Alkali-Based Pretreatment-Facilitated Lignin Valorization: A Review

In an integrated biomass conversion process, pretreatment becomes an indispensable unit operation to enhance the accessibilities of cellulose and hemicelluloses for producing biofuels or chemicals. Besides carbohydrates utilization, lignin valorization is also considered as a key of achieving economic viability of biorefineries. The pretreatments using alkaline reagents exhibit a superior performance on boosting the yield of fermentable sugars by separating the lignin fraction and altering lignin structures and properties, offering the potential to facilitate the covalorization of lignin and carbohydrates. The diverse lignin recovered from alkali-based pretreatment affects the subsequent lignin depolymerization and conversion process due to the varying chemical and physical properties. In this review, the features and performances of different types of alkali-based pretreatments in the modifications to lignin are discussed. Also, the drill-down analysis of the promising methods to depolymerize lignin provides deep insights toward the conversion and upgrading of lignin isolated from alkaline pretreatment.