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
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
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.
iv
To my parents
for their constant support and love
v
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.
vi
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
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
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
ix
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
x
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
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
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
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
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
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.
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
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
4
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).
5
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
6
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
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].
8
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
-
9
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
10
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
11
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
12
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.
13
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.
14
References [1] C. Song, Catal. Today, 86 (2003) 211.
[2] P. Greening, Top. Catal., 16/17 (2001) 5.
[3] Y. Yoshimura, M. Toba, H. Farag, K. Sakanishi, Catal. Surv. Asia, 8 (2004) 47.
[4] T. Fujikawa, H. Kimura, K. Kiriyama, K. Hagiwara, Catal. Today, 111 (2006)
188.
[5] A. Zhou, X. Ma, C. Song, J. Phys. Chem. B., 110 (2006) 4699.
[6] S. Hatanaka, M. Yamada, O. Sadakane, Ind. Eng. Chem. Res. 36 (1997) 1519
[7] W.C. Cheng, G. Kim, A.W. Peters, X. Zhao, K. Rajagopalan, M.S. Ziebarth, C.J.
Pereira, Catal. Rev.-Sci. Eng. 40 (1998) 39
[8] U. Alkemake, T.J. Dougan, in: M. Absi-Halabi, J. Beshara, H. Qabazard, A.
Stanislaus (Eds.), Catalysts in petroleum refinishing and petrochemical industries 1995,
Elsevier, 1996, p. 303.
[9] B.C. Gates, J.R. Katzer, G.C.A. Schuit, Chemistry of catalytic processer,
Mcgraw-Hill: New York, 1979
[10] M. Egorova, R. Prins, Hydrodesulfurization of dibenzothiophene and 4,6-
dimethyldibenzothiophene over sulfided NiMo/γ-Al2O3, CoMo/γ-Al2O3, and Mo/γ-
Al2O3 catalysts, J. Catal. 225 (2004) 417
[11] R. Prins, M. Egorova, A. Röthlisberger, Y. Zhao, N. Sivasankar, P. Kukula,
Mechanisms of hydrodesulfurization and hydrodenitrogenation, Catal. Today 111 (2006)
84.
[12] M. L. Vrinat, Appl. Catal., 6 (1983) 137.
15
[13] H. Topsøe, B. S. Clausen, F. E. Massoth, Hydrotreating Catalysis-Science and
Technology, Springer, Berlin, 1996.
[14] A. N. Startsev, Catal. Rev.-Sci. Eng., 37 (1995) 353.
[15] Th. Weber, R. Prins, R. A. van Santen, Eds., Transition Metal Sulfides, Chemistry
and Catalysis, Kluwer, Dordrecht, 1998.
[16] S. Eijsbouts, Appl. Catal. A: Gen., 158 (1997) 53.
[17] Stud. Surf. Sci. Catal., Vol. 127, Hydrotreatment and Hydrocracking of Oil
Fractions, November 1999, pp. 1-458, Edited by B. Delmon, G.F. Froment and P. Grange.
[18] M. Breysse, E. Furimsky, Catal. Rev.-Sci. Eng., 44 (2002) 651.
[19] D.Zuo, D. Li, H. Nie, Y. Shi, M. Lacroix, M. Vrinat, J. Mol. Catal. A: Chem.,
211 (2004) 179.
[20] Y. Zhao, P. Kukula, R. Prins, J. Catal., 221 (2004) 441.
[21] A. Borgna, E.J.M. Hensen, J.A.R. van Veen, J. Catal., 221 (2004) 541.
[22] C. Dujardin, M.A. Lélias, J. van Gestel, A. Travert, J.C. Duchet, F. Maugé, Appl.
Catal A: Gen., 322 (2007) 46.
[23] R. Prins, M. Egorova, A. Röthlisberger, Y. Zhao, N. Sivasankar, Catal. Today,
111 (2006) 84.
[24] E.J,M. Hensen, V.H.J. de Beer, J.A.R. van Veen, R.A. van Santen, J. Catal., 215
(2003) 353.
[25] L. Vradman, M.V. Landau, M. Herskowitz, Fuel, 82 (2003) 633.
[26] Y.S. Al-Zeghayer, P. Sunderland, W. Al-Masry, F. Al-Mubaddel, A.A. Ibrahim,
B.K. Bhartiya, B.Y. Jibril, Appl. Catal. A: Gen., 282 (2005) 163.
16
[27] H.R. Reinhoudt, R. Troost, A.D. van Langeveld, J.A.R. van Veen, S.T. Sie, J.A.
Moulijn, J. Catal., 203 (2001) 509.
[28] D. Zuo, M. Vrinat, H. Nie, F. Maugé, Y. Shi, M. Lacroix, D. Li, Catal. Today, 93-
95 (2004) 751.
[29] M. Egorova, R. Prins, J. Catal., 225 (2004) 417.
[30] P. T. Vasudevan, J. L. G. Fierro, Catal. Rev.-Sci. Eng., 38 (1996) 161.
[31] T. G. Kaufmann, A. Kaldor, G. F. Stuntz, M. C. Kerby, L. L. Ansell, Catal. Today,
62 (2000) 77.
[32] R. Shafi, G.J. Hutchings, Catal. Today, 59 (2000) 423.
[33] M. Breysse, G. Djega-Mariadassou, S. Pessayre, C. Geantet, M. Vrinat, G. Pérot,
M. Lemaire, Catal. Today, 84 (2003) 129.
[34] T. C. Ho, Catal. Today, 98 (2004) 3.
[35] I. V. Babich, J. A. Moulijn, Fuel, 82 (2003) 607.
[36] Third International Symposium on Molecular Aspects of Catalysis by Sulfides,
Monte Verità, Switzerland, May 23-27, 2004.
[37] XIX Ibero American Catalysis Symposium, Merida, Mexico, Sept. 6-9, 2004.
[38] Fourth International Symposium on Molecular Aspects of Catalysis by Sulfides,
Doorn, The Netherlands, May 13-17, 2007.
[39] Angewandte Chemie, International Edition, Vol 40, 2001, Hydrodesulfurization
and Hydrodenitrogenation. Edited by Toshiaki Kabe, Atsushui Ishihara and Weihua Qian.
[40] Catal. Today, Vol. 107-108, October 2005, pp. 1-956, Edited by José Manuel
Domínguez and Jorge Ramírez.
17
[41] Catal. Today, Vol. 109, Hydroprocessing of Heavy Oil Fractions, Nov. 30, 2005,
pp. 1-218, Edited by Jorge Ancheyta, Mohan S. Rana and Edward Furimsky.
[42] Catal. Today, Vol. 104, Catalysis in Ultra-clean Fuels Production, June 15, 2005,
pp. 1-100, Edited by Noritatsu Tsubaki, Uday T. Turaga and Chunshan Song.
[43] Catal. Today, Vol. 98, International Symposium on Advances in Hydroprocessing
of Oil Fractions (ISAHOF 2004), Nov. 24, 2004, pp. 1-342, Edited by Jorge Ancheyta.
[44] Catal. Today, Vol. 86, Effects of Support in Hydrotreating Catalysis for Ultra-
clean Fuels, Nov. 1, 2003, pp. 1-288, Edited by Yasuaki Okamoto, Michele Breysse,
Gudimella Murali Dhar and Chunshan Song.
[45] Update article, Desulfurization, the new-fashioned way, Chem. Eng. Prog., April
2006, pp. 12-13.
[46] R. R. Chiannelli, M. H. Siadati, M. P. de la Rosa, Catal. Rev.-Sci. Eng., 48 (2006)
1.
[47] J. J. Lee, H. Kim, J. H. Koh, A. Jo, S. H. Moon, Appl. Catal. B: Env., 61 (2005)
274.
[48] H. Kim, J. J. Lee, J. H. Koh, S. H. Moon, Appl. Catal. B: Env., 54 (2004) 33.
[49] Usman, T. Yamamoto, T. Kubota, Y. Okamoto, Appl. Catal. A: Gen., 328 (2007)
219.
[50] Y. Okamoto, Catal. Today, 132 (2008) 9.
[51] R.R. Ramos, C. Bolívar, J. Castillo, J. Hung, C.E. Scott, Catal. Today, 133-135
(2008) 277.
[52] S. E. Skrabalak, K. S. Suslick, J. Am. Chem. Soc., 127 (2005) 9990.
[53] X. Liu, L. Zhang, Y. Shi, H. Nie, X. Long, Cuihua Xuebao, 25 (2004) 748.
18
[54] B. Liu, X. Xha, S. Sheng, W. Yang, G. Xiong, Cuihua Xuebao, 25 (2004) 770.
[55] B. Liu, X. Zha, Q. Meng, H. Hou, S. Gao, J. Zhang, S. Sheng, W. Yang, Cuihua
Xuebao, 26 (2005) 458.
[56] M. Toba, Y. Miki, Y. Kanda, T. Matsui, M. Harada, Y. Yoshimura, Catal. Today,
104 (2005) 64.
[57] T. Fujikawa, M. Kato, T. Ebihara, K. Hagiwara, T. Kubota, Y. Okamoto, J. Japan
Petrol. Inst., 48 (2005) 114.
