BRONSTED ACIDITY OF AMORPHOUS SILICA-ALUMINAS STUDIED BY H-1-NMR

H-1 broad-line (4 K) and MAS (room temperature) NMR have been used to study the acid strength of two amorphous silica-aluminas interacting o r not with adsorbed water. The study is more difficult than for zeolit es, because the acidic SiO(H)Al bridges are reversibly destroyed by de hydration. However, an acidity coefficient value (H3O+ concentration p er Bronsted acid site when one water molecule interacts with each Bron sted site) of 0.34 +/- 10% has been determined. This value is equal to that obtained for H-faujasite and H-mordenite samples with Si/Al rati os high enough for maximum acid strength.