Probing oxygen exchange between UiO-66(Zr) MOF and water using ¹⁷ O solid-state NMR

The Zr-based Metal Organic Framework (MOF) UiO-66(Zr) is widely employed owing to its good thermal and chemical stabilities. Although the long-range structure of this MOF is preserved in the presence of water during several days, little is known about the formation of defects, which cannot be detected using diffraction techniques. We apply here ¹⁷ O solid-state NMR spectroscopy at 18.8 T to investigate the reactivity of UiO-66, through the exchange of oxygen atoms between the different sites of the MOF and water. For that purpose, we have selectively enriched in ¹⁷ O isotope the carboxylate groups of UiO-66(Zr) by using it with ¹⁷ O-labeled terephthalic acid prepared using mechanochemistry. In the presence of water at 50 °C and a following dehydration at 150 °C, we observe an overall exchange of O atoms between COO^- and μ₃ -O^2- sites. Furthermore, we demonstrate that the three distinct oxygen sites, μ₃ -OH, μ₃ -O^2- and COO^- , of UiO-66(Zr) MOF can be enriched in ¹⁷ O isotope by post-synthetic hydrothermal treatment in the presence of ¹⁷ O-enriched water. These results demonstrate the lability of Zr-O bonds and the reactivity of UiO-66(Zr) with water.