[58] J. Escobar, M.C. Barrera, J.A. de los Reyes, J.A. Toledo, V. Santes, J.A. Colín, J.
Mol. Catal. A: Chem., 287 (2008) 33.
[59] V. Costa, K. Marchand, M. Digne, C. Geantet, Catal. Today, 130 (2008) 69.
[60] M.A. Lélias, J. van Gestel, F. Maugé, J.A.R. van Veen, Catal. Today, 130 (2008)
109.
[61] M.S. Rana, J. Ramírez, A. Gutiérrez-Alejandre, J. Ancheyta, L. Cedeño, S.K.
Maity, J. Catal., 246 (2007) 100.
[62] M.S. Rana, E.M.R. Capitaine, C. Leyva, J. Ancheyta, Fuel, 86 (2007) 1254.
[63] S. Texier, G. Berhault, G. Perot, F. Diehl, Fabrice., Appl. Catal. A: Gen., 293
(2005) 105.
[64] N. Frizi, P. Blanchard, E. Payen, P. Baranek, C. Lancelot, M. Rebeilleau, C.
Dupuy, J.P. Dath, Catal. Today, 130 (2008) 32.
[65] N. Escalona, J. Ojeda, P. Baeza, R. Garcia, J. M. Palacios, J. L. G. Fierro, A.
Agudo Lopez, F. J. Gil-Llambias, Appl. Catal. A: Gen., 287 (2005) 47.
[66] J. Ojeda, N. Escalona, J. L. G. Fierro, A. Agudo Lopez, F. J. Gil-Llambias, Appl.
Catal. A: Gen., 281 (2005) 25.
19
[67] R. Hubaut, Appl. Catal. A: Gen., 322 (2007) 121.
[68] Z. Vít, Appl. Catal. A: Gen., 322 (2007) 142.
[69] N. Escalona, M. Vrinat, D. Laurenti, F.J.G. Llambías, Appl. Catal. A: Gen., 322
(2007) 113.
[70] J.A. De Los Reyes, Appl. Catal. A: Gen., 322 (2007) 106.
[71] S.A. Giraldo, M.H. Pinzón, A. Centeno, Catal. Today, 133-135 (2008) 239.
[72] V.G. Baldovino-Medrano, S.A. Giraldo, A. Centeno, Fuel, 87 (2008) 1917.
[73] A. Ishihara, H. Mochizuki, J. Lee, E. W. Qian, T. Kabe, Y. Tatsumi, K. Umehara,
J. Japan Petrol. Inst., 48 (2005) 137.
[74] T.C. Ho, Catal. Today, 130 (2008) 206.
[75] M. Poisot, W. Bensch, S. Fuentes, G. Alonso, Thermochim. Acta, 444 (2006) 35.
[76] M. Poisot, W. Bensch, Thermochim. Acta, 453 (2007) 42.
[77] R. Romero-Rivera, M. Del Valle, G. Alonso, E. Flores, F. Castillón, S. Fuentes, J.
Cruz-Reyes, Catal. Today, 130 (2008) 354.
[78] S. L. González-Cortés, T.-C. Xiao, T.-W. Lin, M. L. H. Green, Appl. Catal. A:
Gen., 302 (2006) 264.
[79] S. L. González-Cortés, T.-C. Xiao, P.M.F.J. Costa, B. Fontal, M.L.H. Green, Appl.
Catal. A: Gen., 270 (2004) 209.
[80] S.L. González-Cortés, T.-C. Xiao, S.M.A. Rodulfo-Baechler, M.L.H. Green, J.
Mol. Catal. A: Chem., 240 (2005) 214.
[81] S.L. González-Cortés, S.M.A. Rodulfo-Baechler, T. Xiao, M.L.H. Green, Catal.
Lett., 111 (2006) 57.
20
[82] A. Olivas, R. Verduzco, G. Alonso, S. Fuentes, Preprints, ACS Div. Petr. Chem.,
50 (2005) 372.
[83] H. Nava, F. Pedraza, G. Alonso, Catal. Lett., 99 (2005) 65.
[84] R. Huirache-Acuña, M.A. Albiter, J. Espino, C. Ornelas, G. Alonso-Nuñez, F.
Paraguay-Delgado, J.L. Rico, R. Martínez-Sánchez, Appl. Catal. A: Gen., 304 (2006) 124.
[85] R. Huirache-Acuña, M.A. Albiter, C. Ornelas, F. Paraguay-Delgado, R. Martínez-
Sánchez, G. Alonso-Nuñez, Appl. Catal. A: Gen., 308 (2006) 134.
[86] H. Nava, J. Espino, G. Berhault, G. Alonso-Nuñez, Appl. Catal. A: Gen., 302
(2006) 177.
[87] Y. Saih, M. Nagata, T. Funamoto, Y. Masuyama, K. Segawa, Appl. Catal. A:
Gen., 295 (2005) 11.
[88] K. Segawa, T. Funamoto, M. Watanabe, Preprints, ACS Div. Petr. Chem., 50
(2005) 454.
[89] P. Rayo, J. Ancheyta, J. Ramirez, Catal. Today, 98 (2004) 171.
[90] J. Ramirez, G. Macias, L. Cedeno, A. Gutierrez-Alejandre, R. Cuevas, P. Castillo,
Catal. Today, 98 (2004) 19.
[91] E. Altamirano, J. A. de los Reyes, F. Murrieta, M. Vrinat, J. Catal., 235 (2005)
403.
[92] E. Altamirano, J. A. de los Reyes, F. Murrieta, M. Vrinat, Catal. Today, 133-135
(2008) 292.
[93] W. Huang, A. Duan, Z. Zhao, G. Wan, G. Jiang, T. Dou, K.H. Chung, J. Liu,
Catal. Today, 131 (2008) 314.
21
[94] U. Usman, M. Takaki, T. Kubota, Y. Okamoto, Appl. Catal. A: Gen., 286 (2005)
148.
[95] U. Usman, T. Kubota, Y. Araki, K. Ishida, Y. Okamoto, J. Catal., 227 (2004) 523.
[96] Usman, T. Kubota, I. Hiromitsu, Y. Okamoto, J. Catal., 247 (2007) 78.
[97] F. Dumeignil, K. Sato, M. Imamura, N. Matsubayashi, E. Payen, H. Shimada,
Appl. Catal. A: Gen., 315 (2006) 18.
[98] S.A. Giraldo, A. Centeno, Catal. Today, 133-135 (2008) 255.
[99] D. Ferdous, A. K. Dalai, J. Adjaye, L. Kotlyar, Appl. Catal. A: Gen., 294 (2005)
80.
[100] M. Sun, D. Nicosia, R. Prins, Catal. Today, 86 (2003) 173.
[101] S.K. Maity, G.A. Flores, J. Ancheyta, M.S. Rana, Catal. Today, 130 (2008) 374.
[102] L. Ding, Z. Zhang, Y. Zheng, Z. Ring, J. Chen, Appl. Catal. A: Gen., 301 (2006)
241.
[103] H. Kim, J.J. Lee, S.H. Moon, Appl. Catal. B: Env., 44 (2003) 287.
[104] D. Mey, S. Brunet, C. Canaff, F. Mauge, C. Bouchy, F. Diehl, J. Catal., 227
(2004) 436.
[105] Y. Fan, J. Lu, G. Shi, H. Liu, X. Bao, Catal. Today, 125 (2007) 220.
[106] H. Mizutani, H. Godo, T. Ohsaki, Y. Kato, T. Fujikawa, Y. Saih, T. Funamoto, K.
Segawa, Appl. Catal. A: Gen., 295 (2005) 193.
[107] Y. van der Meer, E. J. M. Hensen, J. A. R. van Veen, A. M. van der Kraan, J.
Catal., 228 (2004) 433.
[108] N. Kunisada, K.-H. Choi, Y. Korai, I. Mochida, K. Nakano, Appl. Catal A: Gen.,
279 (2005) 235.
22
[109] N. Kunisada, K.-H. Choi, Y. Korai, I. Mochida, K. Nakano, Appl. Catal. A: Gen.,
273 (2004) 287.
[110] C. Leyva, M.S. Rana, J. Ancheyta, Catal. Today, 130 (2008) 345.
[111] Y. Zhou, Z. Zhang, Preprints, ACS Div. Petr. Chem., 50 (2005) 449.
[112] G. Li, W. Li, M. Zhang, K. Tao, Appl. Catal. A: Gen., 273 (2004) 233.
[113] E. Kraleva, D. Paneva, A. Spojakina, I. Mitov, L. Petrov, Reac. Kin. Catal. Lett.,
85 (2005) 283.
[114] M. A. Chaoui, A. El Ouassouli, A. Ezzamarty, M. Lakhdar, C. Moreau, J. Leglise,
A. Travert, J. Physique: Proc., 123 (2005) 319, (REMCES IX, Proceedings of the 9th
International Seminar on the Physical Chemistry of Solid State Materials, 2002).
[115] N. El Azarifi, A. El Ouassouli, M. Lakhdar, A. Ezzamarty, C. Moreau, A. Travert,
J. Leglise, J. Physique: Proc., 123 (2005) 203, (REMCES IX, Proceedings of the 9th
International Seminar on the Physical Chemistry of Solid State Materials, 2002).
