Origin of first-order kinetics during the bimolecular cracking of n-hexane over H-ZSM-5 and H-beta zeolites
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An in situ diffuse FTIR spectroscopy study of n-hexane cracking over H-ZSM-5 and H--zeolites was performed to elucidate the origin of first-order kinetics under bimolecular conditions. Contrary to an earlier proposal, the acidic protons were not extensively complexed with olefins during the reaction. A simple qualitative kinetic model consisting of six elementary reactions were used to rationalize the first-order rate law for n-hexane conversion with only a small fraction of the protons being complexed. Because of competing reactions, the olefin and the protonated-complex concentrations became constant as a function of residence time and was independent of the hexane concentration. The model provided a plausible explanation consistent with the kinetic and IR results. By considering a set of reactions including olefin adsorption/desorption, -scission, and oligomerization, the alkoxide concentration may be nearly independent of n-hexane concentration, even when most of the protons remain uncomplexed. Thus, during bimolecular cracking, first-order kinetics may prevail.
