State of aluminum in dealuminated, nonhydrated zeolites Y investigated by multinuclear solid-state NMR spectroscopy

A series of dealuminated zeolites Y with framework n(Si)/n(Al) ratios of 2.8-6.0 was prepared by steaming and characterized by atomic emission spectroscopy and H-1, Al-27, and Si-29 NMR spectroscopy. The steaming of zeolite H-Y was performed under water vapor pressures of 3.4-81.5 kPa and at a temperature of 748 K. To exclude an additional modification of the dealuminated zeolites Y, the samples were investigated in the nonhydrated state, i.e., without hydration after the dealumination. By Si-29 magic-angle spinning (MAS) NMR spectroscopy, a strong high-field shift of the signals of Si(3Al) and Si(2Al) sites in the spectra of nonhydrated zeolites Y in comparison with those of the hydrated samples was observed. This finding is explained by a change of the Si-O-T bond angles of Si(nAl) sites in the local structure of nonhydrated framework AlO4 tetrahedra. With increasing water vapor pressure during the dealumination, a systematic decrease of the total amounts of framework aluminum atoms in nonhydrated zeolites Y was found by Al-27 spin-echo NMR and 29Si MAS NMR spectroscopies. The amounts of disturbed framework aluminum atoms, probably 3-fold-coordinated species in nonhydrated zeolites Y. were determined by the increase of the concentrations of bridging OH groups after an ammonia adsorption/desorption treatment and by application of 1H MAS NMR spectroscopy. By a quantitative comparison of the amounts of tetrahedrally coordinated framework aluminum atoms, responsible for the occurrence of negative framework charges, and the amounts of charge-compensating residual sodium cations and bridging hydroxyl protons, the mean cationic charge of extraframework aluminum atoms was estimated. This mean cationic charge per extraframework aluminum atom was found to vary from ca. +2 for weakly dealuminated zeolites Y to ca. +0.5 for strongly dealuminated samples.