Efficient CO2 capture by deep eutectic solvents through reducing the reaction between carbenes and CO2

Abstract

Deep eutectic solvents (DESs) have emerged as a class of promising absorbents for carbon capture. Herein, DESs formed using the ionic liquids (ILs) [Emim][Triz] (1-ethyl-3-methylimidazolium 1,2,4-triazolide) and [Emmim][Triz] (1-ethyl-2,3-dimethylimidazolium 1,2,4-triazolide) with ethylene glycol (EG) were applied to CO₂ capture. The CO₂ capture capacities of the absorbents were determined at 25 °C and 1.0 atm. Interestingly, the DES [Emim][Triz]–EG exhibited a higher capacity (∼0.95 mol CO₂ per mol solvents) than [Emim][Triz] (0.77 mol CO₂ per mol IL) under the same conditions. [Emmim][Triz]–EG also exhibited a high capture capacity (∼0.96 mol CO₂ per mol solvents). NMR and FTIR methods were applied to investigate the interactions between CO₂ and the solvents used in this work. The mechanistic studies indicated that CO₂ was bonded to both the –OH group of EG and the C2 position of the [Emim]⁺ cation after CO₂ was captured by [Emim][Triz]–EG DESs, forming EG-based carbonate and Emim-based carbene–CO₂ complexes (1-ethyl-3-methylimidazolium-2-carboxylate), respectively. The amount of carbene–CO₂ complexes in [Emim][Triz]–EG + CO₂ DESs was lower than that in [Emim][Triz] + CO₂. As the EG content increased, the amount of carbene–CO₂ complexes gradually reduced in [Emim][Triz]–EG + CO₂. Moreover, [Emim][Triz] : EG (1 : 2) exhibited a lower desorption temperature (70 °C) than [Emim][Triz] (90 °C), which can be due to the lower amount of carbene–CO₂ complexes in [Emim][Triz]–EG than in [Emim][Triz] after CO₂ capture. In other words, the addition of EG into [Emim][Triz] affected not only the CO₂ capture capacity but also the regeneration temperature of the absorbents. As no carbene–CO₂ complex formed in the [Emmim][Triz]–EG system, the regeneration temperature of [Emmim][Triz]–EG was further reduced to 60 °C.