CO₂ capture from flue gas at high temperatures by new ionic liquids with high capacity

Abstract

Room-temperature ionic liquids (ILs) are widely investigated to capture CO₂ from flue gas and can capture a large amount of CO₂. However, almost all of the previous studies on the absorption of CO₂ by ILs were carried out at low temperatures, less than 60 °C and even at room temperature. When the temperatures reach the real flue gas temperatures, from 110 °C to 140 °C, these ILs might hardly absorb CO₂, as high temperatures decrease the CO₂ capacity. Therefore, it is necessary to design new task specific ILs to capture CO₂ at flue gas temperatures with high absorption capacity. In this work, a new type of polyamine based ILs were developed to absorb CO₂ from simulated flue gas at high temperatures and ambient pressure with large absorption capacities up to 0.944 mole ratio or 0.176 mass ratio of CO₂ to IL at 110 °C. These ILs could be easily synthesized by neutralization of polyamines and organic acids, and the number of amido groups on the ILs can be easily controlled. The CO₂ absorption is influenced by temperature, CO₂ volume fraction in the flue gas, and the number of amido groups on the ILs. Also a possible mechanism for the CO₂ absorption has been proposed.

---

Introduction

Carbon dioxide, which is mainly emitted from the burning of fossil fuels, may cause global warming and climate change. The most efficient way to reduce the emission of CO₂ may be CO₂ capture after the burning of fuels. For years, many kinds of materials have been used to capture CO₂, such as methanol, diethanolamine and N-methyldiethanolamine.^1–3 Although they can capture CO₂ on a large-scale, these materials cannot be used at high temperatures because they have high volatility. Besides, the difficulty of desorption and the high viscosity during absorption may also limit the reuse of these solvents.

Due to their extremely low vapor pressure, tunable structure, high thermal and chemical stability, and excellent solvent power, ionic liquids (ILs) are regarded as a kind of novel materials for CO₂ capture. Until now, many kinds of ILs have been synthesized and used for CO₂ capture. CO₂ can physically be absorbed in normal ILs.^4–8 For example, [Bmim][PF₆] can physically absorb CO₂ at high pressures.⁴ The physical absorption of CO₂ in normal ILs depends on the pressure of CO₂. The mole ratio of CO₂ in ILs decreases when the partial pressure of CO₂ decreases. At ambient pressure, normal ILs can hardly absorb CO₂. As a result, these ILs cannot be used to capture CO₂ in flue gas. Due to these reasons, Bates et al.⁹ synthesized a task-specific IL, 1-n-propylamine-3-butylimidazolium tetrafluoroborate, which can capture CO₂ at ambient pressure. And after that many kinds of task-specific ILs were synthesized, such as tetraalkylammonium amino acids,¹⁰ tetrabutylphosphonium amino acids.^11–14 These ILs are functionalized with an amido group at the anion and/or the cation. And they can capture a large amount of CO₂ at ambient pressure.

As we know, the temperatures of flue gas are very high, for example, 110 °C. But all of the previous studies on ILs absorbing CO₂ were carried out at low temperatures less than 60 °C and even at room temperature. The increase in temperature can decrease the absorption capacities of these ILs. As a result, when temperatures reach the real flue gas ones, these ILs might hardly absorb CO₂. On the other hand, a decrease of flue gas temperatures in order to capture CO₂ by ILs costs much energy. Therefore, it is necessary to design new task specific ILs to capture CO₂ at flue gas temperatures with high absorption capacity. As we know, polyamine has a strong molecular interaction with CO₂ and can absorb a large amount of CO₂, but it is volatile especially at high temperatures. So it is possible to functionalize polyamines onto ILs to combine the extremely low volatility of ILs and the high CO₂ absorption capacity of polyamines.

In this work, a new type of polyamine based ILs were developed to absorb CO₂ from simulated flue gas at high temperatures and ambient pressure with large absorption capacities. The polyamine based ILs could be easily synthesized by neutralization of polyamines and organic acids.

Experimental

Materials

CO₂ (99.995%) and N₂ (99.99%) were supplied from Beijing Haipu Gases. Diethylene triamine, triethylene tetramine, lactic acid and trifluoroacetic acid were purchased from Shanghai Jingchun Reagent Plant (Shanghai, China). All the reagents and solvents were analytical reagents. Before the use of reagents, the water contents of them were determined by Karl Fischer titration (ZDJ-400S, Multifunctional Titrator, Beijing Xianqu Weifeng Company, Beijing, China).

