Homologation and hydrogenolysis of linear and branched butenes and butanes on Ru/SiO₂ catalysts

In order to investigate the possible mechanism(s) by which CC bonds are cleaved or are formed on metallic surfaces, the reactions of linear and branched butenes and butanes over Ru/SiO₂ catalysts are studied. At temperatures above 50-100°C and in the presence of hydrogen, 1-butene, cis-2-butene, trans-2-butene, or isobutane simultaneously undergo isomerization (cis-trans isomerization or double-bond migration), hydrogenation, hydrogenolysis, and homologation reactions. In the same conditions, n-butane and isobutane only undergo hydrogenolysis. The influence of the structure of the starting hydrocarbon, contact time, reaction temperature, and hydrogen/hydrocarbon ratio on conversions and selectivities has been examined. Mechanisms for these reactions of hydrocarbons over metallic surfaces in the presence of hydrogen are envisaged using the known concepts of molecular chemistry: the results suggest that metal-alkyl species are key intermediates. Concerning hydrogenolysis and homologation of butenes, it appears that these two reactions occur at comparable rates, which suggests that they are mechanistically related. These reactions involve: (i) the formation of C₃ and C₁ (probably methylene) fragments from a C₄ olefin and (ii) the reaction of the C₁ fragments with the starting olefin to give C₅ hydrocarbons, in competition with the hydrogenation of these C₁ fragments leading to methane. Two simple mechanisms can explain the simultaneous and reversible nature of these two reactions of CC bond cleavage and formation: (i) the insertion-deinsertion of a metallocarbene into (or from) a metal-alkyl species and (ii) the formation-rearrangement of a dimetallacyclopentane intermediate.