Ternary Deep Eutectic Solvent Enables Mild Lignin Arylation with High β-O-4 Retention and Cellulose Recovery

The persistent trade-off between carbohydrate digestibility and preservation of the native lignin structure hampers integrated lignocellulose valorization. Herein, we designed a reactive ternary deep eutectic solvent (DES) that integrated fractionation and in situ lignin arylation into a single solvent platform under mild conditions. The ternary DES redirected benzylic reactivity toward selective α-arylation, suppressing unordered condensation and yielding well-defined arylated lignin with 85.5% β-O-4' retention. Fourier transform infrared spectroscopy (FTIR), hydrogen nuclear magnetic resonance (¹H NMR), density functional theory (DFT), and an independent gradient model based on Hirshfeld partition analyses consistently supported a reconfigured local hydrogen-bonding topology. The feature is a cooperative dual-donor O-H···Cl environment that likely contributed to enhanced lignocellulose fractionation efficiency and increased arylated lignin yield. The system yielded 21.4 wt % arylated lignin, versus 15.8 wt % for the formic acid-syringol control, together with 92.8% cellulose recovery. This work establishes a solvent-engineering strategy for coupling controlled lignin transformation with high carbohydrate recovery.