Scientists have come one step closer to a Holy Grail of green chemistry, creating a chemical that helps along a reaction and then separates out easily from the product and can be reused. The novel technique may be useful for making compounds important in agriculture and pharmaceuticals with less damage to the environment.
Most industrial reactions require catalysts, which push a reaction forward but are not used up in the process. In the last decade, chemists have worked to develop reusable catalysts that separate out from the end products without the help of other often-toxic and waste-generating chemicals.
Organometallic chemists Vladimir Dioumaev and Morris Bullock of Brookhaven National Laboratory in New York tried to create a catalyst that capitalized on differences between the properties of the ingredients and those of the products. They focused on reactions that create alkoxysilanes--a common ingredient in ceramics and in organic compounds used in agriculture and pharmaceuticals. They knew that a charged, or polar, catalyst would be soluble in the initial polar ingredients but not in the nonpolar product. But many catalysts separate out and stop working as soon as the nonpolar reaction products start to build up. The tricky part was creating a catalyst that would remain soluble and keep working until the very end of the reaction when all of the ingredients had been used up.
They solved the problem, Dioumaev and Bullock report in the 31 July issue of Nature, by using catalytic compounds that tend to form an oily mass in a nonpolar solution before separating out. In the oily phase, the catalyst continues to react with the remaining ingredients until finally, after the ingredients are used up, it solidifies and sinks to the bottom. The product can be poured off and the catalyst used again.
"It's a novel idea," says organometallic chemist William Jones of the University of Rochester in New York. The approach is limited because it only works in liquid reactions that start with polar chemicals and end with a nonpolar product, he says, but it's another tool that chemists could use to cut down on polluting waste. The next step, Jones and Bullock say, is to tailor the catalyst so that it will work in a wider range of reactions.