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Crystal Structure and De- and Rehydration Behavior of Two New Chloride-Containing Zeolitic Imidazolate Frameworks

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journal contribution
posted on 07.08.2019, 07:29 by Stephan Glante, Sebastian Bette, Gianpiero Gallo, Robert E. Dinnebier, Martin Hartmann
A previously unknown solid phase in the zinc-imidazole system relevant for the synthesis of ZIF-8 was obtained by adding a small excess of methylimidazole (Hmim) to a diluted zinc chloride solution. By chemical and thermal analyses, a composition of Zn3(mim)5ClH2xH2O with x = 0.74 was determined and the compound denoted as ZIF-yqt hydrate. The crystal structure was solved from laboratory X-ray powder diffraction (XRPD) data. ZIF-yqt hydrate crystallizes in a C-centered monoclinic unit cell with space group C2/c (15) and lattice parameters of a = 13.1574(3) Å, b = 16.4959(3) Å, c = 13.6403(3) Å, and β = 119.166(2)°. The structure is built up from Zn­(mim)4/2 and Zn­(mim)3/2(Cl0.5H2O0.5)1/1 units forming a self-penetrating yqt1 type net. IR spectroscopy was employed to confirm the presence of water in the coordination sphere of zinc. In addition, noncoordinated water is situated in the voids of this net. By heating, ZIF-yqt hydrate can be completely dehydrated resulting in the formation of anhydrous Zn3(mim)5Cl that is denoted as ZIF-yqt anhydrous. The crystal structure of the latter phase was solved from XRPD data as well. A self-penetrating yqt1 type net almost identical to the structure of the hydrated phase is formed by Zn­(mim)4/2 and Zn­(mim)3/2Cl1/2 units. By temperature and moisture dependent powder XRD measurement, it was demonstrated that the dehydration of ZIF-yqt hydrate and the transformation into ZIF-yqt anhydrous is completely reversible.