Analysis of the Water Adsorption Isotherms in UiO-Based Metal-Organic Frameworks

The present work takes a detailed look at the adsorption of nitrogen gas and water vapor in six related zirconium-based metal-organic frameworks (UiO-66, UiO-66-NH2, UiO-67, UiO-67-NH2, UiO-68-Me-4/PCN-57, and UiO-68-NH2). The work relates the role of defects, linker length, and linker functionality to the gas adsorption properties. The water adsorption isotherms showed no hysteresis consistent with capillary condensation. This suggests a node-based cluster growth mechanism is occurring. Analysis of the water adsorption isotherms illustrated that, prior to the water condensation step in the isotherm, each MOF adsorbed roughly one water molecule per zirconium center. As the linker length increases, the MOF becomes more hydrophobic. The amine functionality increases the hydrophilicity, but the effect of the functional group diminishes as the linker length increases. The latter point is illustrated by calculating the apparent contact angle between the pore wall and condensed water. The apparent contact angle increased from 54.0 to 83.1 degrees from UiO-66 to UiO-68-Me-4/PCN-57 and from 13.0 to 71.0 degrees from UiO-66-NH2 to UiO-68-NH2. From this, the water isotherm was used to construct a pore size distribution consistent with the distribution determined from nitrogen gas adsorption. We further explored the amine-unfunctionalized MOFs for long-term water vapor exposure. Below the water condensation step, the MOFs showed no change in nitrogen gas adsorption capacity/surface area for 100 days.

Automated summary

This study examines the adsorption of nitrogen gas and water vapor in six related zirconium-based metal-organic frameworks, investigating the role of defects, linker length, and functionality on gas adsorption properties. The analysis shows that as the linker length increases, the MOF becomes more hydrophobic, while the amine functionality increases hydrophilicity initially but diminishes with longer linkers. The apparent contact angle between the pore wall and condensed water increases with linker length, indicating changes in hydrophobicity and hydrophilicity.