Photocatalytic Partial Oxidation of Tyrosol: Improving the Selectivity Towards Hydroxytyrosol by Surface Fluorination of TiO2

The photocatalytic partial oxidation of tyrosol to hydroxytyrosol, the most powerful and expensive natural antioxidant currently known, has been studied in irradiated aqueous suspensions of TiO2 as the photocatalyst. Three TiO2 samples, two commercial (Merck and P25® Evonik) and a home prepared one, have been used bare or surface modified. The influence of surface hydroxylation, surface fluorination, and silver nanoparticles decoration on the selectivity towards hydroxytyrosol has been investigated. The higher the hydroxyl group density, the lower the selectivity. On the other hand, fluorination improved the selectivity towards hydroxytyrosol for all of the samples. In particular, selectivity values up to 10%, i.e. ca. 10 times higher than those obtained with bare sample, were obtained in the presence of fluorinated P25 Evonik TiO2. It is proposed that the higher production of hydroxyl radicals and the lower hydrophilicity of the surface of the fluorinated sample favour hydroxylation of tyrosol and desorption of hydroxytyrosol, respectively, thus hindering the overoxidation of tyrosol. The presence of silver on fluorinated TiO2 was detrimental and it did not significantly improve the selectivity even on bare TiO2. Even if the selectivity hereby reported is relatively low, results show that the surface modification of the photocatalyst and the fluidodynamic optimization of the reacting system could make viable this interesting photocatalytic route of synthesis, which can be used also for the enrichment of some commercial polyphenolic solutions in the very valuable hydroxytyrosol.