Multiscale exploration of lignin dissolution mechanism based on novel ternary deep eutectic solvents incorporating p-hydroxybenzoic acid

Abstract

Heightened requirements in terms of environment, economy, and efficiency had been imposed on solvents used in the treatment of lignocellulose by the upgrading of the concept of green chemistry. Limitations were imposed on studies that only focus on enhancing a single interaction within the system due to the lack of understanding of the molecular-level mechanism in the solvent-biomass system. In this work, four novel ternary deep eutectic solvents (DES) were synthesized by adopting a new strategy in which the synergistic effect of hydrogen bonds (H-bonds), van der Waals (vdW) interactions and π-π stacking was used to enhance the delignification ability of DES. It was indicated by the results of quantum chemical calculation based on density functional theory (DFT) and molecular dynamics (MD) simulation that 84.26% and 68.72% of the electrostatic energy and vdW interaction energy in the DES-Lignin system were contributed, on average, by choline chloride (ChCl) and p-hydroxybenzoic acid (PB), with increases of 21.60% and 27.71% successively. This improvement was attributed to the increased molecular polarity resulting from the N⁺ on ChCl, the enhanced dispersion attraction due to the arrangement of methyl groups around the sp³-hybridized N⁺, and the π-π stacking interaction formed between the benzene ring structure of PB and lignin, which resulted in the enhancement of vdW interaction within the system. Affected by the ortho-substituents on the benzene ring, among the six investigated lignin dimers, stronger interactions with DES were exhibited by β-1 and β-O-4. The delignification ability of ChCl-LA-PB (strongest calculated interaction energy, reaching -92.37 kcal/mol) was demonstrated in the experiment of treating xylose residue. The delignification rate of lignin was increased by 1.86 times (up to 61.91%) compared with that of ChCl-LA. It was confirmed by the characterization resulted that the excessive depolymerization and condensation of lignin were avoided by the p-π conjugation effect of the phenolic hydroxyl group on PB through inhibiting the ionization of H⁺. Designing DES from the origin of molecular structure and adopting a strategy of multiple interactions in synergy provided novel insights and green methods for the high-value utilization of lignocellulose.