Turning carbon dioxide into a useful feedstock chemical could help to reduce levels of this greenhouse gas in the atmosphere, as well as providing a cheap source of carbon.

Now Japanese researchers have isolated the active form of a catalyst that turns CO₂ into formic acid using water as a green solvent, a crucial step in developing this technology on an industrial scale.

Seiji Ogo from Kyushu University and co-workers had previously found that water-soluble ruthenium aqua complexes could catalyse the hydrogenation of CO₂ without using large amounts of base, which are usually required for the reaction. Base stabilises the formic acid product against decomposition by forming unreactive sodium formate. 'If CO₂ hydrogenation could only be achieved in the presence of bases that would be a dead end,' explained Ferenc Joo, Chair of the Institute of Physical Chemistry at the University of Debrecen, Hungary.

However, Ogo's team could not isolate the active form of the ruthenium catalyst, thought to be a hydride complex. Using ruthenium, the slowest step of the reaction involves formation of the hydride complex, which is then rapidly used up to make formic acid.

So they switched to a water-soluble iridium complex, making the reaction of hydride with CO₂ the rate-determining step. This means that the active hydride catalyst survives for much longer, allowing them to pin down the active catalyst at the heart of the reaction.

Ogo believes that 'this work provides a valuable strategy to develop new catalytic systems for the hydrogenation of CO₂.'

The next major hurdle is that in most cases, the same metal complex that catalyses the formation of formic acid is also a catalyst for its rapid decomposition, explained Joo. Ideally, the product should be used as it is generated, in reactions such as turning alkenes into carboxylic acids.

'But the results of Ogo and co-workers demonstrate that with suitable catalysts, formic acid can be produced in substantial quantities, opening the way to further synthetic applications,' added Joo.

Joanna Stevens