It is now widely accepted that anthropogenic CO₂, due to its role as a greenhouse gas, is the major contributor to climate change. Other environmental implications of these emissions, such as ocean acidification, are also becoming increasingly apparent and worrisome. Thus, CO₂ management is one of the most challenging issues of our century. Capture and sequestration of CO₂ (CCS) underground has been proposed, but none of the existing technologies has been proven on the enormous scale needed. We also need to make sure that if billions of tons of CO₂ are pumped underground it also remains there and does not leak out over time.

While fossil fuels will be used for as long as they can be easily and economically produced, it should also be clear that their amounts are finite and that they are increasingly depleted. Post-fossil fuel alternative sources of carbon therefore need to be found to fulfill our needs for fuels, hydrocarbons, polymers, and other products presently derived mostly from petroleum oil and natural gas. Instead of considering CO₂ as a problematic and unwanted combustion byproduct, it should be seen as a valuable feedstock for the production of fuels and materials. While the required CO₂ can be presently captured from concentrated industrial sources, eventually it will have to be obtained directly from the atmosphere.

...While about half of the anthropogenic CO₂ emissions are the result of large industrial sources such as power plants and cement factories, the other half originate from small distributed sources such as cars, home heating, and cooking. For those, CO₂ capture at the emission source is not practical and/or economical. A possible pathway to deal with these emissions is to capture CO₂ directly from the air.

—Goeppert et al.

The study of the separation and recovery of CO₂ from ambient air on a larger scale is in its infancy and has only relatively recently attracted increased interest, they note. Adsorbents based on Ca(OH)₂, NaOH, and combinations thereof have been suggested, but their regeneration is generally energy intensive. Amine and polyamine based sorbents either chemically bound or physically adsorbed on a support such as silica, mesoporous solids, and carbon fibers have also been proposed and in some cases tested for CO₂ capture from the air. Hyperbranched aminosilicas (HAS) have also been reported as adsorbents for CO₂ capture from the air.

The Loker group turned to solid materials based on polyethylenimine (PEIs), a readily available and inexpensive polymeric material. They selected branched PEI with a high molecular weight (Mw = 25,000) as an adsorbent material in large part because of its very low volatility and coated it on the surface of fumed silica. The CO₂ adsorption and desorption measurements were performed in an all-glass, grease-free flow system.

Under dry conditions, on FS-PEI-33 the amount of CO₂ adsorbed per g of PEI was 156 mg/g, whereas it was 150 mg/g for FS-PEI-50. In the presence of water, these values were 230 and 124 mg CO₂/g PEI, respectively. PEI use was almost two times more efficient at a lower loading. A similar trend was already reported in the case of adsorption of pure CO₂.

Our reported values under humid conditions are among the highest reported for CO₂ adsorption from the air! The 1.74 mmol/g adsorption of FS-PEI-33 obtained under humid conditions was noticeably higher than the one reported for a hyperbranched aminosilica (1.44 mmol/g) with a comparable organic amine content of 36% and closer to the ones obtained on an HAS with a higher organic content of 42.5% (1.72 mmol/g). Triamine-grafted pore-expanded mesoporous silica (TRI-PE-MCM-41), on the other hand, exhibited a CO₂ adsorption capacity of 0.98 mmol/g. Operating under humid conditions has a further advantage: Recently, the presence of water has been shown to greatly improve the stability of solid amine based adsorbents by avoiding the formation of urea.

—Goeppert et al.

The researchers suggest the materials may be useful on submarines, in smokestacks or out in the open atmosphere, where they could clean up carbon dioxide emissions that comes from small point sources such as cars or home heaters.

Resources

• Alain Goeppert, Miklos Czaun, Robert B. May, G. K. Surya Prakash, George A. Olah, and S. R. Narayanan (2011) Carbon Dioxide Capture from the Air Using a Polyamine Based Regenerable Solid Adsorbent. Journal of the American Chemical Society 133 (50), 20164-20167 doi: 10.1021/ja2100005