Silicalite xenon

Xenon has been considered as the diffusing species in simulations of microporous frameworks other than faujasite (10-12, 21). Pickett et al. (10) considered the silicalite framework, the all-silica polymorph of ZSM-5. Once again, the framework was assumed to be rigid and a 6-12 Lennard-Jones potential was used to describe the interactions between Xe and zeolite oxygen atoms and interactions between Xe atoms. The potential parameters were slightly different from those used by Yashonath for migration of Xe in NaY zeolite (13). In total, 32 Xe atoms were distributed randomly over 8 unit cells of silicalite at the beginning of the simulations and calculations were made for a run time of 300 ps at temperatures from 77 to 450 K. At 298 K, the diffusion coefficient was calculated to be 1.86 X 10 9 m2/s. This... 

Jost S, Bar NK, Fritzsche S, Haberlandt R, and Karger J. Diffusion of a mixture of methane and xenon in silicalite A molecular dynamics study and pulsed field gradient nuclear magnetic resonance experiments. J Phys Chem B 1998 102 6375-6381. 

Fig. 13. HP xenon adsorbed in a 1 mm silicalite single crystal at different temperatures. Channel orientation is shown in the inset. (Courtesy of Igor Moudrakovski. Reprinted from ref 247 with permission. Copyright 2001, American Chemical Society.)...

Rees, L.V.C. and Shen, D., Frequency-response measurements of diffusion of xenon in silicalite-1. J. Chem. Soc., Faraday Trans., 86, 3687 3692, 1990. 

Early molecular dynamics simulations focused on spherically shaped particles in zeolites. These particles were either noble gases, such as argon, krypton, and xenon, or small molecules like methane. For these simulations, the sorbates were treated as soft spheres interacting with the zeolite lattice via a Lennard-Jones potential. Usually the aluminum and silicon atoms in the framework were considered to be shielded by the surrounding oxygen atoms, and no aluminum and silicon interactions with the sorbates were included. The majority of those studies have concentrated on commercially important zeolites such as zeolites A and Y and silicalite (all-silica ZSM-5), for which there is a wealth of experimental information for comparison with computed properties. 

It is instructive to compare the results of simulations obtained using spherical representations of the sorbates with those firom nonspherical (structured) representations. We need also to consider the influence of the rigidity of the lattice, which can be constrained to be completely rigid or flexible. June et al. were able to reproduce experimental self-diffusivities and isosteric heats of adsorption of methane and xenon in silicalite by employing a rigid but structured methane molecule despite the neglect of the Coulomb term in their poten-tial.244 Even when methane is treated as a nonstructured sphere, it was still possible to predict experimentally observed diffusion coefficients well. " ... 

June, R.L. Bell, A.T. Theodorou, D.N. Transition-state studies of xenon and SFe diffusion in silicalite. J. Phys. Chem. 1991, 95, 8866-8878. 

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