DIFFUSION AND VIBRATIONAL-RELAXATION OF A DIATOMIC MOLECULE IN THE PORE NETWORK OF A PURE SILICA ZEOLITE - A MOLECULAR-DYNAMICS STUDY

The vibrational relaxation and the diffusion of diatomic molecules in the zeolite silicalite have been studied through molecular dynamics si mulations in the microcanonical statistical ensemble. The adopted mode l accounts for the vibrations of the framework and sorbed atoms using a harmonic potential for the silicalite and a Morse potential for the diatomic molecule. The results show that the framework favors the rela xation of diatomics oscillating at frequencies near to its characteris tic vibrational frequencies, leading in such cases to lower relaxation times and to an increasing in the energy exchanged per collision. The diffusion of a two-site oscillating molecule representing ethane has been also investigated; the diffusion coefficient and the heat of adso rption agree very well with the experimental data. Arrhenius parameter s for the diffusion have been calculated, and some insights into the d iffusion mechanism have been obtained from log-log plots and by inspec tion of the distribution of the ethane molecules in the silicalite cha nnels. Therefore the simplified model adopted seems to adequately desc ribe the diffusive motion and the guest-host energy exchanges, and it could be useful in order to study simple bimolecular reactions in zeol ites. (C) 1996 American Institute of Physics.