Highly efficient and reversible CO₂ capture through 1,1,3,3-tetramethylguanidinium imidazole ionic liquid

Abstract

1,1,3,3-Tetramethylguanidinium imidazole ([TMG][IM]) ionic liquid was synthesized and its absorption and desorption of CO₂ were investigated, as well as the effect of reaction temperature, N₂, and moisture on capture of CO₂ and the absorption mechanism. The results show that the maximum molar ratio of CO₂ to [TMG][IM] is to achieve 1.01 at 30 °C under atmosphere pressure with a high absorption rate. The capture capability is not obviously affected by the moisture of CO₂, but decreases with temperature increasing from 30 to 50 °C. The absorption mechanism might be that CO₂ reacts with the imidazole anion of [TMG][IM] and then a new carbamate group is formed, which is confirmed by IR and NMR spectra. The enthalpy of CO₂ absorption for [TMG][IM] is −30.3 kJ mol⁻¹. The captured CO₂ can be readily released at 65 °C by bubbling N₂ and recycled with little loss of its capture capability. Considering the efficient and reversible process with [TMG][IM], this method has great potential for the capture of CO₂.

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Introduction

The excessive emission of carbon dioxide (CO₂) has resulted in climate change and it has increasingly affected the life of people on Earth.¹ On the other hand, CO₂ is an important chemical intermediate or supercritical solvent in industry and needs to be separated and recycled.^2,3 Accordingly, the development of novel sorbent material for CO₂ capture is of great significance.⁴ As one promising sorbent and separation candidate media, ionic liquids (ILs) have received great attention in recent years due to their properties, such as high thermal stability, negligible vapor pressure, and tunable physicochemical characteristics.⁵ Related research shows that the CO₂ absorption capacity of ILs is related to their species. For traditional ILs, experimental studies with CO₂ have revealed that it is mainly a physical adsorption process and the absorption capacity is obviously low. Recently, a series of task-specific ILs containing amine-functionalized groups have been synthesized and demonstrated to have much higher capacities for CO₂ due to their reactivity with CO₂.⁶ For example, Davis group proposed a new strategy for the chemical absorption of CO₂ under atmosphere pressure by amine-functionalized cation-based ILs.⁷ In addition to use the functionalized cations ILs as sorbent materials to capture CO₂, some amine-functionalized anion-tethered ILs^8,9 and amino acid anions ILs were also employed to capture CO₂ under atmosphere pressure.¹⁰ Compared with the traditional ILs, the absorption capacity of the task-specific ILs has a certain increase. However, the existence of functionalized groups in the task-specific ILs gives rise to an increase in viscosity, which will slow the sorption kinetics of CO₂ and lead to the formation of solid or highly viscous gel products, and eventually affect the absorption capacity.^6,10,11 Consequently, the maximum capacity of task-specific ILs is generally limited to about 0.50 mol CO₂ per mole of IL under atmosphere.¹² To enhance absorption capacity of ILs, some alternative technologies and approaches were developed,^12–18 as well as the novel ILs were prepared.^19–23 For example, a series of novel phenolic ILs were designed for the efficient and reversible capture of CO₂ by Wang and coworkers, and the maximum capacity was to achieve 0.90 mol CO₂ per mole of IL at 30 °C under atmosphere.²² The important feature of those phenol-based ILs is that much higher absorption capacities for CO₂ and thermal stability. However, the preparation process of the above ILs is relatively complex and the cost of raw materials such as trihexyl(tetradecyl)phosphonium bromide is high, which might limit its application in industrial processes. Consequently, the development of the efficient and reversible CO₂ capture methods using ILs with a higher CO₂ absorption capacity, simple preparation process, and low cost are highly desired.

