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1 FISCHER TROPSCH SYNTHESIS: USING NANOPARTICLE OXIDES AS SUPPORTS FOR COBALT BASED FISCHER TROPSCH CATALYSTS By ROBERT JAMES COLMYER A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2011
2 2011 R obert J ames Colmyer
3 To my parents
4 ACKNOWLEDGMENTS I would like to acknowledge the Florida Department of Agriculture and Consumer Ser vices fo r funding. I would like to thank the students in the lab that have helped with this work, Justin Dodson, Luke Neal, Wei Cheng, Cheng Chun Tsai, and Trent Elkins. I would like to thank all of my friends over the years that have helped me achieve m y goals. I would like to thank Liyan Jin for being there for me over the past three plus years. I would also like to thank my co chair Dr. Jason Weaver for his help. Also I would like to thank my advisor Dr. Helena Hagelin Weaver for all of her help an d support. Finally I would like to especially thank my parents for everything they have done to allow me to succeed.
5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 9 LIST OF FIGURES ................................ ................................ ................................ ........ 11 ABSTRACT ................................ ................................ ................................ ................... 14 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 16 Motivation ................................ ................................ ................................ ............... 16 Fischer Tropsch History ................................ ................................ .......................... 17 Fischer Tropsch Synthesis Background ................................ ................................ 19 Fischer Tropsch Reaction Mechanism ................................ ............................. 20 Product Distribution ................................ ................................ .......................... 22 Fischer Tropsch Catalysts ................................ ................................ ...................... 23 Metal Support Interactions ................................ ................................ ............... 24 Catalyst Supports ................................ ................................ ............................. 26 Al 2 O 3 supported catalysts. ................................ ................................ ......... 27 SiO 2 supported catalysts. ................................ ................................ ........... 28 TiO 2 supported catalys ts. ................................ ................................ ........... 30 ZrO 2 supported catalysts. ................................ ................................ .......... 30 Carbon supported catalysts. ................................ ................................ ...... 30 Other supports. ................................ ................................ .......................... 31 Promoters and Additives ................................ ................................ .................. 32 Alkali metals. ................................ ................................ .............................. 33 Early transition metal oxides. ................................ ................................ ..... 33 Rare earth oxides. ................................ ................................ ...................... 34 Late transition and noble metals. ................................ ............................... 35 Catalyst Preparation and Pretreatment ................................ ............................ 36 Catalyst preparation. ................................ ................................ .................. 37 Catalyst pretreatment. ................................ ................................ ................ 39 Catalyst Deactivation ................................ ................................ ........................ 40 Sulfur and nitrogen poisoning. ................................ ................................ ... 40 Fouling b y heavy hydrocarbons. ................................ ................................ 41 Inactive compound formation. ................................ ................................ .... 41 Catalyst regeneration. ................................ ................................ ................ 42 Catalyst Properties ................................ ................................ ........................... 43 Reactors ................................ ................................ ................................ ........... 44 Nanoparticle Oxide Supports ................................ ................................ .................. 45 2 EXPERIMENTAL SETUP ................................ ................................ ....................... 51
6 Reactor System Design ................................ ................................ .......................... 51 Reaction Experiments ................................ ................................ ............................. 54 Catalyst Synthesis ................................ ................................ ................................ .. 55 Catalyst Characterization Techniques ................................ ................................ .... 56 Brunauer Emmet Teller (BET) surface area measurements ............................ 56 Carbon Monoxide Pulse Titration ................................ ................................ ..... 56 Oxygen Titration Measurement ................................ ................................ ........ 57 Temperature Programmed Reduction ................................ .............................. 58 X Ray Diffraction ................................ ................................ .............................. 59 Transmission Electron Microscopy ................................ ................................ ... 59 3 EFFECTS OF SUPPORT, COBALT PRECURSOR, NANOPARTICLE SUPPORT & ZIRCONIA PROMOTION ................................ ................................ .. 64 Background ................................ ................................ ................................ ............. 64 Experimental Details ................................ ................................ ............................... 66 Results and Discussion ................................ ................................ ........................... 66 Fischer Tropsch Synthesis Results ................................ ................................ .. 66 Unpromoted catalysts ................................ ................................ ................ 66 ZrO 2 promoted catalysts. ................................ ................................ ........... 67 Catalyst Characterization ................................ ................................ ................. 68 BET Surface Area Measurements ................................ ................................ .... 68 Carbon Monoxide Pulse Titration ................................ ................................ ..... 69 Temperature Programmed Reduction Analysis ................................ ................ 69 Oxygen Pulse Titration ................................ ................................ ..................... 71 X Ray Diffraction ................................ ................................ .............................. 71 TEM Results ................................ ................................ ................................ ..... 72 Summary ................................ ................................ ................................ ................ 73 4 EFFECTS OF ADDITIVES ................................ ................................ ...................... 88 Background ................................ ................................ ................................ ............. 88 Experimental Section ................................ ................................ .............................. 94 Catalyst Preparation ................................ ................................ ......................... 94 Catalytic Activity Measurements ................................ ................................ ....... 94 Results and Discussion ................................ ................................ ........................... 95 Fischer Tropsch Synthesis ................................ ................................ ............... 95 BET Surface Area Analysis ................................ ................................ .............. 98 Carbon Monoxide Chemisorption ................................ ................................ ..... 99 Temperature Programme d Reduction Results ................................ ............... 100 Oxygen Pulse Titration ................................ ................................ ................... 102 XRD Results ................................ ................................ ................................ ... 103 Summary ................................ ................................ ................................ .............. 105 5 EFFECTS OF DIFFERENT OXIDE SUPPORTS ................................ .................. 118
7 Background ................................ ................................ ................................ ........... 118 Experimental Details ................................ ................................ ............................. 120 Results and Discussion ................................ ................................ ......................... 121 Fischer Tropsch Synthesis ................................ ................................ ............. 121 BET Surface Area Analysis ................................ ................................ ............ 123 Carbon Monoxide Chemisorption ................................ ................................ ... 123 Temperature Programmed Reduction Resu lts ................................ ............... 124 Oxygen Pulse Titration ................................ ................................ ................... 127 X Ray Diffraction Results ................................ ................................ ............... 127 Summary ................................ ................................ ................................ .............. 128 6 KINETICS OF A FISCHER TROPSCH CATALYST ................................ ............. 135 Background ................................ ................................ ................................ ........... 135 Results and Discussion ................................ ................................ ......................... 138 Effect of Temperature ................................ ................................ ..................... 138 Effect of H 2 /CO Ratio ................................ ................................ ...................... 139 Effect of Total Flowrate ................................ ................................ .................. 139 Effect of Pressure ................................ ................................ ........................... 140 Effect of Carbon Dioxide in the Feed ................................ .............................. 140 Rate Equation Fit of Data ................................ ................................ ............... 140 Summary ................................ ................................ ................................ .............. 141 7 CONCLUSIONS ................................ ................................ ................................ ... 151 APPENDIX A EFFECTS OF CALCINATIONS AND MULTIPLE IMPREGNATIONS .................. 154 Background ................................ ................................ ................................ ........... 154 Results and Discussion ................................ ................................ ......................... 154 Fischer Tropsch Synthesis ................................ ................................ ............. 154 Catalyst Characterization ................................ ................................ ............... 155 BET Surface Area Measurements ................................ ................................ .. 155 Carbon Monoxide Pulse Titration ................................ ................................ ... 155 Temperature Programmed Reduction Analysis ................................ .............. 155 Oxygen Pulse Titration ................................ ................................ ................... 156 Discussion ................................ ................................ ................................ ............ 156 B DEACTIV ATION BEHAVIOR ................................ ................................ ................ 160 Background ................................ ................................ ................................ ........... 160 Results ................................ ................................ ................................ .................. 163 Characterization ................................ ................................ ............................. 163 XRD on spent catalysts ................................ ................................ ............ 163 BET surface area loss ................................ ................................ .............. 163 Regenera tion Attempts ................................ ................................ ................... 163 Discussion ................................ ................................ ................................ ............ 165
8 LIST OF REFERENCES ................................ ................................ ............................. 17 0 BIOGRAPH ICAL SKETCH ................................ ................................ .......................... 186
9 LIST OF TABLES Table page 1 1 Patented rejuvenation techniques  ................................ ................................ 50 1 2 Patented regeneration techniques  ................................ ................................ 50 3 1 Description of the unpromoted catalysts ................................ ............................. 75 3 2 Description of the promoted c atalysts ................................ ................................ 75 3 3 Product Analysis of All Reactions ................................ ................................ ....... 80 3 4 Characterization data for all catalysts ................................ ................................ 81 3 5 Particle sizes calculated from the XRD data using the Scherrer Equation for fresh catalysts using a 99.999% Co precursor and a precalcined support. ........ 85 4 1 Product analysis results for cobalt catalysts with additives on n SiO 2 ............. 110 4 2 Product analysis results for cobalt catalysts with additives on n SiO 2 ............. 110 4 3 Characterization results for additives ................................ ................................ 114 4 4 Particle sizes for n SiO 2 supported promoted catalysts ................................ .... 117 4 5 Particle sizes for Ru/ZrO 2 promoted n SiO 2 supported catalysts. ..................... 117 5 1 Reaction data for different supported catalysts, 20 wt% cobalt, P=20bar, T=230 C, total flo w rate=62.5 ml/min (H 2 :CO:N 2 =6:3:1). ................................ 131 5 2 Characterization data for cobalt catalysts with different supports. .................... 132 5 3 Parti cle sizes calculated from the XRD data using the Scherrer Equation for fresh catalysts using different supports with 20% cobalt loading. ..................... 134 6 1 List of kinetic experiments in literature. ................................ ............................. 143 6 2 List of kinetic experiments in literature, fit rate equations. ................................ 144 6 3 Conversion and selectivity at different temperat ures for 20/Co/ZrO 2 /Ru/n SiO 2 ................................ ................................ ................................ ................ 145 6 4 Conversion values at different H 2 /CO ratios for 20/Co/ZrO 2 /Ru/n SiO 2 at T=205 C, total flowrate=62.5ml/min, P=20 bar. ................................ ................ 145 6 5 Fit rate equations ................................ ................................ .............................. 149
10 A 1 Reaction data for cobalt catalysts with different preparation techniques. ......... 158 A 2 Characterization data for cobalt catalysts with different preparation techniques. ................................ ................................ ................................ ....... 159 A 3 BET surface area comparison of nanoparticle SiO 2 supports calcined at differen t temperatures. ................................ ................................ ..................... 159 B 1 Loss of surface area from reaction. ................................ ................................ ..... 167
11 LIST OF FIGURES Figure page 1 1 FTS reaction scheme 1 carbene mechanism ................................ ..................... 47 1 2 FTS reaction scheme 2 alcohol chain (enolic) mechanism. ............................... 47 1 3 F TS reaction scheme 3 carboxyl group (direct insertion) mechanism. ............... 48 1 4 obability, d=desorption rate ................................ ................................ .............................. 48 1 5 Suggested possible termination steps in the Fischer Tropsch process. ............. 49 1 6 ASF distribution plot of weight fraction vs. chain growth probability f actor, .... 49 2 1 Design drawing of Fischer Tropsch reactor system. ................................ ........... 61 2 2 Diagram of trap setup for liquid product collection. ................................ ............. 62 2 3 Picture of the reactor system for the Fischer Tropsch Synthesis experiments. .. 63 3 1 Carbon monoxide conversion for un pro moted cobalt based catalysts supported on SiO 2 ................................ ................................ .............................. 76 3 2 C 5+ selectivity for un promoted cobalt based catalysts supported on SiO 2 ......... 77 3 3 CO conversion for ZrO 2 promoted cobalt based catalysts supported on SiO 2 ... 78 3 4 C 5+ selectivity for ZrO 2 promoted Cobalt based catalysts supported on SiO 2 .... 79 3 5 TPR for un promoted catalysts a) n SiO 2 supported catalysts b) p SiO 2 supported catalysts. ................................ ................................ ............................ 82 3 6 TPR for ZrO 2 promoted catalysts a) n SiO 2 supported catalysts b) p SiO 2 supported catalysts. ................................ ................................ ............................ 83 3 7 XRD patterns obtained from fresh catalysts prepared using a precalcined support and the 99.999% pure Co precursor. ................................ ..................... 84 3 8 XRD patterns obtained from reduced catalysts prepared using a precalcined support and the 99.999% pure Co precursor. ................................ ..................... 84 3 9 XRD patte rns obtained from spent catalysts (catalysts after exposure to reaction conditions). ................................ ................................ ........................... 85 3 10 TEM Images of 99UnNpPC. ................................ ................................ ............... 86
12 3 11 TEM Images of 99UnPoPC. ................................ ................................ ............... 86 3 12 TEM Images of 99PrNpPC. ................................ ................................ ................ 86 3 13 TEM Images of 99PrPoPC. ................................ ................................ ................ 87 4 1 CO Conversion for promoted catalysts supported on nanoparticle SiO 2 .......... 107 4 2 C 5+ Selectivity for promoted catalysts supported on nanoparticle SiO 2 ........... 108 4 3 Cobalt catalysts with the addition of ZrO 2 and Ru supported on SiO 2 ............. 109 4 4 TPR for cobalt catalysts supported by nanopar ticle SiO 2 ................................ 112 4 5 TPR for cobalt catalysts and double promoted catalysts. ................................ 113 4 6 XRD of fresh promoted catalysts: Unpromo ted, Ru, Fe, Pd, CaO, MgO, CeO 2 ZrO 2 a) full scan b) high resolution scan. ................................ .............. 115 4 7 XRD of reduced catalysts: Unpromoted, Ru, Fe, Pd, CaO, MgO, CeO 2 ZrO 2 a) full scan b) high resolution s can. ................................ ................................ .. 115 4 8 XRD of spent catalysts: Unpromoted, Ru, Fe, Pd, CaO, MgO, CeO 2 ZrO 2 a) full scan b) high resolution scan. ................................ ................................ ...... 116 4 9 XRD of double promoted Ru and ZrO 2 p SiO 2 and n SiO 2 supported. Fresh, reduced, and spent. ................................ ................................ .......................... 116 5 1 Reaction behavior for different supports with 20% cobalt. ................................ 130 5 2 Temperature Programmed Reduction behavior for different supports with 20% cobalt. ................................ ................................ ................................ ....... 131 5 3 XRD of TiO 2 supported catalysts, a) full scans b) hig h resolution scans. ......... 132 5 4 XRD of SiO 2 supported catalysts, a) full scans b) high resolution scans. ......... 133 5 5 XRD of ZrO 2 sup ported catalysts, a) full scans b) high resolution scans. ......... 133 5 6 XRD of Al 2 O 3 supported catalysts, a) full scans b) high resolution scans. ........ 134 6 1 The effect of temperature on CO conversion, catalyst: Co/ZrO 2 /Ru/n SiO 2 P=20bar, F tot =62 5 ml/min, T=205 C, a) CO conversion b) C 5+ selectivity ........ 146 6 2 The effect of H 2 /CO ratios on CO conversion, catalyst: Co/ZrO 2 /Ru/n SiO 2 P=20bar, F tot =62.5 ml/min, T=205 C. ................................ ............................... 147 6 3 The effect of total flowrate on CO conversion data, catalyst: Co/ZrO 2 /Ru/n SiO 2 P=20bar, F tot =62.5 ml/min, T=205 C. ................................ ..................... 147
13 6 4 The effect of total pressure on CO conversion data, catalyst: Co/ZrO 2 /Ru/n SiO 2 P=20bar, F tot =62.5 ml/min, T=205 C. ................................ ..................... 148 6 5 The effect of total CO 2 catalyst: Co/ZrO 2 /Ru/n SiO 2 P=20bar, F tot =62.5 ml/min, T=205 C. ................................ ................................ .............................. 148 6 6 Reaction rate data for Co/ZrO 2 /Ru/n SiO 2 catalyst vs. Equation 6 2 (Power Law Model). ................................ ................................ ................................ ...... 150 A 1 Reaction behavior for different impregnation procedures with 20% cobalt. ...... 157 A 2 Temperature programmed reduction of catalysts prepared by different methods ................................ ................................ ................................ ............ 158 B 1 XRD spectra of 20% Co 2% ZrO2 78% p SiO2 freshly reduced, two spent catalysts and glass beads. ................................ ................................ ............... 167 B 2 Co/Ru/ZrO 2 /nanoparticle SiO 2 1 gram of catalyst and 8 grams of glass beads (V bed =8ml). ................................ ................................ ............................. 169
14 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy FISCHER TROPSCH SYNTHESIS: USING NANOPARTICLE OXIDES AS SUPPORTS FOR COBALT BASED FISCHER TROPSCH CATALYSTS By Robert J ames Colmyer December 2011 Chair: Helena Hagelin Weaver Cochair: Jason Weaver Major: Chemical Engineering The F ischer Trospch (F T) synthesis of liquid hydrocarbon fuels from biomass derived synthesis gas (CO + H2) is a promising way of producing renewa ble ultra clean diesel fuel. The Fischer Tropsch process has been used commercially for many decades using coal or natural gas as a feedstock. Recently the focus has switched to renewable resources such as biomass. Significant effort has been spent dev eloping efficient Fischer Tropsch catalysts, but very little is known about the effects of nanoparticle oxide supports for Fischer Tropsch catalysts. In this research project the objectives were to 1) design and build a F T reactor and 2) use the system t o develop new efficient F T catalysts. A number of different catalysts were prepared carefully characterized a nd tested for F T activity The initial study involved the effects of support pretreatment and catalyst precursor quality In addition ZrO 2 pr omotion was investigated in detail for Co catalysts supported on nanoparticle (n SiO 2 ) and porous (p SiO 2 ) silica supports. The results show that the support pretreatment and catalyst precursor have limited effects on the catalytic activities and selectiv ities. It was also found that ZrO 2 addition
15 increases activity and in some cases selectivity with minimal difference between Co/n SiO 2 and Co/p SiO 2 In addition to ZrO 2 a number of other metal and metal oxide promoters investigated and it was found t hat CeO 2 Ru, and Cu are also promo ters as they increase the activit y of the unpromoted catalyst. The higher activities using the Ru and Cu promoted catalysts may be due to increased reducibilit ies and metal surface area s In contrast, MgO, CaO, La 2 O 3 Fe Pd, and Re all appear to decrease the productivity of the unpromoted cataly st. The reasons for the lower activities of these catalyst s may be due to a reduction in active metal surface area from decreased reducibilities or inactive mixed cobalt oxide fo rmation either during catalyst pretreatment or under the reaction conditions. The effect s of using nanoparticle supports compared to more conventional porous supports for Fischer Tropsch catalysts were investigated. The nanop article support produces simi lar activities as the porous support for SiO 2 and ZrO 2 In the case of alumina, t he Al 2 O 3 support is more productive than n Al 2 O 3 while n TiO 2 is more productive than p TiO 2 The high activities over the Al 2 O 3 supported catalysts are surprising consid ering the low reducibilities observed for these catalysts and indicate that the Co on these catalysts is highly active. A kinetic evaluation of a Co/Ru/ZrO 2 /n SiO 2 catalyst led to a simple power law rate equation. This rate equation shows that the reactio n has approximately a square root dependence on H 2 and that CO is an inhibitor in the reaction. The apparent activation energy is near that of literature values at 90kJ/mol.
16 CHAPTER 1 I NTRODUCTION Motivation The development of alternative fuel s is of cruc With the limited supply of fossil fuels, in particular oil and increasing demands the search for other ways of producing renewable, cleaner fuel s has become a priority. Local renewable fuels are important for economical and environmental reasons and will This pro ject focuses on using woody bio mass to produce high quality diesel fuel. Synthetic diesel fuel produced via the Fischer Tropsch synthesis has a higher cetane valu e compared to petroleum based diesel, since it contains mainly straight chain hydrocarbons and minimal amounts of aromatics [ 1 ] Another benefit is reduced emissions as lower amounts of soot are produced and the hydrocarbon product is almost free of sulf ur. If the synthetic diesel is produced from biomass derived synthesis gas, it is a lso considered a renewable fuel [ 2 ]. This is critical for the state of Florida as it is 3 rd in the nation in energy consumption and 1 st in biomass resources [ 3 4 ] The c limate is perfect for growing biomass that can be u sed for the production of transportation fuels. Overall the synthetic diesel produced from woody biomass releases fewer pollutants and will burn more efficiently than the current petroleum based diesel. The bio diesel will yield 280% more energy than petroleum diesel fuel and lower exhaust emissions over 47% according to the United States Department of Energy and the Department of Agriculture [ 5 ] The main source of the woody biomass would be from Flori d are very common in Florida. To find the optimal biomass source one must identify naturally occurring variants of existing Florida native and breeding populations of slash and loblolly pine
17 and Populus deltoids that have high wood c arbohydrate contents, but require low input (water and fertilizer). The se variants are therefore a good source of biomass for this process. Diesel fuel obtained from Florida pine trees, or other woody biomass, is a non food based bio fuel so the impact o n food supplies will be non existent. In the synthesis pr o c ess of diesel fuel from woody biomass, the wood is first gasified, or steam reformed, at high temperature to produce a synthesis gas, or syngas, which is a mixture of hydrogen (H 2 ) and carbon mono xide (CO) with small amounts of carbon dioxide (CO 2 ) The synthesis gas is then fed to a Fischer Tropsch reactor where the CO and H 2 are converted to hydrocarbons. This step is the main focus of this study, which has two main goals, 1) design, build, tes t, and operate a reactor system to ru n the Fischer Tropsch process and 2 ) develop an optimal Fischer Tropsch catalyst Fischer Tropsch History The Fischer Tropsch process has been around for many years and has been in s focu s. This section will give a brief history of the process and some of the motivation for the current wo rk. According to Bartholomew and Farruto [ 6 ] there are five periods of Fischer Tropsch history. It began with the discovery in 1902 by Sabatier and Sen derens that CO could be hydrogenated over Co balt (Co) Iron ( Fe ) a nd Ni ckel (Ni) to produce methane The next advancement came in 1913 with the discovery of liquid s production under harsh conditions over cobalt catalysts by BASF. Franz Fischer and Hans Tropsch then found the production of oxygenated hydrocarbons using alkalized iron at high temperature and pressure. Shortly after that, in 1925, hydrocarbon liquid and solid (wax) production over Co Fe catalysts at much milder conditions was accomplished In fact in 1926 Fischer and Tropsch secured a patent which had some important conclusions [ 7 ]:
18 Fe, Co, and Ni are the most effective catalysts for hydrocarbon synthesis Co is most active for C 2+ hydrocarbons, Ni for methane Carriers such as zinc oxide ( ZnO ) and chromium oxide ( Cr 2 O 3 ) improve CO c onversion while lowering sintering rates of metals Addition of small amounts of alkali to an Fe catalyst favors selectivity to liquid hydrocarbons Copper (Cu) improves reduction of Fe at low temperatures Syn thesis g as (CO+H 2 s yngas must not contain sulfur to prevent catalyst poisoning. In the second time period there was a push for commercial development and the use of Co as a catalyst instead of Fe In the 1930s and 1940s the commercial production mai nly took place in Germany which used coal as a feedstock for the syn thesis gas. The optimal catalyst for a long time was the Co/ThO 2 /kieselguhr catalyst. The optimal reduction conditions were found during this generation to be 365C for 5 20 hours. Als o it was discovered that the higher pressure reaction above 5 bar, leads to increased stability and more liquid fuel production. The third time period began with the end of World War II when the German interest in synfuels was conti nued in Britain and America. There was a perceived shortage of petroleum which led to continued support for synthetic fuels research and development in the USA, Britain and Germany. This research focused on Fe based catalysts. The fourth generation beg an with the 1973 oil embargo sparking interest in synfuels again This led to a rush to do research in the field, with some research leading to the rediscover y of old work. However, p rogress was made on find ing structure activity relationships. Active site density measured by H 2 chemisorption was found to be related to CO hydrogenation in some cases. This new
19 knowledge led to the design and synthesis of a high surface area, high ly activ e noble metal promoted C o/Al 2 O 3 catalyst. These findings also led to improvements in selectivi ty, avoidance of deactivation, cost efficiency heat removal, and thermal efficiency. Finally the current fifth, time period of Fischer Tropsch synthesis began with the opening of two new plants in Mala y sia and South Africa. The first plant was a 10,000 bbl/day ( Fischer Tropsch l iquid fuel) wax crack ing plant constructed by Shell in Mala y sia and used a Co based catalyst. The other is the 25,000 bbl/da y Sasol natural gas to gasoline plant in Mossel Bay, South Africa us ing Fe based catalysts and circulating fluid bed reactors. There have been several other announcements and construction has begun on several other gas to l iquid plants in the Middle East. Fischer Tropsch Synthesis Background The conversion of hydrogen and car bon monoxide, syngas, to liquid hydrocarbons is called the Fischer Tropsch S ynthesis (FTS) The Fischer Tropsch process involves several possible reactions such as (1 1) and (1 2) [ 8 ]: = 1 65 kJ/mol (per mole CO ) (1 1) = 152 kJ/mol (per mole CO ) (1 2) Reaction ( 1 1) occurs when the H 2 :CO ratio is high and a catalyst with strong hydrogenation power is used, such as cobalt or n ickel. If the H 2 :CO ratio is low and a catalyst with less hydrogenation power (Fe) is used, Reaction (1 2 ) is more likely to take place In the case of an iron catalyst Reaction (1 3) can also occur in addition to Reactio n s (1 1) and (1 2) [ 8 ] The formation of oxygenated products, such as Reaction (1 4), is p ossible over some catalysts, in particular those containing Fe.
20 (1 3) (1 4) The methanation reaction, (1 5 ), is the most likely reaction when using Ni as a catalyst. The Water Gas Shift (WGS) Rea ction and Boudard Reaction (1 6 1 7), are possible side reaction s during the Fischer Tropsch process = 247 kJ/mol (1 5 ) = 41 kJ/mol (1 6 ) = 172 kJ/mol (1 7 ) Reaction (1 7 ) leads to the deposition of carbon on the surface which can be a cause of deactivation Reactions (1 1) through (1 5) are all highly exothermic, with a reduction in volume. This allows the use of relatively low temperatures and elevated pr essures to run this reaction. Results show that increasing pressure, up to 2MPa (20bar) increases conversion and C 5+ selectivity (chain growth probability) [ 9 10 ]. Increasing pressure also increases olefin selectivity, while decreasing methane and branched product selectivity [ 10 ]. Fischer Tropsch Reaction Mechanism Fischer Tropsch synthesis involves reaction between CO and H 2 adsorbed on the catalyst surface. While it is assumed that hydrogen dissociatively adsorbs onto t he surface, it is still debated whether carbon monoxide dissociatively adsorbs or not before it is hydrogenated. There fore, there are three suggested mechanisms for the activation of CO. The most commonly accepted mechanism for the production of hydroca rbons is the carbene mechanism (Figure 1 1) The other two, hydroxyl carbene mechanism (Figure 1 2) and
21 carbonyl insertion mechanism (Figure 1 3) mainly explain the production of alcohols and aldehydes. The carbene mechanism begins with the c hemisorpti on of CO and H 2 Both undergo dissociation and form metal carbide a metal oxide, and metal hydrides. It is suggested that the metal oxide then reacts with the surface hydrogen to form metal hydroxyl groups and then to water, which desorbs from the surfa ce. The metal carbide also reacts with surface hydrogen to form a methyl ene group on the metal surface (Figure 1 1 ) [ 2 6 ]. On some catalysts, the metal oxide can react with a carbon monoxide molecule to form carbon dioxide, which desorbs from the metal surface. The overall reaction can be illustrated as a polymerization reaction [Figure 1 4]. It is suggested that the chain initiates from the suitable activation of a methylene radical or from a methyl radical formed by the attack of a hydrogen atom on a methylene radical [ 8 ] The reaction propagates via methylene group addition and grows from one end The re a re three main termination steps (Figure 1 5): 1) direct loss by hydrogenation of chain, 2) mutual termination, meaning two chains lose H form ing H 2 or 3) d isproportionation between two radical chains producing one olefin and one paraffin. The lost hydrogen atom may also attach to one methyl radical forming methane [ 8 ] The olefins produced from termination steps 2 or 3 can re adsorb to reiniti ate the process and form larger hydrocarbons. Other reactions may occur such as olefin hydrogenat ion, hydrogenolysis (carbon heteroatom bond cleaved by hydrogen) or cracking or CO insertion to form alcohols [ 6 ]. All of these routes are possible under th e wide range of conditions in metal chemistry, additives, and surface structure.
