Selective hydrogenation of α,β-unsaturated aldehydes and ketones using novel manganese oxide and platinum supported on manganese oxide octahedral molecular sieves as catalysts

Abstract

The selective hydrogenation of α,β-unsaturated aldehydes and ketones has been studied using ketoisophorone and cinnamaldehyde as model substrates using manganese oxide octahedral molecular sieve (OMS-2) based catalysts. For the first time, OMS-2 has been shown to be an efficient and selective hydrogenation catalyst. High selectivities for either the C=C or C=O double bond at ≈100 % conversion were achieved by using OMS-2 and platinum supported on OMS-2 catalysts. Density functional theory (DFT) calculations showed that the dissociation of H₂ on OMS-2 was water assisted and occurred on the surface Mn of OMS-2(0 0 1) that had been modified by an adsorbed H₂O molecule. The theoretically calculated activation barrier was in good agreement with the experimentally determined value for the hydrogenation reactions, indicating that H₂ dissociation on OMS-2 is likely to be the rate-determining step. A significant increase in the rate of reaction was observed in the presence of Pt as a result of the enhancement of H₂ dissociative adsorption and subsequent reaction on the Pt or spillover of the hydrogen to the OMS-2 support. The relative adsorption strengths of ketoisophorone and cinnamaldehyde on the OMS-2 support compared with the Pt were found to determine the product selectivity.