Recently, guanidinium-based ILs (GBILs) have been widely applied in sulfur dioxide (SO₂) absorption^24–28 and organic synthesis reaction.^29,30 For example, Li and coworkers reported the efficient and reversible capture of SO₂ by three new GBILs as absorbents including [1,1,3,3-tetramethylguanidinium][phenol] ([TMG][PHE]), [1,1,3,3-tetramethylguanidinium][2,2,2-trifluoroethanol] ([TMG][TE]) and [1,1,3,3-tetramethylguanidinium][imidazole] ([TMG][IM]).²⁸ The results showed that the saturated molar ratio of SO₂ to GBILs varied from 2.24 to 3.16 at 40 °C and the absorbed SO₂ could be removed under vacuum at 100 °C. The above-mentioned GBILs exhibit excellent capture capability of SO₂ and other advantages such as easy preparation, low viscosity, and high thermal stability, which encourage us to consider their capture capability of CO₂. Although Li and coworkers²⁸ briefly mentioned the absorption of CO₂ by GBILs, the information on CO₂ absorption performance by GBILs is still insufficient when compared with SO₂ absorption by GBILs. Moreover, the issues associated with the absorption mechanism, the effect of reaction temperature, N₂ and moisture on the CO₂ absorption of GBILs, and the recycling of GBILs were not investigated. To study the absorption behavior of CO₂ by GBILs in more detail as well as the absorption mechanism, therefore, in this manuscript, [TMG][IM] was synthesized and chosen as an example because of its more excellent capture capability of SO₂ than other GBILs, and the CO₂ absorption behavior of [TMG][IM] as well as the effect of reaction temperature, N₂, and moisture on CO₂ absorption, the absorption mechanism, and the desorption were investigated.

Experimental

Materials

1,1,3,3-Tetramethylguanidine (TMG) (99%, Sigma-Aldrich), trihexyl(tetradecyl)phosphonium bromide ([P₆₆₆₁₄][Br]) (>95%, Sigma-Aldrich), and imidazole (99.5%, Sinopharm Chemical Reagent Co. Ltd.) were used without further purification. Carbon dioxide (99.9%) and high purity nitrogen (99.999%) were purchased from Shanghai Pu Jiang Specialty Gases Co. Ltd.

Synthesis and characterization of [TMG][IM]

[TMG][IM] was obtained by direct neutralization of TMG and imidazole at room temperature (see Scheme 1), and dried under vacuum at 60 °C for at least 24 h before use. The specific procedures for [TMG][IM] preparation were similar to those reported previously.^24,28 The water content in [TMG][IM] determined by Karl Fischer titration (Mettler Toledo DL32, Switzerland) was 180 ppm. Trihexyl(tetradecyl)phosphonium imidazole ([P₆₆₆₁₄][IM]) was prepared according to the literature,²⁰ and its water content was 150 ppm.

Scheme 1 Molecular structure and synthesis of [TMG][IM].

FT-IR spectra of CO₂-free and CO₂-absorbed [TMG][IM] were measured on Nicolet 6700 infrared spectrometer with ATR accessory. ¹H NMR and ¹³C NMR chemical shifts δ of CO₂-free and CO₂-absorbed [TMG][IM] or [P₆₆₆₁₄][IM] were recorded on a Bruker AVANCE III 400 MHz NMR spectrometer using CDCl₃ as solvent and tetramethylsilane (TMS) as an internal reference. To obtain accurate chemical shifts, ¹H NMR of CO₂-free [TMG][IM] were measured through the internal reference method according to our previous work.³¹ NMR chemical shifts δ of CO₂-free [TMG][IM] were given as follows: ¹H NMR (400 MHz; CDCl₃; TMS): δ = 2.65 (12H, s, CH₃), 7.12 (2H, s, CH), 7.79 (1H, s, CH), 9.79 ppm (2H, s, NH₂); ¹³C NMR (400 MHz; CDCl₃; TMS): δ = 39.0, 121.9, 135.7, 167.0 ppm, which were consistent to the chemical shifts data of [TMG][IM] reported previously.²⁸ NMR chemical shifts δ of CO₂-free [P₆₆₆₁₄][IM] were given in ESI data.†

CO₂ absorption and desorption

The experiments on CO₂ absorption of [TMG][IM] or [P₆₆₆₁₄][IM] were carried out under atmospheric pressure at various temperatures, and the experimental diagram was illustrated in Scheme 2. In a typical CO₂ absorption, CO₂ at atmospheric pressure was bubbled through [TMG][IM] about 1.00 g in a glass container with an inner diameter of 10 mm at a flow rate of about 60 ml min⁻¹. The glass container was partly immersed in an oil bath at the desired temperature. The amount of CO₂ absorbed was determined at regular intervals by an electronic balance with an accuracy of ±0.1 mg. The effect of N₂ on CO₂ absorption of [TMG][IM] was carried out at ambient pressure with a CO₂ concentration of around 15%. The desorption was carried out at 65 °C by bubbling N₂ at a flow rate of about 20 ml min⁻¹ through the CO₂-absorbed [TMG][IM].

