جل

-Au/γ-Al2O3 ،1%Au/4%MgO-Al2O3 ،1%Au/8%MgO%1الطريقة نفسها فنتجت الحفازات التالية

Al2O3 . 1حضرت كذلك الحفازات التالية باستخدام نسب مختلفة من الذهب%Au/15%MgO-Al2O3

2%Au/15%MgO-Al2O3 ،4%Au/15%MgO-Al2O3 جففت هذه الحفازات ومن ثم كلست عند درجة ،

550حرارة oC ساعات 4ولمدة .

ساحة السطحية للحفاز وكذا حجم المسام، لتحديد الم BETأجريت عدة اختبارات على هذه الحفازات مثل الـ

لتحديد ( ICP)حللت هذه الحفازات باستحدام تقنية التحليل اآللي . XRDكما فحصت هذه الحفازات باستخدام تقنية الـ

درست خواص هذه الحفازات الحامضية والقاعدية باستخدام . نسبة العناصر الموجودة فيها مثل الذهب والمغنسيوم

، استخدم غاز ثاني أكسيد الكربون لتحديد الخواص القاعدية لهذه (دمصاص بواسطة التسخين المبرم اال) TPDتقنية

ولمعرفة عند أية درجة حرارة يختزل الذهب . الحفازات كما استخدم غاز األمونيا لتحديد خواصها الحامضية

درست . يدروجين لهذا الغرضواستخدم غاز اله( اإلختزال باستخدام التسخين المبرم ) TPRاستخدمت تقنية

بغرض تحديد حجم جسيمات الذهب وكذا توزيع هذه الحجوم، كما درس TEMحفازات الذهب المدعم بواسطة تقنية

لتحديد حالة الذهب على سطح الحفاز إن كان على شكل أكسيد FT-IRعلى سطح الحفاز بواسطة تقنية COامتزاز

.إن كان للذهب كمعدن أو كأكسيد أو على شكل معدن ومعرفة النشاط الحفزي

بروبانول -2ولتحديد كفاءة وفاعلية هذه الحفازات وكذا سلوكها كحفازات قاعدية أو حامضية، استخدم تفكك

ومعلوم أن هذا الكحول يتفكك على سطح الحفاز الحامضي لينت بوبيلين . على سطح هذه الحفازات كمقياس لذلك

xx

درس تفكك هذا . الخواص القاعدية فتنت اسيتون باإلضافة الى الهيدروجينوماء، أما على سطح الحفازات ذات

0،15،15الكحول عند ثالث درجات حراية مختلفة للحمام المائي الذي يحتويه وهي oC بغرض تغيير تركيز الكحول

على سطح الحفاز ومن ثم تحديد ثابت سرعة التفاعل لكل حفاز، كما درس تفكك هذا الكحول عند ثالث درجات حراية

250 ، 255،220مختلفة للمفاعل هي oC 225 ،100،255ومرة أخرى عند

oC بغرض تحديد طاقة التنشيط

حصل عليها امكن تحديد االنتقائية لكل منت تحت ظروف مختلفة للتفاعل كما امكن من خالل البيانات التي . لكل حفاز

. تحديد نشاط الحفز لكل حفاز عند ظروف مختلفة كذلك

درست عملية الهدرجة االنتقائية لالستيلين الى االيثيلين على سطح هذه الحفازات بوجود وعدم وجود

المدعمة على الحفاز % 4، 2، 1نسب متفاوته من الذهب االيثيلين عند درجات حراة مختلفة وباستخدام

.15%MgO-Al2O3 درس تأثير كل من نسبة الهيدروجين الى نسبة االسيتيلين على نسبة الهدرجة وكذا االنتقائية

كما درس فقد الفعالية للحفاز مع الزمن وكذا حددت كمية ونوع الكاربون المتجمع على سطح الحفاز . تجاه اإليثيلين

.تأثير وجود الماغنسيوم في التقليل من كمية نوع الكاربون األشد تأثيرا على فعالية الحفازو

درجة الدكتوراه في العلوم

جامعة الملك فهد للبترول والمعادن

الظهران، المملكة العربية السعودية

1

CHAPTER 1

INTRODUCTION

1.1 SUPPORT ACID-BASE CHARACTER

Surface acidic and basic properties of a catalyst affect the catalytic reactivity since

they alter the adsorption and desorption behavior 'd' orbitals of supported transition metal

such as Ni which in turn enhances or hinders the adsorption of Lewis acid and base such

as CO2 and NH3, respectively. γ –Al2O3 always recognized as amphoteric support because

it behaves as acidic and basic catalyst due to the presence of both acidic and basic sites.

The addition of metal oxides such as K2O, MgO and La2O3 at loadings less than 200

μmol/g converts the stronger acid sites to sites of intermediate strength; where increasing

the loading of the basic oxides further eliminates the stronger acid sites and sites of

intermediate strength [1]. The adsorption of reactant is significantly affected by the

acidic/basic properties of the supports. Recently, it has been reported that Ni/ Al2O3

catalyst behave more acidic than Ni/MgAlO and Ni/MgO catalysts and the adsorption

strength of CO on these catalysts follow the order: Ni/MgO ˃ Ni/MgAlO ˃ Ni/Al2O3[1].

In addition, it is investigated that the hydrogenation of aromatic molecules such as toluene

(Lewis base) favor acidic catalyst such as Ni/Al2O3 due to the enriched electron density

2

on the aromatic ring with π-bonds. Also, it has been suggested that increasing the acidity

of the support causes an increase in methanation activity via transfer of electrons from

transition metal to the support [3].

Supported metal or metal oxide catalysts are widely used in industrially important

catalytic processes [4]. The recent plethora of reports on the catalytic activity of supported

metallic nano gold is mainly focused on the redox properties of the metal [5-7]. With the

exception to a couple of studies on the acid-base properties of gas-phase gold and gold-

oxide [8], no reports exists on the acid-base properties of supported gold particles. Of

particular interest to this study is the influence of the -alumina and magnesia-alumina

supports on the resulting catalytic acidic-basic character of supported metallic gold

catalyst.

Both acid and basic sites are usually simultaneously present on solid surfaces

where the two centers may work independently or in a concerted way [9]. Historically,

focus has mostly been on the acid features of catalytic materials, which are of paramount

importance for the oil-refining and petrochemical industry [4.10]. The search for very

strong acidic catalysts able to activate the C-C bonds of alkanes,[11,12] as well as the

large variety of organic reactions catalyzed by zeolites and other solid acids,[13] act as

driving force for continuing research in the field of acid catalysis. On the other hand, the

pioneering work of Tanabe[14] on the role of basic centers and the occurrence of

bifunctional reaction pathways requiring cooperative action of acid and basic centers have

received relatively less attention [9,15-17].

The formulation of an understanding of the acid-base properties of the supported

3

metal or metal oxide surface is still lacking. The nature and extent of interaction between

support material and dispersed metal catalyst influence the acid-base chemistry of the

catalyst.

1.2 ISOPROPANOL CONVERSION

In this study, the acid-base character of supported metallic form of gold was

evaluated in the 2-paopanol dehydration reaction. The supports used were -alumina and

magnesia-alumina mixtures. The supported metal catalysts were prepared by an a

modified sol-gel method [18,19].

In particular, the basicity of supported nano metallic gold particles was assessed in

this study as a function of its interaction with the above mentioned supports.

Decomposition of 2-propanol is chosen as the test reaction because it is widely used to

characterize the acid-base properties of catalysts.6Over acid catalysts, 2-propanol is

dehydrated (E1-mechanism) with the production of water and propene as the main

components [20]. Over basic catalysts, 2-propanol is dehydrogenated yielding hydrogen

and acetone as products. Therefore, the selectivity for acetone and propene is the factor

that characterizes the basicity of the catalyst.

1.3 ACETYLENE HYDROGENATION

Presence of acetylene in ethylene stream leads to poisoning of the polymerization

catalyst because acetylene adsorbs at the active sites for ethylene and blocks the

4

polymerization process. Therefore, the acetylene content in the ethylene feed has to be

reduced to the low ppm-range. Hydrogenation of acetylene in the presence of ethylene

requires high selectivity to ethylene to prevent hydrogenation of ethylene to ethane.

Although most of the selective acetylene hydrogenation studies to date have

involved Pd-based systems, other catalysts have also been considered [30]. The acetylene

hydrogenation on Au/Al2O3 catalyst was studied and the results revealed that well-

dispersed Au catalysts show extremely high selectivity for ethylene formation. This study

was carried out in a batch reactor (closed gas circulating system); moreover they provided

no information about catalyst stability [30]. The hydrogenation of acetylene in the

presence and absence of ethylene on promoted (Li2O, BaO and CrOx) and un-promoted

Au/Al2O3 catalyst has been studied [31]. it was investigated that the Au particle size and

the pr-treatment condition (hydrogen versus oxygen) as well as the nature of the promoter

play an important role in the acetylene hydrogenation. They investigated that the

deactivation during C2H2 hydrogenation is a reversible process and is due to accumulation

of carbon/coke deposits as a result of C2H2 adsorption on different active Au sites. These

deposits can be easily burned off by a thermal treatment in oxygen. Moreover it is also

reported that the selectivity towards C2H4 can reach 100% when C2H4 is present in the gas

stream, where the C2H2 hydrogenation temperature is slightly shifted to higher

temperatures as compared to hydrogenation of C2H2 in the absence of C2H4[31].

In this study, new gold supported on promoted and unpromoted γ-Al2O3 supports

were synthesized, characterized for their acid-base properties and applied for acetylene

hydrogenation in the presence or absence of ethylene in stream. The MgO was added to γ-

Al2O3 in order to alter the electronic properties of the catalyst and enhance the selectivity

5

towards ethylene formation. MgO is considered as basic additives due to its ability to

donate lone electron and facilitate the charge transfer between the metal oxide support and

the metal gold supported catalyst.

6

1.4 THE STUDY OBJECTIVES

The objective of this proposed study is to investigate the influence of acidic and

basic properties of supported gold catalysts on the hydrogenation reaction of Acetylene.

The influence of the acid base character of supports such as γ-Al2O3, Al2O3-MgO on the

selectivities and catalytic properties of the gold catalysts will be investigated. Therefore,

the general steps that will be followed to meet the objective include;

1) Synthesis of the supports and supported gold catalysts:

Sol-gel method will be used to synthesis γ-Al2O3, 4%MgO-Al2O3, 8%MgO-

Al2O3, 1%Au/ γ-Al2O3, 1%Au/4%MgO-Al2O3, 1%Au/8%MgO-Al2O3,

1%Au/15%MgO-Al2O3, 2%Au/15%MgO- Al2O3 and 4%Au/15%MgO- Al2O3

catalysts.

2) Characterization of the physicochemical properties of the resulting catalysts will

be conducted by various methods.

3) Characterize the extent of catalytic acidity and basicity possessed by the various

proposed catalysts in the 2-propanol dehydration / dehydrogenation reactions.

4) Hydrogenation of acetylene over different catalysts to investigate the effect of

support acid-base properties on the selectivity and catalytic properties of the

catalysts.

7

CHAPTER 2

LITRATURE REVIEW

2.1 CATALYSIS AND CATALYSTS

The application of a catalyst is one of the most fascinating and significant

concepts in science and the world, and is one of the few words that have carried over

broadly outside scientific language. The function of the catalyst is to speeds up a chemical

reaction without being consumed substantially during the reaction. The function of the

catalyst can be summarized briefly as follows:

The catalyst works by forming chemical bonds with reactants, generating

intermediates that react more readily to give products than the reactants would alone - and

giving back the catalyst. So, the catalyst affects the rate of approach to equilibrium of a

reaction but not the position of the equilibrium. It also provides subtle control of chemical

conversions, increasing the rate of a desired reaction but not the rates of undesired side

reactions (i.e. it affects the selectivity of a chemical process). The increase of temperature

could provide a comparable means for increasing reaction rates, but high temperatures are

often unacceptable [23].

The most important issue in a chemical process is the enhancement of selectivity

to one of the products. Unfortunately, each chemical process has its specific needs,

8

including catalyst, and there is not one universal catalyst composition for all reactions.

For example, Pd/Cu catalyzes the oxidation of ethene to vinyl acetate, whereas Ag is used

to obtain ethene oxide [24].

The stability of a catalyst accounts for how long it keeps its activity or selectivity

during operation. The ideal catalyst is infinitely stable however real catalysts suffer

changes causing loss of both activity and selectivity, and must be replaced or regenerated

(at intervals ranging from seconds to years) [25].

The majority of catalysts used in heterogeneous catalysis are solids. The

catalytically active components (i.e. metals, metal oxides or metal sulphides), typically in

a low concentration, are deposited on a support or carrier such as γ-Al2O

3, SiO

2, carbon

or zeolites. In general, this support often lacks catalytic activity. In addition to the active

components, other components, including those with catalytic activity, are added. They

are named promoters or additives. Although chemical promoters are not active

themselves, they improve the effectiveness of the catalyst by chemical means. For

example, the role of alkali metal oxides (K2O, C2O) on Fe- and Ru-based catalysts in

ammonia synthesis is significant [23-26]. It is well established that these promoters help

by lowering the N2

dissociation barrier. During the process, co-catalysts may formed and

these cocatalysts have an active role during the process and act along with the main

catalyst (for example by supplying active oxygen). On the other hand, textural (structural)

promoters play a role in stabilization of the catalyst, or increasing the number of “active

sites”. For instance, they can influence (reduce) the tendency of a porous material to

9

collapse or sinter and lose internal surface area, which is a mechanism of deactivation

[26].

2.2 BRIEF REVIEW ON Au CATALYSIS

It was long believed that Au cannot be used as a catalyst because its

chemisorption ability was too small, compared with, for example, the platinum group

metals. The Tanaka-Tamaru Rule [27] had shown that initial enthalpies of chemisorption

of oxygen and any related molecules are linearly related to the enthalpies of formation of

the most stable oxides. This implies that chemisorbed oxygen atoms are energetically

similar to atoms or ions in bulk oxides, and the nobility of gold, which is due to the

instability of its oxide Au2O3 (ΔH

f = 19.3 kJ mol

-1), also determines its inability to

chemisorb oxygen.

The differences between the 4d and 5d elements and their compounds have been

ascribed to the large stability of the 6s2

electron pair. Due to the large size of its nucleus

the relativistic effects become very important for Au [28]. Because of the increased mass

of the nucleus, the s orbitals and to a lesser extent the p orbitals are contracted, whereas

the d and f orbitals are in fact expanded [30]. As a result, the 6s2

pair is contracted and

stabilized and much of the chemistry (including the catalytic properties [31]) is due to the

5d band. The yellow colour of Au, similar to that of Cu, but different from that of silver,

caused by optical absorption in the visible region, is due to the relativistic lowering gap

between the 5d band and the Fermi level [29, 32]. In the absence of this effect, gold would

be white like Ag and have the same propensity to tarnish and corrode [29].

11

First attempts to use Au as a catalyst were carried out as early as 1906 and Au

gauzes were reported as catalysts for H2

oxidation [33]. In 1925 Au reported as a catalyst

for CO oxidation [34]. During the next 40 years the reports about Au as a catalyst were

rather rare and were reviewed in 1971 [35]. During this period, R. S. Yolles et al.

published an interesting paper indicating that Au acts as a hydrogenation catalyst [36]. It

was reported also that Au deposited on silica and alumina supports is active in alkene

hydrogenation at 100 ºC [37]. G. J. Hutchings [38] made a significant contribution in the

field of Au catalysis. He realised that the very high standard electrode potential of Au

(+1.4 V) would make AuCl3 a very effective catalyst for the hydrochlorination of

acetylene. However, the most important breakthrough came in 1987, thanks to the work of

Haruta, who showed that nanometer-sized gold particles supported on titanium dioxide

act as an effective catalyst for CO oxidation even at sub-ambient temperature [39]. Since

then, a large number of experimental and theoretical studies has been dedicated to the

origin of the enhanced chemical activity of nanosized gold [40]. Au-based catalysts were

tested for other reactions as well, i.e. NOx reduction [43], hydrogenation of unsaturated

hydrocarbons [45], water gas-shift (WGS) reaction [46], NH3

oxidation [47], total

oxidation of volatile organic compounds (VOCs), and preferential oxidation of CO in the

presence of H2 (PROX) [49].

2.3 GOLD AS A WORKING CATALYST

Explanations for the high activity of gold nanoparticles in various reactions can

broadly be classified into four groups, namely, the influence of the preparation method

11

(including the pretreatment conditions), the particle size effect, the separate catalytic role

of the support/additive and the presence of ionic gold. These factors will be briefly

discussed below.

2.3.1 THE PREPARATION METHOD

Traditionally, noble metal-based catalysts are prepared by impregnation. However,

this method is not suitable for Au for at least two reasons. The first reason is that Au has a

relatively low affinity for metal oxides and, thus, the interaction with the support is weak.

Secondly, during calcination of HAuCl4, because of the presence of chloride ions, the

coagulation of Au particles is greatly enhanced and the result is an inactive Au-based

catalyst due to the formation of large inactive Au ensembles. However, recently, a

modified impregnation method was reported by Qing Xu et al. [52]. They reported that

,according to their method, it is possible to obtain Au-based catalysts with an average

particle size around 2.4 nm. In addition, they reported that the activity of these catalysts

at room temperature in CO oxidation was comparable to catalysts prepared by deposition-

precipitation.

Recently, the most widely used method to prepare active Au-based catalysts is

deposition-precipitation [42]. Different ways were reported to deposit Au onto the

support. The first way can be summarized by adjusting the pH of HAuCl4 with Na

2CO

3 or

NaOH [53] and then adding the support. Secondly, the pH value of the HAuCl4 solution

slowly increased in the presence of the support [44]. In each of these approaches, the pH

is very important. By using any of these approaches it is claimed that very small Au

12

particles are obtained, highly dispersed over the support. However, this method is only

applicable for a basic support. If the support has an acidic character, gas phase grafting

(GG) is more suitable [54].

Haruta used alternative method called coprecipitation for its simplicity [42]. Also

in this case the pH at which Au precipitates is very important. In addition to the pH, the

identity of the basic reagent used to precipitate Au was reported to be important [42]. He

reported that Na2CO3 or K2CO3

are more efficient to obtain smaller gold particles than

NaOH or NH4OH.

In order to avoid any contamination of chloride ions originating from the Au

precursor (HAuCl4 or AuCl3), washing the catalyst with deionized water several times is

important for deposition-precipitation method. An alternative method is GG in which

monodispersed Au colloids stabilized by organic ligands or polymer compounds can be

used [55]. This method may be used to deposit Au on carbon, but the size of the Au

particles is relatively large (~ 10 nm). The thermal treatment of the catalyst precursor has

also been reported to have a great influence on the final catalytic properties of supported

gold catalysts. The thermal pretreatment of catalyst depends on the both the catalyst and

the support. For example, it was reported that for Au/TiO2 [56], Au/Fe2O3 [57], and

Au/MnOx [59] catalysts, calcination at mild temperatures (100-200 ºC) results in more

active catalysts than if they were calcined at higher temperatures. Catalyst such as

Au/Al2O3 or Au/Y [60] are reported to be active without calcinations. In addition, a higher

calcination temperature may cause a severe sintering of the gold crystallites [61]. On the

other hand, it has been reported that a higher calcination temperature may positively

influence the catalytic activity of Au/TiO2 in the sense that a stronger interaction between

13

Au and support is created and this phenomenon lead to a higher catalytic efficiency, even

though the gold particles sinter [61]. Regarding the effect of the pretreatment, Dekkers

found that for Au/Al2O3, a reductive pretreatment under hydrogen gives better results than

an oxidative one under oxygen at (300 ºC) [62]. These results confirmed by Grisel [50].

In addition, he reported that smaller Au particles, obtained by calcination at only 300 ºC,

are more active than larger gold particles(~ 10 nm), obtained by calcination at 400 ºC

[50].

2.3.2 PARTICLE SIZE EFFECT

Another origin of very high activities for Au-based catalysts resides in the size of

the Au particles. As the size of gold particle decreases, a number of effects occur: (i) The

fraction of surface atoms increases, and because these vibrate more freely the melting

temperature falls and surface mobility rises. (ii) Because the overlap of electron orbitals

decreases as the average number of bonds between atoms becomes less, the band structure

is weakened, and in particular the surface atoms start to behave more as individuals. (iii)

A larger fraction of the atoms comes into contact with the support, and the length of the

periphery per unit mass of metals rises. (iv) More steps, edge and kink sites are formed on

the particles [64].

It was reported that the size of the Au particles may also affect the shape of the Au

particles [64]. It was reported that the relative amount of corner and edge atoms is larger

on flat particles than on round particles with the same diameter [64]. It was reported also

that the shape of a gold particle depends on the interaction between the gold and the

14

support. So , different supports induce gold particle shapes with a varying amount of edge

and corner atoms.

However, not all differences in catalytic activity of various Au-based catalysts can

be explained by a particle size effect [65]. For example, Schubert et al. proposed that the

metal oxide support may serve as oxygen supplier [66]. They distinguished active support

materials, which can supply oxygen to Au particles (i.e. TiO2

and Fe2O

3), and inert

support materials such as Al2O

3. For the former it was proposed that the size of the gold

particles is not very important.

