Selective Conversion of Lignin to Oxalic Acid via Free-Radical-Mediated Oxidative Cleavage under Alkaline Hydrothermal Conditions

Oxalic acid (OA) is a high-value platform chemical widely used as an electron donor for hydrogen production and fuel cells. However, existing technologies for lignin-to-OA conversion generally suffer from critical drawbacks, such as catalyst dependence, harsh conditions, and low selectivity. Herein, we developed a hydrogen peroxide (H2O2)-mediated alkaline hydrothermal oxidation system that converts lignin to OA in just 10 min and elucidated the free-radical-mediated oxidative ring-opening mechanism. Using guaiacol as a model compound, we confirmed that the oxidation of catechol to o-benzoquinone is the critical step and further clarified the generation mechanism of hydroxyl radicals (·OH), the two ring-opening pathways of o-benzoquinone, and the dominant formation pathway of oxalic acid. Electron paramagnetic resonance (EPR) spin-trapping experiments revealed that NaOH promotes deprotonation of H2O2 to form hydroperoxide anions (HO2−), which further react with H2O2 under hydrothermal conditions to continuously generate hydroxyl radicals (·OH). These ·OH attack the aromatic structure via radical addition, accelerating ring opening and OA formation. Under the optimal conditions, the yield of OA obtained from lignin reached 16.6%. This work establishes a rapid and green approach for lignin valorization, clarifies the radical-mediated aromatic ring-opening pathway, and provides fundamental guidance for targeted oxidative cleavage of lignin into high-value carboxylic acids.