In this work， a teaching experiment was designed to tune the carbon capture performance of deep eutectic solvents （DESs） by using hydrogen bonds. In the experiment， tetraethylammonium 4-fluorophenolate ([Et 4 N] [4-F-PhO]) was used as the hydrogen bond acceptor， and ethylene glycol （EG） and 4-fluorophenol （4-F-PhOH） were used as hydrogen bond donors to synthesize a variety of DESs for CO 2 capture. The CO 2 capacities of DESs were measured， and the CO 2 capture mechanisms by DESs were investigated by using nuclear magnetic resonance （NMR） and Fourier transform infrared （FTIR） spectra. The results indicated that the strength of hydrogen bonds between the anion [4-F-PhO] ─ and hydrogen bond donors had an important influence on the CO 2 capacity of DESs. This experiment is combined with hot topics， which will help stimulate students’ interest in scientific research and improve their practical operation ability.

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-FPhOH). The DESs are obtained by mixing [DBUH][4-F-PhO] with ethylene glycol (EG) or 4-F-PhOH. Surprisingly, [DBUH][4F-PhO]-EG DESs present a much higher CO2 capacity (similar to 1.0 mol CO2/mol solvent) than [DBUH][4-F-PhO]-4-F-PhOH (similar to 0.10 mol CO2/mol solvent) at 25 degrees 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]-4F-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.

Herein, tetraethylammonium 1,2,3-triazolide ([Et4N][Tz]), 1,2,3-triazole (Tz), and ethylene glycol (EG) are used to form DESs for CO2 capture. Surprisingly, [Et4N][Tz]-EG DESs can react with CO2, but [Et4N][Tz]-Tz cannot react with CO2, although both of the two systems contain the same anion [Tz]-. Unexpectedly, with the addition of EG to [Et4N][Tz]-Tz, the formed ternary DESs [Et4N][Tz]-Tz-EG can react with CO2, although neither EG nor [Et4N][Tz]-Tz can react with CO2 before the combination of them. NMR, FTIR and theoretical calculation results disclose that the surprise CO2 absorption behavior mainly depends on the strength of hydrogen bonds (H-bonds) between the anion [Tz]- and H-bond donors (EG or Tz). The strength of the H-bond between [Tz]- and Tz is much stronger than that between [Tz]- and EG. The strong H-bond between [Tz]- and Tz in [Et4N][Tz]-Tz greatly reduces the basicity of [Tz]-, rendering the anion [Tz]- unreactive to CO2. In [Et4N][Tz]-Tz-EG ternary DESs, EG competes with Tz to form a H-bond with [Tz]-, which weakens the strength of the H-bond between [Tz]- and Tz. Moreover, H-bonds also impact the desorption behavior. [Et4N][Tz] : EG (1 : 2) is regenerated at 60 °C, whereas the chemisorbed CO2 by [Et4N][Tz] : Tz : EG (1 : 2 : 2) can be released even down to 30 °C.

In this work, we report the SO2 capture by 2-pyridineethanol (2-PyEtOH). 2-PyEtOH exhibits a high SO2 capacity, up to 1.16 g SO2 per g solvent at 1.0 atm. The effect of temperature and pressure on the SO2 absorption by 2-PyEtOH is investigated. It is found that 2-PyEtOH can capture 0.57 g SO2 per g solvent even at a low SO2 partial pressure of 0.10 atm. Interestingly, the absorbed SO2 by 2-PyEtOH can be released at a low temperature of 50 °C, suggesting that 2-PyEtOH not only has a high capacity but also exhibits a regeneration process of low energy cost. Moreover, 2-PyEtOH also exhibits an excellent reversibility. The NMR and FTIR studies disclose that SO2 reacts with the -OH group of 2-PyEtOH, resulting in the formation of a zwitterionic sulfite. We believe that the findings of this work will be very useful for the design of efficient absorbents for SO2 capture.

Deep eutectic solvents (DESs) formed by bio-phenol-derived superbase ionic liquids (ILs) and ethylene glycol (EG) exhibit a high CO2 capacity, up to 1.0 mol CO2/mol DESs, which is much better than those of the parent ILs. Surprisingly, mechanism results indicate that CO2 reacts with EG, but doesn't react with phenolic anions in the solvent, which is different from other DESs formed by superbase ILs and EG. The reaction pathway between CO2 and DESs used in this work may include two steps. The first step is the acid-base reaction between the phenolic anion and EG, which forms HO-CH2-CH2-O-, and then CO2 is attached to the anion HO-CH2-CH2-O- to form a carbonate species.

Recently, deep eutectic solvents (DESs), a new type of solvent, have been studied widely for CO2 capture. In this work, the anion-functionalized deep eutectic solvents composed of phenol-based ionic liquids (ILs) and hydrogen bond donors (HBDs) ethylene glycol (EG) or 4-methylimidazole (4CH3-Im) were synthesized for CO2 capture. The phenol-based ILs used in this study were prepared from bio-derived phenols carvacrol (Car) and thymol (Thy). The CO2 absorption capacities of the DESs were determined. The absorption mechanisms by the DESs were also studied using nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR), and mass spectroscopy. Interestingly, the results indicated that CO2 reacted with both the phenolic anions and EG, generating the phenol-based carbonates and the EG-based carbonates, when CO2 interacted with the DESs formed by the ILs and EG. However, CO2 only reacted with the phenolic anions when the DESs formed by the ILs and 4CH3-Im. The results indicated that the HBDs impacted greatly on the CO2 absorption mechanism, suggesting the mechanism can be tuned by changing the HBDs, and the different reaction pathways may be due to the steric hinderance differences of the functional groups of the HBDs.