Chemical Shift Anisotropy: A Promising Parameter To Distinguish the 29Si NMR Peaks in Zeolites

Abstract

Using a combination of (i) symmetry-based recoupling in 29Si NMR, (ii) spin dynamics-based simulations, and (iii) DFT-based theoretical calculations, we show how the 29Si NMR peaks of (≡Si–O–Si≡) (Q4(0Al)), Brønsted acid sites (≡Si–OH–Al≡) (Q4(1Al)), and silanols (≡Si–OH) (Q3) 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 (Q4(1Al)) compared to those which are not, i.e., Q4(0Al) and Q3, allowing one to clearly assign the 29Si NMR peaks. The isotropic chemical shift alone cannot resolve such differences.