THE EFFECT OF PHOSPHATE ON THE HYDRODENITROGENATION ACTIVITY AND SELECTIVITY OF ALUMINA-SUPPORTED SULFIDED MO, NI, AND NI-MO CATALYSTS

Al2O3-supported Mo, Ni, NiMo, and Rh catalysts, prepared by sequential aqueous impregnation and in situ sulfidation, were investigated in the hydrodenitrogenation (HDN) of quinoline at 643 K and 3 MPa and in the hydrodesulfurisation (HDS) of thiophene at 673 K and 0.1 MPa. The Ni and Mo catalysts had a very low conversion of quinoline to hydrocarbons which improved only slightly in the presence of phosphate. The Rh catalysts had a high conversion and a high selectivity for propylcyclohexane and showed no deactivation with time. The addition of Ni to MO/Al2O3 and of phosphate to NiMo/Al2O3 and Rh/Al2O3 catalysts increased the HDN conversion significantly. The selectivity for propylbenzene and the apparent HDN activation energy increased with increasing P-loading. Ni increased the thiophene conversion of Mo/Al2O3, but phosphate had almost no influence on the HDS activity of NiMo/Al2O3 and Rh/Al2O3. The effect of phosphate is due to a combination of structural and catalytic factors. Phosphate improves the activity by inducing the formation of the type II NiMoS structure, but also, especially at high Ni loading, lowers the activity by inducing a decrease in the dispersion of the NiMoS phases and a segregation of Ni3S2. Phosphate also promotes the S- and N-elimination reactions, but this only has an influence on the overall catalyst activity if the preceding hydrogenation reactions are not rate determining. © 1991.