2.3.3 THE ROLE OF THE SUPPORT/ADDITIVES

There are different ways to explain the influence of the support/promoter on the

catalytic activity of Au catalysts. One model suggests that the role of additives such as

MgO, Li2O, Rb2O or BaO is to stabilize small gold particles during the preparation and

further thermal treatments [51].

Regarding the direct role of the support, some theoretical studies suggested that it

can influence the electronic structure of Au at the interface [67]. The authors suggested a

charge transfer between the support, particularly negatively charged defects on to sapport

and Au particles. Other theoretical calculations showed that defects on a support such as

TiO2

are crucial for adhesion of Au and in this way both the size and the shape of the Au

particles are affected [68]. However, it was further suggested that these defects are not

necessary oxygen vacancies, since similar results were obtained for nonreducible metal

oxides such as Al2O3, MgO or SiO2 [69]. Of course, the reducible supports have a larger

15

number of oxygen vacancies, useful for anchoring of Au. Moreover, it was suggested that

for this type of easily reducible metal oxides, an additional reaction path is valid, i.e. the

metal oxide may be the supplier of the active oxygen during the reaction [48].

From a practical point of view, it is important to note that the choice of the

support/additive is directly connected with the reaction that should be catalyzed by Au.

This is not for Au catalysts only, but for other metal based catalysts. For instance, CO

oxidation is successfully catalyzed by Au supported on any material except acidic ones,

such as SiO2-Al2O3 and activated carbon. It was reported for example that Mg(OH)2

is the

best support for Au in CO oxidation at sub-ambient temperatures [70], but that

deactivation occurs after 3 months. The effect of the support was explained on the basis of

the structure of the as-prepared catalyst.

If the desired reaction is the complete combustion of hydrocarbons, Co3O4 is the

best support, based on the fact that this support is by itself highly active in complete

oxidation reactions [71]. However, if the goal is the partial oxidation of hydrocarbons,

then the choice become more restricted. Thus, it was reported that only TiO2 and Ti-

silicates act as efficient supports, especially if H2 and O2

are present in the feed [50].

Authors reported that for nitrogen-containing compounds, ferric oxide and nickel ferrites

are the best to lead to the highest activity due to their high affinity to nitrogen [53]. There

are also reports of methanol synthesis over supported Au catalysts and according to

Haruta et al., ZnO is the best support also for Au, not only for the commercial Cu catalyst

[73].

16

2.3.4 THE NATURE OF THE Au ACTIVE SITET

The nature of the active sites was mostly a matter of debate in CO oxidation, the

most frequently studied reaction over Au-based catalysts. However, until now there is no

general consensus about this. As already mentioned in the previous paragraph, for some

systems it is clear that the size of the Au crystallites is not sufficient to explain the high

activity. Therefore, it has been proposed that the perimeter or gold-support interface [41],

or small Au clusters that have nonmetallic electronic properties due to a quantum-size

effect [63], or step sites on the surface and strain defects [74] are more important than

only Au nanoparticles with a size between 2 and 5 nm. On the other hand, other groups

are convinced that Au+3

is responsible for the high activity in either CO oxidation [58],

hydrogenation of ethene [75], ethyne hydrochlorination , or water-gas shift. An

intermediate model was proposed: an ensemble of metallic Au atoms and Au cations with

hydroxyl ligands [76]. This model suggests that the Au cations must be stable in a

reducing environment and also in the neighbourhood of metallic gold. It was further

concluded that Au+ would be able to satisfy these requirements, rather than Au

+3

and

[Au+-OH] has been proposed as the cationic component [77]. It has also been suggested

that the active site for gold catalysis is anionic gold, Au-. These species are mainly formed

if Au is deposited on a defect-rich support, i.e. with an increased concentration of F-

centers [77].

The last factor that is believed by some authors to have a great influence during

the catalytic reactions is water. According to some studies, the moisture effect is directly

connected with CO activation and the nature of the support [78]. More water is required to

17

promote CO oxidation over Au/Al2O3 or Au/SiO2, compared to Au/TiO2 [78]. H2O was

also found to be responsible for the stabilization of Au+

[39, 65], as well as for the

deactivation of the catalyst [55]. It was, for example, suggested that in the presence of

water the dissociation of oxygen on gold particles may be activated and that the oxygen

reacts with water to produce an oxygen radical available for reaction and two surface

hydroxyl groups [79]. A beneficial effect of water on the activation of O2 was also

proposed by Bond and Thompson [76]. Their reaction mechanism proceeds via a reaction

between an OH group that spilled over from the support to Au3+

and the CO adsorbed on

the gold particle, via a carboxylate group. The existence of the OH groups was used to

explain the negative activation energy found for Au/Mg(OH)2

tested under ultra-dry

reaction conditions in CO oxidation [80]. Water was even proposed as an easy way to

regenerate the Au/Al2O3 catalyst [81]. For other reactions reports on the effect of water on

the catalytic activity of Au-based catalysts are rather scare. However, Haruta reported that

some Au-based catalysts were tolerant against moisture in NO reduction by CO [82], and

in reduction of NO with C3H6 in the presence of oxygen [83].

18

2.4 ISOPROPANOL CONVERSION

Bulk gold is chemically inert and often regarded as a poor catalyst. However

when gold is in very small particle with diameter below 10 nm, it become surprisingly

active especially if it is dispersed on metal oxide or activated carbon [84]. It is reported

that the catalytic activity of gold is defined by three factors: contact structure, support

selection and particle size [85].

Since Haruta's discovery that nanometer-sized gold particles supported on titanium

dioxide act as an effective catalyst for oxidation reactions, a large number of experimental

and theoretical studies has been dedicated to the origin of the enhanced chemical activity

of nanosized gold. Recently, Haruta et al.[86] concluded that for high-activity Au

catalysts, the contact structure between the Au particles and the support is more important

than the mean particle size, thus emphasizing the role of the oxide substrate in the

chemistry of supported metal clusters. Haruta et al.[87] reported that the catalytic activity

for methanol synthesis is attributed to the concentration of oxygen vacancies on the

surface of the metal oxide around the primer of the metal particles. This hypothesis could

lead to the prediction that gold and silver should also exhibit catalytic activity for

methanol synthesis. Heiz et al.[88] studied the size-selected Au8 cluster on regular and

defective MgO surfaces and came to the conclusion that the surface defects enhanced the

activity and proposed the presence of oxygen vacancies that efficiently transfer charge to

cluster thus, increasing its chemical activity and enhanced the 2-propanol conversion.

Decomposition of 2-propanol is chosen as the test reaction because it is widely

used to characterize the acid-base properties of catalysts. The activity and selectivity of

19

the decomposition reaction of 2-propanol over γ-Al2O3 depended on the calcinations

temperature of the catalyst and the reaction temperature. J.A. Wang et al. [89]

investigated that there were two types of acidic active sites as well as two types of

basic active sites on the γ-Al2O3 catalyst surfaces. These different active sites are Lewis

acidic and basic active sites as well as brönsted acidic and basic active sites. The

total conversion of 2-propanol decomposition reaction increased with increasing the

reaction temperature. And they reported that the dehydrogenation reaction of 2-

propanol to acetone decreased with the increasing of the reaction temperature

however, the selectivity to dehydration reaction to propylene increased as the reaction

temperature increased. They investigated that the structural vacancies play an important

role in the decomposition of 2-propanol and are probably the main factor for acetone

formation.

M.A. Aramendia, et al.[90] investigated that the dehydration activity was

appreciably correlated to Brönsted and total acidity. Moreover, correlation between the K-

H2/K-H2O ratio and the basic site density was higher than that between K-H2 and this

density. These results revealed that the dehydration of 2-propanol takes place exclusively

at acid sites, whereas its dehydrogenation involves both acid and basic sites [91].

The addition of water to the inlet stream enhanced acetone formation in the 2-

propanol decomposition over a sol-gel magnesia-alumina oxide catalysts through

hydroxyl-assisted reaction pathway [92]. But the addition of water to the inlet stream can

promotes or inhibits the total conversion of 2-propanol depending upon the reaction

temperature. It is investigated that below 300 ºC, some of new hydroxyls were formed on

21

the catalyst surface, which promotes acetone formation; thus, the acetone selectivity is

enhanced. Above 300oC, on the one hand, when water is added, the reaction of

isopropanol dehydration to water must be inhibited from the view point of chemical

reaction equilibrium; on the other hand, it can be also understood that, the added water

may strongly adsorb on the active sites responsible for isopropanol dehydration, this

suppresses dehydration reaction. Scheme 2.1 shows the acetone formation through a

hydroxyl-assisted mechanism on surface of Mg-Al-O solid in the case of water presence.

Shyamal K. et al. studied the acid-base properties of a high surface area

Mo2C catalyst using the decomposition of 2-propanol. This catalyst was very

selective to acetone production indicating that the dehydrogenation of 2-propanol

was predominate. They investigated that both acid and base sites are presence on

Mo2C catalysts and they reported that dehydrogenation and dehydration of 2-

propanol occurred on different sites. The acid-base site may created as a result of

charge transfer between molybdenum and carbon atoms [93].

In the last years the interest of the scientific community in gold-containing catalysts

strongly increased due to their activity at low temperature in a large number of catalytic

reactions, in particular, oxidation ones. In 2000 S. Minico et al [94]. investigated the

catalytic oxidation of 2-propanol, methanol, ethanol, aceton and toluene on coprecipitated

Au/ Fe2O3 in the presence of excess of oxygen. They reported that the gold catalysts have

been found to be very active in the oxidation of tested volatile organic compounds (VOC)

[93]. The highly activity of these systems has been related to the capacity of highly

21

dispersed gold to weaken the Fe—O bond thus increasing the mobility of the lattice

oxygen which is involved in the VOCs oxidation. It is investigated that the higher

catalytic activity depends on the content of gold in the catalyst, the higher the gold content

the lower the light-off temperature. In the oxidation of 2-propanol the formation of

acetone was observed however, total oxidation to CO2 was observed at higher

temperature. Only acetone was observed due to the basic character of the support used

[94].

In 2001 S. Minico et al.[95] investigated the influence of calcinations

pretreatments on the catalytic behavior of the Au/Fe2O3 towards the combustion of some

representative organic compounds (2-propanol, ethanol, methanol, acetone and toluene).

They reported that decreasing or increasing the calcinations temperature affects the

catalytic activity of Au/Fe2O3 samples towards the total oxidation to CO2. In fact, in the

case of the oxidation of 2-propanol the activity towards the intermediate (acetone) is not

relevantly influenced by the pretreatment whereas the conversion to CO2 decrease with

increased calcination temperatures. The XPS and FT-IR characterization data, performed

on the same catalysts showed that oxidized gold species, which are present in the 200oC

calcined Au/Fe2O3 sample, are reduced to metallic gold when the calcinations temperature

increases. It can be pointed out that the catalytic behavior of the Au/Fe2O3 system is

related to the gold and/ or the iron oxide phase [95].

22

Scheme 2.1: Acetone formation through a hydroxyls-assisted mechanism on the surface

of Mg-Al-O solid in the case of water presence in the inlet stream. This reaction pathway

is a modified dehydrogenation route accompanying water formation by dehydration [92] .

23

M. A. Centeno et al. [96] investigated the catalytic combustion of volatile organic

compounds (VOC) on Au/CeO2/Al2O3 and Au/Al2O3 catalysts prepared from the

deposition-precipitation method. In all cases, the presence of gold strongly enhanced the

rate of VOCs oxidation and, gold supported on CeO2/ Al2O3 samples presented

considerably higher activities than that supported on pure Al2O3. For support such as

CeO2/Al2O3, the increasing amount of gold also played an important role in the VOCs

oxidation. The conversion of 2-propanol produces acetone as an intermediate at low

temperature over the three catalysts. However, at higher temperatures complete oxidation

of 2-propanol to CO2 is obtained. It is reported that Ceria participates in the fixation of

gold enhances its final dispersion, thus leading to the stabilization of gold particles with

lower crystallites size. Besides the increase of the activity due to the mentioned fact, the

redox properties of ceria improve the catalytic behavior of the catalysts, probably by

increasing the mobility of the lattice oxygen and by controlling the adequate oxidation

state of gold [96]. Indeed, it is well known that ceria enhances the oxygen storage

capacity of the catalysts, due to its particular ability to undergo deep and rapid

reduction/oxidation cycles according to the reaction CeO2 ↔ CeO2-x + (x/2)O2 upon

interaction with reducing or oxidizing reagents present in the reaction condition [96].

The decomposition of 2-propanol was studied over different metal oxide-

supported gold catalysts (Au/CeO2,Au/Fe2O3,Au/TiO2) [97]. The presence of gold

enhances the rate on these supports. The final products observed are always CO2 and H2O

with increase in reaction temperature, but significant amount of acetone and propene are

observed when 2-propanol is not completely converted [97]. The conversion of 2-

24

propanol over these catalysts found in the following order: 1.6Au/CeO2 > CeO2 >

1.7Au/Fe2O3 > 1.6Au/Al2O3 > Au/TiO2 [97]. Different factors affect the conversion of 2-

propanol over these catalysts such as the nature of the support, gold loading, the particle

size , the oxidation state of gold and the calcinations temperature. The results suggest that

the activity of 2-propanol oxidation is obviously influenced by the appearance of Au(+1),

and the presence of Au(+1) for sample calcined at 300oC is the factor for the high activity

of 2-propanol complete oxidation [97].

The conversion of 2-propanol was studied on gold-supported titanium oxide and

oxynitride catalysts [98]. It is investigated that the supports themselves are not able to

oxidate completely the 2-propanol to CO, although they can oxidize it partially to acetone

at high temperatures (around 400 oC ). The nitrogen concentration in the support does not

affect its oxidative activity. However, the presence of gold strongly increases the catalytic

performance. Three maxima of the 2-propanol conversion and acetone production were

detected, pointing out the existence of more than one type of gold active centers towards

the partial oxidation of 2-propanol [98]. Beside this, it is investigated that the samples

calcined at lower temperature (300 oC ) produce more acetone than that calcined at higher

temperature because the heating process results in a loss of nitrogen and an increase in the

in the average size of the gold particle [98].

Gold supported on γ-Al2O3 showed complete conversion of 2-propanol at 200 oC

and acetone was the only by-product. Whereas, propene was produced on γ-Al2O3. The

selectivity of acetone was the highest on Au/ γ-Al2O3 catalyst activated by H2 and

reached 60%. When 2-propanol concentration was 2.0 g/m3, the complete conversion

25

temperature of 2-propanol was 170 oC, with acetone as the only origin product over the

optimized catalyst, and the mineralization tempetarure of 2-propanol was 260 oC [99].

Decomposition of 2-propanol was used to study the acid-base active sites

requirement for elimination reactions on alkali-promoted MgO catalysts[100]. It is found

that, on MgO-based catalysts, 2-propanol dehydrogenated to acetone and dehydrated to

propylene with zero-order reaction. The results revealed that the addition of promoters

increases the MgO activity for 2-propanol decomposition and the Li/MgO catalyst

exhibits the highest total activity. On all the catalysts, the selectivity to acetone was higher

than 55%, but the product distribution of 2-propanol decomposition depended on the

active site density and strength. It is investigated that the acetone formation rate

effectively increases with increasing the density of medium-strength of base sites

however, the propylene formation is favored on catalysts containing an enhanced

contribution of high-strength base sites. These results can be interpreted by assuming that

rate-limiting steps of the dehydrogenation and dehydration reactions are catalyzed by

Mg(A)–O pairs of intermediate basic strength and by strongly basic O2− sites,

respectively. Figure 2.1 illustrates the proposed mechanism for 2-propanol decomposition

to acetone or propylene over medium-strength brönsted base sites (combined oxygen in

A–O or Mg–O pairs) or aver high-strength brönsted base sites (isolated O-2

)[100].

26

Figure 2.1: Formation of C3= and C3one on MgO and A/MgO catalysts via base-catalyzed

E1cB mechanisms. L: weak Lewis acid site (A+ or Mg

2+cations); B: medium-strength

Brönsted base sites (combined oxygen in A–O or Mg–O pairs); B: high-strength Brönsted

base sites (isolated O2−

) [100].

27


2.5.1 HYDROGEB ADSORPTION

Hydrogen molecule adsorbs physically on the metal surface of most transition

metals and then dissociates [101].

H2 + M ↔ M-H2 (physisorbed)

M-H2 (physisorbed) ↔ H-M-H

The hydrogen may be adsorbed in a bridge mode (between two metal atoms) or adsorbed

atop a single metal atom [101]. The adsorption of hydrogen to metal involves transition of

electrons from metal atom to hydrogen in which the adsorbed hydrogen exist as hydride.

The donation of electrons to hydrogen differ from one metal to another [101]. Different

parameters affect the adsorption of hydrogen atom on the metal surface, however , the

influences of the promoters and supports are the most important. For example, Iron on

potassium carbonate displayed greater binding energy for chemisorbed hydrogen than

unpromoted iron [101]. Moreover, hydrogen atoms chemisorbed on a supported metal can

migrate to the support, which then acts as a hydrogen atom reservoir even though it is

itself incapable of dissociating H2 [101]. It is reported that an oxidation-reduction couple

between hydrogen chemisorbed to the metal and metal oxide at the interface between the

metal and the support may account for the conversion of hydridic hydrogen to proton.

Thus, both hydridic and protonic atom assumed to be available [101].

28

2.5.2 ACETYLENE ADSORPTION AND HYDROGENATION

An ethylene stream from a naphtha cracker unit typically contains about 0.1–1%

of acetylene as an impurity, which is required to be removed to less than 5 ppm because it

poisons the catalyst in ensuing ethylene polymerization processes and eventually degrades

the quality of the produced polyethylene [119,120]

The selective hydrogenation of acetylene found in ethylene is a commercially

important process used to remove trace amounts of acetylene from polymer-grade

ethylene streams. A very high selectivity is required as the acetylene content must be

reduced to a few parts per million without any significant ethylene hydrogenation [102].

An additional difficulty is that the ethylene to acetylene ratio in the feed can be as high as

100. Industrial processes normally use supported palladium catalysts with carbon

monoxide added to the feed as a selectivity promoter [102]. It is well known that

acetylene is not only hydrogenated, but also undergoes hydro-polymerization reactions.

These lead primarily to various C4 compounds but also to higher hydrocarbons

(oligomers), most of which are retained on the catalyst [102]. The retained fraction can

under some conditions eventually form a liquid phase, often referred to as ‘‘green oil.’’

The accumulation of hydrocarbon residues on the catalyst surface leads to interesting

deactivation phenomena [80]. Different strategies have been developed to overcome these

negative effects. These include altering the electronic properties of palladium by adding a

second metal such as Ag [103], Cu [104], Cr [105], Co [106], Ni [107], K [108] and Si

[109]. Another approach was made by modification of the interaction between the metal

29

and the support using TiO2 or SiO2 to improve the selectivity towards ethylene production

[110].

Modification of Pd/SiO2 catalyst with Si resulted in increase in the selectivity of

ethylene while, acetylene conversion was slightly lowered upon Si addition. The added Si

appears to be a promoter for ethylene selectivity rather than for hydrogenation activity for

Pd catalyst. It is investigated that the catalyst deactivation was suppressed on Pd-Si/SiO2

compared to that on Pd/SiO2 catalyst [111].

C. Guimon et al. [112] studied the hydrogenation of acetylene over Ni-Si-Al

mixed oxides prepared by sol-gel method. They investigated the presence of three

different types of coke which are, ordering them according to their oxidability,

filamentous carbon, amorphous coke, and strongly adsorbed hydrogenated carbons. The

formation of the latter is facilitated by the acidity of silica-alumina, while filamentous

carbon is generated by the sites with weak metal–support interactions and amorphous

coke is formed by those with strong metal–support interactions [112].

Although most of the selective acetylene hydrogenation studies to date have

involved Pd-based systems, other catalysts have also been considered [113]. The

acetylene hydrogenation on Au/Al2O3 catalyst was studied and the results revealed that

well-dispersed Au catalysts show extremely high selectivity for ethylene formation [113].

This study was carried out in a batch reactor (closed gas circulating system); moreover

they provided no information about catalyst stability [113]. The hydrogenation of

acetylene in the presence and absence of ethylene on promoted (Li2O, BaO and CrOx)

and un-promoted Au/Al2O3 catalyst has been studied [113]. In this study it is investigated

that the Au particle size and the pr-treatment condition (hydrogen versus oxygen) as well

31

as the nature of the promoter play an important role in the acetylene hydrogenation. They

investigated that the deactivation during C2H2 hydrogenation is a reversible process and is

due to accumulation of C deposits as a result of C2H2 adsorption on different active Au

sites. These deposits can be easily burned off by a thermal treatment in oxygen. Moreover

it is reported that the selectivity towards C2H4 is 100% also when C2H4 is present in the

gas stream, although the C2H2 hydrogenation temperature is shifted to slightly higher

temperatures, as compared to hydrogenation of C2H2 in the absence of C2H4[114].

Acetylene hydrogenation over Au/CeO2 showed selectivity of 100% towards

ethylene even at high level of conversion up to 300 oC. Below this temperature, a

carbonaceous deposit was formed with no change in the selectivity. However, at higher

temperature, deactivation concomitant to a decrease in selectivity was observed due to the

formation of methane in the gas phase with no ethane produced [115]. A. Sarkany et al.

