Structure-sensitivity of hydrodesulfurization of 4,6-dimethyldibenzothiophene over silica-supported nickel phosphide catalysts

The effect of surface area on the hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene (4,6-DMDBT) was studied on a series of supported nickel phosphide catalysts of low (Ni2P/SiO2-L, 96 m2 g−1), medium (Ni2P/SiO2-M, 133 m2 g−1), and high (Ni2P/SiO2-H, 208 m2 g−1) specific surface areas. The activity was based on 240 μmol of active sites (as measured by CO chemisorption) loaded in the reactor and was measured at 573-643 K and 3.1 MPa. The best catalyst, Ni2P/SiO2-H, gave a steady-state conversion of 99+% at 613 K, which was higher than that of the Ni2P/SiO2-M catalyst with a conversion of 94% or the Ni2P/SiO2-L catalyst with a conversion of 76%. The order (H > M > L) correlated with the dispersion of the catalysts as measured from the respective CO uptakes for the samples (125 vs. 99 vs. 59 μmol g−1), and the Ni2P crystallite size as determined from X-ray diffraction (XRD) line-broadening measurements (6.5 vs. 7.8 vs. 10.1 nm). The higher surface area catalysts gave more of the 3-(3′-methylcyclohexyl)toluene and 3,3′-dimethylbicyclohexyl products, indicating that the small Ni2P crystallites favor desulfurization by the hydrogenation route. Increasing the reaction temperature from 573 to 643 K enhanced the HDS activities and at the same time gave more 3,3′-dimethylbiphenyl product, indicating that high reaction temperatures favor the direct desulfurization route. Extended X-ray absorption fine structure (EXAFS) and elemental analysis measurements showed that the nickel phosphide was partially sulfided and probably formed a surface phosphosulfide phase in the course of the HDS reaction. The superior activity and stability of the Ni2P/SiO2-H catalyst was likely due to the better accessibility of the hindered 4,6-DMDBT molecule to the catalyst surface. The HDS reaction for this molecule is structure-sensitive.