Lattice Modification and Morphological Control of Halide-Substituted yqt-Type Zeolitic Imidazolate Frameworks Zn3mim5X, with X = F, Br, Cl, or OH

Abstract

Zeolitic imidazolate frameworks (ZIFs) are a large subgroup of metal-organic frameworks (MOFs). The synthesis of ZIF phases showing a yqt-type framework by the reaction of methylimidazole (Hmim) with a zinc salt in an aqueous solution using different zinc salts like acetates, nitrates, chlorides, fluorides, bromides, iodides, and sulfates yields different morphologies and products: Zn3mim5XH2O·nH2O with X = Br, Cl and α-Zn3mim5X with X = F, OH. By thermal dehydration of the hydrated ZIF phases, a series of isostructural compounds, α-Zn3mim5X with X = Br, Cl, F, or OH, were obtained. The products were characterized by X-ray powder diffraction (XRPD) and vibrational (IR) and two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy. In all structures, zinc exhibits tetrahedral coordination and forms a yqt net. Inorganic anions are incorporated into this net as Zn-X-Zn bridges and show a major impact on the reversible de- and rehydration behavior, as well as on the thermal expansion. The latter property is mainly governed by bending of the Zn-X-Zn bridge. Within this series, only the chloride-containing phase, showing the strongest anisotropic thermal expansion, exhibits two polymorphs: α- and β-Zn3mim5Cl, the α-form showing a bent and the β-form showing a linear Zn-X-Zn bridge.