Through IR microimaging the spatially and temporally resolved development of the CO₂ concentration in a ZIF-8 at 6FDA-DAM mixed matrix membrane (MMM) was visualized during transient adsorption. By recording the evolution of the CO₂ concentration, it is observed that the CO₂ molecules propagate from the ZIF-8 filler, which acts as a transport "highway", towards the surrounding polymer. A high-CO₂-concentration layer is formed at the MOF/polymer interface, which becomes more pronounced at higher CO₂ gas pressures. A microscopic explanation of the origins of this phenomenon is suggested by means of molecular modeling. By applying a computational methodology combining quantum and force-field based calculations, the formation of microvoids at the MOF/polymer interface is predicted. Grand canonical MonteCarlo simulations further demonstrate that CO₂ tends to preferentially reside in these microvoids, which is expected to facilitate CO₂ accumulation at the interface.

A microporous Al trimesate-based metal-organic framework (MOF), denoted MIL-96-(Al), was selected as a porous hybrid filler for the processing of mixed matrix membranes (MMMs) for CO₂/N₂ postcombustion separation. First, the structural model of MIL-96-(Al) initially reported was revisited using a combination of synchrotron-based single-crystal X-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, and density functional theory (DFT) calculations. In a second step, pure MIL-96-(Al) crystals differing by their size and aspect ratio, including anisotropic hexagonal platelets and nanoparticles of about 70 nm in diameter, were prepared. Then, a combination of in situ IR spectroscopy, single-gas, and CO₂/N₂ coadsorption experiments, calorimetry, and molecular simulations revealed that MIL-96-(Al) nanoparticles show a relatively high CO₂ affinity over N₂ owing to strong interactions between CO₂ molecules and several adsorption sites such as Al³⁺ Lewis centers, coordinated water, and hydroxyl groups. Finally, the high compatibility between MIL-96-(Al) nanoparticles and the 6FDA-DAM polymer allowed the processing of homogeneous and defect-free MMMs with a high MOF loading (up to 25 wt %) that outperform pure polymer membranes for CO₂/N₂ separation.