Synthesis of ZIF-8 in a deep eutectic solvent using cooling-induced crystallisation

We report an alternative route for the synthesis of ZIF-8 in a choline chloride-urea deep eutectic solvent (DES). Zinc nitrate hexahydrate and 2-methylimidazole were dissolved in the hot DES. The ZIF-8 sample could precipitate from the DES when the solution was cooled using rapid cooling or programmed cooling. The effect of the synthetic conditions, including the cooling rate of synthesis system and the reaction temperature, on the particle size and morphology of the ZIF-8 sample were investigated. The product was characterised using PXRD, SEM, TG, the particle size analysis and gas-sorption. When the system was chilled via rapid cooling, the product was 0.35 mu m in size, agglomerating to form irregular spheres. After programmed cooling, the product was 5-50 mu m with a rhombic dodecahedron or truncated rhombic dodecahedron morphology, depending on the cooling rate and reaction temperature. In addition, the precipitation of ZIF-8 in DES is also discussed. ZIF-8 synthesised in DES also displayed excellent thermal and chemical stability, as well as good gas adsorption and separation performances. (C) 2014 Elsevier Inc. All rights reserved.