Mechanistic Modeling of n-Heptane Cracking on HZSM-5

A mechanistic model for the catalytic cracking of n-heptane was developed using a novel mechanism-based lumping scheme that exploits the chemical similarities within reaction families. The formal application of 13 reaction family matrices, which correspond to the 11 reaction families in the model, to the matrix representations of the reactants and derived products generated 70 species, 235 elementary steps and 70 ordinary differential equations. The reaction family concept was further exploited to constrain the kinetics within each reaction family to follow a quantitative structure/reactivity Polanyi relationship. Ultimately, four Polanyi relationship parameters and one catalyst specific parameter were optimized using experimental data obtained from the cracking of n-heptane at 500 °C over HZSM-5 with a Si/Al ratio of 21.25. The model correlations were excellent, as were the a priori predictions of experimental results at 450 and 550 °C with an HZSM-5 Si/Al ratio of 21.25 and at 500 °C with HZSM-5 Si/Al ratios of 35.25 and 63.5. The thus validated model was then used to probe the controlling elementary steps of n-heptane cracking. Carbonium ion cracking, β-scission, and hydride transfer were the kinetically significant reactions.