Octane catalysts shape selective cracking

The use of a small pore ZSM-5 co-catalyst with REUSY catalysts increases the octane number of gasoline by a process known as shape-selective cracking. Straight-chain Ce-Cio olefins produced by normal cracking reactions and which are the precursors of low-octane paraffins are selectively cracked by ZSM-5 catalysts. 

Another key step was the demonstration by P.B. Weisz and coworkers (3-5) of the shape selectivity of zeolite catalysts related to molecular sieving (1960). This initiated further research in the synthesis of new zeolites as well as industrial applications based on this property. The first commercial shape-selective process, Selectoforming, was developed by Mobil (1968) and allowed the selective cracking of the low octane (n-alkane) components of light gasoline over a natural zeolite (erionite) (6). 

Chevron introduced a novel concept via the Paragon process that combines high-pressixre hydrocracking with the low-pressure step using ZSM-5 zeolite as a shape-selective catalyst. The second stage provides conditions for the selective cracking of normal and slightly branched paraffins to produce Cj-Cg olefins to further boost the octane number [11]. 

Until the recent discovery of UTD-1 and CIT-5, the largest pore zeolites known were composed of pore structures having 12-MRs or less. Many of these materials such as zeolite Y have enjoyed immense commercial success as catalysts (2). There is some evidence from catalytic cracking data that suggests the inverse selectivity found with the 12-MR pore ( 7.5 A) structure such as for SSZ-24 (Chevron) might be used to enhance octane values of fuel (3). However, small increases in pore size as well as variations in pore shape and dimensionality could further improve the catalysts. Pores with greater than a 12-MR structure might allow the conversion of... 

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