Solvents made from a reactive ionic liquid, with an imidazolium cation and pyrrolide anion, and ethylene glycol at a wide compositional range were studied for separations of CO 2 at low partial pressures (≪0.1 bar up to 1 bar). Thermal analysis and measurements of viscosity and density show compacting of the liquid upon mixing with enhanced stability achieved by hydrogen bonding. A detailed mechanistic study was performed by IR, quantitative NMR, and ab initio calculations that show significant CO 2 absorption capacity below 5000 ppm of CO 2 in N 2 . Three reversible routes are found that yield carbonate (major product), carboxylate (moderate), and carbamate (minor) species. With CO 2 at 100% RH, bicarbonate along with carbonate species form. The CO 2 -ethlyene glycol reaction complex, the carbonate anion, is stabilized by the hydrogen bonding and Coulombic interactions, thus preventing evaporation of the solvent during regeneration. This study demonstrates a promising approach to designer green solvents for CO 2 separations in open systems such as direct air capture.

The task-specific ionic liquid (IL), 1-ethyl-3-methylimidazolium 2-cyanopyrolide ([EMIM]­[2-CNpyr]), was encapsulated with polyurea (PU) and graphene oxide (GO) sheets via a one-pot Pickering emulsion, and these capsules were used to scrub CO2 (0–5000 ppm) from moist air. Up to 60 wt % of IL was achieved in the synthesized capsules, and we demonstrated comparable gravimetric CO2 capacities to zeolites and enhanced absorption rates compared to those of bulk IL due to the increased gas/liquid surface-to-volume area. The reactive IL capsules show recyclability upon mild temperature increase compared to zeolites that are the conventional absorber materials for CO2 scrubbing. The measured breakthrough curves in a fixed bed under 100% relative humidity establish the utility of reactive IL capsules as moisture-stable scrubber materials to separate CO2 from air, outperforming zeolites owing to their higher selectivity. It is shown that thermal stability, CO2 absorption capacity, and rate of uptake by IL capsules can be further modulated by incorporating low-viscosity and nonreactive ILs to the capsule core. This study demonstrates an alternative and facile approach for CO2 scrubbing, where separation from gas mixtures with extremely low partial pressures of CO2 is required.

Summary The desire toward decarbonization and renewable energy has sparked research interests in reactive CO 2 separations, such as direct air capture that utilize electricity as opposed to conventional thermal and pressure swing processes, which are energy-intensive, cost-prohibitive, and fossil-fuel dependent. Although the electrochemical approaches in CO 2 capture that support negative emissions technologies are promising in terms of modularity, smaller footprint, mild reaction conditions, and possibility to integrate into conversion processes, their practice depends on the wider availability of renewable electricity. This perspective discusses key advances made in electrolytes and electrodes with redox-active moieties that reversibly capture CO 2 or facilitate its transport from a CO 2 -rich side to a CO 2 -lean side within the last decade. In support of the discovery of new heterogeneous electrode materials and electrolytes with redox carriers, the role of computational chemistry is also discussed.

In recent years, significant development milestones have been reached in the areas of facilitated transport membranes and ionic liquids for CO 2 separations, making the combination of these materials an incredibly promising technology platform for gas treatment processes, such as post-combustion and direct CO 2 capture from air in buildings, submarines, and spacecraft. The developments in facilitated transport membranes involve consistently surpassing the Robeson upper bound for dense polymer membranes, demonstrating a high CO 2 flux across the membrane while maintaining very high selectivity. This mini review focuses on the recent developments of facilitated transport membranes, in particular discussing the challenges and opportunities associated with the incorporation of ionic liquids as fixed and mobile carriers for separations of CO 2 at low partial pressures (<1 atm).

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.