ROLE OF ZSM-5 AND ULTRASTABLE Y-ZEOLITES FOR INCREASING GASOLINE OCTANE NUMBER

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 C7+ 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 C5 and C6 branched olefins than the REY zeolite which gives higher yields of C7+ branched paraffins. However, the USY catalyst, unlike the ZSM-5 addition case, does not decrease the yields of C7+ olefins. Reaction pathways that affect gasoline composition are different, for the two cases; yet, in both cases, increases in C5 hydrocarbons and decreases in C7+ paraffins contribute significantly to the increase in octane number. © 1991.