Acidity of dealuminated β-zeolites via coupled NH3-stepwise temperature programmed desorption (STPD) and FT-IR spectroscopy

Ammonia stepwise temperature programmed desorption (STPD) and FT-IR spectroscopy have been used to study the acidity of dealuminated beta-zeolites with Si/Al ratios in the range of 14.5 to 132. A carefully optimized temperature profile starting at 150 degrees C (in order to exclude physisorbed NH3) revealed five peaks at about 180 degrees C, 250 degrees C, 350 degrees C, 440 degrees C, and 540 degrees 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 population of strong Bronsted 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 zeolite acid sites. (C) 1998 Elsevier Science B.V.