Photocatalytic oxidation of cresols on zinc oxide

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Abstract
The photocatalytic oxidation of o‐, m‐, and p‐, cresol (2‐MP, 3‐MP, 4‐MP) was compared with ZnO (Vieille Montagne SA) and TiO2 (Société Thann et Mulhouse). The same products were obtained with both photocatalysts. The main identified products are methylhydroquinone, 3‐methylpyrocatechol and pyrocatechol with 2‐MP; methylhydroquinone, 3‐methylpyrocatechol and 4‐methylpyrocatechol with 3‐MP; 4‐methylpyrocatechol and hydroquinone with 4‐MP. They result from o‐ and p‐hydroxylation with or without substitution of methyl group. The rate of photocatalytic transformation increases with increasing pH and with addition of small amounts of hydrogen peroxide. The limited inhibition by ethanol suggests that hydroxyl radicals are the main but not the only oxidizing species involved.