22 The o ther two reaction mechanisms are less commonly accepted The enolic mechanism (Figure 1 2) involves the CO molecule adsorbing and the hydrogen molecule attaches to form a n alcohol on the surface. Th e surface alcohol reacts with an other surface alcohol molecule producing water and forming the chain. The carbon insertion mechanism (Figure 1 3) involves the carbon monoxide inserting into a hydrogen atom on the surface. Subsequently hydrogen is added, water lost, and a methyl group is formed. The process continues to form the hydrocarbon chain as seen in Figure 1 3 Product Distribution The Fischer Tropsch process leads to a broad distribution of par af f ins and olefins. The distribution depends on many variables such as : active metal, promoter(s) support, catalyst preparation, reaction pressure, reaction temperature, and feed properties. The most commonly known distribution is the Anderson Schulz Flory (ASF) distributio n [Figure 1 6 ] This model is based on chain polymerization kinetics. (1 8 ) (1 9 ) Where, is the chain growth probability, k p is the rate of propagation, k t is the rate of termination, and W n is the weight fraction of product containing n carbon atoms. The chain growth probability is defined as the rate of propagation divided by the sum of the rate of propagatio n plus the rate of termination. However, e xperimentally there are deviations from th e ASF distribution. The ASF distribution underestimates methane production, and overestimates C 2 Above C 5 the ASF distribution reasonably describ es
23 the product distribution, although in some cases the C 5+ is slightly underestimated. These deviations can be attributed to 1) superposition of two ASF distributions resulting from multiple growth sites or reaction paths [ 11 ] 2) secondary reaction pro cesses, readsorption of alpha olefins followed by their i nitiation of new chains [ 12 ] and 3) h igher methane is due to secon dary hydrocracking of light par a f fins and an alternate reaction path producing only methane. There are attempts to avoid this ASF b ehavior by designing certain catalyst s or experiments i.e. shape selective supports [1 3 17 ] or unsteady state operation. Importantly it has been shown that cobalt catalysts result in a higher fraction of straight chain paraffins compared with Fe, Ru, and Ni [6 ]. Fischer Tro p s ch Catalysts The goal of designing a Fischer Tropsch catalyst is to make an affordable, highly active and selective, as well as stable catalyst. Some of the most important variables include the selection of the metal precursor, metal loading support promoters and preparation plus pretreatment methods. The four main active metals in Fischer Tropsch catalysts are Co, Fe, Ni, and Ru. Fischer and Tropsch originally used Fe Cu catalysts The current focus is mainly on Fe and Co base d catalysts, since Ni is a very efficient methanation catalyst and methane is an undesir able product for diesel production. While r uthenium is very active it is typically used as a promoter due to its high cost. Comparing Fe and Co, Fe is significantly c heaper than Co, but Co is ab out 2 3 times more active on a site basis and this difference is greater with promot ed catalysts [ 6 ] Other disadvantages with iron catalysts are the high water gas shift activity and reaction inhibition by H 2 O. The higher wat er gas shift activity over iron catalysts results in und esire d oxygenated products, such as alcohols and CO 2 Therefore, the C 5+ selectivities for Co catalysts are 20 30%
24 higher than for Fe catalysts According to Bartholomew the benchmark productivity f or a commercial Co FT catalyst is about 1 gC 5+ /g cat h. A catalyst with that productivity should have between 20 40 wt% Co and 0.1 2.0 wt% Ru, Re, or Pt [ 6 ] Due to the higher activity, hydrocarbon selectivity, carbon efficiency, stability and regenerabil ity, supported cobalt based catalysts with promoters, are favored over iron based catalysts for Fischer Tropsch synthesis. The only downside s to cobalt based catalysts are the higher cost and the higher H 2 /CO ratio required to a void carbon deposition C obalt was chosen as the active metal in this study due to its higher activity relative to Iron. The loading of the active metal for F T catalysts is typically between 5 and 40%. A Co loading of 20% is commonly used in Fischer Tropsch synthesis, although the optimal loading is dependent on the support used [18 ] It has been sho wn that the optimal loading on carbon nanotube supported catalysts is 40% [ 19 2 0 ]. Using a SBA 15 supported catalyst the optimal loading was found as to be 30% [1 5]. The product carbon number is shown to increase with increasing loading (3% to 15%) and decreasing dispersion [ 2 1]. In an effort to limit cost and maintain activity and selectivity, a loading of 20% cobalt was chosen for the majority of the catalysts in this study Metal Support Interactions Active metals are deposited on high surface area supports to increase the surface area of the active metal, and thus also the number of active site s. Using supported metal catalyst s lowers sintering rates due to enhanced therm al stability [6, 21 2 3 ] The interaction between the metal and support must be favorable to achieve a high active metal dispersion on the support. However, t oo strong metal support interactions can affect the catalytic properties of the active metal nega tively, as it can result in mixed oxide formation between the support and the active metal [ 22 ] These
25 mixed oxide compounds cannot be reduced at typical reduction temperatures. They are reduced only at very high temperatures, which cause severe sinterin g of the catalysts, a nd are thus considered irreducible and inactive. Therefore, t he catalyst support can play a major role in the catalyst s behavior. Four major effects of metal support interactions have been identified [6 22 2 4 ] : 1. Modifications of el ectronic and geometric properties of the metal surface by support derived species deposited on the metal during preparation or during reaction. These modifications can act as promoters or inhibitors by creating new sites. These sites can modify the elect ronic properties of metal atoms through a localized charge transfer at the promoter metal interface. 2. Changes in morphology, electronic and adsorption properties of small metal clusters (<2nm diameter) in physical/chemical contact with the support 3. Inhibiti on by unreduced metal oxide or metal support solid solutions such as cobalt oxide or aluminate located near or at metal crystallite surfaces. 4. Introduction of alternate reaction path way s catalyzed by acid sites provided by the support Metal support intera ctions are important in the effectiveness of Fischer Tropsch catalysts particularly since they can also affect the reduction oxidation (redox) properties of the active metals Very strong metal support interactions in Co based Fischer Tropsch catalysts c an lead to inactive mixed metal oxides, a lower active metal activity, and d ecorations of metal crystallites by the support, which also lowers the activity. It has been stated that if the Co metal dispersion is less than 15% the n contamination by the supp ort can be avoided and a high extent of reduction (or
26 reducibility, ability to reduce CoO x ) can be maintained, about 70 80% (i.e. 70 80% of the original cobalt oxide on the catalyst surface can be reduced to active cobalt metal) [ 6 ]. Some authors have sugg est ed that the surface structure is important [2 5 8 ] H owever other authors have found that the initial surface structure is not so important [29 33 ] This is evidence that the catalyst surface, composit ion and structure, is dynamic [34 ]. Therefore, th e support is central to the productivity of the catalyst in the Fischer Tropsch reaction. Catalyst Supports A number of different supports have been investigated for FTS and much work has been done on determining the optimal support. Bartholomew and Farra uto ha ve presented some ideal support properties [6, 35 37 ]: moderately high surface area (100 150m 2 /g) low acid site concentration low react ivity towards Co to limit metal support interactions so there is no formation of inactive compounds high thermal stability during regenerative oxidation high strength and attrition resistance The most commonly, almost exclusively, used supports for Co catalysts are Al 2 O 3 SiO 2 and TiO 2 [ 6 21 22 37 39 ] Other supports which have been used in FTS studies include Z rO 2 [ 40 42 ] carbon based supports (active carbon or carbon nanotubes) [ 19 21 43 46 ], MgO [21 38 ] and CeO 2 [ 47 ]. Early results on the initial specific activity of supported cobalt catalysts indicate that the activity decreases in the following order Ti O 2 >SiO 2 Al 2 O 3 >C>MgO [ 21 22 38 ]. The interaction between cobalt and the support increases from SiO 2 to TiO 2 to Al 2 O 3 which is why SiO 2 is often used in Co based F T
27 catalysts [ 4 8 49 ] It has been suggested that small amounts of water can have a positive effe ct on SiO 2 supported catalysts, with minimal positive effects on TiO 2 supported catalysts, and negative effects, such as deactivation, on Al 2 O 3 supported catalysts [5 0 53 ] In a study of ruthenium promoted catalysts, the SiO 2 supported Co produced a highe r conversion, but lower C 5+ selectivity than the TiO 2 supported catalyst [ 54 ]. Comparing zirconia, titania, and ceria, the activity increased and the chain growth probability decreased in the following order: ZrO 2
28 also low Co dispersion [6 ] Therefore, alum ina supports are often modified, with for example La 2 O 3 [6 ], MgO [ 76 ] or ZrO 2 [ 73, 79 81 ] t o reduce the acidity and increase the stability of the support, and avoid inactive CoAl 2 O 4 f o r mation [6 5 0, 72, 7 3, 76, 80 ] However, some additives have a negative effect, such as Nb 2 O 5 [ 82] and Mo [83 ], by blocking metal atoms and a bimetallic effect decreasing the adsorption capacity and increasing branching [ 83 82 ]. Aluminum p hosphate is an other aluminum based suppo r t which can have a higher activity than Al 2 O 3 [ 84 ]. Porosity is also of importance when us ing Al 2 O 3 supports as higher porosity appears to lead to increased activity [ 66 74, 85 86 ]. The addition of nobl e metal promoters (Pt, Re, Ru) does not limit the deactivation of the catalyst with faster deactivation observed in some cases [ 68, 70, 87 92 ]. The reduction properties along with th e amount of bridged CO species are positively affected by the addition of rhenium [ 68, 71, 93 ] In an attempt to limit Co support compound format ion CO can be added to the reduction gas, which can lead to higher reducibility, dispersion, and activity [ 60 ]. The use of carbonyl derived cobalt can drastically increase reducibility on Al 2 O 3 supporte d catalysts [ 77 ]. Mesoporous aluminas are used in attempt s to use supports with different pore sizes (MCM 41, MCM 22) [94 9 6 ]. Comparing the MCM 41 to an amorphous SiO 2 catalyst, it has significantly stronger metal support interactions [ 94 ]. SiO 2 s upport ed c atalysts. Silica is more easily modified than alumina, to have certain surface properties ; however it has low hydrothermal stability [ 6 97 102 ] Silica supports can break d own in the presence of steam to form c obalt silicates which are inactive in the FTS even at moderate temperatures [ 6 99 103 ] Interestingly, in some cases small amounts of water ha ve shown a positive effect on SiO 2 supported
29 catalysts by increasing the amount of active cobalt available [50, 101 ] Despite its low hydrothermal stability, p orous SiO 2 is a very common ly used support when investigating Fischer Tropsch catalysts [ 9, 79, 101, 103 1 22 ]. Similar to alumina supports, silica supports are also often modified with stabilizers such as ZrO 2 [ 79, 105 10 7], K [123 ] TiO 2 [1 24 ] CeO 2 [ 9,104, 110 125, 12 6 ] or by organic solvents [ 127, 12 8 ] to incr ease the hydrothermal stability. Also, it has been shown that calcination of the support prior to Co deposition can be beneficial [1 4 128 ]. It was shown that, when using the prec ipitation method, a basic medium led to a mesoporous support while a microporous support fo r m s in a n acidic medium [129 ]. Recently the use of mesoporous silica supports, i.e. SBA 15 [ 13 18 95, 130 139 44 ] have been used in the preparation of Co catalys ts to determine the effects of support pore sizes on the catalysts [ 16 ]. The wider pore silica supports lead to larger, more reducible, Co particles, which are more active and selective to the diesel fraction compared with other SiO 2 supports [ 13 130 2 ]. Interestingly another study shows that the optimum pore size is 6 10nm for high conversion and high C 5+ s electivity [ 133 ]. Co/SiO 2 catalysts prepared by a sol gel technique have resulted in catalysts where the activity increases with incr easing specifi c surface area of the support [ 131, 132 ] An eggshell catalyst, avoids internal diffusion limitations, using SiO 2 has been reported as having high dispersions [ 115, 134, 13 5 ]. Hybrid Co/SiO 2 zeolite catalysts have also been prepared to obtain higher ga sol ine selectivities (20 50%) [ 136, 13 7 ]. The zeolite may crack the C 13+ long chain n paraffins to produce more gasoline range and cause isomerizations to branched products. However there is deactivation and a decrease in branched products due to
30 the cok e formed on the zeolites [ 136, 13 7 ] A mixture of metal/zeolite and Co/SiO 2 catalysts has been shown to lead to the production of iso paraffins [ 138 ]. TiO 2 s upported c atalysts. Titania does not react with c obalt below 650C but titania has very low surf ace areas compared to silica and alumina [6 ] Also at moderately high calcination temperatures (400 500C) surface TiO 2 reduce to a mobile TiO x species which can migrate over the Co and block active sites [ 6 ] However, titania supported catalysts have been used in Fischer Trospch with some success [ 47 145 14 7 ]. Stabilizers such as transition metal [ 147 1 53 ] noble metals [ 33, 1 45, 154 ], rare earth oxides or even boron [ 154 15 7 ] can be added to stabilize the support by forming bulk and /or surface com pounds which are resistant to sintering. Titania appears to have the highest hydrothermal stability, compared to alumina and silica [ 50 ] ZrO 2 supported c atalysts. In a study of mixed nanoscale alumina and zirconia supports the alumina supported catal yst has the highest reducibility activity, and H 2 chemisorption [ 158 ] The use of ZrO 2 lower ed the activity compared with mesoporous silicates modified with ZrO 2 [ 141 ]. The optimal pore size for mesoporous zirconia supports is 9.1nm to obtain high cata lyst performance and high diesel fraction, even better than an amorphous SiO 2 supported catalyst [ 40, 159 ]. Zirc onia supports also performed better than Al 2 O 3 supported catalysts in terms of reducibility and activity [ 41 ]. Finally a nickel promoted zi rconia supported catalysts performed on par with traditional cobalt based catalysts [ 42 ]. Carbon supported c atalysts. Different carbon supports, such as active carbon and various carbon nanostructures, have also been investigated for Co based F T catalyst s. Carbon supports are of interest since the Co support interactions are minimized.
31 Carbon nanotubes, compared to a n Al 2 O 3 support, showed the ability to decrease cobalt crystallite size (lower metal support interactions), increase reducibility (also low er reduction temperature), increase dispersion, and increase hydrocarbon yield [ 19 20 ]. However there is a decrease in selectivity in the higher carbon number products [ 19 20 ]. Cobalt catalysts supported on activated carbon [ 43 ] and carbon nanofibers [ 4 6 160 ] have also been used as a support in Fischer Tropsch synthesis Acidic treatments of carbon nanotubes can improve cobalt surface area, reducibility, and activity, but shifts the product distribution to lower molecular weight products [ 44 ]. The act ivity of c arbon nanotube supported catalysts d ecline over time, but pretreatment can reduce this effect [ 44 ]. Other s upports. Synthetic stevensites of Mg and Zn were introduced as supports to try to maintain thermal stability. The results showed signific ant metal support interactions with Co which led to disappointing activity [ 161 ]. A La(Co, Fe)O 3 perovskite support has been synthesized because of its versatility, and ability to allow cationic vacancies. The support was successfully used a catalyst fo r Fischer Tropsch synthesis [ 162 ]. Ceria has been used as a support to try to control metal support interactions and the relative adsorptions of CO and H 2 [163 ] CeO 2 also shows high chain growth probability, but with low activity [ 47 ] A mixture of Al 2 O 3 and SiO 2 as a support led to the formation of a new surface species of cobalt found using SEM and XPS [16 4 ]. A bimodal pore catalyst using alumina and silica can bring an increase in activity and selectivity [165 ]. A mixed TiO 2 SiO 2 supported catalys t led to larger chain hydrocarbons due to the presence of TiO 2 which also limits the activity [ 166 ]. In an investigation of SiO 2 Al 2 O 3 montmorillonite,
32 and zeolites (USY, ZSM 5, and MCM 22), the MCM 22 supported catalyst was optimal in terms CO conversi on [ 167 ]. The use of a Mo 2 C support has shown some positive results, although there is a higher production of alcohols [ 168 ]. Another support that has been used in Fischer Tropsch synthesis is Nb 2 O 5 to try to control the metal support interactions [ 169 ]. The Nb 2 O 5 creat es new active sites involving the metal and partially reduced support formed during high temperature reduction, but this interaction is destroyed over time due to the presence of water [ 169 ] The use of tungsten carbide can be useful beca use of its good mechanical properties, high density, and high thermal conductivity. However, these supports lead to the production of light alkanes and alcohols. C lean tungsten carbide showed better results than tungsten carbide that is protected by free carbon [ 170 ]. Promoters and Additives A number of additives have been investigated for cobalt based Fischer Tropsch catalysts. In general, the additives can be divided into two categories, 1) oxide promoters which are added to stabilize or alter the pro perties of the support and/or promote Co and 2) metal promoters, which are added to increase reducibility of CoO x and induce favorable interactions with Co. However, the additives can alter many catalytic properties and the effects on Co based F T catalys ts from the additives depend on the specific oxide and metal promoter used. The following metals and metal oxides have been used favorably as promoters in Co based catalysts; alkali metal oxides early and late transition metals, noble metals, as well as l anthanid e and actinide series oxides These are used because they may control the surface H/CO ratio through el ectronic interaction with the metal, increase and facilitate CO dissociation at the promoter metal interface, and/or lower support acid ity (pre vents side reactions) [ 6 ].
33 Alkali m etals. The addition of alkali metal oxides can increase activity by donating electrons to the metal and enhancing CO dissociation and metal carbide formation and lowering H/C surface concentration [ 22, 49 17 1] Typica lly alkali metals are used with Fe based c atalysts and were considered po i s ons for Co based catalysts [ 6 37 ]. While alkali promoters in some cases can lead to a lower activity [ 37 ] it appears that the alkene production is increased [ 172 ], in particular 1 alkenes [ 63 ], and the yield of heavier hydrocarbons is increased [ 63 ]. Potassium has been shown to influence the product selectivity by inhibiting methanation and increasing the production of unsaturated hydrocarbons [ 49 173 ]. Magnesia can limit the interaction between Co and alumina and increase activity when used in small amounts [ 76 ]. Early transition metal o xides. The early transition metal oxides can facilitate dissociation of CO increasing activity and C 5+ s electivity (ZrO 2 ZnO, Cr 2 O 3 ) [ 49 62, 79, 81, 105 7 147, 150 152 174 ] can resist ca rbiding and increase olefin/par a f fin ratio (MnO 2 TiO 2 ) [ 15 46, 124, 148 14 9 151 153 174 ] This is due to the fact that CO dissociation is facilitated at the promoter metal interfaces (adlineati on sites) and the formation of inactive mixed oxides (between promoter and support) can resist carbon deposition Furthermore, these promoters can control the surface H:CO ratio by decoration of and electronic interaction with the Co metal surface which lead s to the hydrogen poor olefinic products Other benefits of ZrO 2 include increased Co dispersions on Al 2 O 3 and SiO 2 [ 49 62 73, 80 ] and increased chain growth probability (higher selectivity to higher hydrocarbons) [ 73, 106, 158 ] Molybdenum can d ecrease the metal surface area (covers Co sites), cause faster deactivation due to the interaction between Co Mo (carbon deposition), and decrease the chain growth probability [ 83,
34 17 5]. Adding MnO 2 can increase dispersion, but negatively affect s the redu cibility by interacting with the Co, with mixed minimal effects on activity [ 15 46, 148,14 9, 151 ]. TiO 2 as a promoter, can lead to increased dispersion and metal surface area on a SiO 2 supported catalyst [ 124 ]. Even though zinc is not an early transitio n metal it behaves like the other metals in the early transition metal category, used as oxides. The addition of zinc to a titania supported catalyst appears to form a layer of zinc oxide in between the titania and the cobalt, increasing dispersion, howev er the uncovered zinc particles were active and produced mainly methane [ 147, 150 ]. Rare earth o xides. Lanthanides and Actinides (La 2 O 3 CeO 2 Gd 2 O 3 ThO 2 ) can increase site density, block H 2 ads orption (more sites available for CO adsorption) increase reducibility, decrease methane selectivity, and/or increase branching [ 49 176 ] These additives can also decorate the surfaces of metal crystallites in Group VIII metal catalysts (such as Co) facilitate the dissociation of carbon monoxide and lower th e acidity of the catalyst support limiting side reactions [ 176 ] The addition of ceria can lead to the formation of mixed oxides lowering reducibility, and shift the hydrocarbon selectivities to lower carbon numbers by possibly opening new pathways to t he formation of methane and other low carbon number products [ 9, 104 ]. A separate investigation concluded that CeO 2 work s well as a promoter increasing Co dispersion, decreas ing the electron density of the cobalt particle, and increasing conversion and he avy hydrocarbon selectivity [ 110 125 ]. The deactivation of the CeO 2 promoted catalyst appears to occur through silicate formation by water [ 126 ]. Lanthanum addition (0.2 wt%, optimum loading) increases reducibility and activity, and decreases methane
35 se lectivity on both an alumina support and an activated carbon support [ 43 177 ] but d oes not change the active sites, only increas ing the active site concentration and the c hain growth probability [ 119 ]. The addition of ThO 2 appeared to work as a promoter by reducing particle size, carbon deposit formation, methane selectivity, and increasing activ i ty when added by ethanol impregnation but not w ith aqueous impregnation [ 120 ]. Gadolinium had a minimal effect on an alumina supported catalyst, with small in creases in turnover frequency and reducibility [ 178 ]. Late t ransition and n oble metals Late transition and noble metals (Ru, Re, Pt Ir ) can increase reducibility (H 2 dissociation and spillover mechanism) [ 33, 48 117 176, 179 1 86 ] dispersion, prevent inactive carbon buildup, increase C 5+ s electivity/decrease methane, and enable in situ rejuvenation [6 33 182 18 4 ] More specifically noble metal promoters result in chemical, electronic, and/or structural modifications which not only increase dispers ion (higher active site density) they can also enhance the site activity [ 6 33, 54 ] It has been shown that the use of noble metals significantly lowers the temperature of CoO x reduction [ 48 70, 88 117, 187 ] Overall the additions of noble metals hav e a minimal affect on site activity in some cases (activation energy or kinetics) but they significantly affect the mass based activity, reduction and dispersion [ 6 49 54, 88 117, 188 ] Platinum and p alladium increase the reducibility and reaction rat e, while decreasing the C 5+ selectivity (increases methane selectivity) [ 49 185, 18 6, 189 1 91 ] Ruthenium promotes the catalyst with increasing CoO x reducibility, activity, and C 5+ selectivity, possibly due to a synergist ic effect of Ru and Co [ 49 65, 6 9, 109, 128, 139 187 ]. Ruthenium promotion appears more effective on silica supported catalysts than alumina supported catalysts with the formation of RuCo bimetallic
36 nanoparticles [ 61, 70, 10 9 18 4 187 ]. Rhenium addition can slightly increase activity and chain growth probablility, mainly increasing the reducibility of strongly interacting sites [ 15, 39, 49 53, 93, 108, 10 9 154, 186, 191, 19 2 ]. Scanning transmission electron microscopy shows that the Pt, Re, and Ir promoters are seen mainly as isolate d atoms, but Ru is present in the highly Co concentrated regions [ 188 ]. The use of noble metals on an Al 2 O 3 support has shown to have negative effect on the deactivation of the catalyst [ 39, 68, 87 89 92 ]. Catalyst Preparation and Pretreatment The pre paration procedure is vital for Fischer Tropsch catalysts as it can affect the uniformity of the active metal(s) dispersion the interactions of the promoters with the active phase, the interaction of the active metal with the support and/or the dispersio n of the active metal all of which can influence the catalytic activity and selectivity The most commonly used preparation method is incipient wet ness impregnation methods, using either aqueous or non aqueous solutions [ 6 ] However, other typical catal yst preparation methods have been used, such as regular wet impregnation [ 15, 136 13 7 ] and precipitation [ 104 129 ]. Gel techniques have also been used to make supported Fischer Tropsch catalysts [ 131 13 2 152 193 ]. Careful control over the precipitat ion, such as pH control during precipitation of Co amine by ammonia evaporation and matching of colloids is another technique that has been used [ 83 160 ]. A gl ow discharge plasma treatment for the deposition of Co has been shown to increase Co dispersion with minimal cobalt silicate formation and a small negative e ffect on reducibility [ 194 ]. Catalysts prepared by coating stainless steel with VO x /TiO 2 and Co/SiO 2 have been used but more research needs to be done to de termine their
37 viability [ 38 ]. An in teresting new technique is the use of ultrasound to prepare a SiO 2 supported cobalt catalyst from a cobalt nitrate precursor [ 195 ]. Catalyst p reparation. The choices of support, precursor/solvent pairing, drying, calcinations, and reduction conditions a re important to control the metal support interactions, and thus also the catalyst properties, and avoid inactive compound formation. Calcination of t he support may be necessary prior to impregnation and the cobalt p recursor and solvent for impregnation must be selected carefully The best precursor and solvent to use will be dependent on the choice of support and the catalytic properties are also dependent on the drying and calcination of the resulting catalyst. For example, a cobalt citrate precursor can produce better dispersion than cobalt nitrate when impregnated on a SiO 2 support, while c obalt nitrate resulted in catalysts with higher activities than those obtained from acetate or acetylacetonate precursor [ 15, 74, 84 ]. If the catalyst is dried in air at 110C followed by calcination at 400C highly reducibile crystallies of Co 3 O 4 are formed, while vacuum drying can form stable surface silicates [6 118 ] Cobalt silicate formation can be limited by using a support with a relatively low surface are a (<300m 2 /g), using a precursor favoring a pH below 5 (such as nitrates), solvents with high polarity (water), drying at high temperatures (120 150C) over several hours (10 24) and calcining at lower temperatures (200 300C) [6 35 ] On TiO 2 supported c atalysts cobalt oxalate, acetate and acetyl acetonate increased activity relative to catalysts prepared using cobalt nitrate due to an increase in cobalt dispersion [ 47 ]. However cobalt nitrate and oxalate showed higher chain growth probabilities than o ther precursors On Al 2 O 3 supported catalysts a cobalt carboxylate precursor improved performance when the carboxyl chain
38 length was increased However a cobalt acetate precursor allowed too strong metal support interactions at a 10% loading, but behaved better than the carboxylate when us ing 20% and 30% loadings [ 196 ]. The use of tetracobalt dodecacarbonyl as a precursor increase d reducibility and resulted in a small increase in specific activity [ 77 ]. Cobalt n itrate appears to favor Co 3 O 4 formation ove r cobalt silicate formation especially compared to cobalt a cetate  C obalt nitrate oxidative decomposition is end othermic versus exothermic for cobalt a cetate. The exothermicity of salt decomposition in air and the temperature of the oxidative pretre atment affect the repartition of the phases of cobalt between crystalline Co 3 O 4 and cobalt silicate [ 111, 121, 197 ]. The low temperature decomposition of a cobalt nitrate precursor shows greater dispersion and low reducibility. An acetic acid treatment o f the catalyst produced from cobalt nitrate can be beneficial while cobalt acetate showed poor results [ 114 ]. It has also been shown that the best results for high molecular weight hydrocarbons come from precursors that avoid chloride salts [ 69 ]. Another technique to avoid spinel (mixed cobalt support oxides) formation may be the use of non aqueous solvents for impregnation or contin u ous deposition. The advantages of non aqueous solvents are rapid evaporation of the solvent from the support, a more unifor m deposition, and efficient wetting of hydrophobic supports. The initial catalysts produce results with high activity and selectivity [ 6 176 185 198, 199 ] In attempt s to decrease the amount of silicate species formed the use of chelating agents [ 122, 20 0, 20 1 ] sucrose [ 108 ], and the use of other solvents to prepare the cobalt nitrates have shown some promise [ 120, 202 ]. In summar y, the cobalt nitrate precursor has proven to be the best precursor available [ 195 ].
