The Role of Water in CO₂ Capture by Amine

An in situ IR study unraveled that adsorption of CO₂ on 20 μm TEPA film at 50 °C followed a zwitterion-intermediate pathway: zwitterion → ammonium carbamate. H₂O in the mixed TEPA/H₂O (5:1) film decreased the rate of CO₂ adsorption but increased the amine efficiency. CO₂ preferentially adsorbs on primary amine sites and H₂O on secondary amine sites. The presence of H₂O promotes the formation of carbamic acid and produces a broad IR band centered at 2535 cm^–1, which can be assigned to the ν­(O–H) of hydronium carbamate, −NCOO^–···H–OH₂⁺. The broadness of this 2535 cm^–1 band ranging from 2100 to 2800 cm^–1 persists at 120 °C. These broad components of the band can be ascribed to ν­(N–H) in hydrogen-bonded ammonium carbamate, a R–NH₃⁺/R₁R₂–NH₂⁺···–NCOO^– moiety. The binding strength of adsorbed species on the TEPA film increases in the order adsorbed H₂O < carbamic acid < ammonium carbamate < hydrogen-bonded water/ammonium carbamate.