BRONSTED AND LEWIS ACID SITE CONCENTRATIONS IN FLUORIDED ALUMINA FROM INFRARED SPECTRA OF ADSORBED PYRIDINE SPECIES

Previous studies have shown that the acidity of alumina is altered by fluoriding but have not established whether the acidic sites formed are of the Lewis or Brönsted type. In the present work, infrared spectra of adsorbed pyridine species proved that both types of sites are present in alumina fluorided with hydrogen fluoride vapor or aqueous solutions of hydrogen fluoride or ammonium fluoride. Quantitative measurements showed that many of the Brönsted sites were removed at elevated temperatures. However, an appreciable concentration remained even after dehydration at 538 °. Exposure to water vapor increased the concentration of Brönsted sites and decreased that of Lewis sites. Thus the acidic sites of fluorided alumina appear to undergo a reversible dehydration-rehydration analogous to the interconversion of Brönsted and Lewis sites in silica-alumina. The catalytic roles of the two types of acidic sites remain to be established. © 1969.