The discovery of materials that combine selectively, controllably, and reversibly with CO2 is a key challenge for realizing practical carbon capture from flue gas and other point sources. We report the design of ionic liquids (ILs) with properties tailored to this CO2 separation problem. Atomistic simulations predict that suitably substituted aprotic heterocyclic anions, or “AHAs,” bind CO2 with energies that can be controlled over a wide range suitable to gas separations. Further, unlike all previously known CO2-binding ILs, the AHA IL viscosity is predicted to be insensitive to CO2. Spectroscopic, temperature-dependent absorption, rheological, and calorimetric measurements on trihexyl(tetradecyl)-phosphonium 2-cyanopyrrolide ([P66614][2-CNpyr]) show CO2 uptakes close to prediction as well as insignificant changes in viscosity in the presence of CO2. A pyrazolide-based AHA IL behaves qualitatively similarly but with weaker binding energy. The results demonstrate the intrinsic design advantages of ILs as a platform for CO2 separations.

New experimental thermal conductivity, density, viscosity, glass transition temperature, and heat capacity values were measured for nine ionic liquids (ILs): [emim]­[TFA], [emim]­[OTf], [emim]­[DEP], [emim]­[MeSO3], [emim]­[SCN], [hmim]­[Tf2N], [bDMApy]­[Tf2N], [hDMApy]­[Tf2N], and [hmDMApy]­[Tf2N]. Classical molecular mechanics force fields were developed and used to calculate thermodynamic and transport properties for these ILs using molecular dynamics. Two versions of each force field were developed: one with integer charges of ± 1 and one with all charges scaled by 0.8. The force fields with total charges of ± 0.8 generally gave better agreement with experimental results. Very good agreement was obtained for density and heat capacity. Simulated values for thermal conductivity slightly overpredicted experimental results but captured trends between different ILs very well. Experimental Prandtl numbers were determined as a function of temperature and can exceed 10 000 at low temperature. Prandtl numbers on the order of 100–1000 were observed above 330 K. These values suggest that heat transfer with ionic liquids will be dominated by convective effects.

Mixtures of ionic liquids (ILs) and molecular amines have been suggested for CO2 capture applications. The basic idea is to replace water, which volatilizes in the amine regeneration step and increases the parasitic energy load, with a nonvolatile ionic liquid solvent. To fully understand the thermodynamics of these systems, here experimental excess enthalpies for binary mixtures of monoethanolamine (MEA) and two ILs: 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [hmim][NTf2], and 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [OHemim][NTf2], were obtained by calorimetry, using a Setaram C80 calorimeter, over the whole range of compositions at 313.15 K. Since it is the temperature derivative of the Gibbs energy, enthalpy is a sensitive measure of intermolecular interactions. MEA + [hmim][NTf2] is endothermic and MEA + [OHemim][NTf2] is exothermic. The reliability of COSMO-RS to predict the excess enthalpy of the (MEA+IL) systems was tested based on the implementation of two different molecular models to define the structure of the IL: the IL as separate cation and anion [C+A] and the IL as a bonded single specie [CA]. Quantum-chemical calculations were performed to gain additional insight into the intermolecular interactions between the components of the mixture. For MEA + [hmim][NTf2] both the [C+A] and [CA] models predict endothermic behavior, but the [CA] model is in better agreement with the experimental results. For MEA + [OHemim][NTf2] the [C+A] model provides the best match to the experimental exothermic results. However, what is really surprising is that two different conformations of the cation-anion pair with nearly identical energies in the [CA] model result in completely different (exothermic vs endothermic) predictions of the excess enthalpy. Nonetheless, the results do show that the influence of the structure of the IL on the thermodynamic behavior of the mixture (endothermic vs exothermic) can be attributed to hydrogen bonding between the cation and the MEA molecule. However, this study highlights the importance of carefully selecting the molecular model and conformation in order to obtain even qualitatively correct predictions with COSMO-RS. The fact that even very slightly different conformations of the IL can drastically change the thermodynamic estimations using COSMO-RS is of significant concern. Overall, we believe the present work provides a better understanding of the behavior of mixtures involving amines and ILs, which is an important aspect to consider when evaluating the use of such solvent mixtures in CO2 capture technologies.

The molecular interactions between the ionic liquid trimethylbutylammomium bis(trifluoromethylsulfonyl)imide, [N 1114 ]-[Tf 2 N], and three common solvents, including short chain alcohols (ethanol and 1-propanol) and an aprotic solvent (N,N-dimethylformamide or DMF) were investigated through experimental properties. The excess enthalpies were measured by calorimetry at temperatures from 308.15 K to 323.15 K over the whole composition range and excess volumes were calculated from density measurements. The vapor− liquid equilibrium (VLE) measurements of the [N 1114 ][Tf 2 N] + DMF mixture were obtained using a headspace gas chromatography instrument. It was determined that hydrogen bonds between the alcohol molecules contribute to the positive excess enthalpies, while the favorable interactions between DMF and [N 1114 ][Tf 2 N] might be explained by electrostatic interactions between the ionic liquid and the zwitterionic resonance structure of DMF. The nonrandom two liquid (NRTL) equation was successfully used to correlate the excess enthalpy and VLE results.

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