Selective side-chain oxidation of alkyl-substituted aromatics on TiO2 partially coated with WO3 as a photocatalyst

TiO2 particles partially coated with WO3 (WO3/TiO2) were used as photocatalysts for selective oxidation of toluene to benzaldehyde with molecular oxygen (O2) under photoirradiation at λ > 300 nm. The catalysts loaded with 8–10 wt% WO3 exhibit very high oxidation activity and produce benzaldehyde with ca. 50% selectivity, which is the highest selectivity among the photocatalytic systems reported so far. This is achieved because subsequent photocatalytic decomposition of the formed benzaldehyde is suppressed on the catalysts. On the photoactivated WO3/TiO2, the exposed TiO2 surface behaves as the oxidation site, whereas the WO3 surface is much less active for oxidation. The WO3 loading decreases the amount of benzaldehyde adsorbed onto the TiO2 surface. This thus suppresses photocatalytic decomposition of benzaldehyde on the TiO2 surface, resulting in high benzaldehyde selectivity. The WO3/TiO2 catalyst successfully promotes side-chain oxidation of several kinds of alkyl-substituted aromatics and selectively produces aldehydes and ketones.