Influence of phosphorus on the preparation of CoMo/Al2O3 hydrotreating catalysts

This paper deals with the preparation, testing and characterisation of two series of phosphorus containing, Al2O3 supported cobalt-molybdenum hydrotreating catalysts. Co and Mo loadings were fixed at 3.8 % wt and 11.6 % wt respectively, whereas P content was varied between 0 and 3 % wt. Model reactions studied were the simultaneous hydrodesulfurization (HDS) of dibenzothiophene (DBT) and hydrogenation (HYD) of naphtalene (NAPHT). When P was impregnated prior to Co and Mo (the latter two in one step, double impregnation, D.I.) HDS and HYD activities were lower than for the P-free catalyst and decreased with increasing amounts of P. Catalysts prepared by impregnating P, Co and Mo in one single step (co-impregnation) are more active than the P-free catalyst and HDS and HYD activity increased with P loading. Characterisation by XRD, BET, XPS and SEM/EDX revealed that the P-free and D.I. catalysts exhibited preferential deposition of Mo and Co at the exterior support surface, whereas for the C.I. catalysts a more homogeneous dispersion is achieved, explaining the differences in catalytic behaviour.