Preparation of magnetic composites of MIL-53(Fe) or MIL-100(Fe) via partial transformation of their framework into γ-Fe2O3

A novel two-step approach is proposed to obtain magnetically active composite materials consisting of MIL-53(Fe) or MIL-100(Fe) and gamma-Fe2O3 particles. The first step consists in a partial transformation of the framework into a layer of gamma-FeO(OH) (lepidocrocite) covering the MOF particles. We found that such a transformation can be realized under air-free conditions by hydrolysing the MOFs at pH 6.2 in the presence of FeSO4. In the second step the obtained gamma-FeO(OH)/MOF composite is heated under an air flow at 250 degrees C in order to transform gamma-FeO(OH) to gamma-Fe2O3. The thus prepared composites containing 40 wt% of the magnetic phase were characterized in detail by XRD, HRTEM, FESEM, N-2 adsorption and magnetic measurements. It was found that the diameter of gamma-Fe2O3 crystallites is about 4 nm in both materials but the microstructure of the magnetic layer is different. While in the MIL-53(Fe)-based composite the crystallites of gamma-Fe2O3 form polycrystalline needle-shaped particles, in the case of MIL-100(Fe) the crystallites are present mainly in the isolated form. Both composites are superparamagnetic at ambient temperature, however the saturation magnetization of the MIL-53(Fe)-based composite (12.7 emu g(-1)) is higher than that for the MIL-100(Fe)-based one (6.6 emu g(-1)) probably due to the difference in the microstructure of gamma-Fe2O3. The specific surface area and pore volume of the prepared gamma-Fe2O3/MIL-100(Fe) composite (699 m(2) g(-1) and 0.41 cm(3) g(-1)) make it a promising magnetically active material for catalysis or adsorption.