The reduction and functionalisation of carbon dioxide in a single step yields chemically versatile molecules
Chemists in France have devised a new way to turn carbon dioxide into a useful chemical building block. The work is part of a worldwide effort to develop ways to chemically recycle the large amounts of CO2 that are expected to be captured from power stations and other sources.
Two main routes for chemically recycling CO2 have so far been investigated. ’Vertical’ recycling involves reducing CO2 to compounds such as formic acid or methanol, while ’horizontal’ recycling results in the functionalisation of the carbon atom to form new C-O and C-N bonds.
Now, Thibault Cantat’s team at the French Alternative Energies and Atomic Energy Commission (CEA) has combined the two approaches in a ’diagonal’ approach, both reducing and functionalising CO2 in a single step to produce chemically versatile formamides.
’We wanted to see if we could find a catalyst that could give us new bonds and also reduce the carbon,’ says Cantat. The team selected an amine to functionalise the carbon with a C-N bond, and a silane for the reduction. The catalyst was the organic base 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD).
’The mechanism is not clear, but what we think happens is that the CO2 interacts with the amine, in a way that is well known, to produce a carbamate,’ says Cantat. ’This is then efficiently stabilised by the TBD catalyst. The silane then reacts with the stabilised carbamate, transferring a hydrogen to the carbon atom of the CO2. At the same time oxygen is captured by the silicon of the silane.’ In this way the carbon is both reduced and functionalised to produce formamide, a useful chemical building block that is normally produced from petrochemicals.
Crucially, says Cantat, the catalyst is not dependent upon an expensive transition metal. ’The key now is to find a sustainable way to use a green energy source to regenerate the silane.’
’Carbon dioxide capture and utilisation - CCU - is a fast growing field,’ says Peter Styring, who researches carbon dioxide re-use at the University of Sheffield in the UK. ’However, there are those who say it is the future of chemistry and manufacturing and those who dismiss it out of hand because of the unfavourable thermodynamics and associated energy penalties. This paper appears to look at a compromise: a diagonal approach using CCU combined with existing hydrocarbon feedstocks. It is therefore a short term fix, not a long term solution, as hydrocarbons from fossil fuels will eventually need to be replaced.’
Styring adds that while the catalysts are ’very good’ and do not rely on metals, the fact that they are homogeneous would make them difficult to recover.