USING hot, high-pressure water as a solvent could make the chemicals industry
cleaner and greener. Charles Eckert of the Georgia Institute of Technology in
Atlanta says that water at temperatures of 250 掳C to 350 掳C and
pressures of 50 to 100 atmospheres can dissolve many hydrocarbons and other
compounds that normally require organic solvents.
Under these 鈥渘ear-critical鈥� conditions, about a thousand times as many water
molecules split into hydroxide ions and hydrogen ions as at room temperature.
These ions ensure that organic compounds dissolve more readily. They can also
catalyse some chemical reactions. When the water cools, the ions recombine and
the products drop out of solution. As well as avoiding the use of potentially
polluting organic solvents, this can eliminate the need to separate the final
products鈥攖he most costly phase of many chemical processes.
At last week鈥檚 meeting of the American Chemical Society in New Orleans,
Eckert鈥檚 group described how near-critical water acts as a solvent in the
widely used Friedel-Crafts reaction. This type of reaction forms a common step
in many chemical syntheses, and usually requires large amounts of aluminium
chloride catalyst, which eventually has to be neutralised and disposed of.
Near-critical water eliminates all that waste, Eckert claims.
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