Lignin depolymerization from softwood biomass using integrated protic ionic liquid–enzyme pretreatment

Lignin, a vital component of plant biomass, offers significant potential for advanced biorefineries seeking to produce high-value chemicals and materials. However, maximizing lignin yield while ensuring its efficient valorization remains a substantial challenge in biorefineries. In this study, an integrated approach to producing lignin-derived oligomers and monomers was developed from softwood biomass. Pinus sylvestris was processed using a protic ionic liquid (PIL), triethylammonium methane sulfonate ([N222H][OMS]), to extract lignin, followed by its targeted depolymerization using bacterial laccases. Advanced analytical techniques were employed to investigate the qualitative and quantitative changes in lignin during the optimization and depolymerization stages. The findings highlight the effectiveness of [N222H][OMS] in removing 87.90% of lignin from pine wood at 180 °C. Furthermore, engineered bacterial laccases demonstrated significant catalytic activity, converting 9.2% of aliphatic hydroxyl groups and 73.8% of phenolic hydroxyl groups in lignin into carboxylic acids. Similarly, benchmarked against commercially available kraft lignin, the same depolymerization approach achieved lower conversion rates, transforming 12.4% aliphatic and 44.5% phenolic hydroxyl groups in lignin into carboxylic acids. Thus, this integrated strategy, combining ionic liquid delignification with enzymatic upgrading, presents a scalable and efficient route for maximizing lignin valorization.