Methyl t-butyl ether (2-methoxy-2-methylpropane), often referred to as simply MTBE, has been widely used over the last 30 years to improve the burning characteristics of petrol.
Uses of methyl t-butyl ether
There are three ways of improving the octane rating of petrol. One is to add a volatile lead compound, usually lead tetraethyl, Pb(C2H5)4. However, there is great concern that when lead compounds are released into the atmosphere from the exhaust, they damage our health and their use is now prohibited. They also poison the metal catalysts in catalytic converters.
Another way to reduce knock is to increase the proportion of branched chain alkanes, cycloalkanes (alicyclic) and aromatics in petrol. This is the principal way used in the UK to improve the octane rating. The branched chain alkanes, cycloalkanes and aromatics are produced from straight-chain alkanes in a variety of ways, for example, by catalytic cracking of gas oil and by reforming of naphtha. However, because of its toxicity, the use of benzene, one of the most effective chemicals to help in raising the octane rating, is restricted.
A further way to improve the octane rating is to add an oxygenated compound. Methyl t-butyl ether, MTBE, has been most widely used and its production grew over 20 years from almost zero to 16 million tonnes a year.
However, the second reason for using MTBE in petrol is also important. MTBE burns very effectively to carbon dioxide and water and there are very few other side-products, even in small amounts. With the very stringent anti-pollution legislation in the US, Europe and other developed countries, MTBE was added thus reducing the proportion of hydrocarbons in petrol which in turn means an overall reduction of polluting gases (carbon monoxide and unburnt hydrocarbon) in the exhaust.
Now, there has been a dramatic turnaround. It was found that even very small amounts of MTBE can contaminate drinking water. Unfortunately seepage from storage tanks in petrol stations is not uncommon and damage to the water course occurs. MTBE is much more soluble in water than petrol and so is carried by rainwater through the soil, whereas petrol that leaks remains close to the tank.
Over 20 states in the US (notably California was the first) and Denmark in the EU have banned the use of MTBE in petrol and further legislation in the US is being considered. The amount used in the EU varies from country to country and between suppliers. For example in Finland, the range used is between 10 and 15%, with an average of 12%. In the UK and in Ireland, the average is one of the lowest in the EU, of 1-2% and a range from 0 to 10%.
There is now an increasing amount of ethyl t-butyl ether being used in Europe, particularly in France (see below).
Bioethanol (from biomass) is now the favoured oxygenate in the US in place of MTBE.
Annual production of methyl t-butyl ether
Manufacture of methyl t-butyl ether
MTBE is manufactured from 2-methylpropene (isobutene) and methanol using an acid catalyst. The catalyst is an anionic ion-exchange resin and the reaction is carried out at ca 340-360 K and 8 atm pressure, with methanol in excess. The unused methanol is recovered and recycled.
2-methylpropene is obtained from a variety of sources:
(ii) By dehydrogenation of 2-methylpropane, which is also formed as a by-product in various processes in the petrochemical industry. The vapour is passed over a catalyst (platinum and palladium on an inert support):
(iii) Another route from 2-methylpropane involves its oxidation to 1,1-dimethylethyl hydroperoxide (often called t-butyl hydroperoxide:
This process is carried out in the liquid phase, pure oxygen, under pressure, is passed through the liquid, which has been warmed to about 400 K.
Epoxypropane (propylene oxide) is a very valuable co-product and is used, for example in the production of polyols, used to make polyurethanes.
The alcohol is dehydrated to 2-methylpropene, by passing the vapour over a catalyst:
Ethyl t-butyl ether (ETBE) 2-ethoxy-2-methylpropane) is an alternative to MTBE as an oxygenate to enhance the octane rating of petrol. It is used mainly in Europe, particularly in France. This accounts for over 90% of the world's annual production of ca 3 million tonnes.
It is manufactured from 2-methylpropene and ethanol:
Figure 1 A plant which has been recently commissioned in Geleen in the Netherlands to produce
Date last amended: 18th March 2013