Hydration of Ethene to Ethanol

 
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Students learn to describe the addition of water to ethene resulting in the production of ethanol and identify the need for a catalyst in this process and the catalyst used

Ethanol can be made in two ways: by fermentation of sugars, or by hydration (adding water) to ethene. Due to the low costs of ethene as a raw material, approximately 95% of the industrial ethanol made in the developed world is produced by the second process.

The double bond of ethene is electron rich. and is liable to attack by any chemical species that is poor in electrons. The hydrogen ion, H+, is perhaps the simplest example of such a species, and for this reason strong acids are used to hydrate ethene.

One synthetic route involves the use of concentrated sulfuric acid; H+ adds across the double bond, generating a cation, which receives two electrons from the HSO4- ion to form ethyl sulfate.


Formation of ethyl sulfate by adding concentrated sulfuric acid to ethene

When the ethyl sulfate is removed from the concentrated sulfuric acid and added to water, the -O-SO3 group is replaced by the more powerful base -OH to give ethanol in high yield.


Hydrolysis of ethyl sulfate to ethanol and dilute sulfuric acid

Obviously, reactions that involve the use of huge amounts of sulfuric acid, no matter how cheap the reagents are, pose many problems with safety, environmental impacts, and maintaining plant in an un-corroded state. In recent decades, this reaction has been replaced by the use of highly acidic catalysts made by impregnating zeolites or silica aerogels with phosphoric or tungstic acid. These provide the equivalent 'concentrated acid' regime on a microscale at the surface. These convert ethene to an immobilised phosphate or tungstate which can be hydrolysed in place, regenerating the catalyst and giving ethene in a one-stage process.


Mechanism for hydration of ethene on a solid catalyst

A variety of catalysts are used, with different optimum conditions, but all require elevated pressure (1-9 MPa) and T (>100 °C) to give adequate yields of ethanol. So curiously enough, zeolites very similar to those used for dehydrating ethanol can be used to hydrate ethene!