Pore Perfection vs. Defect Design: Examining the Complex Relation-ship Between Pore Structure and Carbon Dioxide Adsorption in Zr-Based MOFs

Abstract

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 de-fects (–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. Dehydrating the cluster of pristine/defective UiO-66 (–21 kJ/mol) and UiO-67 (–14 kJ/mol) the adsorption capabilities decreased considerably. 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 tetra-hedral pore of UiO-66, hydrogen bonding, and dispersion interaction enhanced by the electron-withdrawing Zr(IV). Post-synthetic 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.