Scheme 2 Experimental diagram for CO₂ absorption of [TMG][IM]. (a) CO₂ gas cylinder; (b) anhydrous calcium chloride dryer; (c) rotary flow meter; (d) temperature controller; (e) oil bath; (f) magnetic stirrer; (g) glass container; (i) pressure gauge; 1, 2, 3, 4, valves.

Effect of moisture on the absorption

The effect of moisture on CO₂ absorption of [TMG][IM] was carried out at ambient pressure and 30 °C. Firstly, the CO₂ saturated with water vapour was bubbled through [TMG][IM] at a flow rate of about 60 ml min⁻¹. The amount of CO₂ and water absorbed by [TMG][IM] was determined at regular intervals by an electronic balance with an accuracy of ±0.1 mg. Secondly, the N₂ saturated with water vapour was bubbled through another the same amount of [TMG][IM] at a flow rate of about 60 ml min⁻¹, and the increment was also determined at regular intervals. The difference between the increment of [TMG][IM] with the CO₂ saturated with water vapour and the N₂ saturated with water vapour could be calculated, which was the CO₂ absorption of [TMG][IM] under the influence of moisture.

Results and discussion

The behaviors of CO₂ absorption at different temperatures

The effect of reaction temperatures on CO₂ absorption of [TMG][IM] under atmospheric pressure was investigated, and the results were shown in Fig. 1. It was clear that the influence of reaction temperatures on CO₂ absorption was significant. As seen in Fig. 1, CO₂ absorption capability decreased steadily as temperature increased from 30 to 50 °C, while capture rate increased as temperature increased. For example, at the beginning of the absorption experiment at 30 °C, the amount of CO₂ in the [TMG][IM] increased almost linearly with time, and the absorption equilibrium was reached within 40 min, showing a high CO₂ absorption rate. The molar ratio of absorbed CO₂ to original [TMG][IM] at 30 °C exceeded 1.00, which was the theoretical maximum for chemical absorption of CO₂, indicating that [TMG][IM] could absorb CO₂ by both chemical and physical interactions. When the absorption temperature was up to 45 °C, the absorption equilibrium was reached more rapidly within 10 min, however, the capture capability reduced to about 0.55 mol CO₂ per mole [TMG][IM]. Therefore, high temperature is not favorable for CO₂ absorption, indicating that the captured CO₂ at low temperature can be easily stripped out partially by heating the CO₂-absorbed [TMG][IM] system. As shown in Fig. 1, the CO₂ absorption capacity of [TMG][IM] is higher than that reported in the literature, which is 0.405 mol CO₂ per mole [TMG][IM] at 20 °C.²⁸ Moreover, rapid CO₂ absorption is a distinct advantage of this method, which may be related in part to the low viscosity of [TMG][IM] (5.4 mPa s at 40 °C).²⁸

Fig. 1 CO₂ absorption of [TMG][IM] at different temperatures under atmospheric pressure; ■, 30 °C; ●, 35 °C; ▲, 40 °C; ▼, 45 °C; ♦, 50 °C.

Mechanism of CO₂ absorption

As seem in Fig. 1, the molar ratio of absorbed CO₂ to original [TMG][IM] at 30 °C exceeded 1.00, which is the theoretical maximum for chemical absorption of CO₂, indicating that both chemical and physical absorption are present. Therefore, it is necessary to study the mechanism of CO₂ absorption. It is known that chemical interaction between [TMG][IM] and CO₂ could be observed in both FT-IR and ¹³C NMR spectra of CO₂-free and CO₂-absorbed [TMG][IM] samples. As seen in Fig. 2, the CO₂-absorbed [TMG][IM] sample shows new absorption bands at 1703.65 and 1291.80 cm⁻¹ at 30 °C compared to the CO₂-free [TMG][IM] sample, which can be assigned to stretching vibrations of C=O and C–O of carbonate group, respectively.^10,20 Furthermore, as shown in Fig. 3, ¹³C NMR spectrum of CO₂-absorbed [TMG][IM] sample at 30 °C shows a new resonance signal at 161.4 ppm compared to the CO₂-free [TMG][IM] sample, which can be attributed to carbamate carbonyl carbon and is in agreement with the reported data by Wang et al.²⁰ As the combination of FT-IR and ¹³C NMR spectra of CO₂-free and CO₂-absorbed [TMG][IM] confirm that there are existed the chemical interactions between [TMG][IM] and CO₂. Based on previous reports^{20–22,32,33} and the observed product, the possible CO₂ absorption mechanism of [TMG][IM] can be proposed that CO₂ reacts with imidazole anion of [TMG][IM], and then a new carbamate group is formed, which is shown in Scheme 3.

