Insights into alkaline choline chloride-based deep eutectic solvents pretreatment for Populus deltoides: Lignin structural features and modification mechanism.

Deep eutectic solvent (DES) is a kind of green solvent for biorefinery, which favors the progress of being more environmentally friendly and effective. A better understanding of structural changes of lignin is necessary to optimize pretreatment conditions and efficient utilization of the resultant lignin. The current study reported the structural features of lignin recovered from alkaline ChCl/imidazole and ChCl/urea DES pretreatment, and the mechanism of lignin modification was revealed. The profiling demonstrated that lignin samples possessed a high purity (>94.4%), low molecular weight ranging from 1544 to 2562 g/mol and an excellent uniformity (PDI < 1.6). Noteworthy, the content of β-O-4' linkages in lignin was over 75% (i.e. 72.2%-77.4% retention); S/G ratio was increased whereas the content of -OCH groups were decreased. It was revealed that slight cleavage of β-O-4' linkages, preferential breakdown of G units, and demethylation reaction were occurred during alkaline ChCl-based DES pretreatment. Specifically, cleavage of ester linkages between PB and lignin macromolecule was taking place during ChCl/imidazole pretreatment at a high temperature; whereas oxidation only appeared in ChCl/urea system. Despite the modification, well β-O-4' preserved and less condensed lignin samples were recovered after low-temperature pretreatment. Consequently, high contents of phenol derivatives (26.3-30.6%) were achieved in lignin oil. The present study provides critical information on alkaline ChCl-based DES pretreatment, which will contribute to the valorization of lignin by-products and will be beneficial to the development of biorefineries.