Electrophoretic Crystallization of Ultrathin High-performance Metal-organic Framework Membranes

We report the synthesis of thin, highly intergrown, polycrystalline metal-organic framework (MOF) membranes on a wide range of unmodified porous and non-porous supports (polymer, ceramic, metal, carbon, and graphene). We developed a novel crystallization technique, which is termed the ENACT approach: the electrophoretic nuclei assembly for the crystallization of highly intergrown thin films (ENACT). This approach allows for a high density of heterogeneous nucleation of MOFs on a chosen substrate via the electrophoretic deposition (EPD) directly from the precursor sol. The growth of well-packed MOF nuclei leads to a highly intergrown polycrystalline MOF film. We show that this simple approach can be used for the synthesis of thin, intergrown zeolite imidazole framework (ZIF)-7 and ZIF-8 films. The resulting 500 nm-thick ZIF-8 membranes show a considerably high H₂ permeance (8.3 x 10^-6 mol m^-2 s^-1 Pa^-1) and ideal gas selectivities (7.3 for H₂/CO₂, 15.5 for H₂/N₂, 16.2 for H₂/CH₄, and 2655 for H₂/C₃H₈). An attractive performance for C₃H₆/C₃H₈ separation is also achieved (a C₃H₆ permeance of 9.9 x 10^-8 mol m^-2 s^-1 Pa^-1 and a C₃H₆/C₃H₈ ideal selectivity of 31.6 at 25 °C). Overall, the ENACT process, owing to its simplicity, can be extended to synthesize intergrown thin films of a wide range of nanoporous crystalline materials.