The climate crisis has continued to worsen in recent years, with one of the primary contributions being rising carbon dioxide (CO2) concentrations in the atmosphere. This increase, a direct result of anthropogenic CO2 emissions, has reached a point where it can no longer be reversed by reaching net-zero carbon emission without severe environmental cost. Negative emission technologies such as direct air capture (DAC) are essential to preventing devastating climate disaster. Anion-functionalized ionic liquids (ILs) have demonstrated excellent CO2 absorption performance under very low partial pressure conditions such as air, but high viscosity has resulted in poor transport, reducing absorption rates. Improving CO2 transport in such sorbents is essential to making them viable DAC materials. The addition of hydrogen bond donors (HBDs) to ILs such as glycols, forming a deep eutectic solvent (DES), has been shown to lower viscosity while maintaining high CO2 capacity at low partial pressures. This study illuminates the CO2 absorption mechanism in DESs made from choline amino acid ILs mixed with glycols. The amino acid anions studied include glycinate (Gly), b-alaninate (BAla), phenylalaninate (Phe), and prolinate (Pro). Amino acid ILs have been shown to react with CO2 in up to 1:1 molar ratio. We observe only 1:2 mole ratio absorption in choline amino acid ILs, and with the primary amino acids (Gly, BAla, and Phe), precipitation of unreacted amino acid is observed which limited reversibility. Choline Pro demonstrated good CO2 capacity with no precipitation. CO2 absorption stoichiometry improved upon dilution with the glycol. Using quantitative 13C-NMR, we assessed the content of various absorption products after exposing the mixtures to feed streams of different CO2 concentrations (410ppm, 2500ppm, 5000ppm, and 1 bar). We paired these experimental results with DFT calculations in order to determine the absorption mechanism and assess the energetics of the system. We found that when reaction sites are hindered by hydrogen bonds, the CO2 capacity is lowered. By removing hydrogen bonding groups in the IL cation and the HBD, we investigated the impact of hydrogen bonding and nucleophilicity in the system on the CO2 absorption products. This work will inform future IL and DES design for DAC processes.

Understanding mechanisms of carbon dioxide absorption in choline amino acid ionic liquids mixed with glycols for direct air capture

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