Water stability of cobalt doped ZIF-8: a quantitative study using optical analyses

Zeolitic imidazolate frameworks (ZIFs), in particular ZIF-8 (made of Zn2+ and 2-methyilimidazolate) and cobalt-doped-ZIF-8, are found important for many energy and environmental applications. It was reported that ZIFs show excellent structural stability in water and thus ideal for aqueous applications. However, recent studies also found some evidence that ZIF-8 undergoes hydrolysis in water. Despite the importance of ZIF's stability in many aqueous applications, the extent of ZIFs' degradation in water is still not yet fully understood. In this study, we report a quantitative study of the water stability of 0–100 at% cobalt-doped ZIF-8, using a new combination of analytical tools. The study demonstrated the importance of analyzing both filtered powders and the filtrate liquid systematically, in particular by using UV–Vis spectroscopy and thermogravimetric analysis. The combination of analytical tools allowed the study on the effects of ZIF concentrations in water, cobalt doping levels, and amounts of ligands in water on the water stability of ZIF samples. The effect of cobalt-doping was investigated by using ZIF particles with identical sizes (200–400 nm), in order to eliminate the effects of particle size on hydrolysis. Unlike other synthesis methods, a mechanochemical ball milling method allowed the production of nano-scale ZIF-8 particles with similar sizes, independent of cobalt-doping levels. The proposed combination of analytical tools including UV–Vis spectroscopy can be applied to the study of the water stability of other MOF materials.