Kinetic modelling of the catalytic cracking of n-hexane and n-heptane over a zeolite catalyst Original Research Article

In this paper, a new parameterised microkinetic model for the catalytic cracking of paraffins is described. This model allows us to simulate the complex network of reactions involved in the catalytic cracking of paraffins over an acidic solid; the model includes six different elementary steps (adsorption, desorption, protolytic scission, β-scission, chain growth and hydride transfer). The reaction scheme is detailed to the carbon atom number of the species and explicitly distinguishes between carbenium ions, paraffins and olefins. The number reaction rate constants required to describe all the individual reactions that are possible within the network is well over one hundred and, by using a system of equations which describes the dependency of the rate constants with the nature of the reaction and the species involved, the number of parameters that are fitted in the model is reduced to fourteen.This microkinetic model was fitted, using a genetic algorithm, to the experimental data for the catalytic cracking of n-hexane over a protonic ZSM-5 catalyst at 450 °C with very satisfactory results. The model parameters obtained from n-hexane data, were then used to simulate the catalytic cracking of n-heptane under similar conditions; this simulation compared very well with actual experimental results on n-heptane cracking. This ability to predict the catalytic behaviour for one reaction based on data obtained for other reactions is a significant step in the goal of obtaining models that are able to describe the transformation of a wide range of compounds using a limited number of parameters.