PLAN DE NEGOCIOS

Introduction

This chapter documents the molecular dynamics simulations o f chloroform adsorbed in zeolite NaY at various temperatures.

Molecular dynamics is a well-established technique for studying diffusive processes, having successfully been utilised in the past for the study of zeolites and other materials such as minerals, perovskite oxides and amorphous solids, including the simulation o f diffusion and migration o f cations, and o f framework stability. For instance, molecular dynamics has been used to study the behaviour o f sodium cations in hollandite, a porous manganese oxide. Work by Cormack' has revealed the cation migration mechanism in materials such as sodium sihcate glasses.

In terms of zeolite science, diffusive processes are an important part of the study of zeolite properties because reactant molecules must reach active sites before any catalytic chemistry can take place. Computational studies in this area have mainly focused on the diffusion o f organic molecules such as methane, butane and benzene and other hydrocarbons, as these are industrially important. However, simulations using argon, krypton and xenon as probe molecules can also give us useful information on molecular diffusion in zeolites, as shown in studies by Calmiano^ and Yashonath et al}'^^ as well as Mosell^. The importance o f and interest in such studies is reflected in the number o f works pubhshed over the last 15 years. Studies on benzene diffusion in NaY by Yashonath et a f. and Bull et a f. have elucidated the migration paths and diffusion coefficients of methane and benzene in zeolite Y.

there has been little mention of cation motion and migration, A feature of early molecular dynamics work (and some recent studies) was the use of a fixed zeolite framework. Computational limitations at the time meant that a fixed framework needed to be used. Studies o f molecular diffusion in zeolite A have concluded that the diffusion coefficients for these molecules are not significantly affected by the framework vibrations and that mutual thermalisation of the guest molecules is sufficient for them to pass through any potential barriers to diffusion, e.g. the apertures of supercages^’*. Earlier work detailing the diffusion of methane in silicalite reached the same conclusion. A number o f authors studying diffusion of guest molecules in other zeolites have concluded that diffusion constants are not affected by whether the fi-amework is flexible or not^’^®, for instance, work by Demontis et al. on methane in silicalite-1 This study was conducted with both a rigid and a flexible framework, and the authors concluded that the diffusion coefficients were not greatly influenced by the fi-amework flexibility, although a flexible framework would affect dynamical properties, such as the damping of the velocity autocorrelation function*. It has been suggested that the zeolite lattice vibrations have a small influence on the diffusion o f guest molecules that are relatively small, and a greater influence on larger molecules that are closer in size to the zeolite pores*.

The molecular dynamics technique has also been applied to the study o f proton and cation motion in framework materials such as perovskite oxides and in the zeolite materials. Both hydrated and dehydrated zeolites have been studied in the past, and the cases where the zeolite framework has been treated both as rigid and flexible have been considered.

Sodium cations in zeolite A have been studied by Faux^^, and Lee et a lP and Shin et a lP Simulations on dehydrated zeolite NaA have also been performed by Demontis and Suf f r i t t i ^Shi n et a lP and Lee et a lP use unusually small charges for the zeolite and cations which will affect the Coulomb sum when calculating the long range electrostatic comtributions to the total energy. Furthermore, the diffusion coefficients calculated by Lee et al could be deemed to be inaccurate due to the short simulation time over which the mean square displacements were calculated (4 ps).

Studies by Faux et a lP , and later by Faux*^ on Na cations in zeolite A have concluded that Na cations sited in six rings are immobile in hydrated and dehydrated Na zeolite A. Other cations, sited in 8 rings and 4 rings exhibited diffusive characteristics. These findings may give some qualitative idea o f the sodium-oxygen interactions in zeolite six rings, although their model of Na in a six ring is questionable. The position of the Na cation at the six-ring site was unusual, as the model predicted the cation to lie in the plane o f the six-ring. Such a position is at odds with crystallographic studies and would probably be energetically unstable, as indicated by energy minimisation calculations of cation positions described in a later chapter of this thesis.

Also, the fact that these cations were immobile in the presence o f water could be constmed as an inadequacy in the forcefield parameters for the water-sodium interactions. Extraframework cations in zeolite structures are held at their sites by coordination to oxygen atoms. In anhydrous zeohtes they are not very mobile, although they are mobile in hydrous zeolites as water enhances their mobility. Hence the use of cation-containing zeolites as ion exchangers, a property facilitated by the presence of water molecules.

As in the case of simulation studies o f molecular difhision through zeolite pores, some works available on cations in zeolite employs a rigid framework model. Although a number o f molecular dynamics studies have suggested that a flexible framework does not make a big difference to the diffusion of guest molecules in zeolites, it could be argued that when studying cation motion in zeolites, the effect of a flexible framework should be included as the cations are located closer to the framework than the guest molecules. For example, sodium cations are coordinated to framework oxygen atoms. Therefore the effect of the vibrations of the framework on those o f the cation would be more significant. Inclusion of a flexible zeolite framework might have the effect of thermalisation o f the cations in a similar way that it is proposed for molecular diffusion. The term ‘thermalisation’ is taken to mean the ability o f a species to lose its excess kinetic energy when moving from a region of high potential energy to one of lower potential energy, in this case energy transfer from cations to the framework'^. In view o f this, a rigid framework might therefore mean that the cations would have unrealistically high velocities, and might well affect the diffusion constants of these species.

The purpose o f the molecular dynamics work described here was principally to investigate the question of the cation migration evident from experimental studies o f halocarbon molecules adsorped in dehydrated NaY and NaX zeolite.