Impact of Hydrogen Bonds on CO₂ Binding in Eutectic Solvents: An Experimental and Computational Study toward Sorbent Design for CO₂ Capture

Choline-based amino acid ionic liquids with anions glycinate, β-alaninate, phenylalaninate, and prolinate were synthesized and mixed with ethylene glycol to form lower-viscosity benign eutectic solvents for CO₂ capture. The highest capacity measured was 0.7 moles of CO₂ per mole of ionic liquid (2 moles CO₂ per kg solvent) for a 1 to 2 mole ratio mixture of choline prolinate to ethylene glycol at 1 bar of CO₂ and 25 °C. Under 5000 ppm of CO₂, half of this capacity was realized. Through a combined study of quantitative ¹³C NMR spectroscopy, molecular dynamics simulations and density functional theory calculations, we show that hydrogen bonding in the eutectic solvent prevents proton-transfer between prolinate anions upon CO₂ absorption, which occurs in the absence of ethylene glycol and deactivates binding sites. Blocking this proton transfer leads to a higher binding capacity compared to neat choline prolinate. This work demonstrates the impact of hydrogen bonding on the CO₂ binding mechanism and energetics, as well as physical and thermal properties in eutectic solvents, thus addressing an unmet need and informing future studies on the development of benign sorbents for capturing CO₂ from dilute streams.