Abstract
Ionic liquids (ILs) with aprotic heterocyclic anions (AHA) are attractive candidates for CO(2) capture technologies. In this study, a series of AHA ILs with 1-ethyl-3-methylimidazolium ([emim](+)) cations were synthesized, and their physical properties (density, viscosity, and ionic conductivity) were measured. In addition, CO(2) solubility in each IL was determined at room temperature using a volumetric method at pressures between 0 and 1 bar. The AHAs are basic anions that are capable of reacting stoichiometrically with CO(2) to form carbamate species. An interesting CO(2) uptake isotherm behavior was observed, and this may be attributed to a parallel, equilibrium proton exchange process between the imidazolium cation and the basic AHA in the presence of CO(2), followed by the formation of "transient" carbene species that react rapidly with CO(2). The presence of the imidazolium-carboxylate species and carbamate anion species was verified using (1)H and (13)C NMR spectroscopy. While the reaction between CO(2) and the proposed transient carbene resulted in cation-CO(2) binding that is stronger than the anion-CO(2) reaction, the reactions of the imidazolium AHA ILs were fully reversible upon regeneration at 80 °C with nitrogen purging. The presence of water decreased the CO(2) uptake due to the inhibiting effect of the neutral species (protonated form of AHA) that is formed.
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