The Synthesis of Petrol (Gasoline)

Probably the most valuable resource on the market is crude oil, chiefly due to the petrol available, after fractional distillation, for use in car engines. It drives the economy and is a key expense in the budgets of households and businesses. If only there were a way of synthesising this precious chemical from cheaper, more abundant ones.

Although petrol consists of hydrocarbons mostly between six and twelve carbon atoms per molecule, the most important compound with regards to combustion is isooctane. This has the best ignition properties of all the constituent compounds in petrol: it doesn’t show knocking, which reduces the efficiency of the engine.

This can, and is, made by the Nexoctane Process. This involves mixing two gases called isobutane and isobutylene dissolved in a strong acid catalyst, usually hydrofluoric acid though sulphuric acid can be used. The advantage for anyone seeking to devise a method of making isooctane commercially viable is that each gas can be made from the other (by hydrogenation or dehydrogenation).

The reason why this already isn’t used to make petrol is that the reaction continues to create further products, though isooctane will have the lowest boiling temperature of these and should be easy to decant or distil, as necessary. This could be countered further by having an excess of isobutane in the reaction mixture – thus increasing the yield of the target compound. The advantage of making isooctane as a replacement for petrol is that it can be used in internal combustion engines without the need for modification.

The Fischer-Tropsch Process is used to turn carbon dioxide and hydrogen into hydrocarbons. The trouble with this approach is the hydrogen is taken from methane via the water shift reaction and methane is the major product of the reaction, so it seems to be unviable as a long-term, sustainable option for providing an alternative to petrol from crude oil.

There is currently research into growing algae which produces butanol as a replacement for regular petrol. There is also great interest in the possibility of fermenting waste via the ABE process using a germ called Clostridium acetobutylicum. This is said to produce a fuel which is nearly as efficient as the petrol you put in your car and is less corrosive to the engine as ethanol. It also mixes with petrol far better than ethanol does. T-butanol cannot be used on its own (it must be mixed with petrol) because of its high melting temperature (26°C) which causes problems in all but the hottest of climates. This isn’t a problem with n-butanol which has a lower melting temperature (-90°C) but a lower octane number. This process also makes propan-2-ol and ethanol, which can be sold off to make this venture more cost-effective.