Molecular dynamics simulation of an activated transfer reaction in zeolites

Abstract

The activated transfer of a light particle between two heavier species in the micropores of silicalite and ZK4 zeolites has been studied through molecular dynamics ͑MD͒ simulations. A three-body potential controls the exchange of the light particle between the heavier ones; an effective barrier of a few k B T separates the two stable regions corresponding to symmetric ''reactant'' and ''product'' species. Harmonic forces always retain the reactants at favorable distances so that in principle only the energetic requirement must be fulfilled for the transfer to occur. The rate constant for the process ͑obtained from a correlation analysis of equilibrium MD trajectories͒ decreases by more than one order of magnitude when the barrier height is increased from 2k B T to 5k B T following an Arrhenius-type behavior. The transfer rates are always lower in ZK4. When the reaction is studied in a liquid solvent the calculated rate constants are closer to those obtained in silicalite. Since with this model the diffusive approach of the reactants is almost irrelevant on the reactive dynamics, only the different ability of each environment to transfer the appropriate energy amount to the reactants and then promote the barrier passage could be invoked to explain the observed behavior. We found that structural, rather than energetic, effects are mainly involved on this point. The lower efficiency of ZK4 seems to arise from the frequent trapping of the reactive complex in the narrow ZK4 windows in which the transfer is forbidden and from the weaker interaction of the reactive complex with the host framework compared to silicalite.

Influence of temperature inhomogeneity on product profile of reactions occurring within zeolites

“…The coupling between reaction and diffusion in such systems normally is via the concentration of the reactant and product species. 33 In contrast, in the present situation, the enhancement in D is a direct consequence of inhomogeneous temperature. The present study demonstrates that such a coupling between reaction and diffusion can arise not just due to concentration, but also due to the increase in local temperature, a fact that could not have been anticipated.…”

Sodium migration pathways in multicomponent silicate glasses: Car–Parrinello molecular dynamics simulations

“…If we consider the additional diffusion parameters produced by the analysis with f m = 0.75, the initial and final coordination shells share approximately one oxygen, on average. The backward jumps are always faster compared to forward jumps, as it would be expected for an activated process when the product state is not rapidly thermalized, 51 but also for a temporary displacement to a metastable position, discussed above. The probability p r that a forward jump is followed by a backward jump, reported in the last column of Table I, confirms that backward correlations of Na ions are an important feature of this glass composition, with a 0.40 probability.…”

A molecular dynamics study of intramolecular proton transfer reaction of malonaldehyde in solution based upon a mixed quantum–classical approximation. II. Proton transfer reaction in non-polar solvent

“…4 The molecular dynamics method, in which solvent molecules are described explicitly, has been a powerful tool to investigate chemical reactions in condensed phases. [5][6][7][8][9][10][11][12][13][14][15] For proton transfer reactions, quantization of nuclei is essential. A number of studies of the quantum dynamics of proton transfer a) Current address: CNRS, CRPP-University of Bordeaux, 115 Ave. Dr. Albert Schweitzer, 33600 Pessac, France.…”