Imidazole-Monoethanolamine-Based Deep Eutectic Solvent for Carbon Dioxide Capture: A Combined Experimental and Molecular Dynamics Investigation

Imidazole (IMI) and monoethanolamine (MEA) are mixed in various molar ratios to form a nonionic deep eutectic solvent (DES). This DES shows promising application for carbon dioxide (CO₂) capture. Solubility of CO₂ in the DES was directly related to changes in pressure while being inversely proportional to change in temperature. The highest CO₂ loading of 0.711 mol CO₂/mol DES was obtained at 30 °C, 10 bar and for a DES molar ratio of 1:4. Interestingly, upon addition of 50 vol % (47.62 wt %) water to the DES, the absorption capacity of the DES was almost doubled to 1.357 mol CO₂/mol DES. The calculated Henry’s constant value and the negative CO₂ absorption enthalpy indicate a strong interaction between the DES and a low regeneration energy requirement. Nonreactive molecular dynamics (MD) simulations were performed to investigate the local microstructure of IMI and MEA in neat and wet DES and the various key interactions responsible for CO₂ absorption identified. The potential of mean force-based free energy MD calculations indicated that in the presence of water, the DES shows increased CO₂ physisorption, consistent with our experimental results. The inclusion of water in the DES weakens the inter- and intramolecular interactions between MEA and IMI, which is observed from the reduction in peak heights for the various pairwise interactions obtained from molecular dynamics simulations. The weakening of the inter- and intramolecular hydrogen-bonding interactions in MEA and IMI in the presence of water results in the exposure of the amine and hydroxyl sites on MEA and the annular NH nitrogen group in IMI, thereby enabling such sites to interact favorably with CO₂ and result in increased absorption. This fundamental study should open many avenues for more indepth investigations involving IMI/MEA-based DES and their potential selective absorption of other flue gases.