Direct cyanomethylation of aliphatic and aromatic hydrocarbons with acetonitrile over a metal loaded titanium oxide photocatalyst

Abstract

A platinum-loaded TiO₂ (Pt/TiO₂) photocatalyst promoted cyanomethylation of aliphatic hydrocarbons, namely cyclohexane and cyclohexene, with acetonitrile, where the photogenerated hole oxidatively dissociates the C–H bond of both the acetonitrile and the aliphatic hydrocarbons to form each corresponding radical species before their radical cross-coupling. The Pt/TiO₂ photocatalyst was more active than the Pd/TiO₂ photocatalyst in these reactions. In contrast, the cyanomethylation of benzene was promoted by the Pd/TiO₂ photocatalyst or a physical mixture of the Pt/TiO₂ photocatalyst and a Pd catalyst supported by Al₂O₃, while it was hardly promoted by the Pt/TiO₂ photocatalyst alone. The temperature dependence of the reaction rate proved that the Pd nanoparticles on the TiO₂ photocatalyst thermally function as a metal catalyst. These results clearly suggest that the Pd metal catalyst is necessary for the cyanomethylation of benzene. However, in the cyanomethylation of aliphatic hydrocarbons, the catalytic effect of the metal particles was not observed, meaning that the radical coupling takes place without the metal catalysis. Thus, it is concluded that in the case of the benzene cyanomethylation the Pd nanoparticles play dual roles, as a catalyst to catalyse the substitution reaction of benzene with the cyanomethyl radical, and as an electron receiver to reduce the recombination of the photoexcited electrons and holes in the TiO₂ photocatalyst, although they could not contribute as a catalyst to the cyanomethylation of aliphatic hydrocarbons.