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Pore Perfection vs Defect Design: Examining the Complex Relationship between Pore Structure and Carbon Dioxide Adsorption in Zr-Based MOFs
This work examines the relationship between defects, pore size, and pore functionalization as it pertains to the enthalpy of adsorption between carbon dioxide and zirconium-based metal–organic frameworks (UiO-66 and UiO-67). When UiO-66 is synthesized without defects, carbon dioxide adsorption is more exothermic relative to when UiO-66 contains defects (−24.3 vs −20.9 kJ/mol). We repeated the experiments with pristine/defective UiO-67 and observed the opposite trend (−16.9 vs −21 kJ/mol), albeit less exothermic. With the exception of defective UiO-66, which had no change in the enthalpy of adsorption, dehydrating the cluster of pristine/defective UiO-66 (−21 kJ/mol) and UiO-67 (−14 kJ/mol) produced materials that were less exothermic upon carbon dioxide adsorption. This work indicates that there is a hierarchy of adsorption interactions that can work independently or in tandem to increase the enthalpy of adsorption. These include the small tetrahedral pore of UiO-66, hydrogen bonding, and dispersion interaction enhanced by the electron-withdrawing Zr(IV). Postsynthetic modification of the node with methanol/methoxy groups had a strong effect on the defect containing UiO-66. In this MOF, the pore sizes appeared nearly identical to the pristine UiO-66 and contained an enthalpy adsorption of −28 kJ/mol; this is the highest value obtained in this work.
