Impact of confinement on zeolite cracking selectivity via Monte Carlo integration

Monte Carlo integration methods employing a classic molecular mechanics for ce field were used to probe the impact of confinement on the selectivity of the n-hexane/3-methylpentane test reaction in 12 different zeolites. The s imulations estimated relative cracking rates and predicted the constraint i ndex (CI) selectivity for both monomolecular and bimolecular cracking mecha nisms. By comparing with experimentally measured values of the CI, informat ion on the dominant mechanism responsible for the observed selectivity was obtained. For small-pore zeolites, the monomolecular mechanism dominated wi th the measured CI attributed to reactant selectivity based on preferential adsorption. For large-pore zeolites, the CI selectivity was attributed to confinement effects imposed on the bimolecular transition states. For zeoli te structures with intermediate-sized pores, such as MTW and TON, the simul ations indicated that both reaction mechanisms may be operative. The experi mentally observed reduction in the CI with temperature in ZSM-5 can be expl ained in terms of the relative importance of steric and entropic factors in the stabilization of the bimolecular transition state. A change in mechani sm from bimolecular to monomolecular, as postulated in the literature, is n ot necessary to explain the experimentally observed temperature dependence of the CI.