Determination of Absorption Rate and Capacity of CO2 in Ionic Liquids

Nov 10, 2011 - Here, a cheap and fast way to measure the CO2 absorption rate and capacity through thermogravimetric analysis (TGA) is proposed...
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Determination of Absorption Rate and Capacity of CO2 in Ionic Liquids at Atmospheric Pressure by Thermogravimetric Analysis Yu Chen,† Jin Han,† Tao Wang,‡ and Tiancheng Mu*,† † ‡

Department of Chemistry, Renmin University of China, Beijing 100872, P. R. China College of Chemical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, P. R. China ABSTRACT: Here, a cheap and fast way to measure the CO2 absorption rate and capacity through thermogravimetric analysis (TGA) is proposed. The absorption of CO2 in 11 ILs varying in anion, cation, alkyl chain length, and C2 methylation was then investigated. Three parameters comprehensively characterizing the absorption capacity and kinetics, including the absorption capacity (x), the initial absorption rate (r10), and the degree of difficulty to reach phase equilibrium (t0.9), were proposed as the standards to evaluate the potential of ILs for CO2 capture. Results show that the correlation between absorption capacity and the degree of difficulty to reach phase equilibrium is complicated. However, ILs with higher absorption capacity usually have a higher initial absorption rate, suggesting a simple way to estimate absorption capacity just by determining initial absorption rate for less than 10 min. More importantly, ILs with the acetate ([Ac]) anions have an advantage in x, r10, and t0.9 over other ILs, indicating that [Ac]based ILs are promising candidates for CO2 capture in practice.

1. INTRODUCTION The increasing emission of greenhouse gas, especially carbon dioxide (CO2), is the main cause of the global warming. It threatens the environment and the future of humankind. Capturing CO2 from fossil fuel combustion is a promising way to reduce CO2 emissions. Amine-based scrubbing was the traditional technology used for the capture of CO2 in industry.1 But this process suffered from inherent drawbacks, such as high energy consumption, solvent loss, and corrosion.2 Ionic liquids (ILs) have received increasing attention as neoteric solvents in recent years because they have unique properties, including low vapor pressure, high thermal stability, and tunable properties.3 They are deemed promising candidates for the capture of CO2.4,5 The physical absorption capacity of CO2 by ILs is limited, usually up to about 3 mol % under atmospheric pressure.6 It can be enhanced by increasing alkyl chain length or adding the fluoroalkyl group. However, by these means, the CO2 solubility increases not more than 10%.6,7 A promising strategy to enhance the absorption capacity of CO2 in ILs is based on chemisorption by task-specific ILs, especially amine functionalization ILs8 10 or superbasederived protic ILs,11 which could reach about a 1:1 stoichiometry (1 mol of CO2 per mole of IL). Attention has been mainly paid to improving the CO2 absorption capacity of ILs, while the absorption rate is another important factor in the evaluation of the industrial application of this technology.12 Some specific ILs, which can absorb CO2 quickly, have been synthesized and investigated.11,13,14 It took only 4 min for poly-ILs to reach their 90% absorption capacities and about 30 min to reach their full capacities.13 CO2 absorption for superbasederived protic ILs could be almost completed within 5 min.11 IL amine solutions reached over their 90% absorption capacity within 15 min, and the reaction was completed after 25 min.14 However, it takes about 3 h to reach equilibrium for most other ILs whether by absorbing CO2 physically or by the amino-functionalization ILs reacting with CO2.8,13 The absorption rate plays an r 2011 American Chemical Society

