Characterization of active oxygen species working in oxygen bleaching

摘要

The reactivities of active oxygen species (AOS) toward a non-phenolic lignin, 1-(3,4-dimethoxyphenyl)ethane-1,2-diol (veratryl glycol, VG), and a carbohydrate, methyl β-D-glucopyranoside (MGP), model compounds were examined and compared under practical oxygen bleaching conditions. In this study, AOS were in situ generated by subjecting a phenolic compound, 2,4,6-trimethylphenol, to oxygen oxidation in order to faithfully simulate practical oxygen bleaching. The results have indicated that the time-dependent reaction selectivity of AOS between VG and MGP, defined as k_(VG)(t)/k_(MGP)(t), in the early stage is 2.1 and 0.3 at the initial pH 11.8 and 13.1, respectively. By comparing this value with the reaction selectivity of hydroxyl radical obtained by Ek el al., it was concluded that AOS working in the early stage of this system is hydroxyl radical. The value of k_(VG)(t)/k_(MGP)(t) at pH 13.1 was especially small, indicating that oxyl anion radical, the conjugate base of hydroxyl radical, predominantly abstracts a hydrogen from aliphatic substructures. Chain type reactions could mainly be responsible for the degradation of the model compounds after the middle stage. The operation of chain type reactions could not greatly be dependent on pH of the medium but on structure of the substrate.