The Chemical Shift Anisotropy: A Promising Parameter To Distinguish The 29 Si NMR Peaks In Zeolites
Résumé
Using a combination of (i) symmetry-based recoupling in 29 Si NMR, (ii) spin dynamics-based simulations and (iii) DFT-based theoretical calculations, we show how the 29 Si NMR peaks of (≡Si-O-Si≡) (Q 4 (0Al)), Brønsted acid sites (≡Si-OH-Al≡) (Q 4 (1Al)), and silanols (≡Si-OH) (Q 3), can be characterized in nanosized ZSM-5 zeolites. Significant differences in the chemical shift anisotropy are calculated theoretically and observed experimentally for silicon nuclei close to aluminum (Q 4 (1Al)) comparing to those who are not i.e. Q 4 (0Al) and Q 3 , allowing one to clearly assign the 29 Si NMR peaks. The isotropic chemical shift alone cannot resolve such differences.
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