A new force field for cations in zeolites was developed which explicitly distinguished Si from Al atoms, as well as different types of oxygens in the framework. The new force field gave excellent agreement with experimental data on cation positions, site occupancies and vibrational frequencies. Energy minimization showed that Na cations in site-I were not at the centers of hexagonal prisms, but were rather in one of two symmetrical SI sites displaced by about 0.6Å along the [111] direction. Molecular dynamics simulations showed that most cations were immobile in Na-X and Na-Y at the molecular dynamics time-scale; even at 1000K. Only Na-X cations in site-III′ exhibited diffusive motion at 1000K, with a molecular dynamics self-diffusivity of 3.6 x 10-10m2/s. The simulations also showed that cation movement was highly correlated; being composed of jumps involving 4 cations or more.

New Force Field for Na Cations in Faujasite-Type Zeolites. E.Jaramillo, S.M.Auerbach: Journal of Physical Chemistry B, 1999, 103[44], 9589-94