Crystals and energy

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The advance of green energy is directly related to the obtaining of new crystalline materials. Zeolites, for example, are porous materials that are used as catalysers in the oil industry and as molecular sieves in the purification of water. The effectiveness of solar energy depends on silicon crystals. And the great challenge of energy accumulation lies in improving the crystallization of semiconductor materials.

Do you know how a solar power plant works? Do you know what a fuel cell and a catalyst are? Can you imagine how a molecular sieve purifies water?

Zeolites are microporous with an extremely high capacity to hydrate and dehydrate due to the presence of pores and channels of a certain size in their structure. Some 200 types of zeolites have been identified according to their structure. The majority of these are synthetic but some exist in nature in the form of minerals, such as Leucite, Heulandite and Stilbite, which was the first known zeolite. These minerals appear in volcanic, metamorphic and sedimentary rocks. Their crystalline structure is formed by tetrahedrons that are joined making a three-dimensional lattice in which each oxygen atom is shared by two atoms of silicon.

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Crystalline structure of Leucite Crystalline structure of Heulandite
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Crystalline structure of Stilbite

Zeolites contribute in various ways to making a cleaner and safer environment. Almost all of their uses are directed at solving problems related to pollution or energy consumption in numerous industrial, technological and domestic processes. For example, thanks to their capacity to absorb other molecules and soften water, they are present in most detergents, substituting polluting compounds based on phosphates, in soil conditioners for agriculture, and in water filters.

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Sodium Zeolite A, shown above, is used to soften water by means of ionic interchange. This helps to improve the efficiency of those detergents that contain it, reducing water pollution and energy consumption during washing.

Some zeolites present molecular selectivity to substances that they absorb, meaning that they are highly specific for some catalytic applications. By definition, catalysers make any chemical process more efficient. Several synthetic zeolites are used as catalysers in the petrochemical industry to break diesel molecules and produce petrol. In the nuclear industry, zeolites are used to trap radioactive fusion products. Their crystalline structure is very stable and resistant to radiation, which means that, after compressing the charged zeolite at heat, the radioactive waste remains trapped in a very stable ceramic form, which reduces the risk of contamination. After accidents like those at Chernobyl and Fukushima Daiichi, large quantities of zeolite were poured in to trap and prevent the radioactive caesium leaking and spreading, in the case of the latter, into the sea. Other zeolites are being developed to be used as storers of biogas, given that it is possible to trap the gas in their cavities with greater efficiency than any other known method.

 

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The pores of zeolites enable the selective absorption and diffusion of molecules with the appropriate form and size. Above is a molecule of paraxylene inside a zeolite channel. This enables the processes of separation and catalysis to make the petrochemical industry cleaner and more efficient.

 

Did you know that…

  • Natural zeolites have been known for two hundred years. Synthetic zeolites have been produced since the 1950s. Currently more than 12000 tonnes of some 100 different zeolites are produced.
  • Synthetic zeolites are widely used as catalysers in the petrochemical industry. This use is considered one of the greatest achievements of twentieth-century chemistry, and it consumes more than 10 billion dollars of these products.
  • However, the greatest use of zeolites is in the manufacture of laundry detergents, which consumes almost 2 million tonnes of zeolites per year.
  • The Fukushima nuclear disaster produced high levels of contamination by Caesium-134, Caesium-137 and other radioactive isotopes. Immediately after the disaster zeolites were poured into the sea around the nuclear plant to trap and immobilise these dangerous radioactive isotopes.
  • Natural zeolites are formed by the alteration of volcanic rocks and ash layers when they react with alkaline groundwater.
  • Many Zapotec buildings near Oaxaca, Mexico, are built from volcanic tuff made up almost exclusively of zeolites. Likewise in many buildings in Naples, which are constructed with “tuffo giallo Napolitano”, also a zeolitic tuff, and in numerous cathedrals and public buildings in Central Europe.
  • Natural zeolites are the most efficient known filters for absorbing the sulphur dioxide produced by the burning of coal with a high sulphur content – the cause of acid rain.

To find out more

  • You can learn more about the use of silicon crystals in solar energy in the panel “Crystals: Technologies of Today and Tomorrow”.
  • The Wikipedia page on zeolites has plenty of information on the subject (http://en.wikipedia.org/wiki/Zeolite).

 

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