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Zeolitic-imidazole framework-8 (ZIF-8) membranes have shown exceptional propylene/propane separation performances. Their commercial applications have, however, been impeded by several challenges. One such challenge is the difficulty of managing microstructural defects ( i.e. , grain boundary defects) in a consistent manner, leading to poor membrane performances and ultimately to a reproducibility issue. Herein, we introduce a new effective strategy to seal the microstructural defects of polycrystalline ZIF-8 membranes using post-synthetic surface polymerization which consists of two steps: (1) introduction of initiator ligands on the membrane surface by post-synthetic ligand exchange and (2) in situ polymerization of poly(methyl methacrylate) (PMMA) via atom transfer radical polymerization. The ZIF-8 membranes were fully covered with ultra-thin PMMA layers of sub-10 nm thickness, increasing the propylene/propane separation factor from ∼60 to ∼106 with unexpectedly increased propylene permeance, effectively improving the membrane reproducibility. The enhanced separation properties of the PMMA-coated ZIF-8 membranes were attributed to the ultra-thin PMMA layers as well as to the possible facilitated propylene transport by Cu ions in the PMMA layers. 

Polymer-modification-enabled in situ metal–organic framework formation (PMMOF) is potentially a paradigm-shifting preparation method for polymer/MOF mixed-matrix membranes (MMMs). However, the actual reaction conditions of the in situ formation of MOFs in a confined polymer free volume are expected to be quite different from that in a bulk solution. ZIF-7 is an interesting filler material not only due to its use in selective light gas separations but also for its three different crystal phases. Herein, we carried out systematic investigations on the in situ confined formation of ZIF-7 phases inside a polymer (6FDA-DAM) by PMMOF. The reaction conditions of ZIF-7 in the polymer free volume were deduced based on a bulk-phase ZIF-7 phase diagram constructed by varying the ZIF-7 precursor concentrations and ratios. Based on the understanding of the reaction conditions, the ZIF-7 crystal phases formed inside the polymer during the PMMOF process were controlled, yielding 6FDA-DAM/ZIF-7 MMMs with three different crystal phases. The ZIF-7 phases had significant effects on the gas transport of MMMs with layered ZIF-7-III fillers exhibiting the highest performance enhancement for H 2 /CO 2 separation ( i.e. , H 2 permeability of ∼1630 Barrer and H 2 /CO 2 selectivity of ∼3.8) among other phases. Furthermore, the MMMs obtained by the PMMOF process showed enhanced H 2 /CO 2 separation performance, surpassing the upper bound.