A METHOD FOR THE ESTIMATION OF RELATIVE REACTIVITY OF ACTIVE OXYGEN SPECIES UNDER OXYGEN BLEACHING CONDITIONS

摘要

The relative reactivity of active oxygen species (AOS) toward a non-phenolic lignin model compound, 1-(3,4-dimethoxyphenyl) ethane-1, 2-diol (veratryl glycol, VG) or 3,4-dimethoxybenzyl alcohol (veratryl alcohol, VA), and a carbohydrate model compound, methyl β-D-glucopyranoside (MGP), was examined and compared under practical oxygen bleaching conditions. In this study, AOS were in situ generated by subjecting a co-existing phenolic compound, 2, 4, 6- trimethylphenol, to oxygen oxidation in order to faithfully simulate practical oxygen bleaching. The results 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 and co-workers, it was concluded that AOS working in the early stage of our reaction system is hydroxyl radical. The value of k_(VG)(t)/k_(MGP)(t) at the 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 of the reaction. The operation of chain type reactions could not greatly be dependent on pH of the medium but on structure of the substrate.