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.