important role in CO2 capture for both common ILs and taskspecific ILs.15 On the other hand, the properties of ILs, particularly the viscosity, are changed after CO2 absorption, which affects further CO2 absorption. The viscosity of some amino-functionalized anion-tethered ILs increased by a factor of 2 when fully combined with CO2.16 But the increasing viscosity of amine-functionalized ILs after absorbing CO2 was due to strong hydrogen-bonded networks, which might be the hurdle preventing the application of ILs as the CO2 absorbent.17 The basic ILs also suffered from similar viscosity increment after CO2 uptake.5 Interestingly, the viscosity of one basic ionic liquid was found to decrease after complexing with CO2, a result that might be related to the absence of hydrogen-bonded networks.5 The viscosity of not only the functionalized ILs but also the common ILs could increase or decrease after mixing with CO2.18 Moreover, the thermodynamic factor affects further CO2 absorption; namely, the absorbed CO2 makes further CO2 uptake difficult. Since both the absorption kinetic and absorption capacity of CO2 by ILs are important, a cheap and fast way to characterize the CO2 absorption is necessary. The volumetric method is the common way to measure CO2 solubility in ILs,9,10,16,19 23 but with the volumetric method, the absorption kinetics are difficult to determine. In this regard, TGA seemed to be a fast and cheap way to measure the CO2 absorption capacity and rate simultaneously. Usually, TGA is used to evaluate the thermal stability of substances, but recently TGA was used to desorb CO2 which was absorbed in ILs.5 TGA was also applied to absorb CO2 in aqueous amine solutions as a function of time. Taking a cue from the special application of the above-mentioned TGA and the efficiency of TGA, we propose to measure the absorption Received: October 7, 2011 Revised: November 6, 2011 Published: November 10, 2011 5810

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Energy & Fuels Table 1. Structure, Name, and Abbreviation of ILs

capacity and absorption rate of CO2 in ILs by TGA. In this work, 11 ILs (Table 1) were screened to absorb CO2 concerning both the capacity and rate. These ILs vary in anion, cation, chain length, and C2 (the carbon between the two nitrogen atoms of the imidazolium ring) methylation. We intend to study the effect of these factors of ILs on the CO2 absorption capacity and rate.

2. EXPERIMENTAL SECTION 2.1. Materials. CO2 (99.999%) and N2 (99.999%) were purchased from Beijing Huayuan Gas Chemical Industry Co., Ltd. (Beijing China). All of the ILs with a purity over 99.9 wt % were purchased from Lanzhou Greenchem ILs, LICP, CAS, China (Lanzhou, China). [BMIM][Ac], [BPy][Ac], and [DMIM][BF4] were dried at 50 °C under vacuum conditions for 96 h before use. The remaining ILs were dried at 60 °C for 48 h under vacuum conditions. The reason for drying [BPy][Ac] and [DMIM][BF4] at the lower temperature 50 °C is that we witnessed a color change from shallow yellow and transparent to black for [BPy][Ac] and [DMIM][BF4] at 60 °C after 48 h of drying, respectively. This might be due to the easy thermal decomposition of ILs with an acetate anion and a long chain length octyl.24 Another reason for drying them so long is that ILs with the anion [Ac] were found to absorb water very strongly;25 long-time drying for [BMIM][Ac] and [BPy][Ac] at a relatively low temperature must be ensured. After drying under vacuum conditions, the purity of all of the ILs was verified by 1H and 13C NMR. No impurities and no degradation products were detected in the NMR spectra. Furthermore, in order to exclude the water absorption from the air during the transfer process from the reagent bottle to the platinum sample pan of the TG apparatus, these ILs, particularly for the [Ac] anions, were purged with N226 at 50 °C in the TG apparatus until the weight change rate was below 10  10 6 mg/min. Since [BMIM][Ac] has a strong water-absorbing ability, its water content was immediately determined by Karl Fischer titration25,27 30 (ZDJ-400S, Multifunctional titrator, Beijing Xianqu Weifeng Company, Beijing, China) from the

