CO 2 Absorption Mechanism by Diamino Protic Ionic Liquids (DPILs) Containing Azolide Anions

: Protic ionic liquids have been regarded as promising materials to capture CO 2 , because they can be easily synthesized with an attractive capacity. In this work, we studied the CO 2 absorption mechanism by protic ionic liquids (ILs) composed of diamino protic cations and azolide anions. Results of 1 H nuclear magnetic resonance (NMR), 13 C NMR, 2-D NMR and fourier-transform infrared (FTIR) spectroscopy tests indicated that CO 2 reacted with the cations rather than with the anions. The possible reaction pathway between CO 2 and azolide-based protic ILs is proposed, in which CO 2 reacts with the primary amine group generated from the deprotonation of the cation by the azolide anion.


Introduction
In recent decades, the amount of carbon dioxide (CO 2 ) accumulated in the air has reached unbelievable levels, 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 (coal and oil) to produce electricity [1]. An urgent demand to curb the atmospheric CO 2 concentration to avoid climate disaster has driven industry and the scientific community to explore efficient CO 2 capture technologies. A current, popular method used for CO 2 capture in industry is the amine-based scrubbing process, which mainly utilizes an 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 and a high energy penalty of absorbent regeneration [3]. Developing 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.
During the past decade, ionic liquids have been widely studied for CO 2 capture because of their attractive properties [4], such as negligible vapor pressure, high thermal stability, and tunable structures [5,6]. Among the ILs used for CO 2 absorption, aprotic ILs [7][8][9], such as azolide-based [10] and hydroxypyridine-based ILs [11], exhibit high CO 2 capacity. However, tedious procedures are needed to synthesize these aprotic ILs, resulting in high costs. Recently, protic ILs [12][13][14] have been investigated to capture CO 2 because they can be easily prepared and exhibit promising capacity.
In a recent article, Oncsik and co-authors reported on CO 2 capture by diamino protic ionic liquids (DPILs) formed by N,N-dimethylethylenediamine (DMEDA) with azoles, including imidazole (Im), 1,2,4-triazole (Tz) and pyrazole (Py). These DPILs showed a high gravimetric absorption capacity for CO 2 [15]. The authors also investigated CO 2 absorption mechanisms of these DPILs. On the basis of NMR and FTIR results, they believed that CO 2 reacted with anions, forming carbamate species, and CO 2 did not react with the diamino cations. However, in contrast, we found that CO 2 reacted with the cations rather than with the azolide anions when CO 2 was captured by these DPILs (Scheme 1). The details are presented in the following sections.