[116] N. Elazarifi, M.A. Chaoui, A. El Ouassouli, A. Ezzamarty, A. Travert, J. Leglise,
L.-C. de Ménorval, C. Moreau, Catal. Today, 98 (2004) 161.
[117] H. Chen, X. Zhou, H. Shang, C. Liu, J. Qiu, F. Wei, J. Nat. Gas Chem., 13 (2004)
209.
[118] L. Chuan, S. Bin, C. Min, S. Hong-yan, Q. Guo-he, J. Fuel Chem. Technol., 35
(2007) 407.
[119] I. Eswaramoorthi, V. Sundaramurthy, N. Das, A.K. Dalai, J. Adjaye, Appl. Catal.
A: Gen., 339 (2008) 187.
[120] H. Shang, C. Liu, Y. Xu, J. Qiu, F. Wei, Fuel Process. Technol., 88 (2007) 117.
23
[121] K. A. Carrado, C. Song, J. H. Kim, N. Castagnola, R. Fernandez-Saavedra, C. L.
Marshall, M. M. Schwartz, Prepr. ACS Div. Fuel Chem., 50 (2005) 344.
[122] K.A. Carrado, J.H. Kim, C.S. Song, N. Castagnola, C.L. Marshall, M.M.
Schwartz, Catal. Today, 116 (2006) 478.
[123] J. M. Herrera, J. Reyes, P. Roquero, T. Klimova, Micropor. Mesopor. Mat., 83
(2005) 283.
[124] R. Silva-Rodrigo, C. Calderon-Salas, J. A. Melo-Banda, J. M. Dominguez, A.
Vazquez-Rodriguez, Catal. Today, 98 (2004) 123.
[125] X. Li, A. Wang, S. Zhang, Y. Chen, Y. Hu, Appl. Catal. A: Gen., 316 (2007) 134.
[126] E. Rodríguez-Castellón, A. Jiménez-López, D. Eliche-Quesada, Fuel, 87 (2008)
1195.
[127] G. M. Dhar, G. M. Kumaran, M. Kumar, K. S. Rawat, L. D. Sharma, R. D. Raju,
K. S. R. Rao, Catal. Today, 99 (2005) 309.
[128] R. Nava, R.A. Ortega, G. Alonso, C. Ornelas, B. Pawelec, J.L.G. Fierro, Catal.
Today, 127 (2007) 70.
[129] S. Garg, K. Soni, G.M. Kumaran, M. Kumar, J.K. Gupta, L.D. Sharma, G.M.
Dhar, Catal. Today, 130 (2008) 302.
[130] O.Y. Gutiérrez, D. Valencia, G.A. Fuentes, T. Klimova, J. Catal., 249 (2007) 140.
[131] T. Klimova, J. Reyes, O. Gutiérrez, L. Lizama, Appl. Catal. A: Gen., 335 (2008)
159.
[132] T. A. Zepeda, J. L. G. Fierro, B. Pawelec, R. Nava, T. Klimova, G. A. Fuentes, T.
Halachev, Chem. Mater., 17 (2005) 4062.
24
[133] S. Zeng, J. Blanchard, M. Breysse, Y. Shi, X. Su, H. Nie, D. Li, Appl. Catal. A:
Gen., 298 (2006) 88.
[134] T.A. Zepeda, B. Pawelec, J.L.G. Fierro, A. Olivas, S. Fuentes, T. Halachev,
Microporous Mesoporous Mater., 111 (2008) 157.
[135] A. Montesinos-Castellanos, T.A. Zepeda, Microporous Mesoporous Mater., 113
(2008) 146.
[136] B. Pawelec, J.L.G. Fierro, A. Montesinos, T.A. Zepeda, Appl. Catal. B: Env., 80
(2007) 1.
[137] T.A. Zepeda, B. Pawelec, J.L.G. Fierro, T. Halachev, Appl. Catal. B: Env., 71
(2007) 223.
[138] H. Nakamura, M. Amemiya, K. Ishida, J. Japan Petr. Inst., 48 (2005) 281.
[139] Q. Wang, H. Nie, X. Long, Cuihua Xuebao, 26 (2005) 399.
[140] J. Chen, Z. Ring, Fuel, 83 (2004) 305.
[141] N. Guernalec, T. Cseri, P. Raybaud, C. Geantet, M. Vrinat, Catal. Today, 98
(2004) 61.
[142] H. Farag, Appl. Catal. A: Gen., 331 (2007) 51.
[143] A. Logadóttir, P.G. Moses, B. Hinnemann, N.-Y. Topsøe, K.G. Knudsen, H.
Topsøe, J.K. Nørskov, Catal. Today, 111 (2006) 44.
[144] M.S. Rana, R. Navarro, J. Leglise, Catal. Today, 98 (2004) 67.
[145] M.S. Rana, J. Ancheyta, P. Rayo, S.K. Maity, Fuel, 86 (2007) 1263.
[146] V. Rabarihoela-Rakotovao, S. Brunet, G. Perot, F. Diehl, Appl. Catal. A: Gen.,
306 (2006) 34.
[147] U.T. Turaga, X. Ma, C. Song, Catal. Today, 86 (2003) 265.
25
[148] P. Zeuthen, K.G. Knudsen, D.D. Whitehurst, Catal. Today, 65 (2001) 307.
[149] T.C. Ho, J. Catal., 219 (2003) 442.
[150] A. Owusu-Boakye, A. K. Dalai, D. Ferdous, J. Adjaye, Energy & Fuels, 19 (2005)
1763.
[151] J. A. Nava, K. R. Ojeda, Chem. Eng. Res. Des., 82 (2004) 208.
[152] K-H. Choi, N. Kunisada, Y. Korai, I. Mochida, K. Nakano, Catal. Today, 86
(2003) 277.
[153] S.D.S. Murti, H. Yang, K.-H. Choi, Y. Korai, I. Mochida, Appl. Catal. A: Gen.,
252 (2003) 331.
[154] G. Shan, H. Zhang, J. Xing, G. Chen, W. Li, H. Liu, Biochem. Eng. J, 27 (2006)
305.
[155] M. Rashtchi, G.H. Mohebali, M.M. Akbarnejad, J. Towfighi, B. Rasekh, A.
Keytash, Biochem. Eng. J., 29 (2006) 169.
[156] Y. Hou, Y. Kong, J. Yang, J. Zhang, D. Shi, W. Xin, Fuel, 84 (2005) 1975.
[157] F. Li, Z. Zhang, J. Feng, X. Cai, P. Xu, J. Biotechnol., 127 (2007) 222.
[158] S. Guobin, Z. Huaiying, X. Jianmin, C. Guo, L. Wangliang, L. Huizhou, Biochem.
Eng. J., 27 (2006) 305.
[159] G. Mohebali, A.S. Ball, B. Rasekh, A. Kaytash, Enzyme Microb. Technol., 40
(2007) 578.
[160] M. Soleimani, A. Bassi, A. Margaritis, Biotech. Adv., 25 (2007) 570.
[161] H. Chen, W.-J. Zhang, J.-M. Chen, Y.-B. Cai, W. Li, Bioresour. Technol., 99
(2008) 3630.
26
[162] H. Chen, W.-J. Zhang, Y.-B. Cai, Y. Zhang, W. Li, Bioresour. Technol., 99 (2008)
6928.
[163] S. Liu, B.Wang, B. Cui, L. Sun, Fuel, 87 (2008) 422.
[164] X. Ma, A. Zhou, C. Song, Catal. Today, 123 (2007) 276.
[165] J.T. Sampanthar, H. Xiao, J. Dou, T.Y. Nah, X. Rong, W.P. Kwan, Appl. Catal. B:
Env., 63 (2006) 85.
[166] A. Chica, A. Corma, M.E. Dómine, J. Catal. 242 (2006) 299.
[167] G. Yu, S. Lu, H. Chen, Z. Zhu, Carbon, 43 (2005) 2285.
[168] F. Al-Shahrani, T. Xiao, S.A. Llewellyn, S. Barri, Z. Jiang, H. Shi, G. Martinie,
M.L.H. Green, Appl. Catal. B: Env., 73 (2007) 311.
[169] S. Mondal, Y. Hangun-Balkir, L. Alexandrova, D. Link, B. Howard, P. Zandhuis,
A. Cugini, C.P. Horwitz, T.J. Collins, Catal. Today, 116 (2006) 554.
[170] J.L. García-Gutiérrez, G.A. Fuentes, M.E. Hernández-Terán, F. Murrieta, J.
Navarrete, F. Jiménez-Cruz, Appl. Catal. A: Gen., 305 (2006) 15.
[171] J.L. García-Gutiérrez, G.A. Fuentes, M.E. Hernández-Terán, P. García, F.
Murrieta-Guevara, F. Jiménez-Cruz, Appl. Catal. A: Gen., 334 (2008) 366.
[172] L.H. Thompson, L.K. Doraiswamy, Ind. Eng. Chem. Res., 38 (1999) 1215.
[173] H. Mei, B.W. Mei, T.F. Yen, Fuel, 82 (2003) 405.
[174] M.-W. Wan, T.-F. Yen, Appl. Catal. A: Gen., 319 (2007) 237.
