A fundamental kinetic model for the catalytic cracking of alkanes on a USYzeolite in the presence of coke formation

The catalytic cracking of alkanes in the presence of deactivation by coke f ormation is presented. Elementary reactions such as protonation, deprotonat ion, hydride transfer, isomerization, beta scission, and protolytic scissio n are explicitly accounted for. A distinction is made between the formation of primary coke by irreversible adsorption of hydrocarbons on catalytic si tes and the formation of coke by further growth on said primary coke. Termi nation of the coke growth occurs when the coke molecules reach the dimensio ns of the zeolite pores. Primary coke molecules are formed out of the react ion of an alkene with a carbenium ion on the catalyst surface. The degree o f coverage of the catalyst surface with carbenium ions is obtained from the pseudo-steady-state approximation. The deactivation effect of coke on each elementary reaction is modeled with empirical exponential deactivation fun ctions. These functions are expressed as a function of the amount of primar y formed coke, this being a measure for the amount of deactivated acid site s. The kinetic parameters are estimated by regression of experimental data of 2,2,4-trimethylpentane cracking on a USY zeolite catalyst between 698 an d 723 K, a hydrocarbon partial pressure between 7 and 15 kPa, and catalyst coke contents between 0.48 and 3.35 wt %. The deactivation effect of coke o n the various elementary reaction steps is different and increases in the o rder deprotonation <protonation; protonated cyclopropane isomerization < (t ,t) beta scission; protolytic scission < hydride transfer on alkanes < beta scission in which secondary carbenium ions are involved.