Primary and secondary alkanolamines react with dissolved CO2 in a two step sequence, forming first a zwitterion (1), which then transfers a proton to an un-ionized amine, forming the corresponding carbamate (2). The reaction of tertiary amines with CO2 proceeds by the formation of a protonated amine and a bicarbonate anion (3).
The chemical structures of three common alkanolamine CO2 sorbents, monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA) are given below:
The Solid Phase Amine CO2 sorbents work by analogous reactions between airborne CO2, water vapor, and the amine bearing functional groups of the polymer. Early work in this area investigated the used of anion exchange resins bearing amine functional groups. Currently more specialized polymers are used. The thermally regenerable CO2 removal system for the Shuttle Extended Duration Orbiter (EDO) uses Amberlite XAD-7 polystyrene-co-divinylbenzene beads coated with polyethyleneimine. Under heat and/or vacuum, the chemisorption reactions are reversed and the polymeric sorbent is regenerated.
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