(192p) Molecular Simulation of Ionic Polyimides and Ionic Liquid Composites for Gas Separation

Abedini, A., The University of Alabama
Turner, C. H., University of Alabama
Bara, J. E., University of Alabama
Crabtree, E., The University of Alabama
Molecular Simulation of Ionic Polyimides and Ionic Liquid Composites for Gas Separation
Asghar Abedini, Ellis Crabtree, Jason E. Bara, and C. Heath Turner*
Industrial gas separation and storage are primary challenges in energy and power related processes. For instance, the existence of acid gases such as H2S and CO2 in natural gas increases the equipment maintenance and separation cost for the energy industry.1 To make energy supply lines more economically feasible and minimize corrosion, these impurities must be removed through natural gas sweetening with precombustion CO2 capturing units prior to routing natural gas to the supply line.2 Furthermore, CO2 is a main component of power plant exhaust gas and is a side product of many processes of the petrochemical industry, such as ammonia production. Palliating carbon dioxide from emission sources is a critical need in industry, and it is necessary in order to meet current and future environmental regulations. Aqueous alkanolamine solvents are the most common process liquids for CO2 absorption units.3 However, high energy demands for recovery4, volatility, corrosivity, and degeneracy to toxic products are the main drawbacks of these solvents in gas sweetening process.5
Adsorption in porous materials is an alternative energy-efficient CO2 separation method, and the development of high carbon dioxide selectivity and adsorption capacity are key factors. There are some traditional adsorbents that have been widely studied such as activated carbon6, zeolites7, metal organic frameworks (MOFs),8 and silica gel.9 The low cost, high surface area, and large sorption capacity of activated carbons make them widely used in industry for similar applications.10 Additionally, polymers of intrinsic microporosity (PIMs) are another potential material that can be used as a membrane for gas separation, and they can provide increased stability, as compared to other adsorbents.11 The porosity of PIMs comes from their unique molecular structure,12 which provides a significant free volume and microstructure resulting from adjacent polymer chains.13, 14, 15 In order to tailor the permeability of these materials, we are exploring the behavior of composite structures composed of a PIM support plus an ionic liquid (IL) coating. The intent is to leverage the microporous structure of the PIM support with the tunability of the IL selectivity. Otherwise, a pure IL solvent suffers from high transport resistance and low free volume.
In the past, ionic liquids have been shown high selectivity for CO216, 17 versus CH4,18 and this makes ILs promising for applications in natural gas sweetening and precombustion CO2 capturing. The energy required for solvent recovery in ILs can be reduced due to the physical absorption mechanism.
In our study, 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim+][Tf2N-]) molecules have been chosen to infuse PIMs because of the structural consistency between PIMs and [C4mim+][Tf2N-] and because this particular IL has a strong tendency to absorb CO2 gas.1 Using a combination of molecular dynamics and Monte Carlo simulations, the solubility of CO2 and CH4 is modeled over a wide range of different pressures within the PIM and PIM + IL materials. Our results are benchmarked against experimental results, and modeling work shows excellent agreement with the experimental solubility for CO2 and CH4. Moreover, the modeling results indicate that the composite (PIM + IL) has the ability to surpass the predicted adsorption behavior that is typically correlated to the FFV of the adsorbent.

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