Molecular Design of High Capacity, Low Viscosity, Chemically Tunable Ionic Liquids for CO2 Capture

The discovery of materials that combine selectively, controllably, and reversibly with CO 2 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 CO 2 separation problem. Atomistic simulations predict that suitably substituted aprotic heterocyclic anions, or “AHAs,” bind CO 2 with energies that can be controlled over a wide range suitable to gas separations. Further, unlike all previously known CO 2 -binding ILs, the AHA IL viscosity is predicted to be insensitive to CO 2 . Spectroscopic, temperature-dependent absorption, rheological, and calorimetric measurements on trihexyl(tetradecyl)-phosphonium 2-cyanopyrrolide ([P 66614 ][2-CNpyr]) show CO 2 uptakes close to prediction as well as insignificant changes in viscosity in the presence of CO 2 . 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 CO 2 separations.