A scanning electron microscope image (left) and a high-resolution transmission electron microscope image (right) show an activated, sulfur-containing, porous carbon material, which can be tuned to balance carbon dioxide sequestration and methane selectivity. Images: Barron Research Group/Rice University.
A scanning electron microscope image (left) and a high-resolution transmission electron microscope image (right) show an activated, sulfur-containing, porous carbon material, which can be tuned to balance carbon dioxide sequestration and methane selectivity. Images: Barron Research Group/Rice University.

Natural gas producers want to draw all the methane they can from a well while also sequestering as much carbon dioxide as possible. At the moment, they can use filters that optimize either carbon capture or methane flow, but no single filter will do both. Thanks to scientists at Rice University, however, they now know how to fine-tune these sorbents for their needs.

According to Rice chemist Andrew Barron, subtle adjustments in the manufacture of a polymer-based carbon sorbent can switch it between being the best-known material for capturing carbon dioxide and balancing carbon capture with methane selectivity. This finding is reported in a paper in Sustainable Energy and Fuels.

"The challenge is to capture as much carbon as possible while allowing methane to flow through at typical wellhead pressures," explained Barron. "We've defined the parameters in a map that gives industry the best set of options to date."

Previous work by the lab determined that carbon filters maxed out their capture ability with a surface area of 2800m2 per gram and a pore volume of 1.35cm3 per gram. They also discovered that the best carbon capture material didn't achieve the best trade-off between carbon and methane selectivity. With this new work, they know how to tune the material for one or the other.

"The traditional approach has been to make materials with ever-increasing pore volume and relate this to a better adsorbent; however, it appears to be a little more subtle," Barron said.

The lab made its latest filters by heating a polymer precursor while also treating it with potassium hydroxide (KOH), which acts as an activation reagent. Baking the polymer with KOH at temperatures over 500°C (932°F) turns it into a highly porous filter, full of nanoscale channels that can trap carbon.

The ratio of KOH to polymer during processing turned out to be the critical factor in determining the final filter's characteristics. Making filters with a 3-to-1 ratio of KOH to polymer gave a surface area of 2700m2 per gram and maximized carbon dioxide uptake under pressures of 5–30 bar (1 bar is slightly less than the average atmospheric pressure at sea level.) Filters made with a 2-to-1 ratio of KOH to polymer had a smaller surface area – 2200m2 per gram -- and a lower pore volume, but resulted in the optimum combination of carbon dioxide uptake and methane selectivity.

The size of the pores was critical as well. Filters with maximum carbon uptake had the largest fraction of pores smaller than 2nm; bigger pores were better for methane selectivity.

"It appears that total pore volume is less important than the relative quantity of pores at specific sizes," Barron said. "Our goal was to create a guide for researchers and industry to design better materials.

"Not only can these materials be used for carbon dioxide separation from natural gas, but they are also models for carbon dioxide sequestration in a natural resource. This is the future direction of our research."

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