Results and Discussion
Primarily, the CO 2 capacities of the protic ILs were investigated. [ (Table S1), suggesting that the protic ILs used in our study were successfully prepared. The absorption capacity of DMEDA was determined by using the DMEDA solution in sulfolane (30 wt.%). One mole of DMEDA could capture 0.90 mole of CO 2 at 1.0 atm and 22 • C. Moreover, the structure of the IL was further studied using NMR spectra. As shown in Figure 1A, the hydrogen peak of -NH 3 (H-4) can be clearly identified in the 1 H NMR spectra of [DMEDAH] [Im], and there was no N-H peak of imidazole in the spectra. The -NH 2 peak of DMEDA was also completely missing from the 1   In order to study the absorption mechanism, we investigated the 1 H NMR and 13 C NMR spectra of [DMEDAH] [Im] before and after CO 2 absorption. As shown in Figure 1A, there were several new peaks (H-1 , H-3 , and H-c') in the 1 H NMR spectrum after CO 2 absorption. H-c' (11.6 ppm) was the N-H hydrogen on the imidazole ring ( Figure S1). Additionally, four new peaks (C-1 , C-2 , C-3 , and C-4 ) can be observed in the 13 C NMR spectrum after CO 2 absorption. C-4 (161.9 ppm) was the peak of carbamate carbon [16,17]. The new peaks in the 1 H NMR (H-1 , H-2 , and H-3 ) ( Figure S2A) and 13 C NMR (C-1 , C-2 , C-3 , and C-4 ) ( Figure S2B) spectra after CO 2 absorption were more obvious when deuterium oxide (D 2 O) was used as the internal solvent to record the NMR spectra.
It would be difficult to explain these new peaks if CO 2 only reacted with the anion [Im] − ; thus, the 1 H-13 C Heteronuclear Singular Quantum Correlation (HSQC) spectra ( Figure 2) and 1 H-13 C Heteronuclear Multiple Bond Correlation (HMBC) spectra ( Figure 3) of [DMEDAH] [Im] after CO 2 absorption were studied in order to identify these new peaks. As can be seen in Figure 2A, H-1 was attached to C-1 , H-2 was attached to C-2 , and H-3 was attached to C-3 . As shown in Figure 3A Figure S6) after CO 2 absorption, correlation between H-3 and C-4 can also be observed, which again suggested that CO 2 was attached to the cation. In order to further confirm the mechanism, the FTIR spectra of [DMEDAH] [Im] with and without CO 2 were investigated. As can be seen in Figure 4, a new peak at 1676 cm −1 , attributed to the C=O stretching of the carbamate, could be identified after CO 2 absorption. However, the C=O stretching peak of the carbamate formed by the reaction between [Im] − and CO 2 was near 1700 cm −1 [18,19]. Therefore, the peak at 1676 cm −1 implied that CO 2 was attached to the amino group in the cation and not attached to the anion [Im] − . The N-H band related to [DMEDAH] + at 1584 cm −1 shifted to 1575 cm −1 after CO 2 absorption. The stretching vibration of N-COO − could be observed at 1310 cm −1 after the reaction, which was different from the N-COO − stretching band (~1293 cm −1 ) of the carbamate formed by the anion [Im] − and CO 2 [18]. These results again confirmed the interaction between CO 2 and the [DMEDAH] + . Furthermore, we also studied the FTIR spectra of DMEDA solution (30 wt.%) in sulfolane (Sulf) before and after CO 2 uptake. As shown in Figure 5, an obvious peak at 1679 cm −1 can be observed after CO 2 uptake, which was the C=O stretching peak of the carbamate formed by CO 2 and the amnio group of the DMEDA, indicating that the peak at 1676 cm −1 of the [DMEDAH] [Im] + CO 2 system was from the DMEDA-based carbamate. These FTIR results again indicated that CO 2 reacted with the cation rather than the anion of [DMEDAH] [Im].   The possible reaction pathway between CO 2 and [DMEDAH] [Im] is shown in Scheme 2. At first, there was an acid-base reaction between the cation and the anion in the ILs. The cation was deprotonated by the imidazolate anion, generating an amino group. When the absorbent interacted with CO 2 , the final product was formed through nucleophilic addition of the amino group to CO 2 [20,21].

Synthesis of Ionic Liquids
DMEDA and azole (1:1, molar ratio) were mixed in a glass vial. Each mixture was stirred at room temperature for 2 h to obtain a homogenous liquid.

CO 2 Absorption
An ionic liquid (~1.0 g) was added to a glass tube with a diameter of 10 mm. The tube was equipped with a rubber lid and two needles. One needle was a CO 2 inlet, and the other one was a CO 2 outlet. The glass tube was partially immersed in a water bath (22 ± 0.2 • C). CO 2 was bubbled into the IL through a needle at a flow rate of~50 mL/min for 60 min. The weight of the tube was measured every 10 min with an analytical balance (±0.1 mg). The mass increase was attributed to the CO 2 captured by the IL.

Conclusions
In summary, the CO 2 absorption mechanism by the protic azolide ILs based on DMEDA has carefully been studied through various NMR and FTIR experiments. The results indicated that CO 2 reacted with the cations rather than with the azolide anions. We believe that the confirmation of the absorption mechanism is very important to the design of protic ILs in the future for CO 2 capture and utilization.