Influence of Hydrogen Bonding on CO2 Capture in Choline and Ethylene Glycol Mixtures

There is a surge in the interest in carbon capture because of increasing global temperatures, which is a consequence of rising CO₂ emissions. One of the most commonly used absorbers for carbon capture is aqueous amine solution because of its high affinity and selectivity for CO₂ even under low concentrations. However, despite this high CO₂ affinity, aqueous amine requires high temperatures for regeneration, while also being highly volatile and corrosive. Therefore, alternative sorbents with low volatility and tunable binding enthalpy, such as eutectic solvents, are being studied for carbon capture. Eutectic solvents relevant to CO₂ capture are often composed of hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs) with the ability to chemisorb CO₂. Given that the HBA and HBD compositions affect key solvent properties, such as volatility and CO₂ capacity, this study focuses on creating a better understanding of the impact of the HBD concentration on the CO₂ separation process, in particular the capacity and regeneration. To achieve this objective, mixtures of ethylene glycol (EG) HBD and choline 2-cyanopyrrolide ([Ch]⁺[CNpyr]^-) HBA were examined as a function of HBA:HBD molar composition (1:3, 1:4, and 1:5). The samples were evaluated in terms of their CO₂ capacity and regenerability by performing absorption experiments at 25 °C and 1 bar of CO₂ followed by desorption at 50 °C. Products and the various CO₂ binding sites contributing to the overall capacity were identified and quantified by ¹³C nuclear magnetic resonance (NMR) spectroscopy. A general decrease of the gravimetric capacity was observed with increasing HBD concentration through NMR analysis. Overall, while the concentration of HBDs did not directly impact the regenerability of the solvent, the increased HBD-HBD and HBA-HBD interactions at higher HBD concentrations resulted in a decrease in both the HBD-CO₂ and HBA-CO₂ adduct formation. This study showcases the design features in eutectic solvents, and primarily the compositional effects, in controlling the CO₂ absorption-desorption capability as relevant to CO₂ capture for emission mitigation and direct air capture efforts.