Mechanism Studies in Electrochemical Depolymerization Processes of Alkali and Organosolv Lignin

Abstract

To better understand the process and mechanism of lignin structural changes and relative product formation during PbO2 anode electrolysis, a Ti/Sb−SnO2/PbO2 composite electrode has been prepared and utilized for the alkaline electrolytic oxidation of both alkali lignin (AL) and bagasse organosolv lignin (OL). By combining electrochemical measurements, in‐situ FTIR, 2D‐HSQC NMR, and product distribution analysis, we not only investigate the impact of influence factors on the process of lignin depolymerization, but also reveal the depolymerization mechanism from the whole process of lignin structural changes in details. Under certain electrolysis conditions, the highest vanillin yield, 10.1 mg g−1, has been obtained through the electrolysis of alkali lignin, whereas the highest syringaldehyde yield of 4.0 mg g−1 has been achieved during the electrolysis of organic solvent lignin. In particular, AL mainly performs depolymerization through the selective cleavage C−C bonds to produce a series of G unit monomer products of vanillin, acetovanillone, and vanillic acid. By contrast, the depolymerization of OL shows significant selectivity in H units. Particularly, the production of the major H unit monomer product of 4‐vinylphenol has been confirmed by IR spectroelectrochemical studies from the oxidation and decarboxylation of p‐coumaric acid unit.