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

Abstract

Herein, the ionic liquid 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 (~0.76 mol CO2/mol DESs) is more than twice that of [MOEN221][Tz]-PIN DESs (~ 0.30 mol CO2/mol DESs) at the same conditions. However, the capacity of [MOEN221][Tz]–PIN DESs (~ 0.30 mol CO2/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.