TÍTULO DE LA PROPUESTA

“ ...Catalysis must always be preceded by a d s o r p t i o n . . . T h e term adsorption is applied to describing the accumulation of material (the adsorbate) at a surface. In constrast, the term absorption refers to a bulk phenonmenon. The term sorption has been coined to cover both adsorption and absorption.** In any case, before any catalytic activity can take place, some sort of sorptive phenomena must occur, and hence the importance placed on the study of sorption in zeolite materials.

The sorptive properties of a given zeolite are partly determined by steric factors.*^ The size and shape of the pores and channels of the zeolite structure will have a say in which type of molecule will be able to enter the cage and be adsorbed on the internal surface of the cage, and which type will not. Therefore, a zeolite has the ability to select sorbates on the basis of shape and size (known as shape selectivity).

The sorptive and diffusive properties are not only governed by steric factors, but also by the composition and the presence of extraframework cations, which will render the

zeolite either acidic or basic. Such factors will also control whether the sorption is physisorption (van der Waals, dipole, induced-dipole or quadmpole interactions) or chemisorption (where chemical bonds might be formed with the framework). The composition will also have an effect on the energetics o f the adsorption. The presence o f the cations and the aluminium atoms will generate an electric field.^^ This will have a contribution to the adsorption energy of any sorbates in the supercage. Other contributions come from the dispersion-repulsion energy (which can be calculated using a Lennard-Jones potential function, as described in chapter 2) and the polarisation energy of the zeolite. Further terms in the energy calculation depend on permanent dipole and quadmpole moments in the sorbate molecules, and also on sorbate-sorbate interactions.*^

One can measure adsorption isotherms by measuring the amount of adsorbate taken up by the zeolite at a fixed temperature. This can be done at different pressures, and so one can gain an idea of the maximum amount of adsorbate that can be adsorbed before the zeolite pores are saturated.*^'*^.

The sorption of various molecules on basic zeolites has been studied by use o f experimental and computational methods. Information on the sorption and diffusion o f small organic molecules and various gases is important to obtain in order to aid the selection of zeolites with suitable stmctures and compositions for specific purposes and to accommodate specific molecules. As such, a vast amount of work has been performed on determining the soiptive and catalytic properties of different zeolite structures. Past work has successfully used computational methods to study the sorption of CO2, N2 and O2 on zeolites containing sodium and calcium cations. For

example, the work of Bell has successfully calculated sorption isotherms and sorption heats for CO2 and N2 adsorbing on NaA zeolite and CaA zeolite using the Grand

Canonical Monte Carlo technique.

The sorption o f krypton on silicalite has also been studied by Calmiano’* using Monte Carlo methods, and again, the results compared well with experimental values o f adsorption heats and isotherms. Molecular dynamics was also used to study the diffusion of small molecules in zeolites, and this allowed the calculation of diffusion coefficients.

Computational methods have also been used to study organic molecules, for example Monte Carlo studies o f methane adsorbing on Na zeolite-Y by Yashonath et al}^ have yielded adsorption heats that compare favourably with experimental values. One can also study the diffusion of sorbates through the zeolite, as such a process must take place for sorbate to reach their sorption sites. The molecular dynamics technique (outlined in chapter 2) can be used to this end; Catlow, Freeman et studied the behaviour of methane and ethene in zeolite ZSM-5 using molecular dynamics. Their model included a flexible framework, and the results of the diffusion coefficient calculations were in good agreement with experimentally determined values. The results therefore indicated that the simulation was a realistic representation of the diffusion process, adding further weight to the case for the use o f these techniques for studying zeolites. Sastre, Catlow and Corma^^ have also used molecular dynamics to study the diffusion of ortho- and para-xylene in siliceous zeolite CIT-1. The authors found that the ortho isomer could not diffuse through the ten-membered ring channels, although they observed some penetration near the channel intersection with the twelve-membered rings, where the molecules can overcome the activation energy to enter the ten-

membered rings. The ortho-xylene molecules can diffuse through the twelve-membered rings, as this diffusion path is energetically favourable. The authors found that the para- xylene molecules showed larger mobility and could diffuse along a longer path through the 1 0 membered rings and the molecule also diffuses through the 1 2 membered rings.

The authors concluded that the interactions between the molecules also play an important role in determining the nature o f the diffusion of organic molecules in the zeolite, as well as the effects of the pore size.

A number of studies have employed ab initio and first principles methods to study sorption and its effects on zeolite models. Some of these works are further discussed in Chapter 6 of this thesis where they serve as examples of the usefulness of this technique

in the study of sorption.