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06.03.2007



Desperately seeking silicon



05 March 2007



Silicon has been blamed for the faulty fuel that caused thousands of UK cars to break down last week, after drivers filled up at supermarket petrol stations in the south east of England. But the announcement has triggered widespread confusion: how can a silvery-grey semi-metal popularly known to be used in computer chips have found its way into petrol? Here, Chemistry World answers your questions about the element.


 


What is silicon?


Silicon atoms alone form a grey metallic powder, the element silicon. When combined with aluminium and oxygen to make silicate minerals, however, they make up common dirt. In fact, silicon is the second most abundant element in the Earth's crust, making up about a quarter of the crust by mass. 


Joining silicon to oxygen, carbon and hydrogen atoms gives long chains of polymers collectively called silicones or polysiloxanes. These compounds have a backbone of alternating silicon and oxygen atoms, with hydrocarbons hanging off each silicon atom. Depending on the lengths of the chains and the connections between them, the final compounds can be gel-like, rubbery, or stiff. They are used in breast implants, but different types are used as antifoaming agents in diesel, or in grease and lubricants, amongst other applications.


 


Who found silicon in petrol?


Lots of chemists. Asda supermarket's fuel supplier Harvest Energy has confirmed unusually high levels of silicon in fuels that were stored in tanks shared with Greenergy, fuel supplier to Tesco and Morrisons. Independent tests from a number of petrochemical companies are coming to the same conclusion.


 


So which form of silicon is responsible for the contamination?


The scientists who tested the petrol aren't saying, perhaps because the basic test for silicon in petrol doesn't tell you anything about what chemical form the silicon is in, explains Mark Barnes, a consultant for US oil analysts Noria. 


The test first vapourises the fuel, and then looks for the characteristic wavelengths of light emitted by each element. That identifies the presence of silicon atoms, so it is correct to state that silicon atoms are in the fuel; but it's not very helpful in uncovering the exact silicon compound responsible. However, since both dirt and elemental silicon are insoluble in petrol, they are unlikely to be the culprits, says Barnes. 


 


So that leaves .


The silicones - certainly the most likely to have caused these problems, says Clifford Jones of the University of Aberdeen, UK, not least because they are soluble in petrol. 


Various tests exist to confirm that silicones were responsible - as officials strongly believe but haven't yet said. Infrared spectrometry, identifying the vibration frequency of the chemical bonds between silicon and oxygen, would be one clear example, said Barnes.


 


How could silicones have damaged the cars' engines?


As their hydrocarbon components were burnt off, the silicones formed silica (also known as silicon dioxide), which is the principle component in glass and sand. This silica would have formed a whitish deposit which would have clogged oxygen sensors, part of the car's engine management system. 'Silicon should never be in petrol,' said Ian Hillier, of the Trading standards institute, whose tests on a fuel sample provided by a Cambridgeshire motorist identified the rogue element.


 


Has this ever happened before?


Possibly. According to explanatory notes about gasoline refining and testing on the US gasoline company Chevron's website: 'Used toluene from manufacturing processes containing soluble silicon compounds, which was supposed to be used as a paint solvent, has found its way into gasoline. The silicon fouled oxygen sensors and plugged exhaust catalysts, causing severe performance problems and expense.' But a gasoline specialist from the American petroleum institute had not heard of any recent problems in the US with gasoline contaminated by silicon. He was aware of cases of gasoline adulteration, but believes these have occurred overseas, mostly in developing countries.


 


Why wasn't this found earlier?


The European and British specification for unleaded fuel doesn't include a test for silicon, as the element wouldn't normally be found in finished grade petrol or blend components. That's why the fuel was described as meeting British industry standards, even while contaminated, which made it harder for experts to find out what was wrong. Harvest Energy say they will now test for silicon 'as a matter of course' and are proposing the inclusion of a silicon test in the BS EN 228 standard for unleaded petrol. Petrol supplied by Harvest Energy and Greenergy is now free of excess silicon, both companies have said. Information is yet to be released on how the silicon got there in the first place. 


 


Richard Van Noorden




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