Critical development
15 Jan 2000
A universal solvent with no harmful properties, and which simplifies and cuts the costs of many of the most important industrial chemical processes. It sounds too good to be true - especially when the solvent in question is common-or-garden water. However, researchers at the Georgia Institute of Technology (GeorgiaTech) believe that water, near its supercritical (SC) point, could be the ideal solvent for many applications in the fine chemical and drugs industries.
At 250-300 degrees C and 1000psi, water is still a liquid but has properties more like alcohol or acetone than room-temperature water. It will dissolve both polar and non-polar substances, from salts to oils. `Molecules that would normally not be soluble in the same solvent become soluble together in near-SC water and can be processed together,' says research leader Charles Eckert. This would not only render obsolete multi-phase, multi-solvent reactions, it would also mean that reaction mixtures would not need stirring or agitation.
Another advantage is that hot water dissociates into acidic hydrogen and basic hydroxide ions. This means that acid- and base-catalysed reactions can be run in near-SC water without adding extra mineral acids or alkalis, which cuts the use of hazardous substances and removes the need for a neutralisation step, which would create waste salts that would have to be disposed of.
The advantages continue beyond the reaction step. The products can be recovered from the solution in a pure form by simply cooling the solution down and reducing the pressure. This would cut the costs of separation, currently the most expensive part of most processes.
The researchers have already perfected a wide range of industrial reactions, including Friedel-Crafts acylation, ester hydrolyses and condensations. Some reactions do not work fast enough, however. Eckert comments: `The point of our research is to find out what is possible in this medium and sift through what is most worthwhile.'
Eckert believes that near-SC water is likely to be most popular in the pharmaceutical and food ingredient industries, where all traces of solvents must be removed from the products. `Water is about as ideal a solvent as you could imagine,' he says. `Not only is it benign, but the public perception is that it is benign.'
