Connecting defects and amorphization in UiO-66 and MIL-140 metal–organic frameworks: a combined experimental and computational study

Abstract

The mechanism and products of the structural collapse of the metal–organic frameworks (MOFs) UiO-66, MIL-140B and MIL-140C upon ball-milling are investigated through solid state 13C NMR and pair distribution function (PDF) studies, finding amorphization to proceed by the breaking of a fraction of metal–ligand bonding in each case. The amorphous products contain inorganic–organic bonding motifs reminiscent of the crystalline phases. Whilst the inorganic Zr6O4(OH)4 clusters of UiO-66 remain intact upon structural collapse, the ZrO backbone of the MIL-140 frameworks undergoes substantial distortion. Density functional theory calculations have been performed to investigate defective models of MIL-140B and show, through comparison of calculated and experimental 13C NMR spectra, that amorphization and defects in the materials are linked.

The Chemistry of Nucleation: In Situ Pair Distribution Function Analysis of Secondary Building Units During UiO‐66 MOF Formation

“…Such effects contribute to a broadening of the peaks and lead to residual intensity between the model and the observed PDF. The PDF shows a strong similarity to the PDF features below 7 Å for amorphous and crystalline UiO‐66, supporting that the observed cluster structure closely resembles the SBU for the UiO‐66.…”

“…[19] It is interesting to compare the observed PDFw ith the pioneering work by Bennett et al and Chapman et al on PDF for both amorphous and crystalline UiO-66. [17,21] The PDF features below 7 are very similar with both amorphous and crystalline UiO-66, [17,21] supporting that the Zr 6 (O) 4 (OH) 4 SBU structure remains intact, and al imited structurald istortion is observed, whereas the PDF features above 7 show differences with that of both amorphous and crystalline UiO-66. These peaks represent the structuralf eatures obtained by the linkage of SBUs.…”

“…Only simple models werea pplied, in order to confirm the presence of the clusters, and no modelling of solvent effects, correlated movements etc.w as undertaken. Such effects contribute to ab roadening of the peaks and lead to residual intensity between the model and the observed PDF.T he PDF shows as trong similarity to the PDF features below 7 for amorphous and crystalline UiO-66, [17,21] supporting that the observedc luster structurec losely resembles the SBU for the UiO-66.…”

Gel-based morphological design of zirconium metal–organic frameworks

“…The main peaks, present in both PDFs at interatomic distances of 2.33 Å, 3.75 Å and 4.89 Å, correspond to UiO-66's intracluster Zr–O and Zr–Zr atom pairs, while those at greater interatomic distances can be attributed to correlations between atoms in neighbouring clusters. 35,60,61 For the peaks observed below interatomic distances of 1 Å, the quality of lab-recorded total scattering data is typically insufficient to allow for a reliable interpretation. 62 The good agreement between both PDFs, as well as with previously reported PDFs for UiO-66, 35,60,61 further corroborates the successful crystallization of UiO-66 during gel formation.…”

“…35,60,61 For the peaks observed below interatomic distances of 1 Å, the quality of lab-recorded total scattering data is typically insufficient to allow for a reliable interpretation. 62 The good agreement between both PDFs, as well as with previously reported PDFs for UiO-66, 35,60,61 further corroborates the successful crystallization of UiO-66 during gel formation. However, while there are no direct indications that the gels contain additional phases, their presence could not be ruled out entirely based on scattering data alone (both PDF and diffraction data), especially since the nearest-neighbour Zr–Zr and Zr–O distances in ZrO 2 are almost identical to those in UiO-66.…”