Fischer-Tropsch synthesis reaction to produce sulphur free hydrocarbons has enjoyed a resurgent in interests due to increases in world oil prices. In this work, the suitability of Mo and Mo-W carbides has been investigated as a possible cost-effective alternative to noble metals in Fischer-Tropsch synthesis. The molybdenum and tungsten monometallic and bimetallic carbides were prepared through precipitation from homogeneous solution to the sulphide followed by carburization with a mixture of propane and hydrogen to produce the resulting metal carbide. A 23 factorial design strategy was employed to investigate the effect of three carburizing variables, namely, time, temperature and gas ratio on the resulting catalyst. In particular, the effect of supports was also examined through four common semiconductor oxide supports, namely: Al2O3, SiO2, TiO2 and ZrO2. Thermogravimetric analysis of the carburization reactions showed that the conversion from metal sulphide to the metal carbides is a multistep process producing different phases of carbides, namely ??-MoC1-x, ??-Mo2C, ?? -WC1-x and ??-W2C, depending on heating rate and temperature. The rate determining step of the carburising reaction is the diffusion of carbon atoms into the metal matrix, hence giving relatively low activation energy values. Statistical analysis of the factorial design revealed that all three carburizing variables affect the final physiochemical makeup of the catalyst. SEM analysis showed that the carbides are well dispersed on the surface of the support and catalyst particles produced are nanoparticles in the range of 25 to 220 nm depending on the support. Fischer-Tropsch activity test showed that monometallic molybdenum carbide is active under Fischer-Tropsch conditions while tungsten carbide is inactive for the conditions studied in this project. However, bimetallic carbide catalyst, consisting of the two mentioned metals gave overall higher reaction rates and decreased methane selectivity. Steady state analysis revealed that there are two active sites on the surface of molybdenum carbide catalyst resulting in two chain growth propagation values when analysed via the Anderson-Schulz-Flory kinetics. Overall, ZrO2 support appeared to be the most suitable support followed by SiO2, TiO2 and Al2O3. Finally, kinetic modelling of data showed that methanation and higher hydrocarbons formation path occurs via combination of the oxygenated intermediate and Eley-Rideal mechanism.

Fischer-Tropsch synthesis reaction to produce sulphur free hydrocarbons has enjoyed a resurgent in interests due to increases in world oil prices. In this work, the suitability of Mo and Mo-W carbides has been investigated as a possible cost-effective alternative to noble metals in Fischer-Tropsch synthesis. The molybdenum and tungsten monometallic and bimetallic carbides were prepared through precipitation from homogeneous solution to the sulphide followed by carburization with a mixture of propane and hydrogen to produce the resulting metal carbide. A 23 factorial design strategy was employed to investigate the effect of three carburizing variables, namely, time, temperature and gas ratio on the resulting catalyst. In particular, the effect of supports was also examined through four common semiconductor oxide supports, namely: Al2O3, SiO2, TiO2 and ZrO2. Thermogravimetric analysis of the carburization reactions showed that the conversion from metal sulphide to the metal carbides is a multistep process producing different phases of carbides, namely ??-MoC1-x, ??-Mo2C, ?? -WC1-x and ??-W2C, depending on heating rate and temperature. The rate determining step of the carburising reaction is the diffusion of carbon atoms into the metal matrix, hence giving relatively low activation energy values. Statistical analysis of the factorial design revealed that all three carburizing variables affect the final physiochemical makeup of the catalyst. SEM analysis showed that the carbides are well dispersed on the surface of the support and catalyst particles produced are nanoparticles in the range of 25 to 220 nm depending on the support. Fischer-Tropsch activity test showed that monometallic molybdenum carbide is active under Fischer-Tropsch conditions while tungsten carbide is inactive for the conditions studied in this project. However, bimetallic carbide catalyst, consisting of the two mentioned metals gave overall higher reaction rates and decreased methane selectivity. Steady state analysis revealed that there are two active sites on the surface of molybdenum carbide catalyst resulting in two chain growth propagation values when analysed via the Anderson-Schulz-Flory kinetics. Overall, ZrO2 support appeared to be the most suitable support followed by SiO2, TiO2 and Al2O3. Finally, kinetic modelling of data showed that methanation and higher hydrocarbons formation path occurs via combination of the oxygenated intermediate and Eley-Rideal mechanism.