39 Catalyst p retreatment. The purpose of the calcination procedure (heating in air at elevated temperatures) is to decompose the precursor and distribute the metal oxide onto the supp ort. The metal oxide crystallite formation is accelerated by the production of water vapor from the decompositio n of the precursor. Therefore, h igh decomposition rates of the precursor can lead to sintering due to high water vapor pressure. Low heating rate s and high air space veloci ties are desired as this may lead to slightly better dispersions (small oxide cryst allites) and higher activities. In non promoted cobalt catalysts, increasing the ca lcination temperature can negatively affect the reducibility due to compound formation [ 189 ] The reduction procedure is an essential and critical step in creating an ac tive catalyst. Also, during the reduction process it is important to limit the amount of water vapor present at the catalyst surface. This was discovered d uring World War II by G erman scientists [ 6 8 ] This can be accomplished by using a high H 2 space velocity during catalyst reduction The reduction of Co 3 O 4 to Co metal is a two step reduction proces s, Co 3 O 4 is reduced to CoO and then to Co metal [ 48 203, 20 4 ] and since the ease of Co 3 O 4 reduction often correlates with the catalytic activity, t emper ature programmed reduction (TPR) is commonly used to characterize the behavior of F T catalysts. The best reduction temperature to use is the one that exhibits a relatively slow reduction rate instead of the TPR peak maximum. Another option is to add som e CO to the reducing gas which may mitigate the water production [ 113 205 ] The addition of 3 5 vol. % CO to the reductant may have a positive effect by blocking strong interactions between the Co and the support, aluminate formation, and decreasing sin tering on an alumina supported catalyst increasing reducibility and activity despite
40 the presence of carbon deposition [ 60 ]. It has also been suggested that re ducing, oxidizing, calcining, and the n reducing again can lead to higher dispersion s [ 206, 20 7 ] Catalyst Deactivation Catalyst deactivation is an issue with the Fischer Tropsch process The five main suggested reasons for deactivation are 1) poisoning by sulfur or nitrogen, 2) fouling by waxes or carbon deposition 3) formation of inactive phases due to metal support compound formation 4) hydrothermal sintering and 5) attrition. In general for Co catalysts the main reasons for deactivation are: a reversible oxidation of active Co species, accumulation of hard waxes, organic acids or reversibl e poisons (N 2 ) and a n irreversible metal support compound formation along w ith the accumulation of surface carbon and more strongly adsorbed poisons (s ulfur) [ 6 ] Sulfur and nitrogen p oisoning. The deactivation from sulfur or nitrogen poisoning usually occurs due to the presence of either one in the syngas feed These compounds are present in normal syngas production streams such as those derived from coal, petroleu m or natural gas. H owever, the sulfur content is very low in synthesis gas derived from biomass feeds. Th is is a significant advantage since poisoning with sulfur is largely i rreversible so the catalyst can not be regenerated If sulfur is present it must be lowered to below 20ppb by using guard beds containing ZnO or a similar adsorbent. Generally t he morphological properties of the catalyst s are unaffected by the pres ence of sulfur in the catalyst, since it adsorbs on the active Co sites. In low quantities (<100ppm) sulfur lower s conversion without a ffecting selectivity but in larger q uantities sulfur decrease s reducibility and activity leads to CO 2 production and decreases the chain growth probability [ 156, 208 2 10 ]. Since biomass does contain some nitrogen, synthesis gas derived from biomass must be treated so that nitrogen contain ing
41 compounds, NH 3 HCN, etc, are below 10ppb This can be done by using catalytic hydrolysis of HCN to NH 3 a water wash and a guard bed containing an acidic solid adsorbent. Nitrogen poisoning is reversible so the catalyst can be regenerated with mil d in situ H 2 treatment. Fouling by heavy hydrocar bons. Heavy waxes with a carbon number of 40 or even higher can be produced during the Fischer Tropsch process [ 6 ] These waxes have low liquid diffusivities and are strongly physically adsorbed so they slowly accumulate in pores and on surfaces [ 6 ] This effectively blocks the adsorption of the reactan ts and desorption of products. A H 2 treatment at reaction temperature or slightly higher (350C) has been shown to efficiently remove these waxes and th e removal appears to be facilitated by the presence of noble metals [ 54 211 ] It is also possible that the intermediates produced during FTS can become surface carbons that block surface sites but most people conclude that carbon deposition is negligibl e wi th low temperature reactions [ 198 211, 21 2 ] The formation of filamentous carbon from large mobile cobalt particles has occurred in some cases [ 84 ] T he removal of the less active carbons, such as graphitic carbon, requires H 2 treatment at much high er temperatures but this is likely to cause sintering and a reduction in catalytic activity. Syntroleum has developed a regeneration procedure that includes a n oxidation step which is able to remove the inactive surface carbon [ 212, 21 3 ] Inactive c omp ound f ormation. For Co catalysts it is believed that inactive oxides and Co support spinel compounds are formed when water is present at high partial pressures due to high conversions. This behavior is a function of water partial pressure, temperature an d support. It has been shown that when the water par tial pressure is
42 above 5 6 atm, typically a conversion above 60%, reducible Co oxides and irreducible Co support compounds are formed [ 6 ] The water helps in the formation of thermodynamically stable Co 2 SiO 4 spinel from metallic Co and silicic acid [6 5 0, 67, 70, 87 89 91, 100, 10 1 116, 121, 126 ] however these effects are minimal in small amounts and high reactant flowrates [ 100, 101 121, 145 ]. These spinel formations are i rreversible. This behavi or can be restrained by using oxides such as ThO 2 ZrO 2 or La 2 O 3 In Co catalysts there has been no observation of the formation of bulk carbides except when H 2 flow was stopped to the reactor [ 6 ] Carbon dioxide impurities in the syngas stream on a Co/ Al 2 O 3 catalyst result in faster deactivation of the catalyst due to CO 2 behaving as a mild oxidizing agent along with water to p artially oxide the cobalt [ 214 ]. Methanol has a deactivating effect on the cobalt catalyst by increasing the water content t he methane content the wax production and the olefin selectivity [ 215 ]. Others suggest that oxidation is not a major factor in the deactivation of Fischer Tropsch catalysts [ 216 ]. The cause of deactivation of one particular Co/SiO 2 catalyst has been s hown to be the loss of active cobalt, and four carbon species were detected on the surface of the catalyst. One species is attributed to active cobalt, another to strongly interacting species, silicates, another to graphitic carbon on the surface, with th e final species unidentifiable [ 217 ]. Catalyst r egeneration. The regeneration of these catalysts is very important as these procedures will need to be done fairly often. They can be classified in two ways: mild rejuvenation and robust regeneration. Sin ce cobalt catalysts are more expensive than iron catalysts, their lifetimes must be long. Table s 1 1 and 1 2 list s ome of t he known regeneration processes that are necessary for Co catalysts There are many patented
43 regeneration techniques, including ear ly patents dealing with the regeneration of Co catalysts using H 2 rich str eams to remove accumulated heavy hydrocarbons. In fixed bed catalysts the hydrocarbon build up can be H 2 treated when a reactor is taken off line The catalysts with hydrocarbon ac cumulation could be run at higher temperatures but it becomes more of a regeneration rather than rejuvenation. Catalyst Properties There has been lots of recent work on trying to obtain structure activity relationships to help in future design of cataly sts. It is assumed that the active sites for Fischer Tropsch are the metal atoms on the surfaces of the crystallites. Structure a ctivity relationships are developed from t he effects of support properties, metal loading and dispersion. There is some dis cussion as to whether the effects are due to changes in metal crystallite size or if it is related to metal support interactions. In fact if surface contamination by support moieties and small metal cluster support interactions are avoided the specific activity is found to be independent of dispersion and support. The extent of reduction appears to show a trend with turnover frequency (TOF) when looking at catalysts that are reduced below 80%. Once it reaches 80% it appears to level off and becomes un affected by increases in reducibility. There are several possible theories for this but there are no known mechanisms. It is also noted by Iglesia and others that the optimal dispersions are between 8 12% and anything above that leads to rapid catalyst deactivation due to reoxidation of the small crystallites by water. It is noted that the use of chemisorption techniques to measure cobalt dispersion may not be completely accurate. This study also determines that the optimal particle size is approximat ely 6 nm [ 217 ]. The carbon monoxide conversion may not be very dependent on dispersion ; however selectivity is dependent on site density, pore radius
44 and pellet diameter. The site density is definitely dependent on metal loading and dispersion. Iglesia et al. have defined a parameter labeled chi that helps with describing the effect of several properties of each catalyst [ 134 ] p 2 p m /r pore (1 10 ) Where R p represents the radius of the catalyst pellet p is the void fraction in the cataly st bed m is the density of Co sites per unit area and r pore is the radius of the pores of the catalyst. The C 5+ 10 16 10 21 m 1 limited CO, decreasing c hain growth rates [ 6 37 ]. There ha ve been ma ny studies investigating the effects of different catalyst properties [ 6 ]. In general an increase in pore size increased the selectivity to higher carbon number products. The same trend in selectivity can be seen with decreasing pellet size. Finally increases in th e extent of reduction appear to lead to increases in h eavier hydrocarbon selectivity. There appears to be a threshold particle diameter catalyst impregnation depth of 0.2 mm above which pore dif fusional resistance causes a significant decrease in C 5+ selectivity. There appears to be a correlation between cobalt cluster size and the activity of a catalyst, lower cobalt cluster diameter increases the intrinsic activity, but these smaller particles are more sensitive to oxidation by water [ 52 ]. Reactors There are two types of Fischer Tropsch synthesis pathways a high temperature and a low temperature process The low temperature Fischer Tropsch (LTFT) process was investigated in this study The two types of reactors generally used in LTFT are tubular fixed bed reactors and slurry bed reactors. For tubular fixed bed reactors typically the syngas is passed downward through the catalyst bed and converted to
45 hydrocarbons. The heat that is evolved is removed through the walls and used to produce steam in the shell side. In the slurry bed the reactants are bubbled through the slurry ph ase which is made up of process derived wax with catalyst dispersed in it. The heat is passed to cooling coils whi ch produce steam. The light hydrocarbons are taken off the top of the reactor while the heavier products are incorporat ed in the slurry and separated by a proprietary process. Some issues with the tubular fixed bed reactor are the pressure drop at high t hroughp uts, it can be more expensive and has a lower capacity [ 2 ]. A dual bed reactor the first bed with potassium promoted cobalt catalyst followed by an unpromoted catalyst, can lower methane production and increase distillates [ 63 ]. The use of a new monolith loop catalytic membrane reactor showed positive results with a phosphorous and platinum promoted alumina supported catalyst [ 218 ]. An interesting reactor setup is a multichannel microstructured packed bed reactor which has shown the ability to r un highly active catalysts safely, however this setup displayed higher deactivation rates compared to a conventional reactor [ 219 22 0 ]. Nanoparticle Oxide Supports The use of nanoparticle oxides as supports for Fischer Tropsch catalysts is a relatively ne w topic The nano particles considered in this study h ave diameters of less than 100 nm and are interesting due to their high surface area per unit mass. Also they have a high density of edge and corner sites with low coordination and this can alter the interactions with the active metal [ 221 ]. While this can result in increased interactions between the active metal and the support compared with more traditional porous oxides, it is also possible that the limited sizes could affect the product distributi on. To avoid formation of inactive mixed cobalt oxides another stabilizing oxide can be added
46 to the nanoparticle oxides while maintaining a high surface area. Our main goal is to investigate the effects of the nanoparticle oxide support on cobalt and co mpare the activity and selectivity of the nanoparticle supported catalysts to cobalt catalysts on more conventional supports. If promising results are obtained, then the nanoparticle supported cobalt catalysts can be deposited on lower surface area suppo rts, such as monolithic substrates or foams to reduce the pressure drop that would be the result of very small catalyst particles However, this is beyond the scope of the current research and will be investigated in the future. The initial study focuses on characterizing cobalt catalysts supported on nanoparticle oxides and compare them to cobalt supported on more conventional high surface area supports to determine if there is an advantage to using nanoparticle oxide s as supports.
47 Figure 1 1. FTS re action s cheme 1 c arbene m echanism Figure 1 2. FTS r eaction s cheme 2 a lcohol c hain ( e nolic) m echanism
48 Figure 1 3. FTS r eaction s cheme 3 c arboxyl g roup (direct i nsertion) m echanism Figure 1 4. FTS reaction a s polymerization d iagra m d=desorption rate [ 8 ]
49 Figure 1 5. Suggested possible t ermination steps in the Fischer Tropsch process. Figure 1 6 ASF distribution plot of weight f raction vs chain growth probability f acto r
50 Table 1 1. Patented rejuvenation t echniques [ 6 ] Process Class Process Description Patent Ref. Mild Rejuvenation (MRJ) In situ in H 2 Treat catalyst in gas or liquid phase with H 2 or H 2 rich gas at temperature close to that of FTS process; vertical draft tube or gas disengaging zone interior to slurry reactor facilitates rejuvenation. BT334251,BT486928, U.S.2289731, U.S.526834 4, U.S.5283216,U.S.581770 2, U.S.6066679 Ex situ in H 2 Treat catalyst in slurry liquid phase with H 2 or H 2 rich gas at temperat ure close to that of FTS process; slurry is fed to external regeneration zones (stages) and treated with rejuvenation gas. U.S.5260239, U.S.582127 0, U.S.6201030 Ta ble 1 2. Patented regeneration t echniques [ 6 ] Process Class Process Description Patent Ref. Robust Regeneration (RRG) Steam stripping at reduced pressure Treat used catalyst with 93% steam/ 7% H 2 at 300C, 3.4atm, 4h; restores 95% act.; treatment in 100% H 2 restores only 10% act. U.S.6486220, U.S.2002/0198096A1 Sequential oxidation reduction R emove catalyst from reactor, strip off HCs/wax, treat at HT in air, reduce catalyst in H 2 return reactor. U.S.4399234, U.S.2002/0183403A1, GB2222531 LT org. acid, NH 3 or NH 4 salt & air oxidation, reduction Strip HCs/wax with H 2 at 290C, add organic acid (or NH 3 or NH 4 salt) solution to catalyst (e.g. acetic acid); oxidize solution with air at RT; dry at 100C, calcine at 300C, reduce in 10atm H 2 at 375C U.S.2003/0144129A1, U.S.2003/0166451A1
51 CHAPTER 2 EXPERIMENTAL SETUP Reactor System Design The ca talytic activity measurements were performed in a reactor system designed for Fischer Trop s ch synthesis of liquid hydrocarbons (Figure 2 1). The ult r a high purity reactant gases, carbon monoxide (CO) and hydrogen (H 2 ), and nitrogen (N 2 ) inert are fed to t he reactor through high stainless steel tubing (Swagelok) The reactor is a vertical stainless steel tube (406.4 mm long with an inner diameter of 11.81 mm) with a washer welded to the inside, approximately 1 78 mm from the bottom of the tube. The washer supports a metal mesh filter (10 microns), which is used together with quartz wool to keep the catalyst in place in the center of the reactor. The reactor tube is placed inside a horizontal Barnstead Type F21 00 Tube Furnace. A dual probe thermocouple is inserted from the top of the reactor and extends down to the catalyst bed. This thermocouple is used to both control the furnace and read the temperature inside the reactor. The liquid products are collected in a series of condensers, two hot traps and two cold traps. The lines from the outlet of the reactor to the first and second series of traps are heated with heating tapes to prevent condensation of liquids in the lines. As product separation between th e hot and cold traps was not possible, water and liquid hydrocarbons were collected in both traps, it was decided to only heat the first hot trap and try to cool the second hot trap to room temperature to collect as much liquid as possible in the hot trap condensers. The second series of traps, the cold traps, were mainly used to trap liquids which did not condense in the hot traps. The two top condensers both hot and cold traps, have the same design They are designed similar to a typical vacuum cold t rap [ 283 ] in that the
52 line enters at the top of an electropolished 150ml cylinder (Swagelok) and runs through to the bottom of the first cylinder (Figure 2 2 ). Gas phase components exit the trap at the top and are sent to the cold traps. The bottom cylin ders function mainly as collection cylinders and were designed so that they can be isolated, emptied, flushed and pressurized w ith inert gas while collecting product in the top traps. However, to avoid disrupting the reaction this was not normally done du ring reaction runs as it may hinder the ability to accurately take data for extended periods of time. The pressure in the reactor system is controlled using a Brooks back pressure controller. Several pressure gauges (P.G.) and thermocouples (T.C.) were added to the system, before the reactor, between the hot and cold traps, before the pressure controller, and before the gas chromatograph (GC), to monitor system pressure and temperature at all times. A reactor bypass line was also added to the system to allow the reactant gases to bypass the catalyst and the traps. While the bypass line can be used to determine the reactant gas composition before r eaction, the line can also be us ed to convert the system to a recirculating reactor by adding check valves a nd a recirculation pump. An expansion cylinder was added to the system after the pressure regulator to assure that ambient pressure was reached before the product gas flow meter (Brooks) and the on line GC. The completed system is shown in Figure 2 3 Th e on line GC for product analysis is a custom built Varian 450GC. It is equipped with three parallel channels. Two channels are equipped with molecular sieve columns ( Molecular ) to separate permane nt gases, such as H 2 CO, CO 2 N 2 and CH 4 These two channels are connected to thermal conductivity detectors (TCDs). One channel is used to
53 determine hydrogen concentrations and uses argon as the carrier gas. The other channel uses helium as the carri er gas and monitors permanent gases other than H 2 The third column ( CP Sil PONA CB ) is used to separate the hydrocarbon products and is connected to a flame ionization detector (FID). This column is also connected to a direct injection port that will al low identification and quantification of the liquid hydrocarbon products. A Labview program was written to automatically control the reactor system and the gas chromatograph. This program communicates electronically with the mass flow and pressure contr ollers and allows the operator to change the reaction pressure and reactant flow rates in the Labview program. The program also monitors the temperature and pressures before the reactor, before the cold trap, before the pressure controller, and before the GC. If the temperature or pressure in the system is outside specific set points, the Labview program will shut off the reactant flows to the system and continue with only inert gas flow until the system has recovered (i.e. temperatures and pressures are within the set point range). For safety, two detectors, a flammable gas and a hydrogen detector, were also added to the system and connected to the Labview program. If a leak is detected by either detector and th e alarm is triggered, then the reaction is shut down by interrupting the flow to the reactor. As a last resort, if the electronic safety system has failed, a pressure relief valve ( Generant with maximum pressure of 60bar) was added to the system that will vent to the hood if the pressure increase s above the maximum allowed pressure (set to 3 times the normal operating pressure).
54 Reaction Experiments Before reaction, the catalyst powders were pressed into pellets using a diameter pellet die under t w o tons of force exerted by a Carver press. The catalyst pellet was then crushed and sieved to obtain particles with s izes between 180 and 250 loaded into the reactor system together with 1 8 g of inert diluents, either catalyst support or glass beads. The catalyst bed is supported from t he bottom by the metal filter and quartz wool. The catalyst samples were activated by reduction in situ in pure H 2 (100 sccm total flow rate) at 400C for 4 hours. After the reduction is completed the flow was switched to pure N 2 (20 sccm), then the rea ctor temperature was reduced to 150C or below. The pressure in the reactor was then increased to 20 bar (or the desired reaction pressure) at this temperature using the reaction flow conditions ( H 2 :CO:N 2 = 6:3: 1 at a total flow rate of 125 sccm or 62.5 s ccm) Once the reaction pressure is reached, readings are taken from the GC until consistent flow rates of N 2 and CO are temperature is raised slowly (up to 200C at 1 2C/min, then up to reaction temperature at 0.5 1C/min) to avoid temperature spikes. During the start up, GC readings were taken every hour to monitor product formation. After 24 hours (or once the activity leveled off), GC readings were collected appro ximately every 3 hours. The activity or conversion is defined as the fraction of CO converted in the reactor (Equation 2 1). The selectivity to a specific product is defined as the fraction of total CO converted that went to the production of that spec ific product (Equation 2 2). The standard deviation in a typical measurement was measured to be 1.8%.
55 (2 1) (2 2) After a completed reaction, the catalysts were cooled to room temperature under a a sample vial and taken for XRD analysis. The system was tested by repeating an experiment from the literature, i.e. using the same reaction conditions and a catalyst th at was made in a similar way to one from the literature [ 18 ]. The results from the test reaction revealed that the system responded similar to 20 wt% Co/SiO 2 literature catalyst. Catalyst Synthesis All catalysts are prepared by the incipient wetness im pregnation technique. The catalyst support is dried overnight at 105 C or calcined at 400C for 3 hours prior to de position of active metal or promoter. The cobalt precursor, Co(NO 3 ) 2 6H 2 O, ( Alfa Aeasar), is di ssolved in an amount of de ionized water equ al to the volume of the pores of the support. This active metal solution is then added to the support drop wise under vigorous stirring until incipient wetness. After drying overnight at 105C, the catalysts are calcined at 350C for 3 hours to decompose the nitrate and form CoO x mainly Co 3 O 4 are added using the same preparation method either before or after the impregnation of cobalt. In most cases the catalysts are si mply dried at 105C overnight in betwee n impregnations and calcination s are only done after the final impregnation.
56 Catalyst Characterization Techniques As part of the research numerous techniques were used to characterize the catalysts in this work The se techniques can give valuable information on important catalyst properties. Some of the techniques are described in detail in various pieces of literature H owever I will describe in detail the specific setups that were used in this work. B runauer E mmet Teller (B ET ) surface area m easurement s The surface areas of the catalysts and supports were characterized by multipoint BET adsorption isotherms using a Quantachrome Nova 1200 instrument. This process is described in detail by Masel [ 222 ]. The sampl es were dried overnight at 105C in a furnace before analysis. The instrument use d nitrogen as an adsorbent and pulse d into an evacuated sample cell containing the sam ple immersed in liquid nitrogen [ 223 ]. A relationship between the volume of the gas pul sed into the cell and the cell pressure is represented in the isotherm at approximately 123K. Th e calculation that determines the volume necessary to create one monolayer of ni trogen molecules (shown in E quation 2 3 ) uses the slope of the isotherm and the zer o pressure intercept. The cross sectional area of a molecule of N 2 is known as 16.2 2 From that the surface area of the sample can be calculated. (2 3 ) Carbon Monoxide Pulse Titration The chemisorption technique, described in detail by Masel [ 222 ], is a common technique for characterizing surfaces. The carbon monoxide chemisorption technique is typical for measuring metal surface area and dispersions [ 47 ] It is important to note
57 that the stoichiometry of carbon monoxide chemisorption on the cobalt metal atoms must be known in order to calculate the amount of Co metal atoms that are on the surface. The catalysts are loaded at 100 200mg and are held at 105C in an inert for approximately 45 minutes prior to the e xperiments. The measurement was performed with the Quantachrome ChemBET 3000 [ 223 ]. The catalysts are then reduced in the 5% :95% H 2 :N 2 mixture. The temperature is raised from room temperature to 400C, the experimental reduction temperature, at a rate of 10C/min. The temperature was held at 400C until no further H 2 uptake was observed (usually within 2 hours). At this point the flow is switched to He (70 ml/min) and the catalyst is cooled down to room temperature for 1 hour The CO chemisorption is m easured by titrating with pulses of CO (270 l) u ntil no further CO is adsorbed. Each pulse is approximately 20 seconds long and takes place every 5 minutes. The amount of CO in the out stream is measured until there are four peaks that are of similar area. These peaks are used as the calibration of the area for no CO uptake. The Co surface area is calculated from the amount of CO adsorbed using a CO:Co stoichiometry of 1:1 and a Co surface density of 14.6/nm 2 [ 224 ]. Oxygen Titration Measurement The oxygen titration measurement is used to calcul ate cobalt oxide on the surface Th e measurement is done using a Quantachrome ChemBET 3000 [ 223 ]. This technique measures the amount O 2 adsorbed on a reduced catalyst and is used to calculate the amount of cobalt which can be re o xidized after reduction [ 225 ].
58 The catalyst (100 200mg) is first heated to 105 C under inert gas and held for about 45 minutes, then reduced (400C) using the Quantachrome ChemBET 3000. Once the reduction is finished for the sample the flow is switched t o helium for approximately 1 hour before oxidizing the surface with O 2 pulses at 400 C The O 2 pulses (1480L) were measured similar ly to the procedure mentioned for CO titration. The amount reduced was calculated based on the number of moles consumed an d assuming that Co 0 is oxidized to Co 3 O 4 [ 66, 214 225 ]. (2 4 ) Temperature Programmed Reduction The reduction profiles from each catalyst are collected using a Quantachrome C hem BET 3000 instrument [ 223 ] The catalysts are lo aded in a quartz U tube with a weight between 100 200mg and degassed The catalyst is then placed under the flow of 5:95 H 2 : N 2 mixture A water trap is placed at the outlet of the tube before the thermal conductivity detector to condense out any water s o it does not reach the detector. The temperature is then raised from room temperature to 8 00C or 10 0 0 C at a rate of 10C/min. The TCD continuously monitors the hydrogen content in the exit stream to determine the hydrogen uptake. In the CO chemisorp tion and oxygen pulse titration measurements the reduction at 400 C was measured. The area under these reduction curves are compared to the area under the curves for the full temperature programmed reduction (TPR) to calculate a value called reducibility. A reducibility typically represents the percent of the total representation of the amount cobalt that is available for adsorption of reactants.