Synthesis of ILs

The ILs used in this work were prepared by neutralization of polyamines in water with different acids. Taking triethylene tetramine lactate ([TETA]L) for example, the synthesis procedure is as follows: 0.5 mol of triethylene tetramine and 100 mL of water were added into a 500 mL three-necked flask. The flask was placed in an ice-water bath at 0 °C and equipped with a reflux condenser under vigorous stirring with a magnetic stirrer. 0.5 mol of lactic acid was added into a pressure funnel and was added dropwise to the flask over about 1 h. The reaction lasted for 4 h at 25 °C. The solvent was removed by evaporation under vacuum. After the drying process, the water fractions determined by Karl Fischer titration were less than 0.005. Using a similar method, we synthesized triethylene tetramine lactate ([TETA]L), triethylene tetramine trifluoroacetate ([TETA][Tfa]), diethylene triamine lactate ([DEA]L), triethylene tetramine bis(lactate) ([TETA]L₂) and diethylene triamine bis(lactate) ([DEA]L₂) for CO₂ capture.

Apparatus and procedures

The CO₂/N₂ gas mixtures, with different CO₂ contents by volume, were prepared by mixing CO₂ and N₂ at ambient pressure. The apparatus consisted mainly of a cylinder containing the pure CO₂ gas, a cylinder containing the pure N₂ gas, a test tube with an inner diameter of 12 mm and a length of 200 mm, two rotameters (Beijing Forth Automation Meter Factory, China) and a constant temperature oil bath. This absorption experiment was carried out at ambient pressure.

In a typical experiment, the pure CO₂ gas and the pure N₂ mixed together, and then went through the [TETA]L loaded in the test tube. The weight of the IL was about 3.2 g. The flow rate of each gas was monitored by rotameters and calibrated by a soap film fluid meter. The total flow rate of gas was 100 mL min⁻¹. The test tube was partly immersed into the oil bath, the temperature of which was maintained within ±1 °C. After a given time for absorption, the mass ratio of CO₂ to IL was determined by the mass difference of the test tube via an electrical balance (BS224S Sartorius) with an accuracy of ±0.0001 g. And then the mole ratio of CO₂ to IL was calculated. The expanded uncertainty of CO₂ mass ratio to IL is ±4.2%.

The desorption of CO₂ was carried at 140 °C with N₂ as the sweeping gas. The desorbing time was about 0.5 h.

In this work, we also have measured the solubility of pure N₂ in [TETA]L at 110 °C, and we found the mass ratio of N₂ to IL is less than 0.001, which can be neglected compared with the absorption amounts of CO₂.

Results and discussion

Effect of IL on the absorption of CO₂

Fig. 1 shows the absorption of pure CO₂ in the five ILs at 110 °C, indicating that all the ILs can absorb CO₂ from simulated gas, but the mole ratios of CO₂ to ILs are significantly different. At a high temperature of 110 °C, [TETA]L, [TETA][Tfa] and [DEA]L show high capacities of CO₂ absorption, from 0.54 to 0.94 mole ratios of CO₂ to IL, and from 0.12 to 0.17 mass ratios of CO₂ to IL, which are higher than that reported in the literature.^13,14 The most efficient IL is [TETA]L, which shows that the mole ratio of CO₂ to IL reaches 0.944, and the mass ratio 0.176. To the best of our knowledge, this is the highest mass ratio of CO₂ to task-specific IL. [TETA][Tfa] can also get a very high value of CO₂ absorption, which shows a mole ratio of CO₂ to IL of 0.901. When [DEA]L₂ or [TETA]L₂ was used to absorb CO₂, the mole ratio of CO₂ to IL decreased. For example, a mole ratio of CO₂ to [DEA]L in equilibrium is 0.541, and that to [DEA]L₂ is just 0.075.

Fig. 1 Absorption of CO₂ in different kinds of ILs with pure CO₂ at 110 °C: ■, [TETA]L; ●, [TETA][Tfa]; ▲, [DEA]L; ◀, [TETA]L₂; ▼, [DEA]L₂.

The key reason for the different CO₂ absorption capacities in these ILs may be the number of amido groups in the cation. With a decrease in free amido groups that are not neutralized by the acid, the mole ratio of CO₂ to IL decreases. Both [TETA]L and [TETA][Tfa] have three free amido groups, and the mole ratios of these two ILs can reach about 0.9. Both [DEA]L and [TETA]L₂ have two free amido groups, and the mole ratios of these two ILs can reach about 0.5. As compared to [TETA]L, the absorption of CO₂ in [TETA]L₂ decreased obviously. When there is only one free amido group left in the IL, [DEA]L₂, the IL has a very low absorption of CO₂ at a high temperature of 110 °C.

