Elucidating the Structure and the Impact of Synthesis Methods on the Flexibility of the Metal-Organic Framework...

Flexible metal-organic frameworks (MOFs) have emerged as a new generation of porous materials and are considered for various applications such as sensing, water or gas capture, and water purification. MIL-88 A (Fe) is one of the earliest and most researched flexible MOFs, but to date, there is a lack in the structural aspects that govern its dynamic behaviour. Here, we report the first crystal structure of DMF-solvated MIL-88 A and investigate the impact of real structure effects on the dynamic behaviour of MIL-88 A (Fe), particularly upon water adsorption. Four differently synthesized materials are studied with powder X-ray diffraction (PXRD), THz Raman spectroscopy, and N₂ sorption. The very high water vapor sorption capacity is probed by utilizing in situ humidity PXRD and calorimetric cycling studies. At least four different crystallographic phases are identified during the sorption measurements and a structural concept and models are developed that explain the changes in the XRD patterns. From this, it is derived that MIL-88 A (Fe) is an excellent material for a new concept of responsive water harvesting technologies.