New in carbon dioxide chemistry

27 December 2016 - No climate scientists here, move on

Seipp2016.PNGCharles S. may or may not be on Donald Trumps hitlist. As a scientist at Oak Ridge National Laboratory he works on carbon dioxide capture and Trump may think of him as one of those climate scientists. Oak Ridge is part of the United States Department of Energy that will soon have a new climate-sceptic boss (Reuters link).
In a recent Angewandte article, S. (this blog has decided to protect the identities of the scientists involved) together with Neil W., Michelle K. and Radu C. report on a new way to capture carbon dioxide. The method is not aimed at CO2 scrubbing at carbon-fuel energy plants but at so-called direct-air capture (DAC) , a real challenge because with DAC CO2 concentrations are really low. The best DAC chemisorbent at the moment is aqueous sodium hydroxide but it is difficult to isolate soluble sodium carbonate and even more difficult to release the gas again, requiring temperatures of up to 800°C. Release is a must if it is eventually permanently stored, for example underground.
The new champion is 2,6-Pyridine-bis(iminoguanidine) (PyBIG) synthesised from 2,6-pyridinedialdehyde and aminoguanidine. Exposing an aqueous solution of the compound to air for several days will result in (PyBIGH2(CO3)(H2O)4) crystals precipitating out. That is each PyBIG molecule sequestering 3 molecules of carbon dioxide. Heating this compound again at 120°C releases all CO2 in one hour. The authors attribute the CO2 sequestering properties of PyBIG to the basicity of its guanidine groups, the poor water-solubility of the complex and the favourable orientation of the carbonate groups in the crystal structure.