How Does the Moisture Affect CO2 Absorption by a Glycinate Ionic Liquid?

Ionic liquids (ILs) have shown great potential in CO2 capture from the exhaust of fossil fuels burning due to their unique structures and properties. Since the flue gas often contains a small amount of water, understanding the effect of water is critical for the direct capture of postcombustion CO2 by ionic liquids. In recent years, the effect of water in CO2 capture by ILs has been studied in some details, but little is known of the new species produced after humid CO2 capture and thus for the system composition as well as the contribution of each absorption site of ILs to the capture capacity of CO2. In this work, a simple amino acid ionic liquid, 1-ethyl-3-methylimidazolium glycinate ([C2mim][Gly]), has been prepared and used to absorb humid CO2 at 25 °C, and a quantitative approach is established to estimate the absorption capacity of CO2 by different absorption sites. It is found that the absorption capacity of CO2 is as high as 0.91 mol CO2 per mol IL in the wet environment, which is nearly double that of dry CO2 by neat IL. Quantitative investigations by multiple spectral techniques and quantum chemical calculations indicate that the inhalation of H2O results in the production of [HCO3]− in the system and activation of the C2 site of the imidazolium cation. It is this activated site that reacts with CO2 to form imidazolium-2-carboxylate (NHC-CO2) and significantly improves the absorption capacity of CO2. This is remarkably different from the absorption of dry CO2, in which anions of the IL are predominant for the absorption of CO2.