Direct Air\nCapture of CO<sub>2</sub> by Amino Acid-Functionalized\nIonic Liquid-Based Deep Eutectic Solvents

Direct\nair capture of carbon dioxide (CO<sub>2</sub>)\nis crucial\nfor lowering the concentration of atmospheric CO<sub>2</sub> and preventing\nglobal climate change. Nonetheless, a significant obstacle remains\nin advancing effective CO<sub>2</sub> capture and separation techniques.\nThe tunable chemical structures, high thermal stability, and nominal\nvapor pressure of ionic liquids (ILs) make them attractive solvents\nfor CO<sub>2</sub> absorption. Deep eutectic solvents (DESs) are broadly\nrecognized as a new class of IL analogs because of their numerous\nsimilarities and features to ILs. In this work, at first, we have\nsynthesized alanine- and lysine-based ILs, tetraethylammonium alaninate\n([N<sub>2222</sub>][Ala]) and tetraethylammonium lysinate ([N<sub>2222</sub>][Lys]), and utilized them to prepare DESs by mixing the\nILs as hydrogen-bond acceptor (HBA) and ethylene glycol (EG) as hydrogen-bond\ndonor (HBD). Those DESs are less viscous and have a higher CO<sub>2</sub> absorption efficiency than their constituent ILs. The vapor–liquid\nequilibrium (VLE) experiment shows that the pure CO<sub>2</sub> absorption\ncapacity of [N<sub>2222</sub>][Lys]-based DES is considerably higher\nthan [N<sub>2222</sub>][Ala] containing DES. We have employed those\namino acid-functionalized IL-based DESs to capture CO<sub>2</sub> from\nthe open air for the very first time, to the best of our knowledge.\nThe direct air CO<sub>2</sub> capture experiment shows that the DESs\nare highly efficient in capturing CO<sub>2</sub> from the open air.\nThe open air saturated CO<sub>2</sub> uptake in [N<sub>2222</sub>][Lys]-based\nDES is 1.06 mol/mol, whereas the uptake value in [N<sub>2222</sub>][Ala] containing DES is 0.47 mol/mol. Those DESs are reversible\ntoward the absorption and desorption of CO<sub>2</sub> up to several\ncycles. We believe that those systems will be superior for eliminating\nCO<sub>2</sub> from the highly polluted air.