MOF-mixed matrix membranes: Precise dispersion of MOF particles with better compatibility via a particle fusion approach for enhanced gas separation properties

Mixed matrix membranes (MMMs) incorporating conventional fillers frequently suffer from insufficient adhesion between the polymer matrix and the fillers. This often results in the formation of non-selective voids at the filler/polymer interface, which decreases the performance of the membrane. A novel approach is presented here to develop metal organic framework (MOF) based MMMs by using the self-assembly of MOF and polymer particles followed by their controlled fusion. MOF-polymer interaction is optimized through this strategy and it overcomes MOF-polymer incompatibility, MOF agglomeration and MOF distribution problems, happening especially at high loadings of MOFs when applying conventional methods. Matrimid^® polymer particles were first prepared by precipitating a Matrimid^® polymer solution in water. The surface of these particles was then modified by the introduction of imidazole groups, enhancing the chemical compatibility with the selected ZIF-8 MOF. ZIF-8 nanoparticles were then grown in-situ to this modified polymer particle suspension by addition of the precursor for ZIF-8 synthesis. The resulted suspension was cast to dryness and annealed in a solvent-vapor environment to induce particle fusion, leading to a dense MMM structure. Scanning electron microscopy (SEM) images showed an excellent dispersion of the ZIF-8 nanoparticles forming a percolating pathway without any agglomeration, even at 40wt% loading of the ZIF-8. Excellent dispersion of ZIF-8 and an excellent ZIF-8-polymer interfacial adhesion resulted in a significant improvement in both CO₂ permeability and CO₂/CH₄ selectivity. The CO₂ permeability of the MMMs increased by 200% and the CO₂/CH₄ selectivity increased by 65% as compared to unfilled Matrimid^®. More detailed analysis of the gas transport performance of the MMMs showed that the CO₂ permeability and the CO₂/CH₄ selectivity are mainly governed by the increase in CO₂ diffusivity. The presented approach is a very versatile MMM preparation route, not only for this specific ZIF and polymer but for a wide range of material combinations.