Pore Perfection vs Defect Design: Examining the Complex Relationship between Pore Structure and Carbon Dioxide Adsorption in Zr-Based MOFs
Mason C. Lawrence, Aidan M. Spoel, Michael J. Katz
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 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.