Biphasic Separation Approach in the DES Biomass Fractionation Facilitates Lignin Recovery for Subsequent Valorization to Phenolics

Herein, we demonstrate a sustainable technique for quality facile lignin recovery by adopting a biphasic separation approach during the deep eutectic solvent (DES) disintegration of biomass for subsequent valorization. The tetrahydrofuran (THF)/aq NaCl combination influenced the attainment of biphasic layer separation, consequently accelerating the movement of DES-solubilized lignin to the organic phase and allowing for the easy recovery of lignin and solvents (both THF and DES) for reuse. The modified protocol facilitated ∼32% wt lignin per wt of sawdust with a 95% purity (based on a Klason analysis), which was nearly 88% of the lignin extracted to the potential lignin of sawdust. This was achieved through the fractionation of sawdust using a choline chloride and lactic acid combination at a 1:2 molar ratio under modest thermal conditions. The obtained results were ∼2-fold higher than those of the conventional DES protocol, employing the H2O–EtOH mixture for lignin precipitation using a similar wood substrate. All of the analytical characterization techniques, including 13C NMR, Fourier transform infrared, gel permeation chromatography, and pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS), established the relevant structural and morphological characteristics, making the resultant lignin an adequate feedstock for the potential production of aromatic chemicals because of the dominance of the β-O-4 content and the limited residual constituents, including sugars and silica. Upon evaluating its suitability for phenolic chemical synthesis via hydrogenolysis, a ∼48% butylated hydroxytoluene yield was obtained as a dominant phenolic product over heterogeneous Ru@V2O5. Overall, the findings indicated that DES is proficient in fractionating lignocellulose for the entire release of lignin (>90%). The maximum recovery of the released lignin was attributed to the superlative performance of the novel THF/aq NaCl combination through the influence of molecular interactions, such as hydrogen bonding and dipole–dipole interactions between the lignin and solvent, thereby establishing an alternative trend for quality lignin extraction for deriving phenolics.