Role of ZSM5 distribution on Co/SiO ₂ fischer-tropsch catalyst for the production of C ₅-C ₂₂ hydrocarbons

Fischer-Tropsch synthesis (FTS) for the production of gasoline and middle distillates in the range of C ₅-C ₂₂ hydrocarbons was investigated using ZSM5-modified Co/SiO ₂ FTS catalysts. The Co/SiO ₂ catalyst was prepared by an impregnation method using a cobalt nitrate precursor in a slurry of SiO ₂, and theh ZSM5-modified Co/SiO ₂ catalyst was subsequently prepared by synthesizing ZSM5 (Si/Al ratio = 40) by in situ hydrothermal synthesis over the Co/SiO ₂ catalyst at different ZSM5/(Co/SiO ₂) weight ratios. The catalytic performance was largely altered by changing acidity and reducibility of cobalt oxides on ZSM5-modified Co/SiO ₂ catalysts due to the different extents of migration of cobalt oxides from SiO ₂ to the ZSM5 surface with different degrees of catalytic olefin cracking reaction. The ZSM5-modified Co/SiO ₂ catalyst with 25 wt %ZSM5 shows a maximum selectivity to C ₅-C ₂₂ hydrocarbons with a high CO conversion due to the smaller cobalt crystallite size possessing a high reducibility and optimum acid site density. The superior catalytic performance on the ZSM5-modified Co/SiO ₂ catalysts is mainly attributed to the modification of the Co/SiO ₂ surface with the presence of adjacent ZSM5 particles and the possible migration of cobalt species from SiO ₂ to the ZSM5 surface during the hydrothermal synthesis of ZSM5.