Insights into the Catalytic Performance of Mesoporous H-ZSM-5-Supported Cobalt in Fischer-Tropsch Synthesis

Mesoporous H-ZSM-5 (mesoH-ZSM-5) was used as a carrier for a series of bifunctional Co-based catalysts for Fischer-Tropsch synthesis with ZrO2 and/or Ru added as promoters. The reducibility of the catalysts was studied in detail by using temperature-programmed reduction and X-ray absorption spectroscopy. A comparison of the catalytic performance of Co/mesoH-ZSM-5 and Co/SiO2 (a conventional catalyst), after 140h on stream, reveals that the former is two times more active and three times more selective to the C5-C11 fraction with a large content of unsaturated hydrocarbons, which is next to -olefins. The acid-catalyzed conversion of n-hexane and 1-hexene, as model reactions, demonstrates that the improvement in the selectivity toward gasoline range hydrocarbons is due to the acid-catalyzed reactions of the Fischer-Tropsch -olefins over the acidic zeolite. The formation of methane over the zeolite-supported Co catalysts originates from direct CO hydrogenation and hydrocarbon hydrogenolysis on coordinatively unsaturated Co sites, which are stabilized as a consequence of a strong metal-zeolite interaction. Although the addition of either ZrO2 or Ru increases the catalyst reducibility considerably, it does not affect the product selectivity significantly.