Screening of deep eutectic solvents (DESs) as green CO2 sorbents: from solubility to viscosity

Abstract

Deep eutectic solvents (DESs) as ionic liquid (IL) analogues show great potential for CO₂ capture. They exhibit favorable solvent properties and are considered to be economical alternatives to conventional ILs. In this study, we prepare 35 DESs and screen them in terms of their CO₂ solubility and viscosity, both crucial factors to be considered when designing efficient CO₂ sorbents. The influence of salt and HBD type and structure, as well their molar ratio on the CO₂ solubility and viscosity of the DESs is investigated. The viscosity and CO₂ solubility of the DESs are compared with those of other DESs and conventional ILs. 15 DESs, which exhibit comparable CO₂ absorption capacity to choline chloride–urea DESs, glycerol DESs and fluorinated ILs, are chosen as the promising ones. The viscosities of the selected DESs are below 200 mPa s and are lower than those of choline chloride-based DESs. Since the viscosity of the DESs is relatively high, on a par with those of conventional ILs, the effect of water as a co-solvent is investigated in order to decrease the viscosity. The addition of water to the glycerol-based DESs improves the kinetics of absorption by decreasing the viscosity, thus increasing the CO₂ absorption capacity. Dry or aqueous DESs that demonstrate a high sorption capacity and low viscosity are chosen for additional analysis and characterization, and further functionalization will be carried out in the future to improve their sorption capacity.