CO2 Absorption Mechanism by the Nonaqueous Solvent Consisting of Hindered Amine 2-[(1,1-dimethylethyl)amino]ethanol and Ethylene Glycol

In this work, we studied the CO2 absorption mechanism by nonaqueous solvent comprising hindered amine 2-[(1,1-dimethylethyl)amino]ethanol (TBAE) and ethylene glycol (EG). The NMR and FTIR results indicated that CO2 reacted with an -OH group of EG rather than the -OH of TBAE by producing hydroxyethyl carbonate species. A possible reaction pathway was suggested, which involves two steps. In the first step, the acid–base reaction between TBAE and EG generated the anion HO-CH2-CH2-O-; in the second step, the O− of HO-CH2-CH2-O− attacked the C atom of CO2, forming carbonate species.


Introduction
In recent decades, the amount of carbon dioxide (CO 2 ), the main heat-trapping gas, accumulated in the air, has reached an unbelievable level, which is viewed as the main contributor to global warming causing severe environmental problems such as the rising atmospheric temperature, intense heat waves and drought. The vast majority of atmospheric CO 2 is mainly emitted from industrial activities by burning fossil fuels such as coal and oil to produce electricity [1]. An urgent demand to curb the atmospheric CO 2 concentration to avoid a climate disaster has driven industry and the scientific community to explore efficient CO 2 capture technologies. A current popularly used method for CO 2 capture in industry is the amine-based scrubbing process, which mainly utilizes the aqueous solution of alkanolamine to chemically absorb CO 2 [2]. However, amine-based sorption systems have several drawbacks, such as high solvent volatility and equipment corrosion as well as high energy penalty of absorbent regeneration [3,4]. To develop new and efficient sorption systems capable of addressing the above-mentioned drawbacks is one of the main challenges in the field of carbon capture and storage.
Im and co-authors reported that the sterically hindered amine 2-[(1,1-dimethylethyl)amino]ethanol (TBAE) in EG solution could capture CO 2 by forming zwitterionic carbonate species, and they believe methylethyl)amino]ethanol (TBAE) in EG solution could capture CO2 by forming zwitterion rbonate species, and they believe that CO2 reacted with the -OH group of TBAE rather than reacte ith the -OH group of EG [18]. However, Xie and co-authors reported that CO2 reacted with EG b rming hydroxyethyl carbonate species and the zwitterionic carbonate species produced from th action between CO2 and TBAE was not the main product when CO2 was captured by the solutio TBAE in EG, which is supported by quantum chemical calculations and mass spectrometry [19 other words, the results reported by these two groups contradicted each other. Therefore, in th ork, in order to clarify the reaction between CO2 and TBAE-EG absorbent, we studied the CO sorption mechanism again using Nuclear Magnetic Resonance (NMR) and Fourier Transform frared Spectrometer (FTIR) spectroscopies. Our results suggested that CO2 reacted with the -O oup of EG with the formation of a carbonate species (Scheme 1), not with the -OH group of TBA hich was consistent with the results reported by Xie [19]. The details of our investigation can b und in the following section.

Results and Discussions
The interaction between CO2 and the TBAE-EG system are characterized using NMR and FTI ectra. The 1 H and 13 C NMR spectra of TBAE-EG system before and after CO2 absorption can be see Figure 1. As shown in Figure 1a, two new peaks at 3.17 (H-1) and 3.43 (H-2) ppm can be found i e 1 H NMR spectrum of TBAE-EG after absorption, and three new peaks appeared at 60.4 (C-1), 66 -2) and 158.6 (C-3) ppm in the 13 C NMR spectrum after absorption ( Figure 1b). The new peaks e NMR spectra cannot be explained if CO2 reacted with the -OH group of TBAE to form zwitterion rbonate species.

