Stability and Degradation of Metal–Organic-Framework Films under Ambient Air Explored by Uptake and Diffusion Experiments

The loading with guest molecules is the key feature for most applications of metal–organic frameworks (MOFs). The limited stability of MOFs against environmental factors, like humid air, is often a severe problem which hinders real-life applications. Here, the stability of four common MOFs, UiO-66, UiO-67, HKUST-1, and ZIF-8, under long-term exposure to humid air and under exposure to water vapor is explored. Transient uptake experiments with toluene and other volatile organic compounds as probe molecules are combined with structural investigations via X-ray diffraction and infrared spectroscopy. In line with previous publications showing the structural stability of ZIF-8 and UiO-66, it is found that its uptake properties are not affected by exposure to air. On the other hand, HKUST-1 shows clear structural decomposition in air and degradation of the uptake properties. Unexpectedly, while the diffraction and spectroscopy data of UiO-67 do not suggest a corrosion of the structure upon air exposure, the guest-loading data show a strong decrease of the uptake amount and a deceleration of the uptake rate. Both features strongly indicate the formation of surface barriers for the mass transfer in UiO-67, like in HKUST-1. ... mehrThe study underlines the importance of transient uptake experiments for characterizing the stability of MOF materials.