Direct Air Capture of CO₂ by Amino Acid-Functionalized Ionic Liquid-Based Deep Eutectic Solvents

Direct air capture of carbon dioxide (CO2) is crucial for lowering the concentration of atmospheric CO2 and preventing global climate change. Nonetheless, a significant obstacle remains in advancing effective CO2 capture and separation techniques. The tunable chemical structures, high thermal stability, and nominal vapor pressure of ionic liquids (ILs) make them attractive solvents for CO2 absorption. Deep eutectic solvents (DESs) are broadly recognized as a new class of IL analogs because of their numerous similarities and features to ILs. In this work, at first, we have synthesized alanine- and lysine-based ILs, tetraethylammonium alaninate ([N2222][Ala]) and tetraethylammonium lysinate ([N2222][Lys]), and utilized them to prepare DESs by mixing the ILs as hydrogen-bond acceptor (HBA) and ethylene glycol (EG) as hydrogen-bond donor (HBD). Those DESs are less viscous and have a higher CO2 absorption efficiency than their constituent ILs. The vapor–liquid equilibrium (VLE) experiment shows that the pure CO2 absorption capacity of [N2222][Lys]-based DES is considerably higher than [N2222][Ala] containing DES. We have employed those amino acid-functionalized IL-based DESs to capture CO2 from the open air for the very first time, to the best of our knowledge. The direct air CO2 capture experiment shows that the DESs are highly efficient in capturing CO2 from the open air. The open air saturated CO2 uptake in [N2222][Lys]-based DES is 1.06 mol/mol, whereas the uptake value in [N2222][Ala] containing DES is 0.47 mol/mol. Those DESs are reversible toward the absorption and desorption of CO2 up to several cycles. We believe that those systems will be superior for eliminating CO2 from the highly polluted air.