MECHANISMS OF C-C 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. © 1991.