Amino acid ionic liquid trihexyl(tetradecyl)phosphonium methioninate [P(66614)][Met] and prolinate [P(66614)][Pro] absorb CO(2) in nearly 1:1 stoichiometry, surpassing by up to a factor of 2 the CO(2) capture efficiency of previously reported ionic liquid and aqueous amine absorbants for CO(2). Room temperature isotherms are obtained by barometric measurements in an accurately calibrated stirred cell, and the product identity is confirmed using in situ IR. Density functional theory (DFT) calculations support the 1:1 reaction stoichiometry and predict reaction enthalpies in good agreement with calorimetric measurements and isotherms.

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.

Ionic liquids (ILs) with aprotic heterocyclic anions, or AHAs, can bind CO2 with reaction enthalpies that are suitable for gas separations and without suffering large viscosity increases. In the present work, we have synthesized ILs bearing an alkyl-phosphonium cation with indazolide, imidazolide, pyrrolide, pyrazolide and triazolide-based anions that span a wide range of predicted reaction enthalpies with CO2. Each AHA-based IL was characterized by NMR spectroscopy and their physical properties (viscosity, glass transition, and thermal decomposition temperature) determined. In addition, the influence of substituent groups on the reaction enthalpy was investigated by measuring the CO2 solubility in each IL at pressures between 0 and 1 bar at 22 °C using a volumetric method. The isotherm-derived enthalpies range between -37 and -54 kJ mol(-1) of CO2, and these values are in good agreement with computed enthalpies of gas-phase IL-CO2 reaction products from molecular electronic structure calculations. The AHA ILs show no substantial increase in viscosity when fully saturated with CO2 at 1 bar. Phase splitting and compositional analysis of one of the IL/H2O and IL/H2O/CO2 systems conclude that protonation of the 2-cyanopyrrolide anion is improbable, and this result was confirmed by the equimolar CO2 absorption in the presence of water. Taking advantage of the tunable binding energy and absence of viscosity increase after the reaction with CO2, AHA ILs are promising candidates for efficient and environmental-friendly absorbents in postcombustion CO2 capture.

Amine-functionalized anion-tethered ionic liquids (ILs), trihexyl(tetradecyl)phosphonium glycinate [P66614][Gly], alanate [P66614][Ala], sarcosinate [P66614][Sar], valinate [P66614][Val], leucinate [P66614][Leu], and isoleucinate [P66614][Ile], were synthesized and investigated as potential absorbents for CO2 capture from postcombustion flue gas. Their physical properties, including density, viscosity, glass transition temperature, and thermal decomposition temperature, were determined. The influence of changing the anion and, more specifically, the length of the alkyl chain is discussed. Furthermore, the CO2 absorption isotherms of [P66614][Gly], [P66614][Ile], [P66614][Sar], and [P66614][Ala] were measured using a volumetric method, and the results were modeled with two different Langmuir-type absorption models. All four ILs reached greater than 0.5 mol of CO2 per mole of IL at CO2 pressures of less than 1 bar. This indicates the predominance of the 1:1 mechanism, where the CO2 reacts with one IL to form a carbamic acid, over further reaction with another IL to make a carbamate (the 1:2 mechanism). The chemical absorption of CO2 dramatically increased the viscosity of the IL, but this can be mitigated to some extent by decreasing the number of hydrogens on the anion available for hydrogen-bonding.

Amine-functionalized anion-tethered ionic liquids (ILs) trihexyl(tetradecyl)phosphonium asparaginate [P(66614)][Asn], glutaminate [P(66614)][Gln], lysinate [P(66614)][Lys], methioninate [P(66614)][Met], prolinate [P(66614)][Pro], taurinate [P(66614)][Tau], and threoninate [P(66614)][Thr] were synthesized and investigated as potential absorbents for CO(2) capture from postcombustion flue gas. Their physical properties, including density, viscosity, glass transition temperature, and thermal decomposition temperature were determined. Furthermore, the CO(2) absorption isotherms of [P(66614)][Lys], [P(66614)][Tau], [P(66614)][Pro], and [P(66614)][Met] were measured using a volumetric method, and the results were modeled with two different Langmuir-type absorption models. The most important result of this study is that the viscosity of [P(66614)][Pro] only increased by a factor of 2 when fully complexed with 1 bar of CO(2) at room temperature. This is in stark contrast to the other chemically reacted ILs investigated here and all other amino acid-based ILs reported in the literature, which dramatically increase in viscosity, typically by 2 orders of magnitude, when complexed with CO(2). The unique behavior of [P(66614)][Pro] is likely due to its ring structure, which limits the number and availability of hydrogen atoms that can participate in a hydrogen bonding network. We found that water can be used to further reduce the viscosity of the CO(2)-complexed IL, while only slightly decreasing the CO(2) capacity. Finally, from temperature-dependent isotherms, we estimate a heat of absorption of -63 kJ/mol of CO(2) for the 1:1 reaction of CO(2) with [P(66614)][Pro], when we use the two-reaction model.