Improvement of the CO2 Absorption Performance Using Ionic Liquid

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Improvement of the CO2 Absorption Performance Using Ionic Liquid [NH2emim][BF4] and [emim][BF4]/[bmim][BF4] Mixtures Mei Wang,†,‡,§ Liqi Zhang,*,† Linxia Gao,§ Kewu Pi,§ Junying Zhang,† and Chuguang Zheng† †

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China ‡ School of Chemical and Environmental Engineering, Wuhan Polytechnic University, Wuhan 430023, People’s Republic of China § School of Chemical and Environmental Engineering, Hubei University of Technology, Wuhan 430068, People’s Republic of China ABSTRACT: Functional ionic liquids (ILs) have potential advantages in reducing CO2 emissions when applied for CO2 absorption. However, the higher viscosity of functional ILs greatly affects the CO2 absorption separation process. To improve the absorption performance, a functional IL [NH2e-mim][BF4] was synthesized and mixed with low-viscosity conventional ILs [emim][BF4]/[bmim][BF4] based on their respective advantages in terms of CO2 reduction, and the CO2 absorption effect and regeneration performance of the binary ILs were investigated. {[NH2e-mim][BF4] + [emim][BF4]/[bmim][BF4]} showed better performance when the mole fraction of [NH2e-mim][BF4] (X[NH2e‑mim][BF4]) was 0.4, and the CO2 absorption performance reduced with the increase in the temperature. Density and viscosity of the binary rose with the increase of X[NH2e‑mim][BF4] and decrease in temperature. The optimal regeneration temperature was approximately 353.15 K when X[NH2e‑mim][BF4] was 0.4 in 0.1 MPa. During five cycles of absorption and regeneration, CO2 absorption capacity of {[NH2e-mim][BF4] + [emim][BF4]/ [bmim][BF4]} was maintained at 75−85% of the first absorption capacity. Moreover, the quality and density changed slightly, and viscosity showed a 5−10% increase in each loop. Thus, the mixed system had an effective regeneration performance.

1. INTRODUCTION Global warming is caused by greenhouse gas emissions, particularly by a large amount of CO2 released into the atmosphere by burning fossil fuel, and this issue continues to receive worldwide attention.1−4 Therefore, economically viable CO2 capture for large-scale reduction is becoming increasingly important. Currently, amine-based scrubbing (e.g., monoethanolamine, N-methyldiethanolamine, etc.) is widely used for post-combustion CO2 capture.5,6 The method has high CO2 absorption efficiency but is limited by regeneration difficulties and high energy consumption. Furthermore, amino is easily oxidized, which leads to the decrease in absorption efficiency. Organic solvents are volatile and cause equipment corrosion as well as add to environmental pollution.7−9 A series of studies showed that ionic liquids (ILs) have efficient CO2 absorption performance, low vapor pressure, adjustable structure, and other relevant characteristics. Therefore, the application of ILs in CO2 emission reduction has gained significant attention.10−16 Conventional and functional ILs are applicable for CO2 capture. In comparison to conventional ILs, functional ILs have a faster absorption rate and higher absorption capacity, and the use of functional ILs facilitates large-scale industrial application.17−19 However, the viscosity of functional ILs is higher, which largely affects the gas−liquid mass transfer, and the CO2 absorption separation process is impeded. In addition, regeneration of functional ILs becomes more difficult and requires more energy than that of conventional ILs.20−23 To improve the CO2 absorption performance of functional ILs, low-viscosity conventional ILs and functional ILs are mixed on the basis of their respective advantages in terms of CO2 © 2012 American Chemical Society

