Investigating water vapour sorption kinetics of aluminium MOFs by powder X-ray diffraction

Aluminium isophthalate CAU-10 [ Al.OH).O2C-C6H4-CO2)] center dot nH2O and aluminium citraconate CAU-15-Cit [ Al2.OH) 4.O2C-C3H4-CO2)] center dot nH2O (where CAU stands for Christian-Albrechts-University) were investigated regarding their de-and rehydration behaviour using powder X-ray diffraction (PXRD) and volumetric gas sorption. The relative humidities (r. h.) for ad-and desorption observed by PXRD agree well with the values observed in volumetric experiments. Moreover, the kinetics of hydration and dehydration were followed by PXRD at different temperatures. For CAU-10, the hydration at 85% r. h. proceeds much faster than the dehydration at 0% r. h., due to the differences relative to the onset of de-/ rehydration at 20% r. h. We could also observe a clear acceleration for both processes between 40 degrees C and 60 degrees C. For the layered CAU-15Cit, the hydration proceeds much slower compared to CAU-10 while the dehydration proceeds faster. This is attributed to the higher hydrophilicity of CAU-10, as well as the associated structural changes during (de-) intercalation in CAU-15-Cit. Eventually, tentative analyses of the kinetics by Sharp-Hancock plots indicate that the sorption processes are in reasonable agreement with (pseudo-) 0th order kinetics.