Fischer-Tropsch Synthesis on Molybdenum Catalysts (Potassium Promotion, Periodic Operation).

Due to the high sulfur resistance and low coke formation, molybdenum catalysts have recently been suggested for industrial Fischer-Tropsch synthesis. On the other h and , molybdenum catalysts have been found to exhibit much lower reactivity as catalysts based on Group VIII metals. In addition, the selectivity toward useful chemical feed stock range (C(,2)-C(,4)) is not high enough. The goal of this research was therefore designed to investigate the feasibility of improving the reactivity and C(,2) to C(,4) selectivity of molybdenum catalysts. Two approaches were tried in this research. The first was based on the development of better molybdenum catalysts by determining the best catalyst preparation and pretreatment conditions. This approach involved the evaluation of catalyst performance under steady state conditions for catalysts prepared at different conditions. The second approach involved the modification of catalyst behaviors through the manipulation of the reaction conditions, namely the periodic operations. Representative catalysts were selected from the first part and tested in controlled periodic operations in order to achieve the highest catalyst performance. In the first part, optimal catalyst pretreatment conditions were established at 773 K and 4 hours reduction with flowing hydrogen. Catalyst performance was investigated against the variables of charcoal support type, precursor type, pH of the preparing solutions, preparation methods, level of metal loading and potassium promoter effect. By employing the optimal combination of these variables, high activity molybdenum catalysts were successfully prepared. These catalysts exhibited activities an order of magnitude higher than that of previously reported molybdenum catalysts. Addition of potassium was found to increase the desired selectivity significantly with a moderate reactivity decrease. In the second part, catalyst performance was evaluated against the periodic operation variables of time period, and duty fraction between feed CO and H(,2). It was found that catalyst reactivity and selectivity could be significantly modified simply by changing the level of these two variables. Experimental observations in this part also provided some evidence for the possible Langmuir-Hinshelwood type mechanisms for hydrocarbons formation, and Eley-Rideal type reaction mechanisms for CO(,2) production.Ph.D.Chemical engineeringUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/161234/1/8702721.pd