Acid–base properties of modified aluminas

Abstract

The acid–base properties of modified aluminas (SO^2–₄, PO^3–₄, 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 < PO^3–₄⩽ Cl^– < SO^2–₄ < F^–, increasing acidity →, â†� increasing basicity Fluorine is distinguished from the other ions as it is able to create Brønsted-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.