Decomposition Mechanisms of Lignin-Related Aromatic Monomers in Solution Plasma

Lignin is a natural aromatic macromolecule present in wood and an abundant resource on Earth, yet it is hardly used. In this study, an aqueous solution plasma treatment was investigated for the catalyst-free production of valuable chemicals from lignin. To elucidate the decomposition mechanism, the aqueous solution plasma treatment was applied to the fundamental lignin aromatic model compounds—phenol, guaiacol, and syringol. The results showed that the decomposition rate followed the order syringol > guaiacol > phenol, indicating that electron-donating methoxy groups enhance reactivity. These aromatic model compounds underwent hydroxylation at the ortho and para positions, oxidative ring cleavage, and fragmentation, leading to the formation of various dicarboxylic acids, primarily oxalic acid. All these reactions were promoted by hydroxyl radicals generated from water. Ultimately, decarbonylation and decarboxylation of carboxyl groups resulted in gasification, mainly producing H₂, CO, and CO₂. These results provide fundamental insights into lignin decomposition and demonstrate that aqueous solution plasma is a promising method for producing dicarboxylic acids from lignin under mild conditions without catalysts.