Metal-organic cages in polyimide and polyetheretherketone thin film composite mixed matrix membranes for gas separation

Membrane-based gas separation has been recognized as one of the most promising and energy-efficient processes for CO₂ capture from industrial gas streams. Remarkably, commercial gas separation membranes typically contain conventional polymers with sub-optimal performance, necessitating the search for better-performing materials. In this work, we developed novel mixed matrix membranes (MMMs) based on the benchmark polyimide Matrimid® 9725 and a soluble poly (ether ether ketone) PEEK-WC. The addition of a zinc-based metal-organic cage (MOC) featuring two calixsalen macrocyclic units significantly improved the transport properties for various gas pairs with an up to 100 % increase in permeability and up to 10 % increase in selectivity at 30 % MOC. SEM and DSC analyses offered valuable insights into the compatibility between the polymer and MOC, revealing excellent dispersion of up to 30 % of MOC with almost complete phase separation from the matrix. The thermal properties and transport properties were successfully described using the Fox equation and the Maxwell model, respectively. Most interestingly, thin film composites (TFCs) performed much better at 3 times higher cage concentrations than the corresponding self-standing thick films because the faster solvent evaporation limited crystal growth to sub-micron size, favouring a fine homogeneous distribution of the MOC in the polymer matrix. Based on their pure and mixed gas permeation, the TFC-MMMs show promise for the future development of new-generation gas separation membranes.