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 work, the CO2 separation characteristics of ionic polyimides are modeled using molecular dynamics simulations in combination with grand canonical Monte Carlo and kinetic Monte Carlo simulations. The performance of neat i-PI systems is evaluated, as well as composite structures containing both i-PIs and various ionic liquids (ILs). The i-PI+IL composites are based on combinations of 1-n-butyl-3-methylimidazolium ([C4mim+]) cations with three common molecular anions: (bis(trifluoromethylsulfonyl)imide ([Tf2N-]), tetrafluoroborate ([BF4-]), and hexafluorophosphate ([PF6-]). It is found that 50 mol% IL inclusion can increase CO2/CH4 selectivity by 16% in [BF4-]-based materials and by 36% in [PF6-]-based materials from mixtures of 5% CO2 / 95% CH4. While the [BF4-]-based system shows higher CO2/CH4 selectivity, the [Tf2N-]-based system shows higher CO2/N2 gas selectivity. A comprehensive structural analysis (fractional free volume (FFV), pore size distribution, surface area, etc.) is used to highlight the underlying differences among the different i-PI+IL systems that lead to the different adsorption properties. The membrane information generated from the molecular dynamics and grand canonical Monte Carlo simulations is used to develop a comprehensive kinetic Monte Carlo model that can be used to make prediction of gas separation performance on experimentally-relevant time scales. To do this, the membrane pore structure is mapped to a lattice, as well as information about the local electrostatic information at each lattice site. Using this lattice model, we can predict permeability, selectivity, and other transport behavior and connect these metrics directly to the experimental performance benchmarks.

Ionic polyimide composite membranes for gas separation: Predicting experimental performance with Kinetic Monte Carlo

2019 Spring National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.