Recent Advances in the Valorization of Lignin: A Key Focus on Pretreatment, Characterization, and Catalytic Depolymerization Strategies for Future Biorefineries

Lignin is the largest renewable source of aromatics with an annual production of approximate to 50 to 70 million tons worldwide. Unfortunately, despite being a rich source of aromatic chemicals, almost 98% to 99% of lignin is burned as a low-value fuel and only 1% to 2% is utilized as high-value chemicals. It is noteworthy that lignin is cross-linked with carbohydrate fractions of lignocellulosic biomass (LCB) through a stable lignin-carbohydrate covalent complex, making it the main hurdle in converting LCB. Moreover, the stability of the inter-unit linkages (C-O-C, C-C) and the strong interactions among the aromatic units, makes the separation into aromatic monomers challenging. Hence, a delignification pathway through a sustainable approach represents a formidable challenge for the development of future biorefineries. A breakthrough in several technological pathways, e.g., LCB fractionation, lignin characterization/depolymerization, and economic analysis are being considered. However, there remains a dearth of in-depth understanding and a thorough know-how. This review navigates readers through the latest cutting-edge innovations in these four interconnected biorefinery aspects, and may inspire others to propagate the growth of biorefineries in the upcoming years. Overall, the objective of this review is to provide insights into lignin valorization technologies which may guide the future research toward sustainable biorefineries.

Automated summary

Lignin is the largest renewable source of aromatics globally, but the majority is still burned as low-value fuel, with only a small percentage used for high-value chemicals. Understanding lignin depolymerization and its application in biorefineries is a key area for future research and development.