Revealing the role of hydrogen bonding interactions and supramolecular complexes in lignin dissolution by deep eutectic solvents

Deep eutectic solvents (DESs) have great potential for lignocellulose valorization, especially in terms of lignin extraction and upgrading. It is important to understand the mechanisms involved in lignin dissolution in DESs in order to design solvents for biomass fractionation and extraction of property-tailorable lignin. Here, we use molecular dynamics simulations to gain mechanistic insights into lignin dissolution in DESs. Three DESs based on choline chloride with different hydrogen bond donors (HBDs) (i.e., ethylene glycol, formic acid, and lactic acid) are examined. The main intermolecular interactions in each DES as well as their interactions with lignin are identified. The association between lignin and cellulose in each DES is also investigated. The simulation results reveal that both the functional groups (i.e., hydroxyl group and carboxyl group) and the number of oxygen atoms in the HBD of a DES determine the characteristics of its hydrogen bonding network and the strength of its hydrogen bonding interactions with lignin. Ethylene glycol tends to form H-bonds with α-OH in lignin and both lactic acid and formic acid prefer γ-OH in lignin. The average interaction energy between chloride ion and lignin is comparable to that between the HBD and lignin, indicating that the role of the anion in lignin dissolution is similar to that of the HBD in terms of thermodynamics. Molecular interactions between lignin and a DES are found to play a key role in dissociating lignin from cellulose. The number of aromatic rings of lignin on the cellulose in CCFA and CCLA are only 77% and 56% of that in CCEG. The insights gained in this study advance the understanding of lignin dissolution at the molecular level in DESs and provide guidance to design effective DESs for lignin extraction and valorization.