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.
Bibliographical noteFunding Information:
This work was partially supported by the Gas Sub-Committee R&D arm of Abu Dhabi National Oil Company United Arab Emirates and partially by funding from the Industrial Partnership for Research in Interfacial and Materials Engineering at the University of Minnesota. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from the NSF through the MRSEC program. The authors would like to thank Professor R. L. Penn for use of the powder X-ray diffractometer.