Water-Modulated Synthesis of MIL-68(Al) and MIL-53(Al) in Protonic and Aprotic Solvents

Abstract Al-based MOFs, MIL-53(Al) and MIL-68(Al), were synthesized in organic solvents (isopropyl alcohol, tetrahydrofuran, methanol, and N,N-dimethylformamide) by adding a small amount of water. The influence of water content (0–20 mol % relative to organic solvent) on crystallization behavior was studied. Pure MIL-53(Al) and MIL-68(Al) were obtained in water-containing organic solvents. With the addition of water, the number of crystals decreased and particle size increased. In addition, the water-modulated MOFs exhibited high crystallinity, as determined by XRD, and large BET surface areas. Water in organic solvents (both protonic and aprotic) plays a role similar to that of monocarboxylic acids in typical modulation techniques, such as the suppression of nucleation frequencies by the competitive coordination of water and ligands with metal ions. The results suggest that, in addition to its typical role, water plays another role in aprotic solvents by promoting crystal growth, possibly through enhanced ligand deprotonation.