[175] O. Etemadi, T.F. Yen, Energy Fuels, 21 (2007) 2250.
[176] D. Zhao, H. Ren, J. Wang, Y. Yang, Y. Zhao, Energy Fuels, 21 (2007) 2543.
[177] P. Liu, J. A. Rodriguez, J. T. Muckerman, J. Molec. Catal. A: Chem., 239 (2005)
116.
27
[178] P. Liu, J. A. Rodriguez, J. T. Muckerman, J. Phys. Chem., 108 (2004) 18796.
[179] B. Diaz, S. J. Sawhill, D. H. Bale, R. Main, D. C. Phillips, S. Korlann, R. Self, M.
E. Bussell, Catal. Today, 86 (2003) 191.
[180] A. Szymańska-Kolasa, M. Lewandowski, C. Sayag, D. Brodzki, G. Djéga-
Mariadassou, Catal. Today, 119 (2007) 35.
[181] A. Szymańska-Kolasa, M. Lewandowski, C. Sayag, G. Djéga-Mariadassou, Catal.
Today, 119 (2007) 7.
[182] H.A. Al-Megren, S.L. González-Cortés, T. Xiao, M.L.H. Green, Appl. Catal. A:
Gen., 329 (2007) 36.
[183] X.-H. Wang, M.-H. Zhang, W. Li, K.-Y. Tao, Catal. Today, 131 (2008) 111.
[184] S. Suppan, J. Trawczynski, J. Kaczmarczyk, G. Djega-Mariadassou, A. Hynaux,
C. Sayag, Appl. Catal. A: Gen., 280 (2005) 209.
[185] W. A. Abdallah, A. E. Nelson, Surf. Sci., 585 (2005) 113.
[186] M Nagai, Appl. Catal. A: Gen., 322 (2007) 178.
[187] H. Tominaga, T. Arahata, M. Nagai, Chem. Eng. Sci., (2008) In Press.
[188] V. Sundaramurthy, A.K. Dalai, J. Adjaye, Appl. Catal. A: Gen., 335 (2008) 204.
[189] E. Furimsky, Appl. Catal. A: Gen., 240 (2003) 1.
[190] E. Ito, J.A.R. van Veen, Catal. Today, 116 (2006) 446.
[191] L.F. Ramírez-Verduzco, E. Torres-García, R. Gómez-Quintana, V. González-
Peña, F. Murrieta-Guevara, Catal. Today, 98 (2004) 289.
28
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].
29
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).
30
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
31
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]
32
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].
33
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).
34
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.
35
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
/ %
36
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
37
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.
38
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
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
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
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
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.
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
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.
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
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.
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
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
..
..
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.
50
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
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
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].
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].
54
References
[1] S.T. Oyama, T. Gott, H. Zhao, Y.K. Lee, Transition metal phosphide
hydroprocessing catalysts: A review, Catal. Today (2008)
[2] B. Aronsson, T. Lundström, S. Rundqvist, Borides, Silicides and Phosphides,
Methuen, London and Wiley, New York, 1965.
[3] D. E. C. Corbridge, Studies in Inorganic Chemistry, Vol. 10, 4th Ed., Elsevier,
Amsterdam, 1990.
[4] M. Baudler, Angew. Chem., Int. Ed. Engl., 26 (1987) 419.
[5] S. T. Oyama, Transition Metal Carbides, Nitrides, and Phosphides, in Handbook
of Catalysis, G. Ertl, H. Knözinger, J. Weitkamp, Eds., Springer-Verlag, Weinheim,
2008.
[6] S. Rundqvist, Colloq. Int. Cent. Nat. Rech. Sci., 157 (1967) 85.
[7] D. C. Phillips, S. J. Sawhill, R. Self, M. E. Bussell, J. Catal., 207 (2002) 266.
[8] S. J. Sawhill, D. C. Phillips, M. E. Bussell, J. Catal., 215 (2003) 208.
[9] R. Iwamoto, J. Grimblot, Adv. Catal., 44 (1999) 417.
[10] S. Rundqvist, New metal-rich phosphides of niobium, tantalum, and tungsten
Nature 211 (1966) 847
[11] S. Rundqvist, The crystal structure of Mo4P3, Acta Chem. Scand. 19 (1965) 393
[12] S. Rundqvist, T. Lundstrom, X-ray studies of molybdenum and tungsten
phosphides, Acta. Chem. Scand. 17 (1963) 37
[13] G. Lewis, C. Myers, Vaporization properties of iron phosphides, 67 (1965) 1289
[14] N. Schonberg, An x-ray investigation of transition metal phosphides, Acta. Chem.
Scand. 8 (1954) 226
55
[15] H. Fjellvag, A. Kjekshus, A.F. Andresen, Magnetic and structural properties of
transition metal substituted MnP. III. Mn1-tFetP (0.00≤t≤0.30) Acta. Chem. Scand. A 38
(1984) 711
[16] H. Fjellvag, A. Kjekshus, A.F. Andresen, Magnetic and structural properties of
transition metal substituted MnP. IV. Mn1-tFetP, Acta. Chem. Scand. A 38 (1984) 719
[17] R.I. Ripley, The preparation and properties of some transition metal phosphides, J.
Les-Common Met. 4 (1962) 496
[18] A.L. Hector, I.P. Parkin, Room temperature initiated routes to titanium and
vanadium pnictides, Inorg, Chem. 33 (1994) 1727
[19] A.T. Rowley, I.P. Parkin, Convenient synthesis of lanthanide and mixed
lanthanide phosphides by solid-state routs involving sodium phosphide, J. Mater. Chem 3
(1993) 689
[20] A.L. Hector, I.P. Parkin, Self-propagating routes to transition-metal phosphides, J.
Mater. Chem. 4 (1994) 279
[21] K.A. Gingerich, Vaporization behavior and phsophorus decomposition pressures
of tungsten monophosphide, J. Phys. Chem. 68 (1964) 768
[22] W. Li, B. Dhandapani, S.T. Oyama, Molybdenum phosphide: Anovel catalyst for
hydrodenitrogenation, Chem. Lett. (1998) 207
[23] J. Gopalakrishnan, S. Pandey, K.K. Rangan, Convenient route for the synthesis of
transition metal pnictides by direct reduction of phosphate, arsenate, and antimonite
prescursors, Chem. Mater. 9 (1997) 2113
[24] T.S. Lewkebanadara, J.W. Proscia, C.H. Winter, Precursor for the low-
temperature deposition of titanium phosphide films, Chem. Mater. 7 (1995) 1053
56
[25] H. Loboué, C. Guillot-Deudon, A.F. Popa, A. Lafond, B. Rebours, C. Pichon, T.
Cseri, G. Berhault, C. Geantet, Catal. Today, 130 (2008) 63.
[26] L. Song, W. Li, G. Wang, M. Zhang, K. Tao, Catal. Today, 125 (2007) 137.
[27] R. Cheng, Y. Shu, L. Li, J. Sun, X. Wang, T. Zhang, Thermochim. Acta, 450
(2006) 42.
[28] R. Cheng, Y. Shu, L. Li, M. Zheng, X. Wang, A. Wang, T. Zhang, Appl. Catal. A:
Gen., 316 (2007) 160.
[29] S. Yang, C. Liang, R. Prins, J. Catal., 241 (2006) 465.
[30] S. Liu, Y. Qian, X. Ma, Mater. Lett., 62 (2008) 11.
[31] S. Liu, X. Liu, L. Xu, Y. Qian, X. Ma, J. Cryst. Growth, 304 (2007) 430.
[32] K. Senevirathne, A.W. Burns, M.E. Bussell, S.L. Brock, Adv. Funct. Mater., 17
(2007) 3933.
[33] S. Burns, J.S.J. Hargreaves, S.M. Hunter, Catal. Commun., 8 (2007) 931.
[34] C. Stinner, Z. Tang, M. Haouas, Th. Weber, J. Catal. 208 (2002) 456.
[35] V. Zuzaniuk, R. Prins, J. Catal., 219 (2003) 85.
[36] K. A. Layman, M. E. Bussell, J. Phys. Chem. B, 108 (2004) 10930.
[37] S. J. Sawhill, K. A. Layman, D. R. Van Wyk, M. H. Engelhard, C. Wang, M. E.
Bussell, J. Catal., 231 (2005) 300.
[38] A.W. Burns, K.A. Layman, D.H. Bale, M.E. Bussell, Appl. Catal. A: Gen., 343
(2008) 68.
[39] Z. Wu, F. Sun, W. Wu, Z. Feng, C. Liang, Z. Wei, C. Li, J. Catal., 222 (2004) 41.
[40] F. Sun, W. Wu, Z. Wu, J. Guo, Z. Wei, Y. Yang, Z. Jiang, F. Tian, C. Li, J. Catal.,
228 (2004) 298.
57
[41] F. Sun, Z. Wei, P.Ying, X. Sun, Z. Jiang, F. Tian, Y.Yang, C. Li, Cuihua Xuebao,
25 (2004) 685.
[42] S. Yang, C. Liang, R. Prins, J. Catal., 237 (2006) 118.
