Solvent free selective oxidation of benzyl alcohol to benzaldehyde using a membrane contactor unit

A new clean catalytic process for the selective oxidation of benzyl alcohol with hydrogen peroxide to benzaldehyde has been studied. Neither solvent nor promoters are needed: a polymeric microporous membrane acts as a barrier to "keep in contact" the two phases: the organic phase, containing the substrate and the product, and the aqueous phase with the oxidant. Being the two phases separated by the membrane, there is no mix of them and dispersion phenomena do not occur. The species are transferred from one phase to the other only by diffusion. The effect of various reaction parameters such as reaction temperature, type of membrane, type of catalyst, the addition mode of hydrogen peroxide was investigated. Three different membranes based on hydrophobic polyvinilidene fluoride (PVDF) were prepared by phase inversion technique induced by nonsolvent and used as membrane contactor. The membranes are characterized by different thicknesses, porosity, tortuosity and hydrophobicity. The catalysts used were ammonium molybdate, (NH4)(6)Mo7O24 and sodium tungstate, Na2WO4. In literature, both these catalysts were reported to be effective in the oxidation of alcohols with H2O2 in combination with a phase transfer catalyst in chlorinated solvents. (C) 2007 Elsevier B.V. All rights reserved.