Using a soft crystal to visualize how absorbed carbon dioxide behaves in liquid

A team of scientists has succeeded in visualizing how carbon dioxide (CO₂) behaves in an ionic liquid that selectively absorbs CO₂. The finding is expected to help develop more efficient methods to capture CO₂ in the atmosphere, one of the major factors causing global warming.

Carbon dioxide (CO₂) levels in the atmosphere — a major factor in global warming — continue to rise every year, creating grave concerns about the future of Earth. To halt global warming, our industrial society needs to emit much less CO₂. One way of achieving this is to separate and collect CO₂ before it is released into the atmosphere. While some such efforts are already underway, they have not been very efficient. There is thus an urgent need to develop technology that can separate and collect CO₂ more efficiently, both to protect the environment but also to promote recycling of CO₂ as a resource.

The use of ion liquids to effectively absorb CO₂ has been the subject of intensive research. Yet more investigation of how CO₂ absorbed in ionic liquids behaves is needed to improve the materials used in the CO₂ separation and collection process. As ionic liquids are a fluid with no regular structure, it has been difficult to directly observe the state of CO₂ absorbed in them.

In the present study, a team of scientists that included Professors Shin-ichiro Noro and Takayoshi Nakamura, both of Hokkaido University’s Graduate School of Environmental Science, focused on a soft crystal, a substance that possesses both the softness of a liquid and the regularity of a crystal. They synthesized a soft crystal containing components of an ionic liquid that absorbed CO₂. As anticipated, the soft crystal maintained its regularity even after it absorbed CO₂, making it possible to conduct X-ray diffraction analysis.

The analysis showed the absorbed CO₂ interacts with both fluorine and oxygen atoms of the bis(trifluoromethylsulfonyl)imide anion, a component of the ionic liquid. Furthermore, the scientists’ theoretical analysis showed that dispersion and electrostatic interactions exist between CO₂ and the framework, creating the force that binds CO₂ to the anion.

The team’s findings are expected to be helpful in designing and developing ionic liquids capable of efficiently separating and collecting CO₂, and will likely accelerate practical applications of such liquids, a pivotal step to alleviating the negative effects of global warming.

Shin-ichiro Noro focuses on the development of porous materials to contribute to environmental restoration and conservation, while Takayoshi Nakamura’s work is focused on the development of molecular devices for a wide variety of applications.

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