Preparation and Characterization of Deep Eutectic Solvents That Can Be Used in CO₂ Absorption Processes ^†

CO₂ emissions are well-known for creating a lot of environmental issues, at a global scale. One of the most-used technologies for CO₂ capture is the post-combustion method, which consists of separating the CO₂ from the combustion gases, using a solvent, usually amine. In this study, we propose alternative solvents: DESs (deep eutectic solvents) [1], based on choline chloride and amine, and switchable hydrophilicity DESs, based on fatty acids and diluted amine solutions. For standard DESs, choline chloride (ChCl) was used as hydrogen bond donor (HBD), and amines (monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA)) as hydrogen bond acceptors (HBAs) [2,3], in different molar ratios: 1:5, 1:6, 1:8, and 1:10. All the components were precisely weighed and mixed at 300 RPM, 60 °C, until a clear solution was obtained. For switchable hydrophilicity DESs, the hydrophobic part is represented by a hydrophobic DES [4], made up of octanoic acid (OA) and other three acids, successively: dodecanoic (DA), myristic (MA), and stearic acid (SA), each in a 3:1 molar ratio. The hydrophilic phase consists of 10% amine solution [5] MEA, DEA, and TEA. The two phases were blended at 1:13 v/v oil–water, and then analyzed. The obtained solvents were in liquid state at room temperature, except for the OA–SA DES. After adding the amine solutions, the OA–SA–amine DESs were also liquid. All DESs were characterized using pH, density, viscosity, electrical conductivity, refraction index, surface tension, FTIR, and NMR. The prepared DESs are used for solubility tests to determine CO₂ absorption capacity. The hybrid DES–amines mixtures have higher absorption capacity than the ChCl-based DESs and the conventional 30% MEA—up to double CO₂ absorption capacity. Comparing the three amine types, MEA gives the best results, even in hybrid DESs. DESs represent a new green solution for CO₂ absorption. Their advantages consist of cost-effective solvent price, easy preparation, easy reuse, and regeneration. Based on the previous observations that amines and DESs can absorb CO₂, we developed combinations between them that improve the CO₂ absorption capacity and that can be tailored for controlled release in various applications.