Selective oxidation of benzyl alcohols to benzaldehydes using a membrane contactor

In this work, we report a novel method for selective oxidation of benzyl alcohols to benzaldehydes. 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, benzyl alcohols and the products, benzaldehydes and the aqueous phase with the catalyst and the oxidant, hydrogen peroxide. This system showed a high selectivity to benzaldehyde (>98%), operating at 1.7x 10 mL/h and 10x10 mL/h flow rates for organic and aqueous phase, respectively. Increasing the organic flow rate to 4.7x10 mL/h, an higher concentration of over-oxidation product (benzoic acid) is observed, lowering the selectivity to 85%. Noteworthy, in the tests with substituted benzyl alcohols as substrates, i.e. 2-chloro benzyl alcohol and 2-methyl benzyl alcohol, the structure-reactivity behaviour has been induced by the substrate interaction with the polymeric membrane.