(13i) Molecular Simulation of Ionic Polyimides and Ionic Liquid Composite Membranes for Gas Selectivity and Adsorption

Asghar Abedini, Joanna Szala-Bilnik, Ellis Crabtree, Jason E. Bara, and C. Heath Turner*

Abstract:

The reduction and control of greenhouse gas emissions is a priority of many industries due to its detrimental environmental and atmospheric effects. Carbon dioxide is one of the most well-known greenhouse gases with many industrial emission sources. To combat emission, several different carbon capture techniques for pre-combustion and post-combustion have been successfully applied.¹ Most carbon capture processes still depend on amine based solvents, such as monoethanolamine (MEA), but these solvents exhibit multiple disadvantages (e.g., corrosion, volatility, toxicity, and high-energy demand for recovery).²

Ionic polyimides (i-IPs) are a new class of polymers for membrane-based CO₂ separations with promising pre-combustion, gas sweetening, and CO₂ post-combustion capture applications (Figure 1).^3,4 This novel class of polymers, like poly(ionic liquids)s (PILs),^5,6 contain ionic liquid species in the backbone of the monomer. However, the combination of an organic linker and ionic liquid (IL) in the i-IP monomer structure provides an additional degree of structural and chemical design in the polymer system. In this study, prototype composite structures are modeled that contain i-IPs with pyromellitic dianhydride (PMDA) as an organic linker. The role of different ILs is investigated, with [C₄mim⁺] as the cation in combination with one of the following anions: bis(trifluoromethylsulfonyl)imide ([Tf₂N^-]), tetrafluoroborate ([BF₄^-]), and hexafluorophosphate ([PF₆^-]).

We analyze the detailed structural changes (e.g., fractional free volume, pore size distribution, surface area, etc.) of the ionic polyimides and their composites (i-PI+IL) during CO₂ adsorption using a combination of molecular dynamic (MD) calculations and grand canonical Monte Carlo (GCMC) simulations. The extracted results shows that in [BF₄^-]-based materials an addition of 50% IL can increase CO₂/CH₄ selectivity by 16%, and in [PF₆^-]-based materials the selectivity is increased by 36%. Overall, it is found that the [BF₄^-]-based system shows higher CO₂/CH₄ gas separation (potential pre-combustion applications), while the [Tf₂N^-]-based i-PI+IL composites shows higher CO₂/N₂ selectivity performance (potential post-combustion applications). Higher gas solubility correlates with higher theoretical surface area values and a higher fraction of larger pore sizes, while the FFV is not a sensitive indicator of the solubility.

Figure 1. Representative monomer structure of a neat i-IP. The anion shown is [BF₄^-]. Specific nitrogen sites of the i-IP are labeled for reference, as well as the head (C_H) and tail (C_T) designation.

References:

(1) Martín, C. F.; Stöckel, E.; Clowes, R.; Adams, D. J.; Cooper, A. I.; Pis, J. J.; Rubiera, F.; Pevida, C. Hypercrosslinked organic polymer networks as potential adsorbents for pre-combustion CO₂ capture. Journal of Materials Chemistry 2011, 21, 5475.

(2) Abdul Halim, H. N.; M. Shariff, A.; Tan, L. S.; Bustam, M. A. Mass Transfer Performance of CO₂ Absorption from Natural Gas using Monoethanolamine (MEA) in High Pressure Operations. Industrial & Engineering Chemistry Research 2015, 54, 1675-1680.

(3) Abedini, A.; Crabtree, E.; Bara, J. E.; Turner, C. H. Molecular Simulation of Ionic Polyimides and Composites with Ionic Liquids as Gas-Separation Membranes. Langmuir 2017, 33, 11377-11389.

(4) Mittenthal, M. S.; Flowers, B. S.; Bara, J. E.; Whitley, J. W.; Spear, S. K.; Roveda, J. D.; Wallace, D. A.; Shannon, M. S.; Holler, R.; Martens, R.; Daly, D. T. Ionic Polyimides: Hybrid Polymer Architectures and Composites with Ionic Liquids for Advanced Gas Separation Membranes. Industrial & Engineering Chemistry Research 2017, 56, 5055-5069.

(5) Morozova, S. M.; Shaplov, A. S.; Lozinskaya, E. I.; Mecerreyes, D.; Sardon, H.; Zulfiqar, S.; Suárez-García, F.; Vygodskii, Y. S. Ionic Polyurethanes as a New Family of Poly(ionic liquid)s for Efficient CO₂ Capture. Macromolecules 2017, 50, 2814-2824.

(6) Qian, W.; Texter, J.; Yan, F. Frontiers in poly(ionic liquid)s: syntheses and applications. Chem Soc Rev 2017, 46, 1124-1159.