Effect of Al−Si−Al and Al−Si−Si−Al Pairs in the ZSM-5 Zeolite Framework on the²⁷Al NMR Spectra. A Combined High-Resolution²⁷Al NMR and DFT/MM Study

Abstract

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 27Al NMR parameters was investigated employing 27Al 3Q MAS NMR spectroscopy and DFT/MM calculations. The presence of an Al 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 27Al 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 27Al isotropic chemical shift, and not even the direction can be predicted without explicit DFT calculations. Our combined experimental and computational approach employing 27Al (3Q) MAS NMR spectroscopy supported by DFT/MM calculations (Sklenak, S.; Dědeček, J.; Li, C.; Wichterlová, B.; Gábová, 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 27Al (3Q) MAS NMR spectroscopy cannot be used to even identify the number of framework T sites occupied by Al.