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platinum sample pan. The water content was found to be lower than 45 ppm. The other less hydroscopic ILs showed water contents less than 34 ppm. Therefore, the mass gain in the TG curve (weight vs. time) can be attributed to the CO2 absorption by ILs, regardless of the influence of the minimal water content in ILs. 2.2. Apparatus and Measurements. In a typical experiment, 10 15 mg of ILs was loaded in the platinum sample pan of the TA Instruments Q50-TG. After the sample was loaded, the TG apparatus was first purged with a N2 atmosphere26 (80 mL/min, 1 bar) at 50 °C in an isothermal mode to remove the possible water absorption from the air during the transfer process. The high N2 flowing rate (80 mL/min) was set to remove the volatile impurities in ILs faster.26 The impurityremoving process was not conducted at a higher temperature but at 50 °C to avoid unexpected thermal degradation, because a color change occurred for [BPy][Ac] and [DMIM][BF4] at 60 °C after drying under vacuum conditions. After the weight time curve remained nearly horizontal and constant (the mass loss rate is less than 10  10 6 mg/min), the sample was then purged in a CO2 atmosphere (5 mL/min, 1 bar) at 50 °C to monitor the CO2 absorption process. The very low CO2 flowing rate (5 mL/min) ensured a minimum buoyancy influence on the CO2 absorption. The volume of the TG column was estimated to be about 30 mL (radius = 1 cm, height = 10 cm). This little volume could ensure that the CO2 replaced the column space quickly (less than 6 min), minimizing the effect on the initial absorption rate (r10). The ILs were spread in as well-distributed a manner as possible in the TG crucible in every experiment to ensure an equal contact area for CO2 capture. It could minimize the effect of surface area of ILs on the absorption capacity and rate. The mass precision of TG is (0.1 μg with the weight ranging from 0 to 1 g. The temperature precision is (0.1 °C. All of the samples were averaged four times with these the microscale TG study methods; the deviations were below (1.2%.

3. RESULTS AND DISCUSSION Capturing CO2 with ILs involves the capacity and absorption rate.12,15 The absorption rate varies with the time, and it is associated with the potential of ILs to absorb CO2 and the ability of ILs to reach phase equilibrium. Thus, we use the initial absorption rate (r10) and degree of difficulty to reach phase equilibrium (t0.9) for ILs to indicate the potential to absorb CO2 and the ability to reach a phase equilibrium state, respectively. Therefore, three parameters were proposed to evaluate CO2 capture by ILs, the absorption capacity (x), the initial absorption rate (r10), and the degree of difficulty to reach phase equilibrium (t0.9). Absorption capacity (x) refers to the mole fraction of CO2 in CO2 ILs mixtures when the phase equilibrium is reached, indicated by the unchanging weight of the mixtures with the time. The purpose of multiplying the mole fraction by 100 here is only for easy description. The initial absorption rate (r10) refers to CO2 absorption capacity during the first 10 min. We find that the CO2 absorption capacity vs time curve is near linear in the first 10 min, consistent with the previous finding;31 thus the absorption capacity in the first 10 min could represent the initial absorption rate. The degree of difficulty to reach phase equilibrium (t0.9) is expressed as the time when 90% of the CO2 absorption capacity is reached. Phase equilibrium of CO2 capture could be hard to reach due to the viscosity of ILs, especially when large quantities of ILs are used in the application, a way that could cause diffusivity difficulty. Moreover, the equilibrium state is not easy to determine and is unnecessary to arrive at in practice. Thus, an equilibrium time at which 90% of CO2 absorption capacity is reached could be deemed a choice to represent the degree of difficulty to reach phase equilibrium. Actually, 90% of the CO2 5811

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Table 2. CO2 Absorption Rate and Capacity Value at 50°C and atmospheric pressure by TGA xc r10a

t0.9b/min

TGA

[BMIM][Ac]

5.207

43.3

9.500

[BMIM][TFO]

0.466

92.9

1.840

[BMIM][PF6] [BMIM][BF4]

0.441 0.425

69.4 83.0

1.133 1.090 0.921

ILs

[BMIM][NO3]

0.279

82.5

[BPy][Ac]

1.627

76.5

4.759

[BPy][BF4]

0.365

76.4

1.098

[HMIM][BF4]

0.402

74.6

1.183

[OMIM][BF4]

0.436

72.9

2.023

[DMIM][BF4]

0.522

79.0

1.914

[BMMIM][BF4]

0.276

66.4

0.986

exptl. 20.430 1.2,32 1.0,34 1.0133 1.232

CO2 absorption capacity during the first 10 min, indicating the initial absorption rate. b The time when 90% of CO2 absorption capacity is reached, indicating the degree of difficulty to reach phase equilibrium. c 100 mol fraction of CO2 in ILs, indicating the CO2 absorption capacity. a

Figure 2. Practical phase equilibrium time at 90% capacity t0.9 and absorption capacity x of CO2 in ILs at 50 °C and atmospheric pressure by TGA.

