ACIDITY OF DEALUMINATED BETA-ZEOLITES VIA COUPLED NH3-STEPWISE TEMPERATURE-PROGRAMMED DESORPTION (STPD) AND FT-IR SPECTROSCOPY

Ammonia stepwise temperature programmed desorption (STPD) and FT-IR sp ectroscopy have been used to study the acidity of dealuminated beta-ze olites with Si/Al ratios in the range of 14.5 to 132. A carefully opti mized temperature profile starting at 150 degrees C (in order to exclu de physisorbed NH3) revealed five peaks at about 180 degrees C, 250 de grees C, 350 degrees C, 440 degrees C, and 540 degrees C. Our investig ations indicate that the first two peaks correspond to Lewis-type acid ity of low strength and that the last three peaks correspond to Bronst ed-type acidity of increasing strength. A one-to-one relation between the total number of ammonia molecules desorbed in the above temperatur e range and that of the Al atoms exists. High strength Lewis sites wer e not observed probably because the extent of dehydroxylation in the a bove temperature range was minimal. By increasing the level of dealumi nation, it was found that the relative population of strong Bronsted s ites increases in comparison with the weak ones. The present stepwise temperature programmed desorption experiments coupled with FT-IR provi de a very accurate tool for the quantitative measurement of the streng th of the zeolite acid sites. (C) 1998 Elsevier Science B.V.