Stability of ZIF-8 in water under ambient conditions

ZIF-8 shows good stability in a humid atmosphere and has also been studied for applications in aqueous solutions. However, inconsistent results were reported about the stability of ZIF-8 in aqueous solutions. To clarify these inconsistent reports, ZIF-8 water stability tests were conducted using ZIF-8 crystallites of different size prepared by different methods. High crystallinity ZIF-8 crystallites were immersed in pure water at different ZIF-8/water ratio for 24 hours under ambient conditions. The solid powders obtained by evaporation of all the tested ZIF-8/water samples at 40 °C show the presence of a new substance of crystalline structure with a dense, leaf-like morphology. The conversion of ZIF-8 to this substance increases from about 20% to 100% as ZIF-8/water decreases from 6 wt.% to 0.02 wt.%. The solid powder and supernatant were also obtained from the tested ZIF-8/water samples by filtration. Such collected solid powder composes mainly of ZIF-8. The solid powder from the supernatant after evaporation at 40 °C has the same morphology and structure of the new substance discussed above. The results show that ZIF-8 crystallites are not stable in water under ambient conditions. The amount of ZIF-8 crystallites that get dissolved depends on ZIF-8/water ratio and contact time. The solid sample collected by filtration may contain residual ZIF-8, which is responsible for the confusion in the literature about the water stability of ZIF-8. The dissolved zinc and imidazolate ions in aqueous solution, upon condensation by evaporation, form the new substance which is not ZIF-8.