[116] reported that Au/FeOx supporting Au particles of 2.5, 3.2, 3.5 and 7.5 nm in average

is highly selective in semi-hydrogenation of acetylene and the ethane formation selectivity

is less than 1% in the 323-473 K temperature range. The hydrogenation is accompanied

with oligomer formation and the C4+ selectivity is averaged between 2-4% [116].

Choudhary T.V. et al. [117] investigated the hydrogenation of acetylene on Au/TiO2,

Pd/TiO2 and Au–Pd/TiO2 catalysts synthesized by temperature-programmed reduction–

oxidation and the conventional incipient wet impregnation method. They reported that the

highly dispersed Au/TiO2 catalyst prepared by the reduction–oxidation of an Au–

phosphine complex on TiO2 showed extremely high ethylene selectivity even at complete

acetylene conversion. However, it suffered from inferior activity and stability as

compared to a mono-metallic Pd catalyst. on the other hand, Pd-promoted Au catalyst

31

prepared by the redox method showed significantly higher ethylene selectivity at 100%

acetylene conversion as compared to a mono-metallic Pd catalyst under similar reaction

conditions. Moreover the Au–Pd catalyst also showed excellent stability over several

hours [117]. X. Liu et al. [118] studied the acetylene hydrogenation over Au/SiO2 in the

presence of large excess of ethylene [118]. They investigated that the high activity of

plasma-treated catalyst was due to production of small sized Au nanoparticle of

approximately 3nm diameter supported on silica and optimized neutral surface charge of

Au nanoparticle, which favors the dissociative adsorption of H2. The obtained selectivity

was less than 100% which claimed in the literature due to the absence of ethylene or low

ethylene to acetylene ratio[118].

2.5.3 FACTORS AFFECTING THEW ACETYLENE HYDROGENATION

Acetylene impurities are usually removed by two methods, that is, adsorption with

zeolite [121] and conversion to ethylene by selective hydrogenation using Pd catalysts

[120], the latter more commonly being used. Several factors affect the selective

hydrogenation of acetylene to ethylene on Pd catalysts. These factors either related to the

additives or to the reaction conditions.

Various additives, such as Ag, Ni, Cu, Pb, Tl, Cr, and K, have been reported to

improve the performance of Pd catalysts, especially in achieving a high selectivity for

ethylene production [122]. The role of additives, or selectivity promoters, is generally

considered to be derived from two factors: geometric and electronic . For example,

Leviness et al. [122] reported the promotion of ethylene selectivity upon Cu addition,

32

which they proposed to be due to a geometric effect. That is, the insertion of Cu into the

Pd matrix decreases the number of multi-coordination sites of the Pd responsible for the

dissociative adsorption of acetylene and suppresses the formation of beta phase Pd

hydride as well; both are damaging to ethylene selectivity. On the other hand, Huang et al.

[123] reported an improved selectivity on Ag-promoted Pd catalysts, suggesting that an

increase in the Pd d-band electron density upon Ag addition was responsible for the

selectivity enhancement.

To design a high selective catalyst in acetylene hydrogenation, it is essential to

consider the reaction mechanism for acetylene hydrogenation, which consists of several

reaction paths, as shown in Figure 2.2 Path I is the partial hydrogenation pathway of

acetylene to ethylene, which is either desorbed as the major gaseous ethylene product

(Path II) or further hydrogenated to ethane (Path III). Consequently, ethylene selectivity

can be improved by promoting Path II and suppressing Path III, for which two methods

are typically used in industrial processes. One is to add a moderator chemical to the

reaction stream, e.g., CO, which adsorbs more strongly on Pd than ethylene and,

consequently, facilitates ethylene desorption from the catalyst surface (Path II) [124]. The

other is to maintain a low H2/acetylene ratio in the feed such that the catalyst surface is

deficient in hydrogen concentration, thus the full hydrogenation of ethylene (Path III) is

retarded [125,126]. Path IV, allowing the direct full hydrogenation of acetylene to ethane,

is insignificant at high acetylene coverage and low partial pressures of hydrogen [127],

which is the case in most industrial processes. The triply adsorbed species, ethylidyne,

had been suggested as an intermediate in Path IV [122,127–129] but was later verified to

33

be a simple spectator of surface reactions [130,131]. Path V, which allows for the

dimerization of the C2 species, eventually leads to catalyst deactivation via the

accumulation of green oil [132,133]. Path V also lowers the ethylene selectivity because it

consumes acetylene without producing ethylene. Accordingly, the mechanism of

acetylene hydrogenation indicates that, for improving ethylene selectivity and suppressing

green oil formation, Paths I and II should be promoted while the other paths should be

suppressed.

Scheme 2.2: Reaction paths of acetylene hydrogenation [119]

The addition of Si to Pd/SiO2 catalyst enhanced the selectivity of ethylene and

decreased the formation of green oil. It is investigated that the addition of Si to Pd/SiO2

catalyst modified the geometric structure of the catalyst. Accordingly, the CO-IR results

provide evidence that the multi-coordination sites of Pd are effectively inhibited by Si

deposition. Therefore, the density of required adjacent sites for ethylene adsorption and

further hydrogenation were reduced and resulted in enhancement of ethylene selectivity.

The addition of Ti to Pd/SiO2 catalyst modified both the electronic and geometric

structure of the catalyst [119]. It is investigated that the Pd surface was modified

geometrically by the Ti species, as confirmed by the CO-IR spectra, such that the

34

decomposition of ethylene was suppressed [119]. Charge transfer from the Ti species to

Pd further reduced the adsorption strength of weakly adsorbed ethylene, thus facilitating

Path II in the reaction mechanism of acetylene hydrogenation, thus increasing the

ethylene selectivity [119]. The addition of Ti to Pd/SiO2 prevent the formation of 1,2

butadiene which is the precursor of green oil formation and is preferentially produced,

especially when acetylene is adsorbed on neighboring Pd sites [119].

Metal oxides such as Nb2O5, TiO2 and La2O3 are reported as promoters in the

hydrogenation of acetylene over Pd/SiO2 catalyst [119]. Among three oxides added as

promoters, La2O3 reduced at 500 ºC showed the largest promotion of the ethylene

selectivity, followed by TiO2 and Nb2O5, which exhibited the same sequence as that for

the extent of interaction between oxides and Pd [119]. It is investigated that the selectivity

of Pd–Nb/SiO2 decreased after reduction at 500 ºC instead of at 300 ◦C, unlike the other

oxides. On the other hand, the selectivity of Pd–La/SiO2 increased after reduction at 500

◦C (Pd–La/500) instead of at 300 ºC [119]. Considering the characteristic behaviors of

La2O3- and Nb2O5-added catalysts, it was proposed that the former catalysts were

advantageous for use in acetylene hydrogenation at high temperatures and the latter at low

temperatures [119].

35

CHAPTER 3

SAMPLE PREPARATTION AND EXPERIMENTAL METHODS

3.1 OUTLINE

This chapter provides a description of all the experimental techniques that were

used in this research. Section 3.2 describes various preparation methods employed to

synthesis the gold-based catalysts. Section 3.3 deals with a description of each technique

used to characterize the catalysts (i.e. ICP-mass, BET, XRD, TEM, FT-IR, TPR, TPO

and TPD). Section 3.4 describes the manner of performing the catalytic reaction tests i.e.

experimental set-up for various reactions, and data evaluation.

The following catalysts: γ-Al2O3, 4%MgO-Al2O3, 8%MgO-Al2O3,15%MgO-

Al2O3 1%Au/γ-Al2O3, 1%Au/4%MgO-Al2O3, and 1%Au/8%MgO-Al2O3,

1%Au/15%MgO-Al2O3, 2%Au/15%MgO- Al2O3, 4%Au/15%MgO- Al2O3 were prepared

by modified Sol-Gel method of alumina synthesis.

36

3.2 SUPPORTS

3.2.1 GAMA-ALUMINA

γ-Al2O3, was prepared by initially stirring 40 g of aluminum iso-propoxide

Al(OCH2CH2CH3)3 dispersed in 200 mL H2O at 343 k for 10 h. To the refluxing solution,

several drops of dilute nitric acid (4-5 M) were added to lower the pH of the dispersion to

~3 followed by rising the temperature to 373 k. Refluxing of the homogeneous solution

was continued at this temperature for ~36 h until a gel was formed.

3.2.2 MAGNESIA-ALUMINA MIXTURE

The same procedure was applied to the preparation of 4%MgO-Al2O3 and

8%MgO-Al2O3 catalysts as shown in Scheme 3.1. 40 g of aluminum iso-propoxide and

7.5 g of Mg(NO3)2.6H2O (for 4%MgO-Al2O3 catalyst) were dissolved in 200 ml water by

stirring at 343 k for approximately 10 h. To the refluxing solution, several drops of dilute

nitric acid (4-5 M) were added to adjust the pH of the dispersion to ~3 followed by rising

the temperature to 373 k. Refluxing of the homogeneous solution was continued at this

temperature for ~16 h until a gel was formed.

For each system prepared, the gel was aged at room temperature for ~48 h to form

a solid gel. The resulting solid-gel was freeze-dried then calcined at 823 k under static air

for 4 h, with a heating rate of 2 k min-1

.

37

Scheme 3.1: The preparation steps for 4%MgO-Al2O3 catalyst synthesis

4%MgO-Al2O3 Catalyst

40 g Al(OCH2CH2CH3)3 + 7.5 g of Mg(NO3)2.6H2O + 200 ml H2O

Stirred and heated at 343 K for ~ 10 hrs

pH lowered to ~ 3 and the temperature raised to 373 K

Refluxing the homogeneous solution until gel formation

Aged at room T for 48 hrs to form solid gel

Freez-dride

Calcined at 823 K under static air for 4 hrs with heating rate of 2C/min

38

3.2.3 GOLD SUPPORTED CATALYSTS

Gold supported catalysts were synthesized using two types of alumina precursors:

aluminium iso-propoxide (Al(OCH2CH2CH3)3) and boehmite (AlO(OH).2H2O.)

3.2.3.1 Using Aluminum Iso-propoxide Precursor (Al(OCH2CH2CH3)3)

The above described method for the synthesis of the supports was also adopted for

the synthesis of the gold- supported catalysts as shown in Scheme 3.2. Briefly, Aluminum

iso-propoxide and Mg(NO3)2.6H2O were dissolved in 200 ml H2O by stirring at 343 k for

approximately 10 h. The pH of the solution was adjusted to approximately 3 using dilute

nitric acid (4-5M) and the temperature was raised to 373 k. The required quantity of gold

to produce 1% nominal loading (0.2 g HAlCl4.3H2O precursor) dissolved in 10 ml water

was added to the homogeneous solution where the system was refluxed for ~ 20-30h until

the gel formed.

For each system prepared, the gel was aged at room temperature for ~48 h to form

a solid gel. The resulting solid-gel was freeze-dried then calcined at 823 k under static air

for 4 h, with a heating rate of 2 k min-1

.

39

Scheme 3.2: The preparation steps for 1%Au/4%MgO-Al2O3 catalyst synthesis

40 g Al(OCH2CH2CH3)3 + 7.5 g of Mg(NO3)2.6H2O + 200 ml H2O

Stirred and heated at 343 K for ~ 10 hrs

pH lowered to ~ 3 and the temperature raised to 373 K

Refluxing the homogeneous solution until gel formation

Aged at room T for 48 hrs to form solid gel

Freeze - dried

Calcined at 823 K under static air for 4 hrs with heating rate of 2C/min

HAuCl4 Solution

1%Au/4%MgO-Al2O3

catalyst

41

3.2.3.2 Using Boehmite Precursor ( AlO(OH).2H2O) 1%Au/Al2O3, 1%Au/15%MgO-

Al2O3, 2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 catalysts were

prepared using boehmite ,AlO(OH).2H2O, as a procursor.

1%Au/Al2O3 catalyst

For 1%Au/Al2O3 catalyst synthesis, 10 g of boehmite, AlO(OH).2H2O, initially

dispersed in 70 ml H2O at 343 K and refluxed for 10 h. To the refluxing solution, several

drops of dilute nitric acid (4-5 M) were added to lower the pH of the dispersion to ~3

followed by rising the temperature to 373 K. Refluxing of the homogeneous solution was

continued at this temperature for ~48h. 13.5 ml of a HAlCl4.3H2O solution (1g/100ml

H2O) was added to the homogeneous solution where the system was refluxed until the gel

formed.

1%Au/15%MgO-Al2O3 and Other Gold Loadings

The above described method for the synthesis of the 1%Au/15%MgO-Al2O3 was

also adopted for the synthesis of the gold-supported catalysts. Briefly, 10g of boehmite

AlO(OH).2H2O initially dispersed in 70 ml H2O at 343 K and refluxed for 10 h. To the

refluxing solution, several drops of dilute nitric acid (4-5 M) were added to lower the pH

of the dispersion to ~3 followed by rising the temperature to 373 K. Refluxing of the

homogeneous solution was continued at this temperature for ~48h. To produce 1%

Au/15%MgO-Al2O3, 16 ml of HAlCl4.3H2O solution (1g/100mL H2O) was added to the

homogeneous solution where the system was refluxed for 2h. Then, 7.8 g of

Mg(NO3).6H2O was dissolved in 5 ml H2O and added to the homogeneous solution at

41

which the gel formed suddenly. To prepare 2%Au/15%MgO-Al2O3 and

4%Au/15%MgO-Al2O3 catalysts, the same quantities of boehmite AlO(OH).2H2O and

Mg(NO3).6H2O were used however, different quantities of gold precursor solutions were

used. 32 ml of HAlCl4.3H2O solution (1g/100mL H2O) was added to the homogeneous

solution to produce 2%Au/15%MgO-Al2O3 catalyst however, 64 ml of this solution was

used to produce 4%Au/15%MgO-Al2O3 catalyst.

3.3 CHARACTERIZATION OF SUPPORTS AND CATALYSTS

3.3.1 ELEMENTAL ANALYSIS

The elemental composition of the catalysts was quantitatively determined by

inductive coupled plasma mass spectrometry (ICP-MS). The supports and the catalysts

were characterized for their textural and physical properties.

3.3.2 X-RAY POWDER DIFFERACTION, XRD

X-ray diffraction is generally used to identify bulk crystalline phases and it can be

used to estimate the particle sizes. The diffraction method is based on Bragg’s law which

is the condition for the constructive interference of scattering from a set of planes with a

d-spacing (d(hkl)

) at an angle (2θ), with a wavelength (λ) according to the equation (3.1)

and as shown in Figure 3.3

λ = 2 . d(hkl) . sin θ (3.1)

42

where λ is the wavelength of the incident beam, θ is the diffraction angle and d(hkl)

is the

distance between two successive planes characterized by Miller indices (h,k,l). Bragg

reflection according to the above equation is illustrated in Scheme 3.3.

Scheme 3.3: Bragg reflection from a set of lattice planes.

43

The particle size may be estimated by using the Scherrer formula:

d =

(3.2)

where d is the average metal particle size (Å), K is the Scherrer constant (0.9) and β is the

full width at the half-maximum (radians) of the peak. This method, although widely used

in characterization of powders, has some disadvantages. First, it estimates an average size

of the metallic particles and gives no information about the size distribution of the

metallic particles. Second, the sensitivity ranges between 3 nm and 50 nm. Thus, very

small particles, below 3 nm are not detected by X-ray diffraction.

The X-ray diffractograms have been recorded using Siemens-D5005

diffractometer using Cu-Kα radiation under 40 kV, 40 mA, and scan range from 10° to

80°. The scan analysis was made with a scan rate of 3° min

-1. The diffractograms were

recorded at room temperature, using air-exposed samples. All the gold-containing samples

were calcened before XRD analysis. In addition, the supports were also investigated by

means of XRD analysis. The results obtained from XRD have also been used to estimate

the dispersion of MgO on Al2O3 and to estimate also the different phases of metallic gold as

shown in Figure 3.1.

44

Figure 3.1: The XRD pattern for Au-based catalyst

0

200

400

600

800

1,000

1,200

1,400

1,600

10 20 30 40 50 60 70 80

inte

nsi

ty a

. u.

2 theta

45

3.3.3 TRANSMISSION ELECTRON MICROSCOPY (TEM)

Some selected gold based-catalysts were subjected to detailed structural analysis

by means of transmission electron microscopy, TEM. This approach is especially useful

when very small particles are involved since their detection is not possible using XRD.

The TEM powder analysis was performed using a JEOL 2010 microscope operated at 200

kV with a point-to-point resolution better than 1.9 Å. The source of electrons was a LaB6

filament. The sample was mounted on a carbon polymer supported copper micro-grid. A

few droplets of a suspension of the ground catalyst in ethanol or isopropyl alcohol were

placed on the grid, followed by drying at ambient conditions. The average gold particle

size and their distribution were determined by counting at least 100-150 particles

3.3.4 BET TOTAL SURFACE AREA

The total surface area of the catalysts has been determined on the basis of the

Brunauer, Emmet and Teller (BET) theory. The amount of a certain gas, physically

adsorbed on the whole surface at very low temperature (-1960

C), is measured, and the

total surface area can be estimated according to:

S = Xm·N·Am·10-20

(m2

g-1

) (3.3)

where xm is the monolayer capacity (moles per gram of solid), N is Avogadro’s constant (

= 6.023·1023

molecules per mole) and Am is the molecular cross-sectional area of the

adsorbate, i.e. the area which an adsorbed molecule occupies on the surface of the solid in

a complete monolayer (Å2). For nitrogen at –196 ºC the value of Am is 16.2 Å

2. The

46

monolayer capacity was determined by applying the BET equation to the experimental

data. The BET equation is given by:

=

+

(3.4)

where P is the equilibrium pressure, P0

is the saturation vapour pressure of the adsorbate

(N2

in the present case), x is the amount of gas adsorbed at the pressure P, c is a constant

and xm

is the amount of the gas adsorbed at saturation (monolayer capacity). According to

this equation, when

is plotted against

a straight line results with slope s =

, and intercept i =

. Finally, the monolayer capacity x

m can be determined and

by applying equation 3.2, the total surface area of the catalyst is found. It has to be

mentioned that the BET theory is valid in the pressure range 0.05 <

< 0.3. The

nitrogen adsorption/desorption isotherm is shown in Figure 3.2.

Analysis of the textural properties of the catalysts were performed by initially pre-

treating the powder materials at 523 K under vacuum for ~3 h, followed by measuring the

samples’ N2 adsorption/desorption isotherms at liquid nitrogen temperature. Calculation

of the BET surface area and micropore analysis by the t-plot method were performed

using the specialized software supplied with the instrument. The pore size distribution

was calculated by Non-Local Density Functional Theory (NLDFT), assuming a

cylindrical pore model and using the desorption branch of the isotherms as shown in

Figure 3.3. A Quantachrome Autosorb-1C system was used to determine multi-point

BET surface area and porosity.

47

0

20

40

60

80

100

120

140

160

0.00 0.20 0.40 0.60 0.80 1.00 1.20

Vo

lum

e @

STP

cc/

g

P/Po

Figure 3.2: Nitrogen adsorption/desorption isotherm.

48

Figure 3.3: The pore width distribution of gold supported catalyst

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

0 10 20 30 40 50 60 70

Dv/

dD

(cm

-1g-1

)

Pore Width (Å)

49

3.3.5 TEMPERATURE PROGRAMMED REDUCTION MEASUREMENTS (TPR)

Temperature programmed reduction (TPR) is a useful technique for the

characterization of metal oxide catalysts. During a TPR experiment, the catalyst is

exposed to a reducing mixture while the temperature is increased in a linear fashion. The

measurements were performed by using gas mixture of hydrogen and argon. The reducing

gas, 5 % H2 in Ar, passed with a flow rate of 30 ml min

-1

through the catalyst bed at a

heating ramp of 5 ºC min-1

. Prior to each experiment the sample was heated in an Ar flow

at 50 ºC for 30 min in order to eliminate the physically adsorbed water. The concentration

of the gas mixture is measured as function of time using a mass spectroscopy technique.

The resulting TPR profile contains information on the oxidation state of the reducible

species present.

3.3.6 TEMPERATURE PROGRAMMED OXIDATION (TPO)

TPO analyses were carried out on a spent catalyst sample. The TPO analysis

follow the same procedure of the TPR analysis except the use of 5% O2 instead of 5% H2

gas used in the TPR analysis. The measurements were performed by using gas mixture of

5% oxygen in argon. The oxidizing gas, 5 % O2

in Ar, passed with a flow rate of 30 ml

min-1

through the catalyst bed at a heating ramp of 5 ºC min-1

. Prior to each experiment

the sample was heated in an Ar flow at 50 ºC for 30 min in order to eliminate the

physically adsorbed water. Then, the temperature was increased to 650 ºC at a ramp rate

of 5 ºC min−1

. Oxidation was monitored by following CO2 production using an online

mass spectroscopic measurement.

