Reaction Mechanism of Aromatic Ring Amination of Benzene and Substituted Benzenes by Aqueous Ammonia over Platinum-Loaded Titanium Oxide Photocatalyst

The reaction mechanism of photocatalytic aromatic ring amination of benzene and the derivatives with aqueous ammonia was clarified by some reaction experiments and electron spin resonance spectroscopy as follows: a platinum-loaded titanium oxide photocatalyst oxidizes an ammonia to form an amide radical (·NH2) and a proton, and the amide radical attacks an aromatic ring to produce an intermediate, followed by the abstraction of the hydrogen atom from it on the platinum sites to produce an aniline. Simultaneously, the photocatalyst also promotes the reduction of a proton to form a hydrogen radical on the platinum sites, and it reacts with the abstracted hydrogen to produce a molecular hydrogen. The photocatalytic aromatic ring amination proceeded for many kinds of monosubstituted benzenes except for phenol, and high selectivity was recorded for benzonitrile and halogenated benzenes. It is noted that the distributions of the aminated isomers were unique, i.e., the para-isomer was predominantly produced in the case of nitrobenzene, and ortho-isomers were preferentially produced in the case of the other substrates, which would depend on the approaching direction of the molecule to the photocatalyst surface.