Fundamental kinetic studies of the reactivity of hydrocarbons over zeolite catalysts

Zeolite catalysts are used widely in the chemical and petrochemical industries. In general, selectivity issues primarily determine the performance of zeolites, which are capable of facilitating a variety of chemical reactions. The constraints imposed on selectivity by strict environmental regulations and the decreasing quality of available feedstocks drive research toward the development of catalysts with enhanced properties. To this end, we utilize reaction kinetics studies of the conversion of isobutane to address how the activity and selectivity of zeolites are controlled by the structure and composition of the catalyst.; We develop a microkinetic model for the conversion of isobutane on USY over a wide temperature range. The reaction scheme involves initiation, adsorption/desorption, oligomerization/β-scission, isomerization and hydride transfer steps. We find that the composite activation barriers of elementary steps determine activity and selectivity and, therefore, these composite barriers are useful to explain differences in catalytic performance.; We use H-mordenite and β-zeolite to evaluate the effect of zeolite structure on catalytic performance. We find that the composite barriers for monomolecular cracking are ∼20–30 kJ mol−1 lower over these catalysts as compared to USY. Our kinetic model provides new evidence that propagation steps (β-scission and hydride transfer) show similar differences in the composite barriers, suggesting that the same interactions responsible for stabilizing the transition states for monomolecular activation are also important in stabilizing the transition states for propagation reactions.; We use USY- and REUSY-steamed catalysts to evaluate the effect of steaming and rare earth additives. We find that the composite barriers of propagation steps are a linear function of the Brønsted site density, regardless of whether the catalyst is modified by RE cations, steamed under different severity conditions, or both. We suggest that higher Brønsted density stabilizes the transition states for propagation steps, decreasing the corresponding barriers and promoting higher reaction rates.