reduction. The mixture is expected to result in a faster absorption rate, higher absorption capacity, and better regeneration performance to meet the demands of large-scale industrial CO2 reduction. In 2002, Bates et al.24 synthesized an amine-functionalized IL [NH2p-bmim][BF4] that could dissolve 7.4 wt % CO2. Brett, Gurkan, and Zhang, among others,25−32 found that aminefunctionalized ILs have advantages in CO2 absorption. The present study selected an amine-functional IL [NH2e-mim][BF4] that has a shorter carbon chain substituted in the cation and mixed it with a series of conventional imidazole ILs that have low viscosity and an effective CO2 absorption performance. Subsequently, the performance of the mixed systems in terms of CO2 trapping was discussed. Previous studies by our group showed that imidazole ILs, which have anions [BF4] and [CH3CO2] can be dissolved with [NH2e-mim][BF4], whereas anion [Tf2N] imidazole ILs are suspended in the mixtures. Moreover, the mixtures of anion [BF4] or [Tf2N] conventional ILs and [NH2e-mim][BF4] showed better CO2 absorption capacity compared to the single IL. CO2 absorption capacity dropped after anion [CH3CO2] ILs were mixed with [NH2emim][BF4]. Hence, the mixtures of binary ILs that are composed of [NH2e-mim][BF4] and [emim][BF4]/[bmim][BF4] will be discussed in terms of their CO2 absorption and regeneration performances. Received: September 19, 2012 Revised: November 30, 2012 Published: November 30, 2012 461

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Figure 1. Experimental apparatus for CO2 absorption/desorption. measured by a DV-II digital display viscometer provided by Brookfield Co. (Middleboro, MA). In this procedure, the number 31 rotor was selected. The appropriate rotor speed and water bath temperature were adjusted, and the values were read several times and then averaged. The average represents the viscosity of the binary system.

2. EXPERIMENTAL SECTION 2.1. Materials. The following reagents were used: CO2 gas (99.99%; Minghui Gas Technology Co., Ltd., China), [emim][BF4]/ [bmim][BF4] (99%; water content ratio < 0.3%; halogen content < 0.5%; Lanzhou Greenchem ILS, LICP, CAS, China), 2-bromine etylamine hydrobromide (C2H7Br2N; 98%; Shanghai Darui Fine Chemical Co., Ltd., China), 1-methylimidazole (C4H6N2; 99%; Shanghai Aladdin Reagent Co., Ltd., China), and sodium borate [NaBF4; chemically pure (CP); Sinopharm Chemical Reagent Co., Ltd., China]. 2.2. Experimental Apparatus and Methods. 2.2.1. Preparation of [NH2e-mim][BF4]. [NH2e-mim][BF4] was synthesized according to the standard two-step method that was developed and reported in the literature24,33−36 using C2H7Br2N, C4H6N2, and NaBF4 as raw materials. The purity and structure of the product was determined by nuclear magnetic resonance (NMR) spectroscopy. The 1H NMR spectra were recorded at room temperature on a Bruker AV400 dual channel digital Fourier superconducting NMR spectrometer. Dimethyl sulfoxide (DMSO) was used as a solvent, and tetramethylsilane (TMS) was used as an internal standard. The bromine content of the product was determined by using an ICS-90 (Dionex Co., Sunnyvale, CA) ion chromatograph. 2.2.2. CO2 Absorption/Desorption Properties of IL Mixtures. The experimental apparatus is shown in Figure 1. Determination of CO2 solubility in aqueous NaOH with this apparatus obtained results that differed by less than 5% from the theoretical value. During the absorption experiment, the CO2 intake speed was controlled at approximately 60 mL/min and the pressure was controlled at 0.1 MPa. CO2 flow was determined by an 810 type mass flowmeter provided by Sierra Flow Measurement and Control Technology Company (Monterey, CA). Recording CO2 mass flow of these two flowmeters was performed at every certain time, then the CO2 absorption solubility of the IL mixture was calculated by the difference of these two mass values (in units of moles of CO2 per moles of mixture, expressed as mol/mol). The pressure-reducing valve and the value were then closed; the water temperature was regulated to range from 338.15 to 358.15 K; and CO2 release was initiated. The CO2 liberation volume was measured by a mass flowmeter and recorded in a computer, and the CO2 desorption efficiency defined the percentage of CO2 rengeration capacity to correspond to CO2 absorption capacity. The IL mixtures were subjected to the steps previously mentioned to carry out absorption−desorption cycle experiments. 2.2.3. Measurement of the Physical Properties of the Binary Systems. The density of the binary system was measured using a pycnometer at a constant temperature, and the average was derived after measuring for several times in parallel. The viscosity was

