Hydrogenation and isomerization of propenylbenzene isomers over Pt/alumina

The hydrogenation and isomerization of the three isomeric propenylbenzenes, allylbenzene (AB), trans-beta-methylstyrene (TBMS) and cis-beta-methylstyrene (CBMS), in the liquid phase, was investigated over a 1% Pt/alumina catalyst at 313 K and 1 barg. When reacted individually, the cis-isomer gave the fastest rate of hydrogenation followed by AB, with the trans-isomer having the slowest rate giving a ratio of rates of CBMS:AB:TBMS of 57:19:1. All isomers gave high selectivity to phenylpropane, with isomerization controlled by thermodynamic constraints. Competitive hydrogenation revealed that AB adsorbed on a different site from CBMS and TBMS and that AB inhibited hydrogenation of the other two isomers. In contrast, isomerization was unaffected. These results indicate that AB adsorption limits the supply of hydrogen to the other isomers such that reductive elimination of the half-hydrogenated alkyl species to the alkane is depressed while beta-elimination of the alkyl to give an olefin is not affected.

Automated summary

This study investigated the hydrogenation and isomerization of three isomeric propenylbenzenes using a 1% Pt/alumina catalyst in the liquid phase. The results showed that the rate of hydrogenation varied among the different isomers, while isomerization was controlled by thermodynamic constraints. Furthermore, it was found that the adsorption of one isomer inhibited the hydrogenation of the other two isomers, but had little effect on isomerization.