Deep hydrotreating of different feedstocks over a highly active Al2O3-supported NiMoW sulfide catalyst

Abstract

The major challenge for deep hydrotreating of diesel fuels is the removal of refractory sulfur-containing compounds and reducing aromatics to meet the stringent environmental regulations. In the present study, the hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and hydrodearomatization (HDA) reactions of various real feedstocks over alumina-supported NiMoW catalyst synthesized by impregnation solution-based combustion method have been examined in order to gain insight into the capability of this preparation method to produce deep hydrotreating catalysts. The results show that a single exothermic peak with an ignition temperature of 272°C decomposes the NiMo-urea redox solution deposited on tungsten oxide-modified alumina surface, a subsequent impregnation with NiMo-urea solution produces an increase of the decomposition period of the metal oxide precursor and urea. Furthermore, the addition of Ni and Mo to WOx-modified alumina facilities the NiO–Mo (or W) Ox interaction and promotes MeOx reducibility. On the other hand, feed with lower density and higher end-boiling point (T95) shows lower HDS reactivity and hence more demanding operating conditions (i.e., high hydrogen pressure, high reaction temperature and low liquid contact time) are required to obtain diesel fuel with low aromatics, high cetane index and ultra low-S and -N concentrations. The strong inhibiting effect of N-containing compounds and weak effect of aromatics over the HDS reactivity of real feedstock is reflected on smaller HDS rate constant than HDN rate constant for γ-alumina-supported NiMo, NiMoW and even CoMo sulfide catalysts. The presence of tungsten in the NiMo catalyst selectively enhances the rates of the HDS reactions compared to the HDN reactions, particularly of refractory S compounds.