Ammonia stepwise temperature programmed desorption (STPD) and FT-IR spectroscopy hale been used to study the acidity of dealuminated !3-zeolites with Si/Al ratios in the range of 14.5 to 132. A carefully optimized temperature profile starting at 150°C (in order to exclude physisorbed NH,) revealed five peaks at about I80 C. 250 'C. 350 C. 440' C, and 540 C. Our investigations indicate that the first two peaks correspond to Lewis-type acidity of low strength and that the last three peaks correspond to Bronsted-type acidity of increasing strength. A one-to-one relation between the total number of ammonia molecules desorbed in the above temperature range and that of the Al atoms exists. High strength Lewis sites were not observed probably because the extent of dehydroxylation in the above temperature range was minimal. By increasing the level of dealumination.
it was found that the relative pnpulation of strong Brnnsted sites increases in comparison with the weak ones. The present stepwise temperature programmed desorption experiments coupled with FT-IR provide a very accurate tool for the quantitative measurement of the strength of the reolite acid sites. T' 1998 Elsevier Science B.V.