Thesis (M.S.)--University of Wisconsin--Madison, 1983. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 129-132).

Thesis (MSc)--University of Stellenbosch, 2005. / Bibliography. Also available via the Internet.

This dissertation research resulted in the development of a Fe based catalyst with Co as a co catalyst, and Ru and ZnO as promoters. The role of Cu and K as promoters and the effect of SiO2 as an alternate support to gamma- Al2O3 were also investigated. A series of Fe-based catalysts for Fischer-Tropsch (F-T) synthesis were prepared. The different promoters were incorporated into the catalyst by impregnation The catalysts were characterized by several methods. The catalytic performance of these materials for F-T synthesis were investigated in a newly designed fixed bed reactor system in the gas phase. It should be noted that the three phase slurry bubble reactors systems are commercially preferred. The reaction conditions were varied for benchmarking the Fe-Zn-K/ gamma- Al2O3 catalyst and for the bimetallic Fe-Co-Zn/ gamma- Al2O3 catalyst and to identify optimal process parameters for further catalyst designs. The H2:CO ratio used in this study was 2. The newly designed catalysts showed significantly high activity towards CO conversion (>70 %), along with low selectivity towards CO2 (5 -15 %) and methane (ND - 3 %). The data show that varying the process conditions, it is possible to achieve narrow distribution of the liquid products. The results employing Fe-Zn-K catalysts showed that an increase in pressure increased the mean carbon chain length. In contrast, an increase in temperature resulted in a decline in the average carbon chain length. Increasing the feed flow rate, or in other words decreasing the residence time of the reactants and the intermediates, resulted in a decrease in the average carbon number in the product hydrocarbons. The evaluation of the effect of process conditions on the performance of Fe-Co-Zn catalysts revealed that the effect of pressure on the carbon chain length was reversed. Increasing the pressure from 250 to 350 psig decreased the carbon chain length. The increase in temperature, however, resulted in a decrease in the carbon chain length as observed in the Fe-Zn-K catalysts. Fe catalysts groups containing different proportions of Co were prepared. It was determined that an Fe:Co ratio of 4:1 is sufficient to obtain high CO conversions with a high selectivity towards liquid hydrocarbons. The hydrocarbon distribution on the other hand remained almost unchanged due to a change in the Co content. The use of silica, as opposed to alumina as the catalyst support, enhanced the CO conversion and the selectivity of the process towards liquid hydrocarbons. The methane and CO2 selectivities on both the supports remained unchanged. However, a significant difference in the liquid hydrocarbon distribution was observed. Addition of K to the catalyst resulted in a change in the liquid hydrocarbon distribution in that a slight increase in the heavier hydrocarbons was observed. A series of Fe4Co1Zn0.04 based catalysts for Fischer-Tropsch (F-T) synthesis, in which the different amounts of Ru are incorporated by the impregnation were also studied. The results showed the incorporation of Ru suppressed the CH4 formation at the cost of increasing the CO2 selectivity.

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Curso de Pós-Graduação em Físico-Química / Made available in DSpace on 2012-10-16T01:18:50Z (GMT). No. of bitstreams: 0Bitstream added on 2016-01-08T15:46:06Z : No. of bitstreams: 1 202998.pdf: 4722121 bytes, checksum: 2b154256d729c37d2fe8bf2aae06aee7 (MD5)