Figure 3. Effect of anion of ILs on the CO2 absorption rate and capacity at 50 °C and atmospheric pressure by TGA.

Figure 1. Initial absorption rate r10 and absorption capacity x of CO2 in ILs at 50 °C and atmospheric pressure by TGA.

uptake capacity has been reported in several research studies.13,14 The values for the three parameters are included in Table 2. Trends of the three parameters with IL types are shown in Figures 1 (x and r10) and 2 (x and t0.9). The absorption capacity (x) and the initial absorption rate (r10) of CO2 in 11 ILs (varying in anion, cation, chain length, and C2 substitution) have nearly the same tendency; i.e., ILs with higher capacity have a higher initial absorption rate (Figure 1). In particular, ILs with the [Ac] anion involving chemical absorption20,29,30 have the advantage in both absorption capacity and the initial absorption rate over the other ILs absorbing CO2 physically. In this way, we may preliminarily estimate the absorption capacity using the initial absorption rate, which can be easily determined by TGA. However, the tendency between the absorption capacity (x) and degree of difficulty to reach phase equilibrium (t0.9) for these 11 ILs is complicated (Figure 2). This may be caused by a different property change, especially viscosity,18 for different ILs after they absorb CO2.

3.1. Effect of Anion. The absorption capacity of CO2 in [BMIM][Ac], [BMIM][TFO], [BMIM][PF6], [BMIM][BF4], and [BMIM][NO3] as a function of time was measured by TGA at 50 °C and atmospheric pressure (Figure 3). All of the ILs have the same cation [BMIM], but the anions span a wide range of chemical types and basicity. The absorption capacity and the initial absorption rate share a similar order: [BMIM][Ac] > [BMIM][TFO] > [BMIM][PF6] > [BMIM][BF4] > [BMIM][NO3] (Figure 1). The degree of difficulty to reach phase equilibrium follows a different tendency: [BMIM][TFO] > [BMIM][BF4] > [BMIM][NO3] > [BMIM][PF6] > [BMIM][Ac] (Figure 2). The absorption capacity for [BMIM][PF6]32 34 and [BMIM][BF4]32 here by TGA are consistent with a gravimetric microbalance measurement, with the deviation ranging from 8.04% to 13.3% (Table 2). The sample weight difference (micro for TGA but macro for the latter) and the CO2 flowing state (5 mL/min for TGA but usually static for the latter) might be reason for those deviations. Even though, the maximum deviation measured in the same way can reach 20% by different reports,32,34 indicating a proper deviation value for our results. However, a remarkable discrepancy occurs for CO2 capture by [BMIM][Ac] (Table 2).30 At first, we attributed this discrepancy to the volatile impurities, especially water, with which ILs with the [Ac] anion easily 5812

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Figure 4. Effect of cation of ILs on the CO2 absorption rate and capacity at 50 °C and atmospheric pressure by TGA.

Figure 5. Effect of alkyl chain length of ILs on the CO2 absorption rate and capacity at 50 °C and atmospheric pressure by TGA.

