Magic-angle Spinning Nuclear Magnetic Resonance Studies of Water Molecules adsorbed on Brginsted- 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 MAS NMR) measurements. A 'H NMR line at ca. 6.5 ppm is caused by water adsorption on Lewis-acid sites. The shift of the 'H NMR line of Brernsted-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. Zeolites and amorphous silica-aluminas, which are denoted in the following as aluminosilicates, may contain Brernsted-acid sites, known to be bridging hydroxyl groups (SiOHAl), and also Lewis-acid sites. If small amounts of water are adsorbed, the water molecules can be involved in the formation of hydroxonium ions. They can be dissociated or strongly adsorbed by Lewis-acid sites. Vega and Luz'- ~ studied zeolites of type H-RHO by 'H MAS NMR. They found after addition of small amounts of water on activated samples a narrow peak at 5.9 ppm and