Unraveling Reactive Ligands and Catalytic Sites on μ ₂ –OH Bridged Zr ₆ O ₈ Chain Nodes of Metal–Organic Frameworks CAU-22 and CAU-61 for Methanol Dehydration

To elucidate how the connectivity of μ2–OH groups and defect/vacancy sites on metal–organic framework (MOF) nodes influences ligand reactivity and catalytic activity, we report results of CAU-22 and CAU-61. Both MOFs feature Zr6O8 chain nodes bridged by paired μ2–OH groups and incorporate defect/vacancy sites that are well separated from μ2–OH groups. Our results show that ligands at node defect and vacancy sites are dominated by formate and acetate derived from modulators. These ligands are reactive with methanol and can be readily replaced by methoxy─a key intermediate for methanol dehydration. In contrast, paired μ2–OH groups are not reactive under the same conditions. Correspondingly, the catalytic activities of CAU-22 and CAU-61 for methanol dehydration to form dimethyl ether are attributed to their node defect or vacancy sites but not μ2–OH sites. The low reactivities and activities of μ2–OH sites on the Zr6O8 chain nodes can be explained by their isolation from those defect/vacancy sites. Further, CAU-22 is much less stable than CAU-61 under catalytic reaction conditions because of its higher number of vacancies and weaker node-linker bonds.