Epoxyethane (ethylene oxide, EO) is a colourless gas at room temperature. The bonds in the ring are easily broken so epoxyethane is very reactive. This, together with it being made directly from ethene (ethylene), a readily available feedstock, makes it an extremely important intermediate in the manufacture of many useful chemicals.
Uses of epoxyethane (ethylene oxide)
The principal use of epoxyethane is in the production of ethane-1,2-diol, the principal constituent of engine coolants, and in other diols (the glycols), which are used to make polyesters. These and other chemicals made from epoxyethane are discussed below.
Figure 1 Uses of epoxyethane.
Figure 1 shows the worldwide uses of epoxyethane by proportion. But these vary country by country. For example much less, by proportion, is used in the US (41%) but more is used to make glycol ethers (10%), ethoxylates (15%), ethanolamines (16%) and polyethylene glycols (12%).
Annual production of epoxyethane (ethylene oxide)
Ethene is mixed with air or oxygen and passed over a catalyst (finely divided silver on an inert support such as alumina) at 520-550 K and under 15-20 atmospheres pressure (Figure 2). Two reactions, partial and complete oxidation, take place simultaneously at the catalyst surface. A small amount of 1,2-dichloroethane is added to the reaction mixture which decreases the unwanted side reaction to carbon dioxide and steam. Residence time in the reactor is 1-4 seconds.
Silver is unique as a catalyst for this reaction but the mechanism is not clear. The selectivity now being achieved is over 90%. As the catalyst ages, selectivity decreases. The lifetime of a catalyst is in the range of 2-5 years.
Figure 2 Two competing reactions for the oxidation of ethene.
Reactions of epoxyethane (ethylene oxide)
Production and uses of ethylene glycols
Epoxyethane is reacted with water under neutral or acidic conditions, to form stepwise a range of products:
Figure 3 Production of ethylene glycols.
When n = 1, the product is ethane-1,2-diol often known as monoethylene glycol) HOCH2CH2OH, used in engine coolants, in heat transfer fluids, and for production of polyesters such as PET (polyethylene terephthalate).
When n = 3, the product is used as a plasticiser.
When n = 4 or more, the product is a poly(epoxyethane), known usually as polyethylene glycol (PEG). These are classified by relative molecular mass, PEG200, PEG400, PEG600, etc. and are used as nonionic surfactants, synthetic lubricants and solvents for paints. They are also in cosmetics and are used as plasticisers in adhesives and printing inks.
Glycol ethers and polyols are made from epoxyethane by reacting with alcohols:
Glycol ethers produced with n = 1 or 2 and lower relative molecular mass alcohols are used as solvents. Trivial names are usually used for these ethers.
Ethylene glycol monomethyl ether (methanol + 1 EO) and diethylene glycol monomethyl ether (methanol + 2 EO) are used as anti-icing additives in jet fuels.
Where R is from a longer chain alcohol and n is 3 or more, the products are known as ethoxylates. Alcohol ethoxylates, produced by reacting a C10 - C15 alcohol
Ethanolamines are produced from epoxyethane by reaction with ammonia:
Figure 5 Production of mono, di and triethanolamine.
They are used in textile finishing, cosmetics, soaps, detergents, gas purification (as bases, they react with, and remove, hydrogen sulfide, carbon dioxide and sulfur dioxide) and corrosion inhibitors.
Diethanolamine is used in a new 'greener' method of manufacturing the herbicide, glyphosate sold, for example, as 'Roundup'.
Date last amended: 3rd October 2013