[43] S. Yang, C. Liang, R. Prins, Stud. Surf. Sci. Catal., 162 (2006) 307.
[44] F. Nozaki, M. Tokumi, J. Catal., 79 (1983) 207.
[45] F. Nozaki, T. Kitoh, T. Sodesawa, J. Catal., 62 (1980) 286.
[46] W. R. A. M. Robinson, J. N. M. van Gastel, T. I. Korányi, S. Eijsbouts, J. A. R.
van Veen, V. H. J. de Beer, J. Catal., 161 (1996) 539.
[47] X. Liu, J. Chen, J. Zhang, Catal. Commun., 8 (2007) 1905.
[48] S. Zhou, J. Chen, X. Liu, J. Zhang, Chin. J. Catal., 28 (2007) 498.
[49] J. Chen, S. Zhou, X. Liu, J. Zhang, Catal. Lett., 122 (2008) 167.
[50] P. Clark, X. Wang, and S. T. Oyama, J. Catal., 207 (2002) 256.
[51] G. Sun, C. Li, F. Li, Z. Zhou, Shiyou Xuebao, Shiyou Jiagong, 21 (2005) 54.
[52] S. J. Sawhill, K. A. Layman, D. R. Van Wyk, M. H. Engelhard, C. Wang, M. E.
Bussell, J. Catal., 231 (2005) 300.
[53] X. Wang, P. Clark, S. T. Oyama, J. Catal., 208 (2002) 321.
[54] S. T. Oyama, P. Clark, V. L. S. Teixeira da Silva, E. J. Lede, F. G. Requejo, J.
Phys. Chem. B, 105 (2001) 4961.
[55] X. Wang, Y. Lee, W. J. Chun, S. T. Oyama, F. Requejo, J. Catal., 210 (2002) 207.
[56] S. T. Oyama, P. Clark, X. Wang, T. Shido, Y. Iwasawa, S. Hayashi, J. Phys.
Chem. B, 106 (2002) 1913.
[57] S. T. Oyama, X. Wang, F. Requejo, T. Sato, Y. Yoshimura, J. Catal., 209 (2002)
1. Priority Communication
58
[58] Y. Shu, Y.-K. Lee, and S. T. Oyama, J. Catal., 236 (2005) 112.
[59] A. Montesinos-Castellanos, T.A. Zepeda, B. Pawelec, E. Lima, J.L.G. Fierro, A.
Olivas, J.A. de los Reyes H., Appl. Catal. A: Gen., 334 (2008) 330.
[60] E. P. W. Ward, I. Arslan, A. Bleloch, J. M. Thomas, P. A. Midgley, P. Alisivatos,
J. Phys.: Conf. Ser., 26 (2005) 207.
[61] G. A. Somorjai, ACS Symp. Ser. 890 (2005) 210.
[62] R. Esparza, J. A. Ascencio, G. Rosas, J. F. Ramirez Sanchez, U. Pal, R. Perez, J.
Nanosci. Nanotech., 5 (2005) 641.
[63] M. S. Chen, D. W. Goodman, Science, 306 (2004) 252.
[64] S. T. Oyama, X. Wang, Y.-K. Lee, W.-J. Chun, J. Catal., 221 (2004) 263.
[65] Y.-K. Lee, S. T. Oyama, ACS Fuel Chem. Div. Prep., 50 (2005) 807.
[66] Y.-K. Lee, S. T. Oyama, Proc. 4th ASIA-PACIFIC Chemical Reaction
Engineering Symposium, 2005. Stud. Surf. Sci. Catal.,159, 2006.
[67] T. Kawai, S. Sato, W. J. Chun, K. Asakura, K. K. Bando, T. Matsui, Y.
Yoshimura, T. Kubota, Y. Okamoto, Y. K. Lee, S. T. Oyama, Physica Scripta,
T115(2004) 822.
[68] T. Kawai, K. K. Bando, Y.-K. Lee, S. T. Oyama, W.-J. Chun, K. Asakura, J.
Catal., 241 (2006) 20, Priority Communication.
[69] Tauster, S. J., Pecoraro, T. A., Chianelli, R. R., J. Catal., 63 (1980) 515.
[70] A. E. Nelson, M. Sun, A. S. M. Junaid, J. Catal., 241 (2006) 180.
[71] M. Boudart, R. A. Dalla Betta, K. Foger, D. G. Loffler, M. G. Samant, Science,
228 (1985) 717.
59
[72] J. P., van den Berg, J. P., Lucien, G., Germaine, G. L. B., Fuel Process. Technol.,
35 (1993) 119.
[73] F. E., Massoth, S. C., Kim, Catal. Lett., 57 (1999) 129.
[74] J.-D. Grunwaldt, M. Ramin, M. Rohr, A. Michailovski, G. R. Patzke, A. Baiker,
Rev. Sci. Instrum., 76 (2005) 054104.
[75] N. Weiher, E. Bus, B. Gorzolnik, M. Möller, R. Prins, J. A. van Bokhoven, J.
Synchrotron Rad., 12 (2005) 675.
[76] J. F. Odzak, A. M. Argo, F. S. Lai, B. C. Gates, K. Pandya, L. Feraria, Rev. Sci.
Instr., 72 (2001) 3943.
[77] E. Larsson, Arkiv. Kemi., 23 (1965) 335.
[78] O. N.Il'nitskaya, L. G. Aksel'rud, S. L. Mikhalenko, Yu B. Kuz'ma,
Kristallografiya 32 (1987) 50.
[79] J. Trahan, R.G. Goodrich, S. F. Watkins, Phys. Rev. B 2, [3]2 (1970) 2859.
[80] T. Fujii, K. Tanaka, F. Marumo, Y. Noda, Mineral J., 13 (1987) 448.
[81] J. B. Parise, Acta Cryst. B, 36 (1980) 1179.
[82] Y. Shu, Y.-K. Lee, S. T. Oyama, Chem. Commun., (2005) 1143.
[83] Y. Shu, S. T. Oyama, Carbon, 43 (2005) 1517.
[84] P. A. Clark, S. T. Oyama, J. Catal., 218 (2003) 78.
[85] A. Wang, L. Ruan, Y. Teng, X. Li, M. Lu, J. Ren, Y. Wang, Y. Hu, J. Catal., 229
(2005) 314.
[86] I.I. Abu, K.J. Smith, Appl. Catal. A: Gen., 328 (2007) 58.
[87] M. Lu, A. Wang, X. Li, X. Duan, Y. Teng, Y. Wang, C. Song, Y. Hu, Energy
Fuels, 21 (2007) 554.
60
[88] T.I. Korányi, Z. Vít, D.G. Poduval, R. Ryoo, H.S. Kim, E.J.M. Hensen, J. Catal.,
253 (2008) 119.
[89] T.I. Korányi, Z. Vít, J.B. Nagy, Catal. Today, 130 (2008) 80.
[90] Y.-K. Lee, Y. Shu, S. T. Oyama, Active phase of a nickel phosphide (Ni2P)
catalyst supported on KUSY zeolite for the hydrodesulfurization of 4,6-DMDBT, Appl.
Catal. A: Gen., 322 (2006) 191.
[91] J. H. Kim, X. Ma, C. Song, Y.-K. Lee, S. T. Oyama, Energy Fuels, 19 (2005) 353.
[92] V. Vanrysselberghe, G. F. Froment, Ind. Eng. Chem. Res., 35 (1996) 3311.
[93] V. Vanrysselberghe, R. L. Gall, G. F. Froment, Ind. Eng. Chem. Res., 37 (1998)
1235.
[94] G. H. Singhal, R. L. Espino, J. E. Sobel, and G. A. Huff, Jr., J. Catal., 67 (1981)
457.
[95] D. D. Whitehurst, T. Isoda, I. Mochida, Adv. Catal., 42 (1998) 345.
[96] H. Farag, D.D. Whitehurst, K. Sakanishi and I. Mochida, Catal. Today, 50 (1999)
49.
[97] V. Meille, E. Schulz, M. Lemaire, M. Vrinat, J. Catal., 170 (1997) 29.
[98] F. Bataille, J. L. Lemberton, P. Michaud, G. Pérot, M. Vrinat, M. Lemaire, E.
Schulz, M. Breysse, S. Kasztelan, J. Catal., 191 (2000) 409.
[99] P. Steiner, E. A. Blekkan, Fuel Proc. Tech., 79 (2002) 1.
[100] C. Kwak, J. J. Lee, J. S. Bae, K. Choi, S. H. Moon, Appl. Catal. A: Gen., 200
(2000) 233.
[101] C. J. Song, C. Kwak, S. H. Moon, Catal. Today, 74 (2002) 193.
[102] W. Zmierczak, G. MuraliDhar, F. E. Massoth, J. Catal., 77 (1982) 432.
61
[103] T. A. Bodrero, C. H. Bartholomew, K. C. Pratt, J. Catal., 78 (1982) 253.
[104] P. Yang, Z. Jiang, P. Ying, C. Li, Chin. J. Catal., 28 (2007) 670.
[105] P. Yang, Z. Jiang, P. Ying, C. Li, J. Catal., 253 (2008) 66.
[106] A. Montesinos-Castellanos, T.A. Zepeda, B. Pawelec, J.L.G. Fierro, J.A. de los
Reyes, Chem. Mater., 19 (2007) 5627.
