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Catalytic Hydrogenation and Hydrodesulfurization of Model Compounds Haiyan Zhao Dissertation submitted to the faculty of Virginia Polytechnic Institute and State University in partial fulfillment of the requirement of the degree of Doctor of Philosophy In Chemical Engineering S. Ted Oyama, Chairman Luke Achenie David F. Cox James Tanko March 19th, 2009 Blacksburg, Virginia Key words: HDS, ROP, HYD, 2MT, H 2 storage, Bimetallic, Phosphides, EXAFS, FTIR Copy right 2009, Haiyan Zhao

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Page 1: Catalytic Hydrogenation and Hydrodesulfurization of Model ... · ix List of Tables Table 1.1 Summary of Recent Advances in Hydroprocessing with Sulfides 5 Table 1.2 Summary of Process

Catalytic Hydrogenation and Hydrodesulfurization of Model Compounds

Haiyan Zhao

Dissertation submitted to the faculty of Virginia Polytechnic Institute and State University in partial fulfillment of the requirement of the degree of

Doctor of Philosophy In

Chemical Engineering

S. Ted Oyama, Chairman

Luke Achenie

David F. Cox

James Tanko

March 19th, 2009

Blacksburg, Virginia

Key words: HDS, ROP, HYD, 2MT, H2 storage, Bimetallic, Phosphides, EXAFS, FTIR

Copy right 2009, Haiyan Zhao

Page 2: Catalytic Hydrogenation and Hydrodesulfurization of Model ... · ix List of Tables Table 1.1 Summary of Recent Advances in Hydroprocessing with Sulfides 5 Table 1.2 Summary of Process

Catalytic hydrogenation and hydrodesulfurization of model compounds

Haiyan Zhao

Abstract

This dissertation describes two related studies on hydrogenation and

hydrodesulfurization of heterocyclic S-containing compounds.

Alkyl substituted thiophenes are promising candidates for hydrogen carriers as the

dehydrogenation reactions are known to occur under mild conditions. Four types of

catalysts including supported noble metals, bimetallic noble metals, transition metal

phosphides and transition metal sulfides have been investigated for 2-methylthiophene

(2MT) hydrogenation and ring opening. The major products were tetrahydro-2-

methylthiophene (TH2MT), pentenes and pentane, with very little C5-thiols observed.

The selectivity towards the desired product TH2MT follows the order: noble metals >

bimetallics > phosphides > sulfides. The best hydrogenation catalyst was 2% Pt/Al2O3

which exhibited relatively high reactivity and selectivity towards TH2MT at moderate

temperatures. Temperature-programmed desorption (TPD) of hydrogen indicated that

the H2 desorption amount was inversely related to the rate of TH2MT formation.

Temperature programmed reaction (TPR) experiments revealed that pentanethiol became

the major product, especially with HDS catalysts like CoMoS/Al2O3 and WP/SiO2, which

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iii

indicates that poisoned or modified conventional HDS catalysts would be good

candidates for further 2MT hydrogenation studies.

The role of tetrahedral Ni(1) sites and square pyramidal Ni(2) sites in Ni2P

hydrotreating catalysts was studied by substitution of Ni with Fe. The Fe component was

deemed as a good probe because Ni2P and Fe2P adopt the same hexagonal crystal

structure, yet Fe2P is completely inactive for hydrodesulfurization (HDS). For this

purpose a series of NiFeP/SiO2 catalysts were prepared with different Ni:Fe molar ratios

(1:0, 3:1, 1:1, 1:3, and 0:1) and investigated in the HDS of 4,6-dimethyldibenzothiophene

at 300 and 340 oC. The uniformity of the NiFe series was demonstrated by x-ray

diffraction analysis and by Fourier transform infrared (FTIR) spectroscopy of adsorbed

CO. The position of substitution of Fe was determined by extended X-ray absorption

fine structure (EXAFS) analysis. It was found that at 300 oC the HDS activity of the

catalysts decreased with increasing Fe content and that this could be explained by the

substitution of Fe at the more active Ni(2) sites. As temperature was raised to 340 oC,

the activity of the Fe-containing samples increased, although not to the level of Ni2P, and

this could be understood from a reconstruction of the NiFe phase to expose more Ni(2)

sites. This was likely driven by the formation of surface Ni-S bonds, which could be

observed by EXAFS in spent samples.

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iv 

 

 

 

 

 

To my parents

for their constant support and love

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Acknowledgement  

I am extremely grateful to my advisor and mentor Dr. S. Ted Oyama for his guidance and

continued words of encouragement. Thank you for preparing me to think independently, to be a

serious research scientist and to seek the perfection and art of science and engineering. Your

advice will be my fortune in my life. I also greatly appreciate the advice and support from my

committee members Dr. David Cox, Dr. James Tanko and Dr. Luke Achenie.

I greatly appreciate the friendships and supports of present and past group members.

Thank you for providing me this unique research environment to help me grow: Dr. Pelin

Hacarlioglu, Dr. Travis Gott, Jason Gaudet, Dr. Hankwon Lim, Dr. Yungeng Gu, Dr. Yuying

Shu and Dr. Yong-Kul Lee. I am also very thankful to Yujung Dong and John Brooks as an

extended support from Dr Cox’s lab. I would also like to thank the Chemical Engineering

Department staff, Riley Chan, Chris Moore, Diane Cannaday, Mike Vaught and Tina Kirk for

their constant support and help during the past five years.

I can never express enough my sincere love and gratitude to my parents for their

unconditional love, encouragement, and sacrifices in my life and studies. Thank you all for

always being there and being my pillar stone.

Finally, special thanks to my English teachers and mentors, Johnny and Nancy McCord,

for their friendship and big hearts. Special thanks also to Jay and Michele Lester in ICF and

friends in BBC bible study group to help me with my life here and also grow spiritually.

 

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Table of content

Chapter 1

Introduction

1.1 Oragnosulfur model compounds 1

1.2 Sulfur containing model compounds reaction mechanism 2

1.3 State of art for catalytic materials and alternative hydrodesulfurization processes 4

1.4 Goals 10

1.5 Dissertation overview 12

Reference 14

Chapter 2

Transition metal phosphides

2.1 General properties 28

2.2 Synthesis of transition metal phosphides 31

2.3 Catalytic properties in hydrotreating 33

2.4 Kinetic pathways in HDS and in HDN 41

2.5 Bimetallic phosphide systems 49

2.6 Conclusions 53

Reference 54

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vii

Chapter 3

Hydrogen Storage Using Heterocyclic compounds: The Hydrogenation of 2-

Methylthiophene

3.1 Introduction 63

3.2 Experimental 68

3.2.1 Materials 68

3.2.2 Metal phosphides synthesis 69

3.2.3 Characterization 70

3.2.4 Reactivity Studies 71

3.2.5 Temperature Programmed Desorption (TPD) and Temperature Programmed

Reaction (TPR) 72

3.3 Results and discussion 73

3.3.1 CO Chemisorption and O2 Chemisorption 73

3.3.2 Reactivity 75

3.3.3 TPD and TPR 84

3.3.3.1 TPD of H2 84

3.3.3.2 TPR of 2MT and H2 87

3.4 Conclusions 92

Reference 97

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viii

Chapter 4

Nature of Active sites in Ni2P Hydrotreating Catalysts as Probed by Iron

Substitution

4.1 Introduction 101

4.2 Experimental 103

4.2.1 Materials 103

4.2.2 Bimetallic Phosiphides Catalysts Synthesis 104

4.2.3 Charaterization 105

4.2.4 Reactivity Studies 107

4.2.5 CO-FTIR 108

4.3 Results and discussion 109

4.3.1 TPR 109

4.3.2 XRD 111

4.3.3 CO Chemisorption and BET Results 112

4.3.4 Infrared Spectroscopy of Adsorbed CO 114

4.3.5 Reactivity Study 117

4.3.6 EXAFS 122

4.4 Conclusions 139

Reference 140

Chapter 5 conclusions 148

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List of Tables

Table 1.1 Summary of Recent Advances in Hydroprocessing with Sulfides 5

Table 1.2 Summary of Process Improvements, New Processes and Catalyst Developments

6

Table 2.1 Physical Properties of Metal-Rich Phosphides 29

Table 2.2 Synthesis of Phosphides 32

Table 2.3 Characteristics of 12 wt% Ni2P/SiO2 Catalysts 34

Table 2.4 Comparison of Rate Constants for Sulfides and Phosphides (573 K and 20.4 atm)

43

Table 2.5 Correlation Between Active Sites and Catalytic Performance 51

Table 3.1 Summary of Catalyst Characterization by CO Chemisorption 74

Table 3.2 Summary of Catalyst Characterization by O2 Chemisorption 74

Table 3.3 Effect of Pressure on the Activity of 0.5% and 2% Pt/Al2O3 80

Table 3.4 Comparison of Active Site Determinations 86

Table 3.5 Product Distribution for TPR of 2MT 88

Table 4.1 Ni2P, Fe2P Lattice Parameter (nm) 111

Table 4.2 d space of the three strongest peaks of XRD for NiFeP/SiO2 samples 112

Table 4.3 Characterization of NiFeP/SiO2 Samples 113

Table 4.4 Infrared Spectrum Data for Reduced Transition Metal Phosphides and Bimetallic Phosphides

117

Table 4.5 The Conversions and Selectivities of the Silica-supported Nickel Phosphides and Nickel Iron Phsphides at 613 K and 3.1 Mpa with Feed b after 110h on Stream.

120

Table 4.6 Ni K-edge EXAFS Best-fit Values for Ni2P/SiO2 127

Table 4.7a Ni K-edge EXAFS Best-fit Values for NiFeP(3:1)/SiO2 129

Table 4.7b Fe K-edge EXAFS Best-fit Values for NiFeP(3:1)/SiO2 130

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Table 4.8a Ni K-edge EXAFS Best-fit Values for NiFeP(1:1)/SiO2 131

Table 4.8b Fe K-edge EXAFS Best-fit Values for NiFeP(1:1)/SiO2 132

Table 4.9a Ni K-edge EXAFS Best-fit Values for NiFeP(1:3)/SiO2 133

Table 4.9b Fe K-edge EXAFS Best-fit Values for NiFeP(1:3)/SiO2 133

Table 4.10 Comparison of Different Models Based on Ni(1) and Ni(2) Substitution for Ni K-edge EXAFS

134

Table 4.11 Fe K-edge EXAFS Best-fit Values for Fe2P/SiO2 134

List of Figures

Figure 1.1 Relationship between the reactivity and the size of sulfur containing model compounds

2

Figure 2.1 Triangular prism and tetrakaidecahedral structures in phosphides 30

Figure 2.2 Crystal Structures of Metal Rich Phosphides 30

Figure 2.3 HDS Activity vs. time at 613 K and 3.1 MPa 35

Figure 2.4 EXAFS Spectra of Fresh Samples 36

Figure 2.5 EXAFS Spectra of Spent Samples 38

Figure 2.6 a) In Situ EXAFS Cell (Lengths in mm) b) EXAFS Data at Reaction 40

Figure 2.7 Reaction Network for 4,6-DMDBT Desulfurization 41

Figure 2.8 In Situ FTIR Spectra of Pyridine on Ni2P and NiMo 45

Figure 2.9 Reaction Network for 2-Methylpiperidine HDN 47

Figure 2.10 Relative Concentrations and Selectivity in the HDN of 2-Methylpiperidine

47

Figure 2.11 Scheme for the HDN of 2-Methylpiperidine 48

Figure 2.12 Structure of Ni2P showing P (left) and Ni (right) Coordination 50

Figure 3.1 Catalytic activity as a function of temperature 76

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xi

a) 2% Pt/Al2O3 b) 2% PdPt (1/4)/SiO2 c) WP/SiO2 d) CoMo/Al2O3

Figure 3.2 Reactions of Catalysts in the Hydrogenations of 2-Methylthiophene

a) Turnover Frequency of 2MT Reaction

b) Rate of Formation of TH2MT

c) Conversion of 2MT

d) Selectivity to TH2MT

77

Figure 3.3 Comparison of theoretical equilibrium constants for HDS and HYD 79

Figure 3.4 TPD of H2 profile 84

Figure 3.5 Relation between turnover frequency and H2 TPD desorption 87

Figure 3.6 Product Distribution for TPR of 2MT 89

Figure 3.7 TPR of 2MT on 2% PdPt/SiO2 profiles 93

Figure 3.8 TPR of 2MT on 2%Pt/Al2O3 profiles 94

Figure 3.9 TPR of 2MT on CoMo/ Al2O3 profiles 95

Figure 3.10 TPR of 2MT on WP/SiO2 profiles 96

Figure 4.1 Ni2P crystal structure with specifications of tetrahedral Ni(1) and pyramidal Ni(2)

103

Figure 4.2

Mass 18 signal from temperature programmed reduction of Ni2P, Fe2P and NiFeP samples

109

Figure 4.3 Mass 18 signal from temperature programmed reduction of Ni2P, Fe2P and NiFeP samples

112

Figure 4.4 Infrared spectra of adsorbed CO on reduced Ni2P/SiO2, NiFeP (3:1)/SiO2, NiFeP (1:1)/SiO2, NiFeP (1:3)/SiO2 and Fe2P/SiO2

115

Figure 4.5 Activity test in HDS of 4,6-DMDBT for Ni2P/SiO2, NiFeP(3:1)/SiO2 and NiFeP(1:1)/SiO2

119

Figure 4.6 Comparison of Fourier Transforms of the Ni K-edge EXAFS spectra for bulk Ni2P, fresh Ni2P/SiO2, fresh NiFeP(3/1)/SiO2, fresh NiFeP(1/1) /SiO2, fresh NiFeP(1/3) /SiO2, bulk NiO

122

Figure 4.7 Cross sectional schematic model of phosphide crystallites in Ni2P/SiO2 and NiFeP(1:1)/SiO2. Blue ball for Ni(2), red ball for phosphorus, yellow ball for sulfur, grey ball for Fe.

124

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xii

Figure 4.8 Comparison of Fourier Transforms of the Fe K-edge EXAFS spectra for fresh NiFeP(3:1)/SiO2, fresh NiFeP(1:1) /SiO2, fresh NiFeP(1:3) /SiO2, fresh Fe2P:SiO2, bulk FeO, Bulk FeS.

125

Figure 4.9 (Left) Ni K-edge EXAFS spectra (symbols) and model (line) from NiFeP/SiO2 samples. (right) Magnitude of the Fourier transform of the Ni K-edge spectra (symbols) and model(line)

128

Figure 4.10 a) Ni K-edge EXAFS spectra for fresh and spent NiFeP(1:1)/SiO2 b) Fe K-edge EXAFS spectra for fresh and spent NiFeP(1:1)/SiO2 c) Ni K-edge difference spectra d) Fe K-edge difference spectra

137

List of Schemes

Scheme 1.1 Thiophene reaction pathways 3

Scheme 1.2 Simplified reaction network for 4,6-DMDBT HDS 3

Scheme 1.3 Simplified sulfur-specific BDS of DBT 8

Scheme 1.4 Simplified network for ODS of DBT 9

Scheme 3.1 Dehydrogenation of aromatic compounds 65

Scheme 3.2 N,N’-dimethyldihydrobenzimidazole dehydrogenation 66

Scheme 3.3 Hydrogenation and dehydrogenation of N-ethyl carbazole 67

Scheme 3.4 Dehydrogenation of 2-pentanethiol 67

Scheme 3.5 Hydrogenation and ring-opening of 2-methythiophene 68

Scheme 3.6 Hydrodesulfurization reaction mechanism for 2-methylthiophene 81

Scheme 3.7 Hydrogenation mechanism for 2-methylthiophene 82

Scheme 3.8 SN2 mechanism for 2MT ring-opening 90

Scheme 3.9 E2 mechanism for 2MT ring-opening 90

Scheme 3.10 Metal hydride ring-opening of 2MT 90

Scheme 3.11 2MT reaction network 91

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xiii

List of Abbreviation

• BDS – biodesulfurization

• BET – Brunauer-Emmett-Teller

• BPy – Pyridine on Bronsted acid sites

• BT – benzothiophene

• CVD – chemical vapor deposition

• 3,3’- DMBP – 3,3’-dimethylbiphenyl

• 3,3’- DMBCH (DMBCH) – 3,3’- dimethylbicyclohexane

• 3,3’ – DMCHB (MCHT) – 3,3’- dimethylcyclohexylbenzen

(methylcylochexyltuluene)

• 4,6 – DMDBT – 4,6- dimethyldibenzothiophene

• DBT – dibenzothiophene

• DDS – direct desulfurization pathway

• E2 – bimolecular elimination

• EXAFS – extended X-ray absorption fine structure

• FTIR – Fourier transform infrared

• GC – gas chromatography

• HDN – hydrodenitrogenation

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xiv

• HDS – hydrodesulfurization

• HYD – hydrogenation pathway

• 2MT – 2-methylthiophene

• ODS – oxidative desulfurization

• ROP – ring opening

• SN2 – nucleophilic bimolecular substitution

• TH2MT – tetrahydro-2-methylthiophene

• TOF – turnover frequency

• TPD – temperature-programmed desorption

• TPR – temperature-programmed reduction / temperatre-programmed

reaction

• TKD – tetrakaidecahedral

• UAOD – ultrasound-assisted oxidative desulfurization

• UHP – ultrahigh purity

• UPC – ultrapure carrier

• XRD – X-ray diffraction

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1

Chapter 1

Introduction 1.1 Organosulfur model compounds

Organosulfur compounds have been drawing attention because of continuing

interest in hydroprocessing of high-boiling petroleum fractions and increasingly stringent

air-quality regulations. Concerning regulations, in 2006 the US sulfur contents in fuels

were mandated to be 30 ppm (with an 80 ppm cap) in gasoline and 15 ppm in diesel [1],

and similar levels were legislated in Europe and Japan [2,3,4]. By 2008, the standards

effectively require diesel to reach 10 ppm S content and every blend of gasoline sold in

the United States to meet the 30 ppm level [5]. Such levels correspond to the removal of

+99.99 % of sulfur from a typical crude containing 1.5 % sulfur, and the removal process

has been termed deep or ultra-deep HDS. Recent research in HDS has been directed to

the study of thiophenic compounds because they are the least reactive organosulfur

compounds in fossil fuels. The sulfur compounds have been studied extensively

including thiols, sulfides, thiophene and alkylthiophenes and benzothiophenes [6,7,8].

As the number of the rings and methyl substituents is increased, the reactivity of the

sulfur compounds from mercaptans to alkyl derivated dibenzothiophenes is greatly

reduced (Fig. 1.1). 4,6-dimethyldibenothiophene is one of the most refractory sulfur

compounds.

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2

Figure 1.1. Relationship between the reactivity and the size of sulfur containing model compounds

1.2 Sulfur containing model compounds reaction mechanism

Sulfur removal occurs either with or without hydrogenation of the heterocyclic

ring. As shown in Scheme 1.1, the HDS of thiophene follows two parallel reaction

pathways [9]. One pathway is direct desulfurization to get 1,3-butadiene. The other is

hydrogenation of thiophene to form tetrahydrothiophene and then followed by

desulfurization.

S

S

S

Increase in size and HDS difficulty

Rel

ativ

e re

actio

n ra

te

mercaptans

R-SH , R-S-S-R

S

S

S

S

thiophenes (T)

T with R at C2/C5

benzothiophenes (BT)

BT with R at C2/C7

dibenzothiophenes (DBT)

DBT with R at C4

DBT with R at C4/C6

gasoline range

jet fuel range

dieselrange

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3

S

SHYD

DDS

Scheme 1.1. Thiophene reaction pathways

The HDS of bigger molecule 4,6-DMDBT also proceeds through two reaction

pathways (Scheme 1.2). The first pathway is the direct desulfurization route (DDS) by

direct hydrogenolysis of the C-S bonds without prior hydrogenation of either phenyl ring

to yield 3,3’-dimethylbiphenyl (3,3’-DMBP). The second pathway is the hydrogenation

route (HYD) in which 4,6-DMDBT is first hydrogenated to the intermediates 4,6-

dimethyl-tetrahydro-DBT (4H-DMDBT) or 4,6-dimethyl-hexahydro-DBT (6H-DMDBT).

The hydrogenated intermediate 6H-DMDBT is subsequently desulfurized to 3,3’-

dimethylcyclohexylbenzene (3,3’-DMCHB) and 3,3’-dimethylbicyclohexane (3,3’-

DMBCH) by further hydrogenation [Error! Bookmark not defined.,10 ,11].

Scheme 1.2. Simplified reaction network for 4,6-DMDBT HDS

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1.3 State of art for catalytic materials and alternative hydrodesulfurization processes

Commercial catalysts for HDS are sulfides based on Mo or W and promoted

with Ni or Co. The subject has been covered in several treatises [12,13-29], and deep

HDS in other reviews [30-33]. More recent coverage of deep desulfurization has been

made in reviews by Song [1], Ho [34] and Babich and Moulijn [35] and has been a

principal topic in recent conferences [36,37,38] and the subject of dedicated journal

volumes [39-44]. As summarized below, the approaches taken have consisted in

optimizing sulfide compositions, using new supports, exploring novel compounds, and

improving processes (Table 1.1).

Various process options for removal of sulfur including standard hydroprocessing

and also alternative and emerging processes such as catalytic distillation, reactive

adsorption, polar molecule adsorption, selective adsorption, selective extraction, selective

alkylation, membrane separation, and caustic extraction were reviewed comprehensively

by Song [1]. In addition to these processes, there are also biodesulfurization, oxidative

desulfurization and ultrasound-assisted oxidative desulfurization [45] (Table 1.2).

