Synthesis, Modification And Hydrotreating Performances Of Nickel Phosphide And Molybdenum Phosphide Catalysts

The challenge of producing clean engine fuels from increasingly low quality petroleum feedstocks has extensively stimulated the investigation and development of high-performance hydrotreating catalysts. In the preparation of metal phosphides (Ni2P, MoP, WP, CoP and Co2P), low concentration O2 (0.5-1.0%) in inert gas is used to passivate the metal phosphides in order to form a protective layer, which prevents the bulk oxidation of the phosphides. Since the oxides are not catalytically active, the catalyst has to be reduced at elevated temperatures prior to the hydrotreating reaction. In order to investigate the effect of passivation on hydrotreating over phosphide catalysts, we employed H2S and NH3 to passivate metal phosphides. For this new series of hydrotreating catalysts, a mixture of 10 mol%H2S in H2 and pure NH3 were used to passivate a fresh Ni2P catalyst. The catalysts passivated by O2, H2S or NH3 were characterized by XRD, TEM, XPS, FT-IR, CO chemisorption, sulfur and nitrogen measurement. In addition, the relationships of sulfur on Ni2P with the catalytic performance in the hydrodesulfurization (HDS) of dibenzothiophene (DBT), in the hydrodenitrogenation (HDN) of quinoline, and in the hydrodeoxygenation (HDO) of phenol were investigated.The H2S-passivated Ni2P/MCM-41 showed higher HDS activity than the O2-passivated counterpart. The H2S-passivated Ni2P/MCM-41 was stable up to 150 days in air. Moreover, compared to typical O2 passivation method, it is not necessary to re-reduce the H2S-passivated Ni2P/MCM-41 prior to HDS reaction. Therefore, we conclude that H2S passivation is superior to O2 passivation in the preparation of nickel phosphide HDS catalysts. NH3-passivated Ni2P/MCM-41 exhibited a dramatically lower HDS activity than the H2S-or O2-passivated counterparts.An alternating HDS-HDN-HDS investigation over H2S-passivated Ni2P/MCM-41 indicated that the replacement of sulfur by nitrogen was much faster than that of nitrogen by sulfur. In the case of consecutive HDS-HDO-HDS, the replacement of sulfur by oxygen was much slower than that of oxygen by sulfur. Ni2P/MCM-41 showed extremely high activity and stability in the HDO of phenol. A correlation of the HDS performance with the sulfur content of the spent Ni2P/MCM-41 catalyst after HDS reaction was established:the catalyst with high sulfur content after HDS showed high HDS performance. It is therefore proposed that sulfur species or phosphosulfides (NiPxSy) serve in part as the HDS active sites. Effects of oxide additives, including TiO2, Ga2O3, La2O3, and Y2O3, on MoP catalysts were also investigated. The effect of TiO2 on the HDN and HDS performance of MoP/MCM-41 was investigated using quinoline, decahydroquinoline (DHQ) and DBT model molecules. The catalysts were characterized by XRD, CO chemisorption, TEM, TPR, and pyridine FT-IR. The results indicated TiO2 is a promising promoter for MoP/MCM-41 in the HDN of both quinoline and DHQ and in the HDS of DBT. Addition of TiO2 enhanced the C-N bond cleavage of MoP/MCM-41 whereas suppressed the dehydrogenation. A maximum HDN activity was observed when the TiO2 loading was 5 wt%. The characterization results indicated that a surface enrichment of TiO2 in Ti-containing catalysts and small amount of these surface TiO2 can be partially reduced to Tin+(n< 4). It is suggested that these Tin+(n< 4) species may be responsible for the promoting effect of TiO2 on the HDN performance of MoP/MCM-41. In addition, Ga2O3 showed promoting effect similar to TiO2 on MoP/MCM-41 catalysts, whereas, La2O3 and Y2O3 had negative effects on the HDN activity of MoP/MCM-41.