Allyl halide induced electrochemical degradation of lignin into double-bonded phenolic monomers

Abstract

Lignin is one of the major macromolecule in nature that contains an aromatic ring structure, and also a potential source of high-value products such as biofuels and chemicals. However, Lignin is a kind of complex heterogeneous polymer which can produce many degradation products during processing or treatment. These degradation products are difficult to separate, making it challenging to use lignin directly for high-value applications. This study proposes an electrocatalytic method to degrade lignin by using allyl halides to induce double-bonded phenolic monomers, while avoiding separation. In an alkaline solution, the three basic structural units (G, S, and H) of lignin were transformed into phenolic monomers by introducing allyl halide, which could effectively expand lignin application space. This reaction was achieved using a Pb/PbO₂ electrode as the anode and copper as the cathode. It was further confirmed that double-bonded phenolic monomers were obtained by degradation. 3-allylbromide has more active allyl radicals and significantly higher product yields than 3-allylchloride. The yields of 4-allyl-2-methoxyphenol, 4-allyl-2,6-dimethoxyphenol and 2-allylphenol could reach 17.21 g/kg-lignin, 7.75 g/kg-lignin, and 0.67 g/kg-lignin respectively. These mixed double-bond monomers can be used as monomer materials for in-situ polymerization without further separation, which lays the foundation for high value-added applications of lignin.

Keywords: Allyl halide; Electro-catalytic degradation; Electrochemical induction; Lignin.