Fig. 2 FT-IR spectra of [TMG][IM] before and after CO₂ absorption at 30 °C.

Fig. 3 ¹³C NMR spectra of [TMG][IM] before and after CO₂ absorption at 30 °C.

Scheme 3 The possible CO₂ absorption mechanism of [TMG][IM].

It is an interesting thing to make clear the absorption mechanism of [TMG][IM]. If CO₂ reacts with the –NH₂ group on the cation of [TMG][IM], then a new –CO₂H group will be formed, which will be detected by FT-IR and ¹H NMR spectra. However, the FT-IR spectra of CO₂-absorbed [TMG][IM] show no evidence of the presence of the –CO₂H group (Fig. 2). To further confirm whether the –CO₂H group is formed or not in CO₂-absorbed [TMG][IM], as well as the CO₂ absorption mechanism of [TMG][IM], ¹H NMR spectrum of CO₂-absorbed [TMG][IM] sample at 30 °C is measured and compared with that of CO₂-free [TMG][IM] sample, which is shown in Fig. 4. As seen in Fig. 4, there is no new resonance signal of the –CO₂H group appearing. The results of the CO₂ absorption for other GBILs also showed that the cation of GBILs did not react with CO₂. For example, the experimental result of the solubilities of CO₂ in 1,1,3,3-tetramethylguanidium lactate ([TMG]L) indicated that the CO₂ absorption of [TMG]L could be due to physical absorption and the –NH₂ group in the cation of [TMG]L might not react with CO₂.³⁴

Fig. 4 ¹H NMR spectra of [TMG][IM] before and after CO₂ absorption at 30 °C.

To validate the CO₂ absorption mechanism of [TMG][IM], [P₆₆₆₁₄][IM] was prepared and its CO₂ absorption capacity was measured at 30 °C, which is about 0.9 mol CO₂ per mole [P₆₆₆₁₄][IM] (ESI data, Fig. S1†). By analyzing ¹³C NMR data of CO₂-free and CO₂-absorbed [P₆₆₆₁₄][IM] listed in ESI data,† it can be found that there is a new carbonyl carbon about 160.3 ppm in CO₂-absorbed [P₆₆₆₁₄][IM], indicating that [P₆₆₆₁₄][IM] can react with CO₂. Because there have no functional groups in the cation of [P₆₆₆₁₄][IM], the imidazole anion of [P₆₆₆₁₄][IM] has high reactive activity with CO₂. Therefore, according to FT-IR, ¹³C NMR, and ¹H NMR spectra of CO₂-free and CO₂-absorbed [TMG][IM], we think that the possible CO₂ absorption mechanism of [TMG][IM] may be that CO₂ reacts with imidazole anion and then a new carbamate group is formed. Moreover, in contrast to CO₂ capture by amino-functionalized ILs, where two reactive place in imidazole anion reacted with one CO₂ molecule, equimolar CO₂ absorption can be achieved for [TMG][IM] according to Scheme 3.

Effect of N₂ and moisture on the absorption

In the industrial process, CO₂ was usually mixed with N₂, it was important to know the selectivity of [TMG][IM]. Thus, the effect of N₂ on CO₂ absorption of [TMG][IM] was carried out at ambient pressure with a CO₂ concentration of around 15%. As shown in Fig. 5, the CO₂ absorption capability decreased with the temperature increasing from 30 to 50 °C. Compared to Fig. 1, CO₂ absorption of [TMG][IM] with a CO₂ concentration of around 15% seemed lower than that with pure CO₂ at the same temperature. This phenomenon may be attributed to a significant decrease in the physical absorption of [TMG][IM] at low partial pressure. According to the saturated CO₂ absorption of [TMG][IM] with a CO₂ concentration of around 15% and van't Hoff's equation, the enthalpy of CO₂ absorption for [TMG][IM] was −30.3 kJ mol⁻¹ (ESI data, Fig. S2†), indicating that captured CO₂ is easy to release.

Fig. 5 CO₂ absorption of [TMG][IM] at different temperatures under atmospheric pressure with a CO₂ concentration of around 15%; ■, 30 °C; ●, 35 °C; ▲, 40 °C; ▼, 45 °C; ♦, 50 °C.