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Table 1.1. Summary of Recent Advances in Hydroprocessing with Sulfides

Improvement of existing sulfides Review: Improving dispersion and morphology + Chiannelli et al. [46] Sonochemical and CVD methods for high dispersion + Moon [47,48], Okamoto [49,50], Ramos [51] Ultrasonic spray pyrolysis for high dispersion + Suslick [52] Microwave method for high dispersion + Liu et al. [53,54,55]

Dispersing agents + Yoshimura [56], Okamoto[57], Escobar [58], Costa [59], Lélias [60], Rana [61,62]

Activation agents + Perot [63], Frizi [64]

Use of non-traditional elements + Fierro [65,66], Hubaut [67], Vít [68], Escalona [69], De Los Reyes [70], Giraldo [71], Centeno [72]

New precursors with Mo-S bonds + Ishihara [73], Ho [74], Bensch [75,76], Cruz-Reyes [77]

Urea-matrix combustion method + Green [78,79,80,81]

Use of unsupported trimetallic compositions + Fuentes [82], Alonso [83], Huirache-Acuña [84,85], Nava [86]

Alternative or improved supports for sulfides

Improvement of Al2O3 with Ti, and Ga + Segawa [87,88], Ramirez [89,90], Vrinat [91,92], Zhao [93]

Improvement of Al2O3 with B + Okamoto [94,95,96], Shimada [97], Centeno [98] Improvement of Al2O3 with B and P + Ferdous [99] F- or PO4

3- bind to Al3+ ions and reduce interactions - Prins [100], Maity [101], Ding [102], Moon [103] Basic additives (K, Li) are beneficial + Diehl [104], Fan [105], Mizutani [106]

Composite supports (SiO2-Al2O3) + van der Meer [107], Mochida [108], Kunisada [109], Ancheyta [110]

Composite supports (TiO2-SiO2, ZrO2-Al2O3) + Zhou, Zhang [111], Li [112] Pure TiO2 supports for FeMo catalysts + Kraleva [113] Hydroxyapatite modified by Zr and/or Al + Travert [114,115,116]

Carbon nanotubes, mesoporous clays o Chen [117], Chuan [118], Dalai [119], Shang [120], Song [121,122]

Mesoporous supports (MCM-41, SBA-15, Ti-HMS) + [123], [124], [125], [126], [127], [128], [129], [130], [131], [132], [133], [134], [135], [136], [137]

+ Positive effect - Negative effect o Neutral or not applicable

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Table 1.2. Summary of Process Improvements, New Processes and Catalyst Developments

Industrial process and catalyst improvements

Review: Recent developments in industry o Babich and Moulijn [35]

Inhibition of deep HDS by H2S and NH3

-

[138],[139],[140], Vrinat [141], Farag [142], Nørskov [143], Rana [144,145], Perot [146], Song [147], Zeuthen [148], Ho [149]

Removal of H2S and NH3 from intermediate stages + Adjaye [150], Nava [151], Mochida [152,153]

Biodesulfurization (BDS)

+

Liu [154], Towfighi [155], Kong [156], Xu [157], Huizhou [158], Mohebali [159], Bassi [160], Li [161,162]

Oxidative desulfurization (ODS)

+

Wang [163], Song [164], Sampanthar [165], Corma [166], Lu [167], Green [168], Collins [169], García-Gutiérrez [170,171]

Ultrasound-assisted oxidative desulfurization (UAOD) + Doraiswamy [172], Yen [173,174,175], Zhao [176],

Nontraditional catalytic materials (carbides and nitrides) Carbides, nitrides

O

Rodriguez[177,178], Bussell [179], Djéga-Mariadassou [180,181], Al-Megren [182], Zhang [183]

Carbides, nitrides O Djega-Mariadassou [184], Nelson[185], Nagai [186,187], Dalai [188]

Review: Carbides and nitrides for hydroprocessing O Furimsky [189] + Positive effect - Negative effect o Neutral or not applicable

An insightful review of novel sulfur removal processes was recently given by Ito and

van Veen [190]. For refractory molecules, the oxidative removal of sulfur from diesel by

biodesulfurization, oxidative desulfurization and ultrasound-assisted oxidative

desulfurization has been covered extensively in the literature (vide supra). Instead of

reducing sulfur compounds to form H2S, these methods oxidize the sulfur species to their

corresponding sulfoxides (1-oxides) and sulfones (1,1-dioxides). Refractory sulfur

compounds such as DBT and 4,6-DMDBT is only slightly more polar than hydrocarbons

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7

of similar structure. However, sulfoxides and sulfones are substantially more polar, thus

permitting their selective removal by a combination of selective oxidation and solvent

extraction or solid adsorption. The development of oxidation methods for sulfur removal

is aimed at providing efficient, cost effective, environmentally benign processes

alternative to conventional hydroprocessing.

Another option for the removal of sulfur is biocatalytic desulfurization, or

biodesulfurization (BDS). Microorganisms require sulfur for growth and several bacteria

can utilize the sulfur in thiophenic compounds and thus reduce the sulfur content in

petroleum. BDS generally operates under ambient conditions of temperature and

pressure and proceeds mainly through two biological pathways. These pathways include

partial or complete degradation of the molecule with C-C bond cleavages or a sulfur-

specific cleavage of only C-S bonds. The more selective sulfur-specific pathway is

preferable to retain the value of fuels. Scheme 1.3 illustrates the simplified sulfur-

specific enzymatic pathway for the BDS of DBT in the presence of oxygen and water to

yield 2-hydroxybiphenyl as non-degradable end-product. Although many bacteria can

utilize sulfur through the sulfur-specific pathway, stability and life-time of the

biocatalysts are two major concerns. In addition, cooling of the petroleum feedstock to

ambient temperature and subsequent removal of the biocatalyst from the treated feed is

costly. Implementation of BDS as an industrial process would require microorganisms

with higher sulfur removal activity, hydrocarbon phase tolerance, removal ability at high

temperatures and longer stability [160].

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Scheme 1.3. Simplified sulfur-specific BDS of DBT

The most promising alternative method for achieving deep HDS is sulfur removal

via oxidative desulfurization (ODS). Analogous to BDS, the divalent sulfur atoms of

thiophenic molecules are oxidized by the electrophilic addition of oxygen atoms to yield

sulfones. The sulfones are subsequently removed in a second step by solid adsorption,

selective extraction or distillation. Various studies on ODS have employed K2FeO4 [163],

molecular oxygen [164], air [165], organic hydroperoxides [166], aqueous H2O2 [167-

171] and many other reagents as oxidizers. However, oxidation of sulfur compounds in

the absence of catalysts is slow and several studies have utilized phase-transfer catalysts

[163], metal salts [164,168], supported metal catalysts [165,170,171], metal-containing

molecular sieves [166], activated carbon [167], metal-ligand activators [169], etc. to

accelerate or facilitate the oxidation. Scheme 2 shows the simplified ODS scheme for

DBT to DBT sulfone through the intermediate DBT sulfoxide. The ODS method offers

several advantages over conventional hydroprocessing. The oxidation reactions can be

SO

SO

O2

Enzyme 1

H2O Enzyme 2

OH

O-

SO

O

OH

DBT DBT sulfone

Hydroxybiphenyl sulfonate2-hydroxybiphenyl

- SO42-

Enzyme 3desulfinase

-

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carried out under mild conditions of temperature and pressure and do not require the

expenditure of valuable hydrogen. However, the ODS process requires large amounts of

oxidizing agent, separation and recovery of the catalysts and low selectivity and activity

towards the sulfur compounds. Both BDS and ODS require long reaction times and the

second separation step in the processes leads to expensive waste management issues and

oil yield losses of 10-20 % [191].

S

OS

O

oxidation

O

S

oxidation

- SO2

Δ

DBT DBT sulfoxide

DBT sulfoneBiphenyl

Scheme 1.4. Simplified network for ODS of DBT

Future employment of ODS processes will be dependent on the sulfur removal

efficiency and capital investment required for implementation. The use of aqueous H2O2

– solid catalyst ODS is a low cost and environmentally benign option. However, such

processes are typically impractical, requiring long reaction times and generally result in

poor sulfur removal efficiency and excessive decomposition of the oxidant. In such

biphasic oil-water systems, introduction of phase-transfer catalysts and ultrasonication in

conjunction with ODS can be used to greatly increase the overall sulfur removal

efficiency [172]. Ultrasound has a mechanical effect on the reaction system by

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promoting rapid mixing, accelerating dissolution and renewing the surface of the solid

catalyst. Several studies have shown improvement in the ODS process upon ultrasound

irradiation including + 99 % sulfur removal efficiency in shorter reaction times [173,175],

complete recovery of solid catalysts [174] and minimal decomposition of H2O2 [173].

Although oxidative desulfurization processes are viable alternatives they require capital

investment for new unit operations. As a result, discovery of novel catalysts and

implementation into existing hydrotreating reactors is the most promising option for

achieving deep HDS.

1.4 Goals

This dissertation will deal with two related subjects, hydrogenation and

hydrodesulfurization of the sulfur-containing compounds, studied with a class of

nontraditional catalysts, transition metal phosphide.

First, hydrogenation will be explored as a means of storing hydrogen in aromatic

organic molecules, which can serve as energy-dense and efficient hydrogen carrier. For

this purpose the hydrogenation of 2-methylthiophene (2MT) will be studied, because it

has been shown that its hydrogenation products can be easily dehydrogenated. Four

groups of catalysts including bimetallic noble metals, noble metals, transition metal

sulfides and phosphides were evaluated for the hydrogenation and ring-opening of 2MT.

Second, hydrodesulfurization (HDS) of another aromatic heterocyclic, 4,6-

dimethyldibenzothiophene, will be studied with the objective of producing clean fuels.

HDS of 4,6-dimethyldibenzothiophene is one of the most refractory because of the steric

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hindrance. New materials, including the bimetallic compounds NiFeP will be studied and

compared to commercial sulfide catalysts.

In order to achieve these goals the following tasks have been carried out:

• Synthesis of WP/SiO2, Ni2P/SiO2, MoP/SiO2, PdP/SiO2, bimetallic catalysts (1/4)

PdPt/SiO2, (4/1) PdPt/SiO2 and (4/1) PdRu/SiO2 for the hydrogenation study of 2-

methylthiophene (2MT), a series of NiFeP/SiO2 to be used for hydrodesulfurization

of 4,6-dimethyldibenzothiophene (4,6-DMDBT)

• Characterization of the synthesized supported transition metal phosphides and

bimetallic catalysts with CO chemisorption, the commercial catalysts transition metal

sulfides MoS/Al2O3, NiMoS/Al2O3, CoMoS/Al2O3 with irreversible chemisorption of

O2 at dry-ice acetone temperature, BET surface area measurements and temperature-

programmed reduction

• X-ray diffraction measurements to identify the structure of the synthesized

catalysts WP/SiO2, Ni2P/SiO2, MoP/SiO2, PdP/SiO2, NiFeP/SiO2, bimetallic catalysts

(1/4) PdPt/SiO2, (4/1) PdPt/SiO2 and (4/1) PdRu/SiO2

• Hydrogenation activity measurements for 2MT over WP/SiO2, Ni2P/SiO2,

MoP/SiO2, PdP/SiO2, bimetallic catalysts (1/4) PdPt/SiO2, (4/1) PdPt/SiO2 and (4/1)

PdRu/SiO2, the commercial catalysts transition metal sulfides MoS/Al2O3,

NiMoS/Al2O3, CoMoS/Al2O3

• Temperature programmed desorption (TPD) and temperature programmed

reaction (TPR) for Pt/Al2O3, PdPt/SiO2, WP/SiO2, and CoMoS/Al2O3 from the four

groups consisting of noble metals, bimetallics and transition metal phosphides and

sulfides to provide insight into the relationship between hydrogen activation and the

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hydrogenation activities of the catalysts and elucidate the reaction mechanism over

the supported catalysts

• Hydrodesulfurization of 4,6-DMDBT activity measurements over NiFeP/SiO2

• X-ray absorption spectra measurements over NiFeP/SiO2

1.5 Dissertation Overview

Chapter 1 describes the sulfur-containing compounds, advances in

hydroprocessing catalysts and process, objectives and dissertation overview.

Chapter 2 provides a review of the physical and catalytic properties of transition

metal phosphides in hydrotreating reactions and related reaction mechanisms in HDS and

background for this research.

Chapter 3 presents a hydrogenation and ring opening study of 2-methylthiophene

as a hydrogen carrier. Best performance catalysts are selected from four groups of

catalysts totally accounting for 16 catalysts. Temperature-programmed desorption (TPD)

of hydrogen reveals that H2 desorption amount was inversely related to the rate of

tetrahydro-2-methylthiophene formation. Temperature-programmed reaction (TPR)

showed that pentanethiol was the major product on the 2MT preadsorbed surface. A

reaction scheme for 2MT is proposed.

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Chapter 4 presents a study on the nature of active sites in Ni2P hydrotreating

catalysts using iron as a probe in a form of novel bimetallic phosphides. There are two

types of Ni sties in the alloy. The HDS activity is evaluated for this type of bimetallic

phosphides. Various characterizations including XRD, FTIR and EXAFS are carried out

to determine the substitution position of iron.

Chapter 5 presents the conclusion of this research.

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Chapter 2

Transition Metal Phosphides

2.1 General Properties

Intensive research effort has concentrated on optimizing current commercial metal

sulfides, using novel supports, exploring new compounds and improving processes. Alternatives

to the metal sulfides are transition metal carbides, nitrides and phosphides. Metal phosphides

recently have received extensive attention due to their high activity and stability in the HDS and

HDN of petroleum feeds and the subject has been reviewed [1]. Phosphorus reacts with most

elements of the periodic table to form a diverse class of compounds known as phosphides. The

bonding in these materials ranges from ionic for the alkali and alkaline earth metals, metallic or

covalent for the transition elements, and covalent for the main group elements. The focus of this

review concerns the metal-rich compounds, MP or M2P, of the transition metals, which have

metallic properties. The phosphorus-rich compositions are semiconducting and are considerably

less stable than the metal-rich compounds.

The nature, structure and synthesis of phosphides have been described in a number of

reviews [2,3,4]. Basically, the metal-rich phosphides have physical properties similar to those of

ordinary metallic compounds like the carbides, nitrides, borides and silicides. They combine the

properties of metals and ceramics, and thus are good conductors of heat and electricity, are hard

and strong, and have high thermal and chemical stability (Table 2.1) [5].

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Table 2.1. Physical Properties of Metal-Rich Phosphides

Ceramic Properties Metallic Properties

Melting point

K 1100 - 1800

Electrical resistivity

μΩ cm 900 – 25,000

Microhardness

kg/mm-2 600 - 1100

Magnetic

susceptibility

106 emu/mol

110 – 620

-Heat of formation

kJ mol-1 30 - 180

Heat capacity

J/mol K 20 – 50

Although the physical and chemical properties of phosphides resemble those of carbides

and nitrides, they differ substantially in their crystal structure. In the carbides and nitrides, the

carbon and nitrogen atoms reside in the interstitial spaces between metal host atoms to form

relatively simple lattices. For the phosphides, however, the atomic radius of phosphorus (0.109

nm) is substantially larger than that of carbon (0.071 nm) or nitrogen (0.065 nm) and P does not

fit into the ordinary octahedral holes formed by closed-packed metal atoms. For this reason in

phosphides (also borides, sulfides, and silicides) the metal atoms form triangular prisms (Fig. 2.1)

where the metal atoms (dark atoms) surround the nonmetal atom (light atoms) [2]. For metal-

rich compositions the number of nearest-neighbors increases to form 9-fold tetrakaidecahedral

(TKD) coordination with additional metal atoms placed near the centers of the vertical faces of

the prism (Fig. 2.1).

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Figure 2.1. Triangular prism and tetrakaidecahedral structures in phosphides

Different arrangements of these building blocks give rise to different structures. A

summary is given in Fig. 2.2 [6].

Figure 2.2. Crystal Structures of Metal Rich Phosphides

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The monophosphide MoP is isostructural with WC, with the nonmetal-containing prisms

stacked on top of each other. The monophosphide VP has the Ni-As structure with the P-prisms

displaced laterally one-half a lattice spacing. The monophosphides NbP and TaP adopt the

closely related NbAs structure, which just differs from VP in the way the prisms are stacked.

The monophosphides of groups 6-10 adopt the MnP and NiP structures which have distorted

NiAs structures where the phosphorus atoms form chains (MnP) or pairs (NiP). Importantly, the

phosphides, unlike the sulfides, do not form layered structures, and so potentially permit greater

access to active corner and edge sites on the crystallite surfaces. The globular morphology of

MoP [7] and Ni2P [8] has been nicely demonstrated by electron microscopy by the group of

Bussell. Phosphides are different from phosphorus-promoted sulfide catalysts, as reviewed by

Iwamoto and Grimblot [9].

2.2 Synthesis of transition metal phosphides

Many techniques have been applied to the synthesis of transition metal phosphides

including combination of metal and phosphorus elements [ 10 - 14 ], disproportionation of

phosphides [15,16], metathesis [17,18,19,20], fused salt electrolysis [13,21], hydrogen reduction

of phosphate[22, 23], chemical vapor deposition of organometallic precursors [24], and reactions

with phosphine [14]. The generalized reactions involved with these methods are summarized in

Table 2.2. Several recent studies have investigated novel synthesis methods for unsupported and

supported transition metal phosphides. These include metal thiophosphates [25] and amorphous

alloys [26] as precursors, citric acid modified precursors [27,28], polymer surfactant assisted

synthesis [29], solvothermal synthesis methods [30,31], solution-phase arrested precipitation [32]

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and novel reducing agents [33]. These methods were aimed at producing monodispersed, high

surface area unsupported phosphides and improving the dispersion of supported phosphides.

Table 2.2. Synthesis of phosphides

Method Reaction

Combination of the elements M0 + xP0(red) MPx

Disproportionation of phosphides MP2 + M 2MP

Metathesis MClx + Na3P ) MP + NaCl

Fused salt electrolysis MOx + NaPOy MP +Na2O

Reduction of the phosphate MPOx + H2 MP + xH2O

Chemical vapor deposition M-P-R MP + PH3 + R

Reaction with phosphine MClx + PH3 MP + HCl + H2

A first study on simultaneous HDS and HDN showed that MoP could be easily

synthesized by temperature- programmed reduction (TPR) of a metal phosphate precursor [22],

and this has been confirmed for other transition metal phosphides by the groups of Prins [34,35],

Bussell [7,8,36,37,38], and Li [39,40,41]. This synthesis method is simple, and requires only

moderate temperatures (773-873 K) and uses inexpensive precursors, compared to direct

phosphidation with PH3 [42,43].

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2.3 Catalytic properties in hydrotreating

The first catalytic studies of phosphides were by the group of Nozaki [44,45] in the 70’s

and 80’s which examined their hydrogenation properties. This was followed 15 years later by a

report on HDN by Robinson, et al. [46], but the supported materials in that study are likely to

have been sulfides. Transition metal phosphides have also been applied to hydrodehalogenation

reactions by Chen, et al. [47,48,49].

In previous work it has been shown that MoP, and WP have moderate activity and Ni2P

has excellent activity in hydroprocessing [50,51,52]. The overall activity was found to be in the

order: Fe2P < CoP < MoP < WP < Ni2P in the simultaneous HDS of dibenzothiophene (3000

ppm S) and HDN of quinoline (2000 ppm N) at 643 K and 3.1 MPa, with the comparison based

on equal sites (240 μmol CO/O2 for phosphides/sulfides) loaded in the reactor [ 53 ,50].

Subsequent studies employed advanced techniques such as X-ray absorption fine structure

(EXAFS) [54], x-ray absorption near-edge spectroscopy (XANES) [55] and nuclear magnetic

resonance (NMR) [56] to characterize the catalysts. Tests were also carried out with a real feed

that confirmed their high activity [57].

Early work on supported Ni2P catalysts had utilized a low surface area silica (90 m2g-1).

The use of high surface area silica provided an opportunity to investigate the effect of particle

size, and interesting results were obtained [58]. Samples of low, medium, and high surface area

were denoted as L, M, and H, and x-ray diffraction (XRD) line-broadening and chemisorption

analysis duly showed that crystallite size (Dc) decreased with support area (Table 2.3).

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Table 2.3. Characteristics of 12 wt% Ni2P/SiO2 Catalysts

Sample Ni2P/SiO2-L Ni2P/SiO2-M Ni2P/SiO2-H

Support area / m2g-1 102 201 333

CO uptakea / μmol g-1 59 99 125

CO uptakeb / μmol g-1 40 72 103

Dc / nm 10.1 7.8 6.5

a Before reaction b After reaction

Hydrotreating activities of the samples were obtained in a three-phase, packed-bed

reactor operated at realistic conditions of 3.1 MPa and 573-643 K with a model feed liquid.

Fig. 2.3 illustrates the time course of HDS activities for the various Ni2P/SiO2 catalysts. The

initial 4,6-DMDBT conversions were uniformly high for all the samples but they declined

greatly for the Ni2P/SiO2-L, slightly for the Ni2P/SiO2-M, and actually grew for the Ni2P/SiO2-H.

The H catalyst gave a high conversion (99+%) even after 100 h of reaction, the M catalyst gave

an intermediate conversion (94%), and the L catalyst gave the lowest conversion (76%). The

feed had 500 ppm S as 4,6-DMDBT, 3000 ppm S as dimethyl disulfide, 200 ppm N as quinoline,

1 wt% tetralin, 0.5 wt% n-octane (internal standard), and n-tridecane (solvent). The high activity

of the high surface area Ni2P catalysts at these conditions indicates that they are effective even in

the presence of nitrogen compounds and aromatics, which are usually inhibiting of HDS in

sulfides.

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Figure 2.3. HDS Activity vs. time at 613 K and 3.1 MPa

(Based on 240 μmol of CO sites loaded in reactor)

The samples were characterized by extended x-ray absorption fine structure spectroscopy

(EXAFS). Fig. 2.4A) shows the Fourier-transfomed Ni K-edge EXAFS spectra of the fresh

silica-supported phosphide samples and Fig. 2.4B) shows the spectra of reference standards. The

bulk Ni2P sample has two main peaks at 0.18 nm and 0.23 nm which correspond to Ni-P and Ni-

Ni distances (Fig. 2.4B a)). The catalysts all display two distinct peaks at distances close to

those of the Ni2P standard, indicating that the predominant phase in these catalysts is Ni2P,

confirming XRD results. There is no indication of the presence of Ni metal (Fig. 2.4B b), Ni

oxide (Fig. 2.4B c) or Ni sulfide (Fig. 2.4B d).

