Probing the water adsorption and stability under steam flow of Zr-based metal–organic frameworks using ⁹¹Zr solid-state NMR spectroscopy

The stability of metal-organic frameworks (MOFs) in the presence of water is crucial for a wide range of applications, including the production of freshwater, desiccation, humidity control, heat pumps/chillers and capture and separation of gases. In particular, their stability under steam flow is essential since most industrial streams contain water vapor. Nevertheless, to the best of our knowledge, the stability under steam flow of Zr-based MOFs, which are among the most widely studied MOFs, has not been investigated so far. We explore it herein for three UiO-like Zr-based MOFs built from the same Zr cluster but distinct organic linkers at temperature ranging from 80 to 200 °C. We demonstrate the possibility of acquiring their ⁹¹Zr NMR spectra using high magnetic field (18.8 T) and low temperature (140 K) and of interpreting them by comparing experimental data with NMR parameters calculated by DFT. NMR observation of this challenging isotope combined with more conventional techniques, such as N₂ adsorption, X-ray diffraction, IR, and ¹H and ¹³C solid-state NMR spectroscopies, provides information not only on the possible collapse of the MOF framework but also on the adsorption of molecules into the pores. We notably show that UiO-66(Zr) and UiO-66-Fum(Zr) built from terephthalate and fumarate linkers, respectively, are stable over 24 h (and even over 7 days for UiO-66(Zr)) under steam flow at all investigated temperatures, whereas UiO-67-NH₂ containing a 2-amino-[1,1'-biphenyl]-4,4'-dicarboxylate linker degrades under steam flow at temperatures ranging from 80 to 150 °C but is preserved at 200 °C. The lower stability of UiO-67-NH₂ stems from its larger pores and its weaker Zr-O coordination bonds, whereas its preservation at 200 °C results from a more limited condensation of water in the pores.