Self-diffusion in rare gases in silicalite studied by molecular dynamics

Abstract

The motion of rare gases adsorbed in silicalite has been simulated by using simple Lennard-Jones atom-atom potentials. The silicalite framework has been assumed to be rigid and was represented by the oxygen atoms. The MD simulations have been carried out in the microcanonical (NVE) ensemble. Four loadings have been considered: infinite dilution and 2, 5, and 9 atoms per unit d (u.c.). The dimtsion coefficients have been computed from the Einstein relation at Merent temperaturea in the range 200-500 K. At 300 K and for 2 atoms/u.c., values of 24,12, and 2 X lo+' m 2 d have been obtained for neon, argon, and xenon, respectively. Values of 2 and 3 kJ.mo1-l for the activation energies have been derived for neon and argon; they are nearly independent of the loading. For xenon, the activation energy varies from 6 kJ.moI-' at 0 atoms/u.c. to 3 kJ.mo1-' at 9 atoms/u.c. The distribution of the adsorbed atoms along the channels has been investigated in relation with the potential energy values inside the void space. The concept of supermobility has been revisited.

A classical molecular dynamics study of recombination reactions in a microporous solid

“…40 The iodine and oxygen atoms of the zeolite structure interacted via a Lennard-Jones ͑LJ͒ potential whose parameters are the same as Xe-O. 41 From these values, using the zeolite oxygen LJ parameters, 42 the combining rules 7 give I-I ϭ4.02 Å and ⑀ I-I ϭ1.998 kJ/mol. These parameters were used to obtain the reported I-CCl 4 parameters Harmonic potential c parameters ͑zeolite atoms͒…”

Recombination Reactions and Diffusive Properties of Diatomic Molecules in Two Different Microporous Structures: Silicalite and ZK4

“…Note that the interaction parameters for Cl-O, Br-O, and I-O are the same as Ar-O, Kr-O, and Xe-O, respectively. 40,41 No direct interactions between the halogen and Si atoms are considered, as usual in MD simulations of zeolites, because it is assumed that the silicon atoms are effectively screened by the larger and more polarizable oxygen atoms covering the surface of cavities. The Morse potential parameters have been extracted from ref 42 with some slight adjustments according to more recent data.…”

“…The recombination of three diatomic molecules has been investigated; the Morse and LJ parameters modeling their intra- and intermolecular interactions (Table ) roughly represent the halogen molecules, chlorine, bromine, and iodine, and in the following they are referred to as Cl 2 , Br 2 , and I 2 . Note that the interaction parameters for Cl−O, Br−O, and I−O are the same as Ar−O, Kr−O, and Xe−O, respectively. , No direct interactions between the halogen and Si atoms are considered, as usual in MD simulations of zeolites, because it is assumed that the silicon atoms are effectively screened by the larger and more polarizable oxygen atoms covering the surface of cavities.

1 (a) Structure of silicalite, view along the straight channels.

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Classical and Quantum Transition State Theory for the Diffusion of Helium in Silica Sodalite

“…12 In this case, the attractive wells between which activated diffusers "jump" need not be centered on single atoms; rather, wells may exist in rings or from the superposed interaction of regions of zeolite channels, as the numerous simulations of guests in silicalite have shown. [13][14][15] The solid in the present study, silica sodalite (see section 4), is nonpolar, with a framework structure that is very similar to the naturally-occuring polar form: the aluminosilicate, sodium sodalite. It has no channels, but apertures lead directly between "sodalite" cages.…”