Formation and involvement of radicals in oxygen delignification studied by the autoxidation of lignin and carbohydrate model compounds

The formation of superoxide and hydroxyl radicals under the conditions of oxygen bleaching and their involvement in the oxidative degradation of lignin and carbohydrate structures have been demonstrated using simple model compounds. It was found that: 1. The first step of autoxidation is a one-electron transfer process yielding a substrate radical and superoxide. Direct coupling between these species in the initial solvent cage is of minor importance. 2. Hydroxyl radicals are formed by a superoxide driven Fenton reaction with a pH-maximum in the range 11-11.5. 3. Of the heavy metal salts investigated CUSO4 and MnSO4 facilitate both the degradation of phenolic substrates and the formation of hydroxyl radicals, whereas FeSO4 has no significant effect. 4. Striking mechanistic analogies between the autoxidative degradation of lignin and carbohydrate model compounds determine the fundamental selectivity of the process, while formation of hydroxyl radicals determine the conditional selectivity imposed on oxygen delignification processes.