Studies on the selective two-electron photo-oxidation of benzene to phenol using polyoxometalates, water and simulated solar radiation

The direct one-step oxidation of benzene to phenol with the vanadium tungsten polyoxometalate NaH(3)PW(11)VO(40) was investigated under various reaction conditions. Photochemical substrate transformations were followed both under continuous wave photolysis with cut-off filtered mercury lamp light and under simulated solar light conditions. For comparison, potential dark reactions were also studied in control experiments. It could be shown that benzene is very selectively photo-oxidized to phenol in the course of a near UV and visible-light driven reaction. This process is accompanied by a reduction of the NaH(3)PW(11)VO(40) catalyst as indicated by a rapid color change from yellow to violet-blue and the formation of a vanadium(IV) multiline EPR signature. Different strategies for the re-oxidation of the reduced polyoxometalate photocatalyst have been investigated in order to further increase the observed turnover numbers for substrate conversion. (C) 2011 Elsevier B.V. All rights reserved.