Synergistic Effect of AlCl3 and Glycerol/ChCl in the Conversion and Dissolution of Lignin in Three-Constituent Deep Eutectics.

"Lignin-first" approach has attracted much attention on biomass utilization. Deep eutectic solvents (DESs) have the advantages of being versatile and tunable, among which three-constituent DES (glycerol/ChCl/AlCl3·6H2O) is found efficient for lignin extraction in biomass fractionation. Herein, how this kind of three-constituent DES affects the fractionation efficiency of lignin is investigated in detail at molecular level. Molecular dynamics simulations indicate that glycerol and ChCl provide strong interaction with the small fragments of lignin. Chloride anion is found to contribute mostly electrostatic interaction to lignin-solvent interactions, while choline cation and glycerol provide strong van der Waals interaction, which also contributes to the higher solubility of the small fragments of lignin. According to DFT calculations, Al3+ is more likely to coordinate with glycerol, forming chelated [Al(C3H8O3)2]3+, which can effectively catalyze the cleavage of βO4 bond of lignin through Lewis acid catalysis, including the isomerization of hydroxyl via E1 and Markovnikov reactions, significantly reducing the energy barrier by about 24 kcal mol-1, favoring the fractionation of lignin. This mechanism is also verified experimentally. This work gives out deep understanding for lignin fractionation in solvent pretreatment, providing guiding clues for the design of more efficient solvent system in biomass utilization.