Base-Free Ozonation of Lignin for Rapid Production of Value-Added Aromatics: Structural Factors and Mechanistic Insights.

Lignin is a renewable aromatic resource, yet its efficient depolymerization requires metal catalysts under high temperature, pressure, and strong base. This study employed ozone as a clean oxidant for lignin depolymerization under mild, catalyst- and base-free conditions. Grass lignin, rich in p-coumaric acid (pCA), ferulic acid (FA) moieties, and β-O-4 linkages, achieved 91.55% conversion with a 15.41% monomer yield and 18.43% carbon efficiency within 3 min at 0 °C, outperforming most reported systems in rate and selectivity. Process optimization and Van Krevelen diagram confirmed that ethanol served a dual role as a green solvent and reactant, enhancing sustainability compared to fossil-derived solvents. Kinetic studies revealed that p-hydroxyphenyl (H)-type monomers exhibit higher stability than guaiacyl (G) and syringyl (S)-type products. Softwood lignin with abundant G units and C─C linkages gave a low monomer yield (2.40%), but high vanillin selectivity (>90%). Hardwood lignin predominantly generated S-type products, including syringaldehyde (selectivity 53.17%). Pearson correlation analysis linked lignin structural features, including unit composition, linkage types, and molecular weight to monomer yields and selectivity. Overall, this study highlights ozone oxidation as a promising route for rapid lignin valorization and demonstrates strong associations between lignin structural features and depolymerization efficiency and product selectivity.