Molecular approach to the mechanisms of CC bond formation and cleavage on metal surfaces: Hydrogenolysis, homologation, and dimerization of ethylene over Ru SiO₂ catalysts

At temperatures above ca. 50 °C, over Ru SiO₂ catalysts and in the presence of hydrogen, ethylene undergoes hydrogenation, hydrogenolysis, homologation, and dimerization reactions. The influence of contact times and reaction temperatures on conversions and selectivities has been examined. At low temperatures (<ca. 150 °C) and low contact times, the major hydrocarbons produced from ethylene are, in addition to ethane, 2-butenes (with a high selectivity for cis-2-butene), which is good evidence for a dimerization reaction. With increasing temperature, hydrogenolysis of ethylene to methane and homologation to propene increase. Hydrogenolysis and homologation seem to be mechanistically related in terms of elementary steps of CC bond cleavage and formation; two mechanisms are proposed which involve either metallocarbene insertion-elimination reactions or formation and decomposition of dimetallacyclic intermediates. Several mechanisms are envisioned for dimerization of ethylene; experimental data seem to support a mechanism which involves formation and coupling of two ethylidene species.