Hydrogen-Free Hydrogenation of a Nitrogen-Containing Ring of Quinolines in an Alcoholic Suspension of a Titanium(IV) Oxide Photocatalyst Modified with Metal Cocatalysts

TiO2 photocatalysts having various metal cocatalysts were prepared and used for photocatalytic hydrogenation of quinoline in 2-propanol suspensions, in which 2-propanol acted as the hydrogen source in addition to the solvent. Hydrogenation over cocatalyst-loaded TiO2 competed with H-2 evolution by the self-coupling of active hydrogen species over the cocatalyst, and the use of TiO2 with a Rh cocatalyst resulted in a large 1,2,3,4-tetrahydroquinoline (THQn) yield and a much larger H-2 yield. In contrast, the use of TiO2 with an inexpensive Cu cocatalyst resulted in a relatively large THQn yield with greatly suppressed evolution of H-2, and the hydrogenation was greatly accelerated at slightly elevated temperatures with the rate being almost the same as that with Rh-TiO2 at the same temperature. Based on the results of a kinetic study, the catalytic properties of Cu-TiO2 and Rh-TiO2 are discussed. Hydrogenation over Cu-TiO2 under solar light was investigated in addition to the expandability and reusability of Cu-TiO2.

Automated summary

TiO2 photocatalysts with different metal cocatalysts were used for the photocatalytic hydrogenation of quinoline in 2-propanol suspensions. It was found that Cu-TiO2 exhibited high efficiency for hydrogenation, while Rh-TiO2 showed higher H-2 yield. The hydrogenation rate of Cu-TiO2 was almost the same as that of Rh-TiO2 at a certain temperature. Solar light-induced hydrogenation over Cu-TiO2 and its expandability and reusability were also investigated.