Synthesis and Characterization of Core-Shell Metal-Organic Framework (ZIF-67@ZIF-8) Particles

Zeolitic imidazolate frameworks (ZIFs) are a novel class of porous materials composed of metal ions and imidazole linkers. Here we focus on core-shell particles, where ZIF-67 core particles are covered with ZIF-8 shells (ZIF-67@ZIF-8). However, the synthesis of ZIF-67@ZIF-8 is generally difficult because the nucleation rate of ZIF-8 is so high that self-nucleation is unavoidable. In this study, we propose the following two approaches to overcome this challenge: (1) decreasing the nucleation rate by changing the water to methanol ratio used as solvents; and (2) intensifying the mixing during the synthesis by using a microreactor. Our experiments demonstrated that an increase in the water ratio suppresses the self-nucleation of the ZIF-8 particles, whereas surfaces of the resultant ZIF-67@ZIF-8 particles become irregular. The ZIF-67@ZIF-8 particles with uneven surfaces experienced an approximately 14% decrease in the adsorption amount as compared with that of the ZIF-67 particles. In contrast, the microreactor successfully produced ZIF-67@ZIF-8 particles with a smooth surface and high surface area. Our study revealed that intensive mixing is key to obtain uniform core-shell particles.