Site energy distribution and catalytic properties of microporous crystalline ALPO₄-5

Abstract

The site energy distribution on crystalline molecular sieve AlPO₄-5 (studied by temperature-programmed desorption of pyridine at different initial sorbate loadings and stepwise thermal desorption of the base from 373 to 673 K) has been found to be very broad. AlPO₄-5 possesses a significant catalytic activity in cracking of hydrocarbons (viz. cumene, n-hexane, isooctane, and butyl benzene isomers), isomerization of o-xylene and disproportionation of toluene, and also in conversion of methanol and ethanol to aromatics at 673 K. However, the catalytic activity in the above reactions varies significantly with the pulse number due to the coke deposition, which has both detrimental and beneficial effects on the catalytic processes. AlPO₄-5 contains both Lewis and protonic acid sites. The acid sites (particularly the protonic ones) are responsible for its catalytic activity. However, its strong acid sites are very small in number as compared to those on zeolites. The crystalline microporous aluminophosphate (AlPO₄-5) possesses much higher acidity and catalytic activity than the amorphous aluminophosphate.