Acid properties of dealuminated beta zeolites studied by IR spectroscopy

The concentration of acid sites in non-dealuminated (Si/Al = 10), and dealuminated (Si/Al = 20-90) H-beta zeolites has been studied by quantitative IR measurements of pyridine sorption. The number of Bronsted sites was found to be less than that of Al atoms. Most probably this was due to the presence of Al in extraframework positions (Al-OH groups and Lewis acid sites were found). Some positively charged extraframework Al species may neutralize the charge of AlO4- and lower the concentration of protonic sites. Dealumination decreased the amount of extraframework Al species and, hence, the difference between the amounts of Al and Bronsted sites. Dealumination also decreased the number of Lewis acid sites. IR studies of pyridine desorption evidenced that, as well as strong Bronsted sites (Si-OH-Al), there were also weak Bronsted sites, not Si-OH-Al; their nature is not clear. The contribution of such weak Bronsted sites decreased with the extent of dealumination. IR studies of pyridine desorption and benzene sorption, evidenced that Si-OH-Al groups were heterogeneous in H-beta zeolites. Dealumination removed the less acidic hydroxy groups first.