On the enhanced catalytic activity of acid-treated, trimetallic Ni-Mo-W sulfides for quinoline hydrodenitrogenation

Aqueous acid treatment of bi- and trimetallic Mo(W)S₂ sulfides removed a majority of Ni sulfides without affecting the intactness of the Mo(W)S₂ slabs. Reaction path analysis of quinoline hydrodenitrogenation (HDN) on these samples suggests identical active sites on all catalysts, unaffected by the composition of the Mo(W)S₂ phase. HDN of quinoline proceeds primarily via full hydrogenation of both rings followed by the removal of nitrogen. The rate-determining step along this route shifts to a later H-addition as the concentration of active site increases, reflected by the increasing reaction order in H₂. While the rate of the hydrogenation was independent of quinoline concentration, the rate of ring opening (the minor route) depended on the quinoline concentration. This difference is attributed to different adsorbed species mediating the two routes, i.e., N-coordinated intermediate to coordinatively unsaturated sites (CUS) for ring opening and protonated intermediate for hydrogenation. We infer that hydrogenation and ring opening require two types of surface sites that are present at nearly identical proportions on the samples studied. The correlation of HDN rates with H₂-D₂ exchange rates led us to conclude that the Mo(W)S₂ phase composition governs the incorporation of Ni into the slab edge, leading to different concentrations of active sites. The concentration of SH groups, indirectly probed by H₂-D₂ exchange, was highest in a W-rich ternary sulfide phase, leading to 5–10 times higher specific HDN activity than bimetallic (Ni-Mo and Ni-W) samples.