(84f) Gas Adsorption Behavior in Ionic Polyimide Composite Membranes

Authors: 
Turner, C. H., University of Alabama
Abedini, A., The University of Alabama
Crabtree, E., The University of Alabama
Bara, J. E., University of Alabama
Industrial gas separation and storage processes present many technical and economic challenges to the natural gas industry and to power plants. In particular, palliating CO2 from emission sources is a critical need in industry, and it is necessary in order to meet current and future environmental regulations. Polyimides are at the forefront of advanced membrane materials for CO2 capture and gas purification processes. Recently, “ionic polyimides” (i-PIs) have been reported as a new class of condensation polymers which combine structural components of both ionic liquids (ILs) and polyimides through covalent linkages. In this study, we report CO2 and CH4 adsorption and structural analyses of an i-PI and an i-PI + IL composite containing [C4mim][Tf2N]. The combination of molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations are used to compute the gas solubility and the adsorption performance with respect to the density, fractional free volume (FFV), and surface area of the materials.

Our results highlight the polymer relaxation process, and its correlation to the gas solubility. In particular, the surface area can provide meaningful guidance with respect to the gas solubility, and it tends to be a more sensitive indicator of the adsorption behavior versus only considering the system density and FFV. For instance, as the polymer continues to relax, the density, FFV, and pore-size distribution remain relatively constant, while the surface area can continue to increase, enabling more adsorption. Structural analyses are also conducted to identify the nature of the gas adsorption once the ionic liquid is added to the polymer. The presence of the IL significantly displaces the CO2 molecules from the ligand nitrogen sites in the neat i-PI to the imidazolium rings in the i-PI + IL composite. Whereas, the CH4 molecules move from the imidazolium ring sites in the neat i-PI to the ligand nitrogen atoms in the i-PI + IL composite. These molecular details can provide critical information for the experimental design of highly selective i-PI materials, as well as provide additional guidance for the interpretation of the simulated adsorption systems.