Comparing multistage liquid–liquid extraction with cold water precipitation for improvement of lignin recovery from deep eutectic solvents

After biomass fractionation using deep eutectic solvents (DES), solvent recovery is an essential step. Recovery of lignin from DES by liquid–liquid extraction (LLX) may provide large energy savings compared to cold water precipitation. Lignin that is dissolved in DES from biomass fractionation is inhomogeneous, meaning it has various fractions with different molar weights and possibly variations in functional group densities. Therefore, it is important to compare recoverability of every lignin fraction by LLX with cold-water precipitation. In this work, the recovery of lignin from a DES comprised of 30 wt% choline chloride and 70 wt% lactic acid was studied. Using as much as 30 wt% choline chloride is beneficial to limit leaching of lactic acid. Three cross-current extractions were performed using 2-MTHF. This method recovered 95% of the lignin fractions around 2000 g/mol and 85% of the lignin fractions around 10,000 g/mol. No inter-aromatic ether bonds were found in the lignin remaining in the DES raffinate by heteronuclear single quantum coherence spectroscopy (HSQC), indicating the remaining lignin in the DES has a highly condensed nature. Cold water precipitation could fully recover the lignin fractions above 4000 g/mol using 3.5 kg water per kg DES. However, only half of the lignin fraction of 1000 g/mol was recovered. Briefly, LLX is more suitable for the recovery of low molar weight fractions, while cold water precipitation is more suitable for the heavy molar weight fractions. For industrial applications, a combination of both approaches is essential for full lignin recovery.