interact.28 But even when we dried the IL in a vacuum drying oven at 50 °C for another 48 h and removed volatile impurities with N226 at 50 °C in TG until the weight time curve remained nearly horizontal, there was no substantial CO2 absorption capacity change. It cannot be ascribed to some extent of decomposition occurring after the drying process, because no decomposition products were detected in 1H and 13C NMR. The possibility of N2 solubility in [BMIM][Ac] was also excluded, for there was no sign of mass gain in the weight time curves after the N2 purge. The abnormal deviation for CO2 capture by [BMIM][Ac] awaits further investigation. After all, measuring the absorption kinetics by TGA is convenient and efficient. In addition to the absorption capacity, it could also reflect the initial absorption rate and the degree of difficulty to reach phase equilibrium. The IL with the [Ac] anion has a considerably higher absorption capacity and initial absorption rate for CO2 than any other four ILs (Figures 1 and 3). Furthermore, its degree of difficulty in reaching phase equilibrium is the shortest (Figure 2), indicating the easiest phase equilibrium when capturing CO2. The low viscosity25 and reaction with CO2 chemically20,29,30 of the [Ac] anion might be the reason. One might expect some physical CO2 absorption for [BMIM][Ac] like the other four ILs, but it could be negligible at atmospheric pressure. The IL containing fluoroalkyl group [TFO] has the second highest absorption capacity and initial absorption rate (Figures 1 and 3). This might be due to favorable interactions between CO2 and the fluoroalkyl substituent on the anion.21 Interestingly, we found that the phase equilibrium of [BMIM][TFO] and CO2 is the most difficult to reach (Figure 2); one possible explanation is that it suffers from some dramatic property changes after interacting with CO2. Furthermore, CO2 has basically the same solubility in two ILs, [BMIM][PF6] and [BMIM][BF4], with the inorganic fluorinated anions, but lower than the former two groups (Figures 1 and 3), although a very slightly greater solubility for [BMIM][PF6] than [BMIM][BF4] could be observed after specific investigation (Figure 3b). The initial absorption rate for [BMIM][PF6] and [BMIM][BF4] is similar too (Figure 1), but a more difficult phase equilibrium with CO2 is observed for [BMIM][PF6] than for [BMIM][BF4] (Figure 2). The absorption capacity and the initial absorption rate of CO2 in the IL with nonfluorinated anion [BMIM][NO3] are the least, as is expected.6,21 The degree of difficulty to reach phase equilibrium is nearly the same with [BMIM][BF4].

3.2. Effect of Cation. Like the absorption capacity and initial absorption rate of the ILs varying in the anion, these properties of the ILs with different cations have the same tendency (Figure 1). Table 2 shows that the solubilities of CO2 in imidazolium and pyridinium [BF4] compounds at 50 °C and atmospheric pressure are virtually identical. The low-pressure solubility of CO2 for the corresponding [Tf2N] salts was also reported to be very similar.35 In these ILs with the physical absorption anion, the primary interactions of the CO2 appear to be with the anions, and the cation plays a less important role. Very unexpectedly, when we investigated the imidazolium and pyridinium [Ac] compounds, the CO2 solubility in [BMIM][Ac] was nearly two times that of [BPy][Ac] (Figures 1 and 4). That means the cation could also play an important role in determining the CO2 solubility for ILs involving a chemical absorption anion. One possible explanation is that a stable adduct of imidazolium carboxylate forms accompanied by a loss of acetic acid when CO2 interacts with the C2 position of the imidazolium ring, but there is no such interaction for the pyridinium ring.30 Also, it is reported that the cation could play a more important role than the anion in CO2 absorption for the poly-ILs.13 Clearly, one can even design ILs directly interacting with CO2 chemically by the cation.8 10 The difficulty to reach phase equilibrium for [BMIM][Ac] shows a significant advantage over [BPy][Ac] (Figure 2). This may be due to the same stable imidazolium carboxylate.30 But for ILs [BMIM][BF4] and [BPy][BF4] with the physical absorption anion, the difference in the difficulty to reach phase equilibrium is not so substantial, probably because of the lack of such a stable imidazolium carboxylate formation.30 3.3. Effect of Alkyl Chain Length. To study the influence of cation alkyl chain length, [BMIM][BF4], [HMIM][BF4], [OMIM][BF4], and [DMIM][BF4] were chosen as the subjects of investigation. The solubility increases slightly as the chain length increases from the butyl to octyl (Figures 2 and 5). The results are consistent with the ILs with [BF4] anions when the alkyl chain length was increased from butyl to octyl.22 Results also show that the solubility of CO2 in [PF6]-based ILs36,23 and [Tf2N]-based ILs21 increased when the alkyl chain length was increased from ethyl to hexyl and from butyl to octyl, respectively. Higher solubility in ILs with longer cation alkyl chains (from butyl to octyl) can be ascribed to the ILs with longer alkyl chains having greater free volume.21 However, the solubility decreases slightly as the chain length increases from octyl to docanyl (Figures 1 and 5). Therefore, an optimum chain length 5813

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Figure 6. Effect of C2 methylation of ILs on the CO2 absorption rate and capacity at 50 °C and atmospheric pressure by TGA.

