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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 1 H, 27 Al, and 29 Si 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 29 Si 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 AlO 4 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 27 Al spin-echo NMR and 29 Si MAS NMR spectroscopies. The amounts of disturbed framework aluminum atoms, probably 3-foldcoordinated 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 1 H 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. † Part of the special issue "Gerhard Ertl Festschrift".

By studying the ammoniation of the silicoaluminophosphate-type zeolites H-SAPO-34 and H-SAPO-37 by in situ 1 H and 27 Al MAS NMR under continuous-flow (CF) conditions, a two-step adsorption process was determined. The first ammoniation step consists of an adsorption of ammonia exclusively at Brønsted acidic bridging OH groups (SiOHAl), leading to the formation of ammonium ions (NH 4 -form). The second ammoniation step, which occurs at a higher ammonia coverage, consists of a coordination of ammonia molecules to framework Al atoms in ≡P-O-Al≡ bridges. This second adsorption step causes a change of the aluminum coordination from a tetrahedral coordination to an octahedral coordination. The ammonia coordination to Al atoms is reversible when the material is purged at 413 K. The hydration of NH 4 -form silicoaluminophosphates (ammoniated bridging OH groups) causes a coordination of water molecules exclusively to Al atoms in ≡P-O-Al≡ bridges, leading to a hydrolysis of the framework. Therefore, a hydrolysis of the silicoaluminophosphate framework is hindered if the bridging OH groups (SiOHAl), as well as the aluminophosphate framework (≡P-O-Al≡), is covered by ammonia. The latter may support the stabilizing effect of preloaded ammonia on H-form silicoaluminophosphates toward hydration and weak hydrothermal treatments, as recently observed for H-SAPO-34.

The kinetics of the conversion of 13 C-labeled n-butane adsorbed on sulfated zirconia (SZ) were monitored by in situ 13 C MAS NMR spectroscopy. Rate constants of n-to isobutane isomerization and of the 13 C-isotope scrambling from the primary to the secondary carbon atoms in n-butane were determined. The monomolecular scrambling of the 13 C-label in adsorbed n-butane has an activation energy of 17 ± 3 kcal mol )1 and occurs faster than the bimolecular process of n-butane isomerization which has an activation energy of 15.1 ± 0.2 kcal mol )1 . The transfer of the selective 13 C-label from the primary to the secondary carbon atom in the adsorbed n-butane seems to consist of two reaction steps: (i) a hydride abstraction by SZ leading to the formation of sec-butyl cations and (ii) a label scrambling in the sec-butyl cations. This two-step process with the formation of sec-butyl cations as intermediate increases the apparent activation energy for the 13 C-label scrambling, which is almost twice as large compared with the activation energy for carbon scrambling of sec-butyl cations in a superacidic solution.

In situ MAS NMR spectroscopy under continuous-flow conditions and spin−echo NMR experiments have been applied to investigate the quadrupolar interactions of framework aluminum atoms in zeolite H−ZSM-5 at elevated temperatures and during conversion of methanol. The strength of the 27Al quadrupolar interactions of framework aluminum atoms in zeolites depends on the electric field gradient caused by the charge distribution in the local structure of the aluminum sites. Adsorption of methanol on bridging OH groups at 295 K under flow conditions leads to weak 27Al quadrupolar interactions, corresponding to a quadrupole coupling constant of QCC = 4.4 MHz. After the temperature was raised to 573 K during methanol conversion, a strong decrease in the 27Al MAS NMR signal of framework aluminum at 54 ppm occurred due to a significant increase in the 27Al quadrupolar interactions. Comparison of 27Al echo NMR spectra of zeolite H−ZSM-5 loaded with methanol and dimethyl ether and data given in the literature showed that this increase in the 27Al quadrupolar interactions is caused by the formation of dimethyl ether (QCC = 11.2 MHz) and methoxy groups (QCC = 16.2 MHz) at bridging OH groups. Experiments performed during purging zeolite H−ZSM-5 with dry nitrogen at 573 K indicated a high hydroxyl proton mobility which has, however, no influence on the local structure of framework aluminum atoms.