Effect of Water on CO2 Capture by Aprotic Heterocyclic Anion (AHA) Ionic Liquids

The reaction between three aprotic heterocyclic anion (AHA) ionic liquids (ILs) (triethyl(octyl)phosphonium 2-cyanopyrrolide [P-2228]-[2CNPyr], triethylphosphonium benzimidazolide [P-2222] [BnIm], and triethyl(octyl)phosphonium indazolide [P-2228][Inda]) and CO2, in the presence of water, is investigated in this study. We propose a reaction mechanism where, in addition to the reaction of the anion with CO, to form carbamate, the anion reacts with water (i.e., hydronium ions in water from either the natural dissociation of water or the formation of carbonic acid) and is reprotonated, leaving CO2 to react with hydroxide to form bicarbonate. This mechanism is confirmed for [P-2228][2CNPyr] and [P-2222] [BnIm] by H-1 NMR, C-13 NMR, heteronuclear single quantum correlation NMR, and heteronuclear multiple bond correlation NMR, which were used to identify and quantify the reaction products. However, unlike the other two ILs, [P-2228] [Inda] exhibits reprotonation of the anion when in contact with water only; i.e., CO, is not needed to form carbonic acid to increase the proton concentration in water. The reversibility of the reprotonation reaction was tested by measuring the CO2 solubility in [P-2228][2CNPyr] and [P-2222][BnIm] in the presence of water for several cycles using a volumetric method. Previously, we have shown that the reaction of AHA ILs with CO2 to form carbamate (in the absence of water) is reversible. Additionally, encapsulated [P-2228][2CNPyr] and [P-2222][BnIm] were subjected to a CO2 solubility recyclability test in the presence of water. Both reactions between AHA ILs and CO2 in the presence of water appear to be fully reversible. The encapsulated ILs showed good CO2 capture capacity in the presence of water and good recyclability results, as well.