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 quant itative IR measurements of pyridine sorption. The number of Bronsted s ites was found to be less than that of Al atoms. Most probably this wa s due to the presence of Al in extraframework positions (Al-OH groups and Lewis acid sites were found). Some positively charged extraframewo rk Al species may neutralize the charge of AlO4- and lower the concent ration of protonic sites. Dealumination decreased the amount of extraf ramework Al species and, hence, the difference between the amounts of Al and Bronsted sites. Dealumination also decreased the number of Lewi s acid sites. IR studies of pyridine desorption evidenced that, as wel l as strong Bronsted sites (Si-OH-Al), there were also weak Bronsted s ites, not Si-OH-Al; their nature is not clear. The contribution of suc h weak Bronsted sites decreased with the extent of dealumination. IR s tudies of pyridine desorption and benzene sorption, evidenced that Si- OH-Al groups were heterogeneous in H-beta zeolites. Dealumination remo ved the less acidic hydroxy groups first.