SBA-15-supported nickel phosphide hydrotreating catalysts

A series of Ni2P and Ni12P5 hydrotreating catalysts supported on SBA-15 ordered mesoporous silica were prepared by impregnation of nickel phosphate precursors followed by reduction in hydrogen at 873 K. The major product was Ni2P with additional phosphate species when a high excess of phosphorus was used (P/Ni = 2). When a stoichiometric amount of P was used (P/Ni = 0.5), the sole product was Ni12P5 without Ni2P and phosphate byproducts. The active site density as determined by CO chemisorption for such Ni12P5 phases was about three times higher than typically found for Ni2P/SiO2 catalysts and in good accord with active site densities following from particle size. The excess phosphorus results in mesopore blocking by unreduced phosphate species, impeding the accessibility of the Ni2P active sites as probed by CO chemisorption. The catalysts exhibited lower hydrodesulfurization (HDS) but similar or somewhat higher hydrodenitrogenation (HDN) activities than reference alumina-supported NiMo or CoMo catalysts in simultaneous thiophene HDS and pyridine HDN, as well as parallel dibenzothiophene HDS and ortho-methyl aniline HDN hydrotreating reactions. In general, the intrinsic activities of the Ni2P catalysts were higher than those of Ni12P5 catalysts. The activities of these phosphide catalysts were found to be stable or increasing with reaction time. X-ray photoelectron studies of reduced catalysts exposed to a sulfiding mixture suggest that this increase is due to in situ sulfidation of the nickel phosphide to nickel phosphosulfide. Thus, it seems reasonable that surface phosphosulfides form the active catalytic surface in these catalysts.