NIOBIUM OXIDE ACIDITY STUDIED BY PROTON BROAD-LINE NMR AT 4 K AND MASNMR AT ROOM-TEMPERATURE

The quantitative study of the Bronsted acidity of niobic acid (Nb2O5 . xH(2)O) using broad-line H-1 NMR at 4 K has been performed by interac ting niobic acid, pretreated at 573 K under vacuum, with water molecul es. The number of oxyprotonated species (H3O+, and H2O ... HO species formed, unreacted acidic OH groups or excess H2O molecules) deduced fr om the simulations of the broad-line H-1 NMR spectra shows a continuou s increase in the number of H3O+ species with adsorbed water molecules . This increase may be due to a classical dilution effect or to a syne rgistic interaction between Bronsted and Lewis acid sites. These resul ts are compared with those of some HY zeolites with or without framewo rk defects.