The reactions of C-6-C-10 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 C-7- 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 (C-5-C-7) 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 C-5-C-6 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 C-5-C-7 olefins), which tend to reduce gasoline yield with less gain in octane. These reactivity patterns can largely explain the higher gasoline selectivity observed commercially fbr 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. (C) 1998 Elsevier Science B.V. All rights reserved.
