Optimizing Solvent Conditions for Reduced-Time Solvothermal Synthesis of Ti-MIL-125

Titanium(IV)-based metal-organic frameworks (MOFs), particularly the highly porous and photo-catalytically active Ti-MIL-125 structure, exhibit significant potential for diverse applications. Conventional solvothermal methodologies often require prolonged synthesis times resulting with a wider particle size range, which poses a limitation for implementation in two-phase microfluidic systems allowing for production of monodispersed smaller particles. This study investigates the influence of different solvent systems on the nucleation and crystallization kinetics of Ti-MIL-125 to achieve reduced synthesis times. While n-methyl-2-pyrrolidone (NMP) as a solvent allowed for temperature-dependent crystallization with optimal results at 220 °C, it did not facilitate substantial time reduction. Incorporation of glycerol to elevate the solution boiling point and allow for use of higher synthesis temperatures without the solvent evaporating, resulted in a deceleration of nucleation kinetics, attributed to the augmented viscosity of the synthesis medium. A ternary solvent mixture of NMP, n,n-dimethylformamide (DMF), and methanol significantly accelerated the synthesis, achieving well-crystalline Ti-MIL-125 at a lower temperature of 162 °C and a reduced residence time of 4 hours. These findings highlight the critical role of solvent selection and its impact on diffusion and precursor assembly, providing valuable insights for optimizing Ti-MIL-125 synthesis, particularly for production in droplet-based microfluidic platforms.