51

3.3.7 TEMPERATURE PROGRAMMED DESORPTION (TPD)

CO2-TEMPERATURE PROGRAMMED DESORPTION (TPD)

Two different procedures were used for the CO2 temperature-programmed

desorption (CO2-TPD) analysis of the samples. Analyses of unreduced samples involved

the following procedure: 0.25-0.30 g sample was pre-treated at 773 k for 2 h in dry N2

(99.9999 %) flow (100 ml/min), and cooled to 303 k, followed by exposure to pure CO2

(99.9999%) for 30 min. After purging the catalyst with N2 for 1 h at 318 k, the TPD plot

was obtained at a heating rate of 5 k min-1

to 1023 k. The second procedure involved the

pre-reduction of the catalysts in hydrogen as follows: 0.25-0.30 g sample was pre-reduced

at 673 k for 1 h in dry hydrogen (99.99 %) flow (100 ml min-1

), and cooled to 303 k, then

exposed to pure CO2 (99.9999%) for 30 min. After purging the catalyst with N2 for 1 h at

318 k, the TPD plot was obtained at a heating rate of 5 k min-1

to 1023 k. The desorbed

CO2 was monitored by a pre-calibrated quadrupole mass spectrometer.

NH3-TEMPERATURE PROGRAMMED DESORPTION (TPD)

NH3-TPD analysis was carried out according to the following procedure: 0.25-

0.30 g sample was pre-reduced at 673 K for 1 h in dry hydrogen (99.99 %) flow (100 ml

min-1

), and cooled to 303 K, then exposed to pure NH3 for 15 min. After purging the

catalyst with N2 for 1 h at 318 K, the TPD plot was obtained at a heating rate of 5 K min-1

to 1023 K as shown in Figure 3.4. The desorbed NH3 was monitored by a pre-calibrated

quadrupole mass spectrometer.

51

Figure 3.4: NH3-TPD profile

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

200 300 400 500 600 700 800 900 1000

µm

ole

NH

3S-1

g-1ca

t

T, K

52

3.3.8 CO2-FT-IR ANALYSIS

Infrared analysis of adsorbed CO2 was carried out on self-standing wafers of H2-

pretreated samples where each sample was initially dried under vacuum at 723 K then

cold before the introduction of CO2 at 323 K. Ft-Ir spectra were recorded after heating

under vacuum at 323, 373, 423, 473, and 523 K.

3.4 CATALYTIC REACTION

3.4.1 ISOPROPANOL CONVERSION

The decomposition reaction of isopropanol was performed in a fixed bed reactor

placed inside a vertical furnace with temperature controller using 250 mg of catalyst at

atmospheric pressures as shown in Scheme 3.5. The catalysts were initially reduced with

hydrogen at 673 k for 2h before lowering the temperature to the reaction temperatures

(448-523 k) under flowing hydrogen. Nitrogen saturated with isopropanol (Fluka,

Chromasolv) vapor by contact in a saturator at partial pressures of 0.021, 0.030 or

0.041atm was introduced with a total flow of 100 ml min-1

. To avoid the condensation of

isopropanol or any reaction product, the transfer line from the reactor to the gas

chromatograph was heated at 393 k. Reactant and products’ concentrations were

measured by an Agilent-6890N gas chromatography with a flame ionization detector and

an Agilent GS-Q capillary column.

53

Thermocouple

Eurotherm 2

N2 soN2

urce

Bubbler

Flowmeter

Valve

Rea

ctor

Eurotherm 1

Thermocouple N2 Source

GC

H2

أ

Valve

Scheme 3.5: Schematic diagram of the flow system used for isopropanol conversion

measurements

54

3.4.2 ACETYLENE HYDROGENATION

Acetylene hydrogenation was carried out in a fixed bed reactor in which typically

0.20 g of catalyst was loaded. The standard reactivation procedure consisted of in situ

heating the catalyst to 350 ◦C for 1 h under a H2 flow (catalyst reduction). The feed gases

were controlled by mass flow controllers and set to a total flow of 47 ml min−1

balanced

by nitrogen gas. The reactant ratio H2/C2H2 was varied and the optimum ratio was used in

the acetylene hydrogenation. Reactant and products’ concentrations were measured by an

Agilent-6890N gas chromatography with a flame ionization detector and an Agilent GS-

Q capillary column. Scheme 3.6 shows the Schematic diagram of the flow system used

for Acetylene hydrogenation measurements. For catalytic activity measurements the

reactant mixture was stabilized for at least 15 min at each experimental temperature. The

temperature was raised to 100 ◦C at which the first reaction was carried out. Afterwards

the temperature raised by 50 ◦C from 100

◦C to 400

◦C.

55

GC

Valve

Thermocouple

Eurotherm 2

Rea

ctor

Eurotherm 1

Thermocouple

H2

C2H2

2 C2H4

N2

MFC1

MFC2

MFC3

MFC4

Scheme 3.6: Schematic diagram of the flow system used for Acetylene hydrogenation

measurements

56

CHAPTER 4


CONVERSION

4.1 OUTLINE

This chapter presents a comprehensive study of the physico-chemical properties of

various gold-based catalysts. Section 4.2 presents results concerning the gold loading

(ICP-MS technique), BET surface area, average size of the gold particles (XRD and

TEM). Sections 4.3, 4.4 deal with temperature programmed desorption TPD. Section 4.5

presents the H2 temperature programmed reduction and section 4.6 deal with adsorbed

CO2 FT-IR measurements. Section 4.7 presents the isopropanol catalytic conversion at

different temperature and different isopropanol vapor pressure.

4.2 GOLD LOADING (ICP-MS), XRD, BET SURFACE AREA AND

TEM RESULT

4.2.1 INDUCTIVE COUPLED PLASMA MASS SPECTROSCOPY ANALYSIS (ICP-

MS)

The elemental composition of the catalysts is listed in Table-1. It’s noticed that

the amount of supported gold is ~73% of the nominal synthesis amounts. This is a

57

consequence of the synthesis method employed where isopropanol, a product of the

hydrolysis of aluminum isoporpoxide precursor, can partially reduce gold at the synthesis

temperature used leading to the some precipitation of metallic gold and its separation

from the homogenous gel [136].

4.2.2 X-RAY DIFFRACTION ANALYSIS

X-ray diffraction analyses of the samples are depicted in Figure 4.1. It can be

noticed that all of the samples have basically the same x-ray diffraction features with a

broad diffraction peaks with maxima at 2 = 32° and 37.5°, belonging to Al2O3 single-

phase oxide, along with two other peaks at 46.0° and 67.0°, which are characteristic peaks

corresponding to the (220), (311), (400) and (440) reflections of -Al2O3 phase [134]. The

intensity of the latter set of peaks remains unchanged despite the addition of MgO. This

might be due to the homogenous dispersion of MgO into Al2O3 without the aggregation of

MgO nanoparticles, resulting in highly-dispersed particles of MgO-Al2O3 binary oxide

composites as a solid solution. The absence of any characteristic MgO reflections is

probably due to the mutual monolayer dispersion, which causes the XRD pattern of the

solid solution to be dominated by the component with the higher concentration [149]. The

gold containing samples have additional reflections occurring at 2 = 38°, 44.4°, 64.8°,

which are characteristic peaks corresponding to the (111), (200) and (220) reflections of

fcc gold [137]. The gold on -Al2O3 is found to have a slightly broader and lower intensity

reflections than for the gold supported on the magnesia containing supports, which could

be attributed to the formation of larger gold nanoparticles on the latter supports [137].

58

Figure 4.1: XRD patterns of calcined (a) γ-Al2O3, (b) 4%MgO-Al2O3, (c) 8%MgO-

Al2O2, (d) 1%Au/ γ-Al2O3, (e) 1%Au/4%MgO-Al2O3, (f) 1%Au/8%MgO-Al2O3

59

4.2.3 BET SURFACE AREA ANALYSIS

Nitrogen adsorption/desorption isotherms of the samples are shown in Figure

4.2. The isotherms formed are, according to the BDDT classification, of type IV with an

H2 type (IUPAC) hysteresis attributed to the presence disordered mesopores [138]. The

main textural parameters of the samples are reported in Table 5.1. It can be observed that

the BET surface area decreases from ~264 to ~207m2/g with the addition of gold and

increasing MgO content in the material. The total pore volume of the materials was also

found to decrease with the addition of gold and MgO, but the micropore volume increased

with the addition of MgO. Mono-modal pore size distribution was obtained for all the

catalysts, as shown in Figure 4.3. The average pore diameter of 5.1 nm was basically the

same for all the catalysts, indicating low mesopore blockage by the gold or magnesia

additions.

61

Figure 4.2: Nitrogen adsorption/desorption isotherms of calcined (a) γ-Al2O3, (b)

4%MgO-Al2O3, (c) 8%MgO-Al2O2, (d) 1%Au/ γ-Al2O3, (e) 1%Au/4%MgO-Al2O3,


61

Figure 4.3: NLDFT-Pore size distribution for calcined (a) γ-Al2O3, (b) 4%MgO-

Al2O3, (c) 8%MgO-Al2O2, (d) 1%Au/ γ-Al2O3, (e) 1%Au/4%MgO-Al2O3, (f)

1%Au/8%MgO-Al2O3

62

Table 4.1: Textural properties and elemental analysis of the catalysts.

Aue

wt.%

Mge

wt.%

Au-PSf

(nm)

Vmp

(cm3/g)

d

Vp

(cm3/g)

c

Db

(nm)

SAa

(m2/g)

Catalysts

- - 0.008 0.292 5.1 264 Al2O3

5..0 - 5 0.004 0.165 5.1 210 1%Au/Al2O3

- 3.83 0.021 0.207 5.1 236 4%MgO-Al2O3

- 7.38 0.020 0.198 5.1 218 8%MgO-Al2O3

5.00 4.20 7 0.017 0.180 4.9 207 1%Au/4%MgO-

Al2O3

5.00 7.67 8 0.022 0.19 5.1 210 1%Au/8%MgO-

Al2O3

a-BET surface Area

b- Average mesopore diameter calculated by NLDFT.

c- Total pore volume measured at P/Po=0.997.

d- Micropore Volume calculated by t-method.

e- Weight percentages of magnesium and gold as determined by ICP.

f-Average Au particle size as determined by TEM.

g- As calculated from respective H2-TPR analysis.

63

4.2.4 TRANSMISSION ELECTRON MICROSCOPYT (TEM)

The morphology and particle size of the reduced gold on the different supports

have been examined by TEM, as seen in Figure 5.4. The gold particles are seen as dark

contrasts on the surface of the supports. TEM micrographs show that there is a slight

difference in the average size of the metallic gold particles with the change of supports.

The average size of the gold particles on the –Al2O3 support of ~5 nm is found to be

slightly smaller than that found on the magnesia containing supports, of ~7-8nm. The size

distributions of gold particles’ on supports prepared in this study are not as uniform as

those typically prepared by the deposition-precipitation method [138]. Nonetheless, the

size distribution on the –Al2O3 samples is slightly better than that of the MgO–Al2O3.

This results is in support of the XRD analysis presented earlier, but are in contradiction to

what has been reported about the effects of support properties where gold particles size

and their size-distribution are reportedly governed mainly by different surface basic

properties [139]. Since magnesia is known to introduce more surface basic sites to the

alumina [135], it is expected that samples containing magnesia to have better gold

particles size distribution and smaller mean diameter [139]. As mentioned earlier, the

observed particle size difference could stem from the synthesis procedure adopted in this

study, which involved the co-precipitation of the gold precursor with the magnesia-

alumina gel.

64

Figure 4.4: TEM micrographs and the corresponding particle size histograms of hydrogen

pre-treated (a) 1%Au/8%MgO-Al2O3, (b) 1%Au/4%MgO-Al2O3, and (c) 1%Au/–Al2O3.

65

4.3 CO2 TEMPERATURE PROGRAMMED DESORPTION (TPD)

Ammonia and CO2 temperature programmed desorption (TPD) analyses were

also utilized to assess the acidic-basic character of the catalysts, respectively. CO2-TPD

analyses were carried out on all of the samples with two different pre-treatment

procedures, as described in the experimental section: pre-treatment under nitrogen and

another involving pre-reduction with hydrogen. Figures 4.5 and 6.4 depict the CO2-TPD

profiles obtained for the catalysts pre-treated by the two described procedures,

respectively. The CO2-TPD profiles exhibit two separate desorption peaks, one centered

in the range of 323-723 K and the other in 723-1063 K temperature range. The higher

temperature TPD peak will be ignored from the analysis due to the fact that it appeared in

Argon TPD studies of alumina-rich samples (data not shown) even without the exposure

to CO2 gas. Hence, this CO2 peak could due to the combustion of remnant carbonaceous

residue that remained in the materials after their initial calcinations at 823 K, or produced

by stronger basic sites generated through the alumina phase transitions occurring at these

temperatures [2]. Table 4.2 lists the results obtained from the CO2-TPD analysis for all of

the catalysts pre-treated by the two described procedure. It is noticed that the temperature

at the maximum of the TPD peak is higher for magnesia containing materials, where for

the case of nitrogen pre-treated catalysts; it is found to decrease with the addition of gold.

While, for the hydrogen reduced catalysts, the temperatures were in general found to be

higher than for the nitrogen-pretreated samples and either increased slightly and/or

remained constant with the addition of gold. Hence, it could be said that the presence of

magnesia led to the formation a stronger CO2 adsorption sites and a similar deduction

66

could be stated for the gold containing samples that were subjected to a hydrogen

reduction pre-treatment. As for the density of CO2 adsorption sites, it is noticed that even

though samples pre-treated with nitrogen did not exhibit a significant catalytic activity in

the isopropanol decomposition reaction, these samples possessed nearly an order of

magnitude greater density of CO2 adsorption sites than the hydrogen pre-treated samples.

Moreover, the presence of gold in the nitrogen pre-treated samples has led to a significant

increase in the density of CO2 adsorption sites. On the other hand, not only did the

hydrogen pre-treated samples possessed a relatively lower density of CO2 adsorption sites

but the presence of gold in the -Al2O3 has led to a decrease in its density of CO2

adsorption sites, while led to a moderate increase in the density of CO2 adsorption sites on

the 4%MgO-Al2O3 sample and a more significant increase in the 8% MgO-Al2O3 sample,

where the latter had a density of CO2 adsorption sites surprisingly comparable to that of

plain -Al2O3 pre-treated with hydrogen. Hence, and as far as these catalysts are

concerned, there seems to be no clear trend or correlation between the density of basic

sites and the catalysts’ selectivity towards acetone, as been reported in the literature [135].

67

Table 4.2 CO2 and NH3 temperature programmed desorption (TPD) analysis results of

calcined catalysts.

Catalyst

Hydrogen pre-treated Nitrogen pre-treated

Tmax-CO2

(K)a

CO2

(mol g-1

)

Tmax-NH3

(K)a

NH3

(mol g-1

)

Tmax-CO2

(K)a

CO2

(mol g-1

)

-Al2O3 379 1792 393 456 385 2771

4% MgO- Al2O3 391 1039 415 3155 381 8372

8%MgO- Al2O3 400 974 417 3005 398 4389

1%Au/-Al2O3 388 522 418 2251 381 4919

1%Au/4%MgO-Al2O3 394 1085 426 1036 381 7314

1%Au/8%MgO-Al2O3 397 1810 432 2265 391 13095

a- Temperature at desorption peak maximum.

68

Figure 4.5: CO2-TPD profiles of nitrogen pre-treated catalyst (a) -Al2O3, (b)

4%MgO- Al2O3, (c) 8%MgO- Al2O3, (d) 1%Au/-Al2O3, (e) 1%Au/4%MgO-

Al2O3, and (f) 1%Au/8%MgO-Al2O3

69

Figure 4.6: CO2-TPD profiles of hydrogen pre-treated catalyst (a) -Al2O3, (b)

4%MgO-Al2O3, (c) 8%MgO- Al2O3, (d) 1%Au/-Al2O3, (e) 1%Au/4%MgO-Al2O3, and


71

4.4 NH2 TEMPERATURE PROGRAMMED DESORPTION (TPD)

NH3-TPD analyses were only carried out on samples pre-treated with hydrogen.

Figure 6.4 depicts the NH3-TPD profiles obtained for the catalysts. Table 4.2 lists the

results obtained from the NH3-TPD analysis of the catalysts. It is noticed that the

temperature at the maximum of the TPD peak is higher for magnesia containing materials

and a further significant increase is observed with the addition of gold, indicative of the

formation of stronger NH3-adsorbing acid sites. The density of the acid sites on these

catalysts is found to increase with the addition of magnesia or gold on plain -Al2O3,

which is in accord with their catalytic results where -Al2O3, 4%MgO-Al2O3, 8%Mg-

Al2O3, and 1%Au/-Al2O3 have all exhibited a rather high selectivity for propene

formation. On the other hand, the presence of gold nanoparticles on magnesia containing

catalysts has led to a significant decrease in their respective acid site density, with the

1%Au/8%Mg-Al2O3 catalysts possessing an acid site density twice as large as that found

for the 1%Au/4%Mg-Al2O3 catalyst. In addition, out of all of the catalysts listed, only the

1%Au/4%Mg-Al2O3 catalyst and the 1%Au/8%Mg-Al2O3 exhibited comparable acid and

base site densities, where the latter containing almost twice as much of both types of sites

as the former. While the other four catalysts exhibited significant differences between

their acid and base site densities.

Therefore, and in support of the previously mentioned reaction results, it can be

stated that the base-catalytic property of reduced gold nanoparticles supported on

magnesia containing -Al2O3 does not solely stem from the presence of basic CO2

71

adsorption sites, but a significant density of NH3 adsorbing sites is also required, as

noticed with the hydrogen pre-treated the 1%Au/4%Mg-Al2O3 and 1%Au/8%Mg-Al2O3

samples which, according to the results listed in Table 4.2, contained comparable and

large densities of both acid and basic catalytic sites, which operated in a concerted fashion

in the decomposition of isorpoanol reaction. While the reason behind the formation of the

large and comparable density of acid-base sites can be clearly traced to a synergy between

magnesia and reduced gold that has led to the formation of relatively stronger acetone-

selective catalytic sites.

72

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

200 300 400 500 600 700 800 900 1000

µm

ole

NH

3S-1

g-1ca

t

T,K

(a)

(b)

(c)

(d)

(e)

(f)

Figure 4.7: NH3-TPD profiles of hydrogen pre-treated catalyst (a) -Al2O3, b) 4%MgO-

Al2O3, c) 8%MgO- Al2O3, d) 1%Au/ -Al2O3, e) 1%Au/4%MgO-Al2O3 f)

1%Au/8%MgO-Al2O3

73

4.5 HYDROGEN TEMPERATURE PROGRAMMED REDDUCTION

(TPR)

In this regard, H2-TPR results, depicted in Figure 3 and listed in Table 4.3,

reveal that the extent of auto-reduction of gold after the catalysts’ calcination in air

increases with increasing magnesia content in the support. By combining the hydrogen-

TPR values with the gold loading as determined by ICP, it can be qualitialively estimated

that ~2x10-3

mol H2/g Au was needed to reduce the gold in the 1%Au/γ-Al2O3 sample

after calcination in air, while ~1x10-3

and 7x10-4

mol H2/g Au was required to reduce the

gold nanoparticles in the 1%Au/4%MgO-Al2O3 and the 1%Au/8%MgO-Al2O3 catalysts,

respectively. In addition, the temperature of the H2-TPR peak minima, shown in Figure

4.8, was found to decrease gradually from 550 K for the 1%Au/γ-Al2O3 sample to ~500 K

with increasing magnesia content in the 1%Au/8%MgO-Al2O3 catalysts. These

observations could be attributed to the possible presence of MgO oxygen-vacancies which

have been reported to mediate the reduction process [145,146]. Hence, it could be stated

that the surface properties introduced by the addition of magnesia has accelerated and

enhanced the auto-reduction of the gold during calcinations in air, and upon further

hydrogen treatment, led to the formation of supported metallic gold with a relatively

larger average particle size.

74

Figure 4.8: H2-TPR profiles of (a) 1%Au/-Al2O3, (b) 1%Au/4%MgO-Al2O3, and (c)

1%Au/8%MgO-Al2O3.

75

Table: 4.3 Temperature programmed reduction analysis (TPR) results of gold-

based catalysts

Catalyst H2-uptake (µmol/g)

a

1%Au/-Al2O

3 16.6

1%Au/4%MgO-Al2O

3 10.3

1%Au/8%MgO-Al2O

3 5.2

76

4.6 CO2-FT-IR SPECTROSCOPY MEASUREMENTS.

The strength of the CO2-adsorption sites was qualitatively studied by examining

the structure of chemisorbed CO2 species on Au supported catalysts by IR measurements

of pre-adsorbed CO2. Figure 4.9 presents the IR spectra obtained on the hydrogen-

pretreated gold containing catalysts after CO2 adsorption at 323 K and sequential

evacuation at 323, 373, 423, 473 and 523 K. Three surface species of adsorbed CO2 were

detected on the Au supported samples: bicarbonate, unidentate and bidentate carbonates

[147, 148]. These species were detected on all of the Au supported samples with similar

qualitative spectra after CO2 evacuation at increasing temperatures, but with a slight shift

to higher wave-number values for the unidentate and bidentate carbonate bands on the

Au/Al2O3 sample. The different CO2 adsorption modes reveal that the surfaces of

Au/MgO-Al2O3 or Au/Al2O3 oxides contain oxygen atoms of different chemical nature.

Unidentate carbonate formation involves isolated surface O2− ions, i.e., low-coordination

anions, such as those present in corners or edges and exhibits a symmetric O–C–O

stretching at 1360–1400 cm−1 and an asymmetric O–C–O stretching at 1510–1560 cm−1.

