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

https://doi.org/10.3390/molecules25235743

“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: (a) ¹H NMR; (b) ¹³C 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: (a) ¹H-¹³C Heteronuclear Single Quantum Coherence (¹H-¹³C HSQC); (b) ¹H-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 (K_eq) of the acid-base reaction can be obtained using the following equations: (see [18,21]).

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

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