NMR and FTIR spectroscopic studies on the acidity of gallia–silica prepared by a sol–gel route

Abstract

High-surface-area amorphous gallia–silica samples (Si:Ga=3, 10, 20, 40) were prepared by a sol–gel route involving hydrolysis of an ethanolic solution of silicon tetramethoxide and gallium nitrate. Nitrogen adsorption (77 K) showed the materials obtained to be basically mesoporous. The most frequent pore radius resulted to be 0.8 nm in all cases, while surface area ranged from 476 to 586 m 2 g −1. Both, NMR spectroscopy and IR spectroscopy of adsorbed probe molecules (CO and pyridine) showed the gallia–silica samples to have distinctive Brønsted and Lewis acidity. According to NMR spectra of both 29Si and 1H, Brønsted acidity should be assigned to Si(OH)Ga groups formed by partial substitution of gallium for silicon in the silica matrix. Lewis acidity is assigned to coordinatively unsaturated Ga 3+ ions.