The impact of framework flexibility and defects on the water adsorption in CAU-10-H

Aluminum-based metal–organic framework (MOF) CAU-10-H is a promising candidate for heat transformation and water harvesting applications due to its hydrothermal stability, beneficial step-wise water adsorption isotherm and low toxicity. In this study, the effects of the framework flexibility and structural defects on the mechanism of water sorption in CAU-10-H were studied by grand canonical Monte Carlo (GCMC) methods. It was shown by the simulations that the rigid ideal MOF framework is hydrophobic. The account of the linker “flapping” motion during the simulations made the framework more hydrophilic due to unblocking of hydroxyl groups that are inaccessible to water molecules for the rigid structure model. However, this model cannot predict the experimental pressure, at which the step on the adsorption isotherm is observed. Based on this result, we suggested that the presence of structural defects could increase the MOF hydrophilicity. The investigation of the water adsorption using several models of defective structures demonstrated that even a small number of defects shift the calculated position of the step on the adsorption isotherm towards the experimental values. The results obtained in this study emphasize that the controlled synthesis of defective structures is one of the most efficient methods of regulating the MOF adsorption properties.