Bidentate carbonate forms on Lewis-acid–Brönsted-base pairs (M–O2− pair site, where M

is the metal cation Mg or Al), and shows a symmetric O–C–O stretching at 1320–1340

cm−1 and an asymmetric O–C–O stretching at 1610–1630 cm−1. Bicarbonate species

formation involves surface hydroxyl groups. Bicarbonates show a C–OH bending mode at

1220 cm−1 as well as a symmetric and an asymmetric O–C–O stretching at 1480 and 1650

cm−1, respectively [148].

77

On the surface of the Au/Al2O3 catalyst, bicarbonate is the most labile species and

disappears after evacuation at 423 K, while both the unidentate and bidentate carbonates

remain on the surface after evacuation at 473 K, but only the unidentate bands are

observed upon evacuation at higher temperatures. On the surface of the Au/4%MgO-

Al2O3 catalysts, bicarbonate disappears after evacuation at 473 K while both the

unidentate and bidentate carbonates remain on the surface after evacuation at 523 K.

These results may suggest the following strength order for surface basic sites: O2− ions >

oxygen in Mg(Al)–O pairs > OH groups, and these sites are found to be relatively

stronger on the magnesia containing sample. On the other hand, the bands for all three

carbonate species on the surface of the Au/8%MgO-Al2O3 are still present even after

evacuation at 523k, as shown in Figure 7. Since the only difference between this catalyst

and the other ones is the amount of magnesia present, the persistence of the bicarbonate

band after evacuation at 523 k points to the enhancement of the strength of basic sites in

general, and of the basicity of the hydroxyl groups in particular with the addition of 8%

magnesia, supporting the above mentioned high selectivity of acetone over the

Au/8%MgO-Al2O3 catalyst. Hence, from the Infrared analysis of chemisorbed CO2 on

the Au supported catalysts, the following order of catalysts basicity: Au/8%MgO-Al2O3 >

Au/4%MgO-Al2O3 > Au/Al2O3 is in direct agreement with the results of the TPD analysis

and isopropanol decomposition reaction results.

78

Figure 4.9: CO2-FT-IR profiles of (a) 1%Au/-Al2O3, (b) 1%Au/4%MgO-Al2O3, and (c)

1%Au/8%MgO-Al2O3 at temperature of (i) 323, (ii) 373, (iii)423, (iv) 473 and (v) 523 K.

79

4.7 CATALYSTS' ACID-BASE FVALUATION: ISOPROPANOL

CONVERSION

4.7.1 INTRODUCTION

Isopropanol decomposition is used as a probe reaction to characterize the surface

acid-base properties of gold based catalysts. Because the alcohol decomposition on solid

catalysts occurs through two pathways: the dehydration which is generally catalyzed by

the acidic sites and the dehydrogenation which is catalyzed by basic sites or both acidic

and basic sites, this kind of decomposition has been used as an indicator of the surface

acidic and basic sites of the solid catalysts [151].

Of particular interest to this study is the influence of the -alumina and magnesia-

alumina supports on the resulting catalytic acidic-basic character of supported gold

catalyst. Alumina is a very mature industrial catalyst support and its pore structure can

vary over a great range to facilitate the dispersion of metals. The stability of alumina is

favorable to resist water and therefore may be helpful to increase the durability of the

catalyst. Moreover, the strength and the amount of basic sites can be adjusted by changing

the type of the supported basic oxide and its loading amount, respectively. While alumina-

supported magnesia because MgO is the most widely studied single-metal- oxide solid

base where it has been discovered and reported that many salts and oxides can disperse

spontaneously onto the surfaces of supports to form a monolayer [152].

The formulation of a comprehensive understanding of the acid-base properties of

the supported metal or metal oxide surface is still lacking. The nature and extent of

81

interaction between support material and dispersed metal catalyst influence the acid-base

chemistry of the catalyst. In this study, the acid-base character of supported metallic form

of gold was evaluated in the 2-paopanol dehydration reaction. The supports used were -

alumina and magnesia-alumina mixtures. The supported metal catalysts were prepared by

modified sol-gel method [134,153].

In particular, the basicity of supported nano metallic gold particles was assessed in

this study as a function of its interaction with the above mentioned supports.

Decomposition of 2-propanol is chosen as the test reaction because it is widely used to

characterize the acid-base properties of catalysts [152]. Over acid catalysts, 2-propanol is

dehydrated (E1-mechanism) with the production of water and propene as the main

components [155]. Over basic catalysts, 2-propanol is dehydrogenated yielding hydrogen

and acetone as products. Therefore, the selectivity for acetone and propene is the factor

that characterizes the basicity of the catalyst.

4.7.2 ISOPROPANOLL CATALYTIC CONVERSION

The catalysts activities were evaluated in the isopropanol decomposition reaction

through acid-catalyzed dehydration route to produce propylene and/or base-catalyzed

dehydrogenation route to produce acetone. The activities of all catalysts were evaluated

after pre-treatment with hydrogen at 623 K.

The catalyzed decomposition of isopropanol in this study is considered as

surface reaction involving single adsorbed molecule which classified as unimolecular

surface reaction. This type of reactions could be treated in terms of the Langmuir

81

adsorption isotherms as follows. The rate is proportional to the fraction θ of surface that is

covered and is thus

R = kθ =

(4.1)

The dependence of rate on [A] shown in Figure 4.10. At surface concentration low

concentration where K[A] ˂˂ 1 the kinetics become first order, however at higher

concentration of A the rate independent of the concentration which means the kinetics are

zero order [158].

82

Figure 4.10: Variation of rate with concentration for simple unimolecualr process.

[A]

v =

v = kK[A]

v =

v

83

The heterogeneous decomposition of 2-propanol has been interpreted in terms of

various mechanisms. The two main paths of conversion are shown in Scheme 4.1

CH3

CH3COCH3 + H2 (Dehydrogenation)

CH OH

CH2= CHCH3 + H2O (Dehydration)

CH3

Scheme 4.1: Paths of 2-propanol conversion

The products are either propene (DHD, due to acid centers) or acetone (DHG, due to

redox or basic centers), both are parallel first-order reactions liable to take place [145].

These were explained by Fikis et al. [146] in terms of intermediates typical of elimination

type mechanisms. Propene may be formed via an E2 type mechanism on adjacent vacant

surface sites as shown in Scheme 4.2.

Scheme 4.2: Propene formation through E2 michanism

84

Such an attachment of the alcohol to the surface through the hydroxyl group and a β-

hydrogen has been reported to involve little strain even on plane surfaces. In a similar

way, acetone could be produced [ DHG ] :

Scheme 5.3: 2-propanol conversion to acetone

A two-point adsorption intermediate has been proposed for acetone formation one site is

a hydroxyl group which may also form as an adsorbed species from the dehydration

reaction. The adsorbed intermediate species will be located at sites offering H or OH, and

consequently will be related primarily to the presence of “acidic” species in the solid

surface [146].

4.7.3 The γ-Al2O3 and 1%Au/γ-Al2O3 CATALYSTS

Figure 4.11 depicts the activity of the γ-Al2O3 and 1%Au/γ-Al2O3 catalysts in the

isopropanol decomposition reaction carried out at 448, 473, and 498 K and at isopropanol

85

vapor pressure of 0.021 atm. The rate of isopropanol decomposition on both catalysts,

measured on stream, was found to decrease with time and increases with increasing

reaction temperature, where it reached 0.35 mmol g-1

min-1

at 498 K. The results show

that the rate of isopropanol decomposition on γ-Al2O3 catalyst was higher than that on

1%Au/ γ-Al2O3 at 473 K, however, there were no significant differences in the rate of

isopropanol decomposition on both catalysts at 498 K.

Figures 4.12 and 4.13 depict the activity of the γ-Al2O3 and 1%Au/γ-Al2O3

catalysts in the isopropanol decomposition reaction carried out at 448, 473, and 498 K and

at isopropanol vapor pressure of 0.030 and 0.041 atm. The rate of isopropanol

decomposition on both catalysts measured on stream, was found to decrease with time

and increases with increasing reaction temperature, where it reached ~0.5 and ~0.6 mmol

g-1

min-1

at reaction temperature of 498 K. The results show that the rate of isopropanol

decomposition over both catalysts increases as the concentration of isopropanol increases

which follow pseudo first order reaction. On the other hand, it is noticed that the catalytic

activity of γ-Al2O3 catalyst in terms of isopropanol decomposition was significantly

higher than that of 1%Au/Al2O3 at reaction temperature of 448, 473 K and isopropanol

vapor pressure of 0.021 and 0.030 atm however, the catalytic activities of supported gold

catalyst was improved at temperature of 498 K as shown in Figure 4.12. The same

behavior was observed in the case of isopropanol decomposition over both catalysts at

different reaction temperature and at isopropanol vapor pressure of 0.042 atm but, there

were no significant differences between these two catalysts at higher temperature as

shown in Figure 4.13.

86

Acetone and propylene were the only products formed under the reactions’

conditions employed. Both of the γ-Al2O3 and 1%Au/ γ-Al2O3 catalysts produced

propylene and acetone, but the rate formation for acetone decreased significantly after 5

min on stream while the rate formation for propylene decreased slightly and reached a

pseudo steady-state after 5 min on stream. The rate formation results of both propylene

and acetone, depicted in Figures 4.14 and 4.15, reveal that the rate formation of acetone

decreases with increasing reaction temperature over 1%Au/γ-Al2O3 catalyst , while there

were no significant differences on the rate formation of acetone over γ-Al2O3 catalyst at

different temperatures. These results indicate the effects of loaded gold particles on the

acid-base character of the catalyst in which the basic active sites diminished by increasing

the reaction temperature of isopropanol decomposition over gold supported catalysts. The

rate formation of propylene on both catalysts was significantly higher than that of acetone

at different temperature and it increased as the temperature increased where it reached

around 0.3 mmol g-1

min-1

at 497 K. Figures 4.15 show that the propylene formation

enhanced significantly by increasing the reaction temperature over Al2O3. On the other

hand, the results show that the formation of propylene over 1%Au/Al2O3 catalyst was

lower than that of Al2O3 catalyst at different temperature which indicate that the loaded

gold nanoparticles neutralized part of the acidic sites responsible for propylene formation

on the catalyst's surfaces. Figures 4.16, 4.17, 18 and 19 show the rate formation of

propylene and acetone at different isopropanol vapor pressure and different reaction

temperature. It is noted that the selectivity to propylene formation enhanced by increasing

the feed of isopropanol during the course of the reaction and it increases as the reaction

temperature increases which indicate the enhancement of the catalyst acidic character.

87

The rate formation results of propylene on both catalysts revealed that γ-Al2O3 catalyst

was more active than 1%Au/ γ-Al2O3 catalyst. Also as listed in Table 4.3, the

propylene/acetone selectivity ratio observed over γ-Al2O3 is larger than that of 1%Au/γ-

Al2O3 at all the reaction temperatures employed. Therefore, the rather high and almost

exclusive selectivity for propylene over these catalysts is indicative of the predominance

of acid-catalyzed dehydration reaction route which is a reflection of the acidic nature of

the γ-Al2O3 and 1%Au/γ-Al2O3 catalysts synthesized in this study, with the latter is

slightly less acidic than the former, i.e., the addition of gold nanoparticles to gamma-

alumina has apparently neutralized some of alumina’s acid sites.

As mentioned above, analyzing the complete set of reaction data by using a simple

kinetic model under the assumption of a Langmuir surface coverage, the rate of

isoporpanol conversion can be expressed as:

(4.2)

where K is the adsorption equilibrium constant, k is the reaction rate constant, P is the

partial pressure, and i corresponds to all reactants and products of the reaction. With a

further assumption of weak adsorption of all products and reactant (1>> , the rate

equation is reduced to the following form:

R = k KIp PIp (4.3)

Expressing the partial pressure of isopropanol in terms of its initial pressure , of the

degree of conversion x, and the total pressure in the reactor Ptotal, Eq. (3) can be rewritten

in the form:

88

(4.4)

Making use of the design-equation of a plug-flow reactor, as in this case:

F dx = R dm (5)

Where F is the feed molar rate in mol min-1

and m is the mass of the catalyst. Substituting

Eq. (4) into Eq. (5) and after integration and rearrangement, yield:

-x-(1 +

)ln(1-x) = m(

)Ptot (4.6)

Considering k = A exp(−Etrue/RT), KIP = exp[(-HIP)/RT], where (-HIP) is the enthalpy

of adsorption of isopropanol and Etrue is the true activation energy which is related to the

apparent activation energy of the process Eapp and the enthalpy of IP adsorption by Eapp =

Etrue + HIP, as well as using the reaction Ptotal = 1 atm and the IP partial pressure PIP=

0.041 atm and taking the natural logarithm to both side of Equation (6) yields:

(4.7)

Thus, plots of the left-side of Eq.(6) versus

Ptot were used, as shown in Figure 5.20. to

obtain the apparent rate constant of the reaction at certain temperature. However, plots of

the left-side of Eq.(7) versus 1000/T were used, as shown in Figure 4.41 to obtain the

apparent activation energy Eapp of the reactions, as listed in Table 4.4.

First order kinetic analyses of isopropanol decomposition reaction, shown in

Figure 4.20, and that of the corresponding Arrhenius plots, shown in Figure 4.41, are

89

summarized in Table 4.4. Activation energy values from ranging 44 to 99 kJ/mol have

been obtained where reported results span a broad spectrum of activation energy values

for this reaction, ranging from 12 to 133 kJ/mol [156-158]. This large variation in the

activation energy values could be due to the different type of interaction between

isopropanol molecules and the catalyst surface active sites. Also diffusion limitations due

to different experimental conditions may reduce the value of the apparent activation

energy. In this study, it appears that the γ-Al2O3 is slightly more active than the gold

loaded equivalent.

91

Table 4.4: First order reaction kinetic parameters for isopropanol conversion over

γ-Al2O3 and 1%Au/Al2O3

Sample T

(K)

IP Vapor

pressure

(atm)

k'

(mole g-1

min-1

atm-1

)a

Eb

(kJ/mol)

A

(mole g-1

min-1

atm-1

)c

Pr/Acd

γ-Al2O3

448

0.021

0.0071

94.31 5.80x108

0.46

0.030 2.00

0.042 3.20

473

0.021

0.0547

25.6

0.030 20.9

0.042 20.1

498

0.021

0.0923

17.7

0.030 26.0

0.042 35.0

1%Au/Al2O3

448

0.021

0.0063

99.42 1.50x109

0.36

0.030 1.12

0.042 0.20

473

0.021

0.0189

8.10

0.030 4.30

0.042 6.70

498

0.021

0.0885

20.1

0.030 17.4

0.042 10.9

a-Apparent first order rate constant b-Apparent activation energy

c- Pre-Exponential Factor

d-Measured at a WHSV of 2.5 h

-1

91

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0 20 40 60 80 100 120

2-P

rop

ano

l Dec

om

po

siti

on

Rat

e, m

mo

l g-1

min

-1

t(min)

Figure 4.11: The rate of isopropanol decomposition as a function of time and reaction

temperatures: ●-498 k, ∎-473 k, ▲-448 k; and isopropanol vapor pressure of 0.021 atm ,

over 1%Au/γ-Al2O3 (dashed-line) and over γ-Al2O3 (solid-line).

92

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0 20 40 60 80 100 120

2-P

rop

ano

l Dec

om

po

siti

on

Rat

e, m

mo

l g-1

min

-1

t(min)

Figure 4.12. The rate of isopropanol decomposition as a function of time and reaction

temperatures: ●-498 k, ∎-473 k, ▲-448 k; and isopropanol vapor pressure of 0.030 atm,


93

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0 20 40 60 80 100 120

2-P

rop

ano

l Dec

om

po

siti

on

Rat

e, m

mo

l g-1

min

-1

t(min)

Figure 4.13: The rate of isopropanol decomposition as a function time and reaction

temperatures: ●-498 k, ∎-473 k, ▲- 448 k; and isopropanol vapor pressure of 0.042 atm,


94

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0 20 40 60 80 100 120

Pro

pen

e Fo

rmat

ion

Rat

e ,m

mo

l g-1

min

-1

t(min)

Figure 4.14: Propylene formation as a function of time and reaction temperatures: ●-

498 k, ∎-473 k, ▲-448 k; and isopropanol vapor pressure of 0.021 atm, over 1%Au/γ-

Al2O3 (dashed-line) and over γ-Al2O3 (solid-line)

95

Figure 4.15. Acetone formation as a function of time and reaction temperatures: ●-498

k, ∎-473 k, ▲-448 k; and isopropanol vapor pressure of 0.021 atm, over 1%Au/γ-Al2O3

(dashed-line) and over γ-Al2O3 (solid-line)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0 20 40 60 80 100 120

Ace

ton

e Fo

rmat

ion

Rat

e ,m

mo

l g-1

min

-1

t(min)

96

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0 20 40 60 80 100 120

Pro

pe

ne

Form

atio

n R

ate

,mm

ol g

-1 m

in-1

t(min)




97

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0 20 40 60 80 100 120

Ace

ton

e Fo

rmat

ion

Rat

e ,m

mo

l g-1

min

-1

t(min)

Figure 4.17: Acetone formation as a function of time and reaction temperatures: ●-498

k, ∎-473 k, ▲-448 k, and isopropanol vapor pressure of 0.030 atm, over 1%Au/γ-Al2O3


98

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0 20 40 60 80 100 120

Pro

pen

e F

orm

atio

n R

ate

,mm

ol g

-1 m

in-1

t(min)




99

0.00

0.05

0.10

0.15

0.20

0.25

0 20 40 60 80 100 120

Ace

ton

e Fo

rmat

ion

Rat

e ,m

mo

l g-1

min

-1

t(min)


k, ∎-473 k, ▲-448 k; and isopropanol vapor pressure of 0.042 atm, over 1%Au/γ-Al2O3


111

0

50

100

150

200

250

1000 1500 2000 2500 3000

-x-(

1+

(Pto

t/P

IP))

ln(1

-x)

(m/F) Ptot

Figure 4.20: First order kinetics of isopropanol decomposition at ●-498 k, ∎-473 k, ⧫-

448 k, over 1%Au/γ-Al2O3 (dashed-line) and over γ-Al2O3 (solid-line).

111

4.7.4 The 4%MgO-Al2O3 and 1%Au/4%MgO-Al2O3 CATALYSTS

The addition of magnesia to alumina to form a 4%MgO-γ-Al2O3 mixture had a

pronounced effect on the catalytic properties of the resulting 4%MgO-Al2O3 support and

its 1% gold supported equivalent. Figure 4.21 depicts the activity of the 4%MgO-Al2O3

and 1%Au/4%MgO-Al2O3 catalysts in the isopropanol decomposition reaction carried out

at 473, 498, and 523 K and at isopropanol vapor pressure of 0.021 atm. It was found that

the rate of isopropanol decomposition decreased with time and it reached pseudo steady-

state after 17 min on stream. The decomposition of isopropanol to propylene and acetone

enhanced by increasing the reaction temperature and increases as the reaction

temperature increases. The results show a significant differences in the catalytic activity

between that of 4%MgO-Al2O3 and 1%Au/4%MgO-Al2O3 at 523K, where the former

appears to be more active at high temperatures. Figures 4.22 And .23 depict the rate of

isopropanol decomposition over 4%MgO-Al2O3 and 1%Au/4%MgO-Al2O3 catalysts at

different reaction temperature and different isopropanol vapor pressure. The results

indicated that the gold supported catalyst "1%Au/4%MgO-Al2O3" was more active at

higher temperature however, the catalytic activity of 4%MgO-Al2O3 catalyst at lower

temperature was significant compared with supported gold catalyst. First order kinetic

analyses of isopropanol decomposition reaction over these two catalysts, shown in Figure

4.01 and summarized in Table 4.4, support the notion that the 4%MgO-Al2O3 is the more

active of the two catalysts at lower temperature. However, the corresponding Arrhenius

plots indicate that the 4%MgO-Al2O3 and gold loaded 4%MgO-Al2O3 catalyst have an

approximate apparent activation energy values for the decomposition of isopropanol, as

112

listed in Table 4.5, and they have an approximate pre-exponential factor, reflecting the

similarity of the dispersion/density of the active sites in their samples.

