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

Abstract

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-NH 2 and UiO-67-CH 3 . 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-CH 3 > UiO-67-NH 2 . 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.

Identifying UiO‐67 Metal‐Organic Framework Defects and Binding Sites through Ammonia Adsorption

“…The interaction with the MOF linker decreases, noted through a reduction in the NHÀ π interaction intensity, as there is little increase in the MOF intrinsic ν(CH) upon adsorption the MOF thermally activated to 573 K (Figures 7 and S17). This suggests that MOF framework collapse begins at around 523 K, consistent with prior results, [40] which hampers NH 3 access to the MOF interior and results in multilayer adsorption on the exterior crystallite surface, similar to that seen for SWCNTs. [44] NH 3 is thus a versatile probe of the MOF surface and interior, giving insight into defect concentrations and porosity.…”

“…The spectra of NH 3 adsorbed upon UiO‐67‐X MOFs also show the appearance of bands in the 2100–2000 (Figure 5) and 660 cm −1 regions (Figure S6), which are neither intrinsic to the MOFs nor to NH 3 [19c,40] …”

“…Typically, UiO-67 MOFs are thermally treated under vacuum to around 473 K to remove residual solvent, as activation beyond 473 K yields irreversible thermal effects, such as dehydroxylation and structure collapse. [40] Dehydroxylation, which occurs when UiO-67 is heated beyond 473 K, is attributed largely to the release of μ 3 À OH from the node through heating. [43] However, since these studies track a mass fragment that can correspond to both μ 3 À OH and H 2 O (present at missing linker defects), the exact source is unknown.…”

“…[43] However, since these studies track a mass fragment that can correspond to both μ 3 À OH and H 2 O (present at missing linker defects), the exact source is unknown. This mass fragment, m/z = 18, typically starts to evolve at around 500 K and has a maximum at around 580 K. [40] H 2 O in Zr MOFs, if present at a linker defect, is typically spectroscopically hidden due to high intensity of the ν(OH) free of the μ 3 À OH, making degradation product identification ambiguous. [39] NH 3 , as seen in Figure 5, is capable of probing H 2 O-terminated defects in MOFs.…”

Well-design and synthesis of a water- and heat-resistant UiO-67@CNTs composite for Congo red efficient capture

“…Secondly, the mass loss appeared at ∼430 °C for UiO-67 and 480 °C for (5.0)UiO-67@CNTs, and the mass loss is accelerated with the collapse of the samples. The second stage is due to the decomposition of organic linkers (Ko et al 2015;Abazari et al 2021;Goodenough et al 2021). At 500 °C, the weight loss of UiO-67 is ∼60%, whereas that for (5.0)UiO-67@CNTs is only ∼15%.…”