Secondary growth synthesis of MOF-5 membranes by dip-coating nano-sized MOF-5 seeds

Continuous, crake-free and ultrathin metal organic framework (MOF-5) membranes were prepared on α-Al₂O₃ supports by the secondary growth method. By changing zinc source from Zn(NO₃)₂ to Zn(O₂CCH₃)₂, the novel nano-sized MOF-5 crystals could be synthesized in a solvo-thermal process. SEM, XRD and N₂ adsorption measurements indicated that the morphology and framework structure of the MOF-5 crystal synthesized by using Zn(O₂CCH₃)₂ matched well with the published SEM image and XRD pattern for the MOF-5, but its crystal size was reduced by two orders of magnitude from ~50μm-100μm to ~500 nm-800 nm with a larger surface area. By seeding the nano-sized MOF-5 crystals, a continuous and uniform seeding layer on the porous α-Al₂O₃ substrate was formed. The SEM micrographs indicated a high density, uniform and adequate coverage of MOF-5 crystals prevailing on α-Al₂O₃ substrate with the thickness of 4μm. After secondary growth, an ultrathin, continuous and crack-free MOF-5 membrane was obtained. XRD pattern for the MOF-5 membranes showed the randomly oriented structure of the MOF-5 crystals. XRD data showed that the obtained MOF-5 membrane was densely-packed with randomly oriented MOF-5 crystals. SEM image showed well inter-grown continuous MOF-5 membrane. The membrane thickness was only about 6μm with invisible inter-crystalline gaps, which was much thinner than the previously reported literatures.