Aqueous-phase Synthesis of MIL-88B(Fe) and Crystal Size Control via Three-Phase W/O Emulsions

We propose a novel aqueous-based preparation process for MIL-88B(Fe), a metal-organic framework (MOF) composed of Fe³⁺ trimer clusters and terephthalic acid (H₂BDC), and investigate the feasibility of controlling its crystal morphology using water-in-oil (W/O) emulsions as reaction media. MIL-88B(Fe) is conventionally synthesized under solvothermal conditions at temperatures above 100 °C using N,N-dimethylformamide as the solvent, owing to the poor solubility of the H₂BDC precursor in water and common organic solvents. In this study, we demonstrate that MIL-88B(Fe) can be successfully synthesized through an aqueous-based process at temperatures below 50 °C by colloidally dispersing H₂BDC via a reprecipitation method. Furthermore, by employing a thermally stable three-phase W/O emulsion as a confined microreaction environment, the crystal size of MIL-88B(Fe) can be reduced to less than one-tenth of that obtained from bulk aqueous synthesis. These results demonstrate that the internal aqueous droplets of the emulsion effectively function as microreactors, enabling efficient crystal size control of MIL-88B(Fe).