Recovery of lignin from deep eutectic solvents by liquid-liquid extraction

After biomass fractionation using deep eutectic solvents (DESs), solvent recovery is an essential step. Laboratory routine is to precipitate lignin using cold water, however large amounts of water are required, resulting in energy intensive operations. Therefore, we propose liquid-liquid extraction as alternative method for industrial applications, and have studied this to recover lignin from a DES consisting of lactic acid and choline chloride, which we applied in various ratios. In this study six solvents were investigated for this purpose, from which 2-methyl tetrahydrofuran (2-MTHF) was selected for further studies. The phase equilibria between the DES and 2-MTHF were determined at 25, 50 and 75 °C. At least 30 wt% choline chloride in the DES was required to form two phases. Addition of more choline chloride decreases the mutual solubility of 2-MTHF and lactic acid. The overall equilibrium lignin distribution between DES and solvent did not change much with varying DES compositions, but the distribution was dependent on the molar mass of lignin. The low molar weight fractions showed a distribution coefficient around 1, while for the heavy fractions the distribution coefficient was below 0.1. Addition of water changes the system greatly, and high molar mass lignin extraction is tremendously enhanced. At 25 wt% water in the DES, the minimum distribution coefficient was 4, allowing effective extraction of lignin from DES by 2-MTHF.