Study Of Hydrodesulfurization Synergy Mechanism Between Cobalt Phosphide And MoS₂ /γ-Al₂O₃

The promotion mechanism of transition metal sulfides catalysts is essential issue to develop new efficient hydrodesulfurization catalysts for the production of low sulfur or sulfur-free fuel has become an urgent task of national researchers. The sulfided hydrotreating catalysts based on Ni-MoS₂ and Co-MoS₂ traditional system, the study of phosphide hydrotreating catalysts focuses on one or binary transition metal phosphides aspects, but there little research of hydrodesulfurization mechanism concerning the two catalysts. So far, the preparation of Co_xP-MoS₂/γ-Al₂O₃catalyst and the promotion mechanism of cobalt phosphate have not been reported.In this paper, the cobalt hypophosphite with high purity was prepared by simple chemical method and the thermal decomposition mechanism was also studied. The bulk and alumina supported cobalt phosphide catalysts were prepared by the decomposition of cobalt hypophosphite at low temperature. The Co_xP-MoS₂/γ-Al₂O₃and CoMoS/γ-Al₂O₃catalyst was prepared by two step incipient impregnateion. Study the effect of cobalt phosphite and cobalt on MoS₂ active phase and hydrodesulfurization activity, the differences and similarities of the promotion mechanism between Co_xP(Co₉S₈) and MoS₂/γ-Al₂O₃was put forward. The results are the theoretical basis for the development of new efficient hydrodesulfurization catalysts.The results show that the cobalt hypophosphite is synthesised successfully when the reaction temperature between 40-60℃and P/Ni(mol) between 2.2-2.5, the yield could reach above 90%. Under nitrogen atomosphere, the"explosive"decomposition of cobalt hypophosphite was occurred at about 270℃. The PH3 produced from the disproportionating reaction of acid radical acted the role of reducer. The solid-phase products were Co2P, CoP and Co(PO3)2, the gas-phase products were H2O,PH3,P and H2. Bulk and Co_xP/Al₂O₃catalysts could be made between 270-350℃and 270-400℃, respectively, and highly dispersed Co_xP-MoS₂/γ-Al₂O₃could be made when the loading is 15%.The role of cobalt phosphite and cobalt in calcinations and hydrogen activation is to disperse the active phase, formation more Type II Co-Mo-S phase, and at the same time, stabilize the structure of MoS₂ clusters, limit the increase in the length of the MoS₂ chips. Co_xP-MoS₂/γ-Al₂O₃catalyst shows superior HDS activity to CoMoS/γ-Al₂O₃catalyst only at high temperature. Cobalt phosphite and cobalt increase both the hydrogenation (HYD) and desulfurization (DDS) rates, but the influence on the latter is more notable, indicating that there is promotion effect between Co_xP (Co₉S₈) and MoS₂.The results of stacked beds show that, promotion was well explained by the formation of spillover hydrogen in the first bed (Co₉S₈ and Co_xP) that migrate to the second bed (MoS₂), the former synergism ratio is less than the latter, when the reaction temperature increases, the lower the synergism ratio. Cobalt phosphite and cobalt increase both the hydrogenation (HYD) and desulfurization (DDS) rates in DBT hydrotreating reaction, but the influence on the former is more notable, from the side proved that both of Co-Mo-S phase and the remote control model have contribution to the HDS activity of the catalyst.