Surface characterization of alumina-supported catalysts prepared by sol–gel method. Part I.—Acid–base properties

Abstract

Surface characterization of Al₂O₃, In₂O₃–Al₂O₃ and Ga₂O₃–Al₂O₃ prepared by sol–gel method was made focusing on the acid–base properties. Cyclohexanol conversion as a model reaction revealed that acid sites rather than basic sites are present predominantly on the surface. The presence of Lewis acid sites was confirmed by pyridine adsorption followed by Fourier transform infrared (FT-IR) spectroscopy. In addition, a certain amount of Brønsted acid sites were also revealed by the adsorption of 2,6-dimethylpyridine (DMP). From the adsorption of CO, it was found that the addition of Ga₂O₃ into alumina leads to a decrease of Lewis acid strength but to an increase of the number of sites, while In₂O₃ addition caused a global decrease of Lewis acidic properties of alumina. Using CO₂ as a probe to study the basicity, the number of surface basic sites on alumina was found to be decreased by the In₂O₃ and Ga₂O₃ additives. A good correlation between the catalytic activity for 3,3-dimethylbut-1-ene isomerisation and the surface acidity estimated by CO and DMP adsorption was supposed. The combination of model reactions and adsorption of specific probe molecules gives us detailed information concerning the surface acid–base properties.