Tuning the selectivity to aldehyde via pH regulation in the photocatalytic oxidation of 4-methoxybenzyl alcohol and vanillyl alcohol by TiO₂catalysts

The influence of pH on the photocatalytic partial oxidation of 4-methoxybenzyl alcohol (MBA) and vanillyl alcohol (VA) to their corresponding aldehydes in aqueous suspension under UVA irradiation was investigated by using poorly crystalline home-prepared and crystalline commercial TiO2 (BDH, Merck and Degussa P25) photocatalysts. The results clearly show as tuning pH can strongly impart selectivity and activity to photocatalytic processes which are often quite unselective in aqueous suspensions. It was found that pH effect on reaction rate and product selectivity strongly depended on TiO2 crystallinity and substrate type. In the case of MBA oxidation, photoreactivity and selectivity were very high at low pH values for all of TiO2 catalysts, and the crystalline samples showed to be more active than the poorly crystalline ones. At pH= 1 the photoactivity of Degussa P25 was the highest one, and 88% selectivity at 50% conversion was determined. At acidic pH values, selectivity and activity were higher in the presence of HCl than H2SO4 or H3PO4. For VA oxidation, high selectivity values were obtained at high pH's for all of the samples, and the crystalline samples showed higher activity at the alkaline pH values with respect to that observed at the acidic ones. Experiments starting from the obtained products, that are p-anisaldehyde and vanillin, showed that the selectivity depends on the resistance of those compounds to be subjected to further oxidation under the experimental conditions used.