Effect of Hydrogen Bonds on CO2 Capture by Functionalized Deep Eutectic Solvents Derived from 4-Fluorophenol

Deep eutectic solvents (DESs) have received a great amount of attention for CO2 uptake due to their unique properties. Here, deep eutectic solvents (DESs) based on 4-fluorophenol-derived superbase ionic liquid are studied for CO2 capture. The ionic liquid used is [DBUH][4-F-PhO], formed by 1,8-diazabicyclo[5.4.0]undecane-7-ene (DBU) and 4-fluorophenol (4-F-PhOH). The DESs are obtained by mixing [DBUH][4-F-PhO] with ethylene glycol (EG) or 4-F-PhOH. Surprisingly, [DBUH][4-F-PhO]-EG DESs present a much higher CO2 capacity (∼1.0 mol CO2/mol solvent) than [DBUH][4-F-PhO]-4-F-PhOH (∼0.10 mol CO2/mol solvent) at 25 °C and 1.0 atm. However, after EG is added into [DBUH][4-F-PhO]-4-F-PhOH, the ternary solvents [DBUH][4-F-PhO]-4-F-PhOH-EG exhibit an unexpected high capacity, although both EG and [DBUH][4-F-PhO]-4-F-PhOH exhibit a low capacity. Moreover, the capacities of ternary solvents [DBUH][4-F-PhO]-4-F-PhOH-EG decrease with increasing concentration of 4-F-PhOH in the solvents. NMR and Fourier transform infrared (FTIR) results demonstrate that CO2 reacts with EG in [DBUH][4-F-PhO]-EG or [DBUH][4-F-PhO]-4-F-PhOH-EG by forming a carbonate species, while [DBUH][4-F-PhO]-4-F-PhOH binary mixtures are chemically inert to CO2. NMR analysis and theoretical calculations evidence that the strength of hydrogen bonds between [4-F-PhO]− and hydrogen-bond donors (EG and 4-F-PhOH) governs the CO2 absorption behaviors, and the strength of the hydrogen bond between the anion [4-F-PhO]− and 4-F-PhOH is much stronger than that between [4-F-PhO]− and EG. Moreover, the desorption behaviors of the DESs studied can also be controlled by tuning the strength of the hydrogen bonds in the solvents. This work highlights the important role of hydrogen bonds in CO2 capture, which may be useful for the rational design of efficient solvents for carbon capture in the future.