Deep Eutectic Solvent-Driven Refining of Lignin: Selective Fractionation, Depolymerization Mechanisms, and Multidimensional Valorization

Deep eutectic solvents (DESs) with tunable hydrogen-bond network and solvation capacity and acid–base properties are a promising medium for selective fractionation of lignocellulose and valorization of lignin. However, efficient lignin removal and depolymerization, together with the cleavage of lignin–carbohydrate complexes (LCCs), remain critical challenges in DES-mediated lignin-first biorefining. Reactive lignin intermediates may become temporarily stabilized within the solvent environment and subsequently undergo recondensation. Recent developments in DES-assisted selective lignin fractionation, structural regulation, catalytic depolymerization and high-value utilization are critically summarized, and the relationship among molecular structures, the reaction microenvironment, conversion pathways and functional performance are emphasized. The impacts of DES composition, H-bond donors/acceptors, acidity/basicity and solvation parameters on lignin dissolution, LCC bond cleavage, and the stability of major linkages such as β-O-4 are highlighted, along with the intensifying effects of ultrasound and microwave fields. DESs are also regarded as multifunctional reaction platforms with capabilities for selective fractionation, in situ stabilization, and chemical functionalization. Bond cleavage, reactive-intermediate evolution, and radical regulation under thermal, photocatalytic, and electrocatalytic conditions are analyzed, along with strategies to suppress lignin recondensation and enhance aromatic monomer yields. High-value applications in nanomaterials, functional and stimuli-responsive materials, energy materials, and high-energy-density bio-oils are also reviewed. Finally, techno-economic analysis and life-cycle assessment are discussed to identify challenges in DES recovery, stability, mass transfer, energy consumption, and scale-up integration, providing guidance for rational DES design and sustainable lignin-first biorefining.