In depth interpret the structural changes of lignin and formation of diketones during acidic deep eutectic solvent pretreatment
Deep eutectic solvents (DES) as a novel and green solvent can extract high-purity lignin from lignocellulose in a high yield, while the further utilization of the extracted lignin in the DES is a severe challenge in biorefinery processes. Nevertheless, the potential of lignin valorization can be precisely reflected by unveiling the chemical transformations of lignin during DES pretreatment. In this study, we aimed to understand the possible transformation path via thoroughly characterization of the regenerated lignin and lignin-oil and transform lignin to value-added products. During process, alkali lignin (AL) isolated from poplar was pretreated with choline chloride-lactic acid (ChCl-LA) and choline chloride -oxalic acid (ChCl-OA) at 80-120 oC for 6 hours to produce the regenerated lignin and lignin-oil. The yield of regenerated lignin ranged from 44-75% after DES pretreatment. During ChCl-LA pretreatment, γ-acetylated groups in the regenerated lignin were observed at low temperatures and then deacylation occurred at high temperatures, which was beneficial to oxidize lignin and produce the phenolic diketones. Additionally, the depolymerization of lignin fractions was the dominant reaction during DES pretreatment, accompanied by partial condensation reactions. More importantly, this is the first report of obtaining S- and G-derived diketones from lignin oil after DES pretreatment, which could be readily applied as versatile intermediates, synthons in synthetic chemistry, antibiotics, and ligands. The degradation mechanism of lignin in the DES system was proposed. This work opens the way to produce value-added chemicals from lignin in the DES, which needs to be researched in the future.