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 post combustion separation. First, the structural model of MIL-96(Al) initially reported was revisited using a combination of synchrotron-based single crystal X-ray diffraction (XRD), solid state Nuclear Magnetic Resonance (NMR) 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 co-adsorption 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.