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

Authors: 
Szala-Bilnik, J., University of Alabama
Abedini, A., The University of Alabama
Crabtree, E., The University of Alabama
Bara, J. E., University of Colorado
Turner, C. H., University of Alabama
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 (CH) and tail (CT) 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.

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