Ball Milling-Coupled Mild Deep Eutectic Solvent Extraction for Lignin with Enhanced Depolymerization Potential

A combined strategy integrating mechanical ball milling pretreatment with a ternary deep eutectic solvent (DES) system was developed to enable efficient lignin isolation under mild conditions while preserving reactive structural motifs. Ball milling significantly disrupted the hierarchical structure of biomass, altering micromorphology, crystallinity, surface roughness, and specific surface area, thereby enhancing lignin accessibility. Under identical DES extraction conditions (ChCl/EG/10% OA, 90 °C, 6 h), birch premilled at 800 rpm for 1 h achieved a lignin recovery of 44.1%, approximately 4 times higher than that of untreated birch. When applied to six representative hardwood, softwood, and herbaceous biomasses, the ball milling–DES strategy increased lignin recovery by 1.2–4.5 times, with more pronounced improvements observed for woody biomass than for herbaceous biomass owing to their intrinsically more compact cell wall structures. The isolated lignin from birch retained 56 β-O-4 linkages per 100 aromatic units and afforded a monophenol yield of 19.2 wt % upon Ru/C catalyzed depolymerization at 240 °C for 4 h. Mechanistic insights are proposed to illustrate how ball milling enhances DES penetration and lignin extraction. Overall, this strategy helps alleviate the common trade-off between high lignin extraction efficiency and preservation of β-O-4 linkages, enabling the recovery of lignin with both high yield and high structural integrity.