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

The catalytic cracking of alkanes in the presence of deactivation by coke formation is presented. Elementary reactions such as protonation, deprotonation, hydride transfer, isomerization, beta scission, and protolytic scission are explicitly accounted for. A distinction is made between the formation of primary coke by irreversible adsorption of hydrocarbons on catalytic sites and the formation of coke by further growth on said primary coke. Termination of the coke growth occurs when the coke molecules reach the dimensions of the zeolite pores. Primary coke molecules are formed out of the reaction of an alkene with a carbenium ion on the catalyst surface. The degree of 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 functions. These functions are expressed as a function of the amount of primary formed coke, this being a measure for the amount of deactivated acid sites. The kinetic parameters are estimated by regression of experimental data of 2,2,4-trimethylpentane cracking on a USY zeolite catalyst between 698 and 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 on the various elementary reaction steps is different and increases in the order deprotonation <protonation; protonated cyclopropane isomerization < (t,t) beta scission; protolytic scission < hydride transfer on alkanes < beta scission in which secondary carbenium ions are involved.