Effect of H2O/DMF mixed solvents on formation of MIL-100(Fe) and dye adsorption

MIL-100(Fe) with microporous structures is often synthesized under conventional hydrothermal conditions using iron ion and trimesic acid. The effect of H2O/dimethylformamide (DMF) mixtures, with variable H2O volume ratios, on the formation of MIL-100(Fe) was investigated under solvothermal conditions. Pure octahedral MIL-100(Fe) could only be formed in neat H2O or DMF. In most cases, the same reaction in mixed H2O/DMF solvents afforded goethite. When a small volume of H2O was used, morphology resembled coexisting octahedra of MIL-100(Fe) and hexagonal platelets of goethite. A subsequent increase in the H2O content yielded pure hexagonal platelets of goethite and then hexagonal bipyramidal structures of hematite. The surface area and pore volume of MIL-100(Fe) were higher than those of goethite. Dye adsorption tests revealed that 80% of methylene blue (MB) was adsorbed on MIL-100(Fe), synthesized in pure DMF, within 5 ​min, whereas only approximately 26% was adsorbed on MIL-100(Fe) synthesized in pure H2O. This can be attributed to the nanoscale hierarchically porous structures with macroporosity in the DMF-synthesized MIL-100(Fe) and enhanced fast diffusion and mass transfer of dye, which demonstrates its potential as an adsorbent for dye removal. Thus, this study provides a new option of solvent for the formation of nanoscale hierarchically porous MIL-100(Fe) with improved performance in various practical applications.