HDS Activities Of Transition Metal Phosphides Catalysts Prepared Via A Sulfidation-reduction Procedure

Transition metal phosphide as a novel class of catalysts has attracted much attention in recent years due to their high activities and stabilities in hydrotreating reactions. In the present study, the preparation and HDS activities of Ni2P, MoP, and WP were investigated using DBT and 4,6-DMDBT as the model sulfur containing compounds. XRD, CO chemisorption, TPR, HRTEM, and XPS techniques were used to characterize the structures of metal phosphides, which provide some insights into the enhanced HDS activities.Siliceous MCM-41 supported-Ni2P, MoP and WP catalysts were prepared by temperature-programed reduction procedure. The effect of metal/P ratio, metal loading, and final reduction temperature on the phosphide structure and HDS activities were investigated. The results showed that the optimum preparation conditions for Ni2P/MCM-41 were Ni/P=1.25, NiO loading 12% and final reduction temperature of 500℃. The optimum preparation conditions for MoP/MCM-41 and WP/MCM-41 were Mo(W)/P=1, Mo(W)O3 loading of 40% and final reduction temperature of 650℃.The HDS performances of Ni2P/MCM-41, MoP/MCM-41, and WP/MCM-41 in the HDS of DBT were compared in the presence and absence of H2S. The results indicated that Ni2P was the most active among the three metal phosphides, but its tolerance to H2S was poor. XRD and XRF measurements of the catalysts before and after HDS reaction indicated that the bulk structure of phosphides did not change in the reactions but sulfur exists on the surface. The sulfidation of fresh phosphides and sequential HDS-HDN experiments revealed that sulfur played a positive role in the HDS reactions over metal phosphides.MoP/MCM-41 with enhanced activities especially in the HDS of 4,6-DMDBT was obtained via a sulfidation-reduction procedure. The much lower HDS activity over mixtures of MoP/MCM-41 and MoS2/MCM-41 excluded the synergetic effect of separate MoS2 and MoP phases. XRD and HRTEM characterization on crystal structure indicated that d-spacing values increased slightly via sulfidation-reduction, which was probably caused by the incorporation of sulfur in phosphide structure. A shift of the Mo 3d5/2 binding energy was observed in the XPS spectra, which might be related with the incorporation of sulfur in phosphide structure. The HDS performances of DBT and 4,6-MDBT over MoP/MCM-41 prepared via different procedures as a function of temperature and weight time were studied to analyze the reaction pathway. The results indicated that the reaction rate of hydrogenation route was accelerated over MoP/MCM-41 prepared by the sulfidation-reduction procedure.The preparation of WP/MCM-41 and Ni2P/MCM-41 was also attempted by the sulfidation-reduction procedure. The formation of WP was found to be more difficult than that of MoP. The resulting WP/MCM-41 catalyst via sulfidation-reduction showed higher activities than MoP/MCM-41 in the HDS of 4,6-DMDBT. Nevertheless, Ni2P could not be synthesized by this procedure. XRD, TPR, and XPS characterization results indicated that a Ni3S2 phase was formed during sulfidation, thus Ni-P alloy with poor activity instead of Ni2P was formed in the subsequent reduction.Ni2P-WP was prepared by a physical-mixing method, which exhibited much higher performance in the simultaneous HDS of DBT and 4,6-DMDBT than Ni2P and WP. The packing configurations of two phosphides in the bed were investigated, and the highest HDS activity was observed in homogeneous mixing together with quartz-sand particles at a Ni2P/WP mass ratio of 2.