0 20 40 60 80 1000

20

40

60

80

100

Ni2P/SiO2-H

Ni2P/SiO2-M

Ni2P/SiO2-L

Time on stream / h

4,6-

DM

DB

T co

nver

sion

/ %

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Figure 2.4. EXAFS Spectra of Fresh Samples

(Room temp., controlled atmosphere cells)

Elemental analysis shows that the high surface catalyst (H) retains more phosphorus than

the medium (M) or low area (L) catalysts (Ni/P ratio = 1/0.81 vs 1/0.62 and 1/0.48). This is

confirmed by line-shape analysis which show Ni-P coordination to be in the order H > M > L.

The smaller crystallites appear to have stronger Ni-P bonding allowing them to retain more

phosphorus, and this accounts for their stability and activity. This is also seen in the lower loss of

chemisorption sites during the course of hydrotreating (Table 4).

0.0 0.1 0.2 0.3 0.4 0.5 0.6

FT m

agni

tude

/ A

.U.

B)

A)

Ni-S

Ni-P

Ni-Ni

Ni-P

Ni-Ni

b) Ni2P/SiO2-L

c) Ni2P/SiO2-M

d) Ni2P/SiO2-H

a) Bulk Ni2PFT

Mag

nitu

de /

A.U

.

b) Ni foil

c) NiO

d) NiS

a) Bulk Ni2P

d / nm

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The finding that smaller Ni2P crystallites have higher activity and stability than larger

crystallites is significant, as it suggests using low loadings for making better catalysts. These

results were confirmed by Zepeda, et al. in the HDS of DBT at 553 K and 3.4 MPa over

MoP/Al2O3 prepared with different Mo loadings. Phosphides prepared with lower metal loading

exhibited higher HDS activity whereas higher loadings resulted in sintering of MoP particles

during reduction and lower activities [59]. Lower loadings are beneficial for practical cost

reasons. The EXAFS results indicate that the origin of the reactivity difference resides in the

electronic structure, as is found in many metallic systems [60,61,62,63]. This is reflected in the

different Ni-P bond strengths and also the different selectivity for products obtained from 4,6-

DMDBT (hydrogenation versus direct desulfurization).

Considerable work has been done to understand the state of the working Ni2P-H catalyst.

This was done by extensive analysis of the catalyst after reaction [64,65,66] and during reaction

[67,68]. The results of EXAFS analysis of spent Ni2P/SiO2 samples of different loading [64] are

shown in Fig. 2.5.

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Figure 2.5. EXAFS Spectra of Spent Samples

(Room temp., controlled atmosphere cells)

The Fourier transforms of the spent samples (Fig. 2.5 a-c) show roughly the two peak

pattern of the fresh samples except that the peaks are broadened and the Ni-Ni distances are less

intense, indicating the presence of a new phase. The results were compared with some reference

compounds (Fig. 2.5 d-h) to identify the phase. The decrease in the intensity of the Ni-Ni peak

was accompanied by a shift to lower distance, while the Ni-P peak itself remained strong. The

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Ni-NiNi-SNi-P

c)

12% Ni2P/SiO

2

18% Ni2P/SiO2

|FT|

of k

3 χ(k

) / a

. u.

a)

24% Ni2P/SiO2

d)

e)

b)

Ni2P

f)

Ni(OH)2

g)

Ni metal

h)

Ni3S5

d / nm

NiS

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39

spectral changes were due to development of intensity in the Ni-S distance region. Even though

no distinct Ni-S peak is seen, a feature in that region would give rise to the broad signal actually

observed. Fig. 2.5 a-c) shows a fit to three Lorentzian line shapes. The middle peak of each

triad fits the position of the Ni-S distance in the sulfide reference compounds (Fig. 2.5 g,h). The

spectra of phosphorus deficient compounds like Ni12P5 do not fit the observed pattern [64].

Thus, the EXAFS data strongly suggest that in the catalysts some Ni-Ni bonds are

disrupted to form Ni-S linkages. However, the surface is not a pure sulfide, as the reactivity of

nickel sulfide is known to be poor [69]. It was deduced that the active phase was a phospho-

sulfide in the outer region of a Ni2P crystallite core. This work was the first to suggest the nature

of the active phosphide catalyst. Recent calculations by the group of Nelson [70] confirm

the stability of the phosphosulfide overlayer.

Although the post-reaction analysis of the samples gave valuable information about the

catalyst, in situ studies were undertaken by Asakura and coworkers to investigate its working

state [67,68]. EXAFS has been applied to the investigation of hydrotreating catalysts, but little

work has been previously done on structural analysis in the liquid-phase, especially at elevated

temperature and pressure [71]. This is because of the strong absorption of x-rays by liquids, and

because of the severe stresses on the window material at realistic reaction conditions. Yet, the

use of liquid-phase conditions for HDS is important, as it is well known that gas-phase

conditions do not reproduce the results that are obtained with liquids [72,73].

A key development for the work was the construction of a low-volume cell with flat

windows transparent to x-rays that were chemically, structurally, and thermally stable (Fig. 2.6)

[67,68]. Previous cells could not handle both high temperatures (>600 K) and pressures (> 20

bar) for liquids [74,75,76]. The window was made of cubic boron nitride, an x-ray transparent

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40

material superior to beryllium or diamond, because it does not oxidize at high temperatures, and

does not give x-ray diffraction peaks.

Figure 2.6. a) In Situ EXAFS Cell (Lengths in mm) b) EXAFS Data at Reaction Conditions

(613 K, 3 MPa, 3% DBT in tetradecane)

A measurement at reaction conditions of 12 wt% Ni2P/SiO2-H was carried out, and after

subtraction of the Ni2P contribution a single oscillation was isolated (Fig. 8b, dotted line). Curve

fitting simulations assuming different bonds such as Ni-P, Ni-S, Ni-C, and Ni-Ni were carried

out, and the calculated curve for a Ni-S bond gave a good fit as shown by the dotted curve. The

calculated distance of 0.228 ± 0.004 nm is longer than the Ni-P bond of 0.221-224 nm found in

phosphides like Ni2P, NiP, Ni8P3, NiP2 [77,78], but is in the range of 0.225-0.240 nm found in

Ni-S bonds in sulfides like NiS [79], NiS2 [80], and Ni3S2 [81]. Therefore, the newly found

bond corresponds to a Ni-S bond, which is formed during the reaction, and is consistent with the

earlier suggested surface phosphosulfide.

Aside from the silica supported catalysts, various other supports were tested for the

phosphides, including carbon [82,83], alumina [66,84], MCM-41 [85,86,87], SBA-15 [88,89],

4 6 8 10-1.0

-0.5

0.0

0.5

1.0

k3 χ(k

)

k / 10 nm-1

X-ray beam

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41

and USY zeolites [90]. It was found that carbon, MCM-41, SBA-15 and USY zeolites were

particularly effective supports.

2.4 Kinetic Pathways in HDS and in HDN

For deep HDS, new catalysts with higher hydrogenation activity are required, and to

evaluate these and to design new deep HDS processes, kinetic data for each reaction pathway of

4,6-dimethyldibenzothiophene (4,6-DMDBT) HDS are important. Considerable work was

carried out to understand the reaction steps involved in the HDS of this compound by Song and

coworkers [91]. The reaction network for dibenzothiophene (DBT) has been studied extensively

on sulfides, and an identical network has been suggested for 4,6-DMDBT (Fig. 9) [92,93]. Two

main pathways are suggested to dominate: a) a direct desulfurization route (DDS) and b) a

hydrogenation route (HYD) (Fig. 2.7). Evidence for these pathways is discussed in several

comprehensive articles [94-99]. There is also an isomerization route where the methyl rings

migrate, but this is likely to operate only with acidic supports [100,101].

Figure 2.7. Reaction Network for 4,6-DMDBT Desulfurization

SH3C CH3

SH3C CH3

SH3C CH3SH3C CH3

H3C CH3H3C CH3 H3C CH3

4,6-DMDBT

3,3'-DMBP

4H-DMDBT

6H-DMDBT

MCHT

12H-DMDBT

DMBCH

Directdesulfurization

pathway

Hydrogenationpathway

k1

k2

Slow

Negligible

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42

The HYD route proceeds through hexahydrodimethyldibenzothiophene (6-H-DMDBT) and

forms methylcyclohexyltoluene (MCHT). The DDS route produces 3,3’-dimethylbiphenyl.

Interconversion between these products is believed to be slow, and many studies have taken the

ratio of these products to be the ratio k1/k2 of rate constants for the HYD and DDS routes.

Further hydrogenation of MCHT leads to the ultimate product dimethylbicyclohexane

(DMBCH), while the fully hydrogenated 12-hydro-dimethyldibenzothiophene is generally not

observed. An accurate determination of the rate constants for the HYD and DDS routes, k1 and

k2, was obtained by Song and coworkers using batch reactors at low conversions. The

concentration data for 4,6-DMDBT fit first-order kinetics and allowed the calculation of k1 + k2

by equation (1).

tkkCC DMDBTDMDBT ⋅+= )(- )/ln( 210 (1)

The ratio of rate constants k1/k2 was obtained by extrapolating the selectivity as defined

by equation (2) to zero conversion. The combination of measurements allowed calculation of the

individual rate constants.

[ ]

[ ]DMBP ofty SelectiviInitialHDMDBTs ofty SelectiviInitial

kk

2

1 = (2)

Comparison was made to two commercial catalysts, CoMoS/Al2O3 (Cr344) and

NiMoS/Al2O3 (Cr424), obtained from the Criterion Catalyst Co. The rate constants are reported

in Table 2.4 on a weight and (active site) basis.

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43

Table 2.4. Comparison of Rate Constants for Sulfides and Phosphides (573 K and 20.4 atm)

Rate Constant

10-5 s-1g·cat-1 or

(s-1active site-1)

CoMo sulfide NiMo sulfide Ni2P/USY Ni2P/SiO2

k1+k2 34.1 (4.0) 83.2 (8.8) 51.5 (15.2) 66.4 (23.7)

k1/k2 1.2 3.2 5.2 10.1

k1 18.8 (2.2) 63.3 (6.7) 43.2 (12.7) 60.4 (21.6)

k2 15.3 (1.8) 19.9 (2.1) 8.3 (2.5) 6.0 (2.1)

On a weight basis the overall activity, given by k1 + k2, followed the order

NiMoS/Al2O3 > Ni2P/SiO2 > Ni2P/USY > CoMoS/Al2O3

However, on the basis of active sites the order was

Ni2P/SiO2 > Ni2P/USY > NiMoS/Al2O3 > CoMoS/Al2O3

The number of active sites was estimated by the chemisorption of CO for the phosphides

and the low-temperature chemisorption of O2 for the sulfides. The CO method is reasonable for

counting the number of surface metal atoms in phosphides [55,84] The O2 method is similarly

reasonable for estimating the sites on sulfides [69,102], as it is applied at dry ice/

acetone temperatures in a pulse manner, so that corrosive chemisorption is minimized [103].

The result that Ni2P has higher activity than the sulfides on a site basis is significant, as in the

initial stages of catalyst discovery the objective is to find materials with high intrinsic rates. In

addition, Ni2P/SiO2 gave a very high k1/k2 ratio, with a value of 10.1, which is much higher than

the value of 1.2 for CoMo sulfide and 3.2 for NiMo sulfide. The indications are that Ni2P is very

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44

effective in the HYD pathway, which is usually slow in the sulfides. Studies on acetonitrile

hydrogenation over MoP by Li, et al. [104,105] confirmed the high hydrogenation activity of

phosphides. In addition, Zepeda, et al. reported a direct relationship between hydrogen

adsorption capabilities of Al2O3-supported MoP and their corresponding HDS activities [106].

A study by the group of Rodriguez of the electronic properties of SiO2 supported Ni2P,

MoP, and MoS2 catalysts using density functional calculations [107] has shown that the electron

density around the metal followed the order, MoS2/SiO2 < MoP/SiO2 < Ni2P/SiO2, which

correlated well with the thiophene HDS activities of the catalysts. It was suggested that the

higher electron density on the metal cation could enhance HDS activity by facilitating the

dissociation of H2 and the adsorption of thiophene [106,108]. The presence of P in Ni2P prevents

the compound from being bulk sulfided and allows it to retain metallic properties for

hydrogenation.

Extensive work has also been done [109,110,111,112] to study the mechanism of

hydrodenitrogenation (HDN). HDN is more difficult than HDS, and generally, for heterocyclic

compounds is preceeded by the hydrogenation of aromatic nitrogen heterorings, before the

hydrogenolysis of C-N bonds [113]. Thus the high hydrogenation activity of phosphides is

beneficial, and actually a key to deep HDS, as nitrogen compounds are inhibitors.

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45

Figure 2.8. In Situ FTIR Spectra of Pyridine on Ni2P and NiMo

The HDN work was carried out using quinoline, pyridine and 2-methyl piperidine as

probe reactants. Infrared measurements at reaction conditions showed that pyridine was readily

hydrogenated on Ni2P/SiO2 but not on NiMoS/Al2O3 [112]. The FTIR spectra in He of pyridine

adsorbed on the Ni2P/SiO2 (Fig. 2.8a) and NiMoS/Al2O3 (Fig. 2.8c) show the same features due

to a pyridinium species with bands at 1604-1608 cm-1 assigned to ν8a (CC(N)), 1485-1490 cm-1

and 1446-1449 cm-1 due to ν19b (CC(N)).

When the flow is switched to H2 the pyridinium signal for the Ni2P/SiO2 (Fig. 2.8b) is

converted to piperidinium (1597, 1575, 1472 cm-1) while that on NiMoS/Al2O3 remains

unchanged (Fig. 2.8d). This confirms the high hydrogenation activity of Ni2P.

Adsorption of CO was also used to characterize the samples. The sulfided NiMoS/Al2O3

showed very weak IR peaks (at 2173 cm-1 and 2117 cm-1) due to physisorbed CO species on

1650 1600 1550 1500 1450 1400

c) NiMoS/Al2O3 Under He flow

0.1

1650 1600 1550 1500 1450

N+

N+N+

N+

1604

d) NiMoS/Al2O3 Under H2 flowb) Ni2P/SiO2 Under H2 flow

Abs

orba

nce

/ A.U

.

16041485

1446

1485

1446

15751597

16081490

1449

1444

1472

a) Ni2P/SiO2 Under He flow

Wavenumber / cm-1

0.05

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46

cationic sites, as found in previous studies [114,115,116]. Only at a very low temperature (140

K) did IR bands appear at 2125 cm-1 with low intensity [116], indicating a low electron density

on the adsorbing metal sites. In contrast to these samples, the Ni2P/SiO2 gave a distinctive and

stable IR band at 2083 cm-1. After sulfidation the peak reappeared at slightly higher frequency

of 2086 cm-1 with the intensity being slightly diminished. This was previously observed in

studies in Bussel’s group on Ni2P/SiO2 [36,37] and was attributed to weakening of the Ni-CO

bond by electron withdrawal by sulfur. This frequency fell in the region between that of Ni+-CO

and Ni0-CO, indicating the presence of π back-bonding in a metal-like state. Also, the IR band

for bridging CO species was not observed, which is understandable as the bond distance given by

EXAFS between Ni-Ni in Ni2P is not close enough to form bridging CO groups compared to that

in Ni metal. Similar results are found for noble metals in a low oxidation state such as Pt and Pd

which are well known as hydrogenation catalysts, and display stable IR bands at low frequency

(~ 2010 cm-1) at room temperature [117,118]. The noble metals are, however, severely

deactivated in a sulfur environment with a loss of active sites [118].

An in-depth study was made of the HDN mechanism of 2-methylpiperidine. As shown

by the group of Prins [119], this is an ideal probe molecule for distinguishing between

elimination and substitution pathways. Piperidine can react by either of two pathways (Fig. 2.9).

The E2 elimination pathway (top) results in the formation of a 6-amino-2-hexene, which is

hydrogenated to 1-aminohexane. The SN2 nucleophilic substitution pathway (bottom) forms 5-

amino-1-hexene, which is hydrogenated to 2-amino-hexane.

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47

Figure 2.9. Reaction Network for 2-Methylpiperidine HDN

Results of a flow reactor study at 3.1 MPa and 450-600 K were carried out using a

Ni2P/SiO2 catalyst [120] (Fig. 2.10). For realistic conditions sulfur (3000 ppm) was included in

the feed.

Figure 2.10. Relative Concentrations and Selectivity in the HDN of 2-Methylpiperidine

N

H2N H2N

NH2NH2

E2

SN2

0 2 4 6 8 10 120

20

40

60

80

100

0 2 4 6 8 10 12

0

20

40

60

80

100

N

N

Sel

ectiv

ity /

%

N

NH2

N

N N

NH2

Rel

ativ

e C

once

ntra

tion

/ %

Site time / μmol h (g)-1

2-MePiperidine Hexane 2-Hexene 2-Aminohexane 1-Hexene 1-Aminohexane 2-Me-tetra-H

-pyridine 2-MePyridine

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48

Analysis of the product distribution as a function of contact time showed that the initial

product was 2-aminohexane, indicating that the reaction proceeded predominantly by an SN2

substitution mechanism, as was found on sulfides [108]. FTIR spectroscopy at reaction

conditions of the 2-methylpiperidine indicated that a piperidinium ion intermediate was formed

[121]. The intermediate was formed on P-OH groups, which showed a clear band at 3668 cm-1.

The assignment of the P-OH group on the Ni2P/SiO2 had already been addressed in a previous

study [110], which showed that the P-OH intensity fell or rose as the piperidine was adsorbed or

desorbed. The following reaction sequence (Fig. 2.11) accounts for the major observations on

the reaction, i.e. the occurrence of an SN2 step and the involvement of a piperidinium ion

intermediate.

Figure 2.11. Scheme for the HDN of 2-Methylpiperidine

Ni Ni

N

NiP

OH

S SNi Ni NiNi

P S SNi

O- +

Ni NiNiP S S

Ni

O-

Ni NiNiP S S

Ni

O-

NHH

H2N

+

H

H2N

+Ni NiNi

P S

O-

+

NH2

Ni NiNiP S S

Ni

O-

+

H

NH2

HS

Ni

H

..

..

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49

At the top left is a depiction of the Ni2P surface. It shows hydroxyl groups associated

with phosphorus as shown by FTIR [110], and the coverage of part of the nickel sites with sulfur

as shown by EXAFS [64-69] and calculations [70]. A 2-methylpiperidine molecule interacts

with this surface to form an adsorbed piperidinium ion, also observed by FTIR. The lone pairs

on a suitably positioned sulfur atom carry out a nucleophilic attack on the open side of the ring.

This is in agreement with the finding that HDN proceeds by a substitution mechanism.

Subsequent steps involve facile C-S bond breaking reactions.

Kinetics indicates that the nucleophilic attack is the likely rate-determining step, and is

supported by the observation of the piperidinium ion, as its high concentration is consistent with

its participation in a slow reaction. The C-S bond in the resulting amine intermediate is weak

and the species can react rapidly in a variety of ways, including elimination, as shown.

2.5 Bimetallic phosphide systems

As was mentioned earlier, Ni2P has been found to be the most active among the mono-

metallic phosphides. Because of the well known synergism of Co and Ni with Mo in sulfide

catalysts, some researchers have investigated mixed metal phosphides like NiMoP and CoMoP

[122-126]. The surprising finding is that these compositions are poorly active. However, little

explanation has been offered, aside from suggestions of alteration of the chemical nature, or

interactions with the support. Insight into this unexpected result can be obtained from examining

the structure of Ni2P itself (Fig. 2.12), as NiMoP has the same structure.

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The structure of Ni2P is hexagonal [127] with space group: m26P , 2h3D , strucktur-bericht

notation: revised C22, and lattice parameters, a = b = 0.5859 nm, c = 0.3382 nm. The unit cell

has two types of Ni and P sites (denoted as Ni(I), Ni(II) and P(I), P(II)), which form two

different trigonal prisms.

Figure 2.12. Structure of Ni2P showing P (left) and Ni (right) Coordination

Although there are equal numbers of Ni(I) and Ni(II) atoms in the unit cell, they have

different surroundings. The Ni(I) site has 4 nearest-neighbor P atoms in a near-tetrahedral

geometry (2 at 0.2209 nm, 2 at 0.2266 nm) and 8 more distant Ni neighbors (2 at 0.2613 nm, 2 at

0.2605 nm, and 4 at 0.26783 nm). The Ni(II) site has 5 nearest-neighbor P atoms in a square

pyramidal arrangement (1 at 0.2369 nm, 4 at 0.2457 nm) and 6 more distant Ni neighbors (2 at

0.2605 nm, 4 at 0.2678 nm).

An EXAFS study indicates that these two Ni sites can be distinguished by line-shape

analysis [128]. Expanding on the previous work on Ni2P/SiO2-L and Ni2P/SiO2-H [58] as

discussed in Figs. 2.4 -2.5, a Ni2P sample supported on a silicious MCM-41 support was

characterized and its activity was compared to that of the previous samples. The coordination

numbers for the first type of Ni (I) are approximately constant, while for the second type of Ni

Ni(l),P(l)

Ni(ll),P(ll)

Ni

P

Ni(l),P(l)

Ni(ll),P(ll)

Ni

P

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51

(II) increase as the surface area of the support increases. This shows that Ni(II) is more prevalent

in the smaller crystallites.

The feed employed was more severe than that used earlier (Fig. 5) and contained 500

ppm (0.05%) S as 4,6-DMDBT, 6000 ppm (0.6%) S as DMDS, 500 ppm (0.05%) N as quinoline,

1% tetralin, and balance tridecane. The conversion at steady-state at 340 oC was 95% for

Ni2P/MCM-41, 82% for Ni2P/SiO2–H, and 62% for Ni2P/SiO2–L (Table 6). The catalytic

activity of the supported samples was largest for the highest surface area samples:

Ni2P/MCM-41 > Ni2P/SiO2–H > Ni2P/SiO2–L

This order follows the number of Ni(II) sites, as deduced from the increasing

coordination of Ni-P(II) sites (Table 2.5). The conclusion is that the square pyramidal Ni(II)

sites are responsible for the high HDS activity, and constitute the active site.

