MAGIC-ANGLE SPINNING NUCLEAR-MAGNETIC-RESONANCE STUDIES OF WATER-MOLECULES ADSORBED ON BRONSTED-ACID AND LEWIS-ACID SITES IN ZEOLITES AND AMORPHOUS SILICA-ALUMINAS

On weakly rehydrated dealuminated zeolites and amorphous silica-aluminas the presence of different types of acidic centres can be ascertained by proton magic-angle spinning nuclear magnetic resonance (H-1 MAS NMR) measurements. A H-1 NMR line at ca. 6.5 ppm is caused by water adsorption on Lewis-acid sites. The shift of the H-1 NMR line of Bronsted-acid sites (bridging OH groups) to lower field for hydrated samples can be interpreted quantitatively by a fast proton exchange between water molecules, bridging OH groups and hydroxonium ions.