AN APPROACH TO THE MECHANISMS OF C-C BOND FORMATION AND CLEAVAGE ON METAL-SURFACES USING MODEL ELEMENTARY REACTION STEPS OF ORGANOMETALLIC CHEMISTRY

Fischer-Tropsch synthesis, and homologation, hydrogenolysis and dimerization of various C2, C3, C4 and C5 olefins were studied on group VIII metal-based supported catalysts (essentially Ru/SiO2). The experimental results were in favor of similar mechanisms of C-C bond formation for both chain growth in Fischer-Tropsch syntheses and olefin homologation. These mechanisms could also account for C-C bond cleavage in hydrocarbon hydrogenolysis. Using model elementary reaction steps of organometallic chemistry, it was possible to envisage two types of mechanism for C-C bond cleavage and formation on metal catalysts: (i) a carbene insertion-deinsertion reaction to (or from) a metal-alkyl fragment or (ii) a carbene-olefin mechanism via dimetallacyclopentane intermediates. For dimerization reactions, a mechanism involving coupling of alkylidene fragments could explain the experimental results.