(684b) Using Multi-Scale Simulations to Understand CO2 Solubility and Separation in Multivalent Ionic Liquids

Carbon dioxide capture and separation from industrial gases (e.g., pre-combustion, post-combustion, and natural gas sweetening) have attracted a lot of recent attention.¹ Ionic liquids (ILs) and related molecules have shown potential application for selective gas absorption due to their tunable thermophysical properties.^1-4 Understanding the molecular-level solubility of CO₂ and its mixtures is essential to the progress of gas capture and separation technologies. A recent review³ of gas solubility in ILs concluded that at the same temperature and pressure conditions, single-component solubility in commonly used monovalent ILs follows the order: SO₂ (H₂S) > CO₂ ≈ N₂O > C₂H₄ > C₂H₆ > CH₄ > Ar > O₂ > N₂ > CO > H₂. However, the performance of multivalent ILs is still relatively unknown, and in particular, the behavior of multicomponent absorption can give rise to complex phenomena (neighbor-neighbor interactions, competitive absorption, etc.).

In this work, we model CO₂, SO₂, N₂, CH₄ and H₂ pure component and binary mixtures absorption characteristics within multivalent ILs. We use multi-scale simulation approaches, including density-functional theory (DFT), molecular dynamics (MD), and grand canonical Monte Carlo (GCMC) simulations. GCMC simulations are performed to obtain the gas solubility from pure and binary mixtures. Gas molecules absorbed within the ILs are simulated using MD to understand the structure, e.g., pore size distribution (PSD), radial distribution function (RDF), fractional free volume (FFV), and electrostatic potential (ESP) environment, as well as interactions between ILs with the gases.⁵ The DFT calculations are focused on correlating the gas solubility and selectivity to the electrostatic characteristics of the IL ions.⁶ Very recently, we proposed a new parameter, the ionic polarity index (IPI), based on ESP surfaces generated from DFT calculations, to quantify the polarity of both monovalent and multivalent ions. The IPI shows a strong connection to the ionic volumes,⁷ and it can be used as a simple approach to qualitatively predict gas solubility in ILs.⁶ Overall, our multi-scale simulations provide deep molecular-level insight into the solubility behavior and separation performance of gases in ILs, and our proposed design rules can provide reliable guidelines for screening high performance ILs for gas capture and separation.

References:

1. Blanchard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F., Green Processing Using Ionic Liquids and CO₂. Nature 1999, 399, 28-29.
2. Zhang, X.; Zhang, X.; Dong, H.; Zhao, Z.; Zhang, S.; Huang, Y., Carbon Capture with Ionic Liquids: Overview and Progress. Energy Environ. Sci. 2012, 5, 6668-6681.
3. Lei, Z.; Dai, C.; Chen, B., Gas Solubility in Ionic Liquids. Chem. Rev. 2014, 114, 1289-326.
4. Zeng, S.; Zhang, X.; Bai, L.; Zhang, X.; Wang, H.; Wang, J.; Bao, D.; Li, M.; Liu, X.; Zhang, S., Ionic-Liquid-Based CO₂ Capture Systems: Structure, Interaction and Process. Chem. Rev. 2017, 117, 9625-9673.
5. Liu, X.; O’Harra, K. E.; Bara, J. E.; Turner, C. H., Molecular Insight into the Anion Effect and Free Volume Effect of CO₂ Solubility in Multivalent Ionic Liquids. Phys. Chem. Chem. Phys. 2020, 22, 20618-20633.
6. Liu, X.; O’Harra, K. E.; Bara, J. E.; Turner, C. H., Solubility Behavior of CO₂ in Ionic Liquids Based on Ionic Polarity Index Analyses. J. Phys. Chem. B 2021, 10.1021/acs.jpcb.1c01508.
7. Liu, X.; O’Harra, K. E.; Bara, J. E.; Turner, C. H., Screening Ionic Liquids Based on Ionic Volume and Electrostatic Potential Analyses. J. Phys. Chem. B 2021, 10.1021/acs.jpcb.0c10259.