Effect of Al-Si-Al and Al-Si-Si-Al Pairs in the ZSM-5 Zeolite Framework on the 27Al NMR Spectra. A Combined High-Resolution 27Al NMR and DFT/MM Study

The effect of the presence of Al-O-Si-O-Al and Al-O-Si-O-Si-O-Al sequences in the ZSM-5 zeolite framework on the local geometry of AlO4- tetrahedra and the Al-27 NMR parameters was investigated employing Al-27 3Q MAS NMR spectroscopy and DFT/MM calculations. The presence of an At atom as a next-nearest (Al-O-Si-O-Al) and next-next-nearest (Al-O-Si-O-Si-O-Al) neighbor can significantly affect both the local geometry of AlO4- tetrahedra as well as Al-27 NMR isotropic chemical shift (up to 4 ppm). There is no systematic contribution of Al in Al-O-Si-O-Al or Al-O-(Si-O)(2)-Al chains to the Al-27 isotropic chemical shift, and not even the direction can be predicted without explicit DFT calculations. Our combined experimental and computational approach employing Al-27 (3Q) MAS NMR spectroscopy supported by DFT/MM calculations (Sklenak, S.; Dedecek, J.; Li, C.; Wichterlova, B.; Gabova, V.; Sierka, M.; Sauer, J. Angew. Chem., Int. Ed. 2007, 46, 7286.) can be employed only for ZSM-5 samples having a low or negligible concentration of Al-O-(Si-O)(n)-Al (n = 1 and 2) sequences in the zeolite matrix, otherwise Al-27 (3Q) MAS NMR spectroscopy cannot be used to even identify the number of framework T sites occupied by Al.