These prism-like crystals spontaneously form when an aqueous solution of a simple guanidine compound absorbs carbon dioxide. Photo: Oak Ridge National Laboratory/Genevieve Martin.
These prism-like crystals spontaneously form when an aqueous solution of a simple guanidine compound absorbs carbon dioxide. Photo: Oak Ridge National Laboratory/Genevieve Martin.

Scientists at the US Department of Energy's Oak Ridge National Laboratory (ORNL) have found a simple, reliable process for capturing carbon dioxide directly from ambient air, offering a new option for carbon capture and storage strategies to combat global warming.

Initially, the ORNL team was studying methods for removing environmental contaminants such as sulfate, chromate or phosphate from water. To remove these negatively-charged ions, the researchers synthesized a simple compound known as guanidine, which is designed to bind strongly to the contaminants and form insoluble crystals that are easily separated from water.

In the process, they discovered a method for capturing and releasing carbon dioxide that requires minimal energy and chemical input. Their results are published in a paper in Angewandte Chemie International Edition.

"When we left an aqueous solution of the guanidine open to air, beautiful prism-like crystals started to form," said ORNL's Radu Custelcean. "After analyzing 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."

Decades of research has led to the development of various carbon capture and long-term storage strategies to lessen or capture power plants' emissions of carbon dioxide, a heat-trapping greenhouse gas contributing to a global rise in temperatures. Carbon capture and storage strategies comprise an integrated system of technologies that collect carbon dioxide from the point of release or directly from the air, then transport and store it at designated locations.

A less traditional method that absorbs carbon dioxide already present in the atmosphere, called direct air capture, is the focus of the research described in this paper, although the method could also be used at the point where carbon dioxide is emitted.

Once carbon dioxide is captured, it needs to be released from the capturing compound, so that the gas can be transported, usually through a pipeline, and injected deep underground for storage. Traditional direct air capture materials must be heated up to 900°C to release the gas – a process that often emits more carbon dioxide than initially removed. The ORNL-developed guanidine material offers a less energy-intensive alternative.

"Through our process, we were able to release the bound carbon dioxide by heating the crystals at 80–120°C, which is relatively mild when compared with current methods," Custelcean said. After heating, the crystals reverted to the original guanidine material, allowing the scientists to recycle the recovered compound through three consecutive carbon capture and release cycles.

While the direct air capture method is gaining traction, according to Custelcean, the process needs to be further developed and aggressively implemented to be effective in combating global warming. Also, the scientists need to gain a better understanding of the guanidine material and how it could benefit existing and future carbon capture and storage applications.

The research team is now studying the material's crystalline structure and properties with the unique neutron scattering capabilities at ORNL's Spallation Neutron Source (SNS), a DOE Office of Science User Facility. By analyzing carbonate binding in the crystals, the team hopes to gain a better understanding of the molecular mechanism of carbon dioxide capture and release, which could prove of use in designing the next generation of sorbents.

The scientists also plan to evaluate the use of solar energy as a sustainable heat source for releasing the bound carbon dioxide from the crystals.

This story is adapted from material from Oak Ridge National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.