Physical
Properties and CO<sub>2</sub> Reaction Pathway
of 1‑Ethyl-3-Methylimidazolium Ionic Liquids with Aprotic Heterocyclic
Anions
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Abstract
Ionic liquids (ILs) with aprotic
heterocyclic anions (AHA) are
attractive candidates for CO<sub>2</sub> capture technologies. In
this study, a series of AHA ILs with 1-ethyl-3-methylimidazolium ([emim]<sup>+</sup>) cations were synthesized, and their physical properties
(density, viscosity, and ionic conductivity) were measured. In addition,
CO<sub>2</sub> 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<sub>2</sub> to form carbamate species. An interesting CO<sub>2</sub> 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<sub>2</sub>, followed
by the formation of “transient” carbene species that
react rapidly with CO<sub>2</sub>. The presence of the imidazolium-carboxylate
species and carbamate anion species was verified using <sup>1</sup>H and <sup>13</sup>C NMR spectroscopy. While the reaction between
CO<sub>2</sub> and the proposed transient carbene resulted in cation-CO<sub>2</sub> binding that is stronger than the anion-CO<sub>2</sub> 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<sub>2</sub> uptake due to the inhibiting effect
of the neutral species (protonated form of AHA) that is formed