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

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

Effect of Cation on Physical Properties and CO₂ Solubility for Phosphonium-Based Ionic Liquids with 2‑Cyanopyrrolide Anions

A series of tetraalkylphosphonium 2-cyanopyrrolide ([P_<i>nnnn</i>]­[2-CNPyr]) ionic liquids (ILs) were prepared to investigate the effect of cation size on physical properties and CO₂ solubility. Each IL was synthesized in our laboratory and characterized by NMR spectroscopy. Their physical properties, including density, viscosity, and ionic conductivity, were determined as a function of temperature and fit to empirical equations. The density gradually increased with decreasing cation size, while the viscosity decreased noticeably. In addition, the [P_<i>nnnn</i>]­[2-CNPyr] ILs with large cations exhibited relatively low degrees of ionicity based on analysis of the Walden plots. This implies the presence of extensive ion pairing or formation of aggregates resulting from van der Waals interactions between the long hydrocarbon substituents. The CO₂ solubility in each IL was measured at 22 °C using a volumetric method. While the anion is typically known to be predominantly responsible for the CO₂ capture reaction, the [P_<i>nnnn</i>]­[2-CNPyr] ILs with shorter alkyl chains on the cations exhibited slightly stronger CO₂ binding ability than the ILs with longer alkyl chains. We attribute this to the difference in entropy of reaction, as well as the variation in the relative degree of ionicity

Phase-Change Ionic Liquids for Postcombustion CO₂ Capture

Phase-change ionic liquids, or PCILs, are salts that are solids at normal flue gas processing temperatures (e.g., 40–80 °C) and that react stoichiometrically and reversibly with CO₂ (one mole of CO₂ for every mole of salt at typical postcombustion flue gas conditions) to form a liquid. Thus, the melting point of the PCIL–CO₂ complex is below that of the pure PCIL. A new concept for CO₂ separation technology that uses this key property of PCILs offers the potential to significantly reduce parasitic energy losses incurred from postcombustion CO₂ capture by utilizing the heat of fusion (Δ<i>H</i>_fus) to provide part of the heat needed to release CO₂ from the absorbent. In addition, the phase transition yields almost a step-change absorption isotherm, so only a small pressure or temperature swing is required between the absorber and the stripper. Utilizing aprotic heterocyclic anions (AHAs), the enthalpy of reaction with CO₂ can be readily tuned, and the physical properties, such as melting point, can be adjusted by modifying the alkyl chain length of the tetra-alkylphosphonium cation. Here, we present data for four tetrabutylphosphonium salts that exhibit PCIL behavior, as well as detailed measurements of the CO₂ solubility, physical properties, phase transition behavior, and water uptake for tetraethylphosphonium benzimidazolide ([P₂₂₂₂]­[BnIm]). The process based on [P₂₂₂₂]­[BnIm] has the potential to reduce the amount of energy required for the CO₂ capture process substantially compared to the current technology that employs aqueous monoethanolamine (MEA) solvents

CO₂ Chemistry of Phenolate-Based Ionic Liquids

We synthesized ionic liquids (ILs) comprising an alkylphosphonium cation paired with phenolate, 4-nitrophenolate, and 4-methoxyphenolate anions that span a wide range of predicted reaction enthalpies with CO₂. Each phenolate-based IL was characterized by spectroscopic techniques, and their physical properties (viscosity, conductivity, and CO₂ solubility) were determined. We use the computational quantum chemical approach paired with experimental results to reveal the reaction mechanism of CO₂ with phenolate ILs. Model chemistry shows that the oxygen atom of phenolate associates strongly with phosphonium cations and is able to deprotonate the cation to form an ylide with an affordable activation barrier. The ATR-FTIR and ³¹P NMR spectra indicate that the phosphonium ylide formation and its reaction with CO₂ are predominantly responsible for the observed CO₂ uptake rather than direct anion–CO₂ interaction