Acetone and propylene were once again the main products formed under the

reactions’ conditions employed over these two catalyst systems. The rate formation results

of propylene and acetone are depicted in Figures 4.24 and 4.25, reveal that the rate

formation of propylene increases with increasing reaction temperature over 4%MgO-

Al2O3 catalyst with a slightly slower increase over the 1%Au/4%MgO-Al2O3, where the

latter catalyst exhibits a significantly lower selectivity for propylene than the former

catalyst. On the other hand, the rate formation of acetone was found to increase with

temperature over both catalysts, where again it increased at a slower rate over 4%MgO-

Al2O3 catalyst. Also, the 1%Au/4%MgO-Al2O3 catalyst was overall significantly more

selective for acetone formation than the 4%MgO-Al2O3 catalyst. Figures 4.26 and 5.27

represent the rate formation of propylene and acetone over 4%MgO-Al2O3 and

1%Au/4%MgO-Al2O3 catalysts at different reaction temperature and different

idsopropanol vapor pressure. It is noticed that the rate formation of both propylene and

acetone increase with the reaction temperature. The results obtained show that the

addition of magnesia enhanced the base character of the 4%MgO-Al2O3 catalyst and the

corresponding supported gold catalyst. The rate formation of propylene over 4%MgO-

Al2O3 catalyst was found to be predominate at 498 and 523 K, however, at 473 K the

dehydrogenation reaction of isopropanol to acetone was found to be the favored routs as

shown in Figures 4.26 and 4.27 and as listed in Table 4.4. These results probably due

to the interaction between water molecules, resulted from the dehydration process at

113

higher temperature, and the active sites on the catalyst surfaces which probably lead to the

formation of hydroxide groups on the catalyst surfaces responsible for the increase in the

acidic properties of the tested catalyst. On the other hand, as shown in Figure 4.26 and

4.27 and Table 4.4 the addition of gold nanoparticles enhanced the basic character of the

catalysts probably due to the electron transfer between the catalyst support and the

supported gold nanoparticles. The same behavior was observed for both catalysts at

higher vapor pressure of isopropanol at different reaction temperature as shown in Figure

4.28 and 4.29 and as listed in Table 4.5. These results are reflected by the values of the

corresponding propylene/acetone selectivity ratio, as listed in Table 4.5. Here, the values

of the propylene/acetone selectivity ratio of 1%Au/4%MgO-Al2O3 is very low compared

with those found for the 4%MgO-Al2O3 catalyst. In addition, and as listed in Table 4.5,

both of the 4%MgO-Al2O3 and its gold loaded equivalent exhibited significantly smaller

values of the propylene/acetone selectivity ratio at low temperature which reflect the base

character of both catalysts at low reaction temperature. It is noticed also that the values of

the propylene/acetone selectivity ratio of both catalysts were significantly lower than

those obtained over the γ-Al2O3 and its gold loaded equivalent. This only means that over

these two magnesia-containing catalysts, the dehydrogenation of isopropanol is more

predominate. Such a result can be traced to the role of magnesia in forming greater

density of basic sites in mixed MgO-Al2O3 catalytic supports. However, through the

addition of gold nanoparticles to the 4%MgO-Al2O3 support in this study, the base

catalytic properties of the resulting gold loaded 4%MgO-Al2O3 catalyst was enhanced and

became more stable under the reaction conditions used.

114

Table 4.5: First order reaction kinetic parameters for isopropanol conversion over

4%MgO-Al2O3 and 1%Au/4%MgO-Al2O3 catalysts

Sample T (K)

IP Vapor

pressure

(atm)

k'

(mole g-1

min-1

atm-1

)

Eb

(kJ/mol)

A

(mole g-1

min-1

atm-1

)

Pr/Ac

4%MgO-Al2O3

473.15

0.021

0.0024

96.72 7.40x107

0.2

0.030 0.35

0.042 0.25

498.15

0.021

0.0101

2.74

0.030 1.32

0.042 1.5

523.15

0.021

0.0226

3.8

0.030 7.3

0.042 11.9

1%Au/4%MgO-

Al2O3

473.15

0.021

0.0174

97.04 6.90x107

0.4

0.030 0.004

0.042 0.002

498.15

0.021

0.0094

0.017

0.030 0.23

0.042 0.23

523.15

0.021

0.0215

0.13

0.030 0.48

0.042 0.3

a-Apparent first order rate constant

b-Apparent activation energy



-1

115

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 20 40 60 80 100 120

2-p

rop

ano

l Dec

om

po

siti

on

Rat

e, m

mo

l g-1

min

-1

t(min)

Figure 4.21 : The rate of isopropanol decomposition as a function of time and reaction

temperatures: ●-523 k, ∎-498 k, ▲-473 k; and isopropanol vapor pressure of 0.021 atm,

over 1%Au/4%MgO-Al2O3 (dashed-line) and over 4%MgO-Al2O3 (solid-line).

116

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0 20 40 60 80 100 120

2-p

rop

ano

l Dec

om

po

siti

on

Rat

e, m

mo

l g-1

min

-1

t(min)


temperatures: ●-523 k, ∎-498 k, ▲- 473 k; and isopropanol vapor pressure of 0.030

atm, over 1%Au/4%MgO-Al2O3 (dashed-line) and over 4%MgO-Al2O3 (solid-line).

117

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0 20 40 60 80 100 120

2-p

rop

ano

l Dec

om

po

siti

on

Rat

e, m

mo

l g-1

min

-1

t(min)


temperatures: ●-523 k, ∎-498 k, ▲- 473 k; and isopropanol vapor pressure of 0.042

atm, over 1%Au/4%MgO-Al2O3 (dashed-line) and over 4%MgO-Al2O3 (solid-line).

118

0.00

0.05

0.10

0.15

0.20

0.25

0 20 40 60 80 100 120

Pro

pen

e Fo

rmat

ion

Rat

e,m

mo

l g-1

min

-1

t(min)

Figure 4.24: Propylene formation as a function of time and reaction temperatures: ●-523

k, ∎-498 k, ▲-473 k; and isopropanol vapor pressure of 0.021 atm, over 1%Au/4%MgO-

Al2O3 (dashed-line) and over 4%MgO-Al2O3 (solid-line)

119

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0 20 40 60 80 100 120

Ace

ton

e Fo

rmat

ion

Rat

e,m

mo

l g-1

min

-1

t(min)

Figure 4.25: Acetone formation as a function of time and reaction temperatures: ●-523 k,

∎-498 k, ▲-473 k; and isopropanol vapor pressure of 0.021 atm, over 1%Au/4%MgO-


111

0.00

0.05

0.10

0.15

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0.25

0.30

0 20 40 60 80 100 120

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pen

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mo

l g-1

min

-1

t(min)




111

0.00

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0.12

0.14

0.16

0.18

0 20 40 60 80 100 120

Ace

ton

e Fo

rmat

ion

Rat

e, m

mo

l g-1

min

-1

t(min)

Figure 4.27: acetone formation as a function of time and reaction temperatures: ●-523 k,



112

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113

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114

0

5

10

15

20

25

30

35

40

45

50

1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000

-x-(

1+(

Pto

t/P

IP))

ln(1

-x)

(m/F) ptot


473 k; over 1%Au/4%MgO- Al2O3 (dashed-line) and over 4%MgO-Al2O3 (solid-line).

115

4.7.5 The 8%MgO-Al2O3 and 1%Au/8%MgO-Al2O3 CATALYSTS

The slight increase in the amount of added magnesia from ~4.2% to 7.6% in the

MgO-γ-Al2O3 mixture had a profound effect on the catalytic activity of the resulting

8%MgO-Al2O3 support and its 1% gold supported equivalent. Figure 4.31 depicts the

activity of the 8%MgO-Al2O3 and 1%Au/8%MgO-Al2O3 catalysts in the isopropanol

decomposition reaction carried out at 473, 498, and 523 and at isopropanol vapor pressure

of 5.521 atm. The rate of decomposition of isopropanol on both catalysts was found to

decrease with time and increased with increasing reaction temperature. However, in this

case it is the 1%Au/8%MgO-Al2O3 catalysts that exhibited a higher catalytic activity than

that of the 8%MgO-Al2O3 at nearly all of the reaction temperatures used, with a greater

extent of activity enhancement at 523 K. Figures 4.32 and 4.33 depict the activity of the

8%MgO-Al2O3 and 1%Au/8%MgO-Al2O3 catalysts in the isopropanol decomposition

reaction carried out at 473, 498, and 523 and at isopropanol vapor pressure of 0.030 and

0.042 atm. The results show that the rate of isopropanol decomposition increased with

increasing isopropanol vapor pressure and the reaction temperature, respectively. As

indicated above, the 1%Au/8%MgO-Al2O3 catalysts exhibited a higher catalytic activity

than that of the 8%MgO-Al2O3 at nearly all of the reaction temperatures and vapor

pressure used, with a greater extent of activity enhancement at 523 K. The results of the

first order kinetic analyses of isopropanol decomposition reaction over these two

catalysts, shown in Figure 4.40 and summarized in Table 4.6, support the notion that the

1%Au/8%MgO-Al2O3 is the more active of the two catalysts. However, the corresponding

Arrhenius plots, shown in Figure 4.41, indicated that the 8%MgO-Al2O3 catalyst has an

116

overall significantly lower apparent activation energy value for the decomposition of

isopropanol, as listed in Table 4.6, but this low activation energy is counterbalanced by a

relatively lower pre-exponential factor, reflecting the lower dispersion/density of the

active sites in the sample. On the other hand, the 1%Au/8%MgO-Al2O3 catalyst still

exhibited a relatively low apparent activation energy value of ~64 kJ/mol, but displayed

more than tow orders of magnitude higher pre-exponential factor value; reflecting its

possession of greater active site density when compared to the 8%MgO-Al2O3 support.

Acetone and propylene were once again the main products formed under the

reactions’ conditions employed over these two catalyst systems with trace amounts of

isopropylether that disappeared after 30 min on stream. The rate formation of propylene

and acetone are depicted in Figures 4.34 and 4.35, reveal that the selectivity to propylene

increases with increasing reaction temperature over 8%MgO-Al2O3 catalyst. On the other

hand, the rate formation for propylene over the 1%Au/8%MgO-Al2O3 is significantly less

at all of the reaction conditions employed. Meanwhile, the rate formation for acetone over

1%Au/8%MgO-Al2O3 decreased slightly with the reaction temperature. On the other

hand, it is noticed that the rate formation of acetone over 8%MgO-Al2O3 decreased

significantly with increasing the reaction temperature. These results indicated that the

basic character of gold supported catalyst slightly affected with increasing the reaction

temperature as shown in Figure 4.35. On the other hand, a significant decrease in the

acidic properties of 1%Au/8%MgO-Al2O3 catalyst was observed. Figure 4.36 and 4.37

revealed that the presence of gold nanoparticles supported on 8%MgO-Al2O3 lead to a

significant change in the acid-base properties of this catalyst. While the acidic

117

dehydration reaction of isopropanol to propylene was predominate over 8%MgO-Al2O3

catalyst, the basic dehydrogenation of isopropanol to acetone over 1%Au/8%MgO-Al2O3

catalyst was highly significant. Similarly, Figures 4.38 and 4.39 show the rate formation

of propylene and acetone over these tow catalysts at different temperatures and at

isopropanol vapor pressure of 0,030 and 0.042 atm, respectively. These results show an

approximate behavior such as what was observed in the previous example in which an

increase in the rate formation of propylene with increasing the reaction temperature on

8%MgO-Al2O3 catalyst was observed. On the other hand, the rate formation of acetone

over this catalyst increased slight slower with increasing the reaction temperature over

this catalyst. Meanwhile, the rate formation of acetone over 1%Au/8%MgO-Al2O3

catalyst decreased slightly as the reaction temperature increased. These results show that

the basic active sites over 1%Au/8%MgO-Al2O3 catalyst slightly deactivated at higher

reaction temperature, while the acidic active sites were more significantly affected. This

may be due to the accumulation of carbonaceous particles over acidic active sites or due

to the adsorption of by products at higher temperature which lead to their poisoning.

These results are summarized by the selectivity propylene/acetone ratios obtained over

these catalysts where again it is noted that the addition of gold nanoparticles to the

8%MgO-Al2O3 support has enhanced the catalyst’s selectivity for acetone up to two

orders of magnitude larger than those of the other catalysts.

118

Table 4.6: First order reaction kinetic parameters for isopropanol conversion over and

over 8%MgO-Al2O3 1%Au/8%MgO-Al2O3 catalysts

Sample T

(K)

IP Vapor

pressure

(atm)

k'

(mole g-1

min-1

atm-1

)

Eb

(kJ/mol)

A

(mole g-1

min-1

atm-1

)

Pr/Ac

8%MgO-Al2O3

473

0.021

0.0051

58.5 94.26

0.30

0.030 0.25

0.042 0.46

498

0.021

0.0083

1.50

0.030 2.60

0.042 1.70

523

0.021

0.0164

4.30

0.030 2.60

0.042 3.00

1%Au/8%MgO-

Al2O3

473

0.021

0.006

64.02 4.80x104

0.007

0.030 0.013

0.042 0.001

498

0.021

0.0139

0.007

0.030 0.019

0.042 0.015

523

0.021

0.0283

0.070

0.030 0.130

0.042 0.103

a-Apparent first order rate constant

b-Apparent activation energy



-1

119

0.00

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0 20 40 60 80 100 120

2-P

rop

ano

l Dec

om

po

siti

on

Rat

e, m

mo

l g-1

min

-1

t(min)


temperatures: ●-523 k, ∎-498 k, ▲-473 k; and isopropanol vapor pressure of 0.021atm,


121

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l dec

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121

0.0

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0.1

0.2

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0.3

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0.4

0 20 40 60 80 100 120

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rop

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l Dec

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po

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on

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mo

l g-1

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-1

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122

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126

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127

0

10

20

30

40

50

60

1200 1700 2200 2700 3200

-x-(

1+

(Pto

t/P

IP))

ln(1

-x)

(m/F) ptot




128


473 k; over 1%Au/8%MgO- Al2O3 (dashed-line) and over 8%MgO-Al2O3 (solid-line).

129

Figure 4.41: Arrhenius Plots for ●-8%MgO-Al2O3 based catalysts, ∎- -Al2O3 based

catalysts, ⧫-4%MgO-Al2O3 based catalysts; with Au (dashed-line) and without Au (solid-

line).

CONCLUSION

0

1

2

3

4

5

6

1.85 1.90 1.95 2.00 2.05 2.10 2.15 2.20 2.25

ln[-

x-(1

+(P

tot/

PIP

))ln

(1-x

)]

1000*1/T K

Al2O3 1%Au/Al2O3 4%MgO-Al2O3 1%Au/4%MgO-Al2O3 8%MgO-Al2O3 1%Au/8%MgO-Al2O3

131

The above reaction results clearly point first of all to the role of magnesia when

added to -Al2O3 in influencing the catalysts’ selectivity for dehydrogenation of

isopropanol to acetone. Even though the presence of magnesia in a mixture with alumina

is known to reduce the alumna’s acidity, the addition of reduced gold nanoparticles to the

MgO-Al2O3 supports introduced a synergy that further enhanced and stabilized the

catalyst’s base-catalytic properties. This synergy was found in this study to become more

pronounced with increasing amounts of magnesia in the MgO-Al2O3 mixture, where the

absence of magnesia in the reduced 1%Au/-Al2O3 led to a much lower selectivity for

acetone. However, the presence of gold in a reduced state cannot be ignored either, since

the same catalysts exhibited a meager catalytic activity in the decomposition of

isopropanol when gold was in a partially reduced form [155].

131

CHAPTER 5


HYDROGENATION

5.1 INTRODUCTYION

Ethylene used for polymerization to polyethylene is produced by cracking of light

alkanes in a steam cracker. The ethylene stream has to be purified and one step in the

purification process is the selective hydrogenation of acetylene to ethylene.

Presence of acetylene in ethylene stream leads to poisoning of the polymerization

catalyst because acetylene adsorbs at the active sites for ethylene and blocks the

polymerization process. Therefore, the acetylene content in the ethylene feed has to be

reduced to the low ppm-range. Hydrogenation of acetylene in the presence of ethylene

requires high selectivity to ethylene to prevent hydrogenation of ethylene to ethane [159].

The following reactions occur in an acetylene hydrogenation reactor:

C2H2 + H2→ C2H4 (1) hydrogenation of acetylene to ethylene

C2H4 + H2→C2H6 (2) hydrogenation of ethylene to ethane

nC2H2 + 1/2nH2 → (C2H3)n (3) dimerisation and oligomerisation (n= 2, 3, 4, ..)

132

Main product of reaction (3) is 1,3-butadiene which can be further hydrogenated to so-

called C4-hydrocarbons: 1-butene, n-butane, cis- and trans-butene. C6-hydrocarbons and

the so called green oil of higher hydrocarbons are formed in small quantities [160-162].

Hydrocarbon decomposition with subsequent formation of carbonaceous deposits on the

catalyst surface lead to deactivation [163] . In recent years, several approaches have been

issued for developing new efficient catalysts for the selective hydrogenation of acetylene.

The Pd/SiO2 catalysts with different transitional-metal oxide promoter (TiO2, CeO2) have

been investigated by Moon et al; significant enhancements in the activity and ethylene

selectivity in acetylene hydrogenation have been reported [164,165]. J. Hong et al.

reported that the addithion of MgO promoter to Pd/TiO2 catalyst enhanced its catalytic

performance in acetylene hydrogenation and increased its stability [166].

In this study, selective hydrogenation of acetylene was investigated over gold-

based catalysts. Gold nanoparticles were supported on gama alumina and modified

magnesia-alumina supports. This modification is expected to enhance the ethylene

selectivity and the catalyst stability.

5.2 GOLD LOADING (ICP-MS), XRD, BET SURFACE AREA AND

TEM RESULTS

5.2.1 INDUCRTIVE COUPLED PLASMA ANALYSIS (ICP-MS)

The elemental composition of the catalysts is listed in Table 5.1 . It’s noticed that

the amount of supported gold is ~83% of the nominal synthesis amounts. This is a

consequence of the synthesis method employed where small part of gold partially reduce

133

at the synthesis temperature used leading to the some precipitation of metallic gold and its

separation from the homogenous gel [167].

5.2.2 X-RAY DIFFRACTION ANALYSIS (XRD)

X-ray diffraction analyses of the samples are depicted in Figure 5.1. It can be

noticed that all of the samples have basically the same x-ray diffraction features with a

broad diffraction peaks with maxima at 2 = 32° and 37.5°, belonging to Al2O3 single-

phase oxide, along with two other peaks at 46.0° and 67.0°, which are characteristic peaks

corresponding to the (220), (311), (400) and (440) reflections of -Al2O3 phase [172]. The

intensity of the latter set of peaks remains unchanged despite the addition of MgO. This

might be due to the homogenous dispersion of MgO into Al2O3 without the aggregation of

MgO nanoparticles, resulting in highly-dispersed particles of MgO-Al2O3 binary oxide

composites as a solid solution. The absence of any characteristic MgO reflections is

probably due to the mutual monolayer dispersion, which causes the XRD pattern of the

solid solution to be dominated by the component with the higher concentration [172]. The

gold containing samples have additional reflections occurring at 2 = 38°, 44.4°, 64.8°,

which are characteristic peaks corresponding to the (111), (200) and (220) reflections of

fcc gold [168]. The average particle size of gold nanoparticles was estimated by Scherrer

formula.

d =

(5.1)

It is found that the average particle size of metallic god nanoparticles on MgO-

Al2O3 support was significantly smaller (3.8-4.7 nm) than that supported on Al2O3

134

(7.7nm). On the other hand, it is found that the increase of gold loading to 4% produces

larger metallic gold nanoparticles with an average particle size of 8.5 nm as shown in

Table 5.1.

135

(a)

(b)

(c)

(d)

(e)

Au(1,1,1)

Au(2,0,0)

Au(2,20)

Au(3,1,1

)

Figure 5.1: XRD patterns of calcined (a) γ-Al2O3, (b) 1%Au/% γ-Al2O3, (c)

1%Au/15%%MgO-Al2O2, (d) 2%Au/15%MGO- -Al2O3, (e) 2%Au/15%MgO-Al2O3.

136

5.2.3 BET SURFACE AREA ANALYSIS

Nitrogen adsorption/desorption isotherms of the samples are shown in Figure

5.2. The isotherms formed are, according to the Brunauer, Deming, Deming, Teller

(BDDT) classification, of type IV with an H2 type (IUPAC) hysteresis attributed to the

presence disordered mesopores [169]. The main textural parameters of the samples are

reported in Table 5.1. It can be observed that the BET surface area decreases from ~240

to ~194 m2/g with the addition of MgO. it is found that the increase of gold content

slightly decreases the BET surface area. The total pore volume of the materials was also

found to decrease with the addition MgO. Mono-modal pore size distribution was

obtained for all of the catalysts, as shown in Figure 5.3. The average pore diameter of

1%Au/Al2O3 catalyst (4.7 nm) found to be larger than that of gold supported on MgO-

Al2O3 (3.7 nm), indicating of mesopore blockage by the magnesia additions.

137

Table 5.1: Textural properties and elemental analysis of calcined catalysts.

Auf

wt.%

Mge

wt.%

Au-PSf

(nm)

Vp

(cm3/g)

c

Db

(nm)

SAa

( m2/g)

Catalysts

5..0 - 7.7 5.02 4.0 245 1%Au/Al2O3

0.78 5.9 4.0 0.23 0.. 194 1%Au/15%MgO-Al2O3

1..1 9.5 3.8 0.23 3.4 201 2%Au/15%MgO-Al2O3

0.5 9.8 8.5 0.22 0.3 135 4%Au/15%MgO-Al2O3

a- BET surface Area

b- Average mesopore diameter calculated by BJH.

c- Total pore volume measured at P/Po=0.974.

e- Weight percentages of magnesium and gold as determined by ICP.

f- Average Au particle size as determined by XRD.

138

Figure 5.2: Nitrogen adsorption/desorption isotherms of calcined (a) 1%Au/Al2O3, (b)

1%/15%MgO-Al2O3, (c) 2%Au/15%MgO-Al2O2, (d) 4%Au/15%MgO-Al2O3.

