CO2 Capture Mechanism by Deep Eutectic Solvents Formed by Choline Prolinate and Ethylene Glycol

The choline prolinate ([Ch][Pro]) as a hydrogen bond acceptor and ethylene glycol (EG) as a hydrogen bond donor are both used to synthesize the deep eutectic solvents (DESs) [Ch][Pro]-EG to capture CO2. The CO2 capacity of [Ch][Pro]-EG is determined, and the nuclear magnetic resonance (NMR) and infrared (IR) spectrum are used to investigate the CO2 capture mechanism. The results indicate that CO2 reacts with both the amino group of [Pro]− anion and the hydroxyl group of EG, and the mechanism found in this work is different from that reported in the literature for the [Ch][Pro]-EG DESs.


Introduction
Carbon dioxide (CO 2 ) emissions have increased at an unbelievable rate, which causes great concern in global society and results in increasing atmospheric temperature and rising sea levels. The unprecedented amounts of atmospheric CO 2 are mainly emitted from the combustion of fossil fuels in industry [1][2][3]. An urgent demand to reduce the rising levels of CO 2 has driven different industries and fields to explore efficient CO 2 capture technologies. The current prominent commercial method for CO 2 capture in the industry is the alkanolamine-based scrubbing process, which mainly utilizes aqueous solutions of alkanolamines to absorb CO 2 chemically [4,5]. However, alkanolamine-based sorption systems have several inherent drawbacks, such as solvent degradations, equipment corrosion, and high absorbent regeneration energy [6,7]. Thus, developing new efficient sorption systems capable of avoiding the above-mentioned drawbacks is one of the main challenges in the field of carbon capture and storage [8].
In the past decades, as an alternative to amine-based absorbents, ionic liquids (ILs) have received a lot of attention in the CO 2 capture field because of their attractive properties, including negligible vapor pressure, high thermal and chemical stability, low flammability, and tunable structures [9,10]. To date, a number of functionalized ILs, which can chemically capture CO 2 , have been investigated for CO 2 capture. Among them, anionfunctionalized ILs, such as azolide-based and amine-based ILs, exhibit high CO 2 absorption capacities [11][12][13]. However, the main shortcoming of the functionalized ILs is their high viscosity, which limits their large-scale industrial application.
In recent years, deep eutectic solvents (DESs), emerging as a new kind of solvent, have gained significant attention because they share many features with ILs, such as very low vapor pressure and tunable properties [14,15]. At present, most DESs are formed by combining hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs), and the intermolecular hydrogen bonds formed between HBDs and HBAs induce a large depression of melting or freezing temperatures of DESs [16,17]. Due to their attractive properties, DESs have been widely investigated in many fields, including organic synthesis, catalysis, biodiesel conversion, electrochemistry, and nanotechnology [18][19][20][21][22][23].
Functionalized DESs containing basic anions in the solvents are also proposed to capture carbon. It is found that anion-functionalized DESs, consisting of EG and solid azolide ILs (tetraethylphosphonium imidazolide ([P 2222 ][Im]), tetraethylphosphonium 1,2,4-triazolide ([P 2222 ][Triz]), etc.), could chemically absorb CO 2 through the reaction between CO 2 and EG [38]. Interestingly, CO 2 absorption behaviors of anion-functionalized DESs based on 1,2,3-triazole (Tz) can be changed by tuning the strength of hydrogen bonds formed between the anion [Tz] − and HBDs (EG or Tz) [39]. The DESs composed of 1-ethyl-3-methylimidazolium 2-cyanopyrrolide ([Emim] ) and EG can also chemically capture CO 2 , and CO 2 can react with both [Emim]  and EG [40]. The CO 2 absorption by phenol-derived anion-functionalized DESs was also studied [41], and the results indicated that the steric hindrance of functional groups of HBDs greatly impacted the absorption mechanisms. For example, when CO 2 [47], and DBN-BmimCl-Im [48]. According to the results reported in the literature, it can be found that the components of DESs greatly affect the CO 2 absorption behaviors of functionalized DESs. Therefore, revealing the interactions between CO 2 and the components of DESs is of great importance to the design of efficient DESs [49].
Recently, the amino-functionalized DESs, consisting of choline prolinate ([Ch][Pro]) and EG, were prepared to capture CO 2 , and the mechanism studies suggested that CO 2 reacted with the anion [Pro] − , forming carbamate acid, but not with EG [50] (Scheme 1). In this work, we also investigate the CO 2 capture mechanism by [Ch][Pro]-EG DESs. However, based on the nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) results, we find that CO 2 can react with both the anion [Pro] − and EG (Scheme 1), and the discussion can be found in the sections below.  [50] and found in this work.

Results and Discussion
The  [Pro]-EG reported in previous work [50] and found in this work.

Results and Discussion
The   [50] and found in this work.

Results and Discussion
The     [39,42,43]. The peak at 66.4 ppm is the methylene carbon of the carbonate species derived from EG. The other methylene carbon (C-2) peak of carbonate is overlapped with the C-d carbon (61.1 ppm).   can be attributed to the stretching of the O-COO bond. [39,40] Similarly, [Ch][Pro]:EG (1:5) system, the new peaks appear at 1627 and 1292 cm −1 after CO (Figure 4b). Similar peaks can also be observed at 1623 and 1292 cm −1 for the [Ch][Pro]:EG (1:2) after capture (Figure 4c). Moreover, two new bands at 1626 a cm −1 can be found as well in the FTIR spectra of [Et4N][Pro]:EG (1:3) after CO2 ab (Figure 4d). Therefore, the FTIR results also provide evidence that CO2 reacts b the anion [Pro] − and EG. The full-window FTIR spectra of the solvents before a CO2 capture are shown in Figures S11-S15.
Molecules 2023, 28, x FOR PEER REVIEW   4%), and 0.14 (19.7%) mol CO 2 /mol DESs, respectively. The values in parenthesis are the percentages of EG-bonded CO 2 to the overall CO 2 capacity. The results indicate that EG-bonded CO 2 rise with increasing the amount of EG in the DESs. The results reported by reference [50] claimed that CO 2 did not react with EG in the DESs [Ch][Pro]:EG (1:2), probably because the intensities of the H and C peaks of EG-based carbonate in NMR spectra were very weak and these peaks can be easily overlooked if they are not carefully identified.
The FTIR spectra of the DESs used before and after CO 2 capture are shown in Figure 4. As seen in Figure 4a (Figure 4d). Therefore, the FTIR results also provide evidence that CO 2 reacts both with the anion [Pro] − and EG. The full-window FTIR spectra of the solvents before and after CO 2 capture are shown in Figures S11-S15.

Absorption of CO 2
DESs (~2 g) were added into a glass tube with an inner diameter of 10 mm. CO 2 (~50 mL/min) was bubbled into the glass tube, which was partially immersed in a water bath at the required temperature. The amount of CO 2 absorbed by solvents can be calcu-Molecules 2023, 28, 5461 8 of 10 lated by the weights of the tube before and after CO 2 absorption, which were determined by an electronic balance with an accuracy of ±0.1 mg. The weight of the tube was measured at regular intervals during the absorption process. If the weight of the tube did not change with time, the CO 2 absorption by DESs was considered to have reached saturation.

Conclusions
The CO 2 capture mechanism by DESs consisting of [Ch][Pro] and EG is studied by using NMR and FTIR methods. The NMR and FTIR results reveal that CO 2