Angew. Chem. Int. Ed. http://doi.org/f3mrc9 (2016)
To limit CO2 emissions, it is imperative to develop efficient technologies for CO2 capture. Ionic liquids (ILs), especially those based on amino acids, have emerged as promising CO2 absorbents in which CO2 chemisorption can occur at the amino groups. However, the carboxylate groups in amino acid ILs are not able to absorb CO2 because the presence of amino groups reduces their alkalinity, and thus their affinity for CO2, through the electron-withdrawing inductive effect. Duan-Jian Tao, Sheng Dai and colleagues in China and the USA have now designed aminopolycarboxylate-based ILs that feature multiple acetate groups attached to the nitrogen of the amino groups, which activates the carboxylate groups leading to exceptionally high CO2 absorption capacity (up to 1.69 mol CO2 per mol IL).
The idea behind their molecular design was to use acetate groups, also electron-withdrawing species, to mitigate the negative inductive effect of the amino groups, allowing the carboxylate groups to interact with CO2 more effectively. Calculations showed that in conventional amino-acid-based ILs, the oxygen atoms of the carboxylate groups have reduced negative charges once a CO2 molecule is absorbed. In aminopolycarboxylate-based ILs, on the other hand, the nitrogen atoms of the amino groups have significantly reduced negative charges while the oxygen atoms of the carboxylate groups remain largely negative so that further CO2 molecules can be absorbed at the carboxylate sites.