A new catalyst that can split carbon dioxide gas could allow us to use carbon from the atmosphere as a fuel source in a similar way to plants.

"Breaking open the very stable bonds in CO₂ is one of the biggest challenges in synthetic chemistry," says Frederic Goettmann, a chemist at the Max Planck Institute for Colloids and Interfaces in Potsdam, Germany. "But plants have been doing it for millions of years."

Plants use the energy of sunlight to cleave the relatively stable chemical bonds between the carbon and oxygen atoms in a carbon dioxide molecule. In photosynthesis, the CO₂ molecule is initially bonded to nitrogen atoms, making reactive compounds called carbamates. These less stable compounds can then be broken down, allowing the carbon to be used in the synthesis of other plant products, such as sugars and proteins.

In an attempt to emulate this natural process, Goettmann and colleagues Arne Thomas and Markus Antonietti developed their own nitrogen-based catalyst that can produce carbamates. The graphite-like compound is made from flat layers of carbon and nitrogen atoms arranged in hexagons.

The team heated a mixture of CO₂ and benzene with the catalyst to a temperature of 150 ºC, at about three times atmospheric pressure. In a first step, the catalyst enabled the CO₂ to form a reactive carbamate, like that made in plants.

Oxygen grab

The catalyst's next useful step was to enable the benzene molecules to grab the oxygen atom from the CO₂ in the carbamate, producing phenol and a reactive carbon monoxide (CO) species.

"Carbon monoxide can be used to build new carbon-carbon bonds," explains Goettmann. "We have taken the first step towards using carbon dioxide from the atmosphere as a source for chemical synthesis."

Future refinements could allow chemists to reduce their dependence on fossil fuels as sources for making chemicals. Liquid fuel could also be made from CO split from CO₂, says Goettmann. "It was common in Second World War Germany and in South Africa in the 1980s to make fuel from CO derived from coal," he adds.

The researchers are now trying to bring their method even closer to photosynthesis. "The benzene reaction currently supplies the energy that splits the CO₂," Goettmann says, "but in plants it is light." The new catalyst absorbs ultraviolet radiation, so the team is experimenting to see if light can provide the energy instead.

Recycled carbon

Joe Wood, a chemical engineer at Birmingham University in the UK, is also researching ways of fixing CO₂. "There's growing interest in using it as a recycled input into the chemical industry," he says.

The Max Planck technique has only been demonstrated on a small scale and it has a low yield of 20%, he points out. "But it looks quite promising," he adds. "The catalyst can be made cheaply and it works at a relatively low temperature."

The products of the technique are well suited to making drugs or herbicides, says Wood, "so hopefully they can improve the efficiency and scale it up."

Reference: Angewandte Chemie (vol 46, p 1) DOI:10.1002/anie.200603478