Abstract
The anomalous behavior of CO2 adsorption in anhydrous and humid conditions in silica-supported, polyethylenimine (PEI)-impregnated sorbents has been elucidated in a new chemical model. New quantum chemical calculations show that the zwitterion, whose existence was called into question by earlier theoretical and spectroscopic studies, can be stabilized by water as well as amines, to an extent that zwitterions can serve as diffusive intermediates for CO2 transport through PEI. Water-stabilized zwitterions are more numerous and have a lower activation energy for deprotonation compared with amine-stabilized zwitterions, leading to the formation of carbamate. Calculations also show that bicarbonate is disfavored in the system compared with hydronium carbamate, in accord with spectroscopic evidence. A reaction–diffusion model based on this new chemistry and quantitatively linked to quantum calculations reproduces the anomalous experimentally observed macroscopic behavior of the sorbents in a quantitative fashion.
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