Zeolites are a class of aluminosilicates which have rigid anionic frameworks containing well defined channels and cavities. These cavities contain metal cations, such as Na+ and K+, which are exchangeable, and also neutral guest molecules, such as water, which can be removed and replaced.
The general formula for a zeolite is Mx/n[(AlO2)x(SiO2)y].mH2O, where n is the valency of the metal ion M which balance the negative charges on the aluminosilicate framework.
|Typical Zeolite Compositions|
|Name||Typical Unit Cell Contents||N(Si):N(Al)|
|ZSM-5||NanAlnSi96-nO192.16H2O(n approximately equal to 3)||30:1 to infinite (as n tends to zero, this tends to hydrated SiO2)|
The Aluminosilicate framework: the structure of Zeolites
Zeolites are made up of corner sharing SiO4 and AlO4 tetrahedra, which form bent M-O-M linkages. Since linked SiO4 tetrahedra are charge neutral in the extended structure, the substitution of Si by Al requires the introduction of a positive charge, typically a sodium cation.
The MO4 tetrahedra can share 1, 2, or 3 oxygen atoms, and so there is a wide variety of structures possible as the network is extended in three dimensions. Commonly, there is only one shared oxygen atom between rings.
An important structural feature is the ability of the network to form rings of the type -M-O-M-O- from the linked tetrahedra, and these rings occur in a range of sizes. These can then be linked to make up large units. The sodalite unit is made up of 4-rings and 6-rings.
|Building blocks in Zeolite structures|
|Bent M-O-M link||6-ring||sodalite unit|
||[each line represents a M-O-M link]|
These sodalite units, which have the structure of truncated octahedra, and other related units with different numbers of 4-rings and 6-rings, such as those based on truncated cuboctahedra, can then be stacked in three dimensions to give the extended structure. The nature of the units making up the extended structure means that there will be cavities and channels in the material, and the material can be tailored and chosen such that the cavities are of specific sizes, which will vary from material to material.
The uses of Zeolites
Zeolites As Dehdyrating Agents
Normal Zeolites contain water in some of the cavities. When heated under vacuum, this water will be lost, and this often causes the free cations, such as Na+, to move and settle on sites with lower coordination numbers.
These dehydrated Zeolites act as very good drying agents, as the adsorption of water means that the cations can return to their favoured high coordination number sites.
Zeolite As Ion exchangers
The Mn+ cations in the zeolite can exchange with the cations in a surrounding solution.
In hard water, the calcium ions exchange with sodium ions from the zeolite, and so the water becomes softer.
Flushing the calcinated zeolite with saline solution makes the exchange proceed in the opposite direction, and the zeolite can be reused as an ion exchanger.
Zeolites As Adsorbents
The use of zeolites as drying agents is one application of their use as adsorbents.
The large number and size of the cavities and channels in the zeolite means that it has a very high surface area, and so it can absorb large amounts of substances, and not just water.
Different zeolites have affinities for different types of molecule. This affinity is determined by the size of the cavities, and also the Si:Al ratio.
When there is a low Si:Al ratio, the number of free cations (Na+) is high, and so the system is hydrophilic. As the Si:Al ratio increases, the number of cations able to interact favourably with the water decreases, and so the hydrophilicity decreases. Zeolites with a very high Si:Al ratio favour the absorption of non-polar molecules such as the hydrocarbons, and so are used in processes such as the cracking of petroleum.
The size of the molecules absorbed is determined by the size of the cavities they will occupy, and also by the size of the pores they must travel through to get into those cavities. For example, in the sodalite cage above, the cavity inside the cage is much bigger than the 4-ring or 6-ring pore through which the absorbed species must move in order to enter the cavity, and it is the pore size which principally determines molecules may be absorbed.
Zeolites As Catalysts
The zeolites are very commonly used as catalysts for a variety of reasons.
The high surface area which made absorption facile also means that there is a large number of active sites where the catalytic action may occur.
The size and shape of the cavities and pores means that the zeolites can be used to provide shape-selective catalysis, and hence control the product distribution of a reaction. For example, in the cracking of petroleum, branched-chain alkenes may be too large to pass through the pores, and so the products will be the straight-chain species which can slip through the holes.
They may be prepared in a very reproducible fashion, and so their catalytic behaviour will also be reproducible.