Three-Dimensionally Ordered Macroporous Mixed Metal Oxide as an Indicator for Monitoring the Stability of ZIF-8

Three-dimensionally ordered macroporous (3DOM) ZnO-CeO2-Al2O3(MMO)@ZIF-8 was synthesized via a vapor-phase pseudomorphic transformation from 3DOM MMO and 2-methylimidazole that maintained the periodic, interconnected pore structure of the starting material, even for relatively small (190 nm diameter) macropores. After structural optimization via the synthesis conditions, this simple, solvent-free method resulted in a thin, uniform ZIF-8 shell on the 3DOM skeleton with a capacity of 47 mg/g for Rhodamine B (RhB) dye adsorption. The 3DOM MMO@ZIF-8 composite material was suitable for monitoring the reactivity and stability of ZIF-8 in different solvent systems simply and effectively by using the included crystalline CeO2 phase as an internal standard for comparing powder X-ray diffraction (XRD) intensities after dye adsorption steps. By taking advantage of the internal standard, it was clearly shown that ZIF-8 dissolves slowly during dye adsorption in an aqueous system, exposing new adsorption sites during recycling. This behavior benefits the reuse of the ZIF-8 adsorbent during a few cycles, but leads to adsorbent loss in the long run.