Effect of Dehydration on the Local Structure of Framework Silicon Atoms in Zeolites Y Investigated by Solid‐State NMR Spectroscopy

Recently, high‐field shifts of the Si(nAl) signals of 2 to 5 ppm occurring in the 29Si MAS NMR spectra of non‐hydrated zeolites Y were discussed to be caused by i) a variation of the local structure of neighboring AlO4 tetrahedra or ii) the presence of multivalent extra‐framework cations. To clarify the reasons for these resonance shifts, zeolites Y with different H‐ and Al‐exchange degrees were investigated by solid‐state NMR spectroscopy. The experimental results indicate that the primary reason for a high‐field shift of 29Si MAS NMR signals is the change of O‐Al‐O bond angles and Al‐O bond lengths during the dehydration of AlO4 tetrahedra in the framework of zeolite Y. In contrast, the presence of extra‐framework aluminum cations leads only to a strong broadening of the Si(nAl) signals, probably due to 29Si‐27Al couplings, and a weak high‐field shift of not more than 1 ppm.