EFFECT OF FRAMEWORK AL CONTENT ON HY ACIDITY AND CRACKING ACTIVITY

The acidic properties and 2-methylpentane cracking turnover frequencie s (TOFs) were compared for HY and two dealuminated HY zeolites with si milar levels of framework Al: HDY that was dealuminated by ammonium he xafluorosilicate, and a (H,NH4)-USY zeolite that was dealuminated by s team. The acid strength distributions were determined by microcalorime try of NH3 adsorption, and the types of acid sites by FTIR spectroscop y. All zeolites possessed Bronsted acid sites with very few Lewis acid sites. The differential heat of adsorption of NH3 on these zeolites w as nearly constant: about 120 kJ/mol for HY and HDY and approximately 7 kJ/mol higher for the (H,NH4)-USY. The cracking TOF for HDY was five times larger than HY, while the TOF of (H,NH4)-USY was 80 times highe r These results suggest that the cracking activity may be affected by other factors in addition to acid strength. (C) 1998 Elsevier Science B.V.