Elucidating the structure and the impact of synthesis methods on the flexibility of MIL 88 A (Fe) during water capture

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 and investigate the impact of real structure effects on the dynamic behaviour of MIL 88 A (Fe), particularly upon water adsorption. We investigate four differently synthesized materials with powder X-ray diffraction (PXRD), THz Raman spectroscopy, and N2-sorption. We probe the very high water vapor sorption capacity by utilizing in situ humidity PXRD and calorimetric cycling studies. We identify at least four different crystallographic phases during the sorption measurements and develop a structural concept and models that explains the changes in the XRD patterns. From this we derive that MIL-88 A (Fe) is an excellent material for a new concept of responsive water harvesting technologies.