CO2 Absorption by Solvents Consisting of TMG Protic Ionic Liquids and Ethylene Glycol: The Influence of Hydrogen Bonds

Herein, the absorption of CO2 by the TMG-based (TMG: 1,1,3,3-tetramethylguanidine) ionic liquids (ILs) and the absorbents formed by TMG ILs and ethylene glycol (EG) is studied. The TMG-based ILs used are formed by TMG and 4-fluorophenol (4-F-PhOH) or carvacrol (Car), and their viscosities are low at 25 °C. The CO2 uptake capacities of [TMGH][4-F-PhO] and [TMGH][Car] are low (~0.09 mol CO2/mol IL) at 25 °C and 1.0 atm. However, the mixtures [TMGH][4-F-PhO]-EG and [TMGH][Car]-EG show much higher capacities (~1.0 mol CO2/mol IL) than those of parent ILs, which is unexpected because of the low CO2 capacity of EG (0.01 mol CO2/mol EG) in the same conditions. NMR spectra and theoretical calculations are used to determine the reason for these unexpected absorption behaviors. The spectra and theoretical results show that the strong hydrogen bonds between the [TMGH]+ cation and the phenolate anions make the used TMG-based ILs unreactive to CO2, resulting in the low CO2 capacity. In the Ils-EG mixtures, the hydrogen bonds formed between EG and phenolate anions can weaken the [TMGH]+–anion hydrogen bond strength, so ILs-EG mixtures can react with CO2 and present high CO2 capacities.