Cracking of hydrocarbons on LaX and PrX zeolite-Part II: The influence of acidity, degree of exchange, temperature and nature of cation

Abstract

The cracking conversion of II-paraffins over modified zeolite catalysts increases with the carbon number. In the case of isoparaffins, the cracking conversion also increases with increase in the number of side chains. The catalytic cracking activity of the zeolite modified with lanthanum and praseodymium increases with the degree of exchange. The cracking conversion increases smoothly upto 40-43 Na(l) ions exchanged per unit cell beyond which the conversion increases sharply and it has been related to strong acid sites which become available only at higher level of exchange. PrX zeolites, being more acidic, are more active than LaX zeolites at all levels of exchange. The cracking conversion is found to increase with temperature which has been attributed to appearance of lewis acid sites by dehydroxylation and increase in concentration of strong Bronsted acid sites with temperature. With the same catalyst. the activation energy decreases in the order pentane > hexane > heptanes > 3-methylhexane > 2.3- dimcthylpentane. At the same level of exchange LaX and PrX give similar product distributions.