Characterization and hydrodesulfurization activity of CoMo catalysts supported on sol-gel prepared Al2O3

A series of CoMo/Al2O3 catalysts was prepared by impregnation on a series of alumina powders synthesized by the sol-gel method with different hydrolysis ratios R (defined as [H2O]/[aluminum-tri-sec-butoxide (ASB)]; R = 3,4,…,12,13). The oxide precursors were characterized and subsequently tested in the hydrodesulfurization (HDS) of thiophene, dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT). Mainly due to their large pore diameters of ca. 6 nm, the catalysts prepared from the alumina with hydrolysis ratio R = 7-10 showed higher HDS activity compared with the activities of the other prepared catalysts. The effect of the pore diffusional limitation was more significant than expected, due to the ink-bottle shape of the pores of the prepared catalysts with hydrolysis ratio R < 8 and R > 10. Due to highly dispersed CoMo active phase, the HDS activity of the prepared catalysts with hydrolysis ratio R = 8 and 9 for thiophene was similar to that of a reference industrial catalyst that was designed and manufactured for deep HDS of diesel fuel fractions. Furthermore, the HDS activity of the prepared catalysts with hydrolysis ratio R > 5 for DBT was higher than that of the reference industrial catalyst. For 4,6-DMDBT, however, the reference industrial catalyst showed higher HDS activity compared with the activities of the prepared catalysts. Relatively high HDS activity was observed for the prepared catalysts with R = 9 and 10 with cylindrical pore shape and with a high proportion of strong acid sites. The strong acidity supposedly enhanced the hydrogenation activity of the catalysts that was essential for the HDS of 4,6-DMDBT.