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.