Photocatalytic conversion of benzene to phenol using modified TiO2 and polyoxometalates

The application of photocatalytic reactions to organic synthesis has attracted interests in view of the development of environmentally benign synthetic processes. This study investigated the effects of various parameters (electron acceptor, surface modification, and the combination of photocatalysts) on the direct synthesis of phenol from benzene using photocatalytic oxidation processes. The OH radicals generated on UV-illuminated TiO2 photocatalyst directly hydroxylate benzene to produce phenol, hydroquinone, and catechol. The addition of Fe3+, H2O2, or Fe3+ + H2O2 highly enhanced the phenol production yield and selectivity in TiO2 suspension. Surface modifications of TiO2 had significant influence on the phenol synthetic reaction. Depositing Pt nanoparticles on TiO2 (Pt/TiO2) markedly enhanced the yield and selectivity. Surface fluorination of TiO2 (F-TiO2) increased the phenol yield two-fold because of the enhanced production of mobile (free) OH radicals on F-TiO2. Polyoxometalate (POM) in phenol synthesis played the dual role both as a homogeneous photocatalyst and as a reversible electron acceptor in TiO2 suspension. POM alone was as efficient as TiO2 alone in the phenol production. In particular, the addition of POM to the TiO2 suspension increased the phenol yield from 2.6% to 11% (the highest yield obtained in this study). Reaction mechanisms for each photocatalytic system were discussed in relation to the phenol synthesis.