3. RESULTS AND DISCUSSION 3.1. Product [NH2e-mim][BF4]. Amine functional IL [NH2e-mim][BF4], which was the product in the synthesis, was a yellow viscous liquid. The liquid had a density of 1472.22 kg/m3 and viscosity of 3589.2 mPa s at 293.15 K and under 1 atm. The 1H NMR values were as follows: [H2Ne-mim][BF4], 1 H NMR (400 MHz, DMSO) δ 9.421 (s, 1H, unsaturated C− H in the imidazole ring, with N connected to the left and right), 7.918 (s, 1H, unsaturated C−H in the imidazole ring), 7.841 (s, 1H, unsaturated C−H in the imidazole ring), 4.207 (m, 3H, H3C−N ring), 3.922 (s, 2H, H2C−N ring), and 1.821 (m, 4H, N−CH2−CH2−N). The NMR spectra analysis showed that the target product was synthesized and had high purity. The bromine content of the product was 852 ppm, as determined by ion chromatography. 3.2. CO2 Absorption Properties in Different IL Proportions. CO2 absorption capacities and absorption rates of [NH2e-mim][BF4] and [emim][BF4]/[bmim][BF4] mixtures were determined at 303.15 K in 0.1 MPa with the X[NH2e‑mim][BF4] changed to a value within the range of 0−0.7. CO2 absorption capacities of mixtures at different molar fractions of [NH2e-mim][BF4] and [emim][BF4]/[bmim][BF4] are presented in Figure 2, and CO2 absorption rates of mixtures are as shown in Figure 3. Figure 2 shows the increase in the CO2 absorption capacity of the mixed system with the increase in X[NH2e‑mim][BF4]. The CO2 absorption capacity increased quickly when X[NH2e‑mim][BF4] was within 0.1−0.4 and then increased slowly when X[NH2e‑mim][BF4] was more than 0.4. Figures 2 and 3 show that {[NH2e-mim][BF4] + [bmim][BF4]} mixtures had a higher CO2 absorption capacity and a faster absorption rate compared to {[NH2e-mim][BF4] + [emim][BF4]} mixtures at a constant X[NH2e‑mim][BF4] of 0.4. The same trend was observed for the CO2 absorption performance of single [bmim][BF4], which was better than that of single [emim][BF4].37−39 462

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On the basis of the CO2 absorption capacity/rate and costs, the binary IL mixtures had better performance when X[NH2e‑mim][BF4] was approximately 0.4. The {[NH2e-mim][BF4] + [bmim][BF4]} mixture had better CO2 absorption properties compared to the {[NH2e-mim][BF4] + [emim][BF4]} mixture. 3.3. CO2 Absorption Properties at Different Temperatures. The CO2 absorption capacity and absorption rate of binary IL mixtures of {[NH2e-mim][BF4] + [bmim][BF4]} at different temperatures from 298.15 to 323.15 K were determined when X[NH2e‑mim][BF4] was 0.4 under 0.1 MPa. The results are shown in Figures 5 and 6. Figure 2. CO2 absorption capacity of mixtures at different X[NH2e‑mim][BF4].

Figure 5. CO2 absorption capacity of the {[NH2e-mim][BF4] + [bmim][BF4]} system at different temperatures when X[NH2e‑mim][BF4] is 0.4. Figure 3. CO2 absorption rate of IL mixtures when X[NH2e‑mim][BF4] is 0.4.

The CO2 absorption rate of the {[NH2e-mim][BF4] + [bmim][BF4]} mixture at different X[NH2e‑mim][BF4] was shown in Figure 4. The CO2 absorption rate of mixed systems increased

Figure 6. CO2 absorption rate of the {[NH2e-mim][BF4] + [bmim][BF4]} system at different temperatures when X[NH2e‑mim][BF4] is 0.4.

