H2O2 oxidation of lignin model dimers catalyzed by copper(II)– phenanthroline

Abstract

This work aims at elucidating some of the key steps of the oxidation pathways of lignin model compounds by hydrogen peroxide under alkaline conditions (0.05M NaOH) in the presence of copper(II)-phenanthroline complex. Phenolic and non-phenolic βO4 dimeric phenyl propane structures were selected as lignin models since they represent the most abundant structures in lignin: 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy) propane-1-ol and 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy) propane-1-one, and their methylated counterparts. After 3h at 80°C the main products in the reaction medium were analyzed by HPLC. The results suggested that the first step was the oxidation of the benzylic OH group to a ketone group. This step would be catalyzed by the copper (II)-phenanthroline complex and favored by the presence of dissolved oxygen. The second step would be the cleavage of the βO4 linkage by a series of nucleophilic attacks initiated by the reaction of the perhydroxyl anion on the carbonyl group which would not be influenced by the presence of the copper complex. This mechanism is in accordance with the formation of acetaldehyde, veratric or vanillic acid, and guaiacol which were identified as the main reaction products of the models oxidation.