THE promise of more environmentally friendly chemicals plants has come closer, now that five major manufacturers have unveiled plans to scale up production of a new kind of solvent.
Since 1995, 鈥済reen鈥� chemists have championed ionic liquids (ILs) as possible replacements for the toxic organic solvents that are used for key reactions in a range of industries from paper making to pharmaceuticals (New Scientist, 5 September 1998, p 13). But their campaign has been in vain, until now. 鈥淭his is a huge enabling step,鈥� says Kenneth Seddon, who studies ILs at Queen鈥檚 University, Belfast.
Ionic liquids consist of simple salts similar to sodium chloride, except that one of the ions is a big unwieldy molecule that doesn鈥檛 stack easily in the solid state. This means that, just like organic solvents, ILs are liquid at room temperature, and are capable of dissolving many of the reagents used in industry. But they are far less volatile than organic solvents, so they don鈥檛 emit dangerous fumes, and they鈥檙e not as flammable. It might also be easier to extract impurities from ILs, so plants could recycle them instead of producing a constant stream of chemical waste.
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But despite the environmental benefits, manufacturers were not willing to scale up production of ILs. Their change of heart is due to recent discoveries showing that ILs could also be more effective than current solvents.
At the meeting, Robin Rogers鈥檚 research group at the University of Alabama in Tuscaloosa presented its findings on an IL called 鈥渂mim chloride鈥�, also known as an imidazolium salt. In 2000, Rogers鈥檚 colleague Rick Swatloski was filtering bmim chloride when he noticed holes in the filter paper. In further tests he found that bmim chloride dissolves not just paper but also cellulose from wood, which is normally considered fairly insoluble. So as well as replacing organic solvents in paper making and other cellulose industries, bmim chloride could remove the need to pretreat cellulose to help it dissolve. This would be a boon for the paper-making industry, which the US Environmental Protection Agency criticised in June for producing too much toxic waste.
Another promising find came from Gary Baker of Los Alamos National Laboratory in New Mexico. He reports that monellin, a berry protein, remains intact in some ionic liquids even when heated up to 99 掳C. That鈥檚 at least twice the temperature that could be reached with an organic solvent. Drugs manufacturers are excited about this because higher temperatures mean higher yields.
Several dozen ILs that are now on the market in small quantities will probably soon be custom-made for industry by the tonne, representatives from Merck and Solvent Innovation in Germany, Sachem USA, Ozark Fluorine in the US and Cytec Canada told the meeting.
However, the environmental benefits can鈥檛 be taken for granted. Baker鈥檚 finding that proteins are so stable in ILs hints that the chemicals could be less toxic than organic solvents. But he and other chemists at the meeting stress that it would be irresponsible to market any chemical as 鈥済reen鈥� without proper environmental assessments. As yet neither these nor basic toxicity studies have been done on any ILs.