Zeolitic imidazolate frameworks (ZIFs) are a subclass of nanoporous metal−organic frameworks (MOFs) that exhibit zeolite-like structural topologies and have interesting molecular recognition properties, such as molecular sieving and gate-opening effects associated with their pore apertures. The synthesis and characterization of hybrid ZIFs with mixed linkers in the framework are described in this work, producing materials with properties distinctly different from the parent frameworks (ZIF-8, ZIF-90, and ZIF-7). NMR spectroscopy is used to assess the relative amounts of the different linkers included in the frameworks, whereas nitrogen physisorption shows the evolution of the effective pore size distribution in materials resulting from the framework hybridization. X-ray diffraction shows these hybrid materials to be crystalline. In the case of ZIF-8-90 hybrids, the cubic space group of the parent frameworks is continuously maintained, whereas in the case of the ZIF-7-8 hybrids there is a transition from a cubic to a rhombohedral space group. Nitrogen physisorption data reveal that the hybrid materials exhibit substantial changes in gate-opening phenomena, either occurring at continuously tunable partial pressures of nitrogen (ZIF-8-90 hybrids) or loss of gate-opening effects to yield more rigid frameworks (ZIF-7-8 hybrids). With this synthetic approach, significant alterations in MOF properties may be realized to suit a desired separation or catalytic process.

Hybrid zeolitic imidazolate frameworks (ZIFs), containing more than one type of imidazolate linker, can allow highly tunable molecular sieving and adsorption. Their crystallization becomes more challenging when the end-member (single-linker) ZIFs crystallize in different crystal systems. We demonstrate the controlled synthesis and detailed characterization of hybrid ZIF-7-90 frameworks containing linkers of ZIF-7 (rhombohedral) and ZIF-90 (cubic). ZIF-7-90 materials with SOD-type topology are obtained in three crystalline phases depending on the linker composition and synthesis technique. The effect of synthesis conditions on the activation-induced phase transition from rhombohedral to other topologies is studied. Nitrogen physisorption at 77 K and CO 2 physisorption at 273 K shows the tunability of the pore-size distribution and the framework flexibility as a function of framework composition. Measurements of water adsorption and butane isomer diffusion illustrate the tunability of diffusivity over seven orders of magnitude and control of hydrophobic to hydrophilic adsorption behavior. V C 2015 American Institute of Chemical Engineers AIChE J, 62: 525-537, 2016