Insights on the Separation of Kraft Lignin from Betaine-Based Eutectic Solvents: A Liquid–Liquid Equilibrium and Partitioning Study

Lignocellulosic biomass, composed predominantly of cellulose, hemicellulose, and lignin, is the main raw material for biorefineries. Eutectic solvents (ESs) are promising green alternatives for biomass delignification due to their low cost, reduced toxicity, and high lignin solubilization capacity. However, challenges associated with lignin recovery and ES recycling hinder their industrial application. This work addresses this challenge by investigating the liquid–liquid equilibrium (LLE) and the Kraft lignin partition (K_Lignin) in biphasic solvent systems composed of betaine-based ES, water, and a cosolvent (1-butanol or ethyl acetate) at 298.2 K. Four hydrogen bond donors (HBDs) were used to prepare the ESs: urea, ethylene glycol, 1,3-propanediol, and 1,4-butanediol. In nearly all systems, lignin was found to preferentially partition to the top, cosolvent-rich phase, supporting the viability of this recovery approach. In general, 1-butanol created larger biphasic regions, and for the systems with 1,3-propanediol and 1,4-butanediol as HBDs, yielded higher lignin partition than ethyl acetate. The system containing 1,4-butanediol-based ES with 1-butanol was identified as the most promising, achieving exceptionally high K_Lignin values (up to 456.5). These results provide fundamental data for designing effective lignin recovery and ES recycling processes in biorefineries.