Characterization of active oxygen species under oxygen-alkali bleaching conditions

The reactivities of active oxygen species toward a nonphenolic lignin model compound, 1-(3,4-dimethoxyphenyl)-1,2-ethanediol (veratryl glycol, VG), and a carbohydrate model compound, methyl beta-D-glucopyranoside (MGP), were examined and compared under practical oxygen bleaching conditions. In this study, active oxygen species were generated in situ by subjecting a phenolic compound, 2,4,6-trimethylphenol, to oxygen oxidation to faithfully simulate practical oxygen bleaching. The results indicated that the time-dependent reaction selectivity of active oxygen species between VG and MGP, k(VG)(t)/ k(MGP)(t), in the early stage of the reaction is 2.1 and 0.3-0.7 at initial pH values of 11.8 and 13.1, respectively. By comparing this value with the reaction selectivity of hydroxyl radical obtained by Ek et al. [Holzforschung 43 (1989) 391] we conclude that the active oxygen species in the early stage of the reaction are hydroxyl radicals. The reaction selectivity at higher pH was much lower, which is explained by the fact that an oxyl anion radical, the conjugate base of the hydroxyl radical, predominantly abstracts hydrogen from aliphatic substructures. Chain-type reactions could be mainly responsible for degradation of the model compounds after the middle stage of the reaction. We suggest that the operation of chain-type reactions is not greatly dependent on the pH of the medium, but on the structure of the substrate.