ESRF Highlights 2023

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1 1 9 I H I G H L I G H T S 2 0 2 3

PRINCIPAL PUBLICATION AND AUTHORS

High-Porosity Metal-Organic Framework Glasses, W. Xu (a,b), N. Hanikel (a,b), K.A. Lomachenko (e), C. Atzori (e), A. Lund (a), H. Lyu (a,b), Z. Zhou (a,b), C.A. Angell (f), O.M. Yaghi (a,b,c,d), Angew. Chem., Int. Ed. 62, e202300003 (2023); https:/doi.org/10.1002/anie.202300003 (a) Department of Chemistry, University of California, Berkeley (USA) (b) Kavli Energy Nanoscience Institute, University of California, Berkeley (USA) (c) Bakar Institute of Digital Materials for the Planet, Division of Computing, Data Science, and Society, University of California, Berkeley (USA) (d) KACST UC Berkeley Center of Excellence for Nanomaterials for Clean Energy Applications, King Abdulaziz City for Science and Technology, Riyadh (Saudi Arabia) (e) ESRF (f) School of Molecular Sciences, Arizona State University, Tempe (USA)

REFERENCES

[1] O.M. Yaghi et al., Introduction to Reticular Chemistry: Metal- Organic Frameworks and Covalent Organic Frameworks, Wiley-VCH (Weinheim, 2019). [2] Y. Zhao et al., J. Am. Chem. Soc. 138, 10818-10821 (2016). [3] F. Farges et al., Phys. Rev. B Condens. Matter. Mater. Phys. 56, 1809-1819 (1997).

To provide a deeper understanding of the MOF glass structure, X-ray absorption near-edge structure (XANES) analysis was conducted at beamline BM23 on both as- synthesised and activated Ti-Fum. The analyses confirmed the predominant presence of 6-coordinated Ti4+ centres in a substantially distorted octahedral environment, as indicated by the position of the Ti K absorption edge and the low intensity of the pre-edge peaks (features A in Figure 97a) [3]. The Ti-Fum spectrum displayed pronounced differences to those of metallic Ti, bulk Ti oxides (rutile and anatase), and smaller Ti-oxo clusters such as [Ti6O6(OiPr)6(AB)6, AB = 4-aminobenzoate] and Ti8O8[OOC(CH3)3]16. Such distinctions rule out the presence of considerable amounts of these phases within the Ti-Fum MOF glass.

Conversely, when comparing the XANES results of the discrete Ti16-oxo cluster, similarities are apparent across all spectral regions with those of Ti-Fum, implying the presence of such units within the structure of the MOF glass. The broadening of the white-line features (peaks B and C) within the Ti-Fum spectrum relative to the reference Ti16O16(OEt)32, can be attributed to an increased degree of local disorder amongst the Ti-oxo clusters present in the glass. These features exhibit a slight sharpening following the activation of the MOF glass, suggesting a minor increase in structural ordering as opposed to degradation. This observation confirms the inherent stability of the Ti-Fum framework. Moreover, the Ti K-edge extended X-ray absorption fine-structure (EXAFS) spectra of the samples also indicate the structural similarity between Ti-Fum (both as-synthesised and activated) and the Ti16O16(OEt)32 cluster (Figure 97b). These observations provide further validation for the initial XANES findings.

Fig. 97: Structural integrity of Ti-oxo clusters in Ti-Fum. a) Ti K-edge XANES spectra of as-synthesised and activated Ti-Fum, isolated Ti-oxo clusters, Ti oxides, and metallic Ti. b) Moduli of the Fourier transforms of the k2-weighted phase uncorrected Ti K-edge EXAFS spectra of Ti-Fum and isolated Ti-oxo clusters.