59 X Ray Diffraction The XRD data was collected using a Philips powder X ray diffractometer with a Bragg Brentano geomet = 1.54 ). Diffraction patterns were obtained for selected fresh reduced and spent catalysts. The catalyst powders were secured onto a glass slide with double side d sticky tape. The average particle sizes were calculated from th e line broadening of th e XRD peaks using the Scherrer E quation (2 5 ) In this eq uation [ 66, 223 226 ]. Cobalt metal particle sizes can be calculated from the metal oxide particle siz es based on the relative molar volumes of metallic cobalt and Co 3 O 4 The conversion factor i s 0.75 and shown in E quation 2 4 [ 53, 55, 75, 104, 107, 194 227 ] (2 6 ) The cobalt metal dispersion (D %) can then be calculated assuming a site density of 14.6 atoms/nm 2 and spherical uniform particles. The dispersi on calculation is described by E quation 2 7 where d is the average metallic particle size in nm [ 51, 53, 55, 75, 104, 107, 109, 111, 190, 194 224, 228 ] (2 7 ) Transmission Electron Microscopy The Transmission Electron Microscopy, TEM, data was collected using a JEOL 2010F transmission electron microscope Along with the micrographs obtained, ED X
60 (energy dispersive x ray spectroscopy) was used to determine the composition of the species in certain areas. This allows for a qualitative look at how well disperse d the metal and/or promoter are on the support. The TEM data can allow for estimations of particle sizes and i dentification and estimation of composition and dispersion of particles.
61 Figure 2 1 Design drawing of Fischer Tropsch reactor system.
62 Figure 2 2 Diagram of trap setup for liquid pro duct collection.
63 Figure 2 3 Picture of the r eactor s ystem for the Fischer Tropsch Synthesis e xperiments. P hoto c ourtesy of Robert Colmyer.
64 CHAPTER 3 EFFECTS OF SUPPORT COBALT PRECURSOR, NANOPARTI CLE SUPPORT & ZIRCONIA PROMOTION Background The Fischer Trop sch synthesis (FTS) was discovered in the early 1900s and has been emergi ng as a commercial operation recently In the current economical and environmental situation the Fischer Tropsch process has come back to the foreground for the production of liquid fuels from renewable energy sources. The process uses synthesis gas, a mixture of H 2 and CO, to produce a wide range of products mainly straight chain paraffinic hydrocarbons Therefore since fossil fuels are not renewable, FTS may be a more economical ly stable and environmentally suitable supply of diesel fuel. Florida has the most biomass resources in the United States. Therefore this biomass can be used as a source for the production of diesel fuel [ 1 ]. The aim is to use a clean, inexpensive and r enewable energy source in the production of liquid fuels. Supported cobalt catalysts are used for Fischer Tropsch synthesis production of diesel fuel because cobalt has a high activity, low water gas shift activity, and is less expensive than ruthenium [ 229 2 30]. The support used in Co catalysts is important not only to provide a high specific surface area but also to control the interaction s between the support and the active metal. The choice of support can affect the properties reactivity an d sel ectivity of these catalysts [ 21 ]. The size of the pores, i.e. the pore diameter of the support appears to affect the degree of CoO x reduction ; increasing pore diameter increases the CoO x particle sizes which facilitate s the reduction [ 231 ]. The best FT activities and C 5+ selectivities were reported for 100nm pore diameter supports. The use of oxide promoters can affect the texture, porosity, the formation of inactive and difficult to
65 reduce mixed oxides, cobalt dispersion and reducibility, mechanical an d attrition resistance, and the chemical stability of the support [ 105 232 ]. ZrO 2 has been suggested and investigate d as an oxide promoter, however its effects are still disputed. The addition of zirconia has been reported to increase the activity of Si O 2 supported catalysts [ 79, 106 233 ], and facilitate the reduction of cobalt [ 106, 10 7 140 ]. Along with increases in activity, the ZrO 2 increases particle sizes and reducibility [ 106 140 ]. The different preparation procedures can influence the way zir conia affects the catalyst. When the zirconia is added first and calcined before cobalt addition the interaction between the cobalt and the support is decreased by replacing the Co SiO 2 with a Co ZrO 2 interaction lowering the reduction temperature of the catalyst [ 107 ]. S imilar results have been shown on alumina supported catalysts with an inhibition to aluminate formation. In contrast, some studies have reported that the addition of zirconia on alumina supported catalysts had no positive effect on re du cibility, dispersion or activity [ 234 ]. Finally it has been stated that the addition of zirconium facilitate s CO dissociation on SiO 2 supported cobalt catalysts [ 79 ]. In the current study, the effect of using nanoparticle silica (n SiO 2 ) as a support fo r cobalt is investigated and compared with cobalt on a conventional porous silica (p SiO 2 ) support in the Fischer Tropsch synthesis of liquid hydrocarbons from synthesis gas (CO and H 2 ). The influence of ZrO 2 additi on to the Co/n SiO 2 and Co/p SiO 2 catal ysts is also investigated together with the effects of cobalt precursor quality and heat treatment of the support. The catalysts were characterized using a number of analytical tools to determine properties of importance for a high catalytic activity.
66 E xperiment al Details A total of 16 catalysts were pre pared to investigat e the effects of support particle size, support pretreatment, cob alt precursor purity, and zirco n i a addition on the catalytic activities and selectivities. To determine the effects of SiO 2 ( Alfa Aesar [ 235 ] ) support was selected and compared with a nanoparticle SiO 2 support (Nano structured and Amorph ous Materials, Inc. [ 226 ] ) As hydroxyl groups on the SiO 2 support can react with Co and form inactive m ixed oxides (cobalt silicates Co 2 SiO 4 ), the effects of preheating the SiO 2 supports in air at 400C for 3 hours prior to the impregnation of cobalt was also investigated. Two different cobalt nitrate precursors, a 99.999% Co(NO 3 ) 2 6H 2 O (Alfa Aesar Pura tronic [ 237 ] ) and a 97.7% Co(NO 3 ) 2 6 H 2 O ( Alfa Aesar [ 238 ] ) were selected to determine the influence of Co precursor purity. This is important as the 97.7% cobalt nitrate precursor is significantly more cost effective than the 99.999% cobalt nitrate precu rsor. Finally the effects of zirconium oxide addition were investigated to determine if it can be used as a catalyst promoter. Table s 3 1 and 3 2 describe all 16 catalysts investigated in detail The prepared catalysts were subjected to catalytic activi ty measurements according to the procedure described in Chapter 2 (Reaction Experiments). All of the catalysts were also characterized using a number of analytical techniques to determine the influence of these parameters on the catalyst properties The analytical techniques used are described in detail in Chapter 2 (Catalyst Characterization Techniques). Results and Discussion Fischer Tropsch Synthesis Results Unpromoted c atalysts The support precalcination and the cobalt precursor quality only sligh tly affect the CO conversion for the Co/n SiO 2 catalysts (Figure 3 1 A ).
67 Precalcining the n SiO 2 support for the 97.7% cobalt precursor results in a slight increase in CO conversion and assures that the average CO conversion for this cobalt precursor is th e same as for the 99.9% cobalt precursor (Table 3 3 ). In contrast, both the cobalt precursor and the support precalcination appear to affect the selectivity to higher hydrocarbons (Figure 3 2A ). The 97.7% cobalt precursor gives a catalyst with a higher C 5 + selectivity compared to the 99.9% cobalt precursor. While the effects of precalcination are insignificant when using a 97.7% cobalt precursor, precalcining the support gives a slightly higher and more stable C 5 + selectivity when using a 99.9% cobalt pr ecursor (Table 3 3 ). The effects of support precalcination and cobalt precursor quality on the CO conversion for the Co/p SiO 2 catalysts appear to be even lower than for the Co/n SiO 2 catalysts (Figure 3 1B ). The highest average yield is obtained from t he Co/p SiO 2 catalysts prepared using an uncalcined support and the 97.7% precursor, but the difference in yield is close to the error bars of the measurements (Table 3 3 ). In this case, support precalcination results in a higher C 5 + selectivity when usin g a 97.7% cobalt precursor, while the opposite is true for the 99.9% cobalt precursor (Figure 3 2 B ). As for the Co/n SiO 2 catalysts, the highest C 5 + selectivity is obtained from the catalyst prepared using the 97.7% cobalt precursor. Compared to the Co/n SiO 2 catalysts, the CO conversions are slightly higher for the Co/p SiO 2 catalysts, while the C 5 + selectivities are slightly lower (Table 3 3) ZrO 2 p romoted c atalysts. Adding ZrO 2 to the Co/n SiO 2 catalyst increases the CO conversion in all cases (Tab le 3 3 ). While the 99.999% cobalt precursor appear s to result in slightly more active catalysts with a higher CO conversion, the catalysts
68 prepared using a 97.7% cobalt precursor appears to be more stable with time on stream (Figure 3 3 A ). Furthermore, t he more active catalysts prepared using the 99.999% cobalt precursor have lower and less stable C 5 + selectivities compared to the catalysts prepared using the 97.7% cobalt precursor (Figure 3 4A) Addition of ZrO 2 to the Co/p SiO 2 catalyst significantly increases both CO conversion and C 5 + selectivity for all catalysts indicating that ZrO 2 is a true promoter for these catalysts (Figures 3 3B and 3 4B ). While precalcining the support appears to result in slightly more active catalysts, the CO conversion a lso appears less stable compared with a catalysts prepared using an untreated support. The best catalyst of the ones under investigation is therefore the 99PrPoPC catalyst. Catalyst Characterization The catalysts were subjected to catalyst characterizat ions using a number of different techniques to measure surface areas, determine reduction oxidation (redox) properties, as well as morphological properties of the catalyst s BET Surface Area Measurements The BET surface areas for the catalysts under invest igation vary between 150 and 250 m 2 /g (Table 3 4) In most cases, the catalysts supported on n SiO 2 have higher surface areas than the catalysts supported on p SiO 2 as would be expected from the higher original surface area of the n SiO 2 support (490 m 2 / g) compared to the p SiO 2 (240 m 2 /g). Precalcining the support before Co deposition results in a slightly smaller BET surface area of the resulting catalysts. This effect is larger on the unpromoted catalysts compared to the ZrO 2 containing catalysts.
69 C arbon Monoxide Pulse Titration The amounts of CO adsorbed on the different catalysts after reduction in hydrogen at 400C and the corresponding metal surface areas are presented in Table 3 4 The amount of CO adsorbed per unit weight of catalyst is smalle r on the catalysts supported on n SiO 2 compared with the catalysts supported on p SiO 2 This appears to be due to larger Co particles on the n SiO 2 support compared with the p SiO 2 support rather than stronger Co support interactions (vide infra). With t he one exception, the 97PrNp catalyst pair precalcining the support results in a n increase in CO adsorption and thus also a larger metal surface area for both n SiO 2 and p SiO 2 supported catalysts. This may be an indication that the precalcination reduc es the cobalt support interactions, and thus result in a more reducible catalyst and a higher Co metal surface area. Addition of ZrO 2 increase s the cobalt metal surface area for all catalysts and th is increase appears to be higher for the n SiO 2 supported catalysts. It appears that the quality of the precursor causes some variation in BET and Co surface areas of the catalysts, but there is not a specific trend observed. The most evident effect is the increase in Co metal surface area from addition of ZrO 2 Temperature Programmed Reduction Analysis The temperature programmed reduction results for the catalysts under investigation are presented in Figures 3 5 and 3 6 All TPR data obtained from the n SiO 2 and p SiO 2 supported catalysts are similar in th at they exhibit a sharp low temperature peak around 400C and a broader high temperatu re peak between 500 and 700C. Literature data suggests that the t wo peaks represent the two step reduction of Co 3 O 4 i.e. Co 3 O 4 to CoO and then CoO to Co [ 48 79, 87, 1 04 109 2 03 20 4 ]. However, since large Co 3 O 4 particles are normally reduced at lower temperatures than small particles,
70 more than two reduction peaks can be present [ 79, 126, 194 239 ]. The main differences between the TPR spectra obtained from the cat alysts are the peak position of the low temperature peak and the peak width of the high temperature peak. The differences in the TPR spectra obtained from the n SiO 2 and p SiO 2 supported catalysts are not as significant as might be expected. The higher surface area n SiO 2 would be expected to result in stronger Co SiO 2 interactions, which in turn would lead to higher reduction temperatures. This is not observed in the TPR spectra. In fact, several catalysts supported on p SiO 2 exhibit a higher temperat ure where the reduction is complete. However, t he low temperature reduction peak is at slightly lower temperatures on the catalysts supported on p SiO 2 versus n SiO 2 On the precalcined n SiO 2 supported catalysts it appears that the first reduction step is less well defined compared to the catalysts supported on the non treated n SiO 2 support. This effect is less evident on the catalysts supported on p SiO 2 Addition of ZrO 2 to the catalysts shifts the low temperature peak to slightly lower temperatures for all catalysts. For all n SiO 2 supported catalysts ZrO 2 also reduces the temperature where the second reduction step is complete. This effect is less pronounced on the p SiO 2 supported catalysts. Comparing the areas under the TPR curves, which can be related to reducibility, some trends can be observed. The reducibilities of the n SiO 2 supported catalysts appear to increase with precalcination This effect is less noticeable on the p SiO 2 supported catalysts. In fact the precalcined n SiO 2 showed t he highest reducibilities, whether promotered or unpromoted. The addition of ZrO 2 appears to increase reducibility on n SiO 2 supported catalysts. For p SiO 2 supported catalysts, ZrO 2
71 increased reducibility when the support is not calcined, and resulted i n the opposite effect when the support is calcined. Oxygen Pulse Titration As the Co dispersions and the amounts of hydrogen absorbed on these catalysts are low, oxygen chemisorption was measured after reduction at 400C to obtain another measure of the degree of CoO x reducibility on the catalysts (Table 3 4) The unpromoted n SiO 2 supported catalysts have the highest oxygen absorption numbers The reducibility is decreased with ZrO 2 addition on the n SiO 2 supported catalysts. The effects of ZrO 2 on th e oxygen uptake are less significant on the p SiO 2 supported catalysts. Precalcination does not have a significant effect on O 2 uptake. Unpromoted n SiO 2 supported catalysts take up more O 2 than p SiO 2 supported catalysts. Precalcination on the p SiO 2 supported catalysts appear to reduce the difference in O 2 uptake. X Ray Diffraction As the catalysts prepared using different Co precursor quality and support precalcination did not reveal significant differences in activity, the differences between the n SiO 2 and p SiO 2 supports as well as the effects of ZrO 2 promotion were investigated in detail using the 99.999% Co precursor and a precalcined support (the 99XXYYPC series of catalysts). The fresh, reduced, and spent 99UnNpPC, 99UnPoPC, 99PrNpPC, and 9 9PrPoPC catalysts were subjected to XRD measurements. On the fresh catalysts (Figure 3 7 ), i t is evident that the main phase is the Co 3 O 4 oxide The peaks due to Co 3 O 4 are sharper and more intense on the n SiO 2 supported
72 catalysts compared with the cata lysts supported on p SiO 2 This indicates that the Co 3 O 4 particle sizes are larger on the n SiO 2 versus the p SiO 2 supported catalysts. Th e particle size analysis using the Scherrer E quation reveal that the average Co 3 O 4 particle sizes on the n SiO 2 sup ported catalysts are around 25nm, while those on the p SiO 2 support on the order of 12nm (Table 3 5 ). In both cases ZrO 2 addition leads to slightly smaller Co 3 O 4 particles. After reduc tion the most prevalent species is Cobalt metal, as expected (Figure 3 8 ) However, small shoulders due to CoO are present on all catalysts. The highest contribution from CoO is observed in the XRD pattern obtained from the promoted nanoparticle supported catalyst (99PrNpPC). The Co particle sizes from the XRD data obtain ed from the reduced catalysts agree well with those calculated based on the Co 3 O 4 particle sizes on the fresh catalysts (Table 3 5 ). The largest deviation is observed on the 99PrNpPC catalyst, as would be expected since it has the highest contribution fro m CoO. The XRD patterns obtained from catalyst after exposure to the reaction conditions are presented in (Figure 3 9 ) Except for the 99PrNpPC catalyst, no bulk oxidation or cobalt silicate formation is observed for the spent catalysts. The SiO 2 suppo rt appears to become more crystalline over the reaction time period. TEM Results The TEM images obtained from reduced 99UnNpPC, 99UnPoPc, 99PrNpPC, and 99PrPoPC catalysts are presented in Figure 3 10 through Figure 3 13 The lower resolution images (l eft) reveal fairly large agglomerates, on the order of 100nm, of smaller Co metal particles. The larger agglomerates appear more uniform, i.e. more well defined and symmetric, on the p SiO 2 support, while they are more spread and out
73 and asymmetric on the n SiO 2 support, irrespective of whether or not ZrO 2 is present. Furthermore, the individual Co particles appear larger on the n SiO 2 support compared with the p SiO 2 support, although the variation in particle size appears greater on the n SiO 2 support. The Co metal particle sizes from the XRD measurements are in reasonable agreement with the average particle sizes observed in the TEM images, i.e. on the order of 20 nm for the Co supported on the n SiO 2 and closer to 10 nm on the p SiO 2 support. Also th e individual Co particles appear slightly smaller on the ZrO 2 containing catalysts compared to the unpromoted ones. The differences in the structure of the support are evident in the high resolution scans (right) in Figures 3 7 through 3 9 Summary The qu ality 97.7% versus 99.999% purity, of the cobalt precursor and the pretreatment of the support appear to only have a n insignificant effect on the catalytic properties under the conditions used in this investigation. Using the higher quality precursor and pretreating the support for catalyst preparation may result in slightly higher CO adsorption capacity and higher conversion compared with catalysts prepared using the lower quality precursor and untreated supports. Obtaining close to the same activity fo r a much cheaper, lower purity, precursor is advantageous and the slight increase in conversion does not justify using the significantly more expensive 99.999% pure precursor. Comparing all catalysts, those supported on p SiO 2 perform better than those s upported on n SiO 2 in that the conversions are slightly higher. However, after addition of ZrO 2 the catalytic activities are higher and the difference in conversion between catalysts on the n SiO 2 and p SiO 2 supports is smaller than for the unpromoted cat alysts. This is consistent with the higher Co metal surface areas measured using CO
74 adsorption on the promoted versus unpromo t e d catalysts. For the Co/n SiO 2 catalyst the increase in conversion with ZrO 2 addition is associated with a small decrease in C 5 + selectivity. In contrast, over the Co/p SiO 2 catalyst, both the activity and the C 5+ selectivity are higher after ZrO 2 addition indicating that ZrO 2 is a true promoter for this catalyst.
75 Table 3 1. Description of the unpromoted catalysts Unpromoted C atalysts Catalyst a Composition Precursor Support Precalcination 99UnNpNC 20/80 Co/n SiO 2 99.999% No 97UnNpNC 20/80 Co/n SiO 2 97.7% No 99UnNpPC 20/80 Co/n SiO 2 99.999% Yes 97UnNpPC 20/80 Co/n SiO 2 97.7 % Yes 99UnPoNC 20/80 Co/p SiO 2 99.999% No 97UnPo NC 20/80 Co/p SiO 2 97.7 % No 99UnPoPC 20/80 Co/p SiO 2 99.999% Yes 97UnPoPC 20/80 Co/p SiO 2 97.7 % Yes a Catalyst description: NNAaBbCC: NN: 99 = 99.999% Co precursor, and 97 = 97.7% Co precursor. Aa: Un = unpromoted and Pr = promoted. Bb: Np = nanopart icle support and Po = porous support. CC: NC = support not calcined and PC = support precalcined. Table 3 2. Description of the promoted catalysts Promoted Catalysts Catalyst a Composition Precursor Support Precalcination 99PrNpNC 20/2/78 Co/ZrO 2 /n SiO 2 99.999% No 97PrNpNC 20/2/78 Co/ZrO 2 /n SiO 2 97.7 % No 99PrNpPC 20/2/78 Co/ZrO 2 /n SiO 2 99.999% Yes 97PrNpPC 20/2/78 Co/ZrO 2 /n SiO 2 97.7 % Yes 99PrPoNC 20/2/78 Co/ZrO 2 /p SiO 2 99.999% No 97PrPoNC 20/2/78 Co/ZrO 2 /p SiO 2 97.7 % No 99PrPoPC 20/2/78 Co/ZrO 2 /p SiO 2 99.999% Yes 97PrPoPC 20/2/78 Co/ZrO 2 /p SiO 2 97.7 % Yes a Catalyst description: NNAaBbCC: NN: 99 = 99.999% Co precursor, and 97 = 97.7% Co precursor. Aa: Un = unpromoted and Pr = promoted. Bb: Np = nanoparticle support and Po = porous support. CC: NC = support not calcined and PC = support precalcined.
76 Figure 3 1 Carbon monoxide conversion for un promoted cobalt based catalysts supported on SiO 2 at P=2 0bar, T = 2 30C, m cat =1g, F tot =125ml/min (H 2 :CO:N 2 =6:3:1) A ) n SiO 2 supported and B ) p SiO 2 supported. Error bars are the difference between repeated runs.
77 Figure 3 2. C 5+ s electivity for u n promoted c obalt based catalysts supported on SiO 2 at P=20bar, T=230C, m cat =1g, F tot =125ml/min (H 2 :CO:N 2 =6:3:1) A ) n SiO 2 supported and B ) p SiO 2 supported.
78 Figure 3 3 CO c onversion for ZrO 2 promoted c obalt based catalysts supported on SiO 2 at P=20bar, T=230C, m cat =1g, F tot =125ml/min (H 2 :CO:N 2 =6:3:1) A ) n SiO 2 supported and B ) p SiO 2 supported.
79 Figure 3 4 C 5+ s electivity for Zr O 2 promoted Cobal t based catalysts supported on SiO 2 at P=20bar, T=230C, m cat =1g, F tot =125ml/min(H 2 :CO:N 2 =6:3:1) A ) n SiO 2 supported and B ) p SiO 2 supported.
80 Table 3 3. Product Analysis of All Reactions Reaction Results Catalyst a Avg Conv. b CH 4 Sel. c C O 2 d C 2 C 4 e C 5+ f Productivity g 99UnNpNC 1 5 % 20% 1% 16% 63% 9. 5 97UnNpNC 14% 15% 0% 11% 74% 10 99UnNpPC 16% 17% 0% 13% 70% 11 97UnNpPC 17% 17% 0% 12% 71% 12 99UnPoNC 18% 14% 1% 13% 72% 13 97UnPoNC 21% 18% 2% 17% 64% 13 99UnPoPC 1 7 % 17% 1% 15% 67% 11 97UnPoPC 18% 13% 1% 13% 73% 13 99PrNpNC 2 3 % 17% 1% 15% 66% 15 97PrNpNC 22% 14% 0% 12% 74% 16 99PrNpPC 25% 20% 1% 19% 60% 15 97PrNpPC 23% 13% 0% 11% 76% 1 8 99PrPoNC 25% 12% 1% 12% 75% 1 9 97PrPoNC 22% 15% 1% 11% 73% 16 99PrPoPC 28 % 14 % 1% 1 5 % 70 % 20 97PrPoPC 2 4 % 11% 1% 9% 80% 19 a Catalysts as described in Table 3 1. b Avg Conv.: Average conversion. c CH 4 Sel.: Methane s electivity d CO 2 : CO 2 s electivity. e C 2 C 4 : Selectivity of hydrcarbons from carbon number 2 to 4 f C 5+ : Selectivity of hydr ocarbons wi th carbon number larger than 5. g Productivity : ( c onversion*C 5+ )* 100 standard deviation of 0.9
81 Table 3 4 Characterization d ata for a ll c atalysts Characterization Data Catalyst a BET Surface Area [m 2 /g] Vol. of CO Ads. [L/g] b Metal Surfac e Area [m2/g] c Reduc ibility d Full Red. Area e [Area/g] Vol. o f O 2 Ads. [ml/g ] f Reduc i Ibility g 99UnNpNC 26 6 413 0.74 78.8% 36.4 46.8 84.4 % 97UnNpNC 116 198 0.35 79.1% 36.4 43.6 78.5 % 99UnNpPC 235 481 0.86 85.8% 43.7 44.4 80.1 % 97UnNpPC 215 375 0. 67 85.7% 47.5 50.7 91.5 % 99UnPoNC 27 2 457 0.81 80.6% 34.8 42. 2 76.2 % 97UnPoNC 160 456 0.81 80.1% 37.7 44.5 8 0.2 % 99UnPoPC 155 777 1. 38 83.5% 33.3 41.6 75.0 % 97UnPoPC 155 600 1.07 65.2% 43.9 40.8 73.6 % 99PrNpNC 271 541 0.96 86.4% 28.7 45.3 81.7 % 97PrN pNC 230 723 1.29 87.0% 36.7 46.6 84.0 % 99PrNpPC 160 545 0.97 83.4% 38.8 37.0 66.8 % 97PrNpPC 215 616 1.1 94.4% 33.9 45.4 8 1.9 % 99PrPoNC 160 674 1.2 83.8% 29.4 37.1 66.9 % 97PrPoNC 155 570 1.01 83.0% 31.5 43.2 77.8 % 99PrPoPC 150 724 1.29 76.9% 30.9 41. 6 75.0 % 97PrPoPC 155 747 1.33 74.8% 37.3 43.0 77.6 % a Catalyst makeup as described in Table 3 1. b Volume of c arbon m onoxide adsorbed at 20 C after reduction at 400 C. c Metal Surface Area measured from CO p ulse t itration (assuming a 1:1 Co:CO ratio). d Amount of H 2 consumed by catalyst during reduction at 400C divided by total H 2 consumed by catalyst in TPR experiment up to 800C or 1000C. e Full Red. Area: (Area of TPR peak for full reduction per gram of catalyst)/100000. f Amount of O 2 to re oxidi ze Co after reduction at 400C (a measure of reducibility). g Reducibility measured from O 2 titration after reduction at 400C.
82 Figure 3 5. TPR for un promoted catalysts a ) n SiO 2 supported catalysts b ) p SiO 2 supported catalysts. Vertical line at 400 C represents experimental reduction temperature.
83 Figure 3 6. TPR for ZrO 2 promoted catalysts a ) n SiO 2 supported catalysts b ) p SiO 2 supported catalysts. Vertical line at 400 C represents experimental reduction temperature.
84 Figure 3 7 XRD patterns obtained from f resh catalysts prepared using a precalcined support and the 99.999% pure Co precursor a) larger 2 range b) high resolution pattern of most intense Co 3 O 4 peaks Figure 3 8 XRD patterns obtained from reduced catalysts prepared u sing a precalcined support and the 99.999% pure Co precursor a) larger 2 range b) high resolution pattern of most intense Co 3 O 4 peaks
85 Figure 3 9. XRD patterns obtained from spent catalysts (catalysts after exposure to reaction conditions) prepared usi ng a precalcined support and the 99.999% pure Co precursor a) larger 2 range b) high resolution pattern of most intense Co 3 O 4 peaks Table 3 5. Particle sizes calculated from the XRD data using the Scherrer Equation for fresh catalysts using a 99.999% Co precursor and a precalcined support Fresh Catalysts: Reduced Catalysts: Spent Catalysts: Catalyst Average Co3O4 [nm] a Average d(Co) b (nm) D c (%) Average d(Co) d (nm) D c (%) Average d(Co) d (nm) D c (%) 99UnNpPC 26.3 19.7 4.9 17.1 5.7 21.2 4.5 99UnPoPC 15.1 11.3 8.5 10.5 9.8 12.2 7.9 99PrNpPC 22.6 16.9 5.7 12.3 7.9 7.4 13.0 99PrPoPC 9.9 7.4 13.0 8.3 12.0 13.0 7.4 a Calculated particle size using Scherrer Equation from f ull scan main XRD peak. b Calculated cobalt metal particle size from Co 3 O 4 particle size d(Co)=0.75d(Co 3 O 4 ). c Dispersion calculated from d(Co) particle size D=96/d(Co) d Calculated particle size using Scherrer Equation from h igh r esolution scan main XRD peak.