Table 2.5. Correlation between active sites and catalytic performance

Samples Surface area

m2g

-1

Dc

nm Ni-P (I) Ni-P (II) Conv. at

613 K Sel.

HYD Sel. DDS

Ni2P (bulk) - - 2.0 1.75 - - -

Ni2P/SiO

2-L 102 10.1 1.99 2.57 62 34 66

Ni2P/SiO

2-H 333 6.5 1.98 3.19 82 50 50

Ni2P/MCM-41 487 3.8 1.99 3.48 95 65 35

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52

Returning to the poor activity of NiMoP, it happens that in NiMoP the Mo sites

substitute in the Ni(II) sites [129], and this readily accounts for the reduction of activity

in this compound. The structure of CoMoP is different [130], but since both CoP and

MoP have relatively poor activity, the lack of exceptional activity in CoMoP can be

rationalized. That peculiar sites will have enhanced activity is not surprising. Rodriguez

and coworkers have examined the role of P sites in single crystal Ni2P(001) surfaces and

found that they enhance the activity of Ni through a ligand effect [131], and the group of

has identified a stable Ni3PS site [70].

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53

2.6 Conclusions

The synopsis presented above described the properties of transition metal

phosphides and presented detailed studies of the most active of the catalysts, Ni2P. The

coverage included structural analysis of the active phase during reaction, and mechanistic

studies which provide a detailed picture of the properties of nickel phosphide catalysts.

It was shown that highly dispersed Ni2P is particularly active for HDS and resistant to S

and N-compounds. It was suggested that a stronger interaction between Ni and P in small

particles could enhance the sulfur resistance. Also, the activation of N-compounds is

probably related to the acidic properties of the Ni2P, which provides sites for the

formation of the protonated N-compounds as intermediate species in HDN [112]. These

results therefore imply that the remarkable activity of the supported Ni2P catalyst has its

origin in the proximity of the Ni and P species which is responsible for the creation of

proximal sites of high activity in hydrogenation and the activation of N-and S-compounds

[132].

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References

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[3] D. E. C. Corbridge, Studies in Inorganic Chemistry, Vol. 10, 4th Ed., Elsevier,

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[16] H. Fjellvag, A. Kjekshus, A.F. Andresen, Magnetic and structural properties of

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Chapter 3

Hydrogen Storage Using Heterocyclic compounds: The Hydrogenation of 2-Methylthiophene

3.1 Introduction

Hydrogen has been suggested as an ecologically clean energy carrier because it does

not produce air pollution or greenhouse gases. It can be produced by conventional or

emerging membrane technology [1,2,3]. Hydrogen has almost three times the energy

content of gasoline based on weight, but only about a quarter based on volume [4].

Lower-cost, lighter-weight and higher-density hydrogen storage is one of the key

requirements for hydrogen energy use.

The US Department of Energy has set technology targets for hydrogen storage for

2010 and 2015. It is desired by 2010 to develop hydrogen storage systems achieving a

gravimetric density of 2 kWh/kg (6 wt. %), a volumetric density of 1.5 kWh/L, and a cost

of $4/kWh, and by 2015, corresponding quantities of 3 kWh/kg (9 wt. %), 2.7 kWh/L,

and $2/kWh [4]. Currently four methods are being considered as promising candidates

for hydrogen storage: compression and storage in conventional cylinders or cryogenic

tanks, adsorption by metal hydrides, adsorption on high surface area materials, and

chemical hydrogen storage (including off-board regeneration) [5].

High pressure storage needs high strength containers and has a limited volume

capacity. A conventional steel hydrogen cylinder can hold only 1% by weight hydrogen

and the boil-off of liquefied hydrogen requires venting, reduces driving range, and

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produces safety problems. Hydrogen liquefaction is also energy intensive at an expense

of 30% of the heating value of hydrogen [6].

Metal hydrides are difficult to apply because they are too thermodynamically stable.

This has two consequences. First, the hydrides have to be heated to an inconveniently

high temperature to release hydrogen. Second, the heat of absorption is so high that a

large amount of heat must be removed during the refueling process. In addition, the

hydride systems contain strongly reducing agents which can react vigorously with air,

and thus must be leak-free [7].

Adsorption of hydrogen onto high surface area materials, such as carbon nanotubes,

has been studied extensively but also has barriers. These include the reproducibility of the

synthesis of the material and the hydrogen storage performance [8].

Compared with the above methods, chemical hydrogen storage provides high

gravimetric and volumetric hydrogen densities. Additionally, it has the advantage that

hydrogen storage and transportation use conventional petrochemical substances [9].

Chemical hydrogen storage materials, also called organic chemical hydrides, employ

hydrogenation-dehydrogenation of cyclic hydrocarbons or heteroaromatic compounds as

a means to store and transport hydrogen. Aromatic compounds such as benzene, toluene,

and naphthalene can be easily hydrogenated by using appropriate metal catalysts under

relatively mild conditions, e.g. about 100°C and 2 MPa. However, the dehydrogenation

of cyclic hydrocarbons is highly endothermic and the reaction is favored only at high

temperatures (Scheme 3.1). Catalytic dehydrogenation under “liquid-film state”

conditions has been reported [10-13], where the reactant is supplied as a liquid so that the

surface of the catalyst is always wetted with a thin film. Equilibrium limits were

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surpassed because of the evaporation of the dehydrogenated reactants. Another method

uses “wet-dry multiphase conditions” to take advantage of multiple phases to get over

thermodynamic equilibrium limitations [14,15]. However, these two processes still

require relatively high temperatures for vaporization of the volatile components of the

process. An important need is also an effective separation of hydrogen from the mixtures

to get a pure hydrogen product and to reuse the hydrogen carrier materials.

+ 3 H2

ΔΗ0=206 kJ mol-1

+ 5 H2

ΔΗ0=319 kJ mol-1

Scheme 3.1. Dehydrogenation of aromatic compounds

Heteroatom aromatic rings for H2 storage were proposed recently because the

addition of electron-donating groups favors H2 release both thermodynamically and

kinetically at moderate temperatures. In the case of indoline, dehydrogenation is possible

at modest temperature (110°C) [16]. Benzimidazolines, including N, N’-

dimethyldihydrobenzimidazole, 1,3-dimethyl-2-phenylbenzimidazoline, and 1,3-

dimethylbenzimidazoline, were studied with different palladium catalysts, and were

found to release H2 even at room temperature [17] (Scheme 3.2).

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Scheme 3.2. N,N’-dimethyldihydrobenzimidazole dehydrogenation

Hydrogen density is a very important factor in hydrogen storage. A lower weight of

the organic framework needs to be developed while maintaining favorable

thermochemical and kinetic parameters. Smaller molecules such as 4-aminopiperidine

and piperidine-4-carboxamide are promising compounds for reversible hydrogen storage

[18]. The dehydrogenation and hydrogenation of 4-aminopiperidine and piperidine-4-

carboxamide occur easily at low temperatures without by-products, such as C-N cleavage

and hydrogenolysis products. Density functional theory calculations suggest that

dehydrogenation is greatly favored in five membered rings over six and by the

incorporation of N atoms into the rings, either as ring substituents or as ring atoms,

particularly in the 1and 3 positions [19]. A number of heteroaromatic ligands have been

used for reversible hydrogenation/dehydrogenation. For example, N-ethyl carbazole is

hydrogenated with Pd at 72 atm and 160 °C and dehydrogenated with Ru at 50-197 °C

[20] (Scheme 3.3).

+ NH

NH OOH

H

CH3CN

PdN

NOO

-

H2

ΔΗ0=7 kJ mol-1

+

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N

CH3

+ 6H2

N

CH3

Scheme 3.3. Hydrogenation and dehydrogenation of N-ethyl carbazole

The Asemblon hydrogen liquid storage system was proposed to utilize substituted

thiophenes as carriers for storage, transport, and release of hydrogen [21]. This is based

on the discovery that finely dispersed supported metal catalysts readily carry out the

cyclization and dehydrogenation reaction of alkyl sulfides to thiophene, at the same time

releasing hydrogen. For example, for the case of 2-pentanethiol the reaction produces 2-

methylthiophene (Scheme 3.4).

CH3 CH3

SH S CH3

+ 3 H2

Cyclization

Au

Scheme 3.4. Dehydrogenation of 2-pentanethiol

A method for catalytic hydrogenation of thiophenes needs to be established to use

thiophenes as hydrogen carriers. The further cleavage of one of the C-S bonds, also

called selective ring-opening (SRO), is more desirable because an additional mole of

hydrogen can be carried than with the closed ring (Scheme 3.5).

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In this work, sixteen total catalysts are evaluated for the hydrogenation and ring

opening of 2MT, including supported noble metals, bimetallic noble metals, transition

metal phosphides [22] and transition metal sulfides. Temperature-programmed desorption

(TPD) of hydrogen provides insight into the relationship between hydrogen activation

and the hydrogenation activities of the catalysts. Temperature-programmed reaction

(TPR) is used to elucidate the reaction mechanism over the supported catalysts.

3.2 Experimental

3.2.1 Materials

Commercial noble metal catalysts including Pt/Al2O3, Pd/Al2O3, and Ru/Al2O3 were

provided by BASF Catalysts, Inc. Transition metal sulfides MoS/Al2O3, NiMoS/Al2O3,

CoMoS/Al2O3 were obtained from Haldor TopsØe. Transition metal phosphides WP/SiO2,

Ni2P/SiO2, MoP/SiO2 and bimetallics, PdPt/ Al2O3 and PdRu/ Al2O3 were synthesized.

S CH3

+ 2 H2

S CH3

HydrogenationCH3 CH3

SH

Ring-opening

H2

ΔH0= -147 kJ mol-1 ΔH0= -55 kJ mol-1

Scheme 3.5. Hydrogenation and ring-opening of 2-methythiophene

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69

The support fumed silica EH-5 was provided by Cabot Corp. The chemicals used in the

synthesis of the catalysts were (NH4)6W12O39·xH2O (Aldrich, 99%), Ni (NO3)2·6H2O

(Alfa Aesar, 99%), (NH4)6Mo7O24·4H2O (Alfa Aesar, 99%), (NH4)2HPO4 (Aldrich, 99%).

Pd(NH3)4Cl2·xH2O (Alfa Aesar, 99.9%), Pt(NH3)4(NO3)2·xH2O (Alfa Aesar, 99.9%) and

RuCl3·xH2O(Alfa Aesar, 99.9%) were the precursors for the bimetallic catalysts. The

chemicals used for the reactivity tests were 2-methylthiophene (Alfa Aesar, 98%) and n-

nonane (Alfa Aesar, 99%). The gases employed were H2 (Airco, Grade 5, 99.99%), He

(Airco, Grade 5, 99.99%), CO (Linde Research Grade, 99.97%), 0.5% O2/He (Airco,

UHP Grade, 99.99%), O2 (Airco, UHP Grade, 99.99%), 10% H2S/H2 (Airco, UHP Grade,

99.99%) and N2 (Airco, Grade 5, 99.99%). Chemical standards for GC and mass

spectrometry were tetrahydro-2-methylthiophene (TH2MT) (Alfa Aesar, 98%),

pentanethiol (Alfa Aesar, 99%), pentane (Alfa Aesar, 99%), 1-pentene (Alfa Aesar, 99%),

2-pentene (Alfa Aesar, 99%).

3.2.2. Metal Phosphides Synthesis

WP/SiO2 [23], Ni2P/SiO2 [24,25,26], MoP/SiO2 [27,28], PdP/SiO2 were prepared by

temperature-programmed reduction (TPR), following procedures reported previously

[29,30]. Briefly, the synthesis of the catalysts involved two stages. First, solutions of the

corresponding metal phosphate precursors were prepared by dissolving appropriate

amounts of (NH4)6W12O39·xH2O, Ni(NO3)2·6H2o, (NH4)6Mo7O24·4H2O,

Pd(NH3)4Cl2·xH2O with ammonium phosphate in distilled water, and these solutions

were used to impregnate silica by the incipient wetness method. The obtained samples

were dried and calcined at 500 °C for 6 h, then ground with a mortar and pestle,

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70

pelletized with a press (Carver, Model C), and sieved to particles of 650–1180 μm

diameter (16/20 mesh). Second, the solid phosphates were reduced to phosphides at 2 °C

min-1 in flowing H2 [1000 cm3 (NTP) min−1 g−1]. Reduction temperatures were 577 °C

for WP/SiO2, 568 °C for Ni2P/SiO2, 494 °C for MoP/SiO2, 350 °C for PdP/SiO2 The

samples were kept at the reduction temperature for 0.5 h, followed by cooling to RT

under He flow [100 cm3 (NTP) min−1], and then passivated at RT in a 0.5% O2/He for 4 h.

The Ni, Mo, W molar loading were all 1.6 mmol g−1 (mmol per g of support),

corresponding to a weight loading of Ni2P of 7.9% with an initial Ni/P ratio of 1/2, MoP

12.8% with initial Mo/P ratio of 1, WP 19.9% with initial W/P ratio 1.

The bimetallic catalysts (1/4) PdPt/SiO2, (4/1) PdPt/SiO2 [31] and (4/1) PdRu/SiO2

of total loading 2 wt. % with Pd/metal weight ratios (in parentheses) were synthesized by

the incipient wetness method from the corresponding precursors dissolved in distilled

water. After impregnation, the samples were dried in vacuum at 100 °C for 6 h, then

calcined at 300 °C for 3 h [32].

3.2.3. Characterization

Irreversible CO uptake measurements were used to titrate the surface metal atoms

and to provide an estimate of the active sites on the catalysts for the noble metals and the

transition metal phosphides. Usually, 0.3 g of a passivated sample was loaded into a

quartz reactor. Noble metal catalysts were reduced in H2 at 325 oC for 2 h while

passivated transition metal phosphides were reduced at 450 oC for 2 h. After cooling in

He, pulses of CO in a He carrier at 43 μmol s−1 [65 cm3 (NTP) min−1] were injected at RT

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71

through a sampling valve, and the mass 28 (CO) signal was monitored with a mass

spectrometer. CO uptake was calculated by measuring the decrease in the peak areas

caused by adsorption in comparison with the area of a calibrated volume (19.5 μmol).

Transition metal sulfides were characterized by the irreversible chemisorption of O2 at

dry-ice acetone temperature using the same techniques. Prior to the measurement the

samples were sulfided in a flow of 10% H2S/ H2 at 400 oC.

3.2.4 Reactivity Studies

Hydrogenation activity was measured with a mixture containing 10% vol. nonane

and 90% vol. 2MT at 2 MPa (300 psig) in a continuous-flow trickle bed reactor. Briefly,

the testing unit consisted of three parallel reactors enclosed in heating ovens. The

dimensions of the reactors were 1.5 cm i.d. x 25.5cm long, and were generally loaded

with 100 μmol active sites or 40 μmol for the samples with low uptakes. To start a

reaction, catalysts were placed in the catalytic reactor and pretreated at the same

conditions as used for chemisorption. After pretreatment, the pressure was set to 2 MPa,

a hydrogen flow rate of 150 cm3 NTP/min was started, and the liquid reactant was fed at

a rate of 2.4 cm3 /h. Reactivity testing was carried out as a function of temperature,

which was started at the highest temperature 325°C and was varied downwards and

upwards with the initial temperature repeated at the end. Hydrotreating products were

collected every few hours in sealed septum vials and quantified by a gas chromatograph

(Hewlett-Packard,5890A) equipped with a 0.32 mm i.d. x 50m fused silica capillary

column and a flame ionization detector. The reactants and products were identified by

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72

their retention time in comparison with commercially available standards and confirmed

by gas chromatography-mass spectrometry (GC-MS) (Hewlett-Packard, 5890-5972A).

Equilibrium calculations were carried out using the CHETAH program (ASTM

International, West Conshohocken, PA).

3.2.5. Temperature Programmed Desorption (TPD) and Temperature

Programmed Reaction (TPR)

The compounds Pt/Al2O3, PdPt/SiO2, WP/SiO2, and CoMoS/Al2O3 were the most

active members of the four groups consisting of noble metals, bimetallics and transition

metal phosphides and sulfides and were selected for further TPD and TPR tests. The

TPD experiments were performed in a U-shaped quartz reactor connected to a mass

spectrometer through a leak valve. The noble metal catalysts were pretreated in a flow of

hydrogen at 325°C for 2 h, and then cooled in hydrogen to 225°C. After a He flush at the

same temperature to remove the hydrogen background, the heating temperature program

was started 5 °C/min to 450°C. The mass 2 signal was monitored by MS. The sample

was then cooled in helium to 225°C and 2MT was introduced to the system in a helium

carrier flow until saturation was reached. Then the system was heated to 550°C at

5 °C/min in hydrogen. A total of 12 masses were monitored: 2, 4, 34, 42, 43, 55, 70

(Pentene, or C5H10 fragment), 72 (Pentane, or C5H12 fragment), 87, 97 (2MT), 102

(TH2MT), 104 (Pentanethiol). The TPD and TPR were done with WP/SiO2 and

CoMoS/Al2O3 in a similar way except that they were pretreated at 450°C in H2 and

400°C in H2 /H2S, respectively.

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73

3.3. Results and Discussion

3.3.1 CO Chemisorption and O2 Chemisorption

Uptakes of CO of the noble metal, bimetallic and transition metal phosphide

catalysts are given in Table 3.1. Earlier studies have shown that uptakes of the SiO2 and

Al2O3 supports were negligible [29,33,34,35]. The CO chemisorption uptakes of the

different samples varied in a wide range from 8 to 200 μmol/g. It was found that for

noble metals, higher loadings samples gave larger CO chemisorption uptakes but the

average particle sizes (2-4 nm) did not vary appreciably. Bimetallic noble metal alloys

were highly dispersed on the silica support surface and the particle sizes were small (1-2

nm). Among the phosphides the order of dispersion was WP/SiO2 < Ni2P/SiO2 <

MoP/SiO2 < PdP/SiO2. For transition metal sulfides dispersion was approximated based

on the O2 chemisorption uptakes (Table 3.2). It was found that higher metal loadings

gave lower dispersion, although the total number of active sites increased with the metal

loading, which is reasonable.

Assuming that each active site adsorbs one CO molecule or one oxygen atom, the

number of active sites in the catalysts followed the order: sulfides > phosphides ~

bimetallics > noble metals.

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74

Table 3.1. Summary of Catalyst Characterization by CO Chemisorption

Catalyst CO uptake (μmol g-1)

Dispersion (%)

Particle size (nm)

0.5% Pd/Al2O3 10 21 4

5% Pd/Al2O3 120 25 4

0.5% Pt/Al2O3 8 31 3

2% Pt/Al2O3 44 42 2

0.5% Ru/Al2O3 23 47 2

2% PdRu(4/1) /SiO2 140 77 1

2% PdPt(4/1) /SiO2 83 55 2

2% PdPt(1/4) /SiO2 99 84 1

Ni2P/SiO2 95 8 11

MoP/SiO2 200 18 5

WP/SiO2 42 4 25

PdP / SiO2 74 56 2

Table 3.2. Summary of Catalyst Characterization by O2 Chemisorption

Catalyst Composition O2 uptake (μmol g-1)

Dispersion

(%)

Particle size (nm)

MoS/Al2O3 (TK-Mo) Mo 2-4.7 % 58 55-23 2-4

NiMoS/Al2O3 (TK-NiMo1) Ni 0.8-2.4 % ; Mo 7-13 % 54 17-9 5-10

NiMoS/Al2O3 ,AlPO4 (TK-NiMo2)

Ni 1.6-3.9 % ; Mo 8-12 % Al2O3 68-80 % AlPO4 5-11 %

68 12-7 9-13

CoMoS/Al2O3 (TK-CoMo)

Co 2.4-4.7 % ; Mo12-16 %Support: 70-80 % 100 12-8 7-11

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75

3.3.2 Reactivity

A model feed containing 10 vol. % nonane and 90 vol. % 2MT was used to test

the HDS and hydrogenation (HYD) activities of the 16 different catalysts. All the

catalysts showed expected responses to temperature with higher conversions at higher

temperatures and reasonable stability over the time course of the reactions. The major

products were TH2MT, pentenes and pentane, but C5-thiols could barely be observed.

For convenience, all of the HDS products will be referred to as C5 HDS products. Fig.

3.1 gives the conversion and selectivity of the four most active catalysts 2% Pt/Al2O3, 2%

PdPt (1/4)/SiO2, WP/SiO2, CoMoS/Al2O3 out of the four groups of catalysts.

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76

Figure 3.1. Catalytic activity as a function of temperature a) 2% Pt/Al2O3 b) 2% PdPt (1/4)/SiO2 c) WP/SiO2 d) CoMo/Al2O3

0 10 20 30 40 50 60 700

20

40

60

80

20

40

60

80

100325oC200oC

250oC225oC175oC275oC150oC325oC

Sele

ctiv

ity /

%

Time / h

TH2MT C5 HDS product Pentanethiol

a) 2% Pt/Al2O

3

Con

vers

ion

/ %

total conversion of 2MT

20

40

60

80

100

0 10 20 30 400

20

40

60

80

d) CoMoS/Al2O3

325oC225oC275oC175oC325oC

Time / h

20

40

60

80

100

0 10 20 30 40 500

20

40

60

80

c) WP/SiO2

175oC225oC275oC175oC 325oC325oC

Time / h

Con

vers

ion

/ %Se

lect

ivity

/ %

20

40

60

80

100

0 10 20 30 40 500

20

40

60

80

b) 2% PdPt (1/4)/SiO2 325oC225oC175oC275oC150oC325oC

Time / h

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77

Figure 3.2. Reactions of Catalysts in the Hydrogenations of 2-Methylthiophene a) Turnover Frequency of 2MT Reaction b) Rate of Formation of TH2MT

c) Conversion of 2MT d) Selectivity to TH2MT

0.5%Pd

5%Pd

0.5%Pt

2%Pt

0.5%Ru

MoS

NiMoS

1

NiMoS

2

CoMoS

2% PdR

u

2% PdP

t(4/1)

2% PdP

t(1/4) PdP Ni2P MoP W

P

0.00

0.02

0.04

0.06

0.08a) Total turnover frequency of 2MT reaction

Tota

l tur

nove

r fre

quen

cy /

s-1

225oC 275oC 325oC

0.5%Pd

5%Pd

0.5%Pt

2%Pt

0.5%Ru

MoS

NiMoS

1

NiMoS

2

CoMoS

2% PdR

u

2% PdP

t(4/1)

2% PdP

t(1/4) PdP Ni2P MoP WP

0.00

0.02

0.04

0.06

0.08b) Rate of formation of TH2MT 225oC

275oC 325oC

Net

rate

of T

H2M

T fo

rmat

ion

/ s-1

0.5%Pd

5%Pd

0.5%Pt

2%Pt

0.5%Ru

MoS

NiMoS

1

NiMoS

2

CoMoS

2% PdR

u

2% PdP

t(4/1)

2% PdP

t(1/4) PdP Ni2P MoP W

P

20

40

60

80

100

120c) Conversion of 2MT 225oC

275oC 325oC

Tota

l con

vers

ion

/ %

0.5%Pd

5%Pd

0.5%Pt

2%Pt

0.5%Ru

MoS

NiMoS

1

NiMoS

2

CoMoS

2% PdR

u

2% PdP

t(4/1)

2% PdP

t(1/4) PdP Ni2P MoP WP

0

20

40

60

80

100

120d) Selectivity to TH2MT 225oC

275oC 325oC

Sel

ectiv

ity to

war

ds T

H2M

T / %

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78

Fig. 3.2 summarizes reactivity results for all catalysts, presented as the total

turnover frequency (TOF) for the 2MT reaction, the TOF of formation of TH2MT, the

conversion of 2MT, and the selectivity to TH2MT. Fig. 3.2a shows that activities of the

catalysts expressed as turnover frequency for the 2MT reaction follow the trend: noble

metals > sulfides > bimetallics > phosphides. Fig. 3.2c shows that in terms of total

conversion, the order is different, sulfides > phosphides ≈ noble metals > bimetallics. Fig.

