A Very Reactive Situation

The technique is based on molecules called Lewis bases, which carry a lone pair of electrons and can bond with so-called Lewis acids to form a molecule called an adduct. In 2008 Doug Stephan at the University of Toronto, Canada, modified Lewis bases and Lewis acids to make them too big to get close enough to form the adduct. "The molecules can't react – they're frustrated," says O'Hare. "And that creates a very reactive situation."

The frustrated Lewis pair are so reactive that when hydrogen gas is added to the mix, the molecules tear apart the hydrogen molecules and bond with the hydrogen ions. The reaction eases the frustration but still leaves two highly reactive molecules. O'Hare and Ashley reasoned that they would be reactive enough to bond with CO₂.

They have now achieved this goal. Their frustrated pair consisted of the Lewis base tetramethylpiperidine or TMP and the Lewis acid tris(pentafluorophenyl)borane. When they added hydrogen gas, the acid and hydrogen ion formed a boron-hydrogen bond which can then be used to shatter a stable CO₂ molecule, which further reacts with hydrogen to form methanol and water.

Scoop

Several other research groups are working on similar schemes, "but we've scooped a few of [them] on this", says O'Hare. He thinks that's because their frustrated Lewis pair doesn't react with carbon monoxide, even though carbon monoxide is much more reactive than carbon dioxide. "You wouldn't have predicted that," he says.

The fact that the reaction is CO₂-specific will work in its favour in practical applications, because the reaction cannot be compromised by carbon monoxide also released from industrial processes.

Stephan, who was not involved with the study, calls it "important and exciting" work. He says it could ultimately lead to systems to reduce greenhouse gases and build alternative energy sources. "Of course, to be viable this process would have to be coupled with a process that generates hydrogen," he cautions.