A comparison of NiMo/Al2O3 catalysts prepared by impregnation and coprecipitation methods for hydrodesulfurization of dibenzothiophene

NiMo/Al2O3 catalysts prepared by impregnation and coprecipitation methods have been compared in catalytic activity for the hydrodesulfurization of dibenzothiophene (DBT) and have been characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) techniques. The combined results of XRD and XPS indicate that the coprecipitation catalysts contained active beta-NiMoO4 phase while NiAl2O4 and the mixing oxidation state of Mo6+ and Mo5+ species were observed on the impregnation catalyst. The formation of beta-NiMoO4 precursor in the unsulfided NiMo/Al2O3 catalyst prepared by coprecipitation is attributed to the enhanced catalytic activity. BET surface area and N-2 adsorption isotherm measurement demonstrate the characteristic of mesopores for the impregnation catalyst and of macropores for the coprecipitation catalyst. SEM images show that the impregnation method partially caused aggregated crystals while coprecipitation produced homogeneous distribution of Ni and Mo active metal species. As compared to the impregnation catalyst, the reaction temperature over the coprecipitation catalyst has been reduced by 40 K at the same DBT conversion (> 99.7%). The coprecipitation catalyst is expected to show a superior durability to the impregnation catalyst because of its macroporous structure as well as the decrease in reaction temperature. The remarkable performance of the coprecipitation catalyst results from the formation of beta-NiMoO4 precursor, high concentration of surface active sites, and uniform dispersion of NiMo components through optimizing the mixing between Ni and Mo species.