Fluorocarbons–common chemicals in which carbon is bound to fluorine–are potent greenhouse gases, and some form toxic compounds that can accumulate in the environment. But neutralizing fluorocarbons has required a process whose high temperature drives up its cost, limiting its adoption. Researchers at Brandeis University report in Science today that they have found a catalyst that breaks the carbon-fluorine bond at room temperature, promising easier and more effective disposal of pesky pollutants.
The strength of the fluorine-carbon bond makes fluorocarbons valuable in chemically resistant and durable materials such as stain repellants, nonstick cookware, and coolants. But it also explains why they are so difficult to dispose of. One type of fluorocarbon, the ozone-destroying chlorofluorocarbons (CFCs), has now been widely banned under the Montreal Protocol, but the two other main types also present environmental problems.
One of them is now used instead of CFCs as a coolant in refrigerators and air-conditioning units. Where such refrigerants leak into the environment, they function as greenhouse gases that are a thousand times more potent than carbon dioxide.
Another type of fluorocarbon is used in many medical applications, including artificial blood. It, too, is a potent greenhouse gas and gets into the atmosphere as a by-product of the aluminum industry. But some species of it are also toxic and accumulate in the food chain, possibly increasing risk of cancer, birth defects, and other health problems.
Brandeis’s Oleg Ozerov, lead researcher of the current Science study, found a way to crack the carbon-fluorine bond by using a silicon-based catalyst that recycles itself, so it can spark the breakdown reaction over and over again.
“The basic idea is that we use three things: the fluorocarbon, a silicon-based hydrogen source, and a catalyst which mediates between the two to replace the fluorine in the fluorocarbon with hydrogen,” says Ozerov. “The active part of the catalyst is a positively charged silicon compound that kicks off the reaction by ripping the fluorine out of the fluorocarbon bond.”
Having a fluorine ripped out, explains Ozerov, causes the former fluorocarbon to pull a hydrogen molecule out of the silicon-based material. Losing a hydrogen, in turn, transforms the silicon-based material into another instance of the catalyst, so the reaction can continue.
To get the initial catalyst to work, Ozerov and his colleague Christos Douvris had to stabilize it by partnering it with a very nonreactive, negatively charged ion that would interfere as little as possible with the target reaction.
The end products of the reaction, hydrocarbons and fluorosilanes, do not have greenhouse-gas properties and are easier to dispose of than fluorocarbons.
Ozerov and Douvris tried out their catalytic method on three fluorocarbon test substrates. In each case, they were able to get almost all of the material to react; one substrate took just six hours to break down completely, at only 25 °C.
Robin Perutz, a catalyst expert at the University of York, in the U.K., says that Ozerov and Douvris’s method is “an impressive discovery. It’s really important to convert problematic fluorocarbons into something fairly harmless, and at the moment this can only be done by extremely high-temperature chemistry. These guys have said we can do an awful lot just at room temperature, and that’s a big step towards getting rid of more unwanted fluorocarbons.”
There are a few challenges to meet before the catalyst could be used to clean up fluorocarbons on a large scale, however. To begin with, cheaper sources would need to be found for the silicon-based reagents, says Ozerov.
Véronique Garny, director of the Fluorinated Chemicals Groups at the European Chemical Industry Council, says that even then it might be hard for the catalyst technique to beat the established methods. Fluorosilanes, Ozerov acknowledges, have some toxicity–although he says that they “can easily be processed further.” But according to Garny, the existing techniques “are simpler, have completely nontoxic end products, and work fine with highly contaminated starting materials, something which Ozerov’s process still needs to show.”
Garny sees more potential for the catalyst method in attacking solid and liquid fluorocarbons that pollute land and water. Perutz points out, however, that these pollutants are often particularly hard to break down because they are extra rich in strong fluorine-carbon bonds. Ozerov and Douvris have not yet tested their method against such recalcitrant fluorocarbons.
“The technique still has quite a long way to go before it can be used widely,” says Perutz. “But it’s certainly a very promising step with a lot of potential.”