Evolution of the acidic properties of silica—alumina gels as a function of chemical composition: infrared approach

Abstract

The adsorption of pyridine and CD₃CN on silica–alumina gels covering the whole composition range (0–100 % Al₂O₃) has been studied by infrared spectroscopy, the amount adsorbed being measured by an electrobalance. The measurement of the maximum intensity reached by the bands of strongly adsorbed species has allowed us to sketch the evolution of the surface Brönsted sites and various types of Lewis sites as a function of the chemical composition of the adsorbant.

The overall concentration of Lewis sites increases smoothly from 0 to 75 % Al₂O₃; the sharper increase in the high Al₂O₃ range is mainly due to weak sites which are revealed by a pyridine band at 1615 cm^–1 while pyridine coordinated with strong Lewis sites gives a band at 1625 cm^–1.

The concentration of acid sites associated with the mixed silica–alumina phase corresponds to the more acidic sites observed by titration in non aqueous media. A high proportion of them are strongly acidic hydroxlys which are detected by absorption of H bond acceptors.

The mixed phase develops as the Al₂O₃ content increases from 0 to 25 %; above 50 % Al₂O₃ it is progressively diluted in a mixture with other phases and is no longer observed above 85 % Al₂O₃.