ORNL crystals capture CO2 directly from air

RESEARCHERS at Oak Ridge National Laboratory (ORNL), US, have discovered that when a solution of guanidine crystallises, it captures CO₂ from ambient air.

The technique, they believe, could be useful particularly in the direct air capture of CO₂, rather than flue gases, although the guanidine solution would be suitable for both. The idea of direct air capture is gaining in popularity, according to researcher Radu Custelcean, as the world seeks to combat climate change. However, conventional direct air capture materials require heating to very high temperatures, up to 900?C, to release the CO₂ and regenerate them. Guanidine does not.

Guanidine, HNC(NH₂)₂, is a strong base. The discovery occurred by chance, when Custelcean and the team were testing guanidine’s potential to remove negatively-charged pollutants such as chromate and phosphate from water, with the idea that a it would bind to the pollutants and form insoluble crystals which could be easily removed.

“When we left an aqueous solution of the guanidine open to air, beautiful prism-like crystals started to form. After analysing their structure by X-ray diffraction, we were surprised to find the crystals contained carbonate, which forms when carbon dioxide from air reacts with water,” said Custelcean.

They found through experimentation that by heating the crystals to just 80–100?C, the CO₂ was released and could be captured, while the crystals reverted to their original guanidine form. The researchers successfully recycled the guanidine three times.

The team is now more fully investigating the structure of the crystals and their properties using RNL’s Spallation Neutron Source (SNS), to determine how the carbonate binds to the guanidine and better understand the release mechanism. They will also seek to find a way to use sustainable solar heat to release the bound CO₂ from the crystal.

Angewandte Chemie International Edition DOI: 10/f3tnq2