86 Figure 3 10 TEM Images of 99UnNpPC. Figure 3 1 1 TEM Images of 99 UnP o PC Figure 3 1 2 TEM Images of 99 PrNp PC.
87 Figure 3 1 3 TEM Images of 99 PrPo PC.
88 CHAPTER 4 EFFECTS OF ADDITIVES Background The use of additives for Fischer Tropsch catalysts has been a well researched topic. Noble metal promoters are expected to: facilitate cobalt oxide reduction, increase cobalt dispersion, provide better resistance to deactivation, form alloys with cobalt provide a high concentration of hydrogen activation site s, and increase the intrinsi c activity of surface sites. Oxide p romoters are used to modify the catalyst texture and porosity, reduce the formation of inactive mixed cobalt oxides, increase cobalt dispersion, cobalt oxide reducibility, and fraction of different cobalt metal crystalline phases, enhance mechanical and ch emical attrition resistance of cobalt FT catalysts, and improve the chemical stability of the support [ 6 8 21 ] Alkali Metal Promoters Alkali metal promoters have been used mainly on iron based catalysts, with the thought that cobalt based catalysts are poisoned by alkali metals [ 6 37 ]. However studies reveal that addition of potassium increases the C 5+ selectivity du olefins leading to heavier products, while significantly increasing the selectivity to unsaturated hydrocarbons (1 alkenes) over cobalt catalysts [ 63 240 ]. Surface science studies on model catalysts have shown that metallic Mg can increase the ability to absorb CO on Co foil and dissociate CO [ 241 ]. The use of small amounts MgO can deter cobalt aluminate formation and can increase activity [ 76 242 ]. The addition of larger amounts of MgO, on Al 2 O 3 led to decreased activity with a n increase in methane and CO 2 production and a higher olefin to paraffin ratio due to poor reducibility [ 76 ]. Magnesia, on Al 2 O 3 and SiO 2 le d to lowered reducibility and higher dispersion, creating new types of active sites (edge sites), with the best re sults at
89 Mg:Co molar ratio of 1:2 [ 230 243 ]. Calcium oxide (0.6 wt%) has shown to increase reducibility by limiting Co Al interactions [ 244 ]. It appears the increase in reducibility comes from facilitating the reduction of the second step, i.e. CoO to C o [ 227 ]. The activity increased and the selectivity was shifted to the diesel fraction at loadings below 2 wt% [2 2 7, 244 ]. Early Transition Metals Transition metals have been used in many attempts to p romote cobalt based Fischer Tr opsch catalysts. Moly bdenum modification of Al 2 O 3 led to a decrease in metal surface area, but increased the strength of adsorption on the cobalt metal, and increased acidity in the reduced catalyst leading to lower chain growth probability and increased branching [ 83 123 ]. However, these catalysts deactivate faster than the unpromoted catalysts An EXAFS study on a Mo modified Al 2 O 3 supported catalyst showed that the molybdenum interacts with support. Also, regardless of order of impregnation the molybdenum segregated to t he surface [ 245 ]. The use of TiO 2 as a promoter, on a SiO 2 supported catalyst, led to increased dispersion and reducibility, as well as the activity in a slurry phase reactor [ 124 ]. Zirconia is a commonly used promoter in Fischer Tropsch, especially for SiO 2 supported catalysts. The zirconia is used to lower the interaction between the SiO 2 and the cobalt, replacing the Co Si interaction with a Zr Si interaction which is s omewhat weaker [ 73, 80, 106, 10 7 239 ]. In some cases this interaction enables re duction at lower temperatu res [ 106, 10 7 ], however, in other cases the reduction temperature is slightly higher [ 49 81, 234 ]. The cobalt is found in clusters on the surface, and the size of the clusters, i.e. the amount of cobalt particles in the cluster, decrease s with increasing Zr O 2 loading [ 106 ]. The cobalt crystallite size increases with increasing Zr O 2
90 loading lowering metal surface area [ 106 ] The positive effect on activity and selectivity increases with increasing loading of Zr O 2 [ 79, 81, 107 234 239 ]. A hydrogen spillover mechanism for sequentially impregnated catalysts led to higher activity, but the co impregnated catalysts led to higher dispersion [ 79 ]. It is also suggested that the Zr O 2 creates an active interface with Co that facilita tes CO dissociation which increases activity [ 7 9]. ZrO 2 is a true promoter since both the activity and the selectivity to heavy hydrocarbon s are increased [ 7 9]. Zirconia appears to have differen t effects on different supports, i.e. on a SBA 15 support t he reducibility, activity, and dispersion were not affected [ 73, 80 228 ]. In the smaller pore MCM 41 the zirconia limited the Co Si interactions forcing the Co 3 O 4 to the outer surface of the support, while increasing Zr O 2 loading led to larger cobalt par ticles, more easily reduced [ 228 ]. The activity reaches a maximum at 5% loading of zirconia on the MCM 41 support, although this is still lower than the activity for the larger pore SBA 15 supported catalysts [ 228 ]. Manganese is another promoter commo nly used in S iO 2 supported Fischer Tropsch catalyst s The catalyst reduction temperature and cobalt particle sizes are decreased from the addition of manganese [ 246 ]. Despite the apparent decrease in reduction temperature, manganese can lower reducibil it y and increase dispersion [ 18, 46, 149, 247 ]. The use of manganese created an interaction between Mn Si to limit the Co Si interaction [ 247 ]. The activity and low hydrocarbon selec tivity is increased while sligh tly lowering alkene selectivity [ 148 9 240 246 ]. Also the water gas shift reaction appears to be promoted by manganese [ 240 ]. Even though zinc is not an early transition metal it behaves like the other metals in the early transition metal category, used as oxides. Zinc can also be used to try to
91 stabilize the support by interacting with support. However the zinc can also be active and produce methane [ 150 ]. When adding zinc first, but calcining after the cobalt impregnation, it led to increased activity [ 150 ]. Rare Earth Oxide Promoters L anthanides can limit the Co Si interactions and possibly create some synergistic effects. Ceria addition appears to increase the dispersion, and lower the reducibility [ 104 ]. The activity is not affected by ceria, but the selectivity is, with a decrease in chain growth probability This is likely due to smaller particle sizes, specific adsorption/desorption properties of Co CeO 2 /SiO 2 and possibly the presence of formate species on partially reduced ceria leading to lower reducibility due to the stabilit y of the formate species [ 104 ]. In other studies ceria have been shown to increase turnover rate and C 5+ selectivity [ 110 125 ]. The cobalt dispersion increased and CO dissociation was facilitated while the electron density of the cobalt particles decr eased with the addition of CeO 2 [ 125 ]. Ceria may introduce a new active site to increase surface active carbon species during reaction [ 125 248 ]. However, the addition of ceria did not appear to limit the formation of cobalt silicate during the reaction [ 126 ]. Lanthanum is a typical support stabilizer for Fischer Trospch catalysts [ 43, 49 119 ]. The interaction of Co Si is limited by the addition of lanthanum, while the nature of active sites is not affected [ 119 ]. The activity is increased due to a h igher number of available active sites and some authors have reported an increase in reducibility [ 43 119 ]. However, in some cases a decrease in reducibility has been observed with La 2 O 3 addition [ 49 ]. Lanthanum can also lead to lower methane selectivit y and increased chain growth probability [ 43 ]. At higher lanthanum concentrations, 10% or above, the olefin to paraffin ratio is increased .
92 Similar to lanthanides, actinides are also used to promote Fischer Tropsch catalysts. Thoria appears to be a promot er when using ethanol instead of an aqueous impregnation The cobalt particle size is slightly smaller which increas e the CO disproportionat ion and the presence of new carbonyl species are observed on the surface of the catalyst Also the methane selectivity is decreased, increasing the heavier hydrocarbon selectivity This behavior may be due to an increase in step and kink sites due to the ThO 2 addition [ 120 ]. Late Transition and Noble Metal Promoters The most often used promoter for SiO 2 s upported catalysts is ruthenium, mainly because it is also a Fischer Tropsch catalyst [ 128 191 ]. Ruthenium significantly increases the reducibility of the catalysts by lowering the reduction temperature due to a hydrogen spillover mechanism [ 48, 4 9 80, 10 9, 117, 191, 249 2 51 252 ]. Using a lower calcination temperature of the catalyst increases the reducibility [ 197 249, 2 50, 253 ]. The activity is significantly increased with the addition of a very small amount of ruthenium, as evidenced in a higher t urnover frequency with no concomitant effect on CH 4 selectivity [ 117 191 ]. The number of metal sites and dispersion [ 69, 250 ] is also increased by ruthenium, leading to an even higher activity [ 108, 197, 253 ]. Other authors have observed that w hile ruth enium increases the active site density leading to an apparent higher turnover frequency it does not appear to affect the activation energy and kinetics [ 54 ]. The adsorption of linearly and bridge type CO is increased when increasing the ruthenium conte nt from 0.05% 0.5% leading to increased activity due to more adsorbed CO [ 80 ]. Promotion with ruthenium generally appears to increase the chain growth probability, however, at high loadings the behavior is reversed [ 49 6 1, 65 ]. Also, the effect of ruth enium
93 appears stronger on supports that have smaller pore sizes [ 49 6 1, 65 ]. The deactivation rate of cobalt based catalysts may be slowe r in the presence of ruthenium [ 54 ]. In contrast to Re, Pt, and Ir promoters, which are present as isolated atoms, R u is only present in high Co concentration particles aberration corrected scanning transmission electron microscopy [ 254 ]. Rhenium promotion also leads to lower reduction temperatures but t he effect of rhenium on reducibi lity is not as significant as ruthenium [ 18, 48 252 ]. It appears that the increase in reducibility only affects the second step of reduction and the Co cluster sizes are lowered [ 53, 93 ]. It is suggested that rhenium increases the concentration of activ e sites [ 18,108 ]. One study suggests that r henium is on the surface blocking or sterically hindering the ability of species transporting to Co active sites [ 71 ]. Platinum promotion appears to increase the methane selectivity, with a minimal increase in reducibility [ 48 88 117 191 ]. The cobalt time yield appears to increase, but the deactivation behavior is not changed with platinum promotion in a CSTR [ 90 ]. The turnover frequency is decreased with increased cobalt dispersion as it forms a well dispe rsed Pt Co alloy [ 117 191 ]. Palladium appears to behave similar to platinum, but is not as common of a promo ter as the other noble metals. There is no effect on reducibility, but the di spersion and methane selectivity are increased and the turnover freq uency is reduced due to the form ation of a Pd Co alloy [ 117 ]. Iridium can also lower reducibility [ 254 ]. Copper appears to increase reducibility but not the active site density [ 255 ]. The addition of metals like Copper work well in low concentrations, b ut becomes negative at higher concentrations by blocking active sites [ 255 ].
94 Overall there are a number of promoters that have been used in cobalt based Fischer Tropsch catalysts The effects on activity and selectivity from these promoters vary and can depend on the catalyst preparation methods and the support under investigation. In the following work several different promoters will be investigated to determine the viability as promoters for F T catalysts supported on n SiO2. Experimental Section C atalyst Preparation Several promoted catalysts were prepared by a sequential incipient wetness impregnation method. With the exception of the Ru promoter, which is added after Co deposition (only when using more than one promoter) the promoter is added f irst using the incipient wetness impregnation method described in Chapter 2. After drying over night, the Co is added to the MeO x /n SiO 2 using the same incipient wetness impregnation method. After calcination at 350C for 4 hours the catalysts are reduce d in situ before catalytic activity testing as described in Chapter 2. The Co loading in all cases is 20% by weight and the promoter loading is 2% by weight, except for ruthenium which is 0.63 % by weight. Catalytic Activity Measurements The catalysts wer e subjected to catalytic measurements as described in Chapter 2. The final two measurements were done using double promoted catalysts and different conditions. In the previous experiments a constant volume of inert material was used along with the cataly st, typically 1 gram of support. However, in this case t he catalyst was loaded with glass beads to a constant volume of 8ml in each case typically 9 grams of glass beads plus a constant 1 gram of catalyst. T he total flow rate was
95 lowered from 125ml/min to 62.5 ml/min for these two catalysts. All other conditions were kept identical for every measurement. Results and Discussion The prepared promoted catalysts were subjected to catalytic activity measurements to determine their behavior in the Fischer Tro psch reaction. Fischer Tropsch Synthesis Metal Oxide Additions The effects of CaO, MgO, La 2 O 3 and Sm 2 O 3 addition to a Co/n SiO 2 catalyst are presented in Figure s 4 1 A and 4 2 A The average conversions and selectivities are summarized in Table 4 1. Ad dition of Ca O resulted in a catalyst with very little activity and also one of the low est C 5+ selectivities observed for the catalysts under investigation While addition of MgO and La 2 O 3 also decrease s both the activity and selectivity they are not as l ow as for the Co/ CaO /n SiO 2 catalyst Also the Co/ MgO /n SiO 2 catalyst exhibits a higher conversion than the unpromoted catalysts, with similar C 5+ selectivity during the early hours of reaction but the catalyst deactivates quickly after 30 hours on stre am. Addition of Sm 2 O 3 does not appear to have a significant effect on the CO Conversion, but the C 5+ selectivity is lower than the unpromoted catalyst. Therefore, none of these oxides promotes the reaction under these conditions. The effects on the ca talytic activity and selectivity of Al 2 O 3 MnO 2 Ce O 2 and ZrO 2 addition to cobalt supported on n SiO 2 catalyst are shown in Figures 4 1 B and 4 2 B While Al 2 O 3 addition appears to increase the CO conversion the Co/Al 2 O 3 /n SiO 2 catalyst is less stable com pared to the unpromoted Co/n SiO 2 catalyst. The CO conversion data varies more than most catalysts and the C 5+ selectivit y decrease s with time on stream. The Co/MnO 2 /n SiO 2 catalyst has a significantly higher CO conversion
96 than the Co/n SiO 2 catalyst dur ing the 72 hours of reaction. However, since the deactivation rate is higher, the activity of the MnO 2 promoted catalyst is close to that of the unpromoted catalyst at the end of the reaction period (roughly 90 hours). During the first 24 hours of reacti on, the C 5+ selectivities are similar for the Co/MnO 2 /n SiO 2 and Co/n SiO 2 catalyst, but after this time the C 5+ selectivity is lower for the Co/MnO 2 /n SiO 2 catalyst. The activity of the Co/CeO 2 /n SiO 2 catalyst is lower than for the Co/MnO 2 /n SiO 2 catalys t, but higher than for the Co/n SiO 2 catalyst. In contrast to the MnO 2 promoted catalyst, the Co/CeO 2 /n SiO 2 is very stable, with no observable decrease in conversion over the 80 hours of operation. The C 5+ selectivity is also very stable and it is highe r than the selectivity of the Co/n SiO 2 catalyst over the course of reaction. ZrO 2 addition results in a catalyst with the highest CO selectivity observed for the metal oxide promoted catalysts. However, the rate of deactivation is similar to that observ ed on the Co/MnO 2 /n SiO 2 catalyst. Therefore, after 80 hours of operation the CO conversions are similar on the MnO 2 ZrO 2 and CeO 2 promoted catalysts. Furthermore the C 5+ selectivity of the Co/ZrO 2 /n SiO 2 catalyst is only slightly lower than that ob tained from the un promoted catalyst. Therefore, the productivity (the amount of C 5+ products produced during the 80 hours of operation) of the ZrO 2 promoted catalyst is slightly higher than the productivity of the CeO 2 promoted catalyst Transition Metal s Additions A number of transition metals were also added to the n SiO 2 support before Co addition to determine their e ffect s on conversion and selectivity. As is evident from Figures 4 1 C and 4 2 C addition of Re and Pd to c o balt supported on n SiO 2 has negative effects on both catalytic activit ies and selectivit ies as the CO conversions for the Co/Re/n SiO 2 and Co/Pd/n SiO 2 catalysts are very low and
97 the selectivities are significantly below that of the unpromoted Co/n SiO 2 catalyst. While the Co/Pd/n SiO 2 catalyst initially has a slightly higher conversion the selectivity is significantly lower than that of the Co/Re/n SiO 2 catalyst. Addition of Fe decreases the CO conversion slightly, while the C 5+ sel ectivity is significantly lower compared to the C o/n SiO 2 catalyst. Furthermore, Fe addition results in a less stable catalyst since both activity and selectivity decrease significantly with time on stream. In contrast, Cu addition results in a slight increase in both CO conversion and C 5+ selectivity compared to the unpromoted Co/n SiO 2 catalyst. However, compared to the unpromoted catalyst the deactivation rate is slightly higher for the Co/Cu/n SiO 2 catalys t. Therefore after 80 hours on stream the performances of the Co/Cu/n SiO 2 and Co/n SiO 2 cat alysts are similar Of the metal SiO 2 catalysts investigated, Ru has the highest initial activity and the selectivity is comparable to the Co/Cu/n SiO 2 catalyst, which is higher than that of the unpromoted catalyst. However, as for the Co /Cu/n SiO 2 catalyst the Co conversion and C 5+ selectivity decrease with time on stream for the Co/Ru/n SiO 2 catalyst. If these catalysts can be stabilized by a second promoter, perhaps a metal oxide, Ru and Cu exhibit the greatest promise as metal promote r s Double Promotion Catalysts To investigate the effects of promoters further, one catalyst was prepared using both a metal oxide and a metal promoter to determine if the the Co/n SiO 2 catalyst could be improved further. Ru and ZrO 2 were selected as th ey exhibited the highest initial activities of the metal and metal oxide promoters tested. Initial experiments using the Co/Ru/ZrO 2 /n SiO 2 catalyst revealed that this was indeed a highly active catalyst. Under the original reaction conditions, it was not possible to control the temperature in the reactor during start up. Therefore, the reaction conditions
98 were modified, by reducing the feed flow rate and adding a larger amount of a glass beads rather than bare support as the inert material. The results are presented in Figure 4 3. As the Co/ZrO 2 /p SiO 2 catalyst exhibited a higher CO conversion and higher C 5+ selectivity compared with the Co/ZrO 2 /n SiO 2 catalyst (Figure 3 3, 3 4 and Table 3 3) a Co/Ru/ZrO 2 /p SiO 2 catalyst was also prepared and tested fo r activity. The n SiO 2 supported catalyst has high activity and selectivity with a slight decrease in activity over time. While t he p SiO 2 supported catalyst has a very high initial activity the deactivation rate is very high on this catalyst Therefore after 70 hours on stream the catalytic activities of the Co/Ru/ZrO 2 /n SiO 2 and Co/Ru/ZrO 2 /p SiO 2 catalysts are the same. The C 5+ selectivities for both catalysts are almost as high as for the unpromoted Co/n SiO 2 catalyst and only decrease by a few percen t with time on stream. It appears that the deactivation rates are similar for catalysts on the same supports, irrespective of promoter used and the rate is higher on the p SiO 2 compared with the n SiO 2 supported catalyst. BET Surface Area Analysis The overall surface areas of all catalysts under investigation were measured and are summarized in Table 4 3 The unpromoted Co/n SiO 2 is included for comparison. As the n SiO 2 support has a very high surface area, 490 m 2 /g, the surface area of the unpromote d Co/n SiO 2 catalyst is also significant at 235 m 2 /g. It is evident from Table 4 3 that metal oxide addition to the Co/n SiO 2 catalyst results in a significant reduction in overall catalyst surface area. Most catalysts with a metal oxide additive have s urface areas below 100 m 2 /g, and they range from 40 m 2 /g for the Co/CaO/n SiO 2 catalyst to ~85 m 2 /g for the Co/MgO/n SiO 2 and Co/Sm 2 O 3 /n SiO 2 catalyst s The only catalysts with surface areas above 100m 2 /g are the ones containing Al 2 O 3 and ZrO 2
99 The over all surface areas of the catalysts after addition of a transition or noble metal are in all cases higher than the catalysts containing an oxide additive In fact with the exception of the Co/Ru/n SiO 2 catalyst, the BET surface areas of the catalysts conta ining Re, Pd, Fe, and Cu are higher than the surface area of the unpromoted Co/n SiO 2 The Co/Ru/ZrO 2 /n SiO 2 catalyst has a surface area similar to the Co/Ru/n SiO 2 catalyst, which is higher than the surface areas for the Co/ZrO 2 /n SiO 2 and Co/Ru/ZrO 2 /p S iO 2 catalysts. Carbon Monoxide Chemisorption Carbon monoxide chemisorption on reduced catalysts is used to obtain a measure of the Co metal surface area s of the catalysts assuming a certain CO:Co stoichiometry (1:1) [ 47 ]. While the specific stoichiometr y is not known for these catalysts, as linearly adsorbed and bridged CO can be present on the surface, the amount of CO adsorbed is compared qualitatively to observe trends between catalysts. It is expected that under certain Fischer Tropsch synthesis con ditions the activity of the catalysts will correlate with the amount of CO adsorbed on the catalysts  In the case of the noble metal promoter addition the CO monoxide chemisorption may be increased due to the chemisorption to those metals instead of an increased CO chemisorption due to a higher Co metal surface area While the increase in CO adsorption may not be due to a higher cobalt dispersion, it is likely that a higher metal surface area is beneficial in the reaction as species may spill over fr om the metal to the cobalt. From Tab le 4 3 it is evident that there is no simple correlation between the measured CO adsorption and the activity under the F T reaction conditions used in this study This is particularly true for the catalysts containing metal oxide additives. Thus, o ther factors must also play a role in determining the activities for these catalysts.
