THE EFFECT OF EXCHANGE CATIONS ON ACIDITY, ACTIVITY, AND SELECTIVITY OF FAUJASITE CRACKING CATALYSTS

Several studies have shown that the removal of exchanged sodium increases activity for paraffin cracking as would be expected of a solid Brønsted acid. However, it has also been observed that at low unit cell sizes the presence of low levels of sodium affects selectivity but not activity for gas oil cracking. This paper describes how cation exchange affects the distribution of acid sites in dealuminated faujasite over a range of unit cell sizes and how this in turn affects the activity and selectivity of USY at a unit cell size of ∼2.440 nm for gas oil and heptane cracking. The acidic properties were determined by NH3/TPD measurements, by pyridine adsorption followed by IR measurements, and by isopropylamine TPD experiments. Activities and selectivities were measured by gas chromatographic analysis of the products of heptane and gas oil cracking. The results show that alkali exchange of dealuminated faujasite has little or no effect on activities for gas oil cracking, but a very strong effect on activity for heptane cracking. While the results show that exchange by alkali cations reduces the number of cracking sites, the major effect is to reduce the intrinsic activity per site by an order of magnitude or more. The deactivating effect of individual cations is a strong function of size. The activity follows the order H+ ∼ Mg2+ ∼ Li+ > Na+ > K+. © 1993 Academic Press, Inc.