A biomass refining strategy combining liquid hot water and deep eutectic solvent (DES) treatment: Acetylation fractionation of uncondensed lignin by acetic anhydride DES

Acetic anhydride/choline chloride deep eutectic solvent (DES) was developed for functional fractionation of lignin from lignocellulose, in which choline chloride unexpectedly acted as a hydrogen bond donor. Acetic anhydride, a hydrogen bond acceptor, was activated by the hydrogen bonding interactions between DES components, thereby protecting the native structure of lignin via acetylation. DES composition was tuned with acetic acid to improve the dissolution of lignin. Detailed structural characterization verified the protection of aryl ether structures through α-/γ-OH acetylation and capping phenolic hydroxyls to suppress re-condensation, producing light-colored lignin with low molecular weight. Combined with choline chloride-assisted liquid hot water treatment, 89.97 ± 0.61 % of hemicellulose was depolymerized into xylo-oligosaccharides (42.69 ± 1.65 %), demonstrating a green biomass refining process. Isolated acetylated lignin microspheres could form tunable morphologies based on DES composition for sunscreen with broad-spectrum UV protection (280–400 nm). The extracted acetylated lignocellulose improved the flexibility of polylactic acid from 2.70 ± 0.91 % to 35.77 ± 3.83 % while imparting it with UV resistance. These results collectively demonstrated the promise of acetic anhydride-based DES for preserving the natural structure of lignin and for functional materials. • Choline chloride-assisted liquid hot water treatment is benefit for xylo-oligosaccharides extraction. • Acetic anhydride in DES takes a role of both modification and fractionation of lignin. • Acetylation of α-/γ-OH of lignin suppressed the recondensation to obtain lighter color lignin with lower molecular weight. • Acetic acid after acetylation is employed to form ternary DES to improve the dissolution of lignin during acetylation. • Isolated DES lignin is self-assembled to tunable morphologies, which is good for sunscreens.