Herein, tetraethylammonium 1,2,3-triazolide ([Et4N][Tz]), 1,2,3-triazole (Tz), and ethylene glycol (EG) are used to form DESs for CO2 capture. Surprisingly, [Et4N][Tz]-EG DESs can react with CO2, but [Et4N][Tz]-Tz cannot react with CO2, although both of the two systems contain the same anion [Tz]-. Unexpectedly, with the addition of EG to [Et4N][Tz]-Tz, the formed ternary DESs [Et4N][Tz]-Tz-EG can react with CO2, although neither EG nor [Et4N][Tz]-Tz can react with CO2 before the combination of them. NMR, FTIR and theoretical calculation results disclose that the surprise CO2 absorption behavior mainly depends on the strength of hydrogen bonds (H-bonds) between the anion [Tz]- and H-bond donors (EG or Tz). The strength of the H-bond between [Tz]- and Tz is much stronger than that between [Tz]- and EG. The strong H-bond between [Tz]- and Tz in [Et4N][Tz]-Tz greatly reduces the basicity of [Tz]-, rendering the anion [Tz]- unreactive to CO2. In [Et4N][Tz]-Tz-EG ternary DESs, EG competes with Tz to form a H-bond with [Tz]-, which weakens the strength of the H-bond between [Tz]- and Tz. Moreover, H-bonds also impact the desorption behavior. [Et4N][Tz] : EG (1 : 2) is regenerated at 60 °C, whereas the chemisorbed CO2 by [Et4N][Tz] : Tz : EG (1 : 2 : 2) can be released even down to 30 °C.

Deep eutectic solvents (DESs) formed by bio-phenol-derived superbase ionic liquids (ILs) and ethylene glycol (EG) exhibit a high CO2 capacity, up to 1.0 mol CO2/mol DESs, which is much better than those of the parent ILs. Surprisingly, mechanism results indicate that CO2 reacts with EG, but doesn't react with phenolic anions in the solvent, which is different from other DESs formed by superbase ILs and EG. The reaction pathway between CO2 and DESs used in this work may include two steps. The first step is the acid-base reaction between the phenolic anion and EG, which forms HO-CH2-CH2-O-, and then CO2 is attached to the anion HO-CH2-CH2-O- to form a carbonate species.

Recently, deep eutectic solvents (DESs), a new type of solvent, have been studied widely for CO2 capture. In this work, the anion-functionalized deep eutectic solvents composed of phenol-based ionic liquids (ILs) and hydrogen bond donors (HBDs) ethylene glycol (EG) or 4-methylimidazole (4CH3-Im) were synthesized for CO2 capture. The phenol-based ILs used in this study were prepared from bio-derived phenols carvacrol (Car) and thymol (Thy). The CO2 absorption capacities of the DESs were determined. The absorption mechanisms by the DESs were also studied using nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR), and mass spectroscopy. Interestingly, the results indicated that CO2 reacted with both the phenolic anions and EG, generating the phenol-based carbonates and the EG-based carbonates, when CO2 interacted with the DESs formed by the ILs and EG. However, CO2 only reacted with the phenolic anions when the DESs formed by the ILs and 4CH3-Im. The results indicated that the HBDs impacted greatly on the CO2 absorption mechanism, suggesting the mechanism can be tuned by changing the HBDs, and the different reaction pathways may be due to the steric hinderance differences of the functional groups of the HBDs.