Acid–base properties of modified aluminas

Abstract

The acid–base properties of modified aluminas (SO2–4, PO3–4, Cl–, F–, Mg2+ and Na+, 0.2–2.0 wt.%) have been investigated by FTIR spectroscopy of probe molecules (pyridine and CDCl3), CO2 TPD and the dehydration of 4-methyl-pentan-2-ol. The results of these methods and those of previous NH3 TPD studies were compared in order to establish a classification of our aluminas. There is very good agreement between the results of the five methods. The various modifying ions have effects as summarised in the following acidity–basicity scale: Na+⩽ Mg2+ < pure γ-Al2O3 < PO3–4⩽ Cl– < SO2–4 < F–, increasing acidity →, â†� increasing basicity Fluorine is distinguished from the other ions as it is able to create Bronsted-acidic sites. CO2 TPD is an adequate method for basicity measurement. NH3 and CO2 TPD give complementary results. Furthermore, FTIR spectroscopy of adsorbed CDCl3 gives valuable information on the effects of ions on the basic strengths.