Interaction between CO2 and TBAE-EG (Hindered Amine 2-[(1,1-dimethylethyl)amino]ethanol and Ethylene Glycol)

“The interaction between CO2 and the TBAE-EG system are characterized using NMR and FTIR spectra. The 1H and 13C NMR spectra of TBAE-EG system before and after CO2 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 1H 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 13C NMR spectrum after absorption (Figure 1b). The new peaks in the NMR spectra cannot be explained if CO2 reacted with the -OH group of TBAE to form zwitterionic carbonate species.”


Figure 1. NMR spectra of TBAE-EG system before and after CO2 absorption: (a1H NMR; (b13C NMR.”

“In order to explain the new peaks in the NMR spectra, the 2D NMR spectra of 1H-13C Heteronuclear Single Quantum Coherence (HSQC) and 1H-13C Heteronuclear Multiple Bond Correlation (HMBC) of the TBAE-EG system after CO2 uptake were studied. As shown in 1H-13C HSQC spectroscopy (Figure 2a), H-1 and H-2 correlated with C-1 and C-2, respectively. In the 1H-13C 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-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 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.”


Figure 2. The 2D NMR spectra of TBAE-EG after CO2 absorption: (a1H-13C Heteronuclear Single Quantum Coherence (1H-13C HSQC); (b1H-13C Heteronuclear Multiple Bond Correlation (1H-13C HMBC).”

“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.”


Figure 3. The FTIR spectra of TBAE-EG before and after CO2 absorption.”

“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-CH2-CH2-O, the conjugate base of EG. The equilibrium constant (Keq) of the acid-base reaction can be obtained using the following equations: (see [18,21]).

pKeq = pKa (EG) − pKa ([TBAEH]+) = 4.0.
Keq = 1.0 × 10−4


Scheme 2. The possible reaction pathway for the reaction between CO2 and TBAE-EG.”

In the second step, the anion HO-CH2-CH2-O reacted with CO2 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 (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 1,5-diazabicyclo [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 1H and 13C 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 1H 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 13C 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 understandable.
The desorption of CO2 was also investigated. CO2 captured by TBAE-EG can be desorbed at 80 °C and the results were characterized by NMR and FTIR. As shown in the 1H and 13C NMR spectra of TBAE-EG after CO2 desorption (Figure S2), the peaks of carbonate anions cannot be observed, suggesting CO2 was released after heating. The asymmetric and symmetric carbonyl stretching frequencies clearly disappeared in the FTIR spectra (Figure S3) after CO2 desorption, indicating again the reaction between CO2 and TBAE-EG was reversible. The results reported by Im and co-authors [18] also showed the good reversibility of TBAE-EG solvent for CO2 capture.


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