The influence of chelating ligands on the sulfidation of Ni and Mo in NiMo/SiO2 hydrotreating catalysts

The influence of the addition of chelating ligands like nitrilotriacetic acid (NTA) during the preparation of NiMo/SiO2 hydrotreating catalysts on catalyst activity and structure was studied by means of thiophene HDS (hydrodesulfurisation) activity measurements, temperature programmed sulfidation, and EXAFS (extended X-ray absorption fine structure) spectroscopy. The HDS activities of dried NiMo/SiO2 catalysts prepared in the presence of NTA were much higher than those prepared by classic pore volume impregnation in the absence of NTA. Calcination led to sintering of the MoO3 on the SiO2 support, and thus to low HDS activities of the corresponding sulfided catalysts. After sulfidation at 493 K of dried but not calcined catalysts, molybdenum was fully sulfided, but not to MoS2. The observed Mo-S and Mo-Mo distances pointed to Mo(V)-S-2-Mo(V) or Mo(IV)-S-2-Mo(IV) structures. MoS2 was formed after sulfidation at 673 K, and its dispersion was independent of the presence of NTA in the original oxidic catalyst precursors. However, NTA had a dramatic influence on the sulfidation behavior of Ni. In the presence of NTA, Ni was sulfided between 493 and 583 K, while in its absence sulfidation started at temperatures below 493 K. It is proposed that the sulfidation of Ni at a relatively low temperature, before MoS2 crystallites are formed, enhances the formation of thermodynamically stable nickel sulfides so that less of the HDS-active, but metastable, NiMoS phase can be formed. The role of NTA and similar chelating ligands is to prevent the sulfidation of nickel at low temperature, and thus increase the formation of the NiMoS phase. (C) 1996 Academic Press, Inc.