Selective oxidation of benzyl alcohol to benzaldehyde in water by TiO2/Cu(II)/UV solar system

Selective oxidation of benzyl alcohol to benzaldehyde in aqueous solution, under acidic conditions, through the TiO2/Cu(II)/solar UV photocatalytic system, was investigated. Different commercial TiO2 samples were tested. The best result found, in terms of yield, was of 35% for benzaldehyde with respect to the initial benzyl alcohol concentration. During a single run, a partial conversion of benzaldehyde to benzoic acid was also observed. By-products, present at trace level, were 2-hydroxy-benzyl-alcohol, 4-hydroxy-benzylalcohol, 2-hydroxy-benzaldehyde and 4-hydroxy-benzaldehyde. On the basis of the formation of these species a production of HO radicals could be thus inferred. The study suggested that different operative parameters, such as the composition and amount of phou(II) reduction tocatalyst, pH, ionic components in water and the initial concentration of Cu(II) ions, played an important role in the photocatalytic selective oxidation of benzyl alcohol. Moreover, the results of the present investigation indicated that, at the end of the process, Cu(II) could be regenerated and reused, through a re-oxidation of Cu(0), produced during the photolytic run, with air in the dark. A general mechanism of oxidation, supported by the experimental results was proposed.