The war on climate change just got a chemical weapon: a way to destroy the carbon-fluorine bonds that make a class of widely used industrial gases so dangerous in the atmosphere.
Gases made from carbon, fluorine and chlorine, called chlorofluorocarbons (CFCs), long used as refrigerants, were banned in the 1990s because they damage the ozone layer that protects the Earth from UV radiation.
But similar compounds that don’t contain chlorine, fluorocarbons, are still widely used today in products including waterproof clothing and Teflon cookware because their strong carbon-fluorine (C-F) bonds make them highly water repellent. They are even a key component of artificial blood thanks to their high oxygen solubility.
However, fluorocarbons are also powerful greenhouse gases. “The real culprits of ozone depletion have been largely eliminated,” says Robin Perutz at the University of York, UK. “But the remaining fluorocarbons do have a lot of global warming potential.”
One fluorocarbon, tetrafluoromethane, is 6,500 times as potent a greenhouse gas as CO2, although it exists in much smaller amounts, and is unreactive enough to persist in the atmosphere for 50,000 years.
Fluorocarbons’ inertness also makes them difficult to clean up. “It’s fundamentally difficult to do anything with these bonds,” says Oleg Ozerov at Brandeis University in Waltham, Massachusetts. “So it’s an interesting challenge to find ways to break them.”
In 2005, Ozerov’s team found that it can be done using extremely powerful “Lewis acids“, which contain positively charged ions that can rip negatively charged fluoride ions from C-F bonds. But the reaction is difficult to sustain for long periods as the acid tends to become exhausted by reacting with other compounds.
Bonds, three bonds
Now, Ozerov and Brandeis colleague Christos Douvris have found a way to sustain the reaction for long periods. Under their reaction, one molecule of Lewis acid can neutralise up to 2700 C-F bonds. This means just 0.5 milligrams of the acid converts 180 mg of fluorocarbons into a safer form in 24 hours, and at room temperature.
The process uses a Lewis acid discovered by Christopher Reed’s team at the University of California in Riverside. This contains silylium – a reactive form of silicon with three, rather than the usual four bonds and a positive charge.
The silylium acts like an molecular bomb that blows a C-F bond apart. A second reagent, triethylsilane, is like a peacekeeper, coming in afterwards to tame and tidy up the highly reactive compounds produced.
The process starts when a silylium molecule rips a fluoride ion from its carbon partner. That produces a stable silicon atom bonded to fluorine, and a highly reactive naked carbon ion is left behind.
This is then neutralised when it grabs a hydrogen ion from the silicon in a triethylsilane molecule, producing a safer carbon-hydrogen bond and creating a fresh molecule of silylium to attack more C-F bonds.
The reaction is a “downhill” process, using very little energy, says Ozerov, and the end products have little impact on the atmosphere.
Perutz, who was not involved in the study, is impressed with the new approach. “I would say that this is a real step change in effectiveness over what was possible before,” he says. But he points out that the process is as yet untested at the kilogram scale needed for it to be industrially useful.
“This is important chemistry both for the beauty of its chemical logic and for its efficiency,” says David O’Hagan, a fluorocarbon expert at St Andrews University, UK. “The selective and efficient removal of fluorine in this way is an unexpected and interesting development, which is of immediate significance.”
Other proposed methods for neutralising fluorocarbons rely on collecting and storing the chemical rather than converting it into safer forms. “But the fluorocarbons effectively have to be stored in perpetuity,” says Ozerov. “If it’s at all economically feasible, it would be better to put the fluorocarbons through a chemical transition rather than store them.”