100 However, the catalysts with the highest catalytic activities, Co/Ru/n SiO 2 Co/Cu/n SiO 2 Co/Ru/ZrO 2 /n SiO 2 and Co/Ru/ZrO 2 /p SiO 2 also ads orbed the highest amounts of CO. Temperature Programmed Reduction Results Temperature programmed reduction measurements on fresh catalysts reveal the ease of Co 3 O 4 reduction which often correlates with activity in the F T reaction [ 48 79, 104 10 9, 203 20 4 ]. The reduction of Co 3 O 4 to Co metal is a two step reduction proces s, in which Co 3 O 4 is first reduced to CoO and then to Co [ 48 53 203, 20 4 ] T ypical results for SiO 2 supported catalyst s show that peaks below 450 C in the TPR spectra are due to th e Co 3 O 4 two step reduction, while peaks above 5 5 0 C are typically assigned to strongly interacting Co Si species [ 126, 194 239 ] Addition of CaO Al 2 O 3 and CeO 2 significantly decreases the temperature at which the reduction is complete compared to the Co/n SiO 2 catalyst (Figure 4 4 A and B ). The reduction is complete at a slightly lower temperature also for the ZrO 2 promoted catalyst compared with the unpromoted Co/n SiO 2 In contrast La 2 O 3 and Sm 2 O 3 addition increases the temperature at which the redu ction is complete resulting in a broader TPR curve compared to the Co/n SiO 2 catalyst. The temperature at which the reduction is complete is also slightly higher for the Co/MnO 2 /n SiO 2 catalyst, since it has a small tail which extends past 700 C Additio n of MgO causes the most significant change in the shape of the TPR curve. The low temperature peak of the Co/MgO/n SiO 2 catalyst is shifted to slightly lower temperatures, while the high temperature peak is much broader than for the unpromoted Co/n SiO 2 catalyst. The reduction is not complete until very high temperatures, close to 1000 C for the Co/MgO/n SiO 2 catalyst. This indicates that MgO addition results in the formation of a mixed cobalt oxide which is difficult to reduce and explains why the act ivity is lower than on the unpromoted
101 catalyst. The lower reducibility of this catalyst compared to the other catalyst with added metal oxides is evident in Table 4 3 where the amount of hydrogen consumed during reduction at 400 C is compared to the tota l amount of hydrogen consumed during ramp up to 1000 C. None of the added oxides significantly alter s the temperature where the reduction is initiated and thus, does not appear to affect the first reduction step significantly, i.e. the reduction of Co 3 O 4 to CoO. The TPR data indicates that CaO, Al 2 O 3 ZrO 2 and CeO 2 facilitates reduction of CoO to Co metal, while complete reduction is more difficult when La 2 O 3 Sm 2 O 3 MgO, and MnO 2 are added to Co/n SiO 2 Compared to the unpromoted Co/n SiO 2 if a metal oxide additive results in a catalyst which is easier to reduce it often has a higher activity and if the catalyst is more difficult to reduce it has a lower activity than Co/n SiO 2 However, this is not the only catalyst property that governs activity sin ce CaO addition results in a catalyst which is easier to reduce than Co/n SiO 2 but the activity is significantly lower. In contrast to the metal oxide additives, addition of Pd and Ru to a Co/n SiO 2 catalyst significantly reduces the temperature of the first reduction peak (Figure 4 4 C ) The temperature where reduction is initiated is also reduced for the Co/Re/n SiO 2 catalyst compared to the unpromoted catalyst, but in the case of Re it is not possible to distinguish two separate reduction peaks. Add ition of Ru, Pd, and R e also significantly reduces the temperature where the reduction is complete indicating that these additives facilitate both reduction steps On the Co/Pd/SiO 2 catalyst, the low temperature peak is well separated from the high tempe rature peak. This separation is not observed on any other catalyst. Addition of Ru results in a split of the low temperature peak into two distinct components. This is similar to the behavior seen in literature and attributed to
102 the reduction of cobalt around Ru or Pd [ 128 191 ]. This behavior is explained by a H 2 spillover effect where facile hydrogen dissociation occurs on the noble metal and a low energy pathway allows hydrogen to spillover onto the cobalt oxide The reduction properties of the Co/F e/n SiO2 and Co/Cu/n SiO2 catalysts are very different from those containing Ru, Re, and Pd. In the case of Fe addition the high temperature peak is shifted to higher temperatures and there is a significant, broad, second high temperature peak Reductio n is not complete until 900 C for the Co/Fe/n SiO 2 catalyst. This again indicates formation of a mixed metal oxide which is difficult to reduce and explains why the catalytic activity is lower than that of the Co/n SiO 2 catalyst. Addition of Cu to the Co /n SiO 2 catalyst alters the shape of the TPR curve, since the intensity of the low temperature peak is significantly lower compared to the high temperature peak Therefore, reduction of the CoO x on the surface of the Co/n SiO 2 catalyst is more difficult a fter addition of both Fe and Cu. Compared to the Co/ Ru/ n SiO 2 catalyst reduction is initiated at a slightly lower temperature and the two low temperature peaks are closer together on the Co/ Ru/Zr O 2 /n SiO 2 catalyst (Figure 4 5) However, more important ly, the high temperature reduction step is completed at a significantly lower temperature (by ~50C) on the catalyst containing both Ru and Zr O 2 Therefore the Co/Ru/ZrO 2 /n SiO 2 catalyst is more easily reduced than the Co/ Ru /n SiO 2 catalyst. Interestingl y, the Co/Ru/ZrO 2 /p SiO 2 catalyst is more difficult to reduce than the Co/Ru/ZrO 2 /n SiO 2 catalyst. Oxygen Pulse Titration As the amounts of CO adsorbed are rather low and formation of inactive mixed metal oxides with cobalt is common for these catalysts, the catalyst reducibilities were probed using oxygen titration after a standard reduction treatment to obtain another
103 measure of reducibility. The results are summarized in Table 4 3 Most of the metal promoted catalysts have very high reducibilities, w hile some of the metal oxide promoted catalysts exhibit lower reducibilities. Increased oxygen adsorption on some of the metal promoted catalyst can be expected due to reoxidation of the metal promoter during the oxygen titration XRD Results Six of the p romoted catalysts were selected for x ray diffraction (XRD) measurements to determine the crystal structure of the catalysts The catalysts were selected to include both promoters which increased activity and promoters which resulted in very low CO conver sions relative to the unpromoted Co/n SiO 2 catalyst. As evident in Figure 4 6, the XRD patterns obtained from the fresh catalyst s are very similar Co 3 O 4 is the only phase present on all catalysts and the main difference between the catalysts is the peak widths, and thus also the particle sizes of the Co 3 O 4 The particle sizes according to the Scherrer Equation have been calculated and listed in Table 4 4 The Co 3 O 4 particle sizes vary between 1 7.5 and 25 nm on the fresh catalysts. All promoted catalys ts have smaller particle sizes than the unpromoted catalyst and, in general, metal promoted catalysts give smaller particle sizes than metal oxide promoted catalysts. The XRD patterns obtained from the reduced catalyst s reveal more variation (Figure 4 7) Co metal is the main crystal phase on all reduced catalysts, but other crystalline compounds are present on most catalysts. For example, CoO, due to incomplete reduction or partial reoxidation of Co, is evident on most catalysts. The smallest contribu tion from CoO is observed on the Co/CeO 2 /n SiO 2 Co/CaO/ n SiO 2 and Co/n SiO 2 catalysts, while the highest CoO concentration is observed on the Co/MgO /n
104 SiO 2 Co/Pd /n SiO 2 and Co/Fe /n SiO 2 catalysts. The Co metal particle sizes vary between 9 and 1 6 nm on the reduced catalysts. In the case of the Co/Fe /n SiO 2 and Co/MgO /n SiO 2 catalysts the Co particle sizes are smaller than expected from the Co 3 O 4 particle sizes on the fresh catalysts, but in these cases it may be due to part of the Co reacting and formi ng inactive mixed cobalt oxides during reduction (as evidenced by the TPR data). The XRD patterns obtained after exposure to the reaction conditions are different from those obtained from the reduced catalysts (Figure 4 8) While Co metal is still pr esent in all catalysts, the contribution from CoO is lower on the spent catalysts compared with the reduced ones for all catalysts except the Co/CaO /n SiO 2 and Co/ZrO 2 /n Si O 2 In fact, the Co/CaO /n SiO 2 is the only catalyst where Co metal is not the main crystal phase observed from the Co containing compounds. Several additional peaks are evident in the XRD patterns obtained from these catalysts, and the Co particle sizes are significantly smaller than those on the reduced catalysts. As was observed in C hapter 3, it appears that some crystal growth of the support occurs during reaction, and this is particularly evident on the Co/Ru /n SiO 2 and Co/ZrO 2 /n SiO 2 catalysts. The XRD patterns obtained from the fresh and reduced Co/Ru/ZrO 2 /n SiO 2 and Co/Ru/ZrO 2 / p SiO 2 catalysts are presented in Figure 4 9. The main difference between the two fresh catalysts is broader peaks due to Co 3 O 4 in the XRD patterns obtained from the Co/Ru/ZrO 2 / p SiO 2 catalyst. Therefore, the Co 3 O 4 particles are smaller on the Co/Ru/ZrO 2 / p SiO 2 catalyst (1 4 nm versus 22 nm on the Co/Ru/ZrO 2 / n SiO 2 catalyst Table 4 5 ) In the XRD pattern obtained from the n SiO 2 supported catalyst, it is evident
105 that the reduction is incomplete as this catalyst still has some Co 3 O 4 phase present in addit ion to the Co metal phase This is not the case for the p SiO 2 supported catalyst, which exhibits no Co 3 O 4 related peaks. As was the case for the Co/ZrO 2 /n SiO 2 and Co/Ru/n SiO 2 catalysts, a small amount of the CoO phase is present in both reduced Co/Ru/ ZrO 2 /SiO 2 catalysts. Summary Th e effects of several different metal and metal oxide additives on a cobalt catalyst supported on nanoparticle silica were investigated in detail The average activity of the Co/n SiO 2 catalyst was improved by the addition of ZrO 2 CeO 2 Al 2 O 3 MnO 2 Ru, and Cu, while a ddition of MgO, CaO La 2 O 3 Fe, P d, and Re decreased the activity compared to the unpromoted catalyst. Only Sm 2 O 3 did not significantly affect the catalytic activity. Most additives decreased t he C 5+ sele ctivit y of all the catalysts compared to the unpromoted catalyst. The only except ions are the CeO 2 Ru, Cu promoted catalysts, which increased both activity and selectivity compared to the Co/n SiO 2 catalyst.
107 Figure 4 1. CO Conversion for promoted ca talysts supported on nanoparticle SiO 2 pre calcined at 400C for 4 hours. Reaction c o nditions: Temperature=230C, P=20atm, F tot =125ml/min (H 2 :CO:N 2 =6:3:1). A ) CaO, MgO, La 2 O 3 Sm 2 O 3 B ) Al 2 O 3 MnO 2 CeO 2 ZrO 2 and C ) Ru, Cu, Fe, Pd, Re.
108 Figure 4 2. C 5+ Selectivity for promoted catalysts supported on nanoparticle SiO 2 pre calcined at 400C for 4 hours. Reaction Co nditions: Temperature=230C, P=20atm, F tot =125ml/min (H 2 :CO:N 2 =6:3:1). A ) CaO, MgO, La 2 O 3 Sm 2 O 3 B ) Al 2 O 3 MnO 2 CeO 2 ZrO 2 and C ) Ru, Cu Fe, Pd, Re.
109 Figure 4 3 Cobalt c atalysts with the addition of ZrO 2 and Ru s upported on SiO 2 precalcined a t 400C for 4 hours. Reaction c onditions: T=230C, P=20atm, F tot = 62.5 ml/min (H 2 :CO:N 2 =6:3:1)
110 Table 4 1 Product a nalysis r esults for c obalt c atalysts with a dditives on n SiO 2 Reaction Data Catalyst a Avg. Conv. b CH 4 c CO 2 d C 2 C 4 e C 5+ f Produ ctivity g 20/80 Co/n SiO 2 16% 17% 0 % 13% 70% 11.2 20/2/78 Co/La 2 O 3 /n SiO 2 9% 21% 0 % 35% 43% 3.9 20/2/78 Co/MgO/n SiO 2 8% 19% 1 % 27% 54% 4.3 20/2/7 8 Co/CaO/n SiO 2 3% 23% 0 % 30% 47% 1.4 20/2/78 Co/CeO 2 /n SiO 2 20% 12% 0 % 15% 72% 14.4 20/2/78 Co/ZrO 2 /n SiO 2 25% 20% 1 % 15% 61% 15.3 20/2/78 Co/MnO 2 /n SiO 2 18% 17% 1 % 27% 55% 9.9 20/2/78 Co/Al 2 O 3 /n SiO 2 19% 21% 1 % 15% 64% 12.2 20/2/78 Co/Sm 2 O 3 /n SiO 2 1 4% 20% 2 % 27% 51% 7.1 20/2/78 Co/Fe/n SiO 2 11% 24% 2 % 26% 48% 5.3 20/2/78 Co/Cu/n SiO 2 17% 17% 1 % 9 % 74% 12.6 20/ 0.6 /7 9. 4 Co/Ru/n SiO 2 19% 16% 0 % 12% 71% 13.5 20/2/78 Co/Pd/n SiO 2 3% 25% 0 % 20% 55% 1.7 20/2/78 Co/Re/n SiO 2 2% 25% 0 % 18% 57% 1.1 a Cat alyst Composition in weight %. b Avg Conv.: Average conversion. c CH 4 : Methane s electivity d CO 2 : CO 2 s electivity. e C 2 C 4 : Selectivity of hydr o carbons from carbon number 2 to 4. f C 5+ : Selectivity of hy drocarbons with carbon number 5 and lar ger. g Productivity = ( c onversion*C 5+ )* 100 standard deviation of 0.9 Table 4 2 Product a nalysis r esults for c obalt c atalysts with a dditives on n SiO 2 Reaction Data Catalyst a Avg. Conv. b CH 4 c CO 2 d C 2 C 4 e C 5+ f Produ ctivity g 20 Co/n SiO 2 29% 9% 4% 8% 79% 22.9 20 Co/p SiO 2 34% 9% 8% 10% 73% 24.8 20/0.6 /2/77. 4 Co/Ru/ZrO 2 /n SiO 2 45% 11% 1 % 11% 77% 34.7 20/0.6 /2/77. 4 Co/Ru/ZrO 2 /p SiO 2 53% 9 % 8 % 7 % 76% 40.3 a Catalyst composition b Avg Conv.: Average conversion. c CH 4 : Methane s electivity d C O 2 : CO 2 s electivity. e C 2 C 4 : Selectivity of hydrcarbons from carbon number 2 to 4 f C 5+ : Selectivity of hydrocarbons with carbon number larger than 5. g Productivity: ( c onversion*C 5+ )/100 standard deviation of 0.9
112 Figure 4 4 TPR for c obalt c a talysts supported by nanoparticle SiO 2 pre calcined at 400C for 4 hours promoted with A ) CaO, MgO, La 2 O 3 Sm 2 O 3 B ) Al 2 O 3 MnO 2 CeO 2 ZrO 2 and C ) Ru Cu, Fe Pd and R e.
113 Figure 4 5. TPR for c obalt c atalysts and d ouble p romoted c atalysts.
114 Table 4 3 Cha racterization r esults for a dditives Catalyst a BET Surface Area [m 2 /g] b Vol. of CO Ads. [L/g] c CO Metal Surface Area [m 2 /g] d Full Red. Area e [Area/g] Redu c Iblity f Vol. of O 2 Ads. [ml/g] g Redu c Iblity h 20/80 Co/n SiO 2 235 480 0.86 33.3 85.6% 44 .4 80.0% 20/2/78 Co/La 2 O 3 /n SiO 2 70 425 0.75 66.6 57.1% 41.8 75.4% 20/2/78 Co/MgO/n SiO 2 80 915 1.63 38.0 72.2% 44.5 80.2% 20/2/78 Co/CaO/n SiO 2 40 645 1.15 47.2 90.5% 45.6 82.2% 20/2/78 Co/CeO 2 /n SiO 2 70 445 0.79 34.4 98.1% 41.9 75.5% 20/2/78 Co/ZrO 2 /n SiO 2 160 770 1.37 38.8 83.4% 37.0 66.8% 20/2/78 Co/Mn 2 O 3 /n SiO 2 70 140 0.25 44.1 100% 45.3 81.6% 20/2/78 Co/Al 2 O 3 /n SiO 2 180 755 1.35 37.7 89.9% 37.5 67.7% 20/2/78 Co/Sm 2 O 3 /n SiO 2 80 685 1.22 41.7 100% 48.7 87.7% 20/2/78 Co/Fe/n SiO 2 280 175 0.32 39 .0 90.5% 49.6 89.5% 20/2/78 Co/Cu/n SiO 2 265 1230 2.18 42.7 90.6% 42.2 76.0% 20/0.6/79.4 Co/Ru/n SiO 2 200 900 1.61 42.5 100% 48.8 87.9% 20/2/78 Co/Pd/n SiO 2 280 615 1.09 36.9 98.4% 48.1 86.8% 20/2/78 Co/Re/n SiO 2 300 190 0.34 40.7 98.8% 49.2 88.7% 20/ 2/0.6/77.4 Co/Ru/ZrO 2 /n SiO 2 190 1020 1.82 31.5 95.0% 45.4 81.9% 20/2/0.6/77.4 Co/Ru/ZrO 2 /p SiO 2 160 1265 2.25 31.0 93.7% 46.2 83.3% a Catalyst: Support used. b BET Surface Area [m 2 /g]: Overall surface area. c Vol. of CO Ads. [L/g]: Volume of CO adsor bed after catalyst is reduced at 400C. d CO Metal Surface Area [m 2 /g]: Surface a rea measured from CO adsorption. Measurement assumes all the adsorption is due to Cobalt. e Full Red. Area: (Area of TPR peak for full reduction per gram of catalyst)/10000 0. f Reducibility: TPR Area for catalyst reduced at 400 C compared to full TPR area. g Vol. of O 2 Ads. [ml/g]: Volume of O 2 adsorbed after reduction at 400 C. h Reducibility: Volume of O 2 used to re oxidize surface relative to theoretical amount of O 2 to oxidize all cobalt.
115 Figure 4 6 XRD of f resh p romoted c atalysts: Unpromoted, Ru, Fe, Pd, CaO, MgO, CeO 2 ZrO 2 A ) f ull scan B ) h igh resolution scan. Figure 4 7 XRD of r educed c atalysts: Unpromoted, Ru, Fe, Pd, CaO, MgO, CeO 2 ZrO 2 A ) f ull scan B ) h igh resolution scan.
116 Figure 4 8 XRD of s pent c atalysts: Unpromoted, Ru, Fe, Pd, CaO, MgO, CeO 2 ZrO 2 A ) f ull scan B ) h igh resolution scan. Figure 4 9 XRD of d ouble p romoted Ru and ZrO 2 p SiO 2 and n SiO 2 supported. Fresh, reduced, and spent. A ) f ull scan B ) high resolution s can, NP=Nanoparticle support, Po=Porous support.
117 Table 4 4 Particle s izes for n SiO 2 supported p romoted c atalysts Fresh Catalysts: Reduced Catalysts: Spent Catalysts: Catalyst Avg. Co 3 O 4 (nm) Avg. Co (nm) Avg. D(%) Avg Co (nm) Avg. D(%) Avg. Co (nm) Avg. D(%) 20/80 Co/n SiO 2 26.3 19.7 4.9 17.1 5.7 21.2 4.5 20/2/78 Co/ZrO 2 /n SiO 2 22.6 16.9 5.7 12.3 7.9 7.4 13.0 20/2/78 Co/CeO 2 /n SiO 2 19.5 14.6 6.6 13.1 7.4 20.2 4.8 20/2/78 Co/MgO/n SiO 2 23.4 17.6 5.5 8.8 10.9 14.2 6 .8 20/2/78 Co/CaO/n SiO 2 24.5 18.4 5.2 14.8 6.5 7.6 12.6 20/ 0.63 /79.37 Co/Ru/n SiO 2 17.8 13.4 7.2 9.0 10.7 9.8 9.8 20/2/78 Co/Pd/n SiO 2 24.4 18.3 5.3 15.6 6.2 16.0 6.0 20/2/78 Co/Fe/n SiO 2 20.5 15.4 6.2 10.5 9.2 12.5 7.7 Table 4 5 Particle s izes for Ru/ZrO 2 p romoted n SiO 2 supported c atalysts. Fresh Catalysts: Reduced Catalysts: Catalyst Avg. Co 3 O 4 (nm) Avg. Co (nm) Avg D(%) Avg. Co (nm) Avg D(%) 20/2/0.6/77. 4 Co/Ru/ZrO 2 /n SiO 2 21.6 16.2 5.9 9.9 9. 8 20/2/0.6 /77. 4 Co/Ru/ZrO 2 /p SiO 2 14.0 10.5 9 .1 14.8 6.5
118 CHAPTER 5 EFFECTS OF DIFFERENT OXIDE SUPPORTS Background A number of different supports have been used in the synthesis of Fischer Tropsch catalysts. The most commonly used supports are Al 2 O 3 TiO 2 and SiO 2 but they all have advantages and disadvantages. There fore, there have been extensive discussion s of optimal support properties for Fischer Tropsch synthesis. A support should facilitate the preparation of a well dispersed, high surface area active phase, and assist in stabilizing the a ctive phas e. However, through metal supp ort interactions, s upports can also affect the properties of the active metal and thus also the catalytic activity and selectivity [ 21 ]. Metal support interactions are particularly important for cobalt based Fische r Tropsch catalysts, as cobalt can interact with the metal oxide supports and form inactive mixed metal oxides under the harsh F T reaction conditions. Therefore, careful support selection is crucial in the design of highly active and stable Fischer Trops ch catalysts. It appears that activity decreases in the following fashion for different cobalt based supported catalysts : TiO 2 >SiO 2 >Al 2 O 3 >C > MgO  Also, it appears that increasing dispersion correlates with lower activity. Increasing dispersions an d lower reducibility lead to a lower C 5+ selectivity [ 256 ]. A major effect of strong metal support interactions is lower reducibility [ 48 ]. An investigation of three different supports found that the order of interaction between the cobalt and the suppo rt is, from weakest to strongest, SiO 2 TiO 2 Al 2 O 3 [ 49 ]. In a gas phase and supercritical phase study, two different silica suppor ted catalysts performed better with a higher activity but also higher methane selectivity, than an
119 alumina supported cataly st [ 257 ]. The metal support interaction s can be greatly affected by water. The effect of water on Al 2 O 3 is fast deactivat ion In contrast small amounts of water may have a positive effect on SiO 2 and TiO 2 with slight increases in activity. However, la rger amounts of water deactivate all catalysts, irrespective of the supports [ 51 3 ]. In a comparison of Al 2 O 3 and ZrO 2 supported catalysts, the ZrO 2 supported catalysts showed higher reducibility and hydrogen adsorption. Also the more reducible catalys t showed better activity and C 5+ Selectivity [ 41 ]. Mesoporous ZrO 2 supported catalyst has been used to produce more active catalyst compared to an amorphous SiO 2 supported catalyst [ 159 ]. ZrO 2 supported catalysts were compared to two other supports with the activity in the following order: ZrO 2 >TiO 2 >CeO 2 . A bimodal support prepared by sols of ZrO 2 and SiO 2 has been synthesized with high activities and low methane selectivities in a slurry reactor [ 258 ]. Titania has been used as a F T catalyst supp ort in the past due to its low reactivity with cobalt and high hydrothermal stability [ 6 50 ]. With promoters, these TiO 2 supported catalysts have shown improved performance, especially in avoiding sintering [ 47 145 147 149, 151 153, 157 ]. The most po pular support for F T catalysts is SiO 2 It is used because it has relatively limited interaction with the active metal, has high surface areas, and is easily modified [ 6 98 100 10 2 ]. The main issue with SiO 2 is the hydrothermal stability, as it forms inactive cobalt silicate under high pressure steam. SiO 2 is often modified with ZrO 2 or CeO 2 to stabilize the support during reaction [9, 79, 104 7 11 0, 125 12 6 ]. A much less commonly used support is ZrO 2 but it has shown some positive results [ 41 ] and is typically used as a stabilizer to one of the other
120 supports [ 154 ]. It has been shown to work well in its nanoscale form when mixed with Al 2 O 3 [ 158 ] as well as a mesoporous support [ 40 159 ]. Al 2 O 3 supported catalysts typically have strong interaction s with the support but are mechanically strong with high surface areas [ 6 56 ]. They are usually calcined and/or modified, i.e. with La 2 O 3 or ZrO 2 to limit the hydration of the surface before being used as supports [ 6 63, 67 75, 78 81 ]. In t he present study several cobalt catalysts supported on nanoparticle and porous metal oxides were prepared to investigate the effects of differe nt types of supports, as well as if there is an advantage to using nanoparticle support s on the catalytic activities and sel ectivities of these catalysts. Experimental Details Eight catalysts were prepared using four different types of support s; three of the most commonly used Fischer Tropsch catalyst supports; SiO 2 Al 2 O 3 and TiO 2 as well as another promising support ZrO 2 to investigate the effects of each support material on the activity and selectivity of the catalysts. For each support type, the effect of support particle size was investigated by using a nanoparticle oxide as well as a more The supports used in the study we re gamma Al 2 O 3 ( Al 2 O 3 ; Alfa Aesar [ 259 ] ), nanoparticle Al 2 O 3 ( n Al 2 O 3 ; Nano Active Materials [ 260 ]) 2 ( p SiO 2 ; Alfa Aesar [ 235 ]) nanoparticle SiO2 ( n SiO 2 ; Nanostructured and Amorphous Materials Inc. [ 236 ]) 2 ( p TiO 2 ; Alfa Aesar [ 261 ] ), nanoparticle TiO 2 ( n TiO 2 ; NanoActive Materials [ 262 2 ( p ZrO 2 ; Alfa Aesar [ 263 ] ), and nanoparticle ZrO 2 ( n ZrO 2 ; Nanostructured and Am orphous Materials, Inc. [ 264 ]). All catalysts discussed were prepared using the incipient wetness impregnation as described in Chapter 2, with a cobalt metal loading of 20%. All of the prepared catalysts were tested for activity in the F T reaction and characterized using a number of
121 analytical techniques to deter mine the influence of various catalyst properties on the catalytic activities and selectivities The analytical techniques used are described in detail in Chapter 2 (Catalyst Characterization). Results and Discussion Fischer Tropsch Synthesis The TiO 2 sup ported catalysts resulted in very low activities. In fact, to detect any activity the temperature had to be increased relative to the reaction temperature used for the other catalysts in the study (Figure 5 1A) The n TiO 2 supported catalyst exhibits a h igher activity and selectivity compared with the p TiO 2 supported catalyst. The activity of the n SiO 2 supported catalyst is significantly higher than that of the Co/n TiO 2 catalyst. The CO conversion is stable just above 30% for the first 35 hours but sl owly drops to 24% after 80 hours on stream. While the initial conversion at ~45% is significantly higher for the p SiO 2 supported catalyst, the deactivation rate is higher compared to the n SiO 2 supported catalyst. Therefore, after 85 hours on stream the activity of the Co/p SiO 2 is lower than that of the Co/n SiO 2 catalyst (Figure 5 1A) Furthermore, while the C 5+ selectivity does not change significantly with time on stream for the n SiO 2 supported catalyst, it drops from an initial value of 80% to 63% after 85 hours on stream for the p SiO 2 supported catalyst (Figure 5 1B) The in i tial CO conversions for the two ZrO 2 supported catalysts are the same (38%) and the value is in between those of the n SiO 2 and p SiO 2 supported catalysts. Of the two ZrO 2 supported catalysts, the p ZrO 2 supported catalyst is slightly more stable than the Co/n ZrO 2 The deactivation rate on the Co/n ZrO 2 catalyst is similar to that of the Co/n SiO 2 catalyst. In contrast, the CO conversion for the p ZrO 2 supported catalyst is reasonably stable and only drops from the initial 38% to 34% during the 85
122 hours on stream. The C 5+ selectivities are high, amongst the highest observed for the catalysts under investigation, for both ZrO 2 supported catalysts. They are also more stabl e than the C 5+ selectivities of the SiO 2 supported catalysts. It appears that the C 5+ selectivity of the n ZrO 2 supported catalyst is slightly more stable as it remains above 80% during the run, while the C 5+ selectivity of the Co/p ZrO 2 catalyst drops be low 80% toward the end of the run. The initial activity of the n Al 2 O 3 supported catalyst is similar to the Co/p SiO 2 catalyst. However, the Co/n Al 2 O 3 catalyst is significantly more stable compared to the p SiO 2 supported catalyst. While the CO co nversion dropped from 45% to 22% for the Co/p SiO 2 catalyst, the CO conversion over the n Al 2 O 3 supported catalyst is 37% even after 85 hours on stream. The deactivation rate of this catalyst is therefore similar to that of the Co/p ZrO 2 catalyst. The mo st active catalyst of the ones under investigation is the cobalt supported on the Al 2 O 3 However, this catalyst also has the highest deactivation rate, as the activity decreases from 90% to 50% during the 8 5 hours on stream Despite the significant drop in activity over time, the Co/ Al 2 O 3 catalyst still exhibits the highest CO con version of the catalyst s under investigations at the end of 85 hours on stream. Initially the C 5+ selectivity is lower compared to most of the other supported catalysts However the C 5+ selectivity increases with time, or decreasing conversion. Therefore after 50 hours on stream the C 5+ selectivity has increased from 65% to 77% and it appears to level out around 75%. It appears that while Co/ Al 2 O 3 catalyst is the most active catalyst it is also the most unstable catalyst under investigation. In contrast, t he n Al 2 O 3 supported catalyst is amongst the most stable
123 catalyst s and it exhibits the second highest CO conversion and one of the highe s t C 5+ selectivit ies BET Surface Area Analysis The overall surface areas of these catalysts reflect the differences in surface areas of the supports and are thus, dramatically different (Table 5 2) The drastic reduction in surface area for the catalys ts supported on n TiO 2 compared to the support surface area, has been observed before and is due to a phase change in the support during calcination (amorphous TiO 2 crystallizes into anatase particles with a concomitant significant reduction in surface ar ea) [ 265 ]. The highest overall surface areas of the supported catalysts are observed on the SiO 2 supported catalysts. The surface areas of these catalysts are an order of magnitude higher than the catalyst with the lowest surface area, the Co/n ZrO 2 Eve n though the initial support surface area for the n SiO 2 is about twice that of the p SiO 2 support, the overall catalyst surface areas are similar to on SiO 2 supported catalysts. The lowest catalyst surface areas are observed for the ZrO 2 supported cataly sts, which is expected since the supports have the lowest surface areas of the ones under investigation. In contrast to the results for n TiO 2 the low surface area of the Co/n Al 2 O 3 catalyst is unexpected [ 263 ], but is likely indicative of cobalt alumina te formation. The surface area of the Co/ Al 2 O 3 catalyst (160 m 2 /g) is closer to what would be expected of a support with a 260 m 2 /g surface area. Carbon Monoxide Chemisorption Considering the low activities of the TiO 2 supported catalysts, the CO adsorption is higher than expected (Table 5 2) The lower CO adsorption on the Co/n TiO 2 catalyst compared to the Co/p TiO 2 is likely due to the drastic reduction in surface area during
