Hydrophobic deep eutectic solvents for the recovery of bio-based chemicals: Solid–liquid equilibria and liquid–liquid extraction

The solid–liquid equilibrium (SLE) behavior and liquid–liquid extraction (LLX) abilities of deep eutectic solvents (DESs) containing (a) thymol and L-menthol, and (b) trioctylphosphine oxide (TOPO) and L-menthol were evaluated. The distribution coefficients (KD) were determined for the solutes relevant for two biorefinery cases, including formic acid, levulinic acid, furfural, acetic acid, propionic acid, butyric acid, and L-lactic acid. Overall, for both cases, an increasing KD was observed for both DESs for acids increasing in size and thus hydrophobicity. Furfural, being the most hydrophobic, was seen to extract the highest KD (for DES (a) 14.2 ± 2.2 and (b) 4.1 ± 0.3), and the KD of lactic acid was small, independent of the DESs (DES (a) 0.5 ± 0.07 and DES (b) 0.4 ± 0.05). The KD of the acids for the TOPO and L-menthol DES were in similar ranges as for traditional TOPOcontaining composite solvents, while for the thymol/L-menthol DES, in the absence of the Lewis base functionality, a smaller KD was observed. The selectivity of formic acid and levulinic acid separation was different for the two DESs investigated because of the acid–base interaction of the phosphine group. The thymol and L-menthol DES was selective towards levulinic acid (Sij = 9.3 +/− 0.10, and the TOPO and L-menthol DES was selective towards FA (Sij = 2.1 +/− 0.28).