Theoretical simulations of zeolite-catalyzed reactions

This hesis has focused primarily on the benchmarking and application of an electrostatically embedded combined quantum mechanics/molecular mechanics (QM/MM) methodology to the study of zeolite-catalyzed reactions.;The adsorption enthalpy of light hydrocarbon molecules in both acidic and neutral zeolite MFI has been investigated with a range of computational methods. The role of cluster model size and density functional theory methodology is examined by comparison with high quality ab initio wave function theory Moller--Plesset 2nd order perturbation theory (MP2) results and experimentally determined heats of adsorption. A hybrid QM/MM method is required to converge calculated thermochemical properties with respect to cluster model size in a manner that is computationally efficient.;The QM/MM method has also been utilized to study the mechanism by which Ni-exchanged Na-X zeolite is activated by propene and the mechanism for the catalytic oligomerization of propene by the activated Ni-complex in Ni-Na-X. The Ni2+ cations in as-prepared Ni-Na-X are shown to locate preferentially in inaccessible positions within the framework of Na-X, and the migration to a catalytically active position is facilitated by propene. The key activation barrier in the proposed mechanism of propene-facilitated Ni activation corresponds to the formation of a Ni-allyl complex, and is driven by enthalpic stabilization of the Ni2+ cation, which can coordinate better with propene than with the rigid Na-X lattice.;The QM/MM method has been combined with the quasiclassical trajectory (QCT) method to study alkene methylation by methanol catalyzed by the zeolite H-MFI. The rate-limiting step for this reaction is the methylation of the alkene. The apparent activation energy for this step calculated ab initio density functional theory agrees well with the value obtained from experiments and previous full QM calculations. Following the ethene methylation transition state toward the products along the intrinsic reaction coordinate reveals the existence of a protonated cyclopropane (PCP+) carbocation intermediate. A similar protonated methyl-cyclopropane (mPCP+) carbocation intermediate is found for propene methylation.;Free energies of activation for zeolite-catalyzed reactions can be done in the blue-moon ensemble by constrained ab initio molecular dynamics simulations using the QM/MM method. We have used thermodynamic integration (TI) to study the water-assisted proton hopping in zeolite H-MFI. The free energy of activation calculated by TI is compared with that calculated with the quasi rigid-rotor harmonic oscillator (q-RRHO) recently proposed by Grimme. The estimate for the correlation time of proton hopping is improved by two orders of magnitude with the thermodynamic integration technique relative that determined using the q-RRHO approximation. In order to achieve close agreement between the predicted activation barrier for proton hopping and that determined from experimental measurements of the correlation time, it is necessary to use a level of theory that is higher than is practical for molecular dynamics. Therefore, in order to obtain a good estimate of the free energy of activation, it is necessary to use the results of thermodynamic integration to correct the estimate of the entropic contribution to the free energy obtained using the q-RHHO approximation obtained from QM/MM calculations done using a high level of theory for the QM portion of the calculations. (Abstract shortened by ProQuest.).