[107] J. A. José A. Rodriguez, J.-Y. Kim, J. C. Hanson, S. J. Sawhill, and M. E. Bussell,
J. Phys. Chem. B, 107 (2003) 6276.
[108] J. A. Rodriguez, J. Phys. Chem. B, 101 (1997) 7524.
[109] J. H. Kim, X. Ma, C. Song, Y. K. Lee, S. T. Oyama, Effect of nitrogen
compounds on deep hydrodesulfurization of 4,6-dimethyldibenzothiophene over NiMo
Sulfide and Ni phosphide catalysts, ACS Fuel Chem. Div. Prep. 2004, 49.
[110] S. T. Oyama, Y.-K. Lee, J. Phys. Chem. B, 109 (2005) 2109.
[111] Y.-K. Lee and S. T. Oyama, ACS Petr. Chem. Div. Prep., 50 (2005) 445.
[112] Y.-K. Lee and S. T. Oyama, J. Catal., 239 (2006) 376.
[113] G. Perot, Catal. Today, 10 (1991) 447.
[114] B. Müller, A. D. van Langeveld, J. A. Moulijn, and H. Knözinger, J. Phys. Chem.,
97 (1993) 9028.
[115] M. Angulo, F. Maugé, J. C. Duchet, and J. C. Lavalley, Bull. Soc. Chim. Belg., 96
(1987) 925.
[116] B. M. Vogelaar, P. Steiner, A. D. van Langeveld, S. Eijsbouts, and J. A. Moulijn,
Appl. Catal. A: Gen., 251 (2003) 85.
[117] L. Hu, G. Xia, L. Qu, M. Li, C. Li, Q. Xin, D. Li, J. Catal., 202 (2001) 220.
[118] B. H. Cooper and B. B. L. Donnis, Appl. Catal. A: Gen., 137 (1996) 203.
62
[119] M. Egorova, Y. Zhao, P. Kukula, R. Prins, J. Catal., 206 (2002) 263.
[120] S. T. Oyama, Y.-K. Lee, J. Phys. Chem. B, 109 (2005) 2109.
[121] Y.-K. Lee and S. T. Oyama, J. Catal., 239 (2006) 376.
[122] C. Stinner, R. Prins, Th. Weber, J. Catal., 202 (2001) 187.
[123] J. A. Rodriguez, J.-Y. Kim, J. C. Hanson, S. J. Sawhill, M. E. Bussell, J. Phys.
Chem. B, 2003, 6276.
[124] D. Ma, T. Xiao, S. Xie, W. Zhou, S. L. Gonzalez-Cortes, M. L. H. Green, Chem.
Mater., 16 (2004) 2697.
[125] I.I. Abu, K.J. Smith, J. Catal. 241 (2006) 356.
[126] I.I. Abu, K.J. Smith, Catal. Today, 125 (2007) 248.
[127] S. Rundqvist, Acta Chem. Scand., 16 (1962) 992.
[128 ] S. T. Oyama, Y.-K. Lee, J. Catal. (2008) In press.
[129] P. R. Guérin, M. Sergent, Acta Cryst. B, 33 (1977) 2820.
[130] P. R. Guérin, M. Sergent, Acta Cryst. B, 34 (1978) 3312.
[131] P. Liu, J. A. Rodriguez, T. Asakura, J. Gomes, K. Nakamura, J. Phys. Chem. B,
109 (2005) 4575.
[132] S. T. Oyama, Transition Metal Carbides, Nitrides, and Phosphides, in Handbook
of Catalysis, G. Ertl, H. Knözinger, J. Weitkamp, Eds., Springer-Verlag, Weinheim, 2008.
63
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
64
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
65
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).
66
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
+
67
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).
68
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
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,
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
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
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.
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.
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
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.
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
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 / %
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
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
80
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
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
82
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
83
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.
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
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.
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.
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
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
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
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
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
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.
93
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
94
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
95
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
96
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
97
References [1] Y.Gu., S. T. Oyama, Adv. Mater. 19 (2007) 1636.
[2] Y. Gu, S. T. Oyama, J. Membr. Sci. 306 (2007) 216.
[3] Y. Gu, P. Hacarlioglu, S. T. Oyama, J. Membr. Sci. 310 (2008) 28.
[4] S. Satyapal, J. Petrovic, C. Read, G. Thomas, G. Ordaz, Catal. Today 120 (2007)
246–256.
[5] Hydrogen storage roadmap, http://www1.eere.energy.gov/vehiclesandfuels
/pdfs/program/ hydrogen_storage_roadmap.pdf.
[6] U. Bossel, B. Eliason, G. Taylor,
http://www.evworld.com/library/h2economyFinalReport.pdf.
[7] D.K. Ross, Vacuum 80 (2006) 1084–1089.
[8] A. Anson, M. Benham, J. Jagiello, M. A. Callejas, A. M. Benito,
W. K. Maser, A. Z. Uttel, P. Sudan, M. T. Martınez, Nanotechnology 15 (2004) 1503.
[9] R. B. Biniwale, S. Rayalua, S. Devottaa, M. Ichikawa, Int. J. Hydrogen Energy 33
(2008) 360–365.
[10] N. Meng, S. Shinoda and Y. Saito, Int. J. Hydrogen Energy 22 (1997) 361-367.
[11] S. Hodoshima, H. Arai, Y. Saito, Int. J. Hydrogen Energy 28 (2003) 197 – 204.
[12] S. Hodoshima, H. Nagata, Y. Saito, Appl. Catal. A: Gen. 292 (2005) 90–96.
[13] S. Hodoshima, S. Takaiwa, A. Shono, K. Satoh, Y. Saito, Appl. Catal. A: Gen.
283 (2005) 235–242.
[14] N. Kariya, A. Fukuoka, T. Utagawa, M. Sakuramoto, Y. Goto, M. Ichikawa, Appl.
Catal. A: Gen. 247 (2003) 247–259.
98
[15] N. Kariya, A. Fukuoka, M. Ichikawa, Appl.Catal. A: Gen 233 (2002) 91–102.
[16] A. Moores, M. Poyatos, Y. Luo, R. H. Crabtree, New J. Chem. 30 (2006) 1675-
1678.
[17] D. E. Schwarz, T. M. Cameron, P. J. Hay, B. L. Scott, W. Tumas, D. L. Thorn,
Chem. Commun. (2005) 5919–5921.
[18] Y. Cui, S. Kwok, A. Bucholtz, B. Davis, R. A. Whitney, P. G. Jessop, New J.
Chem. 32 (2008) 1027–1037.
[19] E. Clot, O. Eisenstein, R. H. Crabtree, Chem. Commun. (2007) 2231–2233.
[20] G. P. Pez, A. R. Scott, A. C. Cooper, H. Cheng, F. C. Wilhelm, A. H. Abdourazak,
US Pat., 7,351,395, April 1, 2008, to Air Products and Chemicals, Inc.
[21] B.D. Ratner, E.D. Naeemi, US Pat., 7,186,396, March 6, 2007, to Asemblon, Inc.
[22] S. T. Oyama, J. Catal. 216 (2003) 343.
[23] P. Clark, W. Li, S. T. Oyama, J. Catal 200 (2001) 140.
[24] S. T. Oyama, X. Wang, F. Requejo, T. Sato, Y. Yoshimura, J. Catal. 209 (2002)
1.
[25] X. Wang, Y. Lee, S. T. Oyama, K. Bando, F. G. Requejo , J. Catal. 210 (2002)
207.
[26] S. T. Oyama, X. Wang, Y.-K. Lee, W.-J. Chun, J. Catal. 221 (2004) 263.
[27] W. Li, B. Dhandapani, S. T. Oyama, Chem. Lett 3 (1998) 208.
[28] S. T. Oyama, P. Clark, V. L. S. Teixeira da Silva, E. J. Lede, F. G. Requejo, J.
Phys. Chem B105 (2001) 4961.
[29] P. A. Clark, X. Wang, S.T.Oyama, J. Catal. 207 (2002) 256.
[30] X. Wang, P. A. Clark, S. T. Oyama, J. Catal. 208 (2002) 321.
99
[31] K. K. Bando, T. Kawai, K. Asakura, T. Matsui, L. L. Bihan, H. Yasuda, Y.
Yoshimura, S. T. Oyama, Catal. Today 111 (2006) 199.
[32] K. K. Bando, T. Kawai, K. Asakura, T. Matsui, L. L. Bihan, H. Yasuda, Y.
Yoshimura, S. T. Oyama, Catal. Today 111 (2006) 199–204.
[33] P. A. Clark, X. Wang, P. Deck, J. Catal. 210 (2002) 216.
[34] P. A. Clark, S. T. Oyama, J. Catal. 218 (2003) 78.
[35] S. T. Oyama, Y.-K. Lee, J. Catal. 258 (2008) 393.
[36] S. T. Oyama, T. Gott, H. Zhao, Y.-K. Lee, Catal. Today (2008),
doi:10.1016/j.cattod.2008.09.019
[37] A. V. Mashkina, L. G. Salakhtueva, Chem. Heterocycl. Compd. 37 (2001) 546-
549.