(C8) for maximum CO2 capture occurs. The viscosity increases with increasing alkyl chain length37,24,3 while the IL becomes too viscous with the longest chain length, which might prevent the free volume space of the long alkyl chain length from holding much CO2. Likewise, the investigation for the CO2 solubility in fluorocarbons also states that there will be an optimum number of fluorine atoms for maximum CO2-philicity, and then continued increases in fluorine may lead to reduced CO2-philicity.38 So, continually increasing the chain length does not necessarily lead to a proportionate increase in CO2 solubility, which is important for the design of task-specific ILs for CO2 capture. In addition, the IL with the octyl chain length most easily reaches the phase equilibrium state with CO2 (Figure 2). But interestingly, the initial absorption rate of [HMIM][BF4] is the slowest for the four ILs varying in alkyl chain length (Figures 1 and 5). It is probably due to ILs with a moderate chain length maybe having both lower viscosity and higher free volume space. 3.4. Effect of C2 Methylation. The IL [BMMIM][BF4] with C2 methylation shows a slightly lower value for the three parameters (the absorption capacity, the initial absorption rate, and the degree of difficulty to reach phase equilibrium) than that with [BMIM][BF4] (Figures 1, 2, and 6). The acidic C2 hydrogen in the imidazolium ring has the potential to interact with the oxygen atoms on the CO2. Methylation of the C2 position could eliminate the C2 H/O interactions,39 thus making the CO2 solubility in [BMMIM][BF4] 9.5% lower than that in [BMIM][BF4] (Table 2). The difference in solubility may become more apparent at a higher pressure.21 Even though [BMMIM][BF4] with C2 methylation has a slightly lower absorption capacity and initial absorption rate than [BMIM][BF4], it can reach phase equilibrium with CO2 quickly (Table 2). This indicates that the C2 H in the imidazolium ring can increase the CO2 affinity, and therefore the initial absorption rate, but could not make its phase equilibrium time shorter.

4. CONCLUSION A cheap and fast way to determine the absorption kinetics and capacity of CO2 in ILs by TGA was provided. Three parameters were proposed to indicate the efficiency of CO2 capture by 11 ILs, including the absorption capacity, the initial absorption rate, and the degree of difficulty to reach phase equilibrium. The absorption capacity and the initial absorption rate of CO2 had almost the same tendency, indicating a convenient way to estimate

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the CO2 absorption capacity just by the r10 value. However, the relationship for the absorption capacity and the degree of difficulty to reach the phase equilibrium was complicated. ILs with the chemical absorption anion [Ac] could capture CO2 faster and have a higher absorption capacity than other ILs investigated. [BMIM][TFO] with the fluoroalkyl group could absorb CO2 with a higher capacity and bigger initial absorption rate than ILs with inorganic fluorinated anions ([BF4], [PF6]) and nonfluorinated anions ([NO3]), but these had the most difficulty reaching phase equilibrium, which was probably due to some property changes upon uptaking CO2. The chemical absorption anion [Ac] could also play an important role in the CO2 solubility and the initial absorption rate. On the contrary, the physical absorption anion [BF4] determined the CO2 absorption capacity and the initial absorption rate, with the cation type (imidazolium and pyridinium) showing a minor effect. Moreover, octyl was the optimal cation alkyl chain length for the maximum CO2 absorption capacity and for the easiest phase equilibrium with CO2, but ILs with a hexyl chain length had the maximum initial absorption rate for CO2. Finally, methylation at the C2 position in the imidazolium ring slightly decreased the CO2 absorption capacity and the absorption rate but slightly increased the degree of difficulty to reach phase equilibrium.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: +86-10-62514925. Fax: +86-10-62516444. E-mail:tcmu@ chem.ruc.edu.cn.

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