Reactivity of Azole Anions with CO2 from the DFT Perspective

Azole anions are key components in CO2 capture materials that include ionic liquids and porous solids. Herein, we use density functional theory (DFT) and a Langmuir-type adsorption model to study azole anion-CO2 interactions. Linear CO2 has to be bent by approximately 45 degrees to form an NC bond within the azole ring. The energy cost of bending renders CO2 absorption much more difficult compared to SO2 absorption. For different azole anions, the number of nitrogen atoms in the ring and the natural bond orbital energy of the reacting nitrogen lone pair, both linearly correlate with the calculated reaction enthalpy and are useful handles for new sorbent designs. Unlike for SO2, the azole parent architecture (unsubstituted) does not allow successive CO2 absorption under mild conditions (<0.12MPa and at room temperature). Experimental CO2 and SO2 absorption isotherms are reproduced by using the Langmuir model parameterized with the calibrated DFT reaction enthalpies. This study provides insight for designing azole-based CO2-capture materials.