Capturing, storing, and utilizing carbon dioxide (CO2) remains an urgent challenge.
As global temperatures continue to rise, preventing CO2 from entering the atmosphere could help limit warming in a world that will still rely on carbon-based fuels.
Creating a Shift
The world has made significant strides toward creating practical, affordable carbon-capture technologies. Carbon-capture liquids, referred to as solvents when present in abundance, can efficiently absorb CO2 molecules from coal-fired power plants, paper mills, and other sources of emissions. However, they all work through the same fundamental chemical mechanism, or so researchers had assumed.
Designing the Carbon Capture Solvent
In new work appearing in the journal Nature Chemistry, scientists were surprised to discover that a familiar solvent was performing even more promisingly than they had originally thought.
New details about the solvent's fundamental structure revealed that the liquid could hold twice as much CO2 as previously believed. The newly revealed structure could also hold the key to creating a suite of carbon-based materials that could help remove even more CO2 from the atmosphere.
The U.S. Department of Energy's Pacific Northwest National Laboratory (PNNL) developed the solvent several years ago. They had been studying it under a variety of conditions. The team had been working to reduce the cost of using the solvent and improve its performance. Last year, the team reported the lowest-energy carbon capture system to date, and it was during that work that they noticed something peculiar.
"The team was trying to do a different kind of high-pressure gas separation and saw that the solution got a lot thicker, and something showed up in our spectrum that shouldn't have been there," said David Heldebrant, a chemist at PNNL and co-author on the study. "That suggested something new was forming, which was completely unexpected, and we knew we had to dig into it." Heldebrant then reached out to collaborators at the University Claude Bernard Lyon 1 in France and the University of Texas at El Paso to help untangle the molecular changes behind the result.
"This work was truly a collaborative and interdisciplinary effort. The questions to be answered required expertise from more than one discipline," said Jose Leobardo Baños-López, a professor at the University of Texas at El Paso. "We looked at the overall structure of the solvent as it interacts with CO2 and found that it had more order than expected, essentially." It appeared that molecules were clustering together when they should have been pairing off. The question became: What did these new, ordered structures mean?
When the team took a fresh look at the solvent-CO2 system using analytical chemistry tools, they found assemblies of solvent molecules self-assembling. At first, the researchers tried to fit the data to a model using just two solvent molecules. Despite the team's initial expectations, the data did not fit.
But using a model with four solvent molecules, the researchers found a match. The four-component assembly was, in fact, the form of the solvent that the team was seeing. The flexible structure could undergo a series of changes to accommodate incoming CO2 molecules.
Ultimately, the CO2 traveled to the core of the assembly, where it found an active site similar to those found inside enzymes. In fact, the overall assembly structure and interactions appeared remarkably protein-like.
The binding site is at the center of the newly observed chemistry. Typically, carbon-capture systems work with a single CO2 molecule that binds and can react to form something else. Limiting everything to reactions involving a single CO2 limits the follow-on carbon transformations that are possible. However, this new molecular assembly was doing something different.
The team found that a new species was forming, one that included two distinct CO2 molecules. The assemblies incorporated CO2 in a stepwise fashion, first capturing and activating one molecule and then a second.
A Challenge with Captured Carbon: What to Do with It
When both CO2 molecules were in the assembly, they could react with each other. This created different carbon-based molecules, potentially expanding the uses for captured CO2.
"What we are doing is changing a fundamental variable in this process," said Heldebrant. "We used to capture one CO2 at a time. By linking two CO2s together, we can potentially double the effective storage capacity of capture systems."
The newly linked molecules have very different properties than CO2. This changes the chemistry needed to separate the captured carbon from the solvent. These CO2-based molecules are larger and are a first step toward making CO2-rich polymers.
A perennial challenge with captured carbon is what to do with it. While long-term storage of CO2 is one option, it presents logistical challenges and can add to the cost of an already expensive capture process. Finding ways to transform captured CO2 into products with economic value could help offset the cost of capture while also creating a path toward a closed carbon cycle.
"There is a great urgency to deploy carbon capture systems," said Julien Leclaire, a professor at the University Claude Bernard Lyon 1 and co-author on the study. "We don't always explore the molecular details of these processes, because of their complexity. But sometimes we can find insights that connect the molecular and the macroscopic behavior."
By linking two CO2 molecules together in the initial capture step, this work has laid out a new approach to carbon capture and utilization. Instead of starting with CO2, researchers may have more options to create new chemicals. This opens the door to different kinds of chemistry that had previously been impractical for CO2 conversion. These potential next steps are only possible because of a focus on the basic science behind carbon capture.
Summary
- New details about a solvent's fundamental structure reveal that it could hold twice as much CO2 as previously believed.
- The newly revealed structure could also hold the key to creating a suite of carbon-based materials that could help remove even more CO2 from the atmosphere.
- The study found that a new species was forming, one that included two distinct CO2 molecules.
- By linking two CO2s together, we can potentially double the effective storage capacity of capture systems.