Electro-Oxidation of Lignin Model Compounds and Synthetic Lignin with Transition-Metal Complexes (Manganese and Iron Complexes)

Transition-metal complexes Mn(acac)2, Mn(bpy)3, Fe(acac)2, and Fe(bpy)3 were investigated as mediators in lignin oxidation with an electrolytic mediator system (EMS). Fe(bpy)3 was screened among them as a mediator in the bulk electrolysis of a lignin dimer model compound (4-ethoxy3-methoxyphenylglycerol-beta-guaiacyl ether: 3G) and guaiacyl-type dehydrogenation polymer (G-DHP) by experiments using the lignin monomer model compound 1-(4-ethoxy-3-methoxyphenyl)ethanol. In Fe(bpy)3-mediated electro-oxidations of compound 3G, both C alpha-C beta cleavage and C alpha-carbonylation proceeded efficiently in 0.1 M LiClO4/70%-CH3CN/H2O, whereas C alpha- carbonylation predominated in 10%-dioxane/0.1 M acetate buffer (pH 4). Fe(bpy)3-mediated electro-oxidation of G-DHP also proceeded efficiently via both C alpha-C beta cleavage and C alpha-carbonylation in 0.1 M LiClO4/70%-CH3CN/ H2O. The oxidation was promoted by the addition of 2,6-lutidine as a base. By contrast, the Fe(bpy)3-mediated electro-oxidations of G-DHP did not proceed efficiently in 10%-dioxane/0.1 M acetate buffer (pH 4) with or without 2,6-lutidine under the present conditions. It was reconfirmed that electrolyte was also one of the important factors in lignin oxidation with EMS. Consequently, Fe(bpy)3 was found to be a promising mediator of lignin EMS for lignin modification and degradation.

Automated summary

Transition-metal complexes were investigated as mediators in lignin oxidation. Fe(bpy)3 was found to be an effective mediator, promoting the oxidation reactions of lignin under different conditions. It shows potential for lignin modification and degradation.