Origin of First-Order Kinetics during the Bimolecular Cracking of n-Hexane over H-ZSM-5 and H-β Zeolites

An in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) study of n-hexane cracking over H-ZSM-5 and H-β zeolites has been carried out to elucidate the origin of first-order kinetics under bimolecular conditions. Contrary to an earlier proposal, the acidic protons are not extensively complexed with olefins during reaction. A simple qualitative kinetic model that consists of six elementary reactions has been 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 concentration, and, hence, the protonated-complex concentration, apparently becomes constant as a function of residence time and is independent of the hexane concentration.