(355n) Mixtures of Amino Acid Salts and Ethylene Glycol for CO2 Capture

The concentration of CO₂ in our atmosphere continues to rise due to anthropogenic emission sources such as fossil fuel powerplants and cars, recently surpassing 400ppm which is nearly double the average value from the last 800,000 years. As a result, researchers continue to develop CO₂ capture materials to combat this drastic increase. Ionic liquids (ILs) have been of particular interest due to their tunable reactive capture of CO₂ and negligible volatility. Since CO₂ is acidic, basic anions in ILs have demonstrated CO₂ reactivity near a 1:1 molar ratio. Another group of solvents known as deep eutectic solvents (DESs) have been of interest as well due to similarly tunable properties to ILs and high CO₂ solubilities. A common hydrogen bond donor in DESs is ethylene glycol which supports a hydrogen bonding network in the liquid and this is credited with the deep melting point depression that is characteristic of DESs. Ethylene glycol has also been shown to react with CO₂ in the presence of basic anions.

In this study, natural DESs made from a choline cation and series of amino acid anions (glycine, alanine, proline, and phenylalanine) were mixed with ethylene glycol to yield CO₂-reactive solvents. The melting points of the mixtures were measured using differential scanning calorimetry. The resulting phase diagrams indicate eutectic behavior. These solvents were tested for CO₂ capacity at various CO₂ partial pressures in nitrogen (400 ppm CO₂ to 100% CO₂ at 1 bar). Choline amino acids show very slow CO₂ uptake because they are extremely viscous (>1000 cP). Therefore, dilution with an active solvent such as ethylene glycol improves mass transport of CO₂ while maintaining a high CO₂ capacity. The CO₂ absorption mechanism of these solvents was assessed via quantitative ¹³C-NMR. These eutectic solvents are made entirely from natural materials that are significantly cheaper and easier to synthesize than most ILs.