Mechanism of Deep Eutectic Solvent Delignification: Insights from Molecular Dynamics Simulations

Based on the field of ReaxFF, the pretreatment of a lignin–carbohydrate complex model in a choline chloride-based deep eutectic solvent was proposed. The results showed that the β-O-4 ether and glycosidic bonds in the model were broken at an optimal temperature of 373 K. The best pretreatment solvent was choline chloride/lactic acid (1:2), and 40% of the model compounds were decomposed. Free protons in deep eutectic solvents were one of the critical factors in the process of biomass delignification, and thus acidic hydrogen binding donors generally had a high lignin removal rate, but there was no significant correlation with the acidity of the hydrogen bonding donors. In the deep eutectic solvents of choline chloride/lactic acid, the delignification effect tended to decrease with the increase of lactic acid content. The delignification effect of monocarboxylic acid hydrogen bond donors decreased with the increasing molecular chain length, while the opposite effect of delignification was observed for dicarboxylic acids. The molecular dynamics simulations showed that choline chloride/glycerol (1:2) with a slightly weaker hydrogen bonding network could bind better and solubilize the product molecules, with interaction energies of −170.89 and −61.99 kJ/mol with carbohydrates and phenolic compounds, respectively.