Effects of Acidic Properties on the Catalytic Performance of CoMo Sulfide Catalysts in Selective Hydrodesulfurization of Gasoline Fractions

Improvement of the selectivity of hydrodesulfurization (HDS) to hydrogenation (HYD) of olefins is crucial to produce sulfur-free gasoline (S < 10 ppm) from fluid catalytic-cracked (FCC) gasoline. CoMo/SiO2 catalyst, which was less acidic than CoMo/Al2O3 catalyst, showed higher HDS/HYD selectivity than CoMo/Al2O3 catalyst under mild reaction conditions, although both HDS and HYD activities were lower for the former catalyst. However, under more severe reaction conditions, HDS/HYD selectivity over CoMo/SiO2 catalyst decreased to a significantly greater extent than that over CoMo/Al2O3 catalyst. NO adsorption measurements indicated that the formation of coordinatively unsaturated sites (CUS) over CoMo sulfides was more dependent on the reaction severity over the SiO2 supports. FT-IR data of adsorbed pyridine clearly showed that the strength of Lewis acidity over CUS, in particular, mainly formed under high-temperature conditions, was greater for CoMo/SiO2 catalyst than for CoMo/Al2O3 catalyst. These results suggest that strong Lewis acid sites over CUS promote a hydrocracking reaction in olefins: however, it is not more disadvantageous than HYD reactions in terms of octane loss. Controlling the Lewis acid properties over CoMo sulfides is effective in selective HDS of FCC gasoline, even when using nonacidic supports.