Mechanistic insight into the heterogeneous catalytic transfer hydrogenation over Cu/Al 2O 3: Direct evidence for the assistant role of support

Abstract

The catalytic transfer hydrogenation (CTH) between alcohol and alkene was investigated over Cu/Al 2O 3 catalyst via mechanical mixture of the supported catalyst and the pure support. It is found that the CTH activities are highly depended on the amount and acid/basic properties of the mechanically mixed support, clearly indicating the assistant role of support. In addition, only alkene with high activity of hydrogenation can really act as hydrogen acceptor. Based on these, a new bifunctional reaction pathway is proposed. Alcohol dehydrogenates to form alkoxide on Al 2O 3 surface (R 1CH(OH)R 2 → R 1CH(O)R 2 + H), and H atom migrates to the metallic Cu patches via reverse hydrogen spillover, while extraction of α-hydrogen forming ketone (R 1CH(O)R 2 → R 1C(O)R 2 + H) occurs on Cu or Cu–Al 2O 3 interface. On the metallic Cu patches, alkene hydrogenates yielding alkane (R 3CH = CHR 4 + 2H → R 3CH 2CH 2R 4). This mechanism can give a better explanation for the observed acceleration or deceleration role of hydrogen acceptor to alcohol dehydrogenation.