Zeolites and the Bulk Chemical Industry

Although the natural zeolites are widely used (around 4 million tpa) they are not particularly valuable as commercial catalysts. This is due to a number of factors including natural variations in crystal size and porosity as well as the actual small pore size, which limits their synthetic usefulness. Natural zeolites do, however, find widespread use in applications such as removal of heavy metals from water, odour removal and building materials e.g. cavity grouting and sprayed concrete). 

To overcome the limitations of natural zeolites a whole range of synthetic zeolites have been manufactured since the 1950s. These have tailored pore sizes and tuned acidities, as well as often incorporating other metal species. The basic synthesis involves mixing a source of silica, usually sodium silicate or colloidal Si02, with a source of alumina, often sodium aluminate, and a base such as sodium hydroxide. The mixture is heated at temperatures up to 200 °C under autogenous pressure for a period of a few days to a few weeks to allow crystallization of the zeolite. 

Zeolite A is by far the most widely produced synthetic zeolite, with an annual production of some 1.3 million tonnes. As may be expected from this large volume its main use is not as a catalyst but as a detergent builder (Box 4.2). Since about 1970 the use of zeolite catalysts has provided huge economic and environmental benefits to the bulk chemical and petroleum refining industries. 

As with many zeolite-based processes, the zeolite is not used alone since it is highly active, resulting in rapid coke deactivation and poisoning 

There are several other examples of ZSM-5 being used commercially to reduce waste and give high product selectivity. One of these is the alkylation of benzene with ethene to produce ethylbenzene selectively. The pore size of ZSM-5 successfully minimizes dialkylation reactions whilst the ability to regenerate the catalyst avoids waste issues associated with older catalysts such as aluminium chloride. 

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