The mechanisms of hydrogenolysis and isomerization of hydrocarbons on metals: I. Hydrogenolysis of cyclic hydrocarbons

Abstract

The hydrogenolysis of methylcyclobutane and various substituted cyclobutanes and cyclopentanes has been studied on metal films and on the corresponding metal-on-alumina catalysts. The product distribution on metal films depends on the metal, the reaction temperature, and the hydrogen pressure; on supported catalysts, it depends on the concentration of the metal on the carrier. At high temperature, on platinum and palladium films and on the supported catalysts with low metal content, a typical distribution is obtained with any hydrocarbons, corresponding to an equal chance to break the different cyclic bonds of the molecules. π-Allylic triadsorbed species are thought to be responsible for this type of hydrogenolysis. On 10% platinum on alumina or platinum film, at low temperature, and on nickel, only the disecondary CH 2CH 2 cyclic bonds of cyclobutanes and cyclopentanes are broken. α,α,β,β-Tetraadsorbed species could be associated with this selective hydrogenolysis. The analysis of the product distributions obtained from methylcyclopentane on platinum-alumina catalysts and the hydrogen pressure dependency of the hydrogenolysis of methylcyclobutane on platinum films suggest that another mechanism takes place on this metal. The similarity of the product distributions on films and supported catalysts shows that the carrier does not play a catalytic role in the reaction. The difference of the product distributions in the hydrogenolysis of cis- and trans-dimethylcyclopropanes is also discussed.