CIEC Promoting Science at the University of York, York, UK

  • Increase font size
  • Default font size
  • Decrease font size
Basic chemicals Methyl tertiary-butyl ether
Methyl tertiary-butyl ether

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.

When oil is distilled, the fraction that can be used as petrol (the naphtha fraction) is made up mainly of C4 to C10 straight chain alkanes, which have suitable volatility. However, these burn in a way that leads to explosion in the engine cylinder at the wrong time (knock).  One way to improve the performance of the petrol (indicated by its octane rating) is to add compounds whose molecules contain oxygen (known as oxygenates) such as ethanol and some ethers. MTBE is widely used in Europe for this purpose and also, increasingly, is its homologue ethyl t-butyl ether (ETBE).

 

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

World 15.2 million tonnes
Europe 2.6 million tonnes

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:

(i) From the cracking and reforming of various fractions from the distillation of oil.

(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.
Propene is then passed into liquid 1,1-dimethylethyl hydroperoxide under pressure at about 400 K with a soluble molybdenum salt as catalyst:

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)

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
ethyl t-butyl ether, ETBE. The reactants are 2-methylpropene and bioethanol.
This product is referred to as bio-ETBE.

By kind permission of SABIC Europe.

 

 

Date last amended: 18th March 2013