3.2c also shows that TH2MT is the major hydrogenation product, followed by the HDS

products, pentane and pentene. Very little pentanethiol was ever observed with all the

catalysts, indicating that once this ring-opening reaction occurred, the thiols underwent

desulfurization. Fig. 3.2d shows the selectivity towards the desired product TH2MT. The

order was noble metals > bimetallics > phosphides > sulfides. The order of selectivity

towards TH2MT was opposite the order of the number of active sites of the catalysts,

which indicates that active sites of the catalysts titrated by CO or O2 chemisorption

grossly favor HDS more than the hydrogenation product TH2MT. The most active

catalysts were the sulfides with up to 80% conversion but with low selectivity to TH2MT

of around 10% (Fig. 3.2a and 3.2d). This is expected, as these commercial sulfides are

optimized for HDS. The noble metals (Pt and Pd) had conversion of 10-20% at lower

temperatures and selectivity of 80-90%. At higher temperatures the noble metals had

conversion of 40-50% but selectivity of 50-60%. The transition metal phosphides had

very low conversions around 5% at low temperature with selectivity of 70-90%. At

higher temperature transition metal phosphides had a conversion of 10-30% with

selectivity of 30-80%. For the bimetallics, the conversions were about 10-15% at lower

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79

temperature with selectivity of 90%, while at a high temperature the conversion increased

to 40-50% while selectivity decreased to 60-80%.

Desulfurized products are favored at high temperatures and TH2MT is favored at

low temperatures for all of the catalysts, which is consistent with theoretical equilibrium

results: HDS was less favored at low temperatures (Fig. 3.3). As reaction temperature

increased, the reactivity of 2MT increased while selectivity towards TH2MT decreased.

Figure 3.3. Comparison of theoretical equilibrium constants for HDS and HYD

100 150 200 250 300 350 400-2

0

2

4

6

8

10

Log

KLo

g of

equ

ilibr

ium

con

stan

t

Temperature / oC

S CH3

+ 2 H2

S CH3

HYD

S CH3

+ 3 H2 HDS

+ SH2

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The results of the effect of pressure on the activity of 0.5% and 2% Pt/Al2O3 are

summarized in Table 3.3. Comparison of the results on 0.5% and 2% Pt/Al2O3 of higher

pressure (3.7 MPa, 550 psig) to those obtained at standard conditions (20 MPa, 300 psig)

shows that the selectivity for hydrogenation of 2-MT was slightly increased at lower

temperatures (225 and 275°C). However, at higher temperature (325°C) the pressure

change almost did not make a difference for the selectivity towards hydrogenation. The

pressure did not show an obvious impact on the TOF either. Therefore, the activity

studies for all the other catalysts were carried out at the lower pressure of 300 psig.

Table 3.3. Effect of pressure on the activity of 0.5% and 2% Pt/Al2O3

Pressure (psi) Catalyst

498K 548K 598K

TOF(s-1) Selectivity (%) TOF(s-1) Selectivity

(%) TOF(s-1) Selectivity (%)

300

0.5% Pt/Al2O3

0.01 92 0.04 83 0.06 53

2% Pt/Al2O3

0.009 94 0.02 84 0.06 62

550

0.5% Pt/Al2O3

0.01 96 0.03 84 0.05 47

2% Pt/Al2O3

0.006 95 0.02 87 0.04 62

Thiophene reactions have been studied extensively in the field of hydroprocessing,

because they are models for desulfurization of sulfur-containing heterocycles in

petroleum feedstocks. However, the goal in hydroprocessing is to remove the sulfur

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81

atom (for a recent review see [36]) and the hydrogenation and ring-opening reactions

have been considered just intermediary steps in the overall process. Studies on

hydrogenation of thiophenes in the gas phase with Mo and W sulfides promoted with Ni

and Co indicate that hydrogenolysis is facile and the hydrogenated products are only

formed in small amounts [37], as found here. At 240°C and 2 MPa, the activities of

metal sulfides for the formation of tetrahydrothiophene are in the order: Pd » Mo > Rh ≥

Ru> Re> W > Co > Ni [38]. The higher activity of the palladium catalyst compared to

the other sulfide catalysts is likely due to the ease of PdS reduction, which favors the

formation of metallic sites for hydrogenation.

The methyl group at the α-carbon of 2MT increases the electron density on the

aromatic ring and facilitates the activation of the ring. As a result, the rate of 2MT

hydrogenation was reported to be almost an order of magnitude higher than thiophene

hydrogenation [38]. The transformation of 2MT is believed to occur by a consecutive-

parallel network on PdS because the selectivity to the hydrogenation product TH2MT and

the hydrogenolysis product pentane are constant until the conversion reached 50% [39]

(Scheme 3.6).

Scheme 3.6. Hydrodesulfurization reaction mechanism for 2-methylthiophene

S CH3

S CH3H2

C5H12 + H2S

H2

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Supports with strong Brönsted acid surface sites like SiO2, aluminosilicates, and

zeolite HNaY showed much larger specific catalytic activity for the hydrogenation of

2MT than Al2O3, TiO2, and carbon [40]. A commercial CoMo sulfide catalyst produced

TH2MT and HDS products (pentenes and pentane) as the primary products [41]. The

product 1-pentanethiol from the SRO of TH2MT was present in very small amounts.

Hydrogen dissociatively chemisorbs on the surface of sulfide catalysts and forms

hydrogen atoms and ions [42]. 2MT is protonated at the α–carbon and produces a

thiophenium cation or a π-complex with coordinatively unsaturated surface cations to

form the thiophenium cation. In the presence of a hydride ion donor, for example, a

metal hydride, thiophene hydrogenation occurs by hydride transfer (Scheme 3.7) [38]. A

similar 2MT hydrodesulfurization mechanism was proposed on metal sulfide catalysts

[43]. The rate-determining step before the rupture of the C-S bond is the transfer of the

first hydrogen to the adsorbed thiophenic compound.

SCH3 SCH3

H

H

+H+

H-

SCH3

Scheme 3.7. Hydrogenation mechanism for 2-methylthiophene

The proton sites are very important for 2MT activation. Hydrides are thought to

be a source of hydrogen for the hydrogenation and hydrogenolysis reactions. The

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TH2MT ring can be protonated and then made to undergo rupture through an elimination

process to produce thiols.

Hydrogenation of aromatic and consecutive ring opening can be carried out on

certain noble metals. Pt, Pd, Ir, Ru and Rh supported on Al2O3 have been found active

and selective for the ring opening of methylcyclopentane to the corresponding C6

paraffins [44]. Ring opening of naphthenes was seen to occur exclusively on the

Brönsted acid sites of zeolites [45]. The number and strength distribution of Brönsted

acid sites [46,47], the crystallite size [48], and the zeolite topology were all important

parameters for the desired ring opening products. When an effective acid functionality is

combined with a high-activity hydrogenolysis metal, the bifunctional catalyst system will

be greatly selective to the ring-opening products [49]. The commercial process for

selective ring-opening involves such bifunctional catalysts. The acidic sites catalyze

dehydrogenation, cracking, isomerization and dealkylation, while the metal sites carry

out adsorption and desorption. Noble metals supported on acidic oxides are highly active

catalysts for selective ring opening [50]. All these transformations though occur on

aromatics not containing heteroatoms, when heteroatoms like sulfur are present these

molecules are removed by hydrogenolysis.

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84

3.3.3. TPD and TPR

3.3.3.1. TPD of H2

2% PdPt(1/4)/SiO2

T=530 K

2% Pt/Al2O3

T=540K

500 550 600 650 700 750Temperature / K

W P/SiO2

T=560K

Mas

s Si

gnal

/ A

.U.

CoMo/Al2O

3

T=550K

Figure 3.4. TPD of H2 profile

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85

Based on their high reactivity and selectivity, 2% PdPt (1/4)/ SiO2, 2% Pt /Al2O3,

CoMo/Al2O3, and WP / SiO2 were chosen for further study by temperature programmed

desorption (TPD) of H2 and temperature programmed reaction (TPR) of 2MT. The TPD

experiments were started 498 K instead of room temperature so as to depopulate the low

energy binding state of H2 [51] and to only examine the effective active sites in the range

of temperatures used for the reaction of 2MT. Hydrogen desorption peak temperatures

for 2% PdPt (1/4)/SiO2, 2% Pt/Al2O3, CoMo/Al2O3, and WP/SiO2 were 530 K, 540 K,

550 K, 560 K respectively (Fig. 3.4), which are comparable with results in the literature

[52]. The order desorption of H2 indicates that hydrogen is held slightly less strongly on

the noble metals than on the transition metal compounds, as expected. Hydrogen was also

observed to desorb over a broad range of temperature on these four catalysts. The broad

desorption spectra are possibly due to the site heterogeneity in the samples [53].

Table 3.4 compares the numbers of active sites measured by using different methods.

The number measured by TPD of H2 were much smaller than those titrated by CO or O2

chemisorption. This is mainly because the lower energy binding states of hydrogen were

excluded by the higher TPD starting temperature as mentioned above. H2 TPD

desorption amounts for these four catalysts followed a different order from the CO and

O2 chemisorption results.

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86

Table 3.4. Comparison of active site determinations

2% PdPt(1/4) / SiO2 2% Pt/Al2O3 CoMo/Al2O3 WP/SiO2

H2 TPD desorption (μmol/g)

5 3 30 12

CO chemisorption (μmol/g) 44 99 -- 42

O2 chemisorption (μmol/g) -- -- 100 --

It was found that the H2 TPD desorption amount was inversely related to the rate of

TH2MT formation (Fig. 3.5). Since the H2 desorption amount reflects the coverage of

hydrogen at reaction conditions, this may indicate that hydrogen was consumed to a

greater degree on the more active catalysts.

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87

2%Pt/Al2O3

2% PdPt(1/4)/SiO2WP/SiO2

CoMo/Al2O30.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

TPD of H2

Turnover Frequency

Turn

over

Fre

quen

cy /

s-1

0

5

10

15

20

25

30

H2 TPD

desorption amount

(umol/g)

Figure 3.5. Relation between turnover frequency and H2 TPD desorption

3.3.3.2. TPR of 2MT and H2

Temperature programmed reaction experiments of 2MT were carried out with 2%

PdPt (1/4)/ SiO2, 2% Pt /Al2O3, CoMo/Al2O3, and WP/SiO2 (Fig. 3.7-3.10) and the

desorption peaks were integrated to quantify the corresponding products (Table 3.5). The

unreacted 2MT desorbed at a low temperature of around 550 K in a narrow peak but over

90% of the adsorbed 2MT reacted in the broad temperature range 498-823 K. The total

amounts of 2MT adsorbed on the catalysts followed the order: CoMo/Al2O3 > WP/SiO2 >

2% Pt /Al2O3 > 2%PdPt (1/4)/SiO2. This order matches the order found for the amount of

desorbed H2 in the TPD experiment, which indicates that the sites that are active for

binding hydrogen are also active for 2MT hydrogenation. The major product in the TPR

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88

of 2MT was the ring-opened product, pentanethiol, with some pentene also formed. The

selectivities to TH2MT were no more than 6% and very little or no pentane was formed

(Fig. 3.6). This is a reasonable finding. Generally the desulfurization of thiophenes

involves two pathways [54]. One is the direct desulfurization pathway which in the case

of 2MT gives pentenes or pentane. The other is the hydrogenation pathway which gives

TH2MT followed by desulfurization of TH2MT. It has been reported that the reactivities

of thiophenes and tetrahydrothiophenes are similar while those of thiols are about 15-40

times more reactive [55]. Thus, the formation of pentene or pentane and TH2MT is

expected, not pentanethiol.

Table 3.5. Product distribution for TPR of 2MT

2% PdPt(1/4) / SiO2

2% Pt/Al2O3 CoMoS/Al2O3 WP/SiO2

TPR of 2MT Tp (K) µmol/g Tp (K) µmol/g Tp (K) µmol/g Tp (K) µmol/g 1-pentene (µmol/g) 590 0.06 670 0.3 560 0.3 560 0.1

Pentane (µmol/g) --- 0 --- 0 550 0.1 --- 0

2MT (µmol/g) --- 0 580 0.02 550 0.04 550 0.3

TH2MT (µmol/g) --- 0 --- 0 640 0.04 640 0.03

Pentanethiol (µmol/g) 630 0.16 720 0.7 640 5 640 4.2

Total product (µmol/g) 0.2 1 5.6 4.6

Conversion (%) 100 98 99 99

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89

2% PdPt(1/4) / SiO2

2% Pt/Al2O3

CoMo/Al2O3WP/SiO2

0

20

40

60

80

100

Sele

ctiv

ity /

%

Pentanethiol TH2MT Pentane 1-Pentene

Figure 3.6. Product Distribution for TPR of 2MT

The products distribution of TPR of 2MT is shown in Fig. 3.6. Pentanethiol

formed through the ring-opening of TH2MT was the major product. There are three

pathways to ring-open TH2MT similar to the mechanisms of aromatic ring opening, such

as naphthalene [49]. Protonation of TH2MT followed by attack by a nucleophile will

open the ring to form 2-pentanethiol because the SN2 process favors the less hindered

carbon (Scheme 3.8). On the other hand, attack by a base will tend to open the ring on

the more substituted side by β elimination (Scheme 3.9). A third potential ring-opening

pathway occurs on metal hydrides with Lewis acid properties. This will also favor attack

on the more substituted side to form a 1-pentanethiolate intermediate (Scheme 3.10). In

the TPR of 2MT, 2MT was adsorbed on the catalyst surface depleted of H2. It will be

less likely for the adsorbed 2MT to be in the protonated form. Previous work has shown

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90

that supported metal catalysts which have one coordination vacancy can adsorb thiophene

by the sulfur atom [56]. The preadsorbed 2MT would compete with hydrogen for sites

on the surface [57]. Either carbon-sulfur scission or sulfur removal can be rate-

controlling steps depending on the reaction conditions [57]. In this study sulfur removal

is likely the rate controlling step because there TPR shows that more pentanethiol

accumulates on the surface.

SH

+ CH3

NuCH3SHNu

Scheme 3.8. SN2 mechanism for 2MT ring-opening

SH

+ CH3

H HBase

SH CH3

Scheme 3.9. E2 mechanism for 2MT ring-opening

S CH3S

CH3

MH

M H

Scheme 3.10. Metal hydride ring-opening of 2MT

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91

It was found that 2MT, pentanethiol and pentene desorbed at much higher

temperatures on the 2% Pt /Al2O3 surface compared with other catalysts. The interaction

between 2% Pt /Al2O3 and these species is stronger than with the other catalysts. Based

on the results from the TPR of 2MT and evidence for the thiophene reaction mechanism

[58], the following 2MT transformation network is proposed (Scheme 3.11)

S CH3

2MT

M

S CH3

M

S CH3

M

+ 2H

+ 2H+ 2H

+ 2H

+ H

TH2MT

S CH3

M

+ H

- H1-Pentanethiol

1-Pentene Pentane

+ H- H

S CH3M

Scheme 3.11. 2MT reaction network

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92

3.4. Conclusions

Sixteen total catalysts were evaluated for the hydrogenation and ring opening of

2MT, including supported noble metals, bimetallic noble metals, transition metal

phosphides, and transition metal sulfides. The major products were TH2MT, pentenes

and pentane, and C5-thiols could barely be observed. The selectivity towards the desired

product TH2MT follows the order: noble metals > bimetallics > phosphides > sulfides.

The order of selectivity towards TH2MT was opposite the order of the number of active

sites of the catalysts, which indicates that the active sites of the catalysts titrated by CO or

O2 chemisorption favor HDS more than the hydrogenation product TH2MT.

Temperature-programmed desorption (TPD) of hydrogen indicated that the H2

desorption amount was inversely related to the rate of TH2MT formation. Temperature-

programmed reaction (TPR) showed that pentanethiol was the major product on the 2MT

preadsorbed surface, especially on excellent HDS catalysts like CoMoS/Al2O3 and WP/

SiO2.

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Figure 3.7. TPR of 2MT on 2% PdPt/SiO2 profiles

450 500 550 600 650 700 750 800 850

Mass=97 2MT

Mas

s S

igna

l / A

.U.

Temperature / K450 500 550 600 650 700 750 800 850

Mass=104 Pentanethiol

Mas

s Si

gnal

/ A.

U.

Temperature / K

450 500 550 600 650 700 750 800 850

Mass=104 TH2MT

Mas

s Si

gnal

/ A.

U.

Temperature / K

450 500 550 600 650 700 750 800 850

Mass=72 Pentane

Mas

s Si

gnal

/ A.

U.

Temperature / K

450 500 550 600 650 700 750 800 850

Mass=70 Pentene

M

ass

Sign

al /

A.U

.

Temperature / K

450 500 550 600 650 700 750 800 850

Mass=55

Mas

s S

igna

l / A

.U.

Temperature / K

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Figure 3.8. TPR of 2MT on 2%Pt/Al2O3 profiles

450 500 550 600 650 700 750 800 850

Mass=104 Pentanethiol

Mas

s Si

gnal

/ A.

U.

Temperature / K

450 500 550 600 650 700 750 800 850

Mass=70 Pentene

Mas

s Si

gnal

/ A

.U.

Temperature / K

450 500 550 600 650 700 750 800 850

Mass=97 2MT

Mas

s S

igna

l / A

.U.

Temperature / K

450 500 550 600 650 700 750 800 850

Mass=102 TH2MT

Mas

s Si

gnal

/ A.

U.

Temperature / K

450 500 550 600 650 700 750 800 850

Mass=97 2MT

Mas

s Si

gnal

/ A.

U.

Temperature / K450 500 550 600 650 700 750 800 850

Mass=55

Mas

s Si

gnal

/ A.

U.

Temperature / K

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Figure 3.9. TPR of 2MT on CoMo/ Al2O3 profiles

450 500 550 600 650 700 750 800 850

Mass=97 2MT

Mas

s Si

gnal

/ A.

U.

Temperature / K

450 500 550 600 650 700 750 800 850

Mass=72 Pentane

Mas

s Si

gnal

/ A.

U.

Temperature / K

450 500 550 600 650 700 750 800 850

Mass=70 PenteneM

ass

Sign

al /

A.U

.

Temperature / K

450 500 550 600 650 700 750 800 850

Mas

s Si

gnal

/ A.

U.

Mass=55

Temperature / K

450 500 550 600 650 700 750 800 850

Mass=104 Pentanethiol

Mas

s Si

gnal

/ A.

U.

Temperature / K

450 500 550 600 650 700 750 800 850

Mass=102 TH2MT

Mas

s Si

gnal

/ A.

U.

Temperature / K

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Figure 3.10. TPR of 2MT on WP/SiO2 profiles

450 500 550 600 650 700 750 800 850M

ass

Sign

al /

A.U

.

Mass=104 Pentanethiol

Temperature / K

450 500 550 600 650 700 750 800 850

Mas

s Si

gnal

/ A.

U.

Mass=102 TH2MT

Temprature / K

450 500 550 600 650 700 750 800 850

Mas

s Si

gnal

/ A.

U.

Mass=97 2MT

Temprature / K

450 500 550 600 650 700 750 800 850

Mas

s Si

gnal

/ A.

U.

Mass=72 Pentane

Temperature / K450 500 550 600 650 700 750 800 850

Mas

s Si

gnal

/ A.

U.

Mass=55

Temperature / K

450 500 550 600 650 700 750 800 850

Mas

s Si

gnal

/ A.

U.

Mass=70 Pentene

Temperature / K

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101

Chapter 4

Nature of Active Sites in Ni2P Hydrotreating Catalysts as Probed by Iron

Substitution

4.1 Introduction

Reduction of the sulfur content in transportation fuels is an area of great current

activity in the refining industry [1-4]. The currently allowed concentration of sulfur in

diesel is 10 ppm worldwide and in 2009 the permitted quantity of sulfur in gasoline has

been reduced to 30 ppm in the United States [5]. This level of sulfur elimination requires

the removal of +99.99% of sulfur from a typical crude containing 1.5% sulfur, and is

referred to as ultra deep hydrodesulfurization (HDS). The processing is challenging

because it requires the HDS of the most refractory sulfur compounds, sterically hindered

dibenzothiophenes, which are unreactive [6,7]. Many approaches are being taken and the

area has been recently reviewed [1].