Effect of temperature on the absorption of CO₂

As we know, the temperature has a great effect on CO₂ absorption by ILs. In this paper, [TETA]L was employed to investigate the effect of temperature on the absorption of CO₂. Fig. 2 shows the absorption of CO₂ in [TETA]L with pure CO₂ at different temperatures. The mole ratio of CO₂ to [TETA]L increases with the decrease in temperature. For instance, when the temperature is 130 °C, the mole ratio of CO₂ to [TETA]L can reach 0.660 in equilibrium. However, when the temperature decreases to 110 °C, the mole ratio of CO₂ to [TETA]L increases to 0.944. Although the increase in temperature can decrease the absorption of CO₂, the capacity of CO₂ in [TETA]L can still reach a high value of 0.660 mol per mol IL. This is similar to the absorption of acid gases in task-specific ILs, such as SO₂.¹⁵ Due to the influence of temperature on CO₂ absorption, we can desorb CO₂ at higher temperatures to reuse the ILs.

Fig. 2 Absorption of CO₂ in [TETA]L with pure CO₂ at different temperatures: ■, 110 °C; ●, 120 °C; ▲, 130 °C.

Effect of the volume fraction of CO₂ in simulated flue gas on CO₂ absorption

Fig. 3 shows the absorption of CO₂ in [TETA]L with different volume fractions of CO₂ at 110 °C. CO₂ absorption capacity decreases with the decrease in CO₂ volume fraction in simulated flue gas. For example, when the CO₂ volume fraction in the simulated flue gas is 0.50, the mole ratio of CO₂ to [TETA]L is 0.804. When the CO₂ volume fraction is 0.15, which is close to the CO₂ concentration in real flue gas, the mole ratio of CO₂ to [TETA]L still reaches 0.547.

Fig. 3 Absorption of CO₂ in [TETA]L with different volume fractions of CO₂ at 110 °C: ■, 100%; ●, 50%; ▲, 25%; ▼, 15%.

The desorption of CO₂ in [TETA]L

In this work, the absorbed CO₂ was released from [TETA]L upon sweeping by nitrogen at 140 °C for about 0.5 h. Fig. 4 shows the absorption and desorption of CO₂ by [TETA]L for three cycles. The mole ratio of CO₂ to [TETA]L in these three cycles were 0.944, 0.922 and 0.931 after the absorption at 110 °C. Experimentally, the IL can be reused to absorb CO₂ with no obvious change in the absorption capacity and absorption rate.

Fig. 4 Absorption and desorption cycles of CO₂ in [TETA]L.

The mechanism of CO₂ absorbed in [TETA]L

To obtain some information about the chemical interaction between CO₂ and the ILs used in this work, FTIR was used to characterize the fresh IL and CO₂ absorbed IL, which is shown in Fig. 5. After absorption of CO₂, a broad peak clearly appears at 2167 cm⁻¹, which can be assigned to the C=O stretch of CO₂. Compared with the pure CO₂ FTIR spectrum (from NIST web), the wave number of the C=O stretch of CO₂ absorbed by the IL decreases from 2350 cm⁻¹. This suggests that CO₂ is chemically interacts with the IL. The peak at 1661 cm⁻¹ appears stronger after absorption, demonstrating the characteristic absorption peak of a carbamate C=O stretch.⁹ It is noted that the IL was spread on potassium bromide pellets and CO₂ from air was absorbed in the thin film of the spread IL to a small extent. Therefore, the FTIR spectrum of fresh IL also shows a little CO₂ information.

Fig. 5 FT-IR spectrum of [TETA]L before and after the absorption of CO₂: solid line, before the absorption; dashed line, after the absorption.

All the above results indicate that the new ILs can absorb CO₂ at high temperatures, which hints that the molecular interaction between the IL and CO₂ is stronger than in normal ILs that can be used only at near room temperature. But the absorbed CO₂ can be recovered by increasing the temperature, suggesting that the chemical interaction is not very strong and is reversible. The absorption capacity is greatly influenced by the volume fraction of CO₂ in flue gas, further demonstrating that a reversible chemical reaction happens for the CO₂ absorption.

Inspired by the absorption mechanism reported by Bates et al.,⁹ we propose a mechanism for the new polyamine based ILs absorbing CO₂ which is shown in Scheme 1.

Scheme 1 Proposed mechanism of the absorption of CO₂. R₁ stands for ionic liquids, R₂ stands for alkyl chains.

