Effect of water extraction on the surface properties of Mo/Al2O3 and NiMo/Al2O3 hydrotreating catalysts

The effect of water extraction on the surface structure and gas oil HDS act ivity of alumina-supported Mo and NiMo catalysts, prepared at acidic and ne ar neutral pH, and by sequential and simultaneous impregnation procedures, was studied by measuring structural characterization using temperature-prog rammed reduction (TPR) and catalytic activity before and after metal extrac tion with water From oxide catalysts, depending on their preparation. signi ficant amounts of Mo and Ni present as heteropolycompounds, and crystalline MoO3 were extracted with water. The increase in catalyst activity observed after water extraction indicates that the water-soluble heteropolymolybdat e species are not the oxide precursors of the active sulfide phase, and tha t a portion of Mo does not contribute significantly to the catalytic activi ty due to its poor dispersion. This effect was more pronounced on the catal ysts prepared at acidic pH, where the formation of bulk MoO3 was observed, therefore, rendering, the catalysts less active. No clear correlation was obtained between HDS activity and TPR data for oxi de catalysts. However, on sulfided catalysts, a satisfactory Linear correla tion was found between the H-2 consumption of the reduction peak observed i n the low-temperature region 390-523 K and the HDS activity of water-extrac ted NiMo catalysts, where Mo species are more homogeneously dispersed on th e support. The nonextracted NiMo catalysts were, however, outside of this c on elation due to the higher heterogeneity of the Mo species. The low-tempe rature reduction peak (390-523 K) of sulfided catalysts is associated with the hydrogenation of nonstoichiometric sulfur species and the adsorption of hydrogen on anionic vacancies. (C) 2000 Elsevier Science B.V. All rights r eserved.