Gas permeation and separation in functionalized polymers of intrinsic microporosity: A combination of molecular simulations and ab initio calculations

Abstract

We combine molecular simulations and ab initio calculations to investigate the permeation and separation of CO2/N2 in polymers of intrinsic microporosity (PIMs) with different functional groups (cyano, trifluoromethyl, phenylsulfone, and carboxyl). A robust equilibration protocol is proposed to construct model membranes with predicted densities very close to experimental data. The fractional free volumes (FFVs) in PIM-1 (with cyano), TFMPS-PIM (with both trifluoromethyl and phenylsulfone), and CX-PIM (with carboxyl) are 45.2%, 42.1%, and 38.7%, respectively. Hydrogen bonding is observed to form among carboxyl groups and contributes to the lowest FFV in CX-PIM. From wide-angle X-ray diffractions, the estimated d-spacing distances agree well with available experimental results, and the chain-to-chain distance in CX-PIM is the shortest among the three membranes. Ab initio calculations reveal that the interaction energies between the functional groups and CO 2 decrease as carboxyl > phenylsulfone > cyano > trifluoromethyl; consistently, the simulated solubility coefficient of CO 2 is the largest in CX-PIM. The simulated diffusion coefficient decreases with reducing FFV and correlates well with FFV. While the sorption selectivity of CO2/N2 increases in the order of PIM-1 < TFMPS-PIM < CX-PIM, the diffusion selectivity remains nearly constant; consequently, the permselectivity follows the same hierarchy as the solubility selectivity. This computational study provides microscopic insight into the role of functional groups in gas permeation and suggests that strong CO 2-philic groups should be chosen to functionalize PIM membranes for high-efficiency CO2/N2 separation. © 2011 American Chemical Society.