Selectivity to cyclohexenes in the liquid phase hydrogenation of benzene and toluene over ruthenium catalysts, as influenced by reaction modifiers

A study has been made of the selective liquid-phase hydrogenation of benzene and toluene to cyclohexene and methyl-substituted cyclohexenes, respectively, at ambient temperature and pressure. Just as in gas-phase hydrogenation, the selectivities to cyclohexene or to substituted cyclohexenes strongly increase when reaction modifiers are added. Compounds exhibiting a reaction modifying action contain a hydroxyl or an amine group. The action of modifiers in the liquid phase is explained along the same lines of reasoning as advanced for the case of gas-phase hydrogenation (P.J. van der Steen and J.J.F. Scholten, Appl. Catal., 58 (1990) 291 and J. Struijk and J.J.F. Scholten, Appl. Catal., 62 (1990) 151), and is based on the formation of a hydrogen bond between cyclohexene and the modifier. In the hydrogenation of toluene a large difference in the selectivity to 1-methylcyclohexene and 3- and 4-methylcyclohexene is observed. An interpretation of this effect is presented.