The demand for adsorbents to capture carbon dioxide (CO₂) is massive as large carbon emitters, like coal and natural gas power plants, scramble to meet emissions targets. These materials need to filter out the greenhouse gas, CO₂, because captured CO₂ can either be reused to make things like plastics, concrete and fuel, or compressed for storage elsewhere.

“For either of those use cases, you have to have an almost pure stream of CO₂ and that’s why the selectivity is so important,” explained James Burrow lead author of a paper in Advanced Materials describing a new method for synthesizing adsorbents from renewable materials.

Adsorbents to capture CO2 are usually made from a material called metal organic frameworks (MOFs). These nano-sized metallic compounds are effective at maximizing both the amount and purity of CO₂ captured. The drawback is they are expensive and difficult to make.

Replacing MOFs with a sustainable, cheaper, easier-to-use material was the challenge motivating Burrow during his Ph.D.at The University of Texas at Austin and he believed the answer to the carbon capture challenge was also carbon.

Using carbon to capture CO₂

Making adsorbents out of carbon-based material and not MOFs is desirable because all sorts of renewable biomass is readily available. “You can use woody biomass, or wheats, or grass,” Burrow said. “It’s really just any carbon source.”

These carbon-based adsorbents are themselves not new, but many believed they couldn’t compete with MOFs and other designs. Burrow set out to prove otherwise. The first step in maximizing performance of a carbon-based adsorbent is to add nitrogen atoms as they make the compound more negatively charged, creating a stronger attraction with the positively charged end of a CO₂ molecule.  

Normally, a process called chemical activation is used to add nitrogen and make carbon-based adsorbents. The carbon and nitrogen precursors are treated with a salt at extreme temperatures of 800 degrees Celsius. “The salt reacts with the material, and basically rips out atoms, exfoliates it, and makes all these pores and increase the surface area,” Burrow explained.

While studying this process in detail, Burrow realized something else was happening. “We found that under certain conditions, a different process occurs, which is not chemical activation. Instead, it’s called molten salt templating.”

In this method, a salt that doesn’t aggressively react with the precursors is used. Instead, at the same extreme temperatures, the salt begins to act like a liquid. Without the molten salt, the carbon and nitrogen in Burrow’s experiments were the common ingredients, sucrose and urea, which tend to form the same crystalline structure again and again. However, the salt increases the fluidity of the precursors, allowing them to form new structures which can enhance carbon capture performance.

Achieving industry standards

Using a powerful new modelling method called design of experiments, simulations and predictions of the types of structures produced based on a given set of starting conditions — in this case, temperature, cooling rate, nitrogen content, and salt ratio — were made.

With this, Burrow could work backwards and optimize the starting conditions and produce adsorbents that meet precise requirements. For example, the industry specifications needed for carbon capture at natural gas fired power plants. “Industry has basically said, you need to hit a certain purity spec, or else the economics of transportation don’t make sense,” he said. “I figured out how to maximize the CO₂ capacity while hitting that minimum purity spec.”

Aside from proving a carbon-based adsorbent, sourced from renewable materials meets industry standards Burrow also showed that he could make an adsorbent that outperforms some MOFs in terms of the purity of the captured CO₂. In some cases, his materials extracted 99% pure CO₂, which according to Burrow suggest they could be useful for other applications like direct from the air capture of CO₂.

According to Burrow, this is an exciting proof of concept that shows how renewable materials, when properly synthesized, are adaptable and perform well across a range of scenarios. It now remains to be seen where these materials can be scalable and economically viable.

Reference: Burrow J., et al., A Data-Driven Approach to Molten Salt Synthesis of N-rich Carbon Adsorbents for Selective CO₂ Capture, Advanced Materials, (2023). DOI: 10.1002/adma.202306275

Feature image credit: Patrick Hendry on Unsplash

This article was updated on September 14, 2023 to correct the spelling of the first author’s name from Burrows to Burrow.