Molecular Modeling Study to the Relation between Structure of LPEI, Including Water-Induced Phase Transitions and CO2 Capturing Reactions

A molecular modeling study was carried out on linear polyethyleneimine (LPEI) to determine the CO2 capturing process taking into account structural changes of the material at various amounts of H2O. Until now, CO2 capture by LPEI was described as a combination of carbamic and carbonic acid formation. It was found that humidification below the melting point of LPEI leads to the formation of an LPEI/0.5H2O type of phase. Such a type of phase leads to CO2 capture exclusively via carbonic acid formation, stabilized by secondary amines via ammonium bicarbonate and H-bridged amine carbonic acid complexes. Both the strong physisorption CO2 complex and the low activation barrier for carbonic acid formation contribute to the overall process. Only in the absence of H2O, carbamic acid formation is possible. Thus, under CO2 capturing process conditions, carbonic acid formation seems to be the only option as H2O will be present in flue gas.