The figures show that the {[NH2e-mim][BF4] + [bmim][BF4]} system had a lower CO2 absorption capacity and a slower absorption rate with the gradual increase in the temperature. The CO2 absorption rate and absorption capacity changed slightly at 308.15−313.15 K and, subsequently, showed a rapid decrease. According to prior studies, the CO2 absorption performance of ILs is better under low temperatures, which is similar to the CO2 absorption performance of the {[NH2e-mim][BF4] + [bmim][BF4]} system under changing temperatures.40,41 3.4. Density and Viscosity of Binary Mixtures. The density and viscosity of the {[NH2e-mim][BF4] + [bmim][BF4]} system under different [NH2e-mim][BF4] molar ratios, temperature of 303.15 K, and pressure of 0.1 MPa are shown in Figure 7. Figure 7 shows that the density of the mixtures

Figure 4. CO2 absorption rate of the {[NH2e-mim][BF4] + [bmim][BF4]} mixture at different X[NH2e‑mim][BF4].

with the increase in X[NH2e‑mim][BF4]. When X[NH2e‑mim][BF4] was less than 0.4, the CO2 absorption rate increased more quickly than when X[NH2e‑mim][BF4] was over 0.4. Figures 3 and 4 show that, at the onset, the CO2 absorption rate of the binary systems was fast. Subsequently, the rate slowed and, finally, achieved saturation. The response at 10 min showed that the CO2 absorption capacity of the mixtures was near 80% of the total, and the balance of the reactions was reached after 40 min. 463

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indicates that the density and viscosity of the mixed system affected the absorption performance. 3.5. Regeneration Performance of the Binary System. 3.5.1. Optimal Conditions for Regeneration. Figure 9 shows

Figure 7. Density and viscosity of the {[NH2e-mim][BF4] + [bmim][BF4]} system at different X[NH2e‑mim][BF4].

increased with the increasing [NH2e-mim][BF4] molar fraction and then slowed when X[NH2e‑mim][BF4] was greater than 0.4. The viscosity of the mixtures rose smoothly and increased perpendicularly when X[NH2e‑mim][BF4] reached 0.4. From our previous study, the CO2 absorption capacity and absorption rate of these mixtures achieved an inflection point when X[NH2e‑mim][BF4] was 0.4. In other words, the CO2 absorption capacity and absorption rate increased rapidly when X[NH2e‑mim][BF4] was less than 0.4 and, subsequently, increased slowly. The observed changes were similar with those of the density and viscosity of the binary systems. Thus, the density and viscosity of the mixed system affected the absorption performance to a certain extent. The density and viscosity of the {[NH2e-mim][BF4] + [bmim][BF4]} system under different temperatures in 0.1 MPa when X[NH2e‑mim][BF4] was 0.4 are shown in Figure 8. In Figure 8,

Figure 9. CO2 desorption rate of the {[NH2e-mim][BF4] + [bmim][BF4]} system at different temperatures when X[NH2e‑mim][BF4] is 0.4.

CO2 desorption rates of the {[NH2e-mim][BF4] + [bmim][BF4]} system at different temperatures from 333.15 to 358.15 K when X[NH2e‑mim][BF4] was 0.4 under 0.1 MPa. Figure 10 illustrates the CO2 desorption efficiency of the {[NH2emim][BF4] + [bmim][BF4]} system at different temperatures when the desorption time was 30 min.

Figure 10. CO2 desorption efficiency of the {[NH2e-mim][BF4] + [bmim][BF4]} system at different temperatures when X[NH2e‑mim][BF4] is 0.4.

In Figure 9, the desorption rate of the {[NH2e-mim][BF4] + [bmim][BF4]} mixed system rose with the increase of the desorption temperature. The regeneration rate increased slowly when the temperature was higher than 348.15 K. The desorption rate at 353.15 K could match the desorption rate at 358.15 K. Figure 10 shows that the CO2 desorption efficiency of the {[NH2e-mim][BF4] + [bmim][BF4]} system is approximately 60% at 338.15 K and reached 100% at 35 8.15 K after 30 min of desorption. Therefore, considering the desorption efficiency and economic factors, the best desorption temperature of the {[NH2e-mim][BF4] + [bmim][BF4]} mixed system was found to be 353.15 K in 0.1 MPa when X[NH2e‑mim][BF4] was 0.4, and CO2 in the binary system could be desorbed completely within 40 min.

