Role of water on deep eutectic solvents (DES) properties and gas transport performance in biocatalytic supported DES membranes

The development of efficient and clean CO2 capture technologies is a topic of utmost importance in the global environmental agenda. In this work, a wide range of analytical procedures (1H and 13C NMR, TGA, DSC, FT-IR, viscosity, density, refractive index, fluorescence anisotropy, UV–Vis) was used to investigate the properties of deep eutectic solvent (DES) composed of choline chloride and levulinic acid (ChCl:LA), conditioned to four different water activity (aw) values. Also, the impact of using the enzyme carbonic anhydrase (CA), known for its ability to reversibly convert CO2 into bicarbonate, was assessed on DES properties. The ChCl:LA-water mixtures showed to be present as a cohesive cluster up to aw = 0.753 and, above that, in a hydrated form with a free water layer that could crystalize at negative temperatures (-40 to −45 °C), suggesting anti-freezing properties of the solvent. DES were then supported in polymeric hydrophilic membranes. Based on CO2 and N2 permeabilities through these membranes, the one that contained ChCl:LA at aw = 0.216 was further studied regarding its long-term stability. This supported liquid membrane showed a consistent behavior for up to 30 repeated cycles and a stable operating behavior for transmembrane pressure differences in the range of 0.50–1.25 bar. These findings evidence the stability of supported DES membranes and the potential for CO2 capture.