Metal-Organic Frameworks (MOF) are robust hybrid materials with highly tunable geometries, allowing for direct control over chemical and physical properties making them suitable for a variety of filtration, catalytic and sensing applications. While the ability to readily tune complex chemical and physical functionality is highly desirable for targeted applications, the thermal stability and its impact on applications must also be considered. In this work, we provide a detailed characterization of the intrinsic thermal behavior of the UiO-67 series MOF and the functional analogs, UiO-67-NH2 and UiO-67-CH3, using temperature-programmed infrared, X-ray diffraction and mass spectrometric techniques under vacuum conditions. Using in situ XRD, we have determined that the UiO-67 MOFs exhibit isotropic negative thermal expansion (NTE) during heating up to 473 K, associated with reversible changes in the MOF IR signature, and we have identified the structural origin of this NTE behavior as the distortion of the carboxylate group on the organic linker. Specifically, we demonstrate that the functional pendant groups on the organic linker of UiO-67-NH2 and UiO-67-CH3 are uniquely impacted by heating and cooling. We find that above the activation temperature, and in the absence of oxygen, activated UiO-67 MOFs undergo two stages of irreversible change during thermal degradation which are intimately tied to deformation of the inorganic node and the isotropic negative thermal expansion of these materials. These results provide a fundamental understanding of how the properties of UiO-67 MOFs govern their thermal stability, critical for the future development of stable MOFs and for the interpretation of interactions with guest-molecules.

Intrinsic thermal framework stability of UiO-67 metal-organic frameworks

ACS Spring Meeting 2021

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