139

0

0.005

0.01

0.015

0.02

0.025

0.03

0 10 20 30 40 50 60 70

Dv/

dD

(cm

-1g-1

)

Pore Width (Å)

1%Au/Al2O3

1%Au/15%MgO-Al2O3

2%Au/15%MgO-Al2O3

4%Au/15%MgO-Al2O3

Figure 5.3: Pore size distribution by BJH of adsorption branch of calcined 1%Au/Al2O3,

1%/15%MgO-Al2O3, 2%Au/15%MgO-Al2O2, and 4%Au/15%MgO-Al2O3.

141

5.2.4 TRANSMISSION ELECTRON MICROSCOPY (TEM)

The morphology and particle size of the reduced gold on the different supports

have been examined by TEM, as seen in Figure 5.4. The gold particles are seen as dark

contrasts on the surface of the supports. TEM micrographs show that there is a slight

difference in the average size of the metallic gold particles with the change of supports.

The measured size of the gold particles on the –Al2O3 support is found to be significantly

larger (~9.5 nm) than that found on the magnesia containing supports, of ~5.6 nm. This

results is in support of the XRD analysis presented earlier, in consistent with what has

been reported about the effects of support properties where gold particles size and their

size-distribution are reportedly governed mainly by different surface basic properties

[171]. Since magnesia is known to introduce more surface basic sites to the alumina

[170], it is expected that samples containing magnesia to have better gold particles size

distribution and smaller mean diameter [171].

141

Figure 5.4: TEM micrographs of (a) , (b) 1%Au/15%MgO-Al2O3 ;(c) , (d)

4%Au/15%MgO-Al2O3

.

142

5.3 NH3-TEMPERATURE PROGRAMMED DESORPTION (TPD)

NH3- temperature programmed desorption (TPD) analyses were carried out on

gold-based catalysts samples pre-treated with hydrogen for 30 min at 50 ºC. Figure 5.5

depicts the NH3-TPD profiles obtained for the catalysts. Table 5.2 lists the results

obtained from the NH3-TPD analysis of the catalysts. It is noticed that the temperature at

the maximum of the TPD peak is higher for 1%Au/Al2O3 catalyst and slightly lower for

magnesia containing supports. A significant decrease in the density of acid cites is

observed with the increase of gold. The density of the acid sites for both 1%Au/Al2O3 and

1%Au/15%MgO-Al2O3 catalysts found to be approximately similar. On the other hand,

the increase in the gold loading on magnesia containing catalysts has led to a significant

decrease in their respective acid site density, with the 1%Au/15%Mg-Al2O3 catalysts

possessing an acid site density 1.25 as large as that found for the 4%Au/15%Mg-Al2O3

catalyst. According to the data listed in Table 5.2, the acid sites density decreased as the

gold loading increases on magnesia containing supports.

143

Table 5.2 NH3 –temperature programmed desorption (TPD) analysis results of calcined

catalysts.

sample ID µmol NH3 g-1

Tmax (K)a

1%Au/Al2O3 4560.8 420

1%Au/15%MgO- Al2O3 4543.6 407

2%Au/15%MgO- Al2O3 3957.9 412

4%Au/15%MgO- Al2O3 3627.8 413

a- Temperature at desorption peak maximum

144

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

250 350 450 550 650 750 850

µm

ole

NH

3S-1

g-1ca

t

T,K

1%Au/Al2O3

1%Au/15%MgO-Al2O3

2%Au/15%MgO-Al2O3

4%Au/15%MgO-Al2O4

Figure 5.5: NH3-TPD profiles of hydrogen pre-treated catalyst; 1%Au/Al2O3,

1%Au/15%MgO- Al2O3, 2%Au/ 15%MgO- Al2O3, 4%Au/ 15%MgO-Al2O3.

145

5.4 ACETYLENE CATALYTIC HYDROGENATION

5.4.1 EFFECTS OF THE H2/C2H2 RATIO ON ACETYLENE HYDROGENATION

Hydrogenation of acetylene was carried out at different H2/C2H2 ratios and at

temperature of 523 K. Figure 5.6. depicts the conversion of acetylene at different

H2/C2H2 ratios over different gold supported catalysts. It was found that the conversion of

acetylene increased with increasing the H2/C2H2 ratio for all catalyst and reached the

maximum at H2/C2H2 ratio of approximately 20 at which the conversion was more than

95% for 1%Au/Al2O3, 2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 catalysts and

more than 85% for 1%Au/15%MgO-Al2O3 catalyst. It is noticed that the 1%Au/Al2O3 and

4%Au/15%MgO-Al2O3 possess the highest catalytic activity followed by

2%Au/15%MgO-Al2O3 and 1%Au/15%MgO-Al2O3 catalysts.

146

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25

Co

nve

rsio

n

H2/C2H2

1%Au/Al2O3

1%Au/15%MgO-Al2O3

2%Au/15%MgO-Al2O3

4%Au/15%MgO-Al2O3

Figure 5.6: Acetylene conversion over 1%Au/Al2O3, 1%Au/15%MgO-Al2O3,

2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 at different H2/C2H2 ratios and

temperature of 523 K.

147

The selectivity towards ethylene is depicted in Figure 5.7. It was found that the

selectivity towards ethylene was found to be around 30% at H2:C2H2 ratio of 2:1 then

jumped to approximately 70% at around H2:C2H2 ratio of 4:1. On the other hand, it is

noted that the selectivity towards ethylene remained constant around 60% despite the

increase in the H2/C2H2 ratio. The results shown in Figures 5.6 and 5.7 revealed the

presence of side reactions producing different by-products at higher conversion such as

1,3-butadiene, which can be further hydrogenated to so-called C4 hydrocarbons 1-butene,

n-butane, cis- and trans-butene; as well as C6-hydrocarbons; and green oil of higher

hydrocarbons [160-162]. These results showed no significant differences in the ethylene

selectivity between gold catalyst supported on γ-alumina or on modified magnesia-

alumina supports. Meanwhile, the effect of the gold loading on these supports on ethylene

selectivity was negligible. On the other hand, the presence of magnesia in magnesia-

alumina mixture supports have slightly affected the activity of these catalysts, especially

for acetylene conversion in which the conversion is depressed slightly over gold

supported on magnesia-alumina supports.

148

0

10

20

30

40

50

60

70

80

90

0 5 10 15 20 25

Eth

yle

ne

Se

lect

ivit

y

H2/C2H2

1%Au/Al2O3

1%!u/15%MgO-Al2O3

2%Au/15%MgO-Al2O3

4%Au/15%MgO-Al2O3

Figure 5.7: Ethylene selectivity over 1%Au/Al2O3, 1%Au/15%MgO-Al2O3,

2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 at different H2/C2H2 ratios and at

temperature of 523 K.

149


HYDROGENATION

The selectivity of acetylene hydrogenation and the conversion are calculated as

follows:

Selectivity (S) =

(5.2)

Conversion (τ) =

(5.3)

Figure 5.8. depicts the conversion of acetylene at H2/C2H2 molar ratio of 10 over

1%Au/γ-Al2O3, 1%Au/15%MgO-Al2O3, 2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-

Al2O3 catalysts. As shown in Figure 5.8, the conversion is enhanced with the increase of

the reaction temperature for all of the catalysts and reached about 90% for 1%Au/Al2O3 at

400oC. All of the catalysts show approximately similar catalytic activities at reaction

temperature higher than 150oC, however, at lower temperature they behave differently.

For instance, while the conversion of acetylene over 1%Au/Al2O3 catalyst was around

80% at 100oC, it was about 63% for 2%Au/15%MgO-Al2O3 catalyst and around 73% for

both 1%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 catalysts. The catalytic activity

for 1%Au/Al2O3 then decreased to about 64% at 150 oC which could indicate active sites

deactivation by accumulation of the intermediates or may be due to the dimerisation and

oligomerisation of olefins on the active sites. This phenomena was also shown over the

4%Au/15%MgO-Al2O3 catalyst but it was not significant over 1 and 2% gold supported

equivelents. On the other hand, the catalytic activity of all of the catalysts increased again

as the reaction temperature was increased to 200 oC, which could be due to either the

151

desorption of adsorbed intermediates or due to further hydrogenation of the adsorbed

materials to higher hydrocarbons that desorbed to the gas phase. Scheme 5.1 shows the

paths that the reaction may follow over the tested catalysts.

151

Path I + 2H

(g) 4H2C

Path IV

Path II

C4-hydocarmons (a) Green Oils (a)

C2H6 (g)

Path V

Ethylidyne (a)

C2H2(a) C2H4 (a)

C2H2 (g)

Scheme 5.1: The reaction paths of acetylene hydrogenation over the tested catalysts

Path III + 2H

152

40

50

60

70

80

90

100

50 100 150 200 250 300 350 400 450

Co

nve

rsio

n

T (oC)

1%Au/Al2O3

1%Au/15%MgO-Al2O3

2%Au/15%MgO-Al2O3

4%Au/15%MgO-Al2O3

Figure 5.8: Acetylene conversion over 1%Au/Al2O3, 1%Au/15%MgO-Al2O3,

2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 catalysts at different reaction

temperature.

153

The selectivity towards ethylene over these catalysts is shown in Figure 5.9. It is

noticed that the selectivity towards ethylene become significant at temperatures between

150 to 200 oC, at which point the selectivity reached around 56%. Beyond this

temperature, the selectivity towards ethylene decreased dramatically until it reached 26%

over 1%Au/Al2O3 catalyst and around 34% for the remaining catalysts. These results

revealed that the decrease in the ethylene selectivity at higher temperature is may be due

to the formation of C4- hydrocarbons, which is subsequently hydrogenated to green oil.

The decrease in the ethylene selectivity may also be due to dissociative adsorption of

ethylene resulting in the oligomerisation process [167]. It is noticed that the decrease in

the ethylene selectivity at higher temperature is not due to further hydrogenation to ethane

because the selectivity towards ethane decreased as well at higher temperature, as shown

in Figure 5.10. This type of by-products (green oil) was detected during the reaction

course especially over 1%Au/Al2O3 catalyst. It is noticed that the addition of magnesia to

the support increases the density of the basic active sites, which play an important role in

enhancing the selectivity towards ethylene and diminished the selectivity towards C4-

hydrocarbons.

154

0

10

20

30

40

50

60

70

50 100 150 200 250 300 350 400 450

Eth

yle

ne

Se

lect

ivit

y

T (oC)

1%Au/Al2O3

1%Au/15%MgO-Al2O3

2%Au/15%MgO-Al2O3

4%Au/15%MgO-Al2O3

Figure 5.9: Ethylene selectivity over 1%Au/Al2O3, 1%Au/15%MgO-Al2O3,


temperature.

155

In addition to the evaluation of ethylene selectivity, the selectivity towards

methane, ethane, and propylene was measured at different temperatures as shown in

Figures 5.10, 5.11, and 5.12. It is noticed that the selectivity towards methane does not

exceed 1% during the course of the reaction at different temperatures, which indicate that

the dissociative adsorption of acetylene molecules on the catalyst active sites was

insignificant. On the other hand, the selectivity towards ethane was significant compared

to methane and propylene. Figure 5.11 shows that the ethane selectivity increased with

increasing the reaction temperature from 100 to 150 oC at which it reached about 8%

then, decreased gradually with increasing the reaction temperature to reach about 2% at

400 oC. These results could be interpreted as follows:

I) At low temperatures between 100 to 150 oC, the desorption energy, which

is responsible for the ethylene desorption from the catalyst surfaces, was

insufficient; leading to the partial hydrogenation of the adsorbed ethylene

to ethane (path III) .

II) The alternative route is the formation of ethylidyne on the catalyst surface

which undergoes further hydrogenation to ethane (path V).

III) At higher temperature, the desorption energy for ethylene become

sufficient, which decreases the probability for the ethylene to undergo

further hydrogenation to ethane or to undergo other side reactions.

Figure 5.12 shows the propylene selectivity over these catalysts. It is noticed that the

propylene selectivity does not exceed 2% at all reaction temperature range used over

1%Au/15%MgO-Al2O3 and 2%Au/15%MgO-Al2O3 catalysts. However, the selectivity

156

towards propylene was slightly higher over 4%Au/15%MgO-Al2O3 catalyst than that of

the other catalysts.

157

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

300 350 400 450 500 550 600 650 700

Met

han

e Se

lect

ivit

y %

T K

1%Au/Al2O3

1%Au/15%MgO-Al2O3

2%Au/15%MgO-Al2O3

4%Au/15%MgO-Al2O3

Figure 5.10: The selectivity for methane over 1%Au/Al2O3, 1%Au/15%MgO-Al2O3,


temperatures.

158

0

2

4

6

8

10

12

14

50 100 150 200 250 300 350 400

Eth

ane

Sele

ctiv

ity

%

T K

1%Au/Al2O3

1%Au/15%MgO-Al2O3

2%Au/15%MgO-Al2O3

4%Au/15%MgO-Al2O3

Figure 5.11: The selectivity for ethane over 1%Au/Al2O3, 1%Au/15%MgO-Al2O3,


temperatures.

159

0

1

2

3

4

5

6

7

300 350 400 450 500 550 600 650 700

Pro

pyl

ene

Sel

ecti

vity

%

T K

1%Au/Al2O3

1%Au/15%MgO-Al2O3

2%Au/15%MgO-Al2O3

4%Au/15%MgO-Al2O3

Figure 5.12: The selectivity for propylene over 1%Au/Al2O3, 1%Au/15%MgO-Al2O3,


temperatures.

161

5.4.3 ACETYLENE HYDROGENATION IN THE PRESENCE OF ETHYLENE

After performing various studies in order to understand the effect of H2/C2H2

ratio, the influence of the reaction temperature, and the effect of the support type on the

catalytic activity, The selective hydrogenation of acetylene in the presence of ethylene

was also investigated. The selective catalytic reduction (SCR) of acetylene in the presence

of ethylene was performed in a similar manner as the hydrogenation of C2H2, using a feed

ratio of C2H2/C2H4/H2 = 1/1/10. The results are shown in Figure 5.13 in terms of C2H2

conversion versus reaction temperature in the presence and absence of C2H4. One general

characteristic of all the catalysts tested in the selective catalytic reduction of C2H2 is that

their conversion/activity shifted to higher temperature compared to C2H2 hydrogenation in

the absence of C2H2. Another observation is the conversion of acetylene over all of the

tested catalysts decreased slightly compared to that observed in the absence of C2H4. This

may be due to partial competition between acetylene and ethylene molecules on the active

sites which may affected the acetylene coverage over the catalyst surfaces. In addition, the

direct hydrogenation of ethylene was also studied for a better understanding of the

hydrogenation of C2H2 and C2H4 using a reactant feed C2H4/H2 = 1/10. The results

obtained from this test (not presented) show that the hydrogenation of ethylene to ethane

versus temperature was negligible over these catalysts. Hence, the key to high ethylene

selectivity when both C2H2 and C2H4 are hydrogenated could be the absence of adsorbed

C2H4 on the Au surfaces. It is clear that the adsorption of the triple bond is preferred over

the double bond and during SCR of C2H2 the two hydrocarbons do not compete for the

same adsorption sites [168]. The results also point to the conclusion that the rate

161

determining step during either C2H2 hydrogenation or selective catalytic hydrogenation of

C2H2 is the activation of hydrogen, most likely with the hydrocarbon associatively

adsorbed on the Au surfaces [168]. It was already shown that in the hydrogenation of

C2H2 a higher concentration of hydrogen improves the conversion of C2H2.

162

40

50

60

70

80

90

100

50 100 150 200 250 300 350 400 450

Co

nve

rsio

n

T oC

1%Au/Al2O3-A

1%Au/15%MgO-Al2O3-A

2%Au/15%MgO-Al2O3-A

4%Au/15%MgO-al2O3-A

1%Au/Al2O3-A+E

1%Au/15%MgO-Al2O3-A+E



Figure 5.13: C2H2 conversion versus temperature during selective catalytic

hydrogenation of acetylene (A) in the presence and absence of C2H4 (E). Reaction

condition 200 mg catalyst, 47 ml/min feed and C2H2/H2/C2H4 = 1/10/1

163


HYDROGENATION WITH TIME ON STREAM, CATALYSTS'

DEACTIVATION.

A general characteristic of all the catalysts active in C2H2 hydrogenation is

deactivation with time on stream. Catalysts based on Pd were extensively studied and

several models have been proposed that can explain the deactivation phenomena . Some

authors claim that during reaction a significant amount of “green oil” is formed, which is

partly related to the identity of the catalyst and reaction temperature [169]. On Au/CeO2,

Azizi et al. [170] found that CH4 and C2H6 are detected in the gas phase, together with

C2H4, while some other species are adsorbed on the surface.

The catalytic activity of the tested catalysts was measured at 250 oC and

atmospheric pressure using 200 mg of catalyst and a mixture gas of C2H2/H2 in a molar

ratio of 1/10 with a total flow rate of 47 ml/min balanced with N2 gas. Figure 5.14 depicts

the activity in C2H2 hydrogenation over 1%Au/γ-Al2O3, 1%Au/15%MgO-Al2O3,

2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 catalysts for about 20 hrs reaction

time. The results shown in Figure 5.12 compare the relative stabilities of these catalysts.

Despite of the different catalytic activity of these catalysts, the general characteristic that

could be noticed is their high stability during the course of reaction. As shown in Figure

5.14, the 1%Au/Al2O3 catalyst deactivated slightly after 600 min on stream where the

conversion decreased from 84% at 500 min to around 77% at the end of the test. The

deactivation of 1%Au/15%MgO-Al2O3 catalyst was observed also in which the

conversion decreased from 67% at the beginning of the measurement to reach 57% at the

164

end of the reaction. It is also noted that while the activity of 4%Au/15%MgO-Al2O3

catalyst remained constant during the course of the reaction, the catalytic activity of

2%Au/15%MgO-Al2O3 increased gradually from acetylene conversion of 53% at the

beginning of the reaction to reach 80% conversion at the end of the test. It should be

mentioned that Au/CeO2 catalyst deactivated significantly during the acetylene

hydrogenation in which more than 40% of its activity was lost within 300 min, as shown

in Figure 5.15 [170]. However, gold nanoparticles supported on either alumina or

modified magnesia-alumina supports showed significant catalytic stability during the 24

hours test.

165

0

10

20

30

40

50

60

70

80

90

0 200 400 600 800 1000 1200

Co

nve

rsio

n

Time (min)

1%Au/Al2O3

1%Au/15%MgO-Al2O3

2%Au/15%MgO-Al2O3

4%Au/15%MgO-Al2O3

Figure 5.14: C2H2 conversion on stream over 1%Au/Al2O3, 1%Au/15%MgO-Al2O3,

2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 catalysts. Reaction condition 0.2g


166

Fig. 5.15. Aging of the catalyst reactant mixture (R H2/C2H2 = 7) at 210 ◦C for 4 h. [170]

167

The selectivity towards ethylene, ethane, methane and propylene were also

monitored during the reaction period. The selectivity of these catalysts towards ethylene is

depicted in Figure 5.16. The results shown in Figure 5.16 indicated that the tested

catalysts are highly selective towards ethylene which is desirable in the industrial

processes of ethylene polymerizations. The results shown in Figure 5.16 show that the

selectivity towards ethylene is enhanced as the content gold loaded on the catalyst

increased, which describes the effect of gold on the catalytic character of the catalysts.

On the other hand, the ethylene selectivity on the 2%Au/15%MgO-Al2O3 and

4%Au/15%MgO-Al2O3 catalysts increased gradually with time until it reached

approximately 95% at the end of the reaction period. Meanwhile, the ethylene selectivity

remained constant around 65% over 1%Au/Al2O3 and 1%Au/15%MgO-Al2O3 catalysts.

These results could be due to the deactivation of active sites responsible for the formation

of (green oil) over 2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 catalysts during

the acetylene hydrogenation, which is reflected in the ethylene selectivity enhancement.

168

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200

Eth

yle

ne

Se

lect

ivit

y

Time (min)

1%Au/Al2O3

1%Au/15%MgO-Al2O3

2%Au/15%MgO-Al2O3

4%Au/15%MgO-Al2O3

Figure 5.16: Ethylene selectivity on stream over 1%Au/Al2O3, 1%Au/15%MgO-Al2O3,

2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 catalysts. Reaction condition 0.2g


169

Figure 5.17 depicts the selectivities for methane, ethane and propylene over these

catalysts. The selectivity for methane did not exceed 2.5% which was highest over the

1%Au/Al2O3 catalyst, as shown in Figure 5.17. While, the selectivity for ethane reached

20-25% for the case of 1%Au/15%MgO-Al2O3 and 2%Au/15%MgO-Al2O3 catalysts

while around 5% and 15% for 1%Au/Al2O3 and 4%Au/15%MgO-Al2O3, respectively, as

shown in Figure 5.18. Despite of the generally low selectivity of propylene over these

catalysts, its slightly higher selectivity over 1%Au/Al2O3 catalyst compared to the other

catalysts indicate the greeter tendency of this catalyst for higher hydrocarbon formations.

Figure 5.19 shows the selectivity of propylene on these catalysts. The results show that

the selectivity of propylene did not exceed 3% over 1%Au/Al2O3 where it was around

1.5% over the remaining catalysts.

171

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 200 400 600 800 1000 1200

Met

han

e Se

lect

ivit

y

Time (min)

1%Au/Al2O3

1%Au/15%MgO-Al2O3

2%Au/15%MgO-Al2O3

4%Au/15%MgO-Al2O3

Figure 5.17: The selectivity on stream for methane over 1%Au/Al2O3, 1%Au/15%MgO-

Al2O3, 2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 catalysts.

