Gasoline selective ZSM-5 FCC additives: Model reactions of C6–C10 olefins over steamed 55:1 and 450:1 ZSM-5 Original Research Article

The reactions of C6–C10 olefins over a 55:1 SiO2/Al2O3 ZSM-5 and a 450:1 ZSM-5 catalyst were studied, in order to understand the origin of the relatively low gasoline loss per octane gain observed when high-silica “gasoline selective” ZSM-5 is added to the FCC. The ratio of isomerization to cracking rates for hexene and octene was higher over the 450:1 catalyst; isomerization to more highly branched olefins can increase gasoline octane rating with no gasoline yield loss. Also, the cracking rates of the higher (e.g. C+8) olefins, compared to cracking of C7− olefins, were higher for the 450:1 ZSM-5. Thus, for a given amount of cracking of the higher gasoline olefins, more of the lighter (C5–C7) olefins will be retained in the gasoline. The cracking of higher olefins can boost octanes through removal of low-octane long-chain olefins, which also serves to prevent formation of very low octane long-chain paraffins from these olefins by hydrogen transfer over the base catalyst, and by formation of high-octane branched C5–C6 olefins.Compared to reactions over the 55:1 ZSM-5, the rates of the fastest reactions (cracking of higher olefins and isomerization), which tend to boost octane with little or no gasoline yield loss, are accelerated over the 450:1 ZSM-5 relative to the slower reactions (cracking, especially of C5–C7 olefins), which tend to reduce gasoline yield with less gain in octane. These reactivity patterns can largely explain the higher gasoline selectivity observed commercially for the high-silica ZSM-5 FCC additives. A likely basis for these differences in reactivity is that the less-active high-silica ZSM-5 is less subject to diffusional limitation of reaction rates.