Acidity of silica-alumina catalysts by amine titration using Hammett indicators and FT-IR study of pyridine adsorption

In this study, characterization and surface acidity of Siral ($SiO_2-Al_2O_3$) compounds were investigated with Hammett acidity functions, the n-butylamine titration method and FTIR analysis of the spectra of pyridine adsorption. All the samples had an acid strength of $H_oleqle$+2.8. The calculated total amount of acid of the samples increased with the increase in $SiO_2$ content up to Siral 40 and then decreased sharply in the case of Siral 80. Maximum amount of acidity was observed in the case of Siral 40 as 1.37mmole/g. No evidence was found for a band at 1540 $cm^{-1}$ on Pural indicating that there were no Bronsted sites on the surface strong enough to react with pyridine. As a result, we can say that the Lewis sites predominate in all the silica-aluminas.