Considerable effort has been placed on optimizing current commercial metal sulfides,

using novel supports, exploring new compounds, and improving processes. Metal

phosphides have recently received extensive attention as a new type of HDS catalyst

because of their high activity and stability in the HDS and HDN of model and real [1]

feeds. Early work showed that the activity of common phosphides follows the order:

Ni2P > WP > MoP > CoP > Fe2P in the simultaneous HDS of dibenzothiophene (3000

ppm S) and HDN of quinoline (2000 ppm N) at 643 K and 3.1 MPa, with the comparison

based on equal sites (240 μmol CO chemisorption for phosphides) loaded in the reactor

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102

[8,9]. A number of bimetallic phosphides such as NixMoyP [10-15], CoxMoyP [11,16] and

NixCoyP [14,17,18] have also been studied because a synergistic effect between the

components was foreseen as found for promoted metal sulfides. Unexpectedly, however,

these bimetallic phosphide phases did not show enhanced activity over the component Ni,

Co or Mo phosphides, except for the case of CoxNiyP [18] where a modest 50% increase in

conversion was found.

The high activity of Ni2P has prompted many studies of its synthesis, structure [19],

and reactivity. The crystal structure of Ni2P is hexagonal with space group P 6͞2m, and

contains two types of Ni atoms, Ni(1) of tetrahedral coordination and Ni(2) of square

pyramidal coordination (Fig. 4.1). A recent study [20] indicated that the pyramidal Ni(2)

type was particularly active for HDS by the hydrogenation route. The compound Fe2P is

isostructural with Ni2P [21,22], and it was of interest to study NiFeP alloys because Fe2P

itself has very low activity, so it was surmised that substitution of Fe for Ni could provide

confirmation of the role of the two types of Ni atoms. This is the first study of the catalytic

activity of NiFeP. In order to gain insight into the nature of the surface sites use was made

of Fourier transform infrared (FTIR) spectroscopy using CO as the probe molecule.

Extended X-ray absorption fine structure (EXAFS) measurements were carried out to

elucidate the position of substitution of the Fe atom in the active phase.

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103

Figure 4.1. Tetrahedral Ni(1) and pyramidal Ni(2) sites inNi2P

4.2 Experimental

4.2.1 Materials

The support fumed silica EH-5 (Surface area = 350 m2g-1) was provided by Cabot

Corp. The chemicals used in the synthesis of the catalysts were Fe(NO3)3·9H2O (Aldrich,

99%), Ni (NO3)2·6H2O (Alfa Aesar, 99%), (NH4)2HPO4 (Aldrich, 99%). The chemicals

utilized in the reactivity study were 4,6-dimethyldibenzothiophene (synthesized, 95%),

dimethyl disulfide (Acros Organics, 99%), quinoline (Aldrich, 98%), tetralin (Aldrich,

97%), n-octane (Acros Organics, 99%), and n-tridecane (Alfa Aesar, 99%). The gases

employed were H2 (Airco, Grade 5, 99.99%), He (Airco, Grade 5, 99.99%), CO (Linde

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Research Grade, 99.97%), 0.5% O2/He (Airco, UHP Grade, 99.99%), O2 (Airco, UHP

Grade, 99.99%), 10% H2S/H2 (Airco, UHP Grade, 99.99%) and N2 (Airco, Grade 5,

99.99%).

4.2.2 Bimetallic Phosphides Catalysts Synthesis

The NiFeP catalysts were prepared by temperature-programmed reduction (TPR),

following procedures reported previously [23,24]. Briefly, the synthesis of the catalysts

involved two stages. First, solutions of the corresponding metal phosphate precursors were

prepared by dissolving appropriate amounts of Fe(NO3)3·9H2O, Ni(NO3)2·6H2O, with

ammonium phosphate in distilled water, and these solutions were used to impregnate silica

EH-5 by the incipient wetness method. The obtained samples were dried and calcined at

500 °C for 6 h, then ground with a mortar and pestle, pelletized with a press (Carver,

Model C), and sieved to particles of 650–1180 μm diameter (16/20 mesh). Second, the

solid phosphates were reduced to phosphides at 2 °C min-1 in flowing H2 [1000 cm3 (NTP)

min−1 g−1]. The reduction temperatures were for 552 °C for Ni2P/SiO2, 562 °C for

NiFeP(3:1)/SiO2, 567 °C for NiFeP(1:1)/SiO2, 572 °C for NiFeP(1:3)/SiO2, 572 °C for

Fe2P/SiO2. The samples were kept at the reduction temperatures for 2 h, followed by

cooling to room temperature under He flow [100 cm3 (NTP) min−1], and then passivated at

room temperature in a 0.5% O2/He for 4 h. The total metal molar loading was 1.6 mmol

g−1 (mmol per g of support) in all cases. Compositions prepared were Ni2P/SiO2,

NiFeP(3:1)/SiO2, NiFeP(1:1)/SiO2, NiFeP(1:3)/SiO2, and Fe2P/SiO2, where the numbers in

parenthesis are molar ratios.

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105

4.2.3 Characterization

Temperature-programmed reduction (TPR) was carried out on pelletized catalyst

samples (typically 0.2 g) placed in quartz U-tube reactors. The samples were heated with

linear temperature ramps in flowing hydrogen to reduce the metal phosphate to metal

phosphide. The temperature was increased from room temperature to 700°C at 2 °C with a

hydrogen flow rate of 200 cm3 (NTP) min-1. A portion of the exit gas flow was sampled

through a leak valve into a mass spectrometer and the masses 2 (H2), 18 (H2O), 31 (P) and

34 (PH3) were monitored during the experiment.

Irreversible CO uptake measurements were used to titrate the surface metal atoms

and to provide an estimate of the active sites on the catalysts for the transition metal

phosphides. Usually, 0.3 g of a passivated sample was loaded into a quartz reactor. The

passivated transition metal phosphides were reduced at 450 oC for 2 h. After cooling in He,

pulses of CO in a He carrier at 65 cm3 (NTP) min−1 were injected at room temperature

through a sampling valve, and the mass 28 (CO) signal was monitored with a mass

spectrometer. CO uptake was calculated by measuring the decrease in the peak areas

caused by adsorption in comparison with the area of a calibrated volume (19.5 μmol).

Surface areas of the samples were obtained using the BET method based on

adsorption isotherms at liquid nitrogen temperature, and using a value of 0.162 nm2 for the

cross-sectional area of a N2 molecule. The measurements were performed in a volumetric

adsorption unit (Micromeritics ASAP 2000). X-ray diffraction (XRD) patterns of the

samples were obtained with a Scintag XDS-2000 powder diffractometer operated at 45 kV

and 40 mA, using Cu Kα monochromatized radiation (λ= 0.154178 nm). The crystallite

size of the supported samples was calculated using the Scherrer equation, Dc = Kλ/β cos(θ),

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106

where K is a constant taken as 0.9, λ is the wavelength of the X-ray radiation, β is the peak

width in radians at half-maximum, corrected for instrumental broadening (0.1◦), and 2θ is

the Bragg angle.

X-ray absorption spectra at the Ni K-edge (8.333 keV) and Fe K-edge (7.112 keV)

of reference and catalyst samples were recorded in the energy range 8.233–9.283 keV at

beam line X18B at the National Synchrotron Light Source at Brookhaven National

Laboratory. The X-ray ring at the National Synchrotron Light Source has a flux of 1 ×

1010 photons s−1 at 100 mA and 2.5 GeV. The monochromator is equipped with a Si(111)

channel-cut single crystal and has an energy range capability of 5.8–40 keV. The crystal

was detuned slightly to prevent glitches due to harmonics. EXAFS spectra were recorded

at ambient temperature in transmission mode using ionization chambers for the detection

of primary (I0, 100% N2) and transmitted (It, 75% N2, balance Ar) beam intensities.

Samples before reaction (labeled fresh) were reduced in hydrogen as for the

reactivity studies, and were loaded into cells with Kapton windows without exposure to the

atmosphere. Samples after reaction (denoted as spent) were removed from the reactor and

placed in a hexane solvent and washed and dried before being transferred to the EXAFS

cell, all without exposure to air. Bulk reference samples were diluted with BN (0.1 g cat +

0.3 g BN). The EXAFS data were analyzed by the program Athena [25]. To fit the

experimental EXAFS spectra for the fresh and spent samples, theoretical EXAFS models

were constructed from the software FEFF [26] based on the crystal structures of Ni2P and

Fe2P [24]. The Fourier transform (FT) spectra are shown without phase correction. The

FT of the measured spectra was fitted with a k-weight of 3.

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107

The EXAFS data from the supported Ni2P and Fe2P were transformed over the data

range from 20 to 120 nm-1 and modeled in the region from 0.1 to 0.3 nm-1. The EXAFS

models based on the crystal structures of Ni2P and Fe2P contains parameters: energy shift

values, coordination number values, σ2. The quantity Reff is the initial reference half path

length (bond length for single scattering path) as calculated by FEFF. The EXAFS best-fit

values for the half path lengths (R) and σ2 are listed in table 4.6-4.11. For the spent and

fresh NiFeP(1:1)/SiO2, difference spectra of the raw absorption data between spent and

fresh samples were obtained to show the presence of a small oscillation.

4.2.4 Reactivity Studies

Hydrotreating activities of the samples were measured in a three-phase, packed-bed

reactor operated at 3.1 MPa and 613K with a model feed liquid containing 500 ppm sulfur

as 4,6-DMDBT, 3000 ppm sulfur as dimethyl disulfide, 200 ppm nitrogen as quinoline, 1

wt.% tetralin, 0.5 wt.% n-octane as internal standard, and balance n-tridecane. The

schematic of the testing system was described in an earlier paper [27]. Briefly, the testing

unit consisted of three parallel reactors immersed in a fluidized sand bath (Techne, Model

SBL-2) whose temperature was controlled by a temperature controller

(Omega, Model 6015 K). The reactors were 19 mm / 16 mm (o.d./i.d.) 316 stainless steel

tubes with central thermocouples monitoring the temperature of the catalyst. The catalysts

were in the form of pellets (16/20 mesh), and were supported between quartz wool plugs in

a 13 mm i.d. 316 stainless steel basket. The hydrogen flow rate was set to 100 μmol s-1

(150 cm3 min-1, NTP) with a mass flow controller (Brooks, Model 5850E), and the feed

liquid was injected by a high-pressure liquid pump (LDC Analytical, Model NCI 11D5) at

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108

a flow rate of 5 cm3 h-1. Quantities of catalysts loaded in the reactors corresponded to the

same amount of CO uptake (240 µmol). Prior to the reactivity testing, the catalysts were

pretreated in H2 at 450 ºC for 2 h. Hydrotreating products were collected every few hours

in sealed septum vials and were analyzed by a gas chromatograph (Hewlett-Packard,

5890A) equipped with a 0.32 mm i.d. x 50 m fused silica capillary column (CPSIL-5CB,

Chrompack, Inc.) and a flame ionization detector.

4.2.5 CO-FTIR

Transmission infrared spectra of pressed wafers (∼50 mg) of catalysts were

collected in situ in a reactor cell placed in a FTIR spectrometer (Bio-Rad Model FTS

3000MX) at a resolution of 4 cm−1 and using 64 scans spectrum. The IR cell was equipped

with water-cooled KBr windows, connections for inlet and outlet flows, and thermocouples

to monitor and control the temperature. Before dosing CO, the samples were reduced in

H2 at 450 °C for 2 h at the same conditions used for CO chemisorption and reactivity

studies, then cooled to room temperature in a He flow and exposed to CO until saturation

was achieved. The samples were then purged in He carrier for 300 s to remove gaseous

and weakly adsorbed CO species. The spectra were obtained in the absorbance mode and

are shown after subtraction of a background spectrum obtained on the freshly reduced

samples to make the spectral features more clear.

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4.3 Results and discussion

4.3.1 TPR

The supported nickel-iron phosphides were prepared in two stages as described in

the experimental part. First, solutions of the nickel-iron and phosphorous components

were impregnated on the silica support and the materials were dried to form supported

phosphate precursors. Second, the phosphates were transformed into phosphides by

temperature-programmed reduction (TPR). The TPR experiments were carried out to

understand the phenomena involved in the reduction process and to determine the optimum

reduction conditions used for large scale catalyst preparation.

400 600 800 1000 1200Temperature / K

Fe2P/SiO

2

Ni2P/SiO2 NiFeP(3:1)/SiO

2

NiFeP(1:1)/SiO2

Mas

s Sp

ectro

met

er S

igna

l / A

.U.

(m=1

8)

NiFeP(1:3)/SiO2

Figure 4.2. Temperature programmed reduction of Ni2P, Fe2P and NiFeP samples (mass

18 signal)

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Fig. 4.2 shows the water evolution (mass 18) during the temperature programmed

reduction stage of the NiFeP/SiO2 synthesis with different Ni to Fe ratios. Only the results

for mass 18 (H2O) are shown, because the other monitored masses provided little

additional information. As Fig. 4.2 shows, the TPR peak for Fe2P/ SiO2 is wider than that

of Ni2P/ SiO2 and has a trailing high-temperature tail which indicates that Fe2P is

intrinsically more difficult to reduce than Ni2P and is probably more heterogeneously

dispersed. Decreasing the relative Ni : Fe ratio (1:0, 3:1, 1:1, 1:3, 0:1) in the samples shifts

the reduction peaks to higher temperature 825 K, 835 K, 840 K, 846 K, 846 K, respectively.

With increasing Fe contents, the TPR traces showed more peaks and a more complicated

overall reduction pattern than that of the higher Ni content samples. The samples with a

single TPR peak are likely to contain a single phase. When Fe reached 75% for the sample

with Ni to Fe ratio 1:3, two distinct TPR peaks were observed. One peak is very close to

the peak temperature of iron phosphide, the other one overlaps the initial nickel phosphate

reduction temperature. Thus, it is possible that small quantities of two different phases are

present in this sample.

4.3.2 XRD

Analysis of the products of TPR was carried out by XRD (Fig. 4.3). The

diffraction pattern for iron phosphide shows a high background because of fluorescence of

the iron. The standard patterns from the powder diffraction file for Ni2P and Fe2P are very

similar. Ni2P and Fe2P both adopt the same hexagonal structure (space group: ,

barringerite shown in Fig. 4.1) [28]. Table 4.1 lists the lattice parameters of Ni2P and Fe2P,

and as can be seen they are close, with the Fe2P unit cell being slightly larger than that of

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Ni2P. As a result, the five silica supported samples Ni2P/SiO2, NiFeP (3:1)/SiO2, NiFeP

(1:1)/SiO2, NiFeP (1:3)/SiO2, and Fe2P/SiO2 showed very similar XRD patterns as well

(Fig. 4.3). Comparison to the PDF for Ni2P and Fe2P indicates that the mixed NiFe

samples NiFeP (3:1)/SiO2, NiFeP (1:1)/SiO2 and NiFeP (1:3)/SiO2 have patterns that are

closer to that of Ni2P, which indicates that the new phase formed in the bimetallic

phosphide system has crystal structure properties mainly dominated by the Ni2P phase.

Table 4.2 lists the d-spacings for the series of compositions, and these show a monotonous

progression consistent with Vegard’s Law, indicating that uniform alloys are formed.

Table 4.1. Ni2P, Fe2P lattice parameters (nm)

a b c Ni2P 0.5859 0.5859 0.3382 Fe2P 0.5867 0.5867 0.3458

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10 20 30 40 50 60 70 80 90

NiFeP(1:3)/SiO2

Inte

nsity

/ A.

U.

Ni2P/SiO2

NiFeP(3:1)/SiO

2

NiFeP(1:1)/SiO2

Fe2P/SiO

2

Ni2P PDF#3-953

Fe2P PDF#33-670

2θ / degrees

Figure 4.3. X-ray diffraction patterns for reduced Ni2P, Fe2P and NiFeP samples

Table 4.2. d space of the three strongest peaks of XRD for NiFeP/SiO2 samples

d-spacing (hkl)/nm (111) (201) (210)

Ni2P/SiO2 0.221 0.202 0.191 NiFeP(3:1)/SiO2 0.219 0.202 0.192 NiFeP(1:1)/SiO2 0.220 0.202 0.190 NiFeP(1:3)/SiO2 0.220 0.203 0.192

Fe2P/SiO2 0.223 0.205 0.193

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4.3.3 CO chemisorption and BET Results

Table 4.3. Characterization of NiFeP/SiO2 samples

Sample BET Surface Area (m2 g-1)

CO uptake (μmol g-1)

Dispersion(%)

Particle sizea/nm

Crystallite size b/nm

Ni2P/SiO2 135 110 10 9 10 NiFeP(3:1)/SiO2 138 96 8 11 7 NiFeP(1:1)/SiO2 153 60 5 18 9 NiFeP(1:3)/SiO2 126 32 3 33 7

Fe2P/SiO2 148 52 4 20 21 a the particle size estimated by 0.9/D based on the CO chemisorption b the crystallite sizes estimated by the Scherrer equation based on the XRD line-broadening The CO chemisorption and BET characterization results are reported in Table 4.3.

The specific surface areas of the supported materials were around 130~150 m2/g which are

considerably lower than that of the support (SiO2, 350 m2/g), which was caused by

sintering during the preparation process. The CO chemisorption results of the samples are

reported in the third column of Table 4.3. Ni2P/SiO2 has the biggest CO uptake among this

series of catalysts while Fe2P/SiO2 has a much lower CO uptake value. As the iron content

increased in the bimetallic phosphides, the CO uptake values decreased steadily. The

uptake of NiFeP(1:3)/SiO2 even went below that of Fe2P/SiO2. The dispersion (D) of the

metal sites was estimated from the CO uptakes and the known loading of the samples (in

all cases 1.6 mmol g-1 of total metal). The particle size (d) was then calculated using the

equation d≈0.9/D. The dispersion of the mixed NiFe samples decreased with iron content

following the trend in the CO uptake. The crystallite sizes were also calculated from the

XRD line-broadening using the Scherrer equation. The Ni2P/SiO2 and Fe2P/SiO2 sample

show good agreement between the particle size obtained from chemisorption and from

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114

line-broadening, which indicates that these two methods work consistently for estimation

of monometallic phosphide particles. The samples containing more Fe show larger particle

sizes estimated by chemisorption, which could indicate that the crystallites in this case are

agglomerated into polycrystalline particles, which reduces the CO uptakes and the

dispersion.

4.3.4 Infrared spectroscopy of adsorbed CO

Fig. 4.4 shows room temperature FTIR spectra of adsorbed CO in He flow on

Ni2P/SiO2, NiFeP (3:1)/SiO2, NiFeP (1:1)/SiO2, NiFeP (1:3)/SiO2 and Fe2P/SiO2. The

first four catalysts share the same absorbance unit scale. The FTIR signal for CO on

Fe2P/SiO2 was enlarged 2.5 times. A distinctive IR band at 2086 cm-1 was observed for

CO adsorbed on reduced Ni2P/SiO2, which is consistent with previous work [29,30]. As

will be discussed, the band position corresponds to that of linearly bonded CO on metallic

sites. The CO absorbance on Fe2P/SiO2 was observed at a substantially lower frequency of

2003 cm-1 with much lower intensity and a considerably broader peak. The red-shift and

broadening of the CO FTIR signal for the Fe2P/SiO2 sample is consistent with multiple

adsorptions on the metal centers as indicated from the dispersion calculations. This could

be due to the formation of crystallites with different habits and exposed faces than those of

Ni2P/SiO2 and its alloys. The FTIR results are summarized in Table 4.4.

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2200 2000 1800

0.5 A

2079

2081

2084

2086

NiFeP(1:3)/SiO 2

NiFeP(1:1)/SiO 2

Fe2P/SiO 2

NiFeP(3:1)/SiO 2

Ni2P/SiO 2

Abso

rban

ce /

A.U

.

W avenum bers / cm -1

0.2 A 2003

Figure 4.4. Infrared spectra of adsorbed CO on reduced Ni2P/SiO2, NiFeP (3:1)/SiO2,

NiFeP(1:1)/SiO2, NiFeP (1:3)/SiO2 and Fe2P/SiO2

Studies of IR spectra for adsorbed CO are useful for characterizing the bonding

properties of transition metal species [31,32,33]. Four types of bonding are reported for

CO on Ni2P: (1) CO adsorbed on Ni bridge sites (1914 cm-1), (2) Ni(CO)4 adsorbed on the

surface of the catalyst (~2056 cm-1), (3) CO adsorbed on atop Ni sites (2083-2093 cm-1),

and (4) P=C=O species on surface P atoms (2177-2187 cm-1). The characteristic band at

2086 cm-1 is attributed to CO on atop Ni sites on the surface of reduced Ni2P/SiO2. The

low frequency and high stability of the IR band has been explained as due to π back

bonding to the antibonding orbitals of CO by d-electrons of the reduced Ni species. In

going to the higher Fe content samples (Table 4.4) the CO frequency shifts monotonically

to lower values. This is consistent with larger amount of backbonding which weakens the

CO bond. It has been suggested that the higher electron density on the metal cation could

improve HDS activity by increasing the dissociation of H2 and the adsorption of

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116

thiophenes [34,35]. Thus, addition of the Fe component helps increase the electron density

on Ni, which might possibly lead to the improvement of the catalysts’ activity. In the case

of Fe2P/SiO2, the observed frequency of 2003 cm-1 is much lower than that expected for a

simple linear CO molecule on a metal phosphide. For example, in Co2P/SiO2 the CO band

appears at 2062 cm-1 [33]. It is also higher than that found for bridged CO species, for

example, 1919 cm-1 on Ni2P [32]. Although multiple CO adsorption is not commonly

reported for metallic Fe in NiFe [36] alloys, in the case of Fe2P, the Fe-Fe distance is larger

than in metallic Fe and may allow for multiple CO molecules to adsorb on a single site.

The best documented case for multiple CO adsorption is for Rh [37], where a gem-

dicarbonyl species is formed. The vibrational frequency of the gem-dicarbonyl is lower

than that of the linear single carbonyl and has contribution from symmetric and

asymmetric modes. This, in addition to a distribution of sites, may account for the breadth

of the CO mode observed on Fe2P/SiO2 (Fig. 4.3).