Results and Discussions
The interaction between CO 2 and the TBAE-EG system are characterized using NMR and FTIR spectra. The 1 H and 13 C NMR spectra of TBAE-EG system before and after CO 2 absorption can be seen in Figure 1. As shown in Figure 1a, two new peaks at 3.17 (H-1) and 3.43 (H-2) ppm can be found in the 1 H NMR spectrum of TBAE-EG after absorption, and three new peaks appeared at 60.4 (C-1), 66.5 (C-2) and 158.6 (C-3) ppm in the 13 C NMR spectrum after absorption (Figure 1b). The new peaks in the NMR spectra cannot be explained if CO 2 reacted with the -OH group of TBAE to form zwitterionic carbonate species. s 2020, 25, x FOR PEER REVIEW 2 of 7 and co-authors reported that the sterically hindered amine 2-[(1,1ylethyl)amino]ethanol (TBAE) in EG solution could capture CO2 by forming zwitterionic ate species, and they believe that CO2 reacted with the -OH group of TBAE rather than reacted e -OH group of EG [18]. However, Xie and co-authors reported that CO2 reacted with EG by g hydroxyethyl carbonate species and the zwitterionic carbonate species produced from the n between CO2 and TBAE was not the main product when CO2 was captured by the solution E in EG, which is supported by quantum chemical calculations and mass spectrometry [19]. r words, the results reported by these two groups contradicted each other. Therefore, in this in order to clarify the reaction between CO2 and TBAE-EG absorbent, we studied the CO2 tion mechanism again using Nuclear Magnetic Resonance (NMR) and Fourier Transform d Spectrometer (FTIR) spectroscopies. Our results suggested that CO2 reacted with the -OH of EG with the formation of a carbonate species (Scheme 1), not with the -OH group of TBAE, was consistent with the results reported by Xie [19]. The details of our investigation can be in the following section.

