Introduction

In this section, w e summarize the experimental background relevant to our simulation studies on hydrocarbon diffusion in zeolites reported in Chapter 5. Zeolites are defined as crystalline, porous aluminosilicalites which are constructed from comer sharing tetrahedra. The silicon or aluminium atoms are co-ordinated to four oxygen anions at the four comers o f the tetrahedron. They are also com m only protonated and contain extra framework cations. Zeolites are microporous materials; recently mesoporous silicas have been synthesized. Microporous zeolites have pore dimensions o f less than 20Â whereas the pore dimensions o f mesoporous silicas are between 30Â<d<60 Â.

Barrer^^’^^ performed some o f the earliest studies on microporous zeolites by characterizing their stmcture, synthesis and their shape selective catalysis applications. Naturally formed zeolites can be described as volcanic minerals formed when volcanic ash was deposited in ancient alkaline lakes. The resulting alkaline silica rich solutions led to zeolite synthesis.

M odem zeolite chemistry has been characterized by the variety o f synthetic zeolites which can now be designed with a view to specific, desirable applications. Molecular diffusion in zeolites is an important area o f industrial significance particularly in the areas o f shape selective catalysis and gas separation. The ion exchange properties^^'^^ are also o f key industrial importance. Typical zeolite stmctures^' are shown in figure 3.4.

Zeolite frameworks can be purely siliceous or o f the aluminosilicalite variety; for example, silicalite is the siliceous form o f H-ZSM5.^^ A high aluminium to silicon framework ratio maybe indicative o f a stable f r a m e w o r k , a s suggested by the work o f Ooms et The variation o f the ratio can lead to changes in the number o f rings present in the lattice.

Zeolites have enormous internal surface areas such that only one gram o f zeolite provides up to several hundred square meters o f internal surface. This characteristic o f zeolites has given them excellent applications as sorbants and promotes their use in the area o f shape selective catalysis.^^ This process is o f importance to the petroleum industry including major application in hydrocarbon cracking.

As noted, zeolites can contain extra framework cations such as Ca^^ or Na^ as a result o f which they have the ability to exchange cations.^^'^^ This substitution o f ions enables them to adsorb selectively certain harmful or unwanted elements from soil, water and air. A classic example is the removal o f calcium from hard water where they exchange sodium for calcium ions. A further application o f zeolites is in landfills and at industrial sites, where they aid the prevention o f the release o f a number o f harmful or unwanted elements into the environment.

m

m

m

ss

PC

Figure 3.4. From top: (A) zeolite zinc silicalite [ZSM -48], (A l) unit cell o f zinc silicalite.; from bottom: (B) zeolite ABW , (B l) zeolite ABW unit cell and middle diagram: (C) zeolite siliceous Faujasite.

M olecular diffusion'^^’’^^ is a key phenom enon in m any systems. In zeolites it is a property o f m ajor importance, influencing shape selectivity in petrochem ical catalysis and o f course, controlling gas separation. For example, the inequality o f diffusion migration coefficients o f two o f the xylene isomers (para and ortho) is exploited in the area o f catalysis where the faster diffusivity o f para-xylene can be leveraged. Typically, ortho-xylene is not able to travel as quickly or via all channel systems as the para-xylene isomer. This feature is modelled and reported in Chapter 5 o f this thesis. Figure 3.5 shows a snapshot taken o f a para-xylene m olecule residing within a

12 MR channel.

Figure 3.5. Snapshot o f a para-xylene m olecule diffusing w ithin a CIT-1 12MR channel [viewed along the [010] projection].

S elf diffusion in microporous zeolites such as CIT-1 used in the present study, can be followed by a variety o f experimental techniques [explained later in this section] or com putationally by tracing the paths o f motion o f a sample o f m olecules in which the

mean square displacement (M SD), is calculated as the average distance m oved per unit o f time in the three co-ordinate directions o f space. The rate o f the m olecule’s se lf diffusivity can be evaluated by equation 3.1, the Einstein relation.

{r^if)) = 6Dt,

(3.1)

where a is the atom or molecule type, The slope o f the M SD versus time, t, graph therefore allows the diffusion coefficient, D to be calculated.

3,4 Experimental techniques and s e lf diffusion

The mechanism by which m olecules diffuse through pores has been shown by various w o r k e r s t o be governed by factors including pore size, pore wall, structure and molecular size and thermodynamics factors.

The experimental techniques*®^'''^ which have been used to determine the diffusivity in microporous zeolites include uptake methods, pulsed field gradient nuclear magnetic resonance (PEG NMR), neutron scattering, zero length column (ZLC) and chromatography. However, chromatography, uptake and ZLC methods are very slow and calculated diffusion coefficients evaluated by the Einstein relation after molecular dynamics simulations*^^ [explained later in this chapter] cannot be directly compared with diffusivités generated from these methods.

Self-diffusion o f m olecules under equilibrium conditions such as benzene through microporous zeolites such as Faujasite, has been determined by the development o f spectroscopic diffusion measurement techniques. These experimental molecular tracer

techniques include NMR gradient methods such as [PFG NM R], as w ell as thermal neutron scattering techniques such as quasi-elastic neutron scattering [QENS]. The results o f experiments using these methods are more comparable with simulations and a more detailed account therefore follows.