An examination of acid sites in H-LTA zeolites

The Bronsted-acid sites in several high-silica LTA samples were examined us ing temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) of alkylamines, as well as n-hexane cracking rates. Three samples wer e prepared with SiO2/Al2O3 ratios of approximately 6, but with crystallite sizes ranging from 0.5 to 8 to 12 mu m. For n-propylamine, it was necessary to adsorb at slightly elevated temperatures, similar to 423 K, in order to saturate the samples. TPD-TGA results for both the H and Na forms of LTA s uggested that the reaction of n-propylamine to form propene and ammonia abo ve 650 K provides a meaningful measure of the Bronsted-site densities, alth ough this site density is significantly less than the framework Al concentr ation for H-LTA. Adsorption measurements with iso-propylamine demonstrate t hat this molecule can enter the cavities and, therefore, cannot be used to determine external site densities. TPD-TGA results suggest that tri-iso-but ylamine is a useful probe of external Bronsted-acid sites. Reaction rates f or n-hexane cracking, using the alpha-test, decreased with increasing cryst allite size. The implications of these results for the use of LTA as acid c atalysts are discussed. (C) 2000 Elsevier Science B.V. All rights reserved.