STUDIES OF ACIDIC SITES ON BORALITES BY TEMPERATURE-PROGRAMMED DESORPTION (TPD) OF NH3, C2H4, AND 1-C4H8

The acidity of[Al]-ZSM-5 is considerably strong leading to isomerization, dimerization, cracking, and coking reactions that may take place concurrently. Fine-tuning the acidity of ZSM-5 by incorporating boron into the zeolite framework is a goal of our research. From the position of boron in the periodic table, it would be expected that the substitution of aluminum with boron in the framework of pentasil zeolites would weaken the acidic strength of the zeolites. Ammonia, 1-butene. and ethylene have been used as probe molecules to study the acidic sites of zeolites. Our results show that three types of acidic sites are present on boron-containing zeolites, i.e., one inactive for reactions of 1-butene, another active for dimerization of 1-butene, and a final one responsible for isomerization of butene. Framework aluminum plays an important role in adsorption of butene or ethylene molecules on boron/aluminum ZSM-5. Butene or ethylene adsorption appears to be nonlinear with the number of acidic sites, and aluminum appears to activate the adsorption of butene or ethylene on boron acidic sites. © 1993 by Academic Press, Inc.