A zeolitic imidazolate framework (ZIF-8) as member of the metal-organic framework family has been crystallized as a thin porous layer on an asymmetric ceramic support. Hydrogen can be selected from other gases by molecular sieving.

In the past 10 years, porous metalÀorganic frameworks (MOFs) have established themselves in materials science. 1À3 The 3-D framework structures are formed by metal clusters or cations connected by organic linker molecules. On the basis of the organicÀinorganic hybrid character, MOFs have promise in a wide range of applications, e.g., gas storage, 4,5 medical applications, 6À8 catalysis, 9 sensor technology, 10,11 and molecular separation. 12À14 Basically, there are three options to use MOFs for molecular separation of liquid or gas mixtures: (i) by retaining one or more species from the liquid or gas phase by preferential adsorption in the pores, (ii) by different diffusivities of the species in the MOF, which is in the extreme case sterical size exclusion (molecular sieving) of one or more species, and (iii) the combination of (i) and (ii). One of the primary features that makes MOFs highly interesting for molecular separation is the so-called isoreticular design. 15 Linker molecules can be modified with functional groups or even completely substituted, maintaining the basic framework structure while altering adsorption and diffusion properties. 16 Membranes in general represent a cost and energy effective solution for industrial gas and liquid separation modified support. The mixture separation performance of membranes is characterized by the separation factor R i , j , which is defined after IUPAC as the ratio of the molar fraction of species i and j in the permeate, divided by the ratio of the molar fraction of species i and j in the retentate. 17 Currently, there are increasing numbers of successful attempts to prepare molecular sieving MOF membranes as thin, polycrystalline layers on top of macroporous support materials, showing separation factors up to 25. 18À27 For the preparation of MOF membranes, usually two basic techniques known from zeolite membrane fabrication are applied: (i) secondary growth crystallization, where in a first a step a seed layer is attached to the support and, subsequently, in a second step, grown to a continuous polycrystalline layer under solvothermal conditions, and (ii) in situ crystallization, where the polycrystalline layer is grown on the bare or chemically modified support in a one-step one-pot solvothermal synthesis. In situ crystallizations seem to be simple and, thus, the favored preparation route. However, they have the disadvantage of critically depending on high rates of heterogeneous nucleation on the support surface to successfully obtain continuous, well-intergrown MOF layers. Whether or not a high surface nucleation rate occurs depends on various factors, e.g., the surface chemistry of the support material (zeta potential, surface acidity, etc.). In secondary growth crystallizations, nucleation and crystal growth are decoupled, and hence, high nucleation rates and chemical interactions with the support material are less crucial.Recently, we reported on the preparation of ceramic-supported zeolitic imidazolate framework (ZIF) membranes. ZIF-type MOFs frequen...

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Time-resolved investigations using in situ energy-dispersive X-ray diffraction in tandem with ex situ scanning electron microscopy revealed that solvothermal crystallisation of ZIF-8 in methanol solvent and in the presence of sodium formate as a simple monodentate ligand (modulator) is a rapid process yielding big, high-quality single crystals in short time (<4 h). Kinetic analysis of crystallisation curves was performed by applying the Avrami-Erofe'ev and Gualtieri models. The analyses revealed that the weakly basic formate modulator acts as a base in deprotonation equilibria (deprotonation of the bridging 2-methylimidazole ligand) rather than as a competitive ligand in coordination equilibria at the metal (Zn 2+ ) centres. This is in contrast to the coordination modulation function of formate in ZIF-8 synthesis at room temperature. Crystal shape evolves with time in the presence of formate from cubes with truncated edges to rhombic dodecahedra. The latter shape represents most likely the stable equilibrium morphology of ZIF-8.