Polymorph formation for a zeolitic imidazolate framework composition - Zn(Im)2

We study the effect of synthesis time and temperature on the crystal formation, morphology and size of zeolitic imidazolate frameworks (ZIFs) with the Zn(Im)2 composition by using the solution mixing method. The crystal structure, morphology and thermodynamic properties of the ZIFs were characterized by powder X-ray diffraction, scanning electron microscopy and nitrogen sorption isotherms. Our results indicate that the synthesis time significantly influences the structure and topology of crystal products. The crystal structure transforms from ZIF-coi to the progressively denser polymorphs (ZIF-zec, ZIF-nog and ZIF-zni) upon increasing synthesis time from 18 to 120 hours at 10 °C, in accordance with Ostwald's step rule. Increasing synthesis temperature does not change the formation of the ZIF-zec crystals but affects their morphologies and porosities. Both ZIF-nog and ZIF-zec exhibit relatively large surface areas (>500 m2/g). Furthermore, heating ZIF-nog and ZIF-zec causes recrystallization to ZIF-zni and subsequent melting. Quenching the MOF-liquid results in Zn(Im)2 glass. This work helps to understand and control the crystal formation of ZIFs, and reveals two new MOF glass formers.