Water Sensitivity in Zn4O-Based MOFs is Structure and History Dependent

Moisture can cause irreversible structural collapse in metal-organic frameworks (MOFs) resulting in decreased internal surface areas and pore volumes. The details of such structural collapse with regard to pore size evolution during degradation are currently unknown due to a lack of suitable in situ probes of porosity. Here we acquire MOF porosity data under dynamic conditions by incorporating a flow-through system in tandem with positronium annihilation lifetime spectroscopy (PALS). From the decrease in porosity, we have observed an induction period for water degradation of some Zn4O-based MOFs that signals much greater stability than commonly believed to be possible. The sigmoidal trend in the degradation curve of unfunctionalized MOFs caused by water vapor has been established from the temporal component of pore size evolution as characterized by in situ PALS. IRMOF-3 is found to degrade at a lower relative humidity than MOF-5, a likely consequence of the amine groups in the structure, although, in contrast to MOF-5, residual porosity remains. The presence of an induction period, which itself depends on previous water exposure of the sample (history dependence), and sigmoidal temporal behavior of the moisture-induced degradation mechanism of MOFs was also verified using powder X-ray diffraction analysis and ex situ gas adsorption measurements. Our work provides insight into porosity evolution under application-relevant conditions as well as identifying chemical and structural characteristics influencing stability.