New material helps capture double the amount of carbon

Finding ways to capture, store, and utilize carbon dioxide (CO₂) remains an urgent issue.

As global temperatures continue to rise, preventing CO₂ from entering the atmosphere could help mitigate warming in places that still rely on carbon-based fuels.

Creating Change

The world has made significant progress in developing practical, affordable carbon capture technologies. Carbon capture liquids, known as solvents when present in abundance, can effectively absorb CO₂ molecules from coal-fired power plants, paper mills, and other emission sources. However, all these substances operate through the same basic chemical mechanism, or so researchers had assumed.

The research team established a carbon capture solvent.

In a new study published in the journal Nature Chemistry, scientists were surprised to discover that a familiar solvent proved to be even more effective than initially predicted.

New details about the solvent’s underlying structure reveal that the liquid can hold twice the amount of CO₂ previously thought. The newly revealed structure may also hold the key to creating a set of carbon-based materials that could help remove even more CO₂ from the atmosphere.

The Pacific Northwest National Laboratory (PNNL) of the U.S. Department of Energy developed this solvent a few years ago. They have studied it in various scenarios, working to reduce the cost of using the solvent while enhancing its efficiency. Last year, the team unveiled the most cost-effective carbon capture system to date and noticed something unusual during their research.

David Heldebrant, a chemist at PNNL and co-author of the study, stated: “The team was attempting to perform a different type of high-pressure gas separation and found that the solution became significantly denser, and something appeared in our spectra.”

This indicated that something new had formed, completely unexpectedly, and we knew we had to get to the bottom of it.” He then contacted colleagues at Claude Bernard University Lyon 1 (France) and the University of Texas at El Paso (USA) to help unravel the molecular changes behind the results.

Professor Jose Leobardo Bañuelos from the University of Texas at El Paso remarked: “This work is truly a collaborative and interdisciplinary effort. The questions to be answered require more than one type of expertise.”

“We examined the overall structure of the solvent when exposed to CO₂ and found that, fundamentally, there was much more order than expected.” It appeared that the molecules were clustering together when they were supposed to be paired. But the question arose: What do these new, orderly structures mean?

As the research team gained new insights into the solvent-CO₂ system using analytical chemistry tools, they discovered self-assembling solvent molecular clusters. Initially, the researchers tried to fit the data into a model using only two solvent molecules. Despite the team’s initial expectations, the data did not match.

However, by using a model with four solvent molecules, the researchers found that the results aligned correctly. The four-component cluster was indeed the form of solvent that the team had observed. This flexible structure can undergo a series of changes to accommodate incoming CO₂ molecules.

Ultimately, the CO₂ reaches the core of the cluster, where there is an active site that may be similar to the sites found within enzymes. In fact, the overall structure of the cluster and its interactions seem to resemble proteins.

The binding site is central to the newly observed chemical activity. Typically, carbon capture systems operate with a single CO₂ molecule binding and reacting to form something different.

Limiting reactions to involve only one CO₂ molecule restricts further carbon transformation steps. However, this new molecular cluster has produced something distinct.

The research team discovered the formation of a new species comprising two different CO₂ molecules. The clusters sequentially combine CO₂, first capturing and activating one molecule, followed by the second.

A pressing issue with captured carbon is what to do with it.

The Urgency of Implementing Capture Systems

When both CO₂ molecules are in the cluster, they can react with each other. This then creates various carbon-based molecules that could expand the utility of captured CO₂.

Heldebrant shared: “What we are doing is changing a key variable in this process. Previously, we captured each CO₂ individually. Linking two CO₂ molecules together could help us double the storage capacity efficiency of capture systems.”

The newly connected molecules have very different properties compared to CO₂. This alters the chemical characteristics necessary to separate captured carbon from the solvent. These carbon-based molecules are larger and represent the first step towards creating CO₂-rich polymers.

A perennial issue with captured carbon is what to do with it. While long-term storage of CO₂ is an option, it poses logistical challenges and may add to the costs of an already expensive capture process.

Finding ways to convert captured CO₂ into economically valuable products could help offset capture costs. At the same time, it creates a step toward a closed carbon cycle.

Professor Julien Leclaire at Claude Bernard University Lyon 1, co-author of the study, stated: “There is a great urgency to implement carbon capture systems. We do not always explore the molecular-scale details of these processes due to their complexity. However, sometimes we can find deep insights connecting molecular behavior to larger scales.”

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