Visible-light induced photofixation of carbon dioxide into aromatic ketones and benzyl halides catalysed by CdS nanocrystallites1

Abstract

Photocatalytic fixation of CO₂ into organic substrates was carried out in a CO₂-saturated DMF (N,N-dimethylformamide) solution using CdS nanocrystallites prepared in DMF (CdS–DMF, mean diameter = 4 nm, hexagonal) as a photocatalyst under visible light irradiation (λ > 400 nm). Benzilic acid (BpCO₂H), atrolactic acid [methyl(phenyl)glycolic; ApCO₂H] and phenylacetic acid (BnCO₂H) were produced from benzophenone, acetophenone and benzyl halides such as benzyl bromide (BnBr) and benzyl chloride (BnCl), respectively. The formation of CO₂ anion radical (CO₂˙^-) was confirmed by EPR spectroscopic measurements by using 5,5-dimethyl-3,4-dihydropyrrole N-oxide (DMPO) as a spin trapping agent. Both the formation of CO₂˙^- and the concurrent one-electron reduction of the organic substrates were indispensable in the photofixation process, which suggests that the photofixation proceeds via their bimolecular coupling on the surface of CdS nanocrystallites.