Lignin processing by deep eutectic solvents: from structural chemistry to its advanced valorization

First-generation biomass fraction technologies have primarily focused on cellulose-first concepts, particularly chemical pulping processes. In these processes, cellulose fibers and some hemicelluloses are isolated for paper and paperboard production, whereas lignin is utilized primarily as a low-cost thermal energy source or discarded as a byproduct. However, lignin serves as a building block for creating a wide array of chemicals and materials, offering significant practical value in enhancing environmental sustainability and economic profitability of biorefineries. However, efficient lignin exploitation is challenging owing to its structural complexity and heterogeneity. This doctoral thesis investigates the effects of green solvent systems, specifically acidic and alkaline deep eutectic solvents (DESs), on lignin dissolution and extraction, structural chemistry, pyrolysis chemistry, and the formation of lignin nanoparticles (LNPs).
Lignin extracted from wheat straw was highly effective using DES pretreatment. Moreover, the purity and chemical activity of acidic DES-lignin were superior to those of alkaline DES-lignin. Through sequential acid precipitation, alkaline DES-lignin was purified to yield high-quality lignin fractions rich in hydrophilic groups. In addition, technical hydrolysis lignin with high molar mass, pretreated with DESs at varying temperatures, selectively produced monophenolic compounds via fast pyrolysis. A computational simulation study offered the key to interpreting lignin dissolution and its self-assembly behavior triggered by DES, focusing on solvent-lignin molecular interactions. In summary, the findings on DES-based processing and DES-lignin characteristics contribute to substantial theoretical groundwork and experimental knowledge for the sustainable valorization of lignin, thereby promoting its utilization in advanced applications.