Quantification of Strong Brønsted Acid Sites in Aluminosilicates

Abstract

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 Brønsted acid sites comparable in strength to the bridging hydroxyl groups in zeolites. The method distinguishes various types and strengths of strong Brønsted acid sites in aluminosilicates (zeolites, clays, ASAs) and can be used to quantify them. The concentration of strong Brønsted 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 Brønsted acid sites, as probed by our method, for steam stabilized zeolites, clays, and ASAs. The strong Brønsted acid sites in these aluminosilicates arises from hydroxyl groups located between aluminum and silicon occupied oxygen tetrahedra. Differences in Brønsted acidity are caused by differences in the concentrations of strong Brønsted acid sites.