Abstract
This chapter discusses the shape selectivity in catalytic cracking. Shape selective catalysis is a term normally reserved to describe reactions that take place over restricted pore molecular sieves. Shape selectivity in zeolite catalysis is characterized by one or any combination of three primary mechanisms: (1) reactant shape selectivity whereby molecules are sterically discriminated based upon their ability or inability to enter the restricted pores of the zeolite, (2) product shape selectivity whereby bulkier molecules are sterically hindered from leaving the zeolite, and (3) spatioselectivity whereby the formation of molecular transition states is restricted by the confines of the zeolite channels, intersections, or cages. These characteristics of shape selective catalysis have been traditionally applied to restricted pore zeolites, such as ZSM-5. The evidence for zeolite shape selectivity is obtained primarily by examining the relative rates of conversion of isoparaffins to normal paraffins. Because isoparaffins have intrinsically higher cracking rate constants, evidence of selective normal paraffin conversion or higher iso-normal ratios in the product is usually evidence for shape selectivity.
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