Ionothermal synthesis of zeolitic imidazolate frameworks and the synthesis dissolution-crystallization mechanism

Abstract A cooling-induced crystallization process was used to synthesize sod (RCSR symbol)- and zni (RCSR symbol)-type zeolitic imidazolate frameworks (ZIFs) ionothermally in 1-ethyl-3-methylimidazolium bromide ionic liquid and a urea-choline chloride deep eutectic solvent. The products were characterized by X-ray diffraction, scanning electron microscopy, nuclear magnetic resonance spectroscopy, infrared spectroscopy and thermal gravimetric analysis. The effect of the synthesis conditions on the crystallinity, size and morphology of the product was studied. The dissolution-crystallization mechanism of ZIFs was discussed. The cooling rate affected the product morphology. When the synthesis solution was cooled by rapid cooling, the shape of the sod-type product was spherical and the morphology of the zni-type product was rod-like or plate-like. With a programmed cooling, the sod-type product was polyhedron in shape and the zni-type product was in the form of clusters.