The Motions of Guest Water Molecules and Cations in Chabazite

The translational self-diffusion, the librations, and the reorientational motions of guest water molecules in the zeolite chabazite are examined by Molecular Dynamics (MD) computer simulations at different temperatures and loadings, including at room temperature, at which the experiments are carried out. Satisfactory agreement is found between the computed and measured translational self-diffusion coefficients. It is, however, furthermore found that the way in which the long-range electrostatic interactions are computed has an effect on the self-diffusion at high loadings and temperatures. The spectral densities of the librational motions of water are found to be similar to those in aqueous salt solutions. The reorientations of the water molecules, on the other hand, are much slower than in the liquids, and very anisotropic. The vector in direction of the molecular dipole moment reorients only very slowly, at the time scale of the simulations, due to the attraction to the almost immobile Ca++-ions and the walls of the zeolite. The other two vectors seem to undergo jump-reorientations rather than reorientations by a diffusion process. Hyper dynamics boost potential method has been applied to the MD simulations to estimate the self-diffusion coefficients of Ca++ ions in dehydrated chabazite. Because of our system is very complicated, the self-diffusion of Ca++ ions can only be roughly estimated. The Ca++ ions diffusion is small enough to confirm that the cation motion can be neglected in the normal MD simulation

The Motions of Guest Water Molecules and Cations in Chabazite

The translational self-diffusion, the librations, and the reorientational motions of guest water molecules in the zeolite chabazite are examined by Molecular Dynamics (MD) computer simulations at different temperatures and loadings, including at room temperature, at which the experiments are carried out. Satisfactory agreement is found between the computed and measured translational self-diffusion coefficients. It is, however, furthermore found that the way in which the long-range electrostatic interactions are computed has an effect on the self-diffusion at high loadings and temperatures. The spectral densities of the librational motions of water are found to be similar to those in aqueous salt solutions. The reorientations of the water molecules, on the other hand, are much slower than in the liquids, and very anisotropic. The vector in direction of the molecular dipole moment reorients only very slowly, at the time scale of the simulations, due to the attraction to the almost immobile Ca++-ions and the walls of the zeolite. The other two vectors seem to undergo jump-reorientations rather than reorientations by a diffusion process. Hyper dynamics boost potential method has been applied to the MD simulations to estimate the self-diffusion coefficients of Ca++ ions in dehydrated chabazite. Because of our system is very complicated, the self-diffusion of Ca++ ions can only be roughly estimated. The Ca++ ions diffusion is small enough to confirm that the cation motion can be neglected in the normal MD simulation