Early stages in the TiO2-photocatalyzed degradation of simple phenolic and non-phenolic lignin model compounds

The TiO2-photocatalyzed degradation of phenolic and non-phenolic benzyl alcohols, which can be considered simple lignin model compounds has been studied in aqueous solution under a variety of conditions, with the aim of acquiring fundamental information on the early stages of the degradation process. The role of the electron scavengers O2, K2S2O8 and N2O on the degradation process as well as the influence of pH have been investigated. A major role in the degradation process is played by OH and O2−, while SO4− plays a minor role. OH plays a fundamental role in the conversion of non-phenolic substrates into phenolic ones, while O2− exerts its role mainly by reacting with phenoxyl radicals to give aromatic ring-opening products. Addition of K2S2O8 as electron scavenger allows substantial substrate degradation also in the absence of O2, while N2O which is an excellent scavenger for the hydrated electron seems to be a very poor electron scavenger under the heterogeneous conditions employed. A general mechanistic scheme which proceeds through the formation of aromatic radical cations via one-electron oxidation of the neutral substrates by TiO2(h+), accounting for all the experimental observations has been proposed.