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

Abstract

A novel two-step approach is proposed to obtain magnetically active composite materials consisting of MIL-53(Fe) or MIL-100(Fe) and γ-Fe₂O₃ particles. The first step consists in a partial transformation of the framework into a layer of γ-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 γ-FeO(OH)/MOF composite is heated under an air flow at 250 °C in order to transform γ-FeO(OH) to γ-Fe₂O₃. The thus prepared composites containing 40 wt% of the magnetic phase were characterized in detail by XRD, HRTEM, FESEM, N₂ adsorption and magnetic measurements. It was found that the diameter of γ-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 γ-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 γ-Fe₂O₃. The specific surface area and pore volume of the prepared γ-Fe₂O₃/MIL-100(Fe) composite (699 m² g⁻¹ and 0.41 cm³ g⁻¹) make it a promising magnetically active material for catalysis or adsorption.