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 , 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鈥檛 contain chlorine, , 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.
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However, fluorocarbons are also powerful greenhouse gases. 鈥淭he real culprits of ozone depletion have been largely eliminated,鈥� says at the University of York, UK. 鈥淏ut the remaining fluorocarbons do have a lot of global warming potential.鈥�
Perpetual problem
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鈥� inertness also makes them difficult to clean up. 鈥淚t鈥檚 fundamentally difficult to do anything with these bonds,鈥� says at Brandeis University in Waltham, Massachusetts. 鈥淪o it鈥檚 an interesting challenge to find ways to break them.鈥�
In 2005, Ozerov鈥檚 team found that it can be done using extremely powerful 鈥溾��, 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 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 team at the University of California in Riverside. This contains silylium 鈥� a reactive form of 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, , is like a peacekeeper, coming in afterwards to tame and tidy up the highly reactive compounds produced.
鈥楽tep change鈥�
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 鈥渄ownhill鈥� 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. 鈥淚 would say that this is a real step change in effectiveness over what was possible before,鈥� he says. But he points out that the process is as yet untested at the kilogram scale needed for it to be industrially useful.
鈥淭his is important chemistry both for the beauty of its chemical logic and for its efficiency,鈥� says , a fluorocarbon expert at St Andrews University, UK. 鈥淭he selective and efficient removal of fluorine in this way is an unexpected and interesting development, which is of immediate significance.鈥�
Other proposed methods for neutralising fluorocarbons rely on collecting and storing the chemical rather than converting it into safer forms. 鈥淏ut the fluorocarbons effectively have to be stored in perpetuity,鈥� says Ozerov. 鈥淚f it鈥檚 at all economically feasible, it would be better to put the fluorocarbons through a chemical transition rather than store them.鈥�
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