Structure-function relationships of deep eutectic solvents for lignin extraction and chemical transformation

Lignin is the most abundant natural aromatic feedstock, and the conversion of lignin to value-added chemicals has drawn immense attention in biorefineries. Deep eutectic solvents (DESs) have been applied to lignocellulosic biomass fractionation and lignin extraction due to their simple procedure, selective solubility of lignin, low cost, and high recyclability. The nature and quantity of functional groups in DESs remarkably influence delignification and structural changes of lignin during treatment. Although many studies have investigated the lignin extraction and following chemical transformations during DESs treatment, no review have illuminated the structure-function relationships between DESs and lignin. This review presents a systematic overview of important studies to provide insight into lignin extraction and chemical transformations by examining the relationship between the type and number of functional groups in DESs constituents during pretreatment. Furthermore, various challenges and opportunities for the development of more sustainable and efficient lignin extraction are provided according to the remaining problems in this field.