Interplay between Intrinsic Thermal Stability and Expansion Properties of Functionalized UiO-67 Metal–Organic Frameworks

The UiO family of metal–organic
frameworks (MOFs) has been
extensively studied for several applications owing to their high stability
rendered by their robust secondary building units. The efficient design
and use of these materials require a fundamental understanding of
their thermal stability and its impact on chemical and structural
functionality. Herein, we provide a detailed characterization of the
intrinsic thermal behavior of the UiO-67 and functional analogues,
UiO-67-NH2 and UiO-67-CH3. Using in
situ temperature-programmed X-ray diffraction, we find that
distortion of the carboxylate group on the organic linker leads to
negative thermal expansion (NTE) of the UiO-67 MOFs during heating.
This NTE behavior is correlated with rich and reversible thermal changes
observed in the MOF infrared spectral signature as samples are heated
to the sample activation temperature (473 K). We find that in the
absence of oxygen, activated UiO-67 samples show higher thermal stability
compared to ambient or inert environments, with temperature-programmed
desorption revealing an overall stability trend: UiO-67 > UiO-67-CH3 > UiO-67-NH2. Two stages of change are observed
during thermal treatment above 473 K, which are directly related to
deformation of the inorganic node and the isotropic NTE behavior of
these materials. Ultimately, these results provide a real-time interpretation
of the fundamental thermoresponsive behavior of UiO-67 MOFs and offer
a foundation for accurate interpretation of MOF interactions with
guest molecules and their temperature dependence.