Physical Properties and CO₂ Reaction Pathway of 1‑Ethyl-3-Methylimidazolium Ionic Liquids with Aprotic Heterocyclic Anions

Abstract

Ionic liquids (ILs) with aprotic heterocyclic anions (AHA) are attractive candidates for CO₂ 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, CO₂ 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 CO₂ to form carbamate species. An interesting CO₂ 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 CO₂, followed by the formation of “transient” carbene species that react rapidly with CO₂. The presence of the imidazolium-carboxylate species and carbamate anion species was verified using ¹H and ¹³C NMR spectroscopy. While the reaction between CO₂ and the proposed transient carbene resulted in cation-CO₂ binding that is stronger than the anion-CO₂ 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 CO₂ uptake due to the inhibiting effect of the neutral species (protonated form of AHA) that is formed