In situ visualization of loading-dependent water effects in a stable metal-organic framework

Competitive water adsorption can have a significant impact on metal-organic framework performance properties, ranging from occupying active sites in catalytic reactions to co-adsorbing at the most favourable adsorption sites in gas separation and storage applications. In this study, we investigate, for a metal-organic framework that is stable after moisture exposure, what are the reversible, loading-dependent structural changes that occur during water adsorption. Herein, a combination of in situ synchrotron powder and single-crystal diffraction, infrared spectroscopy and molecular modelling analysis was used to understand the important role of loading-dependent water effects in a water stable metal-organic framework. Through this analysis, insights into changes in crystallographic lattice parameters, water siting information and water-induced defect structure as a response to water loading were obtained. This work shows that, even in stable metal-organic frameworks that maintain their porosity and crystallinity after moisture exposure, important molecular-level structural changes can still occur during water adsorption due to guest-host interactions such as water-induced bond rearrangements. A stable zinc-based metal-organic framework known to retain its porosity and crystallinity after exposure to moisture has been shown to undergo structural changes at the molecular level on adsorbing water. This dynamic and reversible response to the presence of water, including the rearrangement of bonds, is suggested to be the reason for the hydrolytic stability of this particular metal-organic framework.