Figure 8. Density and viscosity of the {[NH2e-mim][BF4] + [bmim][BF4]} system at different temperatures when X[NH2e‑mim][BF4] is 0.4.

the density and viscosity of the mixtures decreased with the increase in the temperature. The density decreased slowly when the temperature was below 313.15 K. When the temperature increased above 313.15 K, the density decreased quickly. The viscosity of the binary system reached an inflection point at 318.15 K and decreased faster at a temperature less than 318.15 K than that at a temperature higher than 318.15 K. The changes in the density and viscosity of the {[NH2e-mim][BF4] + [bmim][BF4]} system with the temperature were consistent with those of the CO2 absorption performance. The result 464

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3.5.2. Properties of the Binary System after Regeneration. The {[NH2e-mim][BF4] + [bmim][BF4]} mixed system absorbed CO2 at 303.15 K and desorbed CO2 at 353.15 K in 0.1 MPa. The quality and CO2 absorption performance in five absorption/desorption cycles are shown in Figure 11, and the density and viscosity values are shown in Figure 12.

[BF4] to overcome the disadvantages of high viscosity and poor gas−liquid mass-transfer performance during CO 2 absorption separation. The physical and chemical properties, especially the gas−liquid mass-transfer condition, of the IL mixtures were improved significantly. The CO2 absorption and regeneration performances of the binary ILs were investigated. Considering the CO2 absorption performance and economic factors, {[NH2e-mim][BF4] + [emim][BF4]/[bmim][BF4]} mixtures showed better performance when the mole fraction of [NH2e-mim][BF4] (X[NH2e‑mim][BF4]) was 0.4. Meanwhile, the CO2 absorption capacity and CO2 absorption rate decreased with the increase in the absorption temperature. The density and viscosity of binary IL system rose with the increase in X[NH2e‑mim][BF4] and decrease in the absorption temperature. The optimal regeneration temperature of the {[NH2e-mim][BF4] + [bmim][BF4]} system was approximately 353.15 K in 0.1 MPa when X[NH2e‑mim][BF4] was 0.4. After five absorption/regeneration cycles, the CO2 absorption capacity was maintained at 75−85% of the basic absorption capacity. The study results showed that the application of the binary mixtures of conventional ILs and functional ILs is an effective approach to improve the IL CO2 absorption performance.

Figure 11. CO2 absorption capacity and quality of the {[NH2emim][BF4] + [bmim][BF4]} system in five absorption/desorption cycles.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-27-87542417, ext. 8316. Fax: +86-2787545526. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Natural Science Foundation of China (51076056 and 51021065), the National Basic Research Program of China (2011CB707301), and the Foundation of State Key Laboratory of Coal Combustion (FSKLCC1111).

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Figure 12. Density and viscosity of the {[NH2e-mim][BF4] + [bmim][BF4]} system in five absorption/desorption cycles.

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In Figure 11, the CO2 absorption capacity of the second cycle was substantially reduced in comparison to that of the first cycle and then CO2 absorption properties changed slightly in the latter cycles. The CO2 absorption capacity was maintained at 75−85% of the absorption capacity of the first cycle. The change in the quality of the binary system was slight during five absorption/desorption cycles. Thus, the system had no large quality loss. In Figure 12, the density of the {[NH2emim][BF4] + [bmim][BF4]} system increased slightly with the increase in the number of cycles. Viscosity also rose with the increase in the number of cycles. In each cycle, the increase was in the range of 5−10%. In conclusion, repeated use had a particular impact on the performance of the binary system for the absorption/ desorption, but the impact was limited. Thus, the {[NH2emim][BF4] + [bmim][BF4]} system had an effective performance based on the absorption/desorption cycles, and the system could be used in trapping CO2 more than once.

4. CONCLUSION A functional IL [NH2e-mim][BF4] was synthesized and mixed with low-viscosity conventional ILs [emim][BF4]/[bmim]465

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dx.doi.org/10.1021/ef301541s | Energy Fuels 2013, 27, 461−466