Quantification of Strong Bronsted Acid Sites in Aluminosilicates

By following the selective H/D exchange of acidic hydroxyl groups in aluminosilicates by IR spectroscopy, clear evidence is provided for the existence in amorphous silica-aluminas (ASAs) of Bronsted acid sites comparable in strength to the bridging hydroxyl groups in zeolites. The method distinguishes various types and strengths of strong Bronsted acid sites in aluminosilicates (zeolites, clays, ASAs) and can be used to quantify them. The concentration of strong Bronsted acid sites in ASAs is 2-3 orders of magnitude lower than that in zeolites. The acid catalytic activity in n-alkane hydroconversion correlates well with the number of strong Bronsted acid sites, as probed by our method, for steam stabilized zeolites, clays, and ASAs. The strong Bronsted acid sites in these aluminosilicates arises from hydroxyl groups located between aluminum and silicon occupied oxygen tetrahedra. Differences in Bronsted acidity are caused by differences in the concentrations of strong Bronsted acid sites.