An extended dual kinetic model allows to fit the n-heptane cracking results working in a wide range of reaction conditions. The duality of the model is provided by the contribution of monomolecular and bimolecular cracking mechanisms. It takes into account the role played by the olefins formed on the global cracking or added within the feed. Furthermore by means of this model and the kinetic parameters obtained when cracking n-heptane on ZSM-5, it has been observed that, while some characterization techniques show a homogeneous zeolite surface from the point of view of the active sites, rigorous kinetic experiments point to the possibility that the reactant sees a heterogeneous surface with, at least, two groups of cracking active sites. Those differentiated active sites give different cracking rates and different activation energies for the process and, in the case of ZSM-5, could be assimilated to sites pointing to the 10R channels and sites pointing into the crossing of the 10R channels, mainly due to differences in acid site location and confinement effects.
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