Molecular dynamics simulation of an activated transfer reaction in zeolites

The activated transfer of a light particle between two heavier species in t he micropores of silicalite and ZK4 zeolites has been studied through molec ular dynamics (MD) simulations. A three-body potential controls the exchang e 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 "rea ctant" and "product" species. Harmonic forces always retain the reactants a t favorable distances so that in principle only the energetic requirement m ust be fulfilled for the transfer to occur. The rate constant for the proce ss (obtained from a correlation analysis of equilibrium MD trajectories) de creases by more than one order of magnitude when the barrier height is incr eased from 2k(B)T to 5k(B)T following an Arrhenius-type behavior. The trans fer 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 sili calite. Since with this model the diffusive approach of the reactants is al most irrelevant on the reactive dynamics, only the different ability of eac h environment to transfer the appropriate energy amount to the reactants an d then promote the barrier passage could be invoked to explain the observed behavior. We found that structural, rather than energetic, effects are mai nly 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 rea ctive complex with the host framework compared to silicalite. (C) 1999 Amer ican Institute of Physics. [S0021-9606(99)51736-4].