Strong Brønsted Acidity in Amorphous Silica−Aluminas

Monomolecular cracking of propane was used to investigate the activity of the Brønsted acid sites in amorphous silica−aluminas with three different Si/Al ratios. The reaction rates increased with increasing aluminum content, but the apparent activation energies were identical. In comparison to zeolite ZSM5, the ASA catalysts showed much lower activity, both per weight and per total aluminum content. However, after correcting for the heat of adsorption, the intrinsic activation energies of ASA and H-ZSM5 were similar. This indicates that the ability of the active Brønsted acid sites to protonate propane is similar in amorphous and crystalline structures and that the much lower activity of ASA is due to the lower heat of adsorption and the small number of active sites. Few Brønsted acid sites were detected by means of pyridine adsorption followed by infrared spectroscopy; a broad band at around 3600 cm -1 was observed in the infrared region of the hydroxyl stretch vibrations. It is unclear whether this band was related to the catalytically active sites.