As a class of materials, metal–organic frameworks (MOFs) have been frequently applied to membrane-based gas separations in the form of mixed-matrix membranes (MMMs). Unfortunately, to access MOF-like transport, MOF loadings must be sufficiently high to enable percolation, but these high loadings lead to defects and poor mechanical properties. In this work, a novel approach is used to access MOF-like transport at low loadings by forming ZIF-8 into a three-dimensional mesoporous structure. This “branched” structure connects rod-like ZIF-8 particles at common nodes, producing pre-formed percolation networks for the MOF. A side-by-side study of branched ZIF-8 and rhombic dodecahedral ZIF-8 are compared to reveal the role of the branched structure on transport in MMMs. Interestingly, the branched samples demonstrate improvements in permeability and selectivity for several light gas pairs, including H2/CH4, H2/N2, and O2/N2 separations, but counter to expectations, improvements were not observed for propylene/propane separations. NMR, BET, DMA, diffusion studies, and plasticization-pressure experiments suggest that these findings relate to hindered ZIF-8 linker rotation in the branched sample due to enhanced interactions with the polymer and the high surface-area-to-volume ratio of the branched MOF. An in-depth study of pure-gas permeation, mixed-gas permeation, and modeling are used to support the claim of MOF percolation at lower loadings than those achievable with rhombic dodecahedral structures, and the role of polymer confinement to achieve plasticization resistance is discussed. By using pre-formed percolation networks, enhanced transport performance and plasticization resistance can be achieved relative to traditional morphologies of ZIF-8.

MOFs with network nanostructures to enable percolated transport in mixed-matrix membranes

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