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Green plants can extract carbon dioxide gas from the air and turn it into sugar molecules using sunlight and give off oxygen. Sounds like science fiction? No, this is simply the fundamental process underlying photosynthesis, a biochemical process carried out by chlorophyll-containing plants and microbes, and it is essential to the existence of almost all life on earth.
Chemists, on the other hand, have yet to find an efficient method for converting carbon dioxide into materials that might be useful as fuels or in manufacturing. Almost all our efforts rely on complex reaction schemes to produce the starting materials and then are so inefficient that the end product costs far more to produce, in terms of energy and economics, it is worthless.
Now, Markus Antonietti at the Max Planck Institute for Colloids and Interfaces has taken a step toward a more effective process for exploiting the carbon-containing byproduct of burning fossil fuels—carbon dioxide. He and his colleagues have demonstrated how to "activate" using a new type of catalyst that contains no expensive metals, a graphitic carbon nitride.
Chemical activation of carbon dioxide involves splitting, or cleaving, it in a chemical reaction, the researchers explain. Splitting the CO2 basically releases carbon monoxide, a chemically reactive form, and oxygen free radicals, that can then react with other molecules to produce more complex and potentially useful products. This cleavage process is one of the biggest challenges facing synthetic chemistry today.
The problem with attempting to activate carbon dioxide is that the double bonds between the central carbon atom and its two flanking oxygen atoms are very strong and stable. A lot of energy is needed to pull them apart and cleave the molecule. Plants have had millions of years to evolve the most effective way to use sunlight to activate carbon dioxide, but chemists have only had a few decades and, until recently, have expended a lot of energy developing special metal catalysts, which can cleave carbon dioxide, but are notoriously inefficient.
Antonietti and his colleagues figured that rather than ignoring the plants' approach, they would try to mimic it. Photosynthesis in modern green plants, they explain, involves an important intermediate step in which the CO2 molecule is bonded to nitrogen atoms to form carbamates. The German researchers experimented with nitrogen-rich catalysts with structures that allow them to form carbamates. Their new class of catalysts is made of flat, graphite-like layers. The individual layers consist of ring systems involving carbon and nitrogen atoms. This porous material is called graphitic carbon nitride. Graphitic carbon nitride is very heat resistant and so stable that although it can speed up reaction it always reverts to its original form once the reaction is complete—it is an ideal catalyst, in other words.
The researchers found that the catalyst can cleave carbon dioxide forming carbon monoxide and an oxygen radical via a carbamate step. The intermediate could then react with an organic starting material, the aromatic benzene in the trial experiments, to form phenol. Phenol is a useful product in itself, used in disinfection processes, but also acts as an invaluable starting material for a whole range of products such as aspirin, weedkiller, and synthetic resins.
"Our catalyst could make novel, previously unknown chemistry of CO2 accessible," says Antonietti. "It may even be the first step in artificial photosynthesis," he adds.
Angew Chem Int Edn 2007, 46, http://dx.doi.org/10.1002/anie.200603478