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

Abstract

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.

Understanding the Electrode–Electrolyte Interfaces of Ionic Liquids and Deep Eutectic Solvents

“…Electrocatalysis is yet another domain where the interfacial structure is crucial to understand such as the case for the electrochemical CO 2 reduction reaction (CO 2 RR) . In particular, ILs and DESs are known for their high CO 2 solubility − and thus present advantages compared to the aqueous electrolytes with low CO 2 solubilities . The [EMIM]­[BF 4 ] IL was even reported to co-catalyze the CO 2 RR .…”

Formation of choline salts and dipolar ions for CO2 reactive eutectic solvents

“…1 The tunable and benign nature of eutectic solvents makes them appealing for various applications, particularly for separations. [2][3][4][5] A major driver for the recent surge in eutectic solvent research for lowpressure CO 2 separations is their appreciable CO 2 capacities, attainable when functionalized with nucleophilic sites for CO 2 chemisorption. 5,6 Choline-based eutectic solvents, particularly those containing biodegradable choline chloride ([Ch] + [Cl] À ), are readily available and can be derivatized for specific applications.…”

“…[2][3][4][5] A major driver for the recent surge in eutectic solvent research for lowpressure CO 2 separations is their appreciable CO 2 capacities, attainable when functionalized with nucleophilic sites for CO 2 chemisorption. 5,6 Choline-based eutectic solvents, particularly those containing biodegradable choline chloride ([Ch] + [Cl] À ), are readily available and can be derivatized for specific applications. 7 Ethaline is a common example of a eutectic solvent composed of [Ch] + [Cl] À and EG (about 1 : 4 molar ratio).…”

“…7 Ethaline is a common example of a eutectic solvent composed of [Ch] + [Cl] À and EG (about 1 : 4 molar ratio). 8 While ethaline has no CO 2 chemisorption capacity, anion functionalization, such as replacing the [Cl] À anion with prolinate [Pro] À or glycinate [Gly] À , results in the formation of CO 2 reactive eutectic solvents, 5,9 with CO 2 binding to the anion, but not to [Ch] + or EG. These functionalized solvents present appreciable CO 2 capacities at low partial pressures, 6,[9][10][11] which is ideal for CO 2 capture applications, such as direct air capture.…”

“…The addition of EG to viscous ionic liquids (ILs) 2,5 or solid organic salts 6,7 for the formation of eutectic solvents enhances the CO 2 transport properties. However, in the presence of anions that are strong HBAs, EG was reported to deprotonate and provide an alternative binding site for CO 2 .…”