Elucidating the Germanium Distribution in ITQ-13 Zeolites by Density Functional Theory**


ITQ-13 is a medium-pore zeolite that can be prepared in all-silica form and as silicogermanate with Si/Ge ratios as low as 3. Usually synthesised in the presence of fluoride, ITQ-13 is among the very few systems containing fluoride anions in two distinct cage types, cube-like d4r units and [4 . 5(6)] cages. Here, dispersion-corrected density functional theory (DFT) calculations are used to investigate the energetically most favourable Ge distributions for Si/Ge ratios between 55 and 6. The calculations show Ge atoms are incorporated at both the corners of d4r cages and at the basal plane of the [4 . 5(6)] cages, in accordance with F-19 NMR spectroscopy. Two Ge atoms at adjacent corners of [4 . 5(6)] cages are stable at the highest Ge content considered (Si/Ge=6). Such a local environment has not yet been considered in the experimental literature. A calculation of the corresponding F-19 NMR resonance points to overlap with other resonances, which might preclude its clear identification. Additional calculations investigate the variation of the dynamic behaviour of the fluoride anions as a function of the local environment as well as the selective defluorination of the [4 . 5(6)] cages.



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Fischer, M; Bornes, C; Mafra, L; Rocha, J

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M.F. acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), project number 389577027 (FI 1800/5-1). Calculations were performed with resources provided by the North-German Supercomputing Alliance (HLRN), project hbc00030 (Dynamics of fluoride anions in all-silica and silicogermanate zeolites). C.B. acknowledges FCT for Doctoral Fellowship PD/BD/142849/2018 integrated in the Ph.D. program in NMR applied to chemistry, materials, and biosciences (Grant PD/00065/2013). This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, Grants UIDB/50011/2020, UIDP/50011/2020 and LA/P/0006/2020, financed by national funds through the FCT/MEC (PIDDAC). Open Access funding enabled and organized by Projekt DEAL.

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