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

Abstract

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.