Mechanisms of CC bond formation and cleavage on metal surfaces: Formation of butenes and hexenes from linear and branched pentenes over Ru/SiO₂ catalysts

Over Ru/SiO₂ catalysts, at temperatures above 100°-150°C and in the presence of hydrogen, linear and branched pentenes (1-pentene, cis- and trans-2-pentene, 2-methyl-2-butene, 3-methyl-1-butene, and 2-methyl-l-butene) undergo isomerization, hydrogenation, hydrogenolysis, and homologation. The main primary products of these last two reactions of CC bond cleavage and formation are methane, butenes, and hexenes. At low temperature (100-150°C), the formation of methane is reduced and the major products are C₄ and C₆ olefinic hydrocarbons, which are obtained in roughly comparable amounts. The distribution of the butenes isomers and of the hexenes isomers strongly depends on the structure of the starting pentene (linear or branched, terminal or internal). The results confirm that hydrogenolysis and homologation of a C₅ olefinic hydrocarbon occur at comparable rates and involve: (i) cleavage of mainly a terminal CC bond of the pentene isomer leading to C₄ and C₁ fragments, (ii) reaction of this C₁ fragment with the starting C₅ to give C₆ hydrocarbons, and (or) (iii) hydrogenation of the C₁ fragment to methane. Two mechanisms, based on concepts of organometallic chemistry, can account for the results (especially for the distribution of the C₄ and C₆ olefinic isomers): (i) a methylene insertion-deinsertion mechanism or (ii) a mechanism that involves formation and decomposition of dimetallacyclic intermediates. Several experimental results seem to be in favor of the last proposed mechanism.