Lignin fractionation from lignocellulosic biomass using deep eutectic solvents and its valorization

Lignin, the most abundant natural aromatic feedstock, is one of the most renewable resources for many biofuels, chemicals, and smart materials. Recent progress in deep eutectic solvents (DESs), a class of green solvents exhibiting appealing properties, suggests promising pathways for lignin fractionation and further valorization. Despite considerable efforts that have been devoted to their development, the fractionation and valorization of lignin using DESs are still in their nascent stage. This review presents a comprehensive summary of recent progress in lignin fractionation and valorization using DESs. The introduction of emerging type V DESs, and the relationship between Kamlet–Taft solvatochromic parameters and lignin fractionation and solubility are emphasized. Key factors that affect the lignin fractionation yield and lignin structure have been identified. Particularly, recent growth in computation-based studies to determine which DESs are appropriate for lignin biorefinery and the investigation of underlying mechanisms are also highlighted. Furthermore, the properties of DESs-fractionated lignin and its further valorization routes are systematically summarized. Finally, future perspectives on the development of more efficient and sustainable lignin fractionation and valorization are provided. The present work will provide valuable guidance on the fractionation and valorization of lignin by using renewable DESs.