Effect of molecular structure on the hydrogenation and isomerisation of propenylbenzene isomers

The hydrogenation and isomerisation of allylbenzene (AB), trans-beta-methyl styrene (TBMS) and cis-beta-methyl styrene (CBMS), in the liquid phase, was investigated over a 2.5% Rh/silica catalyst. When reacted individually, the cis-isomer gave the fastest rate of hydrogenation followed by allylbenzene, with the trans-isomer having the slowest rate giving a ratio of rates of CBMS: AB: TBMS of 4.2 : 2.8 : 1. The isomerisation reaction followed thermodynamic control. When co-hydrogenated, allylbenzene inhibited the hydrogenation of both CBMS and TBMS but allowed isomerisation: the rate of allylbenzene hydrogenation was unaffected. This behaviour is in keeping with terminal alkenes hydrogenating at edge/corner sites while internal alkenes hydrogenate on terrace faces. The terminal alkene inhibits fast diffusion of hydrogen into the sub-surface and hence prevents hydrogenation of the CBMS and TBMS. The model of sub-surface hydrogen, observed in palladium catalysis, being key for hydrogenation but not isomerisation, was found to explain the results in rhodium hydrogenation catalysis.