Chemical Shift Anisotropy: A Promising Parameter To Distinguish the <sup>29</sup>Si NMR Peaks in Zeolites
Eddy Dib, Svetlana Mintova, Georgi N. Vayssilov, Hristiyan A. Aleksandrov, Marina Carravetta
Abstract
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)) compared to those which 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.