Sonication-induced Ostwald ripening of ZIF-8 nanoparticles and formation of ZIF-8/polymer composite membranes

The effect of typical membrane processing conditions on the structure, interfacial morphology, and gas separation performance of MOF/polymer nanocomposite membranes is investigated. In particular, the ZIF-8/Matrimid® nanocomposite membrane system is examined, and it is shown that ultrasonication – a commonly employed particle dispersion method – induces significant changes in the shape, size distribution, and structure of ZIF-8 particles suspended in an organic solvent during membrane processing. Dynamic light scattering and electron microscopy reveal that ZIF-8 nanoparticles undergo substantial Ostwald ripening when subjected to high intensity ultrasonication as often required in the formation of MOF/polymer nanocomposite membranes. Other characterization techniques reveal that the ripened particles exhibit lower pore volumes and lower surface areas compared to the as-made material. ZIF-8/Matrimid® composite membranes fabricated using two sonication methods show significant differences in microstructure. Permeation measurements show significant enhancement in permeability of CO2 and increased CO2/CH4 selectivity in membranes fabricated with high-intensity sonication. In contrast, composite membranes prepared with low-intensity sonication are found to be defective. A careful evaluation of MOF membrane processing conditions, as well as knowledge of the properties of the MOF material after these membrane processing steps, are necessary to develop reliable processing–structure–property relations for MOF-containing membranes.

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► Ultrasonication induces Ostwald-ripening of ZIF-8 nanoparticles in THF solvent.
► Two sonication methods yield composite membranes with different microstructures.
► Composite membrane fabrication–structure–property relations are obtained.
► This allows estimation of permeation properties of ZIF-8 particles in the membrane.
► These ZIF-8 particles show better separation properties than previously reported.