Role of ZSM-5 and ultrastable Y zeolites for increasing gasoline octane number

Abstract

Octane number of gasoline made in a fluid catalytic cracker can be catalytically enhanced by using an ultrastable HY zeolite instead of one containing rare earth cations or by adding small amounts of ZSM-5 to a Y zeolite catalyst. ZSM-5 addition catalyzes both normal and branched olefin cracking to give mainly propylene, butenes, 2-methyl 1-butene, and 2-methyl 2-butene. Increase in octane number is mainly due to an increase in yields of CS hydrocarbons, an increase in aromatic concentration, and a decrease in yields of C 7 + paraffins and straight olefins. The decrease in paraffin yield is due to the removal of olefins which would otherwise undergo secondary hydrogen addition. The USY zeolite gives a lighter gasoline with higher yields of C 5 and C 6 branched olefins than the REY zeolite which gives higher yields of C 7 + branched paraffins. However, the USY catalyst, unlike the ZSM-5 addition case, does not decrease the yields of C 7 + olefins. Reaction pathways that affect gasoline composition are different, for the two cases; yet, in both cases, increases in C 5 hydrocarbons and decreases in C 7 + paraffins contribute significantly to the increase in octane number.