CO2 capture by 1,2,3-triazole-based deep eutectic solvents: the unexpected role of hydrogen bonds

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.