124 calcination, which is likely to block Co sites. Considering the low ZrO 2 surface areas compared to for example the SiO 2 supports, the CO adsorption is higher than expected. This explains why the ZrO 2 supported catalysts have activities in the same range as the SiO 2 supported catalysts. However, there is no simple correlation between the calculated Co metal surface ar eas from the CO adsorption measurements and the catalytic activities. Other factors must also influence the activity. The low CO adsorption on the n Al 2 O 3 supported catalyst is consistent with CoAl 2 O 4 formation. However, the catalytic activity is surpri singly high on this catalyst considering the low Co metal surface area, which means that the small amount of Co present on the surface of this catalyst is highly active. Temperature Programmed Reduction Results Temperature programmed reduction experiments are typically performed on Fischer Tropsch catalysts to determine the ease of Co 3 O 4 reduction to Co metal. This is important since the ease of reduction often correlates with the catalytic activity [ 48 203, 20 4 ]. The TPR measurements will also reveal if strong metal support interactions are present and if they appear to result in the formation of inactive mixed cobalt oxides. The reduction of Co 3 O 4 to Co metal is a two step reduction proces s, where Co 3 O 4 is first reduced to CoO and then to Co metal [ 48 2 0 3, 20 4 ] Due to varying metal support interactions, the cobalt oxide reduction behavior is dependent on the support oxide. However, since the metal support interactions are also dependent on the preparation method and pretreatment procedure, it is dif ficult to identify specific trends even within a specific group of oxides. In general i t ha s been suggested that the peaks below 50 0 C are due to the two step reduction of Co 3 O 4 while reduction peaks above 500 C are due to cobalt species strongly interac ting with the support [ 76, 79, 126, 153, 161, 194 196,
125 239 ]. However, there are indications that the Co 3 O 4 reduction is complete below 425 C for some catalysts [ 59 60 66 244 ]. As the reductive pretreatment before reaction is done at 400 C lity was calculated by comparing the reduction at 400 C with the one up to 800 C or 1000 C. The calculated reducib i lities for the catalysts under investigation are listed in Table 5 2. The TPR results from the catalysts under investigation are presente d in Figure 5 2. All catalysts exhibit at least two reduction peaks, one sharper peak at lower temperatures and one broader peak at higher temperatures. T he reduction profile s of the TiO 2 supported catalysts reveal rather high temperatures of reduction For both catalysts the low temperature peak position is well above 400 C, which is higher than most other catalysts. Compared to the p TiO 2 supported catalyst, the peak intensity is shifted from the low temperature to the high temperature peak for the Co /n TiO 2 catalyst. This suggests that the Co 3 O 4 reduction is more difficult on the Co/n TiO 2 catalyst and the lower reducibility at 400 C for the Co/n TiO 2 compared to the Co/p TiO 2 catalyst is evident in Table 5 2 However, the reduction is not complete until a slightly higher temperature for the Co/p TiO 2 catalyst, which could indicate a small fraction of cobalt species with stronger support interactions compared to the Co/n TiO 2 catalyst. The CoO x species the SiO 2 supported catalysts are evidently mor e reducible than on the TiO 2 supported catalysts. The first reduction peak appears at a lower temperature and the reduction is also complete at a lower temperature for the Co/n SiO 2 catalyst compared to the Co/n TiO 2 catalyst. As for the TiO 2 catalysts, reduction is complete at a lower temperature for the nanoparticle supported catalyst, and there is a
126 slightly higher reducibility of the n SiO 2 versus the p SiO 2 supported catalyst evident in Table 5 2 The low temperature peaks are similar on the n SiO 2 and n ZrO 2 supported catalysts, but reduction is complete at a lower temperature on the Co/n ZrO 2 catalyst. This indicates that the reduction of Co 3 O 4 to CoO is not significantly different on the two support s but the Co ZrO 2 interactions are slightly weaker than the Co SiO 2 interactions. As is evident in Table 5 2 the ZrO 2 supported catalysts have the highest reducibilities observed for the catalysts under investigation. The TPR profile obtained from the Co/p ZrO 2 catalyst is different from those ob tained from the other catalysts under investigation. More reduction peaks are evident on this catalyst. Both the low and high temperature peaks have contributions from two states and the additional states appear to be on the low temperature side of the t ypical TPR peaks. This indicates that additional CoO x states are present on this catalyst, and they are easier to reduce compared to the typical supported CoO x states. This said, there are also some Co ZrO 2 interactions that appear stronger on the Co/p Z rO 2 than on the Co/n ZrO 2 catalyst and this is the reason the reducibility is slightly lower on the Co/p ZrO 2 catalyst compared with the Co/n ZrO 2 The CoO x species on the Al 2 O 3 support s are the most difficult to reduce of the catalysts under investigatio n (Figure 5 2 and Table 5 2 ) The low temperature peak is shifted to higher temperatures for both Al 2 O 3 supported catalyst s and reduction is not complete until above 1000 C. These results are consistent with Al 2 O 3 supports inducing the strongest cobalt s upport interactions. For the Al 2 O 3 supported catalysts, the n Al 2 O 3 results in the strongest cobalt support interactions as reduction is not complete even at
127 1000 C. The broad high temperature peaks observed for the Co/n Al 2 O 3 and Co/ Al 2 O 3 catalyst s ar e most likely due to inactive cobalt aluminate species. Oxygen Pulse Titration Analyzing the reducibility through oxygen pulse titration provides a measure of reducibility The Al 2 O 3 supported catalysts adsorbed the least O 2 indicating poor reducibility, with the n Al 2 O 3 supported catalyst only adsorbing about 66% of O 2 volume adsorbed by the Al 2 O 3 supported catalyst. The reduced n TiO 2 supported catalyst ad sorbed about twice as much O 2 as the reduced n Al 2 O 3 supported catalyst. T he reduced p ZrO 2 supported catalyst adsor bed a larger O 2 volume compared with the reduced Al 2 O 3 and n TiO 2 supp orted catalysts, but lower than the reduced p SiO 2 n ZrO 2 and p TiO 2 supported catalysts, which all show similar O 2 adsorption values. The highest O 2 adsorption or reducibility was observed for the n SiO 2 supported catalyst. X Ray Diffraction Result s T he XRD patterns obtained from the fresh and reduced catalysts are presented in Figures 5 3 to 5 6 All spectra obtained from the fresh catalysts reveal peaks due to Co 3 O 4 The Co 3 O 4 particle sizes calculated using the Scherrer Equation are dependent o n the support and varies between 12 and 28 nm. The smallest Co 3 O 4 particles are observed on the Al 2 O 3 supported catalysts and this is likely due to CoAl 2 O 4 formation during catalysts preparation. This will not only reduce the amount of Co available for C o 3 O 4 formation, it will also broaden the XRD peaks (the XRD patterns due to Co 3 O 4 and CoAl 2 O 4 are very similar as they both have spinel type structures) which will give smaller particle sizes according to the Scherrer Equation The Co 3 O 4 particle sizes on the SiO 2 and ZrO 2 supported catalysts are similar, with 15 16 nm for the porous supports and 26 28 nm for the nanoparticle supports. On the TiO 2 supports the
128 opposite trend is observed, i.e. the Co 3 O 4 particle size is 22 nm for the porous and 15 nm for t he nanoparticle support. In addition to the Co 3 O 4 peaks, there are also XRD peaks present in the spectra due to the different supports, s u ch as peaks from the monoclinic ZrO 2 anatase TiO 2 Al 2 O 3 and AlO(OH). After redu ction, Co metal is expected on all catalysts as has been previously observed for the SiO 2 supported catalysts However, Figures 5 3 to 5 6 reveal that only the XRD patterns obtained from the SiO 2 and T iO 2 supported cata lysts exhibit significant Co metal peaks. In the spectra obtained from the ZrO 2 supported catalysts the Co metal peaks overlap with the ZrO 2 related peaks and those from the Al 2 O 3 supported catalysts reveal incomplete reduction as CoO is the main phase. This is consistent with the low reducibility of the Co/Al 2 O 3 catalysts. Summary The TiO 2 supported catalysts both had low activity and selectivity. These catalysts show ed higher temperature reduction and the n TiO 2 supported catalyst exhibit ed a low red ucibility and an average metal surface area. Interestingly, the p TiO 2 supported catalyst show ed a high reducibility and metal surface area, but low overall surface area. The p SiO 2 supported catalyst display ed high act ivity and selectivity but it did no t appear stable, as the activity and selectivity decreased over time. This catalyst had high reducibility, metal surface area, and overall surface area. The n SiO 2 supported catalyst show ed similar behavior, without the initial activity and large deactiv ation but with higher C 5+ selectivity. Characterization results illustrate d similar reducibility and surface areas between the SiO 2 supported catalysts The ZrO 2 supported catalysts show ed more stable activity and high, stable C 5+ selectivity. These
129 tw o catalys ts display ed relatively high reducibility and the p ZrO 2 has high metal surface area. The Al 2 O 3 supported catalysts demonstrate d impressive activity. However, despite t he high activity, these catalysts show ed limited reducibility, surprisingly. The Al 2 O 3 supported catalyst has a relatively high metal surface area. Concomitant with the high activity behavior wa s a lower selectivity Furthermore, the activity dropped over the reaction time even though it still remained above the other catalys ts at the end of 90 hours
130 Figure 5 1. Reaction behavior for different supports with 20% c obalt at P=20bar, T=230C, m cat =1g, F tot = 62.5 ml/min (H 2 :CO:N 2 =6:3:1) n= n anop articl e support, p= p orous support A ) c arbon m onoxide c onversion B ) C 5 + s electivit y.
131 Table 5 1. Reaction d ata for different suppor ted catalysts, 20 wt% cobalt, P=20 bar, T=230 C, total flow rate= 62.5 ml/min (H 2 :CO:N 2 =6 :3:1) Reaction Data Support a Avg Conv. b CH 4 c CO 2 d C 2 C 4 e C 5+ f Productivity g Al 2 O 3 73% 12% 8% 9% 71% 51.8 n Al 2 O 3 41% 10% 1% 6% 83% 34.0 p SiO 2 34% 9% 8% 10% 73% 24.8 n SiO 2 29% 9% 4% 8% 79% 22.9 p ZrO 2 37% 9% 2% 8% 81% 30.0 n ZrO 2 34% 8% 1% 8% 82% 27.9 p TiO 2 2% 4% 12% 8% 74% 1.4 n TiO 2 14% 9% 0% 13% 78% 10.9 a Suppot: C atalyst support. b Avg Conv.: Average conversion. c CH 4 : Methane s electivity d CO 2 : CO 2 s electivity. e C 2 C 4 : Selectivity of hydr o carbons from carbon number 2 to 4 f C 5+ : Selectivity of hydrocarbons with carbon number larger than 5. g Productivit y : (c onversion*C 5+ )/100 standard deviation of 0.9 Figure 5 2. Temperature Programmed Reduction behavior for different supports with 20% c obalt
132 Table 5 2 Characterization data for cobalt catalysts with different supports. Characterization Data Support a Support BET Surface Area [m 2 /g] Catalyst BET Surface Area [m 2 /g] b Vol. of CO Ads. [L/g] c CO Metal Surface Area [m 2 /g] d Full Red. Area e [Area/g] Reduc Iblity f Vol. of O 2 Ads. [ml/g] g Reduc Iblity h Al 2 O 3 260 16 0 5 40 0.96 25.9 39.6 % 17 1 30.7 % n Al 2 O 3 695 90 120 0.21 27.2 39.2 % 11. 7 20.9 % p SiO 2 240 27 0 455 0.81 34.8 75.3 % 42.2 76.2 % n SiO 2 490 265 415 0.74 36.4 78.8% 46.8 84.4% p ZrO 2 100 35 675 1.2 0 26.2 88.7 % 38.0 68.6 % n ZrO 2 35 20 390 0.7 0 28.9 96.7 % 45.1 81.4 % p TiO 2 120 40 780 1.39 36.8 87.0 % 44.3 79.9 % n TiO 2 505 80 385 0.69 34.4 47.4 % 24.5 44.3 % a Support : Catalyst s upport used. b BET Surface Area [m 2 /g]: Overall surface area. c Vol. of CO Ads. [L/g]: Volume of CO adsorbed after catalyst is reduced at 400C. d CO Met al Surface Area [m 2 /g]: Surface a rea measured from CO adsorption. e Full Red. Area: (Area of TPR peak for full reduction per gram of catalyst)/100000. f Reducibility: TPR ar ea for catalyst reduced at 400 C compared to full TPR area. g Vol. of O 2 Ads. [ ml/g]: Volume of O 2 adsorbed after reduction at 400 C. h Reducibility: Volume of O 2 used to re oxidize surface relative to theoretical amount of O 2 to oxidize all cobalt. Figure 5 3. XRD of TiO 2 supported catalysts A ) f ull scan s B ) h igh resolution s can s.
133 Figure 5 4. XRD of SiO 2 supported catalysts, A ) f ull scan s B ) h igh resolution scan s. Figure 5 5. XRD of ZrO 2 supported catalysts A ) f ull sca ns B ) hi gh resolution scan s.
134 Figure 5 6 XRD of Al 2 O 3 supported catalysts A ) f ull scan s B ) high res olution scans Table 5 3 Particle sizes calculated from the XRD data using the Scherrer Equation for fresh catalysts using different supports with 20% c obalt loading Fresh Catalysts: Reduced Catalysts: Support Average Co 3 O 4 (nm) a Average Co (nm) b Av erage D(%) c Average Co (nm) d Average D(%) e p ZrO 2 16.6 12.4 7.7 6.9 14.0 n ZrO 2 28.5 21.3 4.5 17.1 5.6 p TiO 2 21.8 16.4 5.9 12.7 7.6 n TiO 2 15.0 11.2 8.6 16.2 5.9 y Al 2 O 3 11.5 8.6 11.1 4.1 23.7 n Al 2 O 3 18.5 13.8 6.9 3.2 30.0 p SiO 2 15.1 11.3 8.5 10. 5 9.1 n SiO 2 26.3 19.7 4.9 17.1 5.6 a Calculated Co 3 O 4 particle size using Scherrer Equation from average of f ull and high resolution scans. b Calculated cobalt metal particle size from Co 3 O 4 particle size d(Co)=0.75d(Co 3 O 4 ). c Dispersion calculated from d( Co) particle size D=96/d(Co) d Calculated Co particle size using Scherrer Equation from average of full and high resolution scans.
135 CHAPTER 6 KINETICS OF A FISCHE R TROPSCH CATALYST Background The use of cobalt based catalysts for Fischer Tropsch synthesis h as been a heavily researched area [52, 64, 146, 151, 266 277] The understanding of the kinetics that drive s the reaction is important and the studies have proposed many different expressions to descri be the kinetics of the reaction under a wide range of conditions In general, t he Fischer Tropsch process over cobalt catalysts produces mainly n alkanes via the approximate reaction stoichiometry in Equation 6 1. = 1 65 kJ/mol (per CO mole) (6 1) In the following work the rate of the Fischer Trospch reaction is defined as the moles of carbon monoxide converted per time per mass of unreduced catalyst. This rate can easily be converted to rate of carbon monoxide plus hydrogen by a constant, as seen in Equation 6 1. Tab le 6 1 represents some of the kinetic expressions that have been used in literature. The simplest expression is a power law equation Equation 6 2. ( 6 2 ) The results from this expression typically lead to a negative value for c and a positive value for b This suggest s inhibition by adsorbed CO. One of the first equations, Equation 6 2 .1 (from Table 6 2, all equations in table are listed as 6 2.XX) was developed based on a ThO 2 and MgO promoted catalyst and over a short temperature range. Equation 6 2 .4 was developed by postulating the reaction rate is proportional to desorption of the hydrocarbon chains, [ 266 ]. This
136 theory does not work at high values of possibly due to h ot spots [ 266 ]. However Equation s 6 2.1 and 6 2 .4 do not describe the kinetics over the full range of inlet H 2 /C O ratios Equation 6 2 .5 is a result of examining the possible rate determining steps. It was found that the rate limiting step is CO dissocia CH 2 surface intermediate. Equations 6 2 .6 and 6 2. 7 come from two separate mechanisms representing the same data. Both mechanisms assume a bimolecular surface reaction with one stat ing that the CO molecule dissociates and reacts, the other postulates that CO is adsorbed but does not dissociate. The data is regressed non linearly and the residual sum of squares an d the two equations (6 2.6 and 6 2 .7) in T able 6 2 fit best [ 266 ]. The carbide insertion theory led to the deve lopment of Equation 6 2 .9 and it fit s well to data obtained from a Co/Zr/SiO 2 catalyst [ 233 ]. The activation energy for the rate constant in Equation 6 2 .9 was found to be 97kJ/mol. Yates and Satterfield simplified their approach by limiting the rate ex pression to two adjustable parameters  These two parameters represent a surface rate constant and an adsorption coefficient, from Langmuir Hinshelwood kinetics [ 266 ]. It was concluded that Equation 6 2 .8 was the best fit for their data at 240C. T he estimated activation energies for k (rate constant) and b (adsorption coefficient) were 92.7 and 94.5 kJ/mol respectively Ma et al. worked on kinetic parameters based on water content, particle size, pore size, and cobalt content  The decrease i n cobalt cluster led to a more active catalyst with a higher cobalt surface density. The smaller particles sizes were more sensitive to water. Finally the rate express ion is described as Equation 6 2 .12, with a water effect constant m. The values of a an d b were support dependent a and a / b were
137 higher for Al 2 O 3 supported catalysts versus SiO 2 supported catalysts, suggesting a greater inhibition by adsorbed CO [ 52 ] An investigation 29 different rate expressions for a TiO 2 supported c obalt ma n ganese cata lyst show that two expressions fit the data both derived from the Langmuir Hinshelwood Houngen Watson (LHHW) mechanism [ 151 ]. The equations (6 2.13 and 6 2 .14) were fit using a non linear regression method using the Levenberg Marquardt algorithm A study of an Al 2 O 3 supported cobalt ruthenium catalyst compares models including a strong inhibition term for CO, one empirical rate law and four LHHW variations. The apparent activation energies were found to be between 94 103 kJ/mol. The rate law that appear ed to fit the data best is Equation 6 2 .15 [ 64 ] A kinetic analysis of a titania supported catalyst led to a simple power law expression that fit wel l ( Equation 6 2 .16) However, the best fit was a simple Langmuir Hinshelwood rate Equation ( Equation 6 2 .1 7) [ 146 ]. Most current models use rate expressions based on Langmuir Hinshelwood or Eley Rideal mechanisms or power law expressions. In these models the dependence on c arbon m onoxide is negative meaning the adsorption of CO is strong at high concentration s and can inhibit the reaction. Many of the models for Co show x arbon m onoxide in the denominator squared which shows a Langmuir Hinshelwood model [ 6 ] In this study a qualitative relationship between the reaction conditions and the reaction rates are first established. Then the reaction data is fit to several literature rate expressions to determine the best fit equation to represent the activity of the particular novel catalyst in this study.
138 Results and Discussion In this study, experiments were p erformed in a fixed bed reactor. The equipment setup is described in Chapter 2 (Reactor System). The catalyst used is a Co/Ru/ZrO 2 on nanoparticle SiO 2 This is a novel catalyst prepared by incipient wetness impregnation with a precalcined n SiO 2 suppor t and calcinations after each impregnation step. Also the ruthenium was impregnated after the cobalt deposition The composition of the catalyst is 20 wt% Co, 2 wt% ZrO 2 0.63 wt% Ru, with the remainder being n SiO 2 The preparation, reactor loading, a nd reaction conditions are descr i bed in Chapter 2 (Fischer Tropsch Synthesis). The ranges of conditions tested are: P=10 30 bar, T=180 245 C, H 2 /CO ratio=0.5 15, total flow rates of 62.5 125ml/min and the addition of CO 2 Effect of Temperature The effect of temperature on the rate of reaction for the Co/Ru/ZrO 2 /n SiO 2 catalyst is shown in Table 6 3 for the range of 180C to 245C (Figure 6 1) Since there is a small initial activity peak and decline right after the temperature is reached for these r eacti ons, the maximum and average were noted. The maximum is usually in the first few hours and then it declines to a steady state which is noted by the average after the spike The conversion increase d slowly from 3 to 8% in the temparture range from 180 200 C However, as soon as the temperature reaches 210C the conversion spikes to 58% and levels out at 20%. The conversion appears to maintain around 25 30% conversion until the temperature reaches 23 5 C. At this temperature there is again a large increase in conversion to 45% and the average conversion at this temperature is around 40%. Above this temperature the conversion appears to start to level off just
139 above 50% conversion. At temperatures above 245C significant deactivation starts to occur. Th e C 5+ selectivity of the catalyst is ve ry high at the low conversions and low temperatures. Interestingly after the initial drop in selectivity from 86% 74%, the selectivity is steady, between 75 82% all the way up to 240C. The methane selectivity fluct uates around 10% up to 240C also. Once the temperature is raised above 240C the catalyst starts to produce significant amounts of methane and carbon dioxide. Effect of H 2 /CO Ratio In Table 6 4 the results for average conversions and selectivities for different H 2 : CO ratios are tabulated. The carbon monoxide conversion is very low at low H 2 : CO ratios (Figure 6 2 ) It slowly increases from 2% to 7% as the ratio is raised 0.5 to 3. Once the ratio is raised above the 3 the conversion jumps to 13%. The co nversion increases dramatically to almost 100% at the 15:1 H 2 :CO ratio. At the low H 2 :CO ratios the selectivity is very high for C 5+ hydrocarbons, as expected. However, as the H 2 :CO is increased the C 5+ selectivity decreases. Therefore while the CO con version is very high at high H 2 :CO ratios the excess hydrogen forms mainly methane. Effect of Total Flowrate The effect of changing the flowrate at different H 2 :CO ratios is shown in Figure 6 3 At the lower H 2 : CO ratios the effect is stronger. However at a ratio of 5 : 1 it appears the conversion is almost the same for both flowrates. It appears that the lower space velocity is detrimental at lower H 2 /CO flowrates. This could be due to limited hydrogen which at low flow rates may cause carbon depositio n
140 Effect of Pressure From Figure 6 4 it is clear that the effect of pressure on carbon monoxide conversion is minimal under the conditions of the experiment The conversion is around 5.5% at all three pressures slightly higher for a pressure of 20 bar The higher pressure is known to lead to heavier hydrocarbons; however it is surprising that dropping the pressure to 10 bar does not significantly lower the C 5+ selectivity. Effect of Carbon Dioxide in the Feed The addition of carbon dioxide to the fee d had a minimal effect on the activity of the catalyst (Figure 6 5 ) There appears to be an insignificant decrease in conversion when the CO 2 is added. However, this decrease is essentially the same regardless of the amount of CO 2 added. It appears that CO 2 in the feed does not inhibit Fischer Tropsch synthesis for this catalyst. Rate Equation Fit of Data In an attempt to simplify the rate equation it is assumed the reactor is a differential reactor. Each of the rate laws shown in the table, except for those using a water dependence, where examined. They were compared using a MATLAB minimum search function to fit the parameters to the data. The results show that a simple power law expression fits the data reasonably well (Figure 6 6). (6 3) This equation shows a relatively weak dependence on CO, but it appears as an inhibitor. Equation 6 2.19 fits the data; however the adsorption coefficient b dominates the equation and is not reasonable relative to known values for CO ads orption on cobalt For example the activation energy is 1600 kJ/mol, which cannot be representative of carbon monoxide adsorption. Similar results from other equations,
141 showing unreasonable values, were seen. To combat this, the equations with unreason able solutions were constrained to more reasonable values for the activation energies of the reaction rate constant and the adsorption coefficient These constraints were only useful in cases which show the unrealistic values, as the other cases would giv e the same results whether constrained or unconstrained. The constraints used were 40000< Ea (reaction rate constant)<150000 and 200000< Eb (adsorption coefficient)< 50000. In all cases where the equations are constrained the results all show the maximum err or no matter the values input for the energies (Table 6 5) This indicates that these rate equations will not fit the data when constraining certain parameters to relevant values. The power law expression sufficiently fits the data and produces a reasonab le estimation of the activation energy that is similar to those from literature, 90.3kJ/mol. These result s suggest that the rate limiting step may be the addition of the H atom after dissocation of H 2 to the surface carbon. This is interesting because it suggests that the amount of hydrogen available actually drives the reaction, not the amount of carbon. Summary The kinetic effects of changing several different parameters on a nanoparticle silica supported cobalt catalyst promoted with ZrO 2 and Ru w ere evaluated. There is no major effect by changing the pre ssure with this catalyst. Increasing total flowrate decreases conversion without altering the selectivity significantly. When the ratio of hydrogen to carbon monoxide is modified the effects are significant with an obvious product distribution effect along with a change in activity. The condition of excess hydrogen leads to higher activity and lower selectivity. At lo wer hydrogen concentrations there is minimal conversion and higher selectivity The addition of
142 carbon dioxide in the feed appears to have a very minimal negative effect on the activity and selectivity. This indicates that having some carbon dioxide in the feed will not be a serious issue in the production of liquid fuels from bio mass derived synthesis gas The temperature effects are more complicated as the reaction rate appears to increase rapidly once the temperature reaches 230 C. The reaction data was well represent ed by a simple power law model T he best fit model is the power law expression Equation 6 3. The rate constant activation energy is estimated as 90.3kJ/mol, which is similar to previous reported values. The reaction is weakly dependent on CO, as an inhibitor. It is possible that the rate limiting step is the t ransfer of an H atom to the surface carbon. This suggests that the availability of the hydrogen is controlling the reaction rate.
143 Table 6 1. List of kinetic experiments in literature. Catalyst Reactor Type T (C) P (bar) H 2 /CO Ratio Ref. Co/ Mg/ThO 2 /kieselguhr fixed bed 185 200 1 2 267 Co/ThO 2 /kieselguhr fixed bed 186 207 1 0.9 3.5 268 Co/CuO/Al 2 O 3 fixed bed 235 270 1.7 55 1.0 3.0 269 Co/La 2 O 3 /Al 2 O 3 Berty (low c onv ) 215 4.9 8.0 2 2 70 Co/Al 2 O 3 fixed bed (low conv ) 250 1 0.2 4.0 271 Co/B/ Al 2 O 3 fixed bed (low conv ) 181 1.0 2.0 0.25 4.0 272 Co/kieselguhr Berty (low c onv ) 190 2.0 15.0 0.5 8.3 273 Co/kieselguhr Berty (low c onv ) 190 2.0 15.0 0.5 8.3 274 Co/MgO/SiO 2 s lurry 220 240 5.0 15.0 1.5 3.5 266 Co/ZrO 2 /SiO 2 slurry 240 260 233 Co /Al 2 O 3 fixed bed 210 235 8 25 1.8 2.7 275 Co/Al 2 O 3 & Co/SiO 2 CSTR 205 220 20.3 1 2.5 52 Co/MnO 2 /TiO 2 fixed bed 190 280 1 10 1 3 151 Co/Ru/ Al 2 O 3 slurry 200 240 20 35 1.0 2.5 64 Co/TiO 2 f ixed bed 200 10 2.0 146 Co/Ru/ Al 2 O 3 fixed bed 235 250 45 65 1. 7 2.3 276 Co/Pt/ Al 2 O 3 slurry 230 5 40 1.6 3.2 277
144 Table 6 2. List of kinetic experiments in literature, fit rate equations Eqn Ref Rate Equation 6 2.1 267 6 2.2 268 6 2.3 269 6 2.4 270 6 2.5 271 6 2.6 272 6 2.7 273 6 2.8 274 6 2.9 266 6 2.10 233 6 2.11 275 Elementary rate equations for each species 6 2.12 52 6 2.13 151 6 2.14 64 6 2.15 146 6 2.16 276 6 2.17 277 6 2.18 267 6 2.19 268 6 2.20 269
145 Table 6 3 Conversion and selectivity at different t emperatures for 20 /Co/ ZrO 2 /Ru/ n SiO 2 Th e maximum and average at t emperatures from 180 245 C. P=20 bar, t otal f lowrate=62.5ml/min, H 2 :CO=2:1. Conversion (%) Selectivity (%) Temp Max Average C 5+ C 1 CO 2 180 C 7.5 3.3 8 6.0 8.5 0.0 190 C 9.9 5.2 74. 4 12.5 0.0 200 C 9.4 8.3 73.8 12. 1 0.0 210 C 57.7 25.0 81. 7 8.5 0.2 220 C 34.1 29.5 75. 8 1 2.4 0. 9 225 C 31.6 29.9 80. 8 9.8 0. 6 230 C 33.7 32.7 74. 7 12.8 1.2 235 C 45.1 40.5 76.8 11. 3 1.2 240 C 59.0 51.9 66. 5 15. 8 5.2 245 C 60.01 52.3 58. 4 17.2 11.1 Table 6 4 Conversion v alues at d ifferent H 2 /CO r atios for 20/Co/ZrO 2 /Ru/n SiO 2 at T=205 C t otal f lowrate=62.5ml/min, P=20 bar. Ratio Conversion Selectivity H 2 :CO CO H 2 C 5+ C 1 CO 2 1:2 1.98 3.52 84.99 9.71 0 1:1 2.75 2.05 82.60 10.06 0 2:1 5. 10 3.57 73.74 11.75 0 3:1 7.13 3.71 62.44 20.68 0 5:1 13.66 5.81 54.68 27.45 0 10:1 56.05 12.60 4 0.12 39.97 0 15:1 97.13 15.38 9.86 74.46 0
146 Figure 6 1. The e ffect of t emperature on CO c onversion, c atalyst: Co/ZrO 2 /Ru/n SiO 2 P=20bar, F tot =62.5 ml/min, T=205 C, A ) CO conversion B ) C 5+ selectivity.
