Thermally Responsive Biphasic Deep Eutectic Solvents Enable One-Pot Three-Fraction Birch Wood Fractionation

<jats:p>Simultaneous fractionation of lignocellulosic biomass into its three major components in a single vessel, without petroleum-derived solvents, remains a central challenge in sustainable biorefinery. We report a thermally responsive biphasic deep eutectic solvent (DES) system for one-pot three-fraction fractionation of birch wood (Betula pendula). A hydrophobic DES (HDES; thymol: capric acid, 1:1) is combined with one of three hydrophilic DES (h-DES) to exploit temperature responsive miscibility at 110°C; the two phases merge into a single reactive medium; cooling to 25°C with water addition triggered the phase separation, directing lignin to the HDES phase, hemicellulose-rich phase to the hydrophilic phase, and cellulose as a solid residue. HDES was selected from five bio-based candidates by Hansen solubility parameter analysis (RED = 0.530 to syringol). Choline chloride: lactic acid combined with HDES gave the strongest fractionation performance: 21.6% lignin removal, 36.4% hemicellulose removal, and 88.2% cellulose-rich solid recovery, exceeding the single-phase h-DES treatment on all three metrics. RED analysis confirmed a dual mechanism in which the h-DES drives acid-catalyzed β-O-4 bond cleavage while the HDES acts as a thermodynamic sink for freed lignin and 5.26 folds lignin-to-cellulose selectivity. The lignin-containing HDES phase was separated and applied directly to wood as a bio-based surface hydrophobic coating agent, raising the water contact angle to 51.2° without lignin isolation, nanoparticle formation, or chemical modification.</jats:p>