Hyperbranched Aluminosilica Captures Carbon Dioxide

Researchers have developed a new, low-cost material for capturing carbon dioxide (CO2) from the smokestacks of coal-fired power plants and other generators of the greenhouse gas. Produced with a simple one-step chemical process, the new material has a high capacity for absorbing CO2 and can be reused many times.

Combined with improved heat management techniques, the new material could provide a cost-effective way to capture large quantities of CO2 from coal-burning facilities. Existing CO2 capture techniques involve the use of solid materials that lack sufficient stability for repeated use or liquid adsorbents that are expensive and require significant amounts of energy.

"This is something that you could imagine scaling up for commercial use," said Christopher Jones, a professor in the School of Chemical and Biomolecular Engineering at the Georgia Institute of Technology. "Our material has the combination of high capacity, easy synthesis, low cost, and a robust ability to be recycled – all the key criteria for an adsorbent that would be used on an industrial scale."

Details of the new material, known as hyperbranched aluminosilica (HAS), are scheduled to appear in the March 19 issue of the Journal of the American Chemical Society. The research was supported by the U.S. Department of Energy's National Energy Technology Laboratory.

Once removed from the stack gases, the CO2 might be sequestered in the deep ocean, in mined-out coal seams, or in depleted petroleum reservoirs. If the CO2 capture and sequestration process can be made practical, America's large resources of coal could be used with less impact on global climate change.

Working with Department of Energy scientists Daniel Fauth and McMahan Gray, Jones and graduate students Jason Hicks and Jeffrey Drese developed a way to add CO2-adsorbing amine polymer groups to a solid silica substrate using covalent bonding. The strong chemical bonds make the material robust enough to be reused many times.

Production of the HAS material is relatively simple and requires only the mixing of the silica substrate with a precursor of the amine polymer in solution. The amine polymer is initiated on the silica surface, producing a solid material that can be filtered out and dried.

To test the effectiveness of their new material, the Georgia Tech researchers passed simulated flue gases through tubes containing a mixture of sand and HAS. The CO2 was adsorbed at temperatures ranging from 50 to 75 degrees Celsius. Then the HAS was heated to between 100 and 120 degrees Celsius to drive off the gas so the adsorbent could be used again.

The researchers tested the material across 12 cycles of adsorption and desorption and did not measure a significant loss of capacity. The HAS material can adsorb up to five times as much CO2 as some of the best existing reusable materials.

The HAS material works in the presence of moisture, an unavoidable by-product of the combustion process.

Adsorption of the CO2 generates considerable amounts of heat, which must be managed and thermally recycled. Removal of the carbon dioxide requires heating the adsorbent.

"How to manage this heat is one of the most critical issues controlling the economics of a potential large-scale process," Jones added. "You must control the production of heat by the adsorption step, and you don’t want to put any more energy into the desorption process than necessary."

Because of their chemical structure, the amine groups provide three different classes of binding sites for CO2, each with a different binding energy. Optimizing the production of binding sites is a goal for future research, Jones said.

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