Probable reaction mechanism and nature of active sites in the catalytic cracking of hydrocarbons over zeolites LaX and PrX-Part I

Abstract

Catalytic cracking activity of lanthanum and praseodymium exchanged zeolite X has been studied and an attempt is made to elucidate the reaction mechanism and nature of active sites in zeolite. Cracking conversion up to 96% is achieved with PrX (99.4%) zeolite. Similar products, totally paraffinic, are obtained irrespective of catalysts used. However, with rise in temperature, the product distribution has changed. Olefins formed in primary reaction get immediately converted to paraffins by secondary hydrogenation reaction, the source of hydrogen being the catalyst itself as well as the coke. The mechanism of catalytic cracking is primarily a carbonium ion type. Active sites responsible for the cracking of hydrocarbons are-(a) hydroxyl group acting as Bronsted acid site, (b) oxide vacancies creating lewis acidity and (c) the rare earth La(ll) and Pr(ll) ions acting through their uncompensated electrostatic field. Under, the strong electrostatic potential (e/r) of La(ll) and Pr(lll), water molecules dissociate to give strong Bronsted acid sites, At higher temperatures dehydroxylation in zeolite produces lewis acid sites which bring about further increase in strong Bronsted acid site concentration due to induction effect intervening the zeolite lattice. Hence most pronounced activity at higher temperature is a result of synergistic effect of strong Bronsted and lewis acid sites