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I'm reading this article about carbon capture from the combustion of fossil fuels. In this article, it says that amines are used as a chemical absorbent to capture the $\ce{CO2}$ from the exhaust of the combustion.

In particular, I have read that N-Methyl diethanolamine (MDEA) is used (a reference lists the formula as $\ce{CH3N (C2H4OH)2}$). I am curious if anyone can explain what happens during the reaction with $\ce{CO2}$ and how it absorbs the $\ce{CO2}$ from the flue gas.

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In addition to the reaction in permeakra's answer, some amines may react with $\ce{CO2}$ in absence of water also. This type of reaction is a Lewis acid-base reaction forming a zwitterionic complex: $\ce{R3N + CO2 -> R3NCO2}$.

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MDEA adds some stability to this complex. The alcohol groups can stabilize the carboxylate group through hydrogen bonding.

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All amines are proton bases. $\ce{CO2}$ in water containing solutions acts as a weak acid. An overall reaction is $\ce{NR3 + H2O + CO2 -> [NHR3][HCO3]}$ and is possible for all amines. Aminoalcohols are cheap as they are produced from easily available oxirane (ethylene oxide) and have a high boiling point, so for them this reaction can be reversed by heating, with the regeneration of working solution.

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  • $\begingroup$ I don't think this is clear enough for someone who obviously does not have much chemical knowledge. Also, your reaction is wrong, and should contain charged species. $\endgroup$ – CHM Sep 19 '12 at 0:27
  • $\begingroup$ I see nothing wrong with permeakra's reaction. The reaction does contain charged species, $\ce{NHR3+}$ and $\ce{HCO3-}$. However, like all ionic formulae, when you write the two ions together, we often choose not to write the charges. For simplicity, we often write $\ce{HCl + NaOH -> H2O + NaCl}$ and not the full ionic equation. $\endgroup$ – Ben Norris Sep 21 '12 at 11:21

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