Developments and perspectives on lignin-first biomass pretreatment for efficient enzymatic hydrolysis and isolation of lignin with minimized degradation

The utilization of lignocellulosic biomass promotes the concept of sustainability and contribute to carbon neutrality. In contrast to traditional biomass pretreatment, lignin-first biomass pretreatment considers both the integrity of lignin and sugar production. There has been increasing interest in applying the concept of lignin-first biomass pretreatment to traditional pretreatment processes. Progress has been made in the field in the past three years or so. It is timely to offer a systematic analysis of the current state of lignin-first biomass pretreatment, and identify existing challenges and future trends in the field. Herein, we first present a brief introduction to biomass recalcitrance and pretreatment, followed by a discussion of lignin repolymerization/condensation during pretreatment and different strategies to stabilize lignin. Next, we focus on biomass pretreatments with active protection of the β-O-4 bonds in lignin. Ionic liquid (IL) pretreatment, deep eutectic solvent (DES) pretreatment, acidified organosolv pretreatment, and organic acid pretreatment are the main pretreatment technologies that have been studied with the active protection of the β-O-4 linkages. We emphasize the importance of retention of high β-O-4 content in valorizing lignin by depolymerization approach. Finally, we provide our perspectives on the challenges and opportunities in this field, in support of efforts to advance lignin-first biomass pretreatment.