CO₂ capture by anion-functionalized deep eutectic solvents: the effect of steric hindrance of -OH groups

Herein, the ionic liquid (IL) N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium 1,2,3-triazolide ([MOEN₂₂₁][Tz]) and ethylene glycol (EG) or pinacol (PIN) are used to form deep eutectic solvents (DESs) for CO₂ capture. Surprisingly, the capacity of [MOEN₂₂₁][Tz]-EG DESs (∼0.77 mol CO₂ per mol DESs) is more than twice that of [MOEN₂₂₁][Tz] (0.32 mol CO₂ per mol IL) under the same conditions. However, the capacity of [MOEN₂₂₁][Tz]-PIN DESs (∼0.30 mol CO₂ per mol DESs) is much lower than that of [MOEN₂₂₁][Tz]-EG DESs. The capacity of the ternary DESs [MOEN₂₂₁][Tz]-PIN-EG is also much higher than that of binary DESs [MOEN₂₂₁][Tz]-PIN. NMR, FTIR and theoretical calculation results disclose that these unexpected CO₂ absorption behaviors may depend on the strength of hydrogen bonds between the anion [Tz]^- and alcohols used, which is mainly governed by the steric hindrance of -OH groups. Furthermore, in the ternary DESs, CO₂ is attached to the -OH of EG, not the -OH of PIN, revealing that the steric effects can also impact carbon capture mechanisms.