Effect of potassium promotion on the structure and performance of alumina supported carburized molybdenum catalysts for Fischer-Tropsch synthesis

Fischer-Tropsch synthesis produces light olefins and long chain hydrocarbons from syngas generated from fossil and renewable feedstocks. Molybdenum carbide exhibits catalytic properties approaching those of metal catalysts in a number of hydrogenation reactions. The present paper focuses on the effect of promotion with potassium on the structure and performance of alumina supported carburized molybdenum catalysts in Fischer-Tropsch synthesis. Molybdenum carbide catalysts were synthesized by temperature programmed carburization in methane and hydrogen. Molybdenum was highly dispersed in both calcined and carburized alumina supported catalysts. Mixed oxides of potassium, molybdenum and aluminium were uncovered in the calcined promoted catalysts. These oxides are converted in molybdenum carbide species on their carburization. Potassium promotion strengthens interaction between molybdenum and aluminium. The potassium promoted catalysts exhibited higher selectivity to light olefins and long chain hydrocarbons but lower Fischer-Tropsch reaction rate compared to the unpromoted counterpart. The observed lower Fischer-Tropsch reaction rate on the promoted catalysts was attributed to more difficult carburization of the molybdenum species and electronic effects due to the presence of potassium. The light olefin and C5+ selectivities decrease at higher carbon monoxide conversion probably because of secondary olefin hydrogenation and hydrocarbon hydrogenolysis.