Understanding the Interactive Relationship between Aliphatic Series Deep Eutectic Solvents and Lignocellulosic Dimer Model Compounds

Clean, efficient, and selective separation of lignin from lignocellulosic biomass is a challenging yet popular topic of study to achieve biomass conversion and high-value utilization. Unlike conventional acid, alkaline water, and most organic solvents, the main components of lignocellulosic biomass were found to selectively dissolve in certain classes of deep eutectic solvents (DESs). However, the mechanism of selective dissolution is still not well defined. In this paper, we started from the perspective of the interactive mechanism of DESs and the structural unit of lignocellulose and conducted a comprehensive analysis of the relationship between different types of DESs and complexes of DESs with four kinds of lignocellulosic dimer model compounds (interaction energy, hydrogen bond) using the molecular dynamics method. Some main findings are as follows: for the same atomic number of linear carbon chains, interaction energies of 2c-DESs or 3c-DESs are more conspicuous in the presence of an aliphatic HBD with double −COOH group compared with single −COOH, single −COOH + single OH, or double −OH; DESs with a short carbon chain HBD are more compatible with four lignocellulosic dimer model compounds (LDMCs) than DESs with a long carbon chain HBD; two-constituent DESs are more compatible with GG lignin models compared with SS when the HBD contains double −COOH or single −COOH groups. When analyzing the hydrogen bond of DESs or the complexes of DESs with lignocellulosic dimer model compounds, the coordination number of hydrogen bonds was in the order: dicarboxylic acids > hydroxy acids > diols > monocarboxylic acids in C2, C3, and C5 chain HBDs. This work intends to achieve a deeper and more comprehensive mechanism knowledge of the interaction between DESs and different components of lignocellulose and aims to help with screening the DESs for better selective conversion of components of lignocellulosic biomass, as well as opening new perspectives in the biorefinery green process