Enhancing CO capture of an aminoethylethanolamine-based non-aqueous absorbent by using tertiary amine as a proton-transfer mediator: From performance to mechanism.

Non-aqueous absorbents (NAAs) have attracted increasing attention for CO capture because of their great energy-saving potential. Primary diamines which can provide high CO absorption loading are promising candidates for formulating NAAs but suffer disadvantages in regenerability. In this study, a promising strategy that using tertiary amines (TAs) as proton-transfer mediators was proposed to enhance the regenerability of an aminoethylethanolamine (AEEA, diamine)/dimethyl sulfoxide (DMSO) (A/D) NAA. Surprisingly, some employed TAs such as N,N-diethylaminoethanol (DEEA), N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA), 3-dimethylamino-1-propanol (3DMA1P), and N,N-dimethylethanolamine (DMEA) enhanced not only the regenerability of the A/D NAA but also the CO absorption performance. Specifically, the CO absorption loading and cyclic loading were increased by about 12.7% and 15.5%-22.7%, respectively. The TA-enhanced CO capture mechanism was comprehensively explored via nuclear magnetic resonance technique and quantum chemical calculations. During CO absorption, the TA acted as an ultimate proton acceptor for AEEA-zwitterion and enabled more AEEA to form carbamate species (AEEACOO) to store CO, thus enhancing CO absorption. For CO desorption, the TA first provided protons directly to AEEACOO as a proton donor; moreover, it functioned as a proton carrier and facilitated the low-energy step-wise proton transfer from protonated AEEA to AEEACOO. Consequently, the presence of TA made it easier for AEEACOO to obtain protons to decompose, resulting in enhanced CO desorption. In a word, introducing the TA as a proton-transfer mediator into the A/D NAA enhanced both the CO absorption performance and the regenerability, which was an efficient way to "kill two birds with one stone".