Reaction Mechanism of Aromatic Ring Hydroxylation by Water over Platinum-Loaded Titanium Oxide Photocatalyst

Direct aromatic ring hydroxylation of benzene derivatives by using water as an oxidant over platinum-loaded titanium oxide photocatalyst proceeded with high selectivity under the light of selected wavelength. Two kinds of electrophilic active species, which depended on the reaction condition, were proposed: a surface oxygen radical in neutral or acidic conditions and a hydroxyl radical in basic condition, both of which were produced by a photoformed hole on the surface of titanium oxide. In both cases, these active species can attack the aromatic ring to form an intermediate, followed by the formation of a hydroxylated product. Thus, the reaction proceeds through the addition-elimination mechanism. On the other hand, a photoformed electron reduces a proton to form a hydrogen radical, which reacts with the intermediate to produce a hydroxylated product and a molecular hydrogen or with another hydrogen radical to form a molecular hydrogen.