Effect of exchange cations and silica to alumina ratio of faujasite on coke selectivity during fluid catalytic cracking

Abstract

Hydrothermally treated fluid-cracking catalysts containing zeolites of varying chemical composition were used in cracking experiments with commercial gas oils and a model hydrocarbon compound. An observed linear relationship between coke yields for a given catalyst and a second-order kinetic conversion parameter was used to assess coke selectivity as a function of zeolite composition as measured by the unit cell constant. Coke selectivity was sensitive to the unit cell constant in the 24.57- to 24.33-Å range. Both parallel and consecutive coking reactions were suppressed in the presence of the smaller unit cell zeolite catalyst. Below 24.33 Å, coke selectivity was less sensitive to the unit cell constant. Exchange cations like sodium and mixed rare earths did not independently influence coke selectivity at a given unit cell constant. Reduction in coke selectivity for the zeolites with smaller unit cell constants can be attributed to the lower density of acid sites in those zeolites.