Common but differentiated flexible MIL-53(Al): role of metal sources in synthetic protocol for tuning the adsorption characteristics

The property tuning of metal-organic frameworks (MOFs) has been an active pursuit in both academia and industry. In this work, structural properties of a promising flexible MOF, MIL-53(Al), were finely tuned via a metal source-based synthetic protocol. Varying degrees of framework flexibility and hydrophilicity have been achieved using water-insoluble metal sources, such as alumina, aluminum hydroxide, boehmite, and traditional aluminum nitrate for synthesis. MIL-53(Al) prepared from alumina was the most rigid and hydrophilic as is confirmed by powder X-ray diffraction, vapor adsorption, and diffuse reflectance infrared Fourier transform spectroscopy. Magic-angle spinning nuclear magnetic resonance results revealed that utilizing insoluble metal sources entailed different reaction mechanisms during MOF synthesis and introduced uncoordinated carboxyl into the framework. Through selection of metal sources, the adsorption characteristics of MIL-53(Al) were successfully tuned. The samples prepared from insoluble metal sources showed increased adsorption capacities toward iodine and bisphenol A. The maximum capacity toward iodine in water and n-hexane was one and six times higher than that of conventional MIL-53(Al), respectively. This finding offers excellent prospects for the structural regulation and property tuning of MOFs.