SIGNIFICANCE OF SIDE CHAIN OXIDATION IN THE REACTION OF β-O-4 TYPE LIGNIN MODEL COMPOUNDS WITH ACTIVE OXYGEN SPECIES UNDER OXYGEN BLEACHING CONDITIONS

The purpose of this research is to prove that not only phenolic but also non-phenolic units in lignin can be extensively oxidized under oxygen bleaching conditions. A dimeric beta-O-4 type lignin model compound without phenolic aromatic ring, 2-(3,5-difluorophenoxy)-1-(3,4-dimethoxyphenyl)propane-1,3-diol (VDF) or 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)propane-1,3-di of (VG), was treated together with a phenolic compound, 4-hydroxy-3-methoxybenzyl alcohol (VA), under oxygen bleaching conditions. VA reacts with molecular oxygen and generates active oxygen spices (AOS). The attack of AOS on VDF or VG can result in the beta-O-4 bond cleavage and the liberation of 3,5-difluorophenol (DFPh) or 2-methoxyphenol, respectively. The detection of these compounds can confirm the conversion of the non-phenolic into phenolic moiety. The amount of DFPh accompanying the oxidative degradation of VDF was larger in treatments together with VA than those without VA, which confirmed the possible occurrence of the conversion of non-phenolic into phenolic lignin moieties and suggested the participation of AOS in the conversion. VG showed a similar result, which indicats that the fluoride groups in VDF do not largely affect its reaction with AOS. Several other dimeric model compounds were treated together with VA to examine the effect of the presence and kind of hydroxyl group on their reactions with AOS.