Issue 6, 2012

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

Abstract

The hydrogenation and isomerisation of allylbenzene (AB), trans-β-methyl styrene (TBMS) and cis-β-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.

Graphical abstract: Effect of molecular structure on the hydrogenation and isomerisation of propenylbenzene isomers

Article information

Article type
Paper
Submitted
23 Feb 2012
Accepted
21 Mar 2012
First published
28 Mar 2012

Catal. Sci. Technol., 2012,2, 1287-1291

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

L. C. Begley, K. J. Kakanskas, A. Monaghan and S. D. Jackson, Catal. Sci. Technol., 2012, 2, 1287 DOI: 10.1039/C2CY20105D

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