Towards Understanding Structure-Activity Relationships of Ni-Mo-W Sulfide Hydrotreating Catalysts

The relation between activity and selectivity of a series of unsupported Ni–Mo, Ni–W, and Ni–Mo–W sulfides and the physicochemical properties of the sulfides and oxide precursors were explored. Bimetallic oxide precursors were a crystalline molybdate with layered structure ((NH$_4$)HNi$_2$(OH)$_2$(MoO$_4$)$_2$) or a wolframite-type nickel tungstate (NiWO$_4$). Trimetallic precursors were mixed Mo–W–Ni phases appearing amorphous in XRD analysis, in which metal cations had environments similar to those in the bimetallic precursors. The XRD-amorphous layered structure (obtained for a Ni–Mo–W precursor) led to the fastest sulfidation of Ni cations. The proximity of metal cations in precursors was retained after sulfidation, which lead to intralayer mixed Mo$_{1−x}$W$)_x$S$_2$ (in trimetallic materials) and varying concentrations of Ni at the perimeter of the Mo(W)S$_2$ slabs. At MoS$_2$ and WS$_2$ edges, Ni has a square-pyramidal coordination, which is significantly distorted at the Mo–W mixed edges. The catalyst with the lowest Mo/W–Ni coordination had the lowest rates for hydrodenitrogenation of o-propylaniline and hydrodesulfurization of dibenzothiophene. Among the materials with significant Ni–Mo(W) coordinations, the most active sulfide catalyst at low temperatures was Ni–W with highly defective slabs, at high temperatures the Ni–Mo–W sulfide with the highest Ni concentration at the perimeter was the most active catalyst