Abstract
Ultrafiltration was employed in the purification of spent Deep Eutectic Solvent (DES, a mixture of choline chloride and lactic acid, 1:10, respectively) used in the extraction of lignin from lignocellulosic biomass. The aim of this was to recover different lignin fractions and to purify spent solvent. The results revealed that the commercial regenerated cellulose membranes—RC70PP and Ultracel 5 kDa UF membranes—could be used in the treatment of the spent DES. The addition of cosolvent (ethanol) to the spent DES decreased solvent’s viscosity, which enabled filtration. With two-pass ultrafiltration process with 10 kDa and 5 kDa membranes about 95% of the dissolved polymeric compounds (lignin and hemicelluloses) were removed from the spent DES. The utilized membranes also showed the capability to fractionate polymeric compounds into two fractions—above and under 10,000 Da. Moreover, the 10 kDa cellulose-based membrane showed good stability during a continuous period of three weeks exposure to the solution of DES and ethanol. Its pure water permeability decreased only by 3%. The results presented here demonstrate the possibility to utilize cellulose membranes in the treatment of spent DES to purify the solvent and recover the interesting compounds.
Highlights
Accepted: 11 January 2022Deep eutectic solvents (DESs) are mixtures of compounds with a melting point significantly lower (>50 ◦ C) than the melting points of each of the individual compounds [1,2].The melting point depression occurs due to the shift of the electric charge between a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD)
The HBA is represented by a quaternary ammonium salt, while the HBD is often represented by amides, carboxylic acids, and alcohols [3]
Viscosity measurements showed that the viscosity values of DES in ethanol solutions of different concentrations were close to each other when the temperature was high (Figure 2)
Summary
The melting point depression occurs due to the shift of the electric charge between a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD). This delocalization moves from an ion, which is the HBA, to a hydrogen-donor moiety, which is the HBD, along with the hydrogen bonding between them. Since the first description of DESs by Abbott et al [1], they have been studied as “green solvents” for various applications. Due to their unique physical and chemical properties, such as thermal stability, nonvolatility, and nontoxicity [4], DESs pose a significant interest
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