Molecular insight into the mechanism of lignin dissolution in acid choline chloride–Based deep eutectic solvents

Acid choline chloride (ChCl)–based deep eutectic solvents (DESs), are widely used as effective green solvents of lignin, which contributes to the high–value application of biomass. Understanding the underlying microscopic-scale lignin dissolution mechanism of different acid ChCl–based DESs is important for further DES design due to the heterogeneous and refractory structure of lignin. In this study, we employed molecular dynamics and quantum mechanics methods to investigate the behavior of lignin dimers (guaiacyl–guaiacyl [GGM] and guaiacyl–syringyl [GSM] dimers) in three acid ChCl–based DESs ([i] lactic acid [Lac]:ChCl [2:1], [ii] levulinic acid [Lev]:ChCl [2:1], and [iii] oxalic acid [Oxa]:ChCl [1:1]). Results showed that the dissolution tendencies of different lignin dimers in the three solvents followed different orders. Microstructural analysis revealed that the microstructure of the hydrogen bond (HB) between DESs and lignin dimer played a major role in dissolution. The high dissolution abilities of the Lac:ChCl and Oxa:ChCl solvents for GGM and GSM, respectively, can be attributed to the strong HB interactions between acid–GGM and Chol–GSM, respectively. Moreover, the large extended structure of the lignin dimer would increase the active area (β–O–4 had a large exposed area) in DES solvents. The highest occupied molecular orbital and lowest unoccupied molecular orbital gap between lignin dimers and choline molecules in the three solvents suggested that GGM and GSM had higher reactivities in Lac:ChCl and Oxa:ChCl solvents, respectively, than in other solvents. Finally, the statistical analysis of average surface potential charges further confirmed the different solubilities of the three solvents for different lignin dimers and their effect on β–O–4 bonds.