The chemistry of catalytic cracking with molecular sieve catalysts

Abstract

In the commercial catalytic cracking of high-boiling petroleum fractions, the crystalline molecular sieve type catalysts exhibit three outstanding differences in comparison to the noncrystalline silica-alumina catalysts. (1) They are more active. (2) The gasoline contains more paraffins and aromatics and fewer olefins. (3) A greater proportion of the petroleum can be cracked before cracking (“over-cracking”) of the gasoline begins. It is proposed that the primary cracking step is the same for both catalysts and the differences are ascribed to differences in secondary reactions of a hydrogen transfer nature. With the molecular sieve catalysts, the gasoil molecules are too large to enter the catalyst cavities, so cracking occurs on the external surface, but the gasoline molecules can enter the cavities and the secondary reactions occur there. With the large pores of the noncrystalline silica-alumina, there is no separation of sites for the primary and secondary reactions. The observed differences are rationalized using these concepts.