Effects of solvents in the depolymerization of lignin into value-added products: a review

The depolymerization of lignin to produce renewable value-added building block chemicals has received increased attention from both industry and academia, as its facilitation is a key in the development of future biorefinery technologies. The main challenge in lignin depolymerization relies on its complex structure. Numerous efforts have shown that catalysts, solvents, reaction parameters, and physiochemical properties of lignin significantly affect the depolymerization reaction of lignin. Solvents are crucial components of the reaction system and affect the performance of the lignin depolymerization process. A good solvent can promote the solubility of lignin and enhance the catalyst-lignin interactions, and could also stabilize the reactive intermediates. However, the relationship and dependence of the molecular structure of lignins with its solubility and reactivity in different solvents are poorly understood. Thus, it is important to understand the influence of solvents on the reaction mechanism, the thermodynamic state of lignin, intermediates, products, and catalysts' stability. In this article, various solvents used in the depolymerization of different types of lignin reactions are extensively overviewed. Special attention is paid to comparing the conversion of lignin, yield of bio-oil, and yield of monomers in the presence of different polarity-categorized solvents and binary solvents.