Meanwhile, the effect of moisture on CO₂ absorption of [TMG][IM] was also investigated and shown in Fig. 6. As seen in Fig. 6, moisture of the CO₂ can affect the absorption of [TMG][IM], especially at the beginning of absorption experiment. At the end of absorption experiment, the effect of moisture on CO₂ absorption capacity of [TMG][IM] is not obvious, the absorption capacity of [TMG][IM] decreases slightly with the increase the moisture of CO₂.

Fig. 6 CO₂ absorption of [TMG][IM] at 30 °C under atmospheric pressure; ■, pure CO₂; ●, CO₂ saturated with water vapour.

The recycling of [TMG][IM] for absorption

Combined with the effect of reaction temperature on the CO₂ absorption and the enthalpy of CO₂ absorption for [TMG][IM], it can be deduced that the captured CO₂ may be easy to release by heating the [TMG][IM]. Therefore, we investigated the absorption/desorption process using [TMG][IM] (Fig. 7). Results show that the captured CO₂ can be readily released by heating and bubbling N₂. For example, the captured CO₂ could be stripped out at 65 °C under bubbling N₂ at a flow rate of about 20 ml min⁻¹, and the CO₂ release was essentially completed within 30 min (Fig. 7). The respective color change of [TMG][IM] before and after CO₂ absorption at 35 °C, and stripped 65 °C under N₂ was shown in the ESI data.† Two cycles of absorption and release of CO₂ from [TMG][IM] were studied, and IR spectra were recorded and listed in Fig. 8. The IR spectra show, from bottom to up, CO₂-free [TMG][IM], CO₂-absorbed [TMG][IM] at 35 °C for 40 min, release of CO₂ at 65 °C under bubbling N₂ for 30 min, and CO₂ absorption by [TMG][IM] at 35 °C for 40 min again. As seen in Fig. 8, the new peaks at about 1700 cm⁻¹ and 1290 cm⁻¹ are visible when CO₂ is absorbed and carbamate salt is formed, while those peaks disappear when stripped at 65 °C under bubbling N₂. To further investigate the effect of recycling on [TMG][IM], five cycles of absorption and desorption were preformed, which was also shown in Fig. 7. Compared with the CO₂ absorption capability during the five cycles, it could be found that good capacity and rapid absorption rate persisted, thereby demonstrating the reversibility of CO₂ capture by [TMG][IM].

Fig. 7 The three cycles of CO₂ absorption of [TMG][IM] at 35 °C and release at 65 °C under N₂. ▲, absorption; ▼, desorption.

Fig. 8 The two cycles of absorption and release of CO₂ by [TMG][IM]. (A) CO₂-free [TMG][IM]; (B) CO₂ absorption at 35 °C; (C) stripped at 65 °C under N₂; (D) CO₂ absorption at 35 °C again.

Conclusions

In this manuscript, a highly efficient and reversible CO₂ capture method was developed using [TMG][IM] as sorbent martial. The absorption of CO₂ by [TMG][IM] occurs rapidly, and the CO₂ capture capacity is about 1.01 mol per mole [TMG][IM], which decreases slightly with the increase the moisture of CO₂. Meanwhile, temperature is found to have a strong influence on the absorption, and the capture capacity decreases with temperature increasing. The absorption mechanism might be that CO₂ reacts with imidazole anion and then a new carbamate group is formed, which is proved by IR and NMR spectra. Based on the results of the effect of N₂ on the CO₂ absorption for [TMG][IM] from 30 to 50 °C, the enthalpy of CO₂ absorption for [TMG][IM] was obtained. Furthermore, the captured CO₂ is easy to release and recycle with a sight loss of its absorption capacity.

Acknowledgements

The authors are grateful for the financial supports from the National Natural Science Foundation of Zhejiang Province (no. Y4090453) and Undergraduate Scientific and Technological Innovation Project of Zhejiang Province (no. 2013R426004).

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Footnote

† Electronic supplementary information (ESI) available: NMR data and CO₂ absorption of [P₆₆₆₁₄][IM]. Variation in the natural logarithm equilibrium constant of [TMG][IM] with temperature. Appearance of [TMG][IM] before and after CO₂ absorption at 35 °C, and stripped at 65 °C under N₂. See DOI: 10.1039/c3ra47524g

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