Ion-Specificity in Atmospheric CO2 Capture by Diamino Carboxylate PILs; Effect of Diluents

Sharply rising level of atmospheric carbon dioxide is one of the biggest environmental concerns facing our civilization, resulting in an increasing importance of alternative modes of capturing this gas arising from anthropogenic emissions. Numerous materials have been proposed to capture and convert carbon dioxide into more precious materials, however none of these offers an energy efficient process. An alternative industrial method which would be appropriate in every respect is still lacking. In the present research project, several diamino carboxylate protic ionic liquids (PILs) were synthesized and tested for CO2 capture. Capacities as high as 16 %w/w are observed; this is well above the industrial standard monoethanolamine (MEA)/water mixture. Besides the neat ionic liquids, the effects of diluents such as water, the corresponding parent amine and MEA on the absorption capacities were investigated. IR and NMR spectroscopies were employed as analytical techniques to confirm the behavior of the different systems. Varying the amino-functionalities, as well as the alkyl chain length of the carboxylate anions, results in different absorption behavior where several trends were observed. Increasing the size of the anion produces a decrease in the absorption capacity. It was interesting to note that with addition of water, the trend remains the same in the case of the different anions; however, a systematic decrease in uptake indicates non-favorable conditions. PILs containing excess amine show higher uptakes, in contrast to the neat, which can be explained by the increased basicity of the reaction media. Addition of MEA diminishes the ion-specificity in case of the different cations, which can be attributed to the competing reactions between the CO2 and the two different amines in each system. The results generated from this project may help in understanding the CO2 capture mechanisms and be relevant in the development of novel type of absorbents for both flue gas capture and also future large-scale atmospheric capture technologies.