A comparison between the CO uptake by chemisorption and the CO relative

intensity by FTIR is reported in Table 4.4. The results are consistent for Ni2P/SiO2,

NiFeP(3:1)/SiO2, NiFeP(1:1)/SiO2, and NiFeP(1:3)/SiO2 since the CO uptake and relative

intensitydecrease in the same order. However, Fe2P/SiO2 is an exception: the CO uptake

for Fe2P/SiO2 is close to that of NiFeP(1:1)/SiO2 but the CO relative density measured by

FTIR is the lowest among this series of catalysts. This could be due to a much lower

extinction coefficient for multiple CO bonding than for linear CO.

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Table 4.4. Infrared spectrum data for reduced transition metal phosphides and

bimetallic phosphides

aIntegrated absorbance/mg catalyst 4.3.5 Reactivity Study

Reactivity studies of simultaneous 4,6-dimethyldibenzothiophene (4,6-DMDBT)

HDS and quinoline HDN were carried out at two different temperatures 300 and 340 ºC

(573, 613 K) at the same pressure (3.1 MPa, 450 psig). Fig. 4.5 shows the 4,6-DMDBT

conversion for the Ni2P/SiO2 (panel a), NiFeP(3:1)/SiO2 (panel b) and NiFeP(1:1)/SiO2

(panel c) samples. The Fe2P/SiO2 sample deactivated in the course of reaction, as found

earlier [24], and results for this sample cannot be presented. Various liquid feeds with the

different compositions were introduced in sequence in the course of the reaction test.

Initially (Fig. 4.5, section i), 0.05% S in form of 4,6-DMDBT, 0.3% S as DMDS, 0.02% N

as quinoline dissolved in tridecane were introduced at 573 K (300 oC). The Ni2P/SiO2

catalyst exhibited high reactivity and stability with a 4,6-DMDBT conversion of 97-99%.

The NiFeP(3:1)/SiO2 sample had an HDS conversion of around 90% and showed a slight

decline in activity The NiFeP(1:1) /SiO2 sample had a HDS conversion of only about 40%

and also exhibited deactivation. With increase in Fe content, the activity of the catalysts

became lower. This is consistent with an earlier study of dibenzothiophene HDS which

Sample νCO cm-1

Relative CO site intensitya

CO uptake μmol g-1

Ni2P/SiO2 2086 1.6 110 NiFeP(3:1) / SiO2 2084 1.1 96 NiFeP(1:1) /SiO2 2081 0.9 60 NiFeP(1:3)/SiO2 2079 0.3 32

Fe2P/SiO2 2003 0.2 52

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118

showed that Fe2P had a much lower intrinsic activity than Ni2P [24]. After 80 h on stream

time the temperature was increased up to 613 K (340 oC) under the same feed (Fig. 4,

section ii), and this did not affect the HDS activity of Ni2P/SiO2, which was already

operating at very high conversion. However, the higher temperature increased the HDS

activity of NiFeP(3:1)/SiO2 to a conversion level of 95% and led to a substantial

enhancement in HDS activity for NiFeP(1:1)/SiO2 to a conversion level of 90 %. After 85

h a feed with an additional 0.3 % S as DMDS was introduced (Fig. 4.5, section iii). The

activity of the Ni2P/SiO2 did not show much difference, however, the 4,6-DMDBT

conversions with NiFeP/SiO2 (3:1) and NiFeP/SiO2 (1:1) decreased to 90% and 85%

respectively. The small effect of sulfur on the performance of Ni2P/SiO2 had been reported

earlier [38,39,40].

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119

020

406080

100

40 60 80 100 120 140 160 1800

20

40

60

80

100

020406080

100

0.02% N0.6% S

iii) 0.05% S

0.02% N0.3% S

ii) 0.05% S

0.02% N0.3% S

i) 0.05% S4,6-DMDBT: DMDS:Quinoline:

340 oC300 oC

c) NiFeP(1:1)/SiO2

b) NiFeP(3:1)/SiO2

HD

S co

nver

sion

/ %

Time on Stream / h

a) Ni2P/SiO2

Figure 4.5. Activity test in HDS of 4,6-DMDBT for Ni2P/SiO2, NiFeP(3:1)/SiO2 and

NiFeP(1:1)/SiO2

The activation of the NiFeP(3:1)/SiO2 samples when the temperature was raised

from 513 to 613 K (300 to 340 °C) was relatively gradual, requiring more than 30 h, and

was dramatic, especially for the case of the higher Fe content NiFeP(1:1)/SiO2 (Fig. 4.5,

panel iii). The length of time indicates that there was a substantial structural

transformation in the samples, rather than a simple compositional change on the surface. It

is likely that a major reconstruction occurred, leading to perhaps a Ni terminated surface,

because the activities of the NiFeP/SiO2 increased to close to the level of the Ni2P/SiO2

sample. The active surface of Ni phosphide catalysts is actually a phosphosulfide

[32,38,41] and there is hydrogen present in the feed, so a driving force for the

reconstruction may be the formation of Ni-S bonds at the expense of Fe-S bonds, which

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120

would be less stable in the presence of H2 at high temperature. When sulfur concentration

is increased (Fig. 4.5, section iii), this favors the formation of Fe-S, and the activity is

decreased, because of the intrinsically lower activity of Fe2P.

Table 4.5. Conversion and selectivity for silica-supported nickel phosphide and nickel iron phosphides at 613 K and 3.1 Mpa after 110 h on stream.

Reactants

Type Conversion/%

Products Selectivity/%

4,6-DMDBT

Ni2P /SiO2

NiFeP(3:1) /SiO2

NiFeP(1:1) /SiO2

Ni2P /SiO2

NiFeP(3:1) /SiO2

NiFeP(1:1) /SiO2

HDS 99 99 96

3,3`-Dimethybiphenyl 12 69 85 3-(3`-

Methylcyclohexyl)toluene 53 21 11

3,3`-Dimethylbicylohexyl 35 10 4

Quinoline HDN 100 100 100 Propylcyclohexane 74 45 37 propylbenzene 26 55 63

The conversions and product distributions at 613 K and 3.1 MPa are summarized in

Table 4.5. The table lists the reactants, the reaction types, and the corresponding

conversions and selectivities for the samples. As discussed earlier, Ni2P/SiO2 displayed

the highest activity with a 4,6-DMDBT conversion level of 99%. The HDS activity of the

NiFeP(3:1)/SiO2 and NiFeP(1:1)/SiO2 catalysts were also very high giving a 4,6-DMDBT

conversion over 95%. According to the published literature [42,43], there are three major

products formed from 4,6-DMDBT conversion on the catalysts: (1) 3,3’-dimethylbiphenyl

(DMBP), (2) 3-(3’-methylcyclohexyl)toluene (MCHT), and (3) 3,3’-dimethylbicyclohexyl

(DMBCH). As a first approximation DMBP can be considered to result from a direct

desulfurization (DDS) pathway, whereas the MCHT and DMBCH products from a

hydrogenation (HYD) pathway [44,45]. Table 4.5 shows that the Ni2P/SiO2 gave a low

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121

DMBP selectivity of 12% and high MCHT and DMBCH selectivity totally accounting for

88%, indicating that Ni2P/SiO2 favors the HYD pathway. On the other hand,

NiFeP(3:1)/SiO2 and NiFeP(1:1)/SiO2 catalysts showed higher DMBP selectivity than that

of the hydrogenation products MCHT and DMBCH. The NiFeP(1:1)/SiO2 sample even

gave higher DMBP selectivity of 85% than the 69% selectivity with NiFeP(3:1)/SiO2. This

suggests that Fe intrinsically favors the DDS pathway more than HYD pathway. As the

DDS pathway is intrinsically more difficult than the HYD pathway for HDS of 4,6-

DMDBT, this rationalizes the decrease in HDS activity in the samples of higher Fe

content.

The HDN of quinoline does not occur directly because of the strength of the C–N

bonds and is a complex sequential reaction involving hydrogenation of the N-ring,

hydrogenolysis of the aliphatic C–N bond (CNH), hydrogenation of the C6-ring (HYD),

and elimination of ammonia [46]. This may be followed by hydrogenation of the C6-ring

to form propylcyclohexane (PCH) or dehydrogenation to form propylbenzene (PB). In the

case of the phosphides studied here it is found that Ni2P/SiO2 favors formation of PCH, as

also reported earlier [38,47], but increasing iron substitution in the NiFeP(3:1)/SiO2 and

NiFeP(1:1)/SiO2 catalysts gives higher selectivity to PB, indicating that the hydrogenation

ability of the Ni is suppressed by the addition of Fe.

On sulfides detailed kinetic studies show that the rates of HYD and CNH are of a

similar order of magnitude and no single rate-limiting step is operative [48,49]. On

phosphides similar studies have not been carried out, but it is likely that again no single

rate-limiting step is involved, and that CNH will be one of the key steps. The CNH

reaction is a complex reaction and requires multiple sites [50,51,52] among them an acid

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122

site to bind the nitrogen compound and a proximal basic site to carry out a β-H attack.

Thus, the reaction is structure sensitive [38], and is expected to be influenced by the

substitution of Fe for Ni at the surface.

4.3.6 EXAFS

Figure 4.6. Comparison of Fourier transforms of the Ni K-edge EXAFS spectra for bulk Ni2P, fresh Ni2P/SiO2, fresh NiFeP(3:1)/SiO2, fresh NiFeP(1:1) /SiO2, fresh NiFeP(1:3)

/SiO2, bulk NiO

EXAFS spectra at the Ni K-edge of the bulk and reduced supported Ni2P and

NiFeP samples are shown in Fig. 4.6. The figure displays the Fourier-transformed EXAFS

spectra of the fresh Ni2P/SiO2, NiFeP(3:1)/SiO2, NiFeP(1:1)/SiO2, and NiFeP(1:3)/SiO2

samples together with some reference standards, bulk Ni2P and NiO. The bulk Ni2P

reference sample shows two peaks at distances of 0.18 nm and 0.23 nm corresponding

0.0 0 .2 0.4 0.6

N i-P N i-MN i K -edge

B ulk N iO

B ulk N i2P

Fresh N i2P /S iO 2

Fresh N iFeP (1:1 )/S iO 2

Fresh N iFeP (1:3)/S iO 2

Fresh N iFeP (3:1)/S iO 2

FT M

agni

tude

/ A

.U.

d / nm

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123

roughly to Ni–P and Ni–Ni distances. The supported Ni2P samples show two distinctive

peaks at similar positions relative to the bulk Ni2P except that the Ni-P signal is stronger

than the Ni-Ni signal. This has been observed before [20,30] and is due to the small size

of the Ni2P crystallites and the presence of excess Ni(2) atoms on the surface which have a

pyramidal five fold coordination of phosphorous. The termination of the crystallites with

the Ni(2) atoms as opposed to the tetrahedral Ni(1) atoms results in high phosphorus level

in the sample. The samples with Fe content generally show similar bond distances. A Ni

atom probed by EXAFS can have Ni or Fe neighbors, and therefore these elements are

denoted as M. Although there is some variation from sample to sample, as the Fe content

increases the Ni-M peak intensity also grows while the Ni-P intensity decreases. This is

consistent with a lower number of Ni(2) sites at the surface of the crystallites, which would

be realized if Fe substituted for the Ni(2) site positions. Since these Ni(2) sites are found

to be more active [20Error! Bookmark not defined.], this would account for the initial

low activity of the samples containing Fe (Fig. 4.5, section i). A model of the fresh surface

is presented in Fig. 4.7 [53].

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124

EXAFS spectra at the Fe K-edge are shown in Fig. 4.8. The figure shows the

Fourier-transform intensities of the fresh NiFeP(3:1)/SiO2, NiFeP(1:1)/SiO2,

NiFeP(1:3)/SiO2, and Fe2P/SiO2 samples together with some reference standards, FeS and

FeO. The supported Fe2P sample shows two peaks at distances of 0.18 nm and 0.23 nm

roughly corresponding to Fe–P and Fe-Fe distances, which are similar to those of Ni2P

because Ni2P and Fe2P have the same crystal structure. The supported NiFeP samples

show two peaks at similar positions to supported Fe2P. However, in the higher Ni content

catalysts the Fe-P bond length is reduced from 0.18 nm to 0.17 nm and the peak intensity

becomes stronger. This indicates that the Fe in the low Fe content sample substituted in

the Ni(2) pyramidal sites in which the Fe-P distance is smaller. Overall, the peak

b) Working Ni2P/SiO2 a) Fresh Ni2P/SiO2

Figure 4.7. Cross sectional schematic model of phosphide crystallites in Ni2P/SiO2 and NiFeP(1:1)/SiO2

c) Fresh NiFeP(1:1)/SiO2 d) Working NiFeP(1:1)/SiO2

Ni(2) terminated phosphosulfide Ni(2) terminated

Fe in Ni(2) sites

Ni(2) terminated phosphosulfide

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125

intensities of the Fe-P bond were higher than those of the Fe-M bond, as found for the

Fe2P/SiO2 sample, indicating that the Fe was fully phosphided.

0 .0 0 .2 0 .4 0 .6

F e -P

F e-M

F e K -edg e

B u lk F eS

B u lk F eO

F resh F e 2P /S iO 2

F resh N iF eP (1 :1 )/S iO 2

F resh N iF eP (1 :3 )/S iO 2

F resh N iF eP (3 :1 )/S iO2

FT M

agni

tude

/ A

.U.

d / nm

Figure 4.8. Comparison of Fourier Transforms of the Fe K-edge EXAFS spectra for fresh

NiFeP(3:1)/SiO2, fresh NiFeP(1:1) /SiO2, fresh NiFeP(1:3) /SiO2, fresh Fe2P:SiO2, bulk FeO, Bulk FeS

The EXAFS signal could be described by χ function as a summation over all sine

waves scattered off of all neighboring atoms as a simple form by equation (1):

(1)

Fj(k) is for effective scattering amplitude, Φ(k) for effective scattering phase shift,

λ(k) for mean free path, R0 for initial path length. These 4 parameters are calculated

theoretically. The parameters including Ni (degeneracy of path), S02 (passive electron

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126

reduction factor), E0 energy shift, ΔR (change in half-path length), σi2 (mean squared

displacement) also known as Deby-Waller coefficient, are often determined from a fit to

data.

The EXAFS data of the Ni and Fe K-edge from the samples Ni2P/SiO2, NiFeP

(3:1)/SiO2, NiFeP (1:1)/SiO2, NiFeP (1:3)/SiO2, and Fe2P/SiO2 were Fourier transformed

over the data range from 20 to 120 nm-1 (2 to12 Ǻ-1) and modeled within the region from

0.1 to 0.3 nm

A Ni K-edge EXAFS model for the Ni2P/SiO2 was built using FEFF, based

on the crystal structure of Ni2P, and gave a reasonable fit as shown by the low R value in

Table 4.6. R stands for the residual, defined in equation (2).

∑∑

=

=−

= N

i

N

i theo

iy

iyiyR

1 exp

1 exp

)(

)()0( ( 2)

The model used 7 paths including both Ni(1) and Ni(2) as absorbers, so it accounts

for the known structural details from the crystallography of the compound. The model

determines two energy shift parameters for M-P and M-M paths respectively, 4 Debye-

Waller parameters, an expansion/contraction term [54] for the distances to these

neighboring atoms, and a scaling factor F(Ni2P) for the coordination number, which totally

contains 8 parameters. Because the number of independent fitting parameters is limited by

14 as given by the formula below [55], this model with 8 parameters is well constrained

and permissible.

Nind = 2 · k · R/π + 2 (3)

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127

Table 4.6. Ni K-edge EXAFS best-fit values for Ni2P/SiO2

Path desribption EXAFS parameters Path CN Reff(Ǻ) CN R(nm) CN σ2(nm2.104) R

Path for Ni2P S02=0.9

Ni1-P1 2 2.209 F(Ni2P).2 0.217±0.004 1.2±0.19 0.7±0.8 0.009 Ni1-P1 2 2.266 F(Ni2P).2 0.223±0.004 1.2±0.19 0.4±0.3 Ni1-Ni2 2 2.605 F(Ni2P).2 0.256±0.005 1.2±0.19 1.1±0.4 Ni1-Ni1 2 2.613 F(Ni2P).2 0.257±0.005 1.2±0.19 1.1±0.4 Ni1-Ni2 4 2.678 F(Ni2P).4 0.264±0.005 2.4±0.38 1.3±0.6 Ni2-P1 1 2.369 F(Ni2P).1 0.233±0.004 0.60±0.10 0.7±0.8 Ni2-P2 4 2.457 F(Ni2P).4 0.242±0.005 2.4±0.38 0.4±0.3

The EXAFS spectra of the NiFeP (3:1)/SiO2, NiFeP (1:1)/SiO2, and NiFeP

(1:3)/SiO2 were fit with a model combining pure Ni2P and Fe-substituted Ni2P (Tables 7a-

9a), and the resulting FT spectra for the best fit models are plotted in Fig. 4.9. The Fe

substitution was introduced by replacing Ni with Fe in the pure Ni2P calculation [56]. Two

types of Fe replacement, in the Ni(1) and Ni(2) positions, were carried out to determine

which position is favored for Fe substitution. The best fit models are compared through

the residual (R) and reduced chi-square (Chi) as summarized in Table 4.10. The Fe K-

edge fitting results are shown in Tables 7b-9b. The Ni component is introduced in a

similar way as the Fe substitution in the Ni K-edge EXAFS calculation. The same two

types of M(1) and M(2) replacements are calculated but only M(1) substitution model

could converge or give acceptable results with reasonable error range. Thus no

comparisons presented for Fe K-edge data fitting.

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128

Figure 4.9. (Left) Ni K-edge EXAFS spectra (symbols) and model (line) from NiFeP/SiO2

samples. (right) Magnitude of the Fourier transform of the Ni K-edge spectra (symbols) and model (line)

For the NiFeP (3:1)/SiO2 sample a best-fit model for the Ni K-edge EXAFS data of

contains two expansion/contraction terms for the distances to the neighboring atoms, two

σ2 terms for the M-M and M-P paths, two scaling factors F(Ni2P) and F(NiFeP) for the

coordination numbers and one energy shift parameter for all paths, resulting in a total of 7

variables for the model. The number of independent data points from the EXAFS Ni K-

edge is 14 as calculated by Equation 2, which means this model is well constrained. The

replacement of Ni in the M(2) and M(1) positions for the NiFeP (3:1)/SiO2 sample (Table

4.7a), shows the same residual and not much difference in the reduced-chi-square value,

which indicates that the Ni atoms do not have a preference for the M(1) and M(2) positions

in this sample. For the the Fe K-edge spectra of this same NiFeP (3:1)/SiO2 sample the

best-fit model (Table 4.7b) shows that 4 out of a total 6 Fe(1) atoms are replaced by Ni

0.2 0.4 0.6 0.8 1.0 1.2

k3 χ(k

)

k / nm-1

Ni K-edge fitting

Ni2P/SiO2

NiFeP(1:1)/SiO2

NiFeP(1:3)/SiO2

NiFeP(3:1)/SiO2

0.0 0.2 0.4 0.6

Ni2P/SiO2

NiFeP(1:1)/SiO2

NiFeP(1:3)/SiO2

d / nm

FT

Mag

nitu

de /

A.U

.

NiFeP(3:1)/SiO2

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129

atoms and so the Fe atoms favor the M(2) positions. Totally 7 parameters are used for this

model including two σ2 terms for the M-M and M-P paths, two scaling factors F(Ni2P) and

F(NiFeP) for the coordination numbers, two contraction / expansion terms and one energy

shift parameter for all paths.

Table 4.7a. Ni K-edge EXAFS best-fit values for NiFeP(3:1)/SiO2

Path desribption EXAFS parameters Path CN Reff(Ǻ) CN R(nm) CN σ2(nm2.10-4)

Model based on Ni(2) substitution Path for Ni2P S0

2=0.9 Ni1-P2 2 2.209 F(Ni2P).2 0.217±0.002 1.09±0.11 0.6±0.2 Ni1-P1 2 2.266 F(Ni2P).2 0.222±0.002 1.09±0.11 0.6±0.2 Ni1-Ni1 2 2.613 F(Ni2P).2 0.256±0.002 1.09±0.11 0.6±0.2 Ni2-P1 1 2.369 F(Ni2P).1 0.232±0.002 0.55±0.05 0.6±0.2 Ni2-P2 4 2.457 F(Ni2P).4 0.241±0.002 2.18±0.21 0.6±0.2 Ni2-Ni1 2 2.605 F(Ni2P).2 0.256±0.002 1.09±0.11 0.6±0.2 Ni2-Ni1 4 2.678 F(Ni2P).4 0.262±0.003 2.18±0.21 0.5±0.2 Ni1-Fe2 2 2.678 F(NiFeP).2 0.244±0.003 1.70±0.62 0.5±0.2

Model based on Ni(1) substitution Path for Ni2P S0

2=0.9 Ni1-P2 2 2.209 F(Ni2P).2 0.218±0.001 1.08±0.13 0.5±0.3 Ni1-P1 2 2.266 F(Ni2P).2 0.223±0.001 1.08±0.13 0.5±0.3 Ni1-Ni1 2 2.613 F(Ni2P).2 0.257±0.002 1.08±0.13 0.6±0.1 Ni2-P1 1 2.369 F(Ni2P).1 0.233±0.001 0.54±0.07 0.5±0.3 Ni2-P2 4 2.457 F(Ni2P).4 0.242±0.001 2.16±0.26 0.5±0.3 Ni2-Ni1 2 2.605 F(Ni2P).2 0.256±0.002 1.08±0.13 0.6±0.1 Ni2-Ni1 4 2.678 F(Ni2P).4 0.264±0.002 2.16±0.26 0.6±0.1 Ni2-Fe1 2 2.678 F(NiFeP).2 0.245±0.002 1.96±0.35 0.6±0.1

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Table 4.7b. Fe K-edge EXAFS best-fit values for NiFeP(3:1)/SiO2

Fe Kedge fitting model Path description EXAFS parameters

Path CN Reff(Ǻ) CN R(nm) CN σ2(nm2.104) Path for Fe2P S0

2=0.9 R Fe2-P2 1 2.371 F(Fe2P).2 0.233±0.002 1.33±0.25 0.9±0.3 0.012 Fe2-P1 4 2.485 F(Fe2P).2 0.244±0.002 5.31±0.98 0.9±0.3 Chi Fe2-Fe1 2 2.644 F(Fe2P).2 0.260±0.002 2.65±0.49 1.6±0.4 1136 Fe2-Fe1 4 2.702 F(Fe2P).2 0.266±0.002 5.31±0.98 1.6±0.4 Path for substitution phase Fe2-P2 1 2.371 F(NiFeP).1 0.214±0.002 1.11±0.20 0.9±0.3 Fe2-P1 4 2.485 F(NiFeP).4 0.225±0.002 4.45±0.82 0.9±0.3 Fe2-Fe1 2 2.644 F(NiFeP).2 0.239±0.002 2.22±0.41 1.6±0.4 Fe2-Ni1 4 2.702 F(NiFeP).4 0.244±0.002 4.45±0.82 1.6±0.4

For the NiFeP(1:1)/SiO2 sample the Ni K-edge spectra were modeled with two

structures in which half of either the Ni(2) and Ni(1) atoms in Ni2P were replaced with Fe

(Table 4.8a). As shown in Table 4.10, the model based on the Ni(2) replacement results in

about 20% improvement in the reduced-chi-square value compared to the model based on

the M(1) replacement. Thus, more Ni(2) atoms are expected to be replaced than Ni(1) in

NiFeP (1:1)/SiO2 sample. For the NiFeP(1:1)/SiO2 the Ni K-edge EXAFS also indicate

that Ni favors the M(1) position in the Ni2P phase, implying that Fe favors the Ni(2)

position. The best-fit model for NiFeP(1:1)/SiO2 Fe K-edge spectra are based on Fe2P

with Ni replacing half of the Fe(1) atoms (Table 4.8b). It also shows that Fe atoms favor

the M(1) sites in the Fe2P phase.

