Electro-oxidation of synthetic lignin with different mediators for the laccase mediator system (3-hydroxyanthranilic acid, 4-hydroxybenzoic acid, phenol red)

Abstract

Three mediators for the laccase mediator system, 3-hydroxyanthranilic acid (3-HAA), 4-hydroxybenzoic acid (HBA), and phenol red (PR) were investigated as mediators in an electrolytic mediator system (EMS) for the degradation of guaiacyl synthetic lignin (G-DHP). All the electron-oxidations of G-DHP with 3-HAA, HBA and PR in the absence of 2,6-lutidine proceeded to give the electrolysis residues in moderate yields. The significant β-β and β-5 linkage loss was found in all the electrolysis residues, especially the residue in the electro-oxidation with PR was significant. The addition of 2,6-lutidine as a base increased the yields of the electrolysis residues and influenced the relative ratio of β-O-4, β-5 and β-β linkages to some extent, that is, increase of β-O-4 linkage loss and decrease of β-β linkages loss (in the electro-oxidation with 3-HAA), increase of β-O-4 linkage loss (in that with 3-HBA), increase of β-5 linkages loss (in that with PR at 0.35 V) and decrease of β-O-4 and β-β linkages loss (in that with PR at 0.70 V). Thus, the base such as 2,6-lutidine was also one of the critical factors for reaction efficiency and reaction selectivity in the EMS. Consequently,3-HAA, HBA, and PR could be used as mediators in EMS for lignin degradation, especially 3-HAA is the most preferable because of the low applied potential.