Abstract: NH_4Y, USY, PrHY, USPrHY and FSPrHY were prepared. Their compositions and physical properties were determined. The difference among their hydroxyl group has been compared on the IR system. The acid strength of every sample was deter-mined by IR-Pyridine method. The order of Bronsted acid strength are FSPrHY>FSY>USPrHY>USY>PrHY>NH_4Y. ~(29)Si MAS NMR spectra,showed that Si(0Al) content in USPrHY and FSPrHY is more than in PrHY and NH_4Y. These results show that the 0-NNN and 1-NNN strong acidity site is more than 2-NNN and 3-NNN weak acidity site on USPrHY and FSPrHY Zeolites. All the results of IR ~(39)Si MAS NMR and activity test are consistent with a model of strong acidity in which both the presence of 0-NNN strong acidity site and polyvalent cations in the β-cages are important. Praseodymium ions in the form of (?) and Pr(OH)~(2+) species are believed to be responsible for the withdrawal of electrons from the framework hydroxyl groups, thus making the protons more acidic, the cracking disproportionation and dealkylation activity were heightened.

Key words: Rich silicon Y zeolite, Ultrastability PrHY acidity, Catalyst activity, ²⁹Si MAS NMR