Anomalous behavior of cumene cracking in small-pore catalysts

Abstract

The reaction sequence in small-pore catalysts has been postulated to be as follows: adsorption of the reactant from the gas phase to the outside surface of the catalyst to form a condensed-type phase; diffusion of the condensed molecules along the pore walls analogous to surface diffusion, then reaction on active sites, followed by diffusion of the products to the external surface of the catalyst; finally, desorption of the products. This is in contrast to the conventional diffusion, adsorption, and reaction model which is suitable for larger-pore catalysts. Kinetic data from the cumene cracking reaction over REY zeolites are used in this work. The data were obtained at a wide range of temperatures, from 225 to 575°C, sufficient to include the regions of kinetic control and strong pore diffusion. Under these conditions and using the above model, the kinetic and diffusion parameters for cumene cracking are obtained. The intrinsic reaction rate constant and diffusion coefficient over REY zeolites are 2.5 × 10 6 cm 3/g/s and 1.9 × 10 −9 cm 2/s at 500°C, with activation energies of 33 and 15 kcal/mol, respectively.