Zeitschrift für Physikalische Chemie Neue Folge, Bd. 152, S. 171-182 (1987) © by R. Oldenbourg Verlag, München 1987 0044-3336/87 $3.00 + 0.00 Investigation of Acidic Properties of Zeolites by MAS NMR* By D. Freude, M. Hunger and H. Pfeifer Sektion Physik der Karl-Marx-Universität Leipzig, DDR-7010 Leipzig, German Democratic Republic (Received July 22, 1986) -MAS NMR / "AI-MAS NMR / Zeolites / Bronsted acidity Proton magic angle spinning nuclear magnetic resonance has been applied to study hydroxyl groups in hydrogen zeolites of ZSM-5, mordenite and faujasite type. Five different lines can be resolved in the -MAS NMR spectra of these samples. The signals of the non-acidic hydroxyl groups contain information about structure defects in the zeolites. The Bronsted acidity of the samples can be described by the signals of bridging OH groups. The two signals observed in zeolites HY are correlated with the so- called high and low frequency bands in infrared spectroscopy. The value of the chemical shift of the isolated acidic hydroxyl groups is a measure of their strength of acidity and increases with the Si/Al ratio for faujasites but remains constant above Si/Al a; 10. The accessibility of OH groups could be proved by their interaction with deuterated Pyridine causing a line shift of the proton resonance. A reammoniation of the zeolites gives rise to an increase of the intensity of the ammonium line and a decrease of the line of the acidic hydroxyl groups proving that all bridging OH groups can be reammoniated. Combining and 27A1-MAS NMR the hydrothermal dealumination process in hydrogen zeolites was studied. No exception could be found for the rule: The number of bridging hydroxyl groups is equal to the number of fourfold coordinated framework aluminium atoms. An additional line in the -MAS NMR spectrum due to OH groups associated with non-framework aluminium appears only in the case of strong hydrothermal dealumination. The Si/Al ratio in the zeolite framework is determined by means of "Al-MAS NMR. Three types of non-framework aluminium atoms can be distinguished. ^-MAS NMR-Messungen an Hydroxylgruppen in den -Formen der Zeolithe erlauben Aussagen sowohl über Zahl und Stärke azider Brönsted-Zentren als auch über Struktur- defekte, an denen Silanolgruppen entstehen. Es wird am Beispiel verschiedener Zeolithe gezeigt, daß die NMR durch Anwendung der schnellen Rotation um den magischen Winkel im hohen Magnetfeld eine mit der Infrarotspektroskopie vergleichbare Auflösung * Presented as a Plenary Lecture at the IV. International Symposium on Magnetic Resonance in Colloid and Interface Science, Münster, FRG, July 21 —25, 1986. 429