Supramolecular dimers drive the reaction between CO2 and alkanolamines towards carbonate formation

Abstract

In this work we revisited the mechanisms for CO2 absorption in pure alkanolamines (monoethanolamine, diethanolamine, 2 −amino−2 −methyl−1 −propanol, and ethylenediamine) and in their 30% (v/v) aqueous solution, leading to the corresponding carbonate or carbamate. Fourier-transform infrared and 13C nuclear magnetic resonance spectroscopies strongly suggest that in the aqueous medium the main product is carbamate, whereas in the pure alkanolamines or in an acetonitrile solution carbonate is formed. Density functional theory simulations indicate that when alkanolamine monomers are used, carbamic acid is the most stable product. On the contrary, alkanolamine dimers led mainly to a zwitterionic carbonate. The effect of the solvent was also studied showing that in aqueous medium alkanolamines react as monomers with CO2 to produce carbamates. In pure alkanolamines or in the presence of nonpolar solvents, the main species are the alkanolamine supramolecular dimers, which react with CO2 to form carbonates.