(532f) Flow Microreactor Synthesis of ZIF-8 Particles with Controlled Size, Shape, and Adsorption Properties

Significant progress has been made in research and development of metal-organic frameworks (MOFs), which consist of metal ions and organic ligands. Their chief features include large surface area, highly ordered porous network, and structural flexibility. The flexibility leads to a structural transition of the host framework from a non-porous to a porous phase that causes a step-wise increase in guest molecules at a threshold pressure, which is called â??gateâ? adsorption. Because of this unique adsorption property, soft MOFs are promising for various applications not only to gas storage and separation, but also to molecular sensors. To exploit full potential of soft MOFs, their particle size and shape must be monodispersed because their unique adsorption property can depend on their size and shape. In the present study, we synthesized zeolitic imiazolate framework-8 (ZIF-8), which is a subclass of soft MOFs and is composed of Zn ions and 2-methylimidazole linkers. We mixed aqueous Zn(NO₃)₂ and 2-methylimidazole solutions in a central-collision type microreactor, which utilizes shear forces at the collision of two inlet fluids to realize rapid and homogeneous mixing. The mixed solution from the outlet was kept in a glass vessel for an hour. The nucleation starts upon mixing the solutions in the microreactor, followed by the particle growth in the glass vessel. Typical particle size is around 300 nm, and resultant particle size and shape distributions synthesized with the microreactor were more uniform than the case of a batch reactor. This would be due to a high mixing performance of the microreactor that can divide the nucleation process and the particle growth process. Moreover, we synthesized ZIF-8 particles for varied ratio of 2-MeIM to Zn²⁺ (R), concentration of Zn²⁺ ([Zn²⁺]) and temperature. The average size of ZIF-8 decreases with the R, and a higher [Zn²⁺] results in smaller size at a fixed R, which is because of a higher degree of supersaturation at higher R and [Zn²⁺] conditions. Prepared ZIF-8 particles have BET surface area of 1500 m²/g and exhibit the gate adsorption behaviour with different transition pressures depending on the particle size and shape. We also confirmed the microreactor synthesis is applicable to a different type of soft MOFs (ELM-12). In this manner, our flow process using the microreactor is demonstrated to be a simple but versatile technique for the synthesis of monodispersed MOF particles.