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 Si-29 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 Si-29 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 Si-29-Al-27 couplings, and a weak high-field shift of not more than 1 ppm.