147 Figure 6 2 The e ffect of H 2 /CO r atios on C O c onversion c atalyst: Co/ ZrO 2 /Ru/n SiO 2 P=20bar, F tot =62.5 ml/min, T=205 C. Figure 6 3 The e ffect of t otal f lowrate on CO c onversion d ata, c atalyst: Co/ZrO 2 /Ru/n SiO 2 P=20bar, F tot =62.5 ml/min, T=205 C.
148 Figure 6 4 The e ffect of t otal p ressu re on CO c onversion d ata c atalyst: Co/ZrO 2 /Ru/n SiO 2 P=20bar, F tot =62.5 ml/min, T=205 C. Figure 6 5 The e ffect of t otal CO 2 c atalyst: Co/ZrO 2 /Ru/n SiO 2 P=20bar, F tot =62.5 ml/min, T=205 C.
149 Table 6 5 Fit r ate e quations Rate Constant Activatio n Energies Power Dependence Eqn a Error b Ea c Ea b d Ea c e Ea d f CO g H 2 h 6 1.1 82.6 157 .0 6 1.2 32. 1 150 1 67 3 6 1.3 37. 4 107 .2 6 1.4 29.0 94 .9 6 1.5 40.5 8 7.8 460 00 6 1.6 34.2 103 .3 6 1.7 59.0 2. 3 3.2 5. 1 1. 8 6 1.8 3 8.3 24 .8 32 .8 133 .4 6 1.9 26.3 41 .0 45 4 6 1.13 30.2 83 .0 45 .9 6 1.14 23.4 27 .7 1000 772 5 6 1.15 22.4 31 .8 82 .2 6 1.15* 407 40.0 150.0 6 1.16 28.5 93 .5 6 1.17 22. 8 20 .6 7 3.0 6 1.17 407 90.0 40.0 6 1.18 71.8 41 .7 21 .3 0.8 8 1.8 7 6 1.19 10.3 6 5.0 1600 0.05 1.2 2 6 1.19* 407 150 198 0.05 1.2 2 6 1.20 26. 1 43 .4 39 .7 65 .8 6 1.20* 407 90.0 150.0 65.8 Power Law 27. 8 90 .3 0.17 0.54 a Eqn.: Equation as described in Table 6 2. b Error: Sum of Squared Error when comparing to all runs. c Ea: Activation Energy of reaction rate constant k in all equations d E a b: Activation Energy of reaction rate constant b in equations which it is used. e E a c: Activation Energy of reaction rate con stant c in equations which it is used. f E a d: Activation Energy of reaction rate constant d in equations which it is used. g CO: Power dependence on the pressure of CO. g H 2 : Power dependence on the pressure of H 2 *Constrained results: In all cases the constrained equations converge to the same Error value no matter the energies used.
150 Figure 6 6 Reaction rate data for Co/Zr O 2 /Ru/n SiO 2 catalyst vs. Equation 6 2 (Power Law Model).
151 CHAPTER 7 CONCLUSION S In the work above the use of nanoparticle o xides of supports for the Fischer Tropsch process was investigated. The use of nanoparticle oxides brings about unique interactions between the support and the active metal. Also the nanoparticle oxides typically have much larger surface areas. The use of nanoparticle SiO 2 was compared to porous SiO 2 as supports for cobalt based Fischer Tropsch catalysts. It was found that the porous SiO 2 supported performed slightly better than the nanoparticle SiO 2 supported catalysts. However the effects were less pronounced when ZrO 2 was added to the catalyst. Also, pre treating the support and using a lower quality cobalt precursor appeared to have minimal effects on the catalysts. The addition of zirconia to the catalyst had a positive effect on both activity a nd selectivity. The use of other additives such as Ru, Cu, and CeO 2 appeared to promote the productivity of the catalysts. Typically this is due to an increase in reducibility metal surface area and/or a decrease in Co particle sizes Some other addit ives such as La 2 O 3 and MgO lowered productivity due to a decrease in the reducibility of the catalyst. Re and Fe addition lowered productivity due to the fact the catalysts had extremely low metal surface area s The use of Pd and CaO negatively affect pr oduction of hydrocarbons over these catalysts While reducibility of these catalysts appear to be higher than the unpromoted case, the XRD patterns indicate that these catalysts have larger cobalt particle size s are not completely reduced or reveal forma tion of inactive mixed oxides during reaction. One study included the comparison between cobalt on typical Fischer Tropsch catalyst supports and nanoparticle supported catalysts The four supports used were Al 2 O 3 SiO 2 TiO 2 and ZrO 2 and for ea ch one both a porous and a
152 nanoparticle oxide are included The TiO 2 support ed catalysts have the lowest productivit ies of the catalysts under investigation The n TiO 2 supported catalyst gives higher productivity than the p TiO 2 supported catalyst despi te having lower reducibility and metal surface area. From literature it appears that TiO 2 can effectively block Co active sites by interacting with the active metal Both SiO 2 supported catalysts show similar catalytic properties, with minimal difference s between the nanoparticle and porous supports. However the cobalt particle sizes are larger for the n SiO 2 support. The ZrO 2 supported catalysts are more active than the SiO 2 supported catalysts, and while the initial activities are similar on the two Z rO 2 supported catalysts, the Co/p ZrO 2 is slightly more stable. The p ZrO 2 supported catalyst had lower cobalt particle sizes, and thus a higher metal surface area, but exhibited a slightly lower reducibility. The highest activity was observed over the A l 2 O 3 supported catalysts, despite their low reducibilities. Of the two catalysts, the cobalt supported on Al 2 O 3 exhibited the higher productivity, which may be due to the higher metal surface area compared to the nanoparticle support ed catalyst Howeve r the Co/n Al 2 O 3 is the second most active catalyst and it is more stable than the Co/ Al 2 O 3 Under the conditions used in this study the activities of the supported cobalt catalysts are strongly depend ent on the support and decrease in the following ord er : Al 2 O 3 >ZrO 2 >SiO 2 >TiO 2 The kinetic evaluation of a zirconia and ruthenium promoted cobalt catalyst supported on nanoparticle silica led to a simple power law rate equation This rate equation shows a negative dependence on CO, meaning it is an inhibi tor and the hydrogen dependence is about 0.5. This also suggests the rate limiting step may be the
153 insertion of a H atom to the surface carbon. The apparent activation energy is 90kJ/mol, which falls in line with literature values. The Fischer Tropsch reaction over supported catalysts is clearly complex and the use of nanoparticle oxides as catalysts warrants further study. This project has revealed that nanoparticles can be used as catalyst supports without resulting in too strong cobalt support inte ractions or unstable catalysts which would limit their viability. In fact, the results indicate that the nanoparticle alumina support is promising and investigating how promoters behave or enhance the productivity of this and/or ZrO 2 supports would be int eresting. In the case of nanoparticle alumina, previous research has shown that zinc aluminate forms easily on this support  and this mixed oxide could have interesting properties for use as a support for cobalt in a F T catalyst as this would decrea se the active metal support interactions. An in depth investigation into the deactivation behavior can also increase the understanding of these catalysts. There are a number of modifications that could be done to the preparation and pretreatment methods that could affect the catalyst properties, such as cobalt precursors (other than cobalt nitrate, impregnation solvent, impregnation sequence, calcination procedures, and catalyst preparation technique (precipitation, wet impregnation, etc.). Also, work ca n be done on deriving a better relationship between catalyst properties and the F T productivity through more detailed characterization techniques and analysis. Finally an investigation into depositing the nanoparticle oxide supported catalysts onto foams or monoliths would be very interesting.
154 APPENDIX A EFFECTS OF CALCINATI ONS AND MULTIPLE IMP REGNATIONS Background In the synthesis of Fischer Tropsch catalysts many different preparation steps are crucial to the behavior of the catalysts An importan t part of the preparation process is the deposition of the active metal on the suppor t surface. The typical process is incipient wetness impregnation. However, the impregnation can be done in one step or multiple steps. In the three step impregnation me thod the catalysts can also be calcined in between impregnations. Also the treatment of the support prior to This treatment is typically in the form of high temperature calcination and can range fr om 300 700 C. Results and Discussion An alternative preparation method was investigated to determine if an improved catalyst could be obtained. The method involve s calcining the support at higher temperatures up to 700 C instead of 400 C prior to impregn ation. Another difference between the two methods is that instead of a one step cobalt impregnation with final calcination at 350 C the alternative method uses a three step impregnation process with calcinations at 370C in between each impregnation. Eac h catalyst is prepared by incipient wetness impregnation with a cobalt metal loading of 20% Fischer Tropsch Synthesis Two catalysts were prepared with SiO 2 supports, both nanoparticle and porous, by multiple impreg nations to a total cobalt loading of 20% t These catalysts were compared to the catalysts made with one impregnation of 20% cobalt. From the Figures below (Figure A 1) it is evident that there is an increase in the catalytic activity,
155 as well as a slight increase in selectivity T herefore, t he productivity is slightly higher for the three step impregnation (Table A 1) Interestingly the CO 2 production is lower on the n SiO 2 supported catalysts. Catalyst Charac t erization The catalysts were subjected to catalyst characterizations using a numb er of different techniques to measure surface areas, determine reduction oxidation (redox) properties, as well as morphological properties of the catalysts. BET Surface Area Measurements It appears that the surface area of the nanoparticle supported cata lyst was decreased by the three step impregnation process (Table A 2) Whereas the p SiO 2 supported catalysts had almost t he same overall surface areas. An investigation of the surface areas of the supports calcined at different temperatures show that ca lcining at the higher temperature did not affect the surface area of the support for the nanoparticle supported catalysts (Table A 3) Carbon Monoxide Pulse Titration For both the p SiO 2 and n SiO 2 supported catalyst there is a slight increase in t he amou nt of CO chemisorbed on these catalysts (Table A 2) However, the increase is not sufficient to warrant a three step preparation procedure. As has been observed previously, the amount of CO adsorbed on the p SiO 2 supported catalysts is hig her than that a dsorbed on the Co / n SiO 2 catalysts. Temperature Programmed Reduction Analysis The temperature programmed reduction behavior of the catalysts is shown in Figure A 2. Th e data reveals that multiple impregnations increases the temperature at which reductio n is complete This effect is more pronounced on the Co/p SiO2 catalyst
156 and it appears that the multiple impregnations increase the cobalt support interactions on this catalyst. Oxygen Pulse Titration The oxygen titration experiments confirm that the orig inal Co/ n SiO 2 has the highest reducibilit y, and that multiple impregnations on this support result in a catalyst that is more difficult to reduce than the Co/n SiO 2 catalysts. For the n SiO 2 supported catalysts the multiple impregnations do not affect re ducibility. Discussion In these cases it appears that the pretreatment at higher temperatures and the multiple impregnation method are not significantly b eneficial. Using a pretreatment at 400C is sufficient and raising it to 700C does not seem to imp rove the performance of the catalyst as shown by the surface area data for the nanoparticle supported catalysts and the lack of a positive effect on the metal support interactions The metal surface areas are increased slightly, but t his does not warrant the use of a three step preparation In fact the reduction and overall surface area appeared to be negatively affected by these modifications to the preparation technique.
157 A B Figur e A 1 Reaction behavior for different impregnation procedures wit h 20% c obalt at P=20bar, T=230C, m cat =1g, F tot =125ml/min(H 2 :CO:N 2 =6:3:1) n= n anop article support, p= p orous support, *=three step impregnation method A ) c arbon m onoxide c onversion B ) C 5 + s electivity.
158 Table A 1 Reaction data for cobalt catalysts with diffe rent preparation techniques. Reaction Data Support a # of Imp. b Avg Conv. c CH 4 d CO 2 e C 2 C 4 f C 5+ g Productivity h p Si O2 3 21 % 15 % 1 % 14 % 69 % 14.5 n Si O2 3 19 % 15 % 0 % 11 % 74 % 14.1 p S iO2 1 18 % 17 % 1 % 15 % 67 % 12.1 n S iO2 1 16 % 17 % 0% 13 % 70 % 11.2 a Suppot: Catalyst support. b # of Imp.: Number of impregnations c Avg Conv.: Average conversion. d CH 4 : Methane s electivity e CO 2 : CO2 s electivity. f C 2 C 5 : Selectivity of hydr o carbons from carbon number 2 to 5 g C 5+ : Selectivity of hydrocarbons w ith carbon number larger than 5. h Productivity: ( c onversion*C 5+ )/100. Figure A 2 Temperature programmed reduction of catalysts prepared by different methods. n= nanoprarticle support, p=p orous support, (3) =three step impregnation method (1)= o ne step impregnation method
159 Table A 2. Char a cterization data for cobalt catalysts with different preparation techniques. Support a # of Imp b BET Surface Area c [m 2 / g] Vol. of CO Ads d Metal Surface Area e [m 2 /g ] Reduc Ibility f Full Red. Area g [Area/ g] Vol. of O 2 Ads. h [ml/g] Reduc i Ibility i n SiO 2 1 235 481 0. 86 85. 8 % 43.7 44.4 80.1 % p SiO 2 1 155 777 1. 38 83.5 % 33.3 41.6 75.0 % n SiO 2 3 180 587 1.04 92.5% 31.9 42.2 73.8% p SiO 2 3 160 1009 1. 80 87.0% 34.7 40.9 76.2% a Support : Catalyst s upport used. b # of Imp.: Number of impregnations. c BET Surface Area [m 2 /g]: Overall surface area. d Vol. of CO Ads. [L/g]: Volume of CO adsorbed after catalyst is reduced at 400C. e CO Metal Surface Area [m 2 /g]: Surface a rea measured from CO adsorption. f Reducibility: TPR Area for catalyst reduced at 400 C compared to full TPR area. g Full Red. Area [Area/g]: The total area of the reduction peaks when the catalyst is fully reduced up to 1000C/100000. h Vol. of O 2 Ads. [ml/g]: Volume of O 2 adsorbed after reduction at 400 C. i Table A 3. BET surface area comparison of nanoparticle SiO 2 supports calcined at different temperatures. Support Surface Area [m 2 /g] dried n SiO 2 370 350 C calcined n SiO 2 310 600C calcined n SiO 2 370 700C calcined n SiO 2 375
160 APPENDIX B DEACTIVATION BEHAVIO R Background As most catalysts exhibit some type of slow, continuous deactivation during the time on stream, it was decided to investigate this in detail. While t he catalyst s are expected to deactivate over time it is important to slow down this process and increase the life time of the catalysts Therefore, determining the cause of th e deactivation is very important in the design of a longer lasting more active catalyst. A review of the different mechanisms of deactivation for Fischer Trospch catalysts states several possible mechanisms [89, 99, 116, 212, 278 282] The first method is poisoning mainly by nitrogenated compounds or sulfur. This method can be reversible using mild in situ H 2 treatment to regen erate the catalysts. Some severe cases are irreversible therefore it is necessary to keep sulfur and nitrogenated compounds out of the system. It is also noted that the presence of alkali metals in certain high concentrations can work against the product ivity of the catalyst [ 278 ]. Another mechanism is sintering which is affected by the mobility of the crystals on various supports. Sintering is caused by surface energy minimization of the crystallites leading to a decrease in surface area [ 278 ]. Carbo naceous species can also form on the surface and form inactive species. It can be due to polymeric carbon, graphitic carbon, refractory carbon or other carbonaceous species. There are several different deactivation mechanisms that can be cause d by carbon covering the surface Eliason et al. investigated the reaction and deactivation kinetics on an iron based catalyst [ 212 ]. Moodley et al. studied the amount of carbon deposits on the surface of a catalyst that was used under commercially
161 relevant conditi ons [ 279 ]. They used temperature programmed hydrogenation and oxidation, energy filtered transmission electron microscopy, high sensitivity low energy ion scattering and hydrogen chemisorption to examine these carbonaceous species on the surface. These t echniques showed an increase of carbon on the surface over reaction time and it is on both the alumina support and the cobalt. It is suggested that at least some the long term deactivation is due to carbon deposition. The presence of water can lead to re oxidation of the cobalt to deactivate the catalyst In previous work by van Berge et al. oxidation of the cobalt was researched as a possible cause of deactivation in Fischer Tropsch catalysts  It was proposed that in a slurry phase batch reac tor the presence of water allows for the oxidation of cobalt creating inactive sites. It was found that there is a mixed production of reducible and non reducible sites formed by the oxidation of cobalt from the production of water during the Fischer Trop sch reaction [ 280 ]. Saib et al. looked at the effect oxidation on wax coated samples using XANES to show that cobalt crystallites of 6nm size supported by alumina were stable against oxidation [ 281 ]. The presence of high pressure steam is a suggested c ause for deactivation behavior. In one case it is suggested the presence of water causes the SiO 2 supported catalyst to lose a significant amount of surface area and H 2 chemisorption ability [ 99 ]. It is also noticed by Huber et al. that the deactivation is irreversible  There is also some evidence of an increase in the amount of cobalt silicate or hardly reducible sites in the presence of steam [ 116 ]. The deactivation appears to be slower at lower conversions relative to a high rate of deactivatio n at high conversions. This is most likely due to the limited amount of steam produced at lower conversions.
162 In the case of alumina supported cobalt catalyst the effect of water showed a deactivation behavior. This behavior was enhanced by the additi on of Rhenium. A strongly interacting phase was present in both Rhenium promoted and un promoted cases, however there was another oxide phase present as well [ 89 ]. Re oxidation can occur in other ways as well. However, the oxidation of the catalyst as d eactivation mechanism has seen some opposition. It was shown that it cannot be the major deactivation mechanism on several different supports despite the literature dealing with oxidation as a deactivation mechanism [ 216 ]. Finally attrition is another d eactivation mechanism that may occur. The Fischer Tropsch reaction can have severe effects on the catalyst and can cause fragmentation or abrasion o f the catalyst particles [ 278 ]. Saib et al. found that oxidation is not the main mechanism for deactivatio n for Fischer Tropsch Synthesis for supported Co catalysts with crystallite size in excess of 2 nm. Three mechanisms were found to deactivat e the catalyst, sintering of Co active phase, carbon deposition and surface reconstruction. In response to findin g a three step regeneration procedure was implemented involving de waxing, oxidation, and reduction. It is proposed that this reverses sintering, carbon deposition and surface reconstruction [ 282 ]. As mentioned there are many different possible mecha nisms for deactivation for Fischer Tropsch catalysts. The previous work has mostly been on Fe based catalysts. The work done on cobalt based catalysts has not focused on nanoparticle SiO 2 The deactivation mechanism for cobalt based nanoparticle SiO 2 su pported catalysts is investigated in some detail. Also the regeneration or rejuvenation techniques are also approached to try to regain the activity of the catalyst.
163 Results Characterization XRD on spent c atalysts While some catalysts displayed evide nt signs of compound formation (see for example 99PrNpPC Figure 3 9 and Co/CaO/n SiO2 Figure 4 8 ), in most cases, the XRD data did not reveal any signs of mixed cobalt oxides after exposure to the reaction conditions (a typical example is shown in Figure B 1). In a few cases t here is evidence of cobalt oxide, CoO present in the catalyst after reaction (Figure B 1). Since XRD is a bulk sensitive technique which relies on long term order (crystalline compounds with crystal domains larger than 1 2 nm), it is possible that some surface oxidation and some amorphous mixed metal oxides are formed. However, these deactivation pathways are not sufficiently severe to be detectable with XRD. BET surface a rea l oss The spent catalyst s from a few runs were investiga ted and compared to the same fresh catalyst. The data reveal that the catalyst l ose s a significant amount of its surface area (Table B 1) A fter close to 150 hours on stream the catalyst has lost more than half of its original surface area Continued o peration decrease s the surface area further and after more than 400 hours on stream the surface area is only 15% of its original value. This behavior can be due to several deactivation mechanisms including blockage of sites by the presence of wax or heavy hydrocarbons, formation of irred ucible cobalt silicates, or sintering particle growth Regeneration Attempts To investigate the deactivation pathways further, several regeneration treatments were investigated. If the catalysts are deactivating due to a slow oxidation to CoO, a simple reduction treatment should recover most of the activity. This treatment is not
16 4 expected to be very efficient to remove heavy hydrocarbons blocking the active sites. An oxidative treatment at elevated temperatures would be better to remove hydrocarbons from the surface. No regeneration method is available for deactivation due to the formation of inactive mixed metal oxides, as the catalysts are destroyed by severe sintering at the temperatures needed for reduction of these oxides. The catalyst was initially run at the normal conditions, 230C. After approximately 75 hours the temperature is raised to 240C for 75 hours then to 245C for 85 hours After 85 hours at 245C the catalyst is brought back to 230C to see th e decreas e in activity, i.e. the catalyst deactivation. The activity after those changes in temperatures had dropped from 40% to approximately 18% at 230C. After this there were several different attempts to regain the activity. The first attempt to re cover the activity involved using H 2 at the reaction temperature. This is to reduce any CoO species on the surface as well as to force hydrocarbons off the surface of our catalyst. Since this treatment failed to recover any activity the temperature was raised to the typical reduction conditions (400C) As expected, the use of H 2 to force hydrocarbons off does not work for regeneration of this catalyst and it does not appear that the The secon d attempt was to use O 2 to react carbonaceous species from the surface of the cobalt catalysts. There was some reaction occurring at reaction temperature (230C), but the amount of CO 2 exiting the reactor was very small. T he temperature was therefore inc reased to 350C (the typical calcination temperature) and the CO 2 coming out of the reactor was measureable under these conditions After the treatment the catalyst was subsequently reduced at 400C for 4 hours in pure H 2 at
165 100ml/min (normal reduction co nditions ) Some activity was recovered after these conditions, but the catalyst could not be brought back to its original activity. Therefore it appears that the deactivation of this catalyst after exposure to temperatures of 245C is due both to a revers ible carbon species blocking the active sites and an irreversible mixed metal oxide formation or cobalt metal sintering. Furthermore, the C 5+ Selectivity appeared to recover to the initial selectivity of the fresh catalyst although the CO 2 selectivity is significantly higher after regeneration treatments Discussion In an attempt to characterize the surface of a spent catalyst the cause for the deactivation behavior of the catalysts shown in this document was investigated. The results show that the mos t likely cause is the buildup of hydrocarbons on the surface of the active metal, blocking active sites. The results of XRD analysis showed no sign of cobalt silicate formation. Cobalt silicate formation is a common suggested reason for the disappearance of activity for silica supported cobalt catalysts. The formation is due to the presence of high pressure steam and is worse for high ly active catalysts The catalyst shows the high initial activity and then a decline which follows that explanation. How ever there is no sign of the formation of these inactive sites in the XRD of the spent catalyst. Surface area reduction can be explained by almost any deactivation mechanism presented. However the catalyst desorbed visible wax from a small amount of the spent catalyst when heated in H 2 Finally the attempts to regain activity were more telling. The reduction of the spent catalyst did not regain activity, therefore simple oxidation of the catalyst does not appear to be the cause of deactivation. Howeve r when treating the catalyst with oxygen at high temperatures some of the activity is regained. Therefore it appears a major cause for deactivation may be due to the
166 coverage of the active sites by hydrocarbons or carbon. However the actual cause for dea ctivation is most likely a mixture of a few mechanisms. The loss in surface area, appearance of wax when heating the spent catalyst, lack of the presence of silicates in XRD analysis, and the regeneration of activity after high temperature oxygen treatmen t suggests this conclusion.
167 Figure B 1. XRD spectra of 20% Co 2% ZrO2 78% p SiO2 freshly reduced, two spent catalysts and glass beads. Run 51, 1 gram of catalyst+1 gram of support, 424 hours at T=230C P=20 bar Ftot=125ml/min (H2/CO/N2=6/3/1). Run 78 1 gram of catalyst + 8 grams of glass beads (Vbed =8ml) 136 hours at T=230C P=20 bar Ftot=62.5ml/min (H2/CO/N2=6/3/1) The glass beads XRD pattern is included to show the pattern of the dilutent mixture in the Run 78 spent catalyst patterns. The Run 51 s pent spectra uses the support, porous SiO2, as the dilutent. Table B 1. Loss of surface area from r eaction. Hours on Stream Catalyst BET Surface Area [m 2 / g] Fresh 0 20/2/78 Co/ZrO 2 / p SiO 2 154 Spent 136 20/2/78 Co/ZrO 2 / p SiO 2 79 Spent 424 20 /2/78 Co/ZrO 2 / p SiO 2 20
168 A B
169 C Figure B 2 Co/Ru/ZrO 2 / nanoparticle SiO 2 1 gra m of catalyst and 8 grams of glass beads (V bed =8ml) 136 hours at T=230C P=20 bar F tot =62.5ml/min (H 2 /CO/N 2 =6/3/1). Different types of regeneration t echniques and t here results on the A ) Conversion and B ) C 5+ selectivity C ) CO 2 selectivity
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186 BIOGRAPHICAL SKETCH Robert James Colmyer was born in Pennsylvania in 1983. He was raised in Hamilton Township, New Jersey. He moved in 1996 to De laware and attended h igh s chool at Caravel Academy in Bear, Delaware. He graduated from Caravel Academy in 2001 and began attending undergraduate school at the Pennsylvania State University in State College, Pennsylvania. He graduated from Pennsylvania State University in 2005 wi th a Bachelor of Science in c hemical e ngineering. In 2005 he began attending the Villanova University for graduate school. He completed his thesis on the thermal decomposition of molecules at Villanova University in 2007 and graduat ed with a Master of Sc ience degree in c hemical e ngineering. In August of 2007 he attended the University of Florida to begin his pursu it of his Ph D in c hemical e ngineering.