01 May 2009 Spectroscopy Now

Recycling Carbon Dioxide

A research team in Singapore has developed an alternative to simply burying carbon dioxide captured from sources such as coal-fired power stations. Their experiments used NMR spectroscopy to track the catalytic conversion of carbon dioxide into methanol under very mild reaction conditions.

Power station.

Power stations provide raw material for the chemical industry

Carbon capture and sequestration are high on the green agenda as one approach to climate change amelioration. However, some chemists believe that simply trapping and burying carbon dioxide exhausted by power stations is a waste of a potentially valuable resource. And, given that various countries are investigating carbon-capture technologies for next generation electricity-generating plants it would seem timely to find ways to fulfil the potential of this otherwise wasted resource.

Nature, of course, solved the problem of carbon capture millions of years ago in the process of photosynthesis. Chemists have always envied green plants because they are perfectly tuned to sequester carbon dioxide directly from the air and using the energy of sunlight convert the gas into carbohydrates, releasing oxygen as a by-product. In an urgent sense, carbon dioxide is today very much a renewable resource, unlike the fossil fuels and natural gas from the burning of which a proportion of atmospheric carbon dioxide is derived.

Despite its reputation, there couldn't be a more environment friendly raw material for the chemical industry. Unfortunately, although plants succeed very well in ripping apart carbon-oxygen bonds and rearranging the atoms into sugar molecules, chemists have found that the bonds do not break easily, at least without the expenditure of huge amounts of energy; which would defeat the object of recycling the gas in the first place.

Now, Yugen Zhang and Jackie Ying at the Institute of Bioengineering and Nanotechnology in Singapore , and their colleagues have developed a novel reaction scheme through which carbon dioxide can be efficiently converted into the simple oxygenated organic compound methanol under very mild reaction conditions. Their NMR tracking of the process reveals details of how their N-heterocyclic carbene catalyst and the silane reducing agent operate.

The team's N-heterocyclic carbene is essentially a five-membered ring comprising two nitrogen and three carbon atoms. The fact that one of the carbon atoms has just two bonds instead of the usual four means this molecule has two spare electrons in a lone pair arrangement that make this chemical species highly reactive. So reactive in fact that it can attack even the stable C=O double bonds in carbon dioxide.

The researchers produce their carbene catalyst in situ from a precursor compound an imidazolium carboxylate, for instance. The carbene "activates" the carbon dioxide, but then splits off again to end the reaction cycle in its original state. The formal reaction partner is a hydrosilane, specifically diphenylsilane, an organosilicon compound that acts as the reducing agent in the reaction.

A final reaction step, involving addition of an alkali, sodium hydroxide solution, then collects the intermediary activated carbon dioxide species as methanol. "Methanol was typically produced in over 90% yield (based on silane), as characterized by gas chromatography against an external standard," the researchers say. Methanol, they add, is an important starting material for many chemical syntheses and serves as an alternative fuel and as a raw material for the production of energy in methanol fuel cells.

The big advantage of this organic approach to carbon dioxide conversion is that unlike previous attempts it neatly side-steps the need for expensive and toxic precious or other metal-containing catalysts. It also simply uses dry air as the source of the carbon dioxide, which is made possible by the carbene catalyst being insensitive to oxygen. Conventional transition-metal catalysts for carbon dioxide reduction with silanes are usually highly oxygen-sensitive, which makes them of only limited practical application. Moreover, the carbene is much more efficient than earlier metal-containing catalysts and requires only very mild reaction conditions.

One of the drawbacks of the approach is that hydrosilanes are expensive, but the team is currently working on finding less expensive sources of the requisite hydrogen atoms needed to convert oxidised carbon dioxide to reduced methanol. "This approach offers a very promising protocol for chemical carbon dioxide activation and fixation," the team says.

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Source: Article by David Bradley in Spectroscopy Now

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