Unravelling the origin of enhanced CO₂ selectivity in amine-PIM-1 during mixed gas permeation

Previously, it has been reported that amine-PIM-1, a polymer of intrinsic microporosity obtained by reduction of nitrile groups of PIM-1 to primary amine groups, shows enhanced CO₂ selectivity during mixed gas permeation studies with respect to single gas measurements for gas pairs involving CO₂. This distinct and potentially useful behaviour was ascribed to the affinity of CO₂ for the polymer amine groups. Here, we demonstrate that enhanced selectivity originates from both CO₂physisorption and chemisorption. A combination of ¹³C and ¹⁵N solid-state NMR spectroscopic analyses of a CO₂-loaded amine-PIM-1 membrane allowed the identification and quantitative determination of both chemisorbed and physisorbed species and the characterization of polymer-CO₂ interactions. Experiments with ¹³C isotopically enriched CO₂ unequivocally demonstrated the conversion of 20% of the NH2 groups into carbamic acids at 298 K and a CO₂ pressure of 1 bar. Chemisorption was supported by the strong heat of CO₂ adsorption for amine-PIM-1 that was estimated as 50 kJ mol-1. Molecular Dynamics simulations with models based on the experimentally determined polymer structure gave a detailed description of intra- and interchain hydrogen bond interactions in amine-PIM-1 after chemisorption, as well as of the effect of chemisorption on polymer porosity and physisorption.