Optimizing both the CoMo/Al2O3 catalyst and the technology for selectivity enhancement in the hydrodesulfurization of FCC gasoline

Herein, a series of alumina supports were prepared by rehydration-dehydration of the traditional gamma-Al2O3 under different hydrothermal temperatures. After loading CoMo species, it was found the selectivity factor of corresponding CoMo/Al2O3 catalysts was improved greatly from 1.21 to 2.51 with the increased hydrothermal temperature of alumina supports in the hydrodesulfurization (HDS) of a model fluid catalytic cracking (FCC) naphtha. The improved HDS selectivity is attributed to the weakened metal-support interaction and lowered dispersion of MoS2 particles, resulting in catalysts with larger edge-to-corner ratios of CoMoS slabs ((f(e)/f(c))(CoMo)). Besides, the recombination mercaptans formed by reaction of produced H2S and olefins were found as a big challenge to reach a high HDS conversion (S < 10 ppm) when only using the selective HDS catalyst (one-unit process). Therefore, a two-unit process has been developed by using a selective HDS catalyst to remove the most of the refractory sulfur compounds firstly, after which using another mercaptan-removing catalyst to deal with the recombination mercaptans specially. The catalytic results show that the two-unit process has a smaller olefin loss (4.38 v%) than that of one-unit process (7.08 v%) when reducing the sulfur concentration of a heavy FCC fraction to less than 10 ppm, thus resulting in a 0.7 unit less of the research octane number loss. The present work shows that high HDS conversion and low olefin hydrogenation conversion in selective HDS of FCC gasoline can be achieved through optimization design of both the selective HDS catalyst and the whole technology.