Hydrodesulfurization And Hydrodenitrogenation Of Silica-supported Molybdenum Phosphides

Interest in the development of novel catalysts for hydrodesulfurization has been spurred because of the more stringent environmental legislation and continuous decline in quality of petroleum feedstocks. Other than traditionally used sulfide catalyst, Mo nitride and carbide catalysts have been extensively studied , but their hydrodesulfurization (HDS) and hydrodenitrogenation(HDN) activities are initially high but decline with time in the presence of sulfur-containing compounds.More recently, transitional metal phosphides were reported to have superior activities to traditional Mo sulfide catalysts in hydrotreating processes. Among them, molybdenum phosphides ,bulk and supported ,attracted more attention as they show higher HDS and HDN activities and in some cases even exhibit increased activities with reaction time .Silica-supported molebdenum phosphides, MOP/SiO2, with weight loading from 8 to 35wt% are prepared by in-situ TPR reduction of silica-supported molybdenum phosphate precursors in a fixed bed reactor. The precursors were obtained by incipient wetness impregnation of the support with aqueous solution of (NH)6Mo7O24.4H2O and (NH4)2HPO4 . The precursors are charaterized by TPR to investigate the reactions involved in the preparation of MoP catalysts, and the structure of the passivated MoP catalysts were characterized by means of XRD. The HDS performance of the prepared molebdenum phospide catalyst is evaluated in the same fixed bed reactor using 0.8 wt% dibenzothiophene in decalin as the feed at temperature 360 ℃, pressure 5.0 MPa and WHSV 21 h-1. Highest performance is observed at a total loading of 25 wt%. In comparison with the HDS activity of passivated 25 wt% MoP/SiO2 catalyst, it is found that the supported MoP catalysts prepared by in-situ reduction method is superior to that by the conventional reduction-passivation-reduction method in HDS of DBT.An extensive scrutiny of in-situ reduction conditions ,including heating rate, flow rate of reducing agent H2, total pressure and reduction temperature were studied. It is shown that the in situ reduction conditions have great influence on the HDS performance of the prepared catalysts. The favorable conditions of the in situ reduction are as follows, heating rate ≤ 5℃min-1, H2 flowrate ≥80ml min-1, total pressure ≥1.0 MPa and reduction temperature ≥600 ℃.Effect of the precursor composition is also investigated. It is shown that the HDS activity of molebdenum phosphide with initial Mo/P=1 is almost equal to that with initial Mo/P=1/2, but much higher than that with Mo/P=2. Silica supported binary and ternary transitional-metal phosphides(Ni2P/SiO2,NiMOP/SiO2 ,CoMoP/SiO2,and VMOP/SiO2)are also prepared by in-situ method and evaluated under the above mentioned conditions. The activities of the phosphides follow the order VMoP/SiO2