Score at least a partial victory for green chemistry, the campaign to make industrial processes more environmentally benign. In the 3 March issue of Science, researchers report a way to clean up a commonplace family of chemical reactions--turning alcohols into aldehydes, ketones, and carboxylic acids, starting materials for everything from pharmaceuticals to fragrances. If adopted by industry, the new process could displace thousands of tons of hazardous heavy metal waste every year.
To transform alcohols--short hydrocarbons that harbor an extra oxygen and hydrogen atom--chemists must oxidize them by stripping off two or three hydrogen atoms. Chromium oxide is a master at such reactions, so thirsty for electrons that it readily swipes a pair of electrons from an alcohol's hydrogen atoms and pulls the protons along with them. The problem is that once satiated, the toxic metal is unable to give up the hydrogens again, so waste is generated as fast as the desired product is.
Organic chemist Roger Sheldon and his colleagues as Delft University of Technology in the Netherlands sought a similar compound that wouldn't be used up in the process. Other researchers had shown that a palladium atom linked to an organic group called phenanthroline--a trio of hydrocarbon rings--could do the trick. The palladium atoms initially snatch hydrogens from the alcohols but later give them up to oxygen atoms in the solvent, generating water and returning the catalyst to its original state. Thus, far less of it is needed to run the reaction. Palladium is also less toxic than chromium.
But the process still required dangerous organic solvents, and recovering the reaction products from the mix of solvent and catalyst required turning up the heat to distill the solvent, a treatment that could destroy the catalyst. So Sheldon and his colleagues set out to coax similar palladium catalysts to work in a friendlier solvent: water. By linking palladium to phenanthrolines modified with sulfur-containing groups, they made the catalyst water soluble. When the researchers added various alcohols, they found that the catalyst worked well, especially when helped by a common basic compound such as sodium hydroxide, which speeds the reaction by plucking hydrogens off the alcohol. And because the oily reaction products float atop the solution, chemists can easily siphon them off while keeping the catalyst intact.
Terry Collins, a chemist at Carnegie Mellon University in Pittsburgh, Pennsylvania, says several practical problems will have to be worked out before the method is ready for large-scale use in industry. Yet, "It's very interesting chemistry," he says. "Getting rid of chromium is a great thing to be doing."