Catalytic functionalities of supported sulfides: V. CN bond hydrogenolysis selectivity as a function of promoter type

Two types of γ-Al2O3-supported catalysts, containing Group VI–VIII transition metals as active components, were prepared and their selectivities for CN bond hydrogenolysis determined. These were (a) sulfided MMo catalysts (where M = Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt, Re, or Cr), containing 25 mmol M and 77 mmol Mo/100 g Al2O3; and (b) sulfided M catalysts, without Mo, containing 25 mmol M/100 g Al2O3. Pseudo-first-order rate constants for two simultaneous, competing reactions, i.e., CN bond hydrogenolysis of indole (k1) and ring hydrogenation of naphthalene (k2), were determined (at 350 °C and 137 atm H2 pressure) for the above catalysts as a function of the periodic table position of M. It was found that the C-N hydrogenolysis vs ring hydrogenation selectivity of the MMo catalysts, as expressed by the k1k2 ratio, was in the order RuMo > IrMo > CrMo > PtMo > CoMo > FeMo > NiMo > ReMo > RhMo > PdMo. The high selectivity of the RuMo, IrMo, and PtMo catalysts is due to a favorable balance of moderate CN hydrogenolysis activity and very low ring hydrogenation activity. RhMo, NiMo, and CoMo show highest CN hydrogenolysis activity, but relatively low selectivity due to their high ring hydrogenation activity. The sulfided M catalysts, except Pt and Pd, show markedly lower CN hydrogenolysis activities than the corresponding MMo catalysts. Comparison of the actual CN hydrogenolysis activities of MMo catalysts with their “additive” activities (calculated by assuming independent activities of the M and Mo components) shows that Ni, Rh, Co, Fe, and Ru strongly promote, whereas Pt and Pd strongly depress the CN hydrogenolysis activity of the corresponding MMo catalysts. Similar promoting (or depressing) effects of the M components are also observed in comparing the actual and “additive” ring hydrogenation activities of the MMo catalysts, indicating that CN hydrogenolysis and ring hydrogenation reactions possibly occur on the same or similar type of active sites. The high CN hydrogenolysis selectivity of RuMo, IrMo, and PtMo points to the feasibility of their optimization for possible use as improved catalysts in HDN of heavy oils, involving lower hydrogen consumption than that found with conventional Ni- and Co-promoted catalysts.