lts and Discussions
he interaction between CO2 and the TBAE-EG system are characterized using NMR and FTIR . The 1 H and 13 C NMR spectra of TBAE-EG system before and after CO2 absorption can be seen re 1. As shown in Figure  In order to explain the new peaks in the NMR spectra, the 2D NMR spectra of Heteronuclear Single Quantum Coherence (HSQC) and 1 H-13 C Heteronuclear Multiple Correlation (HMBC) of the TBAE-EG system after CO2 uptake were studied. As shown in HSQC spectroscopy (Figure 2a), H-1 and H-2 correlated with C-1 and C-2, respectively. In the HMBC spectroscopy (Figure 2b), the C-3 carbon did not correlate with the H-d hydrogen of T indicating that CO2 did not react with the -OH group of TBAE, and there was also no corre between the C-3 carbon and the H-c hydrogen of TBAE, indicating CO2 was not attached the group of TBAE. The HMBC results revealed that CO2 did not directly react with TBAE. Ther CO2 should react with EG, the other species in the TEAE-EG system, which was supported b HMBC results. As presented in Figure 2b, the C-3 carbon correlated with the H-2 hydrogen an In order to explain the new peaks in the NMR spectra, the 2D NMR spectra of 1 H-13 C Heteronuclear Single Quantum Coherence (HSQC) and 1 H-13 C Heteronuclear Multiple Bond Correlation (HMBC) of the TBAE-EG system after CO 2 uptake were studied. As shown in 1 H-13 C HSQC spectroscopy (Figure 2a), H-1 and H-2 correlated with C-1 and C-2, respectively. In the 1 H-13 C HMBC spectroscopy (Figure 2b), the C-3 carbon did not correlate with the H-d hydrogen of TBAE, indicating that CO 2 did not react with the -OH group of TBAE, and there was also no correlation between the C-3 carbon and the H-c hydrogen of TBAE, indicating CO 2 was not attached the amine group of TBAE. The HMBC results revealed that CO 2 did not directly react with TBAE. Therefore, CO 2 should react with EG, the other species in the TEAE-EG system, which was supported by the HMBC results. As presented in Figure 2b, the C-3 carbon correlated with the H-2 hydrogen and the C-1 carbon also correlated with the H-2 hydrogen, suggesting that CO 2 directly reacted with the -OH group of EG by forming a carbonate species. The peaks of C-1 and C-2 can be ascribed to the carbon atoms of the carbonate species derived from EG, which were similar to those found in the AMP-EG-based nonaqueous solution after carbon capture [9]. The C-3 carbon was the carbonyl carbon in the EG-derived carbonate species [20]. Moreover, the peak of C-b carbon attached to the N atom of TBAE shifted downfield from 50.0 to 55.8 ppm after CO 2 absorption, which indicates that the amine group of TBAE also plays a role in CO 2 capture. On the basis of the above results, it can be concluded that CO 2 reacted with the -OH of EG by producing carbonate species and the amine group of TBAE obtains a proton after CO 2 uptake.
HMBC spectroscopy (Figure 2b), the C-3 carbon did not correlate with the H-d hydrogen of TBAE, indicating that CO2 did not react with the -OH group of TBAE, and there was also no correlation between the C-3 carbon and the H-c hydrogen of TBAE, indicating CO2 was not attached the amine group of TBAE. The HMBC results revealed that CO2 did not directly react with TBAE. Therefore, CO2 should react with EG, the other species in the TEAE-EG system, which was supported by the HMBC results. As presented in Figure 2b, the C-3 carbon correlated with the H-2 hydrogen and the C-1 carbon also correlated with the H-2 hydrogen, suggesting that CO2 directly reacted with the -OH group of EG by forming a carbonate species. The peaks of C-1 and C-2 can be ascribed to the carbon atoms of the carbonate species derived from EG, which were similar to those found in the AMP-EGbased nonaqueous solution after carbon capture [9]. The C-3 carbon was the carbonyl carbon in the EG-derived carbonate species [20]. Moreover, the peak of C-b carbon attached to the N atom of TBAE shifted downfield from 50.0 to 55.8 ppm after CO2 absorption, which indicates that the amine group of TBAE also plays a role in CO2 capture. On the basis of the above results, it can be concluded that CO2 reacted with the -OH of EG by producing carbonate species and the amine group of TBAE obtains a proton after CO2 uptake. The FTIR spectra of the TBAE-EG system with and without CO2 were also studied. Two new peaks can be observed at 1635 and 1290 cm −1 (Figure 3), which can be ascribed to the asymmetric and symmetric carbonyl stretching frequency of C=O in R-O-COO − species [6]. However, the two peaks were different from those of TBAE-CO2 adduct (1641 and 1295 cm −1 ) reported by Im and co-authors The FTIR spectra of the TBAE-EG system with and without CO 2 were also studied. Two new peaks can be observed at 1635 and 1290 cm −1 (Figure 3), which can be ascribed to the asymmetric Molecules 2020, 25, 5743 4 of 7 and symmetric carbonyl stretching frequency of C=O in R-O-COO − species [6]. However, the two peaks were different from those of TBAE-CO 2 adduct (1641 and 1295 cm −1 ) reported by Im and co-authors [18], suggesting that CO 2 was bonded to the O atom of EG, not the O atom of TBAE. Therefore, the FTIR results confirmed again the reaction between CO 2 and -OH of EG.
The FTIR spectra of the TBAE-EG system with and without CO2 were also studied. Two new peaks can be observed at 1635 and 1290 cm −1 (Figure 3), which can be ascribed to the asymmetric and symmetric carbonyl stretching frequency of C=O in R-O-COO − species [6]. However, the two peaks were different from those of TBAE-CO2 adduct (1641 and 1295 cm −1 ) reported by Im and co-authors [18], suggesting that CO2 was bonded to the O atom of EG, not the O atom of TBAE. Therefore, the FTIR results confirmed again the reaction between CO2 and -OH of EG. (1) In the second step, the anion HO-CH2-CH2-O − reacted with CO2 to form the carbonate species. On the basis of the above discussion, we think that the reaction pathway involves two steps (Scheme 2). At first, there was an acid-base reaction between TBAE and EG, forming the anion HO-CH 2 -CH 2 -O − , the conjugate base of EG. The equilibrium constant (K eq ) of the acid-base reaction can be obtained using the following equations: (see [18,21]). pK eq = pK a (EG) − pK a ([TBAEH] + ) = 4.0.
(1) It is reasonable to anticipate that the proton of the OH group of TBAE can transfer to the amino group by forming zwitterionic species (CH3)3-C-N + (H2)-CH2-CH2-O − in TBAE-EG absorbent, which may react with CO2 to form zwitterionic carbonates. However, NMR results did not show any signals of zwitterionic carbonates, suggesting that the reaction pathway forming zwitterionic carbonates was not preferable in the TBAE-EG system. In order to further confirm the reaction between CO2 and EG, we studied the reaction between CO2 and a nonaqueous solvent consisting of superbase 1,5diazabicyclo [5.4.0]-5-undecene (DBU) and EG. As reported in the literature, CO2 reacted with the -OH group of alcohol when CO2 was absorbed by the DBU-alcohol mixtures [22]. The 1 H and 13 C NMR spectra of DBU solution (30 wt%) in EG before and after CO2 absorption were shown in Figure  S1. There were two new peaks at 3.15 and 3.39 ppm in the 1 H NMR spectra of DBU-EG after absorption ( Figure S1a). Three new peaks appeared at 60.0 (C-1), 65.8 (C-2) and 157.9 (C-3) ppm in the 13 C NMR after absorption ( Figure S1b). These new peaks in the DBU-EG-CO2 system were consistent with those in the TBAE-EG-CO2 system, suggesting that CO2 reacted with EG. These results again suggested that the CO2 absorption mechanism of TBAE-EG presented in this work was Scheme 2. The possible reaction pathway for the reaction between CO 2 and TBAE-EG.
In the second step, the anion HO-CH 2 -CH 2 -O − reacted with CO 2 to form the carbonate species. It is reasonable to anticipate that the proton of the OH group of TBAE can transfer to the amino group by forming zwitterionic species (CH 3 ) 3 -C-N + (H 2 )-CH 2 -CH 2 -O − in TBAE-EG absorbent, which may react with CO 2 to form zwitterionic carbonates. However, NMR results did not show any signals of zwitterionic carbonates, suggesting that the reaction pathway forming zwitterionic carbonates was not preferable in the TBAE-EG system. In order to further confirm the reaction between CO 2 and EG, we studied the reaction between CO 2 and a nonaqueous solvent consisting of superbase 1,5-diazabicyclo [5.4.0]-5-undecene (DBU) and EG. As reported in the literature, CO 2 reacted with the -OH group of alcohol when CO 2 was absorbed by the DBU-alcohol mixtures [22]. The 1 H and 13 C NMR spectra of DBU solution (30 wt%) in EG before and after CO 2 absorption were shown in Figure S1. There were two new peaks at 3.15 and 3.39 ppm in the 1 H NMR spectra of DBU-EG after absorption ( Figure S1a). Three new peaks appeared at 60.0 (C-1), 65.8 (C-2) and 157.9 (C-3) ppm in the 13 C NMR after absorption ( Figure S1b). These new peaks in the DBU-EG-CO 2 system were consistent with those in the TBAE-EG-CO 2 system, suggesting that CO 2 reacted with EG. These results again suggested that the CO 2 absorption mechanism of TBAE-EG presented in this work was understandable.
The desorption of CO 2 was also investigated. CO 2 captured by TBAE-EG can be desorbed at 80 • C and the results were characterized by NMR and FTIR. As shown in the 1 H and 13 C NMR spectra of TBAE-EG after CO 2 desorption (Figure S2), the peaks of carbonate anions cannot be observed, suggesting CO 2 was released after heating. The asymmetric and symmetric carbonyl stretching frequencies clearly disappeared in the FTIR spectra ( Figure S3) after CO 2 desorption, indicating again the reaction between CO 2 and TBAE-EG was reversible. The results reported by Im and co-authors [18] also showed the good reversibility of TBAE-EG solvent for CO 2 capture.

