HOLEY CARBON [+]Enlarge Credit: ACS Appl. Mater. Interfaces

Raw natural gas contains carbon dioxide and other gases that must be removed before the fuel can be transported through a pipeline. A new porous material, derived from asphalt, could help perform that separation cheaply and efficiently. The nitrogen-doped solid can hold more than 100% of its weight in pressurized carbon dioxide (ACS Appl. Mater. Interfaces 2014, DOI: 10.1021/am508858x).

When energy companies extract natural gas from wells, as much as 70% of what comes out is carbon dioxide. Engineers send the raw gas through a solution of liquid amine to capture the carbon dioxide. They then heat the amine to drive off the captured gas and regenerate the liquid for another cycle of capture. To reduce the energy used during carbon capture, scientists are creating porous solids that contain and release gas with changes in pressure. Some of these solids are metal-organic frameworks, or MOFs, but these materials can be expensive to make.

Scientists have started working on carbon-based absorbent materials, which should be cheaper. The stability and thermal conductivity of these materials might also make them well suited for large-scale carbon capture operations at commercial plants.

James M. Tour at Rice University and his colleagues have synthesized sulfur- and nitrogen-containing carbon materials to capture carbon dioxide (Nat. Commun. 2014, DOI: 10.1038/ncomms4961). The sulfur and nitrogen atoms serve as binding sites for the gas molecules. Now they report a low-cost source for such materials: asphalt.

Asphalt contains complex aggregates of aromatic hydrocarbons. To turn asphalt into a porous solid, the researchers heated it to 700 °C with potassium hydroxide. This generated carbon dioxide and carbon monoxide, which bubbled out of the material and left holes in the solid. Next, the researchers introduced nitrogen atoms into the carbon skeleton by heating the solid with ammonia and then baking it with hydrogen gas.

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To test the new material, the researchers exposed it to carbon dioxide gas at a pressure similar to that of a well containing 10% carbon dioxide. After the exposure, the weight of the material increased 114%. This uptake was greater than that of other activated carbon materials, Tour says. The solid released its captured gas when the researchers lowered the surrounding gas pressure.

Sheng Dai of Oak Ridge National Laboratory is amazed by the storage capacity of this material. Previously, other teams had shown that nitrogen-doped carbon materials are good at capturing carbon dioxide at low pressures. This work, he says, nicely demonstrates that nitrogen doping can also enhance uptake at high pressures.