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131

Table 4.8a. Ni K-edge EXAFS best-fit values for NiFeP(1:1)/SiO2

Path desribption EXAFS parameters Path CN Reff(Ǻ) CN R(nm) CN σ2(nm2.104)

Model based on Ni(2) substitution Path for Ni2P S0

2=0.9 Ni1-P2 2 2.209 F(Ni2P).2 0.217±0.002 0.89±0.18 0.6±0.2 Ni1-P1 2 2.266 F(Ni2P).2 0.223±0.002 0.89±0.18 0.6±0.2 Ni1-Ni2 2 2.605 F(Ni2P).2 0.256±0.002 0.89±0.18 0.2±0.2 Ni1-Ni1 2 2.613 F(Ni2P).2 0.257±0.002 0.89±0.18 0.2±0.2 Ni1-Ni2 4 2.678 F(Ni2P).4 0.263±0.002 1.78±0.37 4.5±3.9 Path for Fe dopant Ni2P Ni1-P1 2 2.209 F(NiFeP).2 0.227±0.002 0.79±0.12 0.6±0.2 Ni1-P1 2 2.266 F(NiFeP).2 0.233±0.002 0.79±0.12 0.6±0.2 Ni1-Ni2 2 2.605 F(NiFeP).2 0.268±0.002 0.79±0.12 0.2±0.2 Ni1-Ni1 2 2.613 F(NiFeP).2 0.269±0.002 0.79±0.12 0.2±0.2 Ni1-Fe2 4 2.678 F(NiFeP).4 0.276±0.002 1.57±0.25 4.5±3.9

Model based on Ni(1) substitution Path for Ni2P S0

2=0.9 Ni1-P2 2 2.209 F(Ni2P).2 0.222±0.001 1.28±0.26 0.6±0.3 Ni1-P1 2 2.266 F(Ni2P).2 0.227±0.001 1.28±0.26 0.6±0.3 Ni1-Ni2 2 2.605 F(Ni2P).2 0.261±0.001 1.28±0.26 1.0±0.2 Ni1-Ni1 2 2.613 F(Ni2P).2 0.262±0.001 1.28±0.26 1.0±0.2 Ni1-Ni2 4 2.678 F(Ni2P).4 0.268±0.001 2.56±0.53 2.8±1.3 Path for Fe dopant Ni2P Ni2-P1 1 2.369 F(NiFeP).1 0.239±0.003 0.36±0.18 0.9±0.5 Ni2-P2 4 2.457 F(NiFeP).4 0.248±0.003 1.45 ±0.72 0.9±0.5 Ni2-Ni1 2 2.605 F(NiFeP).2 0.263±0.003 0.72±0.36 0.4±0.2 Ni2-Ni1 1 2.678 F(NiFeP).1 0.270±0.003 0.36±0.18 0.4±0.2 Ni2-Fe1 3 2.678 F(NiFeP).1 0.270±0.003 1.09±0.53 0.6±0.2

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Table 4.8b. Fe K-edge EXAFS best-fit values for NiFeP(1:1)/SiO2

Fe Kedge fitting model Path desribption EXAFS parameters

Path CN Reff(Ǻ) CN R(Ǻ) CN σ2(nm2.104) Path for Fe2P S0

2=0.9 R Fe1-P1 2 2.222 F(Fe2P).2 0.2.188±0.018 1.62±0.56 0.8±0.4 0.014 Fe1-P2 2 2.287 F(Fe2P).2 2.252±0.018 1.62±0.56 0.8±0.4 Chi Fe1-Fe1 2 2.592 F(Fe2P).2 2.552±0.021 1.62±0.56 6.4±4.3 583 Fe1-Fe2 2 2.645 F(Fe2P).2 2.604±0.021 1.62±0.56 6.4±4.3 Fe1-Fe2 4 2.702 F(Fe2P).4 2.661±0.022 3.25±1.11 6.4±4.3 Path for substitution phase Fe2-P2 1 2.371 F(NiFeP).1 2.312±0.017 0.41±0.16 0.8±0.4 Fe2-P1 4 2.485 F(NiFeP).4 2.423±0.018 1.62±0.64 0.8±0.4 Fe2-Fe1 2 2.644 F(NiFeP).2 2.579±0.019 0.81±0.32 6.4±4.3 Fe2-Fe1 1 2.702 F(NiFeP).1 2.635±0.020 0.41±0.16 6.4±4.3 Fe2-Ni1 3 2.702 F(NiFeP).3 2.635±0.020 1.22±0.48 0.8±0.4

For the NiFeP(1:3)/SiO2 sample the best-fit result for a Ni K-edge spectra model

based on the replacement with Fe of 6 Ni(2) atoms out of a total 8 Ni atoms leads to a 60%

improvement in reduced-chi-square value (Table 4.10) compared with a model with

replacement of Ni(1) atoms (Table 4.9a). Thus the model based on the Ni(2) replacement

is much better, which indicates that Fe atom substitution is more likely to occur in the M(2)

position. For the same sample the best-fit model for the Fe K-edge is based on Ni

substitution of 2 Fe(1) out of total 6 Fe atoms as shown in Table 4.9b. The best fit for

Fe2P/SiO2 is shown in Table 4.11 by using a similar model for Ni2P with the only

difference in lattice parameters (Table 4.1).

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133

Table 4.9a. Ni K-edge EXAFS best-fit values for NiFeP(1:3)/SiO2

Path desription EXAFS parameters

Path CN Reff(Ǻ) CN R(Ǻ) CN σ2(nm2.104) Model based on Ni(2) substitution

S02=0.9

Ni1-P1 2 2.209 F(NiFeP).2 0.221±0.001 1.90±0.23 1.0±0.2 Ni1-P1 2 2.266 F(NiFeP).2 0.227±0.001 1.90±0.23 1.0±0.2 Ni1-Fe2 2 2.605 F(NiFeP).2 0.261±0.001 1.90±0.23 0.8±0.1 Ni1-Ni1 2 2.613 F(NiFeP).2 0.261±0.001 1.90±0.23 0.8±0.1 Ni1-Fe2 4 2.678 F(NiFeP).4 0.268±0.001 3.80±0.46 5.4±3.9

model based on Ni(1)substitution Ni2-P1 1 2.369 F(NiFeP).1 0.237±0.001 0.93±0.11 3.4±2.8 Ni2-P2 4 2.457 F(NiFeP).4 0.246±0.001 3.72±0.44 3.4±2.8 Ni2-Ni1 2 2.605 F(NiFeP).2 0.261±0.001 1.86±0.22 1.0±0.3 Ni2-Fe1 4 2.678 F(NiFeP).4 0.268±0.001 3.72±0.44 1.0±0.3

Table 4.9b. Fe K-edge EXAFS best-fit values for NiFeP(1:3)/SiO2

Fe Kedge fitting model Path desribption EXAFS parameters

Path CN Reff(Ǻ) CN R(nm) CN σ2(nm2.104) Path for Fe2P S0

2=0.9 R Fe2-P2 1 2.371 F(Fe2P).1 0.214±0.001 0.58±0.10 0.9±0.5 0.009 Fe2-P1 4 2.485 F(Fe2P).4 0.224±0.002 2.31±0.40 0.9±0.5 Fe2-Fe1 2 2.644 F(Fe2P).2 0.239±0.002 1.15±0.20 0.8±0.2 Chi Fe2-Fe1 4 2.702 F(Fe2P).4 0.244±0.002 2.31±0.40 0.8±0.2 327 Path for substitution phase Fe2-P2 1 2.371 F(NiFeP).1 0.230±0.019 0.63±0.12 0.9±0.5 Fe2-P1 4 2.485 F(NiFeP).4 0.241±0.020 2.53±0.47 0.9±0.5 Fe2-Fe1 2 2.644 F(NiFeP).2 0.256±0.022 1.27±0.23 0.8±0.2 Fe2-Fe1 2 2.702 F(NiFeP).2 0.262±0.022 1.27±0.23 0.8±0.2 Fe2-Ni1 2 2.702 F(NiFeP).2 0.262±0.022 1.27±0.23 0.8±0.2

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Table 4.10. Comparison of different models based on Ni(1) and Ni(2) substitution for Ni

K-edge EXAFS

Table 4.11. Fe K-edge EXAFS best-fit values for Fe2P/SiO2

Path desribption EXAFS parameters Path CN Reff(Ǻ) CN R(nm) CN σ2(nm2.104) R

Path for Fe2P S02=0.9

Fe1-P1 2 2.222 F(Fe1).2 0.226±0.002 2.8±0.60 1.1±0.6 0.010 Fe1-P2 2 2.287 F(Fe1).2 0.233±0.002 2.8±0.60 1.0±0.6

Fe1-Fe 1 2 2.592 F(Fe1).2 0.264±0.003 2.8±0.60 0.8±0.3 Fe1-Fe2 2 2.645 F(Fe1).2 0.270±0.003 2.8±0.60 4.1±2.0 Fe1-Fe2 4 2.702 F(Fe1).4 0.275±0.003 5.6±1.2 0.8±0.3 Fe2-P2 1 2.371 F(Fe2).4 0.247±0.004 1.2±0.44 1.1±0.6 Fe2-P1 4 2.485 F(Fe2).4 0.259±0.004 4.6±1.7 1.0±0.6

Catalyst Best fit with Ni(1) replaced Best fit with Ni(2) replaced

NiFeP(3:1)/SiO2 R 0.005 0.005

Chi 1549 1528

NiFeP(1:1)/SiO2 R 0.005 0.005

Chi 428 355

NiFeP(1:3)/SiO2 R 0.013 0.008

Chi 686 282

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135

In summary, the substitution of Fe for Ni most likely occurs on the Ni(2) sites in

the catalysts NiFeP (1:1)/SiO2, NiFeP (1:3)/SiO2. Since the Ni(2) sites are more active

than the Ni(1) sites, this explains the decrease in HDS activity of NiFeP (1:1)/SiO2

compared to that of Ni2P/SiO2 (Fig. 4.5, section i). The two types of Fe substitutions with

Ni(1) and Ni(2) show similar possibility in the catalyst NiFeP(3:1)/SiO2. According to the

principle of efficient space filling [57,58], the larger metal atoms should preferentially

occupy the M(2) position with the highest coordination number. The metallic bond radii of

Ni and Fe are 0.125, 0.126 nm respectively, which are fairly close and make the situation

complex to determine Fe position in the alloys. It has been reported that the equilibrium

distribution of iron atoms over the tetrahedral site M(1) and pyramidal site M(2) will be

dependent on the chemical composition of FexNi1-xP(0<x<1) [57]: for the Fe-rich materials

(x ≥0.45), Fe atoms favors more M(2) sites than M(1); for the Ni-rich materials (x ≤

0.15), for example, FeNiP(0.15:0.85), the Fe will occupy exclusive M(1) sites. This

coincides with NiFeP/SiO2 samples in this study. Moreover, samples with smaller ratio of

Fe less than 15% are worthwhile for further investigation.

Fig. 4.10 shows the EXAFS data for the NiFeP(1:1) /SiO2 sample before and after

reaction together with FeS as reference. This sample was chosen for analysis because it

showed the greatest change in activity in the course of activation (Fig. 4.5, section ii). For

the Ni-edge (Fig, 10a) the fresh sample shows a strong Ni-M peak intensity relative to the

Ni-P peak, while the spent sample shows a reduced Ni-M peak intensity with maintenance

of the Ni-P intensity. For the Fe K-edge (Fig. 4.10b) the fresh sample shows a stronger Fe-

P peak intensity relative to the Fe-M peak, while the spent sample shows a decreased Fe-M

intensity with maintenance of the Fe-P intensity. The lowering of the Ni-M and Fe-M

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136

intensity is consistent with disruption of metal-metal bonds because of formation of a

surface sulfide. A schematic of the transformation to the active phase is depicted in Fig

4.7. In order to analyze the data further [59] a difference spectra of the Fe and Ni K-edge

spectra are taken between the fresh and spent NiFeP(1:1)/SiO2 sample as shown in Fig.

4.10c and 4.10d. A set of oscillations is obtained. It is assumed here that the structure of

the catalyst in the working state does not change very much on quenching and that the

catalyst sample referred to as spent retains its most important features. Indeed, the in situ

EXAFS studies of an active Ni2P catalyst show that the Ni2P structure is retained in the

bulk and that Ni-S bonds are observed on the surface [60]. The fact that the oscillations in

Fig. 4.10 are centered around zero indicate that the subtraction has been successful. The

difference spectra are fitted with Feff using Fe-S and Ni-S bonds at distances of 0.226 and

0.238nm, which are typical of these bonds. The bond length of FexSy varies between

0.221-0.246 nm (0.226 nm for FeS [61], 0.221 [62] and 0.225 nm in FeS2 [63], 0.214 and

0.246 nm in Fe3S4 [64]). In typical NixSy compounds, Ni–S bonds are found in the range

of 0.225–0.240 nm (0.238 nm for NiS [65], 0.236 nm for NiS2 [66], and 0.225 and 0.229

nm for Ni3S2 [67]). The good fit clearly shows that Fe-S and Ni-S bonds are present in the

spent sample, confirming the formation of a phosphosulfide. Many studies [68,69,70]

have shown that the active phase is a phosphosulfide. It has been found that small amounts

of Co in NixCoyP catalysts are promoters [18], and it should be interesting to probe the

reactivity of small amounts (< 15 mol%) of Fe in NiFeP catalysts.

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137

Figure 4. 10. a) Ni K-edge EXAFS spectra for fresh and spent NiFeP(1:1)/SiO2 b) Fe K-edge EXAFS spectra for fresh and spent NiFeP(1:1)/SiO2 c) Ni K-edge difference spectra

d) Fe K-edge difference spectra

0.4 0.6 0.8 1.0

d) Fe-Kedge difference spectra

k3 χ

(k)

k / nm-1

fitting experimental

0.4 0.6 0.8 1.0

k / nm-1

k3 χ(k

)

c) Ni-Kedge difference spectra

fitting experimental

0.0 0.2 0.4 0.6

b) Fe k-edge

d / nmFT

Mag

nitu

de /

A.U

.

Fresh NiFeP(1:1)/SiO2 Spent NiFeP(1:1)/SiO2 FeS

0.0 0.2 0.4 0.6

a) Ni k-edge

Fresh NiFeP(1:1)/SiO2 Spent NiFeP(1:1)/SiO2

FT M

agni

tude

/ A.

U.

d / nm

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138

In summary, the nature of the active sites in Ni2P HDS catalysts was studied by

forming NiFeP alloys and by correlating the reactivity behavior with the surface properties

and the compositions of the different phases. The Ni2P structure (hexagonal, ) has

two types of Ni, a tetrahedrally coordinated Ni(1) and a square pyramidal Ni(2), and

previous work [20] had indicated that the Ni(2) site was the active center for HDS by the

hydrogenation route. Fe2P has the same crystal structure as Ni2P, but is an inactive phase

probably because it forms strong Fe-S bonds, so Fe was deemed as a good diluent in the

Ni2P phase to probe the sites. Indeed FTIR spectra of adsorbed CO showed a gradual

progression to lower wavenumber as Fe was added to the Ni2P phase. In its reactivity

behavior, at a low temperature of 300°C, Ni2P/SiO2 was duly found to be more active than

NiFeP(3:1)/SiO2 and NiFeP(1:1)/SiO2 catalysts for HDS. This was explained by analysis

with extended x-ray absorption fine structure (EXAFS) spectroscopy which indicated that

Fe substituted in the active Ni(2) sites. As temperature was increased to 340°C, the

NiFeP(3:1)/SiO2 and NiFeP(1:1)/SiO2 samples both showed much higher activity, with

NiFeP(3:1)/SiO2 giving 99% 4,6-DMDBT HDS conversion, which was comparable to that

of Ni2P/SiO2. This suggested that a restructuring occurred at 340 oC which favored the

exposure of Ni(2) sites.

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139

4.4 Conclusions

The purpose of this work was to understand the nature of the active sites in Ni2P

hydrodesulfurization (HDS) catalysts by the alloying with Fe, an inactive element. The

catalysts studied were Ni2P/SiO2, NiFeP(3:1)/SiO2,, NiFeP(1:1)/SiO2, NiFeP(1:3)/SiO2, and

Fe2P/SiO2. The main findings were as follows.

1) Characterization of the catalysts by Fourier transform infrared (FTIR)

spectroscopy using CO as a probe showed a gradual diminution in the CO stretching

frequency, indicating that the Fe component has electron donating properties.

2) At low reaction temperature (300 oC) Ni2P has excellent activity for the

simultaneous HDS of 4,6-dimethyldibenzothiophene (conversion 99%) and the

hydrodenitrogenation of quinoline (conversion 100%), while Fe substitution decreases the

activity.

3) Analysis by extended x-ray absorption fine structure (EXAFS) analysis indicates

that the Fe preferentially substitutes for square pyramidal Ni sites (denoted as Ni(2) sites)

instead of tetrahedral Ni sites (denoted as Ni(1) sites).

4) The higher activity of Ni2P over the NiFeP alloys at 300 oC indicates that Ni(2)

sites are the active sites for HDS.

5) A reconstruction of the bimetallic catalysts occurs at higher temperature (340 oC)

that exposes the Ni(2) sites and results in increased activity of the catalysts.

6) EXAFS analysis of spent samples shows the formation of a surface phosphosulfide

phase with both Ni-S and Fe-S bonds.

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140

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148

Chapter 5

Conclusions

Transition metal phosphides, as a new class of hydrotreating catalysts, have received

extensive attentions because of their high activity and stability in the HDS and HDN of model

and real feeds. The transition metal phosphides have physical property similar to ceramics at the

same time retain electronic and magnetic properties as metal conductors. The activity of

common phosphides follows the order: Ni2P > WP > MoP > CoP > Fe2P. Ni2P is the most active

phosphide which shows excellent performance in HDS and HDN. With the HDS refractory

heteroaromatics like 4,6-DMDBT, Ni2P displays great activity by favoring the hydrogenation

pathway which is elucidated by the selectivity of the HYD and DDS products. The active phase

for Ni2P in the working state is the nickelphosphosulfide overlayer, which is more active than

nickelsulfide. Two types of Ni atoms, tetrahedral and square pyramidal are presented in Ni2P

crystal structure while square pyramidal sites are responsible for the high HDS activity and are

the active sites according to the EXAFS and reactivity study.

Hydrogenation and ring opening study of 2MT was carried out on sixteen total catalysts,

including supported noble metals, bimetallic noble metals, transition metal phosphides, and

transition metal sulfides. The major products were TH2MT, pentenes and pentane, and C5-thiols

could barely be observed. The selectivity towards the desired product TH2MT follows the order:

noble metals > bimetallics > phosphides > sulfides. The order of selectivity towards TH2MT

was opposite the order of the number of active sites of the catalysts, which indicates that the

active sites of the catalysts titrated by CO or O2 chemisorption favor HDS more than the

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149

hydrogenation product TH2MT. Temperature-programmed desorption (TPD) of hydrogen

indicated that the H2 desorption amount was inversely related to the rate of TH2MT formation.

Temperature-programmed reaction (TPR) showed that pentanethiol was the major product on the

2MT preadsorbed surface, especially on excellent HDS catalysts like CoMoS/Al2O3 and

WP/SiO2.

A study was conducted to understand the nature of the active sites in Ni2P

hydrodesulfurization (HDS) catalysts by the alloying with Fe, an inactive element. The catalysts

studied were Ni2P/SiO2, NiFeP(3:1)/SiO2,, NiFeP(1:1)/SiO2, NiFeP(1:3)/SiO2, and Fe2P/SiO2.

Characterization of the catalysts by Fourier transform infrared (FTIR) spectroscopy using CO as

a probe showed a gradual diminution in the CO stretching frequency, indicating that the Fe

component has electron donating properties. At low reaction temperature (300 oC) Ni2P has

excellent activity for the simultaneous HDS of 4,6-dimethyldibenzothiophene (conversion 99%)

and the hydrodenitrogenation of quinoline (conversion 100%), while Fe substitution decreases

the activity. The higher activity of Ni2P over the NiFeP alloys at 300 oC indicates that Ni(2) sites

are the active sites for HDS. A reconstruction of the bimetallic phosphides catalysts occurs at

higher temperature (340 oC) that exposes the Ni(2) sites and results in increased activity of the

catalysts. Analysis by extended x-ray absorption fine structure (EXAFS) analysis indicates that

the Fe preferentially substitutes for square pyramidal Ni sites (denoted as Ni(2) sites) instead of

tetrahedral Ni sites (denoted as Ni(1) sites). EXAFS analysis of spent samples shows the

formation of a surface phosphosulfide phase with both Ni-S and Fe-S bonds.