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

摘要

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 (Ni_2P/SiO_2-L,96 m~2 g~(-1)),medium (Ni_2P/SiO_2-M,133 m~2 g~(-1)),and high (Ni_2P/SiO_2-H,208 m~2 g~(-1)) specific surface areas.The activity was based on 240 mumol 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,Ni_2P/SiO_2-H,gave a steady-state conversion of 99+% at 613 K,which was higher than that of the Ni_2P/SiO_2-M catalyst with a conversion of 94% or the Ni_2P/SiO_2-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 mumolg ),and the Ni_2P 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 Ni_2P 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 Ni_2P/SiO_2-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.