[38] A. V. Mashkina, L. G. Sakhaltueva, Kinet. Catal. 43 (2002) 107–114.
[39] A. A. Zirka, A. V. Mashkina, Kinet. Catal. 41 (2000) 388-393.
[40] A.V. Mashkina, A. A. Zirka, Kinet. Catal. 41 (2000) 575-581.
[41] D. Mey, S. Brunet, C. Canaff, F. Maugé, C. Bouchy, F. Diehl, J. Catal. 227 (2004)
436–447.
[42] U. Roland, F. Roessner, Stud. Surf. Sci. Catal. 112 (1997) 191-200.
[43] A. Daudin, A.F. Lamic, G. Pérot, S. Brunet, P. Raybaud, C. Bouchy, Catal. Today
130 (2008) 221-230.
[44] F. G. J. Gault, Adv. Catal. 30 (1981) 1.
[45] M. A. Arribas, P. Concepcion, A. Martinez, Appl. Catal. A 267 (2004) 111-119.
[46] M. A. Arribas, J. J. Mahiques, A. Martinez, Stud. Surf. Sci. Catal. 135 (2001) 303.
[47] M. A. Arribas, A. Martinez, Appl. Catal. A 230 (2002) 203.
100
[48] M. A. Arribas, A. Martinez, G. Sastre, Stud. Surf. Sci. Catal. 142 (2002) 1015.
[49] G. B. Mcvicker, M. Daage, M. S. Touvelle, C. W. Hudson, D. P. Klein, W. C.
Baird, Jr. B. R. Cook, J. G. Chen, S. Hantzer, D. E. W. Vaughan, E.S. Ellis, O.C. Feeley,
J. Catal. 210 (2002) 137-148.
[50] H. Du, C. Fairbridge, H. Yang, Z. Ring, Appl. Catal. A: Gen. 294 (2005) 1–21.
[51] J. R. Anderson, K. Foger, R. J. Breakspere, J. Catal. 57 (1979) 458.
[52] P. G. Menon, G. F. Froment, Appl. Catal., 1 (1981) 31.
[53] S. Smeds, T. Salmi, L. P. Lindfors, O. Krause, Appl. Catal. A: Gen. 144 (1996)
177-194.
[54] H. Topsoe, B.S. Clausen, F.E. Massoth, Hydrotretaing catalysis, Catalysis-
Science and Technology, Springer-Verlag, Berlin, 1996.
[55] S. Kolboe, Can. J. Chem. 47 (1969) 352.
[56] D. L. Sullivan, J. G. Ekerdt, J. Catal. 172, (1997) 64-75.
[57] M. Neurock, R. A. Santan, J. Am. Chem. Soc. 116 (1994) 4427-4439.
[58] D. L. Sullivan, J. G. Ekerdt, J. Catal. 178 (1998) 226-233.
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
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.
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
104
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.
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),
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.
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
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.
109
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)
110
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
111
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
112
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
113
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
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.
115
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
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.
117
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
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].
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
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
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
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
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].
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
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
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)
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.
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
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
130
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.
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
132
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).
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
134
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
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
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.
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
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.
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.
140
References [1] S.T. Oyama, T. Gott, H. Zhao, Y.K. Lee, Transition metal phosphide
hydroprocessing catalysts: A review, Catal. Today (2009)
doi:10.1016/j.cattod.2008.09.019.
[2] C. Song, An overview of new approaches to deep desulfurization for ultra-clean
gasoline, diesel fuel and jet fuel, Catal. Today 86 (2003) 211.
[3] A. Marafi, A. Al-Hindi, A. Stanislaus, Deep desulfurization of full range and low
boiling diesel streams from Kuwait Lower Fars heavy crude, Fuel Process Technol. 88
(2007) 905.
[4] R.G. Leliveld, S.E. Eijsbouts, How a 70-year-old catalytic refinery process is still
ever dependent on innovation, Catal. Today 130 (2008) 183.
[5] A. Zhou, X. Ma, C. Song, Liquid-phase adsorption of multi-ring thiophenic sulfur
compounds on carbon materials with different surface properties, J. Phys. Chem. B. 110
(2006) 4699.
[6] H. Schulz, W. Bohringer, F. Ousanov, P. Waller, Refractory sulfur compounds in
gas oils, Fuel Proc. Tech. 61 (1999) 5.
[7] T.C. Ho, Deep HDS of diesel fuel: chemistry and catalysis, Catal. Today 98 (2004)
3.
[8] X. Wang, Y. Lee, W. J. Chun, S. T. Oyama, F. Requejo, Effect of phosphorus
content in nickel phosphide catalysts studied by XAFS and other techniques, J. Catal., 210
(2002) 207..
141
[9] P. Clark, X. Wang, S.T.Oyama, Characterization of silica-supported Molybdenum
and Tungsten phosphide hydroprocessing catalysts by 31P nuclear magnetic resonance
spectroscopy, J. Catal. 207 (2002) 256.
[10] F. Sun, W. Wu, Z. Wu, J. Guo, Z. Wei, Y. Yang, Z. Jiang, F. Tian, C. Li,
Dibenzothiophene hydrodesulfurization activity and surface sites of silica-supported MoP,
Ni2P, and Ni-Mo-P catalysts, J. Catal. 228 (2004) 298.
[11] C. Stinner, R. Prins, Th. Weber, Binary and ternary transition metal phosphides as
HDN catalysts, J. Catal. 202 (2001) 187.
[12] J. A. Rodriguez, J.-Y. Kim, J. C. Hanson, S. J. Sawhill, M. E. Bussell, Physical and
chemical properties of MoP, Ni2P, and MoNiP hydrodesulfurization catalysts: Time-
resolved X-ray diffraction, density functional, and hydrodesulfurization activity studies, J.
Phys. Chem. B (2003) 6276.
[13] D. Ma, T. Xiao, S. Xie, W. Zhou, S. L. Gonzalez-Cortes, M. L. H. Green, Synthesis
and structure of bimetallic nickel molybdenum phosphide solid solutions, Chem. Mater. 16
(2004) 2697.
[14] I.I. Abu, K.J. Smith, The effect of cobalt addition to bulk MoP and Ni2P catalysts
for the hydrodesulfurization of 4,6-dimethyldibenzothiophene, J. Catal. 241 (2006) 356.
[15] I.I. Abu, K.J. Smith, Hydrodenitrogenation of carbazole over a series of bulk
NixMoP catalysts, Catal. Today 125 (2007) 248.
[16] V. Zuzaniuk, R. Prins, Synthesis and characterization of silica-supported transition-
metal phosphides as HDN catalysts, J. Catal. 219 (2003) 85.
142
[17] I.I. Abu, K.J. Smith, HDN and HDS of model compounds and light gas oil derived
from Athabasca bitumen using supported metal phosphide catalysts, Appl. Catal. A: Gen.
328 (2007) 58.
[18] A.W. Burns, A.F. Gaudette, M.E. Bussell, Hydrodesulfurization properties of
cobalt-nickel phosphide catalysts: Ni-rich materials are highly active, J. Catal. 260 (2008)
262.
[19] Y. Shu, Y.K. Lee, S.T. Oyama, Structure-sensitivity of hydrodesulfurization of 4,6-
dimethyldibenzothiophene over silica-supported nickel phosphide catalysts, J. Catal., 236
(2005) 112.
[20] S.T. Oyama, Y.K. Lee, The active site of nickel phosphide catalysts for the
hydrodesulfurization of 4,6-DMDBT, J. Catal 258 (2008) 393.
[21] J.B. Goodenough, “Transition metal compounds, Crystallographic and magnetic
properties of solid solutions of the phosphides M2P, M = Cr, Mn, Fe, Co, Ni”, J. Appl.
Phys. 40 (1969) 1250.
[22] J.P. Sénateur, A. Rouault, P. L’Héritier, A. Krumbügel-Nylund, R. Fruchart, D.
Fruchart, P. Convert, E. Roudaut, “La selectivite des substitutions dans les phases MM’P
etude de l’ordre par diffraction neutronique dans NiCoP“, Mat. Res. Bull. 8 (1973) 229.
[23] J.F. Deng, H. Li, W. Wang, Progress in design of new amorphous alloy catalysts,
Catal. Today 51 (1999) 113.
[24] X. Wang, P. Clark, S. T. Oyama, Synthesis,characterization, and hydrotreating
activity of several iorn group transition metal phosphides, J. Catal. 208 (2002) 321.
[25] B. Ravel, M. Newville, Athena, artemis, hephaeatus: data analysis for X-ray
absorption spectroscopy using IFEFFIT, J. Synchrotron Rad. 12 (2005) 537-541.
143
[26] J.J. Rehr, R.C. Albers, Theoretical approaches to x-ray absorption fine structure,
Rev. Mod. Phys. 72 (2000) 621.
[27] S. Ramanathan, S.T. Oyama, New catalysts for hydroprocessing: carbides and
nitrides, J. Phys. Chem. 99 (1995) 16365.
[28] H. Fujii, S. Komura, T. Takeda, T. Okamoto, Y. Ito, J. Akimitsu, Polarized neutron
diffraction study of Fe2P single crystal, J. Phy. Soc. Jap. 46 (1979) 1616.
