Catalytic cracking of isobutane and 2-methylhexane over USY zeolite: Identification of kinetically significant reaction steps
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abstract
Reaction kinetics analyses were conducted for isobutane and 2-methylhexane conversion over USY zeolite (both calcined and steam-treated) at 773 K, where reaction is initiated by activation of the reactant paraffin. These analyses indicate that increasing the severity of catalyst steam treatment leads to an increase in the composite activation barriers of kinetically significant steps (initiation and hydride transfer), except for the reaction step involving dehydrogenation of the reactant paraffin, which may be enhanced by extraframework aluminum species introduced via steam treatment. We use sensitivity analyses to probe the degrees of rate control for all reaction steps as a function of the extent of conversion, reaction temperature, and size of the reactant molecule. Initiation steps (i.e., monomolecular reactions) have a high degree of rate control for both isobutane and 2-methylhexane at low conversions and high temperature, whereas the degree of rate control for hydride transfer steps becomes more significant at higher conversions and at lower temperatures. The β-scission steps have relatively lower degrees of rate control. The initiation steps have greater kinetic significance than the hydride transfer and β-scission steps for the larger molecule 2-methylhexane as compared to isobutane. Extrapolation of the kinetic model to higher temperatures, 848 K, suggests that initiation steps dominate over the whole conversion range, especially for the larger 2-methylhexane molecule.
