Abstract
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.
Highlights
Deep eutectic solvents (DESs) are composite solvents that exhibit deep eutectic behavior upon mixing the constituents
The lignin concentrations in the solvent phases were determined by gel permeation chromatography (GPC) and the concentrations in the DES phases were calculated by mass balance
Because the lignin remaining in the DES could not be extracted by 2-MTHF, we conjecture that the remaining lignin has a different chemical nature compared to the lignin that was extracted by 2-MTHF
Summary
Deep eutectic solvents (DESs) are composite solvents that exhibit deep eutectic behavior upon mixing the constituents. Many mixtures that have been reported as being DESs do have a much lower melting point than the pure components. These melting points are often close to ideal melting points that can be calculated thermodynamically [3]. The melting points determined for these mixtures may differ significantly upon the methods used. The melting point of the DES comprised of choline chloride and lactic acid in a 1:10 ratio was determined by Crespo et al [3], and by Francisco et al [4] Crespo et al [3] found the melting point to be close to the ideal melting point of 11 °C using a capillary method, while Franciso et al [4] found a melting point of −66 °C for the same mixture by differential scanning calorimetry (DSC). The hygroscopic nature of DESs may make characterization difficult since water strongly influences phase behaviour [6]
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