UC Riverside

Carbon capture and gas separation is of great importance in addressing climate change, as greenhouse gas emissions continue to rise with major contributions from transportation and electricity production. Some materials have shown great affinity for CO2 capture, like metal-organic frameworks, zeolites, and functionalized ionic liquids. Limitations in these materials, such as regeneration costs, make it advantageous to develop porous media that are liquids at room temperature. This work employs computational chemistry to study two liquids used for gas separations: an ionic liquid with a polyethylene glycol-linked dication, and a novel porous ionic liquid with a permanent pore large enough to house small gas molecules. Using classical molecular dynamics, we are able to understand the behavior of these liquids on the atomic level, and design new materials to effectively tune their properties. The dicationic ionic liquid, [DBU-PEG][Tf2N]2, was synthesized previously, though not studied computationally. We find that our model well-replicated experimentally determined density, viscosity, and powder x-ray diffraction spectra. Our work reveals that the geminal dication, connecting two bicyclic amidine groups with a polyethylene glycol linker, plays a unique role in CO2 absorption, resulting from both the dication’s shape and charge distribution. We also present an anionic covalent cage-based porous liquid—the cage featuring a 4.25 Å pore. We simulate the bulk liquid with CO2, CH4, and N2 molecules, and find a strong preference for CO2 capture from the gas-cage interactions. This is supported by analysis of a gas molecule’s time-of-stay inside a cage, diffusivity of the gases in the system, and predicted solubilities from grand canonical Monte Carlo methods. The liquid also presents energetic and thermodynamic favorability for CO2 over the other gases studied. This work investigates two unique liquids’ performances in capturing carbon, and encourages further development of tunable liquids for gas separations. The geminal design of the cation in [DBU-PEG][Tf2N]2 inspires a similar approach of using dianions in ionic liquids, while the modular synthesis of the porous liquid can be easily modified to feature other functional groups. Such changes may lead to considerable effects in gas uptake and selectivity, and should be studied further.