THE BOND-ENERGY MODEL FOR HYDROTREATING REACTIONS - THEORETICAL AND EXPERIMENTAL ASPECTS

Recently, we have shown that the periodic variations in the HDS activi ties of different transition metal sulfides can be explained by a Bond Energy Model (BEM) based on theoretically calculated metal sulfur bon d energies. The model differs from many previous explanations for the observed Balandin-type volcano curves. Specifically, the BEM model sug gests that under most conditions, the variations in HDS activities are dominated by differences in the concentration of coordinatively unsat urated sites (CUS). The model has also been used to estimate the metal sulfur-bond strength of Co-Mo-S and Ni-Mo-S and the promotion is expl ained in terms of the estimated bond energies for the different types of surface structures. EXAFS and TPR-S results are seen to provide exp erimental evidence for the BEM model. Besides coordinatively unsaturat ed sites, surface SH groups are also present and new evidence is prese nted based on H-1 MAS NMR measurements. SH groups may also play an imp ortant catalytic role in the supply of hydrogen and for acid catalyzed reactions. The relative importance of the SH groups and the CUS sites appears to depend critically on the metal-sulfur bond energy. Further more, the BEM model is shown to provide a useful starting point for un derstanding various reactivity trends observed under different reactio n conditions.