Aliphatic carbonyl reduction promoted by palladium catalysts under mild conditions

The catalytic reduction of aliphatic aldehydes (propanal, pentanal and hexanal) and ketones (pentan-2-one, pentan-3-one and cyclohexanone) to the corresponding alcohols promoted by palladium catalysts,such as Pd/CoO, Pd/NiO, Pd/ZnO, Pd/Fe2O3 and Pd/CeO2, was performed under mild conditions (0.1MPa H2 and 323 K) for the first time. All the catalysts were obtained by the co-precipitation technique and characterized by BET, TPR, XRD,TEM and XPS. The co-precipitation method allows, after reduction, formation of bimetallic ensembles (Pd/CoO and Pd/ZnO in less extent) or alloys (Pd/NiO) thus changing the electronic properties of the palladium on the surface, increasing the d-orbital energy at the Fermi level and permitting the activation of the C=O bond also in aliphatic carbonyls. Accordingly the reactivity of Pd/Fe2O3 and Pd/CeO2 towards aliphatic aldehydes is attributed to a redox interaction of Fe3+ or Ce4+ with the oxygen moiety of the carbonyl bond, leading to the * orbital energy decrease. Analogous reactions, carried out with Pd/CoO and Pd/Fe2O3, prepared by impregnation, gave a very slow reduction. Additional catalytic tests were performed with aromatic carbonylic compounds in order to compare their reactivity with that of aliphatic systems: the differences were interpreted taking into account the energy value between and * orbitals, much lower in aromatic carbonyls, that favours an easier activation of the aromatic C=O bond.