Gasoline conversion: reactivity towards cracking with equilibrated FCC and ZSM-5 catalysts

Abstract

Cracking of a straight-run FCC gasoline using either a steamed ZSM-5 catalyst or a base, FCC equilibrium, catalyst shows that the only significantly reactive components in the gasoline fraction are normal and branched olefins with a carbon number of seven and higher. Overall, the reactivity of gasoline is one to two orders of magnitude smaller than that of a normal FCC feedstock. The ZSM-5 catalyst produces light olefins (LPG-range and some ethene) through cracking of the gasoline-range olefins. The base catalyst produces these light olefins in lower amounts than ZSM-5 does. Further, the base catalyst produces small amounts of paraffins and products that are heavier than the gasoline feedstock. The overall reason for the differences between the two catalysts is a shape-selective mechanism; in the small pores of ZSM-5 only monomolecular cracking reactions take place, while in the larger pores of the FCC base catalyst also a bimolecular reaction mechanism is operative. As a result of the absence of bimolecular reactions, with ZSM-5, the average size of the products and the amount of hydrogen transfer products is smaller than with the base catalyst. On the other hand, because of the relatively small pore size of ZSM-5, the interaction between the catalytic surface of ZSM-5 and the reactants is larger resulting in a higher conversion of linear olefins and a higher production of ethene than with the base catalyst.