171

0

5

10

15

20

25

30

35

40

0 200 400 600 800 1000 1200

Eth

ane

Sele

ctiv

ity

Time (min)

1%Au/Al2O3

1%Au/15%MgO-Al2O3

2%Au/15%MgO-Al2O3

4%Au/15%MgO-Al2O3

(b)

Figure 5.18: The selectivity on stream for ethane over 1%Au/Al2O3, 1%Au/15%MgO-


172

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 200 400 600 800 1000 1200

Pro

pyl

en

e S

ele

ctiv

ity

Time (min)

1%Au/Al2O3 1%Au/15%MgO-Al2O3 2%Au/15%MgO-Al2O3 4%Au/15%MgO-Al2O3

Figure 5.19: The selectivity on stream for propylene over 1%Au/Al2O3, 1%Au/15%MgO-


173


HYDROGENATION WITH TIME ON STREAM IN THE PRESENCE OF

WATER IN STREAM, DEACTIVATION EVALUATION.

The catalytic activities of these catalysts were studied in the presence of water in

stream to investigate the effect of water on the acid-base properties of these catalysts on

the acetylene hydrogenation. The gas mixture, containing the same quantities of the

reactants reported in the experimental section, passed through a bubbler containing

deionized water set at ~ 5oC before entering the reactor. Figure 5.20 depicts the

catalytic activities of these catalysts in the presence of water in fed stream. The results in

Figure 5.20 show a significant effect caused by the presence of water on the catalysts

activities. The general observation for all of these catalysts is the steep deactivation they

exhibited during the course of reaction. After 35 min on stream the conversion of

acetylene dropped suddenly from about 90% to around 25%, which reflect the significant

deactivation of the catalyst active sites. On the other hand, the catalytic activities of tested

catalysts kept nearly constant conversion of 25% during the course of the reaction, as

shown in Figure 5.20. The water molecules may dissociatively adsorbed on the catalyst

and formed strong acidic active sites. The acidic active sites may enhanced the

accumulation of adsorbed hydrocarbons, which in turn poisoned and significantly

deactivated the catalysts.

174

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200 1400 1600

Co

nve

rsio

n

Time (min)

1%Au/Al2O3

1%Au/15%MgO-Al2O3

2%Au/15%MgO-Al2O3

4%Au/15%MgO-Al2O3

Figure 5.20: C2H2 conversion on stream over 1%Au/Al2O3, 1%Au/15%MgO-Al2O3,

2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 catalysts in the presence of H2O in

stream. Reaction condition 0.20 g catalyst, 47 ml/min feed and C2H2/H2 = 1/10

175

5.4.6 TEMPERATURE PROGRAMMED OXIDATIONT (TPO)

Temperature programmed oxidation (TPO) of the spent sample is shown in Figure

5.21. The results show three different desorption peaks for CO2. The first peak is located

at ~ 423 K which is attributed to the desorption of loosely CO2 adsorbed on the catalyst

surfaces because, there was no depletion in the O2 concentration on stream. The second

peak is located at around 723 K for 1%Au/Al2O3, and at around 650 K for

2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 catalyst. This peak could be

associated with the combustion of amorphous cokes, a precursor of graphitic carbon that

has a structure of oligomeric hydrocarbon, CxHy [170]. The third TPO peak is due to an

acidic carbon, the polymerization of which is facilitated by ionic sites. In general, this is a

graphitic carbon held firmly on the support. Its accumulation provokes considerable steric

hindrance, which reduces the availability of hydrogen and/or acetylene [170]. The results

show a significantly higher combustion temperature of this type of coke when adsorbed

on 1%Au/Al2O3 catalyst, 797 K. While the peak is found at around 756 K for

2%Au/15%MgO-Al2O3 and 4%Au/15%MgO-Al2O3 catalysts and about 736 K for

1%Au/15%MgO-Al2O3 catalyst. It can be said that the ionic sites on 1%Au/Al2O3 catalyst

have significantly facilitated the formation of acidic carbon on the catalyst surface.

Moreover, the results shown in Figure 5.21 and the value of the percent of surface carbon

listed in Table 5.3 reveal that higher content of coke over 1%Au/Al2O3 catalyst in

contrast to the other catalysts in which the basisity of magnesia-alumina supports could

have played an important role in inhibiting the acidic carbon formation by possessing

smaller number and/or weaker acid sites.

176

Table 5.3 CO2-Temperaure programmed oxidation (TPO) analysis results of spent

catalysts

sample ID μmol CO2 g-1

C % Tmax (K)a

1%Au/Al2O3 1050.724638 1.661225 797

1%Au/15%MgO-Al2O3 469.3567251 0.89829 736

2%Au/15%MgO-Al2O3 704.5714286 1.31764 755

4%Au/15%MgO-Al2O3 759.9099099 1.12026 757

a- Temperature at combustion peak maximum.

177

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

250 350 450 550 650 750 850 950

µm

ole

CO

2 S

-1g-1

cat

T,K

1%Au/Al2O3

1%Au/15%MgO-Al2O3

2%Au/15%MgO-Al2O3

4%Au/15%MgO-Al2O3

Figure 5.21: TPO for spent catalysts after aging under reaction mixture of H2/C2H2 = 10

for 24 hrs.

178

5.4.7 PROPPOSED MECHANISM OF C2H2 HYDROGENATION

Based on our results, we can propose a mechanism for the hydrogenation of

acetylene on Au based catalysts. This mechanism, illustrated in Scheme 6.2, involves

several features that explain the high selectivity toward ethylene and the formation of

methane, ethane, as well as propylene, along with the formation of carbon species that

may be harmful to the reaction. The high selectivity can be explained by the difference in

the strength of adsorption of acetylene onto the gold surface compared with ethylene. The

formation of ethane and a trace amount of propylene is an indication of further

hydrogenation of adsorbed ethylene as well as its dissociation into two adsorbed CH2

groups (surface carbene) which couple to adsorbed ethylene resulting in propylene

formation. Indeed, direct hydrogenation of ethylene did not occur over of all of the

catalysts, indicating that the adsorption of ethylene was unfavorable but as reported

above, subsequent hydrogenation of ethylene was noted in the case of adsorbed

acetylene. One possible explanation for the difference in adsorption between C≡C and

C=C bonds has been provided by a DFT study, which reported significant adsorption and

activation of only C≡C on Au nanoparticles [172]. Below 200 ◦C, ethylene desorbs from

the surface, but above 200 ◦C, it dissociates by carbon–carbon cracking to form what is

thought to be a surface carbene. This species would explain the formation of methane as

well as propylene. The reaction of the carbene with hydrogen is a function of the partial

pressure of hydrogen, because a high concentration of hydrogen was lead to favor the

formation of methane. In another reaction pathway, and at high reaction temperature,

carbene dehydrogenates and polymerizes to form green oil that poisons the reaction.

179

Scheme 6.2: Proposed reaction pathway for acetylene hydrogenation.

181

CHAPTER 6

CONCLUSION AND RECOMMENDATIONS

6.1 CONCLUSION

The results obtained indicate that the presence of magnesia and reduced gold

nanopartilces has imparted the catalysts, 1%Au/4%Mg-Al2O3 and 1%Au/8%Mg-Al2O3,

with significant base-catalytic properties. These properties apparently stem from the

formation of comparable and large amounts of acid and base sites on the surface of these

two catalysts, where the base-catalytic properties seems to emanate from a concerted

mechanism in the between these two catalytic sites in the decomposition of isopropanol.

While the reason behind the formation of the large and comparable density of acid-base

sites can be clearly traced to a synergy between magnesia and reduced gold that has led to

the formation of relatively stronger acetone-selective catalytic sites.

We have undertaken an extensive study of C2H2 hydrogenation in the presence and the

absence of C2H4 over Au-based catalysts. A number of parameters were varied: H2/C2H2

ratio, the gold loading, the reaction temperature effects, the effect of water on stream.

The deactivation of the catalysts during C2H2 +H2 was also studied in detail.

Our findings are summarized below:

181

- The conversion of acetylene enhanced with increasing the H2/C2H2 ratio and

reached about 96% at H2/C2H2 ratio of 20.

- The acetylene conversion enhanced with increasing the reaction temperature and

reached about 95% at 400ºC.

- The selectivity towards C2H4 reached a maximum value ,~ 60%, between 150-200

ºC and, beyond this temperature, the selectivity towards C2H4 decreased

dramatically.

- The hydrogenation of acetylene in the presence of ethylene indicated that, the

adsorption of the triple bond is preferred over the double bond and during SCR of

C2H2 the two hydrocarbons do not compete for the same adsorption sites.

- Gold nanoparticles supported on either alumina or modified magnesia-alumina

supports showed significant catalytic stability during the 24 hours test.

- The results obtained revealed that the higher content of coke over 1%Au/Al2O3

catalyst in contrast to the other catalysts in which the basisity of magnesia-

alumina supports could have played an important role in inhibiting the acidic

carbon formation by possessing smaller number and/or weaker acid sites.

6.1 RECOMMENDATIONS

The following recommendations might be made for future related studies:

- The effects of different calcination temperatures on the particle size distribution

and catalytic activities.

182

- The effects of different pre-treatment of the catalysts on the catalytic activities and

selectivities.

- Modification of Al2O3 support with other additives such as K2O3, La2O3 and SiO2

- The effects of gold loading at lower contents

- The hydrogenation and oxidation of carbon monoxide over this type of catalysts.

183

y = 5E-09x - 4E-05 R² = 0.9989

0.00E+00

2.00E-05

4.00E-05

6.00E-05

8.00E-05

1.00E-04

1.20E-04

1.40E-04

1.60E-04

1.80E-04

2.00E-04

0 10000 20000 30000 40000 50000

nu

mb

er

of

mo

les

peak area

APPENDIX

Figures A-1, A-2 and A-3 depicts GC-calibration curves for isopropanol,

acetone and propylene, respectively. These calibration curves were established at the

beginning of this study to be used for isopropanol, acetone and propylene determination,

respictively.

Figure A-1: Isopropanol calibration curve.

184

y = 6E-09x - 0.0001 R² = 0.9957

0.E+00

2.E-04

4.E-04

6.E-04

8.E-04

1.E-03

1.E-03

0.E+00 5.E+04 1.E+05 2.E+05 2.E+05 3.E+05

nu

mb

er

of

mo

les

peak area

Figure A-2: Acetone calibration curve.

185

y = 1E-09x - 4E-05 R² = 0.9927

0.0E+00

5.0E-05

1.0E-04

1.5E-04

2.0E-04

2.5E-04

3.0E-04

3.5E-04

4.0E-04

4.5E-04

0.E+00 1.E+05 2.E+05 3.E+05 4.E+05 5.E+05

nu

mb

er

of

mo

les

Peak Area

Figure A-3: propylene calibration curve.

186

y = 3E-12x + 5E-09 R² = 0.9979

0.E+00

1.E-07

2.E-07

3.E-07

4.E-07

5.E-07

6.E-07

7.E-07

0.0E+00 5.0E+04 1.0E+05 1.5E+05 2.0E+05 2.5E+05

mo

le/M

eth

ane

Peak Area

Figures A-4, A-5, A-6, A-7, and A-8 depicts GC-calibration curves for methan,

ethylene, acetylene , ethane and propylene respectively. These calibration curves were

established at the beginning of this study to be used for isopropanol, acetone and

propylene determination, respictively.

Figure A-4: The calibration curve for methane.

187

Figure A-6: The calibration curve for ethylene.

y = 5E-12x - 9E-09 R² = 0.9984

0

1E-08

2E-08

3E-08

4E-08

5E-08

6E-08

7E-08

0.E+00 2.E+03 4.E+03 6.E+03 8.E+03 1.E+04 1.E+04 1.E+04 2.E+04

mo

l/Et

hyl

ene

Peak Area

188

y = 3E-12x - 6E-09 R² = 0.9959

0.E+00

2.E-08

4.E-08

6.E-08

8.E-08

1.E-07

1.E-07

1.E-07

0.E+00 5.E+03 1.E+04 2.E+04 2.E+04 3.E+04 3.E+04 4.E+04 4.E+04 5.E+04

mo

l/A

cety

len

e

Peak Area

Figure A-7: The calibration curve for acetylene.

189

y = 5E-12x - 5E-10 R² = 0.9992

0

5E-10

1E-09

1.5E-09

2E-09

2.5E-09

3E-09

3.5E-09

4E-09

4.5E-09

0 200 400 600 800 1000 1200

mo

l/Et

han

e

peak area

Figure A-8: The calibration curve for ethane.

191

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211

VITA

Abdullah Abdoh Ahmed Manda

Born in Yemen, 13/6/1974

Nationality: Yemeni

Received B.Sc. degree in chemistry from Baghdad University in July 1996.

Worked as a staff member in Ibb university from 1997-1999.

worked as laboratory instructor in Ibb university from 1999-2002.

Joined King Fahd University of Petroleum & Minerals in August 2002.

Received Master of Science degree in Chemistry from King Fahd University of

Petroleum & Minerals in August 2007

Received PhD in Science, Physical Chemistry, from King Fahd University of

Petroleum & Minerals in August 2012.

Present address: King Fahd University of Petroleum & Minerals, Dhahran, Saudi

Arabia, 21361 P. O. Box 1208

Permanent address: Ibb University – Ibb-Yemen

E-mail [email protected]

[email protected]

Tel: 00966508181834

00967711150422

mailto:[email protected]

mailto:[email protected]

211

TABLE OF CONTENTS

CHAPTER 1 ............................................. Error! Bookmark not defined.

INTRODUCTION ..................................... Error! Bookmark not defined.

1.1 SUPPORT ACID-BASE CHARACTER. ................................... Error! Bookmark not defined.

1.2 ISOPROPANOL CONVERSION .............................................. Error! Bookmark not defined.

1.3 ACETYLENE HYDROGENATION. ........................................ Error! Bookmark not defined.

1.4 THE STUDY OBJECTIVES.. ................................................... Error! Bookmark not defined.


LITRATURE REVIEW............................. Error! Bookmark not defined.

2.1 CATALYSIS AND CATALYSTS. ............................................. Error! Bookmark not defined.

2.2 BRIEF REVIEW ON Au CATALYSIS. .................................... Error! Bookmark not defined.

2.3 GOLD AS A WORKING CATALYST. ..................................... Error! Bookmark not defined. 2.3.1 THE PREPARATION METHOD…………. .................. Error! Bookmark not defined.

2.3.2 PARTICLE SIZE EFFECT …………… ......................... Error! Bookmark not defined.

2.3.3 THE ROLE OF THE SUPPORT/ADDITIVES……….Error! Bookmark not defined.

2.3.4 THE NATURE OF THE Au ACTIVE SITET………….. Error! Bookmark not defined.

2.4 ISOPROPANOL CONVERSION .............................................. Error! Bookmark not defined.

212


………………………………………………………………Error! Bookmark not defined. 2.5.1 HYDROGEB ADSORPTION…………. .................... Error! Bookmark not defined.

2.5.2 ACETYLENE ADSORPTION AND HYDROGENATION……… Error! Bookmark

not defined.

2.5.3 FACTORS AFFECTING THEW ACETYLENE HYDROGENATION………….

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SAMPLE PREPARATTION AND EXPERIMENTAL METHODSError!


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3.2 SUPPORTS………………….. ................................................... Error! Bookmark not defined. 3.2.1 GAMA-ALUMINA…………. .................................... Error! Bookmark not defined.

3.2.2 MAGNESIA-ALUMINA MIXTURE………….. ............. Error! Bookmark not defined.

3.2.3 GOLD SUPPORTED CATALYSTS……………. ....... Error! Bookmark not defined.

3.2.3.1 Using Aluminum Iso-propoxide Precursor (Al(OCH2CH2CH3)3)…………. . Error!


3.2.3.2 Using Boehmite Precursor ( AlO(OH).2H2O)…………………………………..

Error! Bookmark not defined.

1%Au/Al2O3 catalyst .................................................. Error! Bookmark not defined.

1%Au/15%MgO-Al2O3 and Other Gold Loadings ...... Error! Bookmark not defined.

3.3 CHARACTERIZATION OF SUPPORTS AND CATALYSTS

…………………………………..Error! Bookmark not defined. 3.3.1 ELEMENTAL ANALYSIS………….. ....................... Error! Bookmark not defined.

3.3.2 X-RAY POWDER DIFFERACTION, XRD…………… ..........Error! Bookmark not

defined.

3.3.3 TRANSMISSION ELECTRON MICROSCOPY (TEM)………….Error! Bookmark

not defined.

3.3.4 BET TOTAL SURFACE AREA…………… ................... Error! Bookmark not defined.

3.3.5 TEMPERATURE PROGRAMMED REDUCTION MEASUREMENTS

(TPR)………Error! Bookmark not defined.

3.3.6 TEMPERATURE PROGRAMMED OXIDATION (TPO)………..Error! Bookmark

not defined.

213

3.3.7 TEMPERATURE PROGRAMMED DESORPTION (TPD)…………. .............. Error!


3.3.8 CO2-FT-IR ANALYSIS………….. ............................. Error! Bookmark not defined.

3.4 CATALYTIC REACTION……….. ........................................... Error! Bookmark not defined. 3.4.1 ISOPROPANOL CONVERSION……………. ........... Error! Bookmark not defined.

3.4.2 ACETYLENE HYDROGENATION……………. ....... Error! Bookmark not defined.



CONVERSION ......................................... Error! Bookmark not defined.

4.2 GOLD LOADING (ICP-MS), XRD, BET SURFACE AREA AND TEM RESULT. ....... Error! Bookmark not defined.

4.2.1 INDUCTIVE COUPLED PLASMA MASS SPECTROSCOPY ANALYSIS (ICP-

MS)…………………………………………………………………………………………………………………………………..


4.2.2 X-RAY DIFFRACTION ANALYSIS…………… ....... Error! Bookmark not defined.

4.2.3 BET SURFACE AREA ANALYSIS………….. .......... Error! Bookmark not defined.

4.2.4 TRANSMISSION ELECTRON MICROSCOPYT (TEM)…………… .......... Error!


4.3 CO2 TEMPERATURE PROGRAMMED DESORPTION (TPD).Error! Bookmark not defined.

4.4 NH2 TEMPERATURE PROGRAMMED DESORPTION (TPD).Error! Bookmark not defined.

4.5 HYDROGEN TEMPERATURE PROGRAMMED REDDUCTION (TPR). .. Error! Bookmark not defined.

4.6 CO2-FT-IR SPECTROSCOPY MEASUREMENTS.. ................ Error! Bookmark not defined.

4.7 CATALYSTS' ACID-BASE FVALUATION: ISOPROPANOL CONVERSION. ........... Error! Bookmark not defined.

4.7.1 INTRODUCTION……… .......................................... Error! Bookmark not defined.

4.7.2 ISOPROPANOLL CATALYTIC CONVERSION……… ........Error! Bookmark not

defined.

4.7.3 The γ-Al2O3 and 1%Au/γ-Al2O3 CATALYSTS………. Error! Bookmark not defined.

4.7.4 The 4%MgO-Al2O3 and 1%Au/4%MgO-Al2O3 CATALYSTS……… ........... Error!


4.7.5 The 8%MgO-Al2O3 and 1%Au/8%MgO-Al2O3 CATALYSTS……….. .......... Error!



214


HYDROGENATION ................................. Error! Bookmark not defined.

5.1 INTRODUCTYION…………… ................................................ Error! Bookmark not defined.

5.2 GOLD LOADING (ICP-MS), XRD, BET SURFACE AREA AND TEM RESULTS... .... Error! Bookmark not defined.

5.2.1 INDUCRTIVE COUPLED PLASMA ANALYSIS (ICP-MS)……… ............. Error!


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5.2.3 BET SURFACE AREA ANALYSIS…………. ........... Error! Bookmark not defined.

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5.3 NH3-TEMPERATURE PROGRAMMED DESORPTION (TPD).. .......... Error! Bookmark not defined.

5.4 ACETYLENE CATALYTIC HYDROGENATION................... Error! Bookmark not defined. 5.4.1 EFFECTS OF THE H2/C2H2 RATIO ON ACETYLENE

HYDROGENATION…….Error! Bookmark not defined.


HYDROGENATION……………………………………………… ......Error! Bookmark not

defined.

5.4.3 ACETYLENE HYDROGENATION IN THE PRESENCE OF ETHYLENE……...

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HYDROGENATION WITH TIME ON STREAM, CATALYSTS' DEACTIVATION.



HYDROGENATION WITH TIME ON STREAM IN THE PRESENCE OF WATER

IN STREAM, DEACTIVATION EVALUATION………………. .....Error! Bookmark not

defined.

5.4.6 TEMPERATURE PROGRAMMED OXIDATIONT (TPO)………… ........... Error!


5.4.7 PROPPOSED MECHANISM OF C2H2 HYDROGENATION………… ........ Error!



CONCLUSION AND RECOMMENDATIONS ....... Error! Bookmark not

defined.

215

6.1 CONCLUSION…………………………………………………………………………………… Error! Bookmark not defined.

6.1 RECOMMENDATIONS............. ............................................... Error! Bookmark not defined.

APPENDIX ............................................... Error! Bookmark not defined.

References ................................................. Error! Bookmark not defined.

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