Activity-Coefficient-Guided Heuristics for Deep Eutectic Solvent Fractionation of Kraft Lignin

The valorization of technical lignin requires fractionation strategies that deliver chemically and thermally distinct lignin streams with predictable structure-property relationships while minimizing hazardous solvent use. Here, we propose a preliminary, exploratory thermodynamic design framework for lignin fractionation based on deep eutectic solvents (DESs), using activity coefficients as quantitative descriptors of solvent-solute interactions. Six chloride-based DESs were synthesized and characterized through freezing-point depression, FTIR spectroscopy, and Schröder-Le Chatelier analysis. All eutectic compositions exhibited negative deviations from ideality (γ < 1), consistent with strong hydrogen-bond interactions. Importantly, lignin solubility and solvent stability were highest among the systems studied within an apparent, exploratory activity-coefficient window (0.2 < γ < 1), supporting rational DES pre-screening that should be validated across a broader range of DES chemistries without extensive empirical screening. Guided by these heuristics, two optimized, sulfur-free DESs (ChCl/levulinic acid and tetrabutylammonium chloride/ethylene glycol) were applied in sequential fractionation of industrial softwood kraft lignin, yielding four distinct fractions. Structural and elemental analyses suggested reduced apparent molecular weight, lower methoxyl content, and increased hydrogen content, while thermal analysis showed substantially decreased glass transition temperatures (<100 °C). These results suggest that activity-coefficient-guided DES selection offers a promising strategy for upgrading heterogeneous kraft lignin into thermally compatible fractions, representing an early step toward predictive solvent engineering for lignin valorization.