Mechanistic studies of the oxidation of a non-phenolic lignin model compound by the laccase/I-hydroxybenzotriazole redox system

Abstract

The use of a laccase-mediator system is one of the promising possibilities for an environmentally benign pulp-bleaching process. Thus a better understanding of the mechanism for the oxidation of lignin by a laccase-mediator system would facilitate the commercialization of such a system for pulp bleaching. By using I-hydroxybenzotriazole (I-HBT) as a mediator, it was found to be partly regenerated during the oxidation of a lignin dimer, 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propan-1,3-diol (I). A free radical of I-HBT, generated by laccase, was probably responsible for the oxidation of I. The free radical of I-HBT was, however, transformed to benzotriazole, which could not mediate the oxidation of I. The hypothesis that I-HBT acted as an activator or a co-substrate for laccase was not confirmed. Active oxygen species such as hydroxyl free radical (HO°) and superoxide (O2/°) were also found not to be involved in the oxidation of I.