Infrared study of the adsorption of benzene and acetonitrile on silica—alumina gels: Acidity properties and surface heterogeneity

Abstract

The adsorption of benzene and acetonitrile on silica—alumina gels covering the whole composition range (0–100% SiO 2) has been studied by infrared spectroscopy, the adsorbed amount being measured by an electrobalance. The quantitative interpretation of the stretching bands of hydroxyls interacting with the adsorbate indicates that a part of the surface is a mixed silica—alumina phase, which does not change appreciably as a function of the global chemical composition but which is progressively diluted by an alumina phase when the Al 2O 3 content increases above 25%. Two populations of OH groups are present on the surface of the mixed phase: hydroxyls of one type are similar to those of silicagel while the other ones are much more acidic. Hydroxyls on the surface of the alumina phase give less intense stretching bands and also appear to be distributed in two populations, those having the same acidity as silicagel hydroxyls and those that are less acidic. The CN stretching bands of adsorbed CD 3CN indicate the presence of Lewis sites associated mainly to the alumina phase.