Lignin-derived deep eutectic solvents (DESs), composed of choline chloride and phenolic constituents having different numbers of functional group(s), are comprehensively investigated. The low viscosity, low net basicity, and high thermal stability of the DESs assist in improving the biomass pretreatment efficiency. The recyclability of DES offers a sustainable approach as a future design solvent in biomass processing.

Lignin-derived deep eutectic solvents (DESs) have been investigated as sustainable green media for biomass processing. However, the properties and processability of DESs have not been fully understood with the chemical structures of their constituents for biomass fractionation. In this article, the properties of the phenolic DESs are discussed with different numbers of functional groups, such as –OCH₃ and –CHO in their hydrogen bond donor (HBD) structures. The formation of DES is significantly related to the hydrogen bond between its constituents, identified by nuclear magnetic resonance (NMR) analysis and density functional theory calculation (DFT). Lower viscosity and net basicity of DES are achieved with fewer –OCH₃ on HBD structures, resulting in enhanced processability and fractionation efficiency. The thermal stability of the DES is also influenced by the –OCH₃ and –CHO of HBD, as indicated by its onset temperature. The recyclability of the phenolic DES is confirmed by the fractionation performance of the recycled DES. Understanding the structural impacts of DES constituents on the properties and performance is crucial for designing solvents in biorefinery applications.