Defects in metal–organic frameworks (MOFs) play important roles in MOF reactivity and catalysis. Now, we report evidence of the reactivity and the quantitative characterization of the missing linker defects on the Zr12O22 nodes in the MOF hcp UiO-66 (these are paired Zr6O8 nodes bridged by OH groups) and those on the Zr6O8 nodes of the MOF UiO-66. The defect sites catalyze the ring-opening reactions of epoxides with alcohols, and new sites formed by removal of bridging OH groups on the Zr12O22 nodes also participate in the catalysis. The hcp UiO-66 was synthesized from UiO-66 and from molecular precursors, and, under various synthesis conditions, the nodes incorporated acetate ligands, where linkers were missing, and the number of these ligands was controlled by the synthesis conditions. These ligands are inhibitors of the catalytic reactions, and their removal by reaction with, for example, methanol (to form, for example, methyl acetate) preceded catalysis on the defect sites. The former MOF incorporated more defect sites than the latter, correspondingly being a more active catalyst. The defect sites on the Zr12O22 nodes are 2–6 times more active per site than those on the isolated Zr6O8 nodes, with the node-bridging OH groups increasing the catalytic activity of the neighboring node defect sites because new sites are formed by their removal. The results help point the way to the design and control of catalytic sites on metal oxide-like MOF nodes by tuning of the number and reactivity of the defect sites.

Elucidation of catalytic sites and mechanisms of reactions on metal oxides is hindered by the complexity and heterogeneity of the surfaces. In contrast, the metal oxide cluster nodes of metal–organic frameworks (MOFs) have well-defined, essentially molecular structures that provide excellent platforms for understanding catalytic reactions in depth. We report an experimental and density functional theory (DFT) investigation of methanol dehydration on the Al8O12 nodes of the MOF CAU-1, presenting catalyst performance data obtained with vapor-phase methanol contacting MOF particles in a flow reactor at temperatures in the range of 180–250 °C. These data and infrared spectra of the MOF characterizing its reactivity with methanol and water show that the reactivities of node μ2-OH groups, influenced by the coordination environments of the node aluminum atoms, react with methanol to form catalytic sites incorporating methoxy ligands that bridge paired aluminum atoms, with the dehydration mechanism engaging nearby MOF linker amine groups that bond to methanol. DFT investigations of the mechanism suggest that the SN2-type methanol dehydration reaction is energetically favored to occur at MOF linker amine groups, which allow for a near-linear transition state (TS) geometry; in contrast, the central cavity of the Al8O12 node adsorbs methanol strongly but, because of geometric constraints, renders the TS for methanol dehydration energetically unfavorable.