Conclusions
The results indicated that CO 2 reacted with -OH group of EG in nonaqueous solvent consisting of TBAE and EG rather than reacting with -OH group of TBAE when CO 2 was captured by the mixture consisting of TBAE and EG. We believe the confirmation of the absorption mechanism is very important to the design of new nonaqueous absorption systems in the future.
The FTIR spectra were recorded on a PerkinElmer Frontier spectrometer (PerkinElmer Corp., Waltham, MA, USA). 1 H NMR (600 MHz) and 13 C NMR (151 MHz) data were obtained from a Bruker spectrometer (Bruker Biospin, Karlsruhe, Germany) and DMSO-d 6 was used as the external reference. The NMR spectrometer was equipped with a 5 mm PABBO probe. The experiment temperature was 25 • C. For the 1 H NMR experiment, the relaxation delay was 1.0 s, and the acquisition time was 2.75 s. In the 13 C NMR experiment, the relaxation delay was 2.0 s, and the acquisition time was 0.92 s. For the HMBC experiment, the relaxation delay was 1.5 s, and the acquisition time was 0.18 s. For the HSQC experiment, the relaxation delay was 1.5 s, and the acquisition time was 0.18 s.

Absorption of CO 2
The method of CO 2 absorption was similar to that reported by Im and co-authors. TBAE solution (30 wt%) in EG (~2 g) was added into a glass tube with a diameter of 10 mm equipped with a rubber lid. At first, the tube was partially placed into an oil bath at 100 • C and N 2 (50 mL/min) was bubbled into the solution through a needle for 10 min to remove any volatile compounds. Then, CO 2 (50 mL/min) was bubbled into the solution in the tube at 40 • C for 60 min, and the weight of the tube during absorption was recorded using a balance (±0.1 mg).
Supplementary Materials: The following are available online. Figure S1: The 1 H (a) and 13 C NMR (b) spectra of DBU-EG before and after CO 2 absorption, Figure S2: The 1 H (a) and 13 C NMR (b) spectra of TBAE-EG after CO 2 desorption, Figure S3: The FTIR spectra of TBAE-EG after CO 2 desorption.
Author Contributions: Investigation, R.L.; data curation, R.L. and C.W.; writing-original draft preparation, C.W. and D.Y.; writing-review and editing, C.W. and D.Y.; supervision, D.Y.; funding acquisition, D.Y. All authors have read and agreed to the published version of the manuscript.