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The Bronsted-acid sites in several high-silica LTA samples were examined using temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) of alkylamines, as well as n-hexane cracking rates. Three samples were prepared with SiO2/Al2O3 ratios of approximately 6, but with crystallite sizes ranging from 0.5 to 8 to 12 μm. For n-propylamine, it was necessary to adsorb at slightly elevated temperatures, ∼423 K, in order to saturate the samples. TPD–TGA results for both the H and Na forms of LTA suggested that the reaction of n-propylamine to form propene and ammonia above 650 K provides a meaningful measure of the Bronsted-site densities, although this site density is significantly less than the framework Al concentration for H-LTA. Adsorption measurements with iso-propylamine demonstrate that this molecule can enter the cavities and, therefore, cannot be used to determine external site densities. TPD–TGA results suggest that tri-iso-butylamine is a useful probe of external Bronsted-acid sites. Reaction rates for n-hexane cracking, using the α-test, decreased with increasing crystallite size. The implications of these results for the use of LTA as acid catalysts are discussed. |
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