This dissertation research aimed at evaluating a fly ash zeolite (FAZ) catalyst support for use in heterogeneous catalytic processes. Gas phase Fischer-Tropsch Synthesis (FTS) over a fixed-bed of the prepared catalyst/FAZ support was identified as an appropriate process for evaluation, by comparison with commercial catalyst supports (silica, alumina, and 13X). Fly ash, obtained from the Wabash River Generating Station, was first characterized using XRD, SEM/EDS, particle size, and nitrogen sorption techniques. Then, a parametric study of a two-step alkali fusion/hydrothermal treatment process for converting fly ash to zeolite frameworks was performed by varying the alkali fusion agent, agent:flyash ratio, fusion temperature, fused ash/water solution, aging time, and crystallization time. The optimal conditions for each were determined to be NaOH, 1.4 g NaOH: 1 g fly ash, 550 °C, 200 g/L, 12 hours, and 48 hours. This robust process was applied to the fly ash to obtain a faujasitic zeolite structure with increased crystallinity (40 %) and surface area (434 m2/g). Following the modification of fly ash to FAZ, ion exchange of H+ for Na+ and cobalt incipient wetness impregnation were used to prepare a FTS catalyst. FTS was performed on the catalysts at 250 - 300 °C, 300 psi, and with a syngas ratio H2:CO = 2. The HFAZ catalyst support loaded with 11 wt% cobalt resulted in a 75 % carbon selectivity for C5 - C18 hydrocarbons, while methane and carbon dioxide were limited to 13 and 1 %, respectively. Catalyst characterization was performed by XRD, N2 sorption, TPR, and oxygen pulse titration to provide insight to the behavior of each catalyst. Overall, the HFAZ compared well with silica and 13X supports, and far exceeded the performance of the alumina support under the tested conditions. The successful completion of this research could add value to an underutilized waste product of coal combustion, in the form of catalyst supports in heterogeneous catalytic processes.

The focus of this research was to investigate the role of carbon nanotubes as active catalysts in the Fischer-Tropsch reaction to derive liquid fuels from synthesis gas. Carbon nanotubes (CNTs) have unique structural and mechanical properties that make them ideal catalyst supports, but they also exhibit catalytic potential as well. This study implored the use of multi-walled CNTs on different substrates and single-walled CNTs grown from various precursors to analyze the effectiveness of the CNTs in FT synthesis. Multi-walled nanotubes (MWNTs) were tested on two different substrates: alumina pellets and inconel. The MWNTs on the alumina substrate yielded nearly all alkane and alkene products, with very little aromatic products. The amount of converted syngas reached 97% but had a high liquid product selectivity to methane, at roughly 57%. The MWNTs on inconel substrate produced nearly 80% aromatic products in one stage of the experiment, while the other three stages produces almost all alkane products with little oxygenates. Much of the liquid product yield (upwards of 73%) was between C10 and C21, which is ideal for diesel fuel. Single-walled nanotubes (SWNTs) were also tested in the FTS. All of the SWNTs were tested under a series of 6 temperatures, 300psig, and a syngas ratio of 1:1. Iron, nickel, and cobalt, which have all been proven as effective FT catalysts, were tested in trace amounts with CNTs. Fe-SWNTs (ferrocene assisted SWNTs) yielded a product of 100% C7 and C8 carbon species at two of the temperatures while 3 of the temperatures held a combination of longer chained alkanes, of C18 and longer. However, the last temperature converted 100% of the feedgas into methane and CO2. The product selectivity to CH4 and CO2 posed a problem with the Fe-SWNTs catalyst, where in all temperatures the selectivity exceeded 80%. Ni-SWNTs (nickellocene assisted SWNTs) yielded slightly better results with a higher selectivity to C2-C7, but no selectivity to longer chained hydrocarbons. Co-SWNTs (cobaltocene assisted SWNTs) tested under the same parameters yielded similar results as the Fe-SWNTs, with a very high selectivity to CH4 and CO2. Only at temperatures of 300 and 250°C were there any selectivity to compounds other than CH4 and CO2, but less than 10% selectivity to those alkanes (C2+). The final experiment consisted of a catalyst prepared from a feed solution containing a mixture of ferrocene and nickellocene. The Fe+Ni-SWNT catalyst underwent the same conditions as the other SWNT catalysts, this combination yielded favorable results with over 98% conversion of syngas over all temperatures and a high selectivity to shorter chain length hydrocarbons, namely alkanes of chain lengths between C2 and C7. Although the higher temperatures did show a selectivity to methane (roughly 45%), the CO2 selectivity was rather low, below 10% (except at 450°C, which pushed 20%).