One CO₂ molecule interacts with two amino groups on the IL molecules to form a complex. Based on the above analysis, these ILs absorb CO₂ following the chemical equilibrium in eqn (1).

CO₂ + 2 amino-IL = CO₂(amino-IL)₂ (1)

In eqn (1), amino-IL stands for the free amino group on the IL, and CO₂(amino-IL)₂ for the complex. The chemical reaction equation has an equilibrium constant. Obviously, at a given temperature, such as 110 °C, the equilibrium constant is fixed, and CO₂ absorption capacity in the IL increases with the increase in CO₂ partial pressure. The equilibrium constant is a function of temperature. From Fig. 2, a decrease in temperature favors CO₂ absorption, which means the equilibrium constant increases with a decrease in absorption temperature. An increase in the number of amino groups in the IL can increase the amino group concentration resulting in the increase of CO₂ absorption amount. However, during desorption of CO₂via vacuum and heat treatment, the increase in temperature decreases the equilibrium constant, and the decrease in CO₂ partial pressure decreases the concentration of CO₂(amino-IL)₂, both resulting in CO₂ desorption from the IL quickly.

Conclusion

A new type of ILs has been easily synthesized by neutralization of polyamines and organic acids and they can capture a large amount of CO₂ from simulated flue gas at high temperatures and ambient pressure with low contents of CO₂. The CO₂ absorption is influenced by temperature, CO₂ volume fraction in flue gas, and IL. The CO₂ absorption capacity of the ILs is increased by the number of amido groups on the ILs, which give an IL absorption function and can be adjusted by the mole ratio of polyamines to acids during synthesis. A possible mechanism of the CO₂ absorption has been proposed.

Acknowledgements

The authors thank Prof. Chengyue Li, Prof. Zhenyu Liu and Dr Qingya Liu for their help. The project is financially supported by the Natural Science Foundation of China (21176020) and Program for New Century Excellent Talents in University (NSET-08-0710).

References

1. N. MacDowell, N. Florin, A. Buchard, J. Hallett, A. Galindo, G. Jackson, C. S. Adjinman, C. K. Williams, N. Shah and P. Fennell, Energy Environ. Sci., 2010, 3, 1645–1669.
2. J. M. Navaza, D. Gomez-Diaz and M. D. L. Rubia, Chem. Eng. J., 2009, 146, 184–188.
3. M. Kanniche and C. Bouallou, Appl. Therm. Eng., 2007, 27, 2693–2702.
4. L. A. Blanchard, D. Hancu, E. J. Beckman and J. F. Brennecke, Nature, 1999, 399, 28–29.
5. L. A. Blanchard, Z. Y. Gu and J. F. Brennecke, J. Phys. Chem. B, 2001, 105, 2437–2444.
6. J. L. Anthony, E. J. Maginn and J. F. Brennecke, J. Phys. Chem. B, 2002, 106, 7315–7320.
7. M. B. Shiflett and A. Yokozeki, Ind. Eng. Chem. Res., 2005, 44, 4453–4464.
8. M. B. Shiflett and A. Yokozeki, J. Phys. Chem. B, 2007, 111, 2070–2074.
9. E. D. Bates, R. D. Mayton, L. Ntai and J. H. Davis Jr., J. Am. Chem. Soc., 2002, 124, 926–927.
10. Y. Y. Jiang, G. N. Wang, Z. Zhou, Y. T. Wu, J. Geng and Z. B. Zhang, Chem. Commun., 2008, 505–507.
11. J. M. Zhang, S. J. Zhang, K. Dong, Y. Q. Zhang, Y. Q. Shen and X. M. Lv, Chem.–Eur. J., 2006, 12, 4021–4026.
12. G. R. Yu, S. J. Zhang, X. Q. Yao, J. M. Zhang, K. Dong, W. B. Dai and R. Mori, Ind. Eng. Chem. Res., 2006, 45, 2875–2880.
13. B. E. Gurkan, J. C. Fuente, E. M. Mindrup, L. E. Ficke, B. F. Goodrich, E. A. Price, W. F. Schneider and J. F. Brennecke, J. Am. Chem. Soc., 2010, 132, 2116–2117.
14. Y. Q. Zhang, S. J. Zhang, X. M. Lu, Q. Zhou, W. Fan and X. P. Zhang, Chem.–Eur. J., 2009, 15, 3003–3011.
15. W. Z. Wu, B. X. Han, H. X. Gao, Z. M. Liu, T. Jiang and J. Huang, Angew. Chem., Int. Ed., 2004, 43, 2415–2417.

---