[29] S.J. Sawhill, K.A. Layman, D.R. Van Wyk, M.H. England, C. Wang, M.E. Bussell,
Thiophene hydrodesulfurization over nickel phosphide catalysts: effect of the precursor
composition and support, J. Catal. 231 (2005) 300.
[30] Y.K. Lee, S.T. Oyama, Bifunctional nature of a SiO2-supported Ni2P catalyst for
hydrotreating: EXAFS and FTIR studies, J. Catal. 239 (2006) 376.
[31] K.I. Hadjiivanov, G.N. Vayssilov, Characterization of oxide surfaces and zeolites
by carbon monoxide as an IR probe molecule, Adv. Catal. 47 (2002) 307.
[32] K.A. Layman, M.E. Bussel, Infrared spectroscopic investigation of CO adsorption
on silica-supported nickel phosphide catalysts, J. Phys. Chem. B 108 (2004) 10930.
[33] A.W. Burns, K.A. Layman, D.H. Bale, M.E. Bussell, Understanding the
relationship between composition and hydrodesulfurization properties for cobalt phosphide
catalysts, Appl. Catal. A: Gen. 343 (2008) 68.
[34] H. TopsØe, B.S. Clausen, F.E. Massoth, J.R. Anderson, M. Boudard. Catalysis
Science and Technology, vol. 11. Springer-Verlag, New York, 1996.
[35] J.A. Rodriguez, Interaction of hydrogen and thiophene with Ni/MoS2 and Zn/MoS2
surfaces: A molecular orbital study, J. Phys. Chem. B 101 (1997) 7524.
144
[36] E. Boellaard, A.M. van der Kraan, J.W. Geus, Preparation, reduction and CO
chemisorptions properties of cyanide-derived NixFe/Al2O3 catalysts, Appl. Catal. A: Gen.
224 (2002) 1.
[37] M. Lepage, T. Visser, F. Soulimani, A.M. Beale, A.Iglesias-Juez, A. M. J. van der
Eerden, B. M. Weckhuysen, Promotion effects in the oxidation of CO over zeolite-
supported Rh nanopartciles, J. Phys. Chem. C 112 (2008) 9394.
[38] S.T. Oyama, X. Wang, Y.K. Lee, K. Bando, F.G. Requejo, Effect of phosphorus
content in nickel phosphide catalysts studied by XAFS and other techniques, J. Catal. 210
(2002) 207.
[39] S.T. Oyama, Novel catalysts for advanced hydroprocessing: transition metal
phosphides, J. Catal. 2106 (2003) 343.
[40] Y. Shu, Y.K. Lee, S.T. Oyama, Structure-sensitivity of hydrodesulfurization of 4,6-
dimethyldibenzothiophene over silica-supported nickel phosphide catalysts, J. Catal. 236
(2005) 112-121.
[41] A.E. Nelson, M. Sun, A.S.M. Junaid, On the structure and composition of the
phosphosulfide overlay on Ni2P at hydrotreating conditions, J. Catal. 241 (2006) 188-188.
[42] V. Vanrysselberghe, G. F. Froment, Hydrodesulfurization of dibenzothiophene on a
CoMo/Al2O3 catalyst: Reaction network and kinetics, Ind. Eng. Chem. Res., 35 (1996)
3311..
[43] V. Vanrysselberghe, R. L. Gall, G. F. Froment, Hydrodesulfurization of 4-
methyldibenzo-thiophene and 4,6-dimethyldibenzothiophene on a CoMo/Al2O3 catalyst:
Reaction network and kinetics, Ind. Eng. Chem. Res., 37 (1998) 1235.
145
[44] J. H. Kim, X. Ma, C. Song, Y.-K. Lee, S. T. Oyama, “Kinetics of two pathways for
4,6-dimethyldibenzothiophene hydrogenation over NiMo, CoMo sulfide, and Nickel
phosphide catalyst”, Energy & Fuels, 19 (2005) 353.
[45] R. Prins, M. Egorava, A. Rothlishberger, Y. Zhao, N. Sivasankar, P. Kukula,
Mechanisms of hydrodesulfurization and hydrodenitrogenation, Catal. Today 111 (2006)
84.
[46] G. Perot, The reactions involved in hydrodenitrogenation, Catal. Today 10 (1991)
447.
[47] R. Prins, M. Jian, M. Flechsenhar, Mechanism and kinetics of
hydrodenitrogenation, Polyhedron 16 (1997) 3235.
[48] B. K. Hodnett, B. Delmon, Stud. Surf. Sci. Catal. 27 (1986) 53.
[49] I. A. van Parijs, G. F. Froment, B. Delmon, Bull. Soc. Chim. Belg. 93 (1984) 823.
[50] J. A. Marzari, S. Rajagopal, R. Miranda, bifunctional mechanism of pyridine
hydrodenitrogenation, J. Catal. 156 (1995) 255.
[51] M. Jian, R. Prins, Mechanism of the hydrodenitrogenation of quinoline over
NiMo(P)/Al2O3 catalysts, J. Catal. 179 (1998) 18.
[52] F. Rota, R. Prins, Mechanism of the C-N bond breaking in the
hydrodenitrogenation of methylcyclohexylamine over sulfided NiMo/γ-Al2O3, J. Mol.
Catal. A 162 (2000) 359.
[53] J. Ren, C.F. Huo, X.D. Wen, Z. Cao, J. Wang, Y.W Li, H. Jiao, Thiophene
adsorption and activation on Mo(001), MoN(100), and NiP(001): Density functional
theory studies, J. Phys. Chem B 110 (2006) 22563-22569.
146
[54] S.D. Kelly, N. Yang, G.E. Mickelson, N. Greenlay, E. Karapetrova, W. Sinkler,
S.R. Bare, Structural characterization of Ni-W hydrocracking catalysts using in situ
EXAFS and HRTEM, J. Catal. (2009), doi:10.1016/j.jcat.2009.01.019.
[55] E.A. Stern, Phys. Rev. B 48 (1993) 9825.
[56] B. Ravel, EXAFS analysis using FEFF and FEFFIT, 2000.
[57] Y. Maeda, Y. Takashima, Mossbauer Studies of FeNiP and related compounds, J.
Inorg. Nucl. Chem. 35 (1973) 1963.
[58] R.C. Buchanan, T. Park, Materials crystal chemistry, Mrcel Dekker, Inc., 1997
[59] T. Kawai, K.K. Bando, Y.-K. Lee, S.T. Oyama, W.-J. Chun, K. Asakura, EXAFS
measurements of a working catalyst in the liquid phase: An in situ study of a Ni2P
hydrodesulfurization catalyst, J. Catal. 241 (2006) 20.
[60] T. Kawai, W.J. Chun, K. Asakura, Y. Koike, M. Nomura, K.K. Bando, S.T. Oyama,
H. Sumiya, Design of a high-temperature and high-pressure liquid flow cell for x-ray
absorption fine structure measurements under catalytic reaction conditions, Rev. Sci. Instr.
79 014101 (2008) 1.
[61] A.R. Lennie, S.A.T. Redfern, P.F. Schofield, D.J. Vaughan, Synthesis and Rietveld
crystal structure refinement of mackinawite, tetragonal FeS, Mineral. Mag. 59 (1995) 677.
[62] M.J. Buerger, The crystal structure of marcasite Locality: Joplin, Missouri, USA,
Am. Mineral. 16 (1931) 361-395.
[63] M. Rieder, J.C Crelling, O. Sustai, M. Drabek, Z. Weiss, M. Klementova. Arsenic
in iron disulfides in a brown coal from the North Bohemian Basin, Czech Republic, Int. J.
Coal Geol. 71 (2007) 115.
147
[64] B.J. Skinner, R.C. Erd, F.S. Grimaldi, Greigite, the thio-spinel of iron; a new
mineral Czech Republic, Am. Mineral. 49 (1964) 543.
[65] J. Trahan, R.G. Goodrich, S.F. Watkins, X-ray diffraction measurements on
metallic and semiconducting hexagonal NiS, Phys. Rev. B 2 (1970) 2859.
[66] T. Fujii, K. Tanaka, F. Marumo, Y. Noda, Polarized neutron diffraction study of
Fe2P single crystal, Miner. J. 13 (1987) 448.
[67] J.B. Parise, Structure of hazelwoodite (Ni3S2), Acta Crystallogr. B 36 (1980)
1179.
[68] S.T. Oyama, X. Wang, Y.K. Lee, W.J. Chun, Active phase of Ni2P/SiO2 in
hydroprocessing reactions, J. Catal. 221 (2004) 263.
[69] T. Kawai, K.K. Bando, Y.K. Lee, S.T. Oyama, W.J. Chun, K. Asakura, EXAFS
measurements of a working catalyst in the liquid phase: An in situ study of a Ni2P
hydrodesulfurization catalyst, J. Catal. 242 (2006) 20.
[70] Y.K. Lee, Y. Shu, S.T. Oyama, Active phase of nickel phosphide (Ni2P) catalyst
supported on KUSY zeolite for the hydrodesulfurization of 4,6-DMDBT, Appl. Catal. A:
Gen. 322 (2007) 191.
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
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.
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