ABSTRACT This thesis presents the results from research conducted on Fischer-Tropsch synthesis (FTS) in supercritical CO2 from syngas (H2:CO =1:1) typically produced from coal gasification and using a Fe-Zn-K catalyst. Experiments were conducted with syngas alone at different pressures (200 psi - 1050 psi) and temperatures (275, 350 and 375 oC). Experiments were also conducted with a syngas pressure of 200 psi and at different partial pressures of an inert diluent (N2) such that the total pressure varied from 200 psi to 1050 psi. Finally, experiments were conducted with CO2 as a diluent and at a syngas pressure of 200 psi. The CO2 partial pressure was increased from 0 psi to 1400 psi (non critical to supercritical conditions). The data show an enhancement in the hydrocarbon selectivity and reduction in the parasitic loss of carbon efficiency due to CO2 formation along with significant improvement in the conversion rates. The experiments were conducted in a unique reactor setup that can conduct gas phase or supercritical phase FT synthesis in both batch or flow modes. The use of the supercritical CO2 (ScCO2) inhibited both CH4 and CO2 selectivities while enhancing the rates of synthesis. In addition, the use of supercritical CO2 is expected to prolong the life of the catalyst presumably by removing the heat of reaction from the catalyst's surface and solubilizing the waxes that tend to deposit on the surface. Although not within the scope of this thesis, the products from such a reactor system can be easily separated without the need of an additional unit process simply by tuning the pressure and temperature. The product spectrum and the selectivities for the different products are presented for each set of experiments. The effects of process parameters such as temperature, pressure, N2 partial pressure, and CO2 partial pressure on the product spectrum are also discussed. The clear increase in CO conversion at H2:CO ratio of 1:1 in supercritical phase as compared to gas phase reaction, the decrease in CO2 and CH4 selectivity, and an overall shift in the product distribution towards higher hydrocarbons have been demonstrated. Thus the use of supercritical CO2 has the potential through the FT process to convert coal to liquid fuels using Fe based catalysts, especially since the reactions can be conducted in a two phase regime without losing the benefits of the 3-phase slurry reactor systems

With increasing oil prices and attention towards the reduction of anthropogenic CO2, the use of supercritical carbon dioxide for Fischer Tropsch Synthesis (FTS) is showing promise in fulfilling the demand of clean liquid fuels. The evidence of consumption of carbon dioxide means that it need not to be removed from the syngas feed to the Fischer Tropsch reactor after the gasification process. Over the last five years, research at SIUC have shown that FTS in supercritical CO2 reduces the selectivities for methane, enhances conversion, reduces the net CO2 produces in the coal to liquid fuels process and increase the life of the catalyst. The research has already evaluated the impact of various operating and feed conditions on the FTS for the once through process. We believe that the integration of unreacted feed recycle would enhance conversion, increase the yield and throughput of liquid fuels for the same reactor size. The proposed research aims at evaluating the impact of recycle of the unreacted feed gas along with associated product gases on the performance of supercritical CO2 FTS. The previously identified conditions will be utilized and various recycle ratios will be evaluated in this research once the recycle pump and associated fittings have been integrated to the supercritical CO2 FTS. In this research two different catalysts (Fe-Zn-K, Fe-Co-Zn-K) were analyzed under SC-FTS in different recycle rate at 350oC and 1200 psi. The use of recycle was found to improve conversion from 80% to close to 100% with both catalysts. The experiment recycle rate at 4.32 and 4.91 was clearly surpassing theoretical recycle curve. The steady state reaction rate constant was increased to 0.65 and 0.8 min-1 for recycle rate of 4.32 and 4.91 respectively. Carbon dioxide selectivity was decreased for both catalyst as it was converting to carbon monoxide. Carbon dioxide consumption was increased from 0.014 to 0.034 mole fraction. This concluded that CO2 is being used in the system and converting which created the concentration of the feed gas higher inside the reactor. The research has provided the best conditions for the enhanced conversion while minimizing CO2 formation. Though this research was not able to provide the optimal recycle rate it have created the path for the future research to proceed in the right direction. This reduction and utilization of CO2 will help to reduce the cost of carbon dioxide removal and saves the environment from carbon dioxide emission.