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New Catalytic Pathway for the Reduction of CO2 to CO Under Mild Conditions

The reaction for carbon dioxide splitting into carbon monoxide with aromatic aldehydes as oxygen acceptors. Credit: ACS, Gu and Zhang. Click to enlarge.

A team of researchers in Singapore have developed a process for the catalytic reduction of carbon dioxide (CO2) to carbon monoxide (CO) under mild conditions, using aromatic aldehydes as reductants and N-Heterocyclic Carbenes (NHCs) as organocatalysts.

The resulting CO can be used to convert water to hydrogen via the water gas shift reaction. The reaction also shows a new economical way to oxidize aromatic aldehydes, and could be applied in pharmaceutical synthesis. A paper on the work was published online 29 December in the Journal of the American Chemical Society.

There is increasing attention being focused on the conversion and utilization of CO2 for the production of fuels and chemicals; the 238th American Chemical Society (ACS) national meeting in August featured a day-long symposium on advances in the area. (Earlier post.)

Carbon dioxide, however, is highly stable, and splitting the O=C(O) bond to generate carbon monoxide requires large energy input. Authors Liuqun Gu and Yugen Zhang from the Institute of Bioengineering and Nanotechnology note that there are currently four basic categories of carbon dioxide splitting methods:

  1. Enzyme carbon monoxide dehydrogenase/ acetyl-CoA synthase (CODH/ACS);
  2. Photoreduction;
  3. Electrochemical reduction; and
  4. Use of metal complexes or metal oxides to abstract the oxygen from carbon dioxide to form carbon monoxide in low turnover.

...reduction of carbon dioxide with organocatalysts remained widely undeveloped until our group reported the first hydrosilylation of carbon dioxide using NHC catalyst under mild conditions recently. In our efforts to look for cheaper and more accessible reductants for CO2 reduction, the new reaction for carbon dioxide splitting into carbon monoxide with aromatic aldehydes as oxygen acceptors was successfully developed. To our best knowledge, this is the first case in the reduction of carbon dioxide to form carbon monoxide using organocatalysts.

—Gu and Zhang

The calculated free energy profile showed that the overall reaction is an exothermic process with a small negative energy difference of ΔE= -7.0 kcal/mol.


  • Liuqun Gu and Yugen Zhang (2009) Unexpected CO2 Splitting Reactions To Form CO with N-Heterocyclic Carbenes as Organocatalysts and Aromatic Aldehydes as Oxygen Acceptors. J. Am. Chem. Soc., Article ASAP doi: 10.1021/ja909038t


Stan Peterson

This is all interesting but rather useless chemical research, in the short term. Re-creating hydrocarbons from completely oxidized components is not a useful exercise, except when Energy is virtually free and unlimited. This state of affairs is unlikely to happen in this third of a century, but such a state of affairs is possible then.

Many unscientifically trained people think this will lead to making petroleum from CO2, with little energy expenditure. They seem to hope that we can then burn the manufactured petroleum and be a net energy generator.

Sorry the Second law of Thermodynamics doesn't allow perpetual motion machines.

Mankind may well want to manufacture certain hydrocarbons, as it does today, to synthesize certain pharmaceuticals, but it will always take more energy to do so, than was released to oxidize such hydrocarbons in the first place.

It is amazing the things that will be come possible once mankind has clean, cheap and virtually inexhaustible, and prodigious amounts of energy available from controlled Fusion.


"...except when Energy is virtually free and unlimited. This state of affairs is unlikely to happen in this third of a century, but such a state of affairs is possible then."

Only 23 years left in this scenario. I would venture this could happen sooner even. Especially if former energy czars don't grok the fate they're heading toward by retarding real innovation.

Clean, cheap, virtually inexhaustible energy is ubiquitous throughout the universe. Earth is poised right now to acquire its share.


It's not completely useless if the overhead can be lowered. As it stands now, it takes about 60 kW-hrs of electricity to synthesis 1 kg of H2 by electrolyzing water. This is to produce a barely storable gas containing 39 kW-hrs (HHV) of energy. Can we do better? What about the overhead of converting H2 to something more useful like methane, methanol, or heptane? What about the COST of the conversion equipment such that if it sits idle sometimes (due to intermittent renewable energy sources) then it's not a big deal? Still lots of stuff to do, even if we don't have free energy lying around.


I'm not sure what qualifies as an aromatic aldehyde (not enough chemistry too long ago), but if e.g. formaldehyde and CO2 can be converted to CO and formic acid, biomass processes which yield aldehydes can be used to upgrade CO2 in further steps.  The dehydration of formic acid yields CO, so there is another step.


Aromatics all have rings in them. Like benzene. I guess the ring is what makes them smell good, hence the name.


Controlled Fusion was slated to be generating electricity in just 20 years 40 ago.

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