Crystallization and phase selection of zeolitic imidazolate frameworks in aqueous cosolvent systems: The role and impacts of organic solvents

Zeolite Imidazolate Frameworks (ZIFs) can be used in a wide range of applications, such as water treatment, molecular sieving, gas adsorption, drug delivery, and catalysis. However, their fabrications in primarily aqueous environments have succumbed to large chemical consumption and become very challenging in sustainable manufacturing. However, the water-organic cosolvent system enables ZIF fabrications at lower chemical consumption (e.g., lower ligand-to-metal ratios and contents in the synthesis system) and resulting materials with interesting nanostructures and surface properties. Herein, aqueous-based fabrications were successfully employed with various cosolvents, including alcohols, ketones, and amides. Their impacts on ZIF (ZIF-L/ZIF-8 as examples) nucleation, crystallization, phase transformation, and morphological evolution were systematically studied. Each solvent environment enhanced the coordination of the metal sites to different degrees. Specifically, ZIF-8 formation was favoured in cosolvent concentrations of 50 v/v%, 25 v/v%, 5 v/v%, 10 v/v%, 10 v/v%, and 25 v/v% for methanol, ethanol, n-propanol, isopropanol, acetone and DMF, respectively. Moreover, the preferential formation towards ZIF-8 in each cosolvent environment resulted in similar BET surface areas, in the range of 1350–1376 m2 g−1, highlighting good reproducibility of cosolvent-assisted syntheses. In this work, a formation mechanism was drawn for various solvents in terms of solvent properties, metal site interactions, and deprotonation capabilities. This newly proposed mechanism of ZIF growth provides vital information regarding the solvents' roles in aqueous-based fabrications and their impacts on crystallinity, structure purity, and crystal morphology as well as new insights for researchers working on the synthesis and structure control/modifications of ZIFs & their composite materials.