determination of absorption rate and capacity of co 2 in ionic liquids...

Published: November 10, 2011

r 2011 American Chemical Society 5810 dx.doi.org/10.1021/ef201519g | Energy Fuels 2011, 25, 5810–5815

ARTICLE

pubs.acs.org/EF

Determination of Absorption Rate and Capacity of CO2 in Ionic Liquidsat Atmospheric Pressure by Thermogravimetric AnalysisYu 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 theninvestigated. Three parameters comprehensively characterizing the absorption capacity and kinetics, including the absorptioncapacity (x), the initial absorption rate (r10), and the degree of difficulty to reach phase equilibrium (t0.9), were proposed as thestandards to evaluate the potential of ILs for CO2 capture. Results show that the correlation between absorption capacity and thedegree of difficulty to reach phase equilibrium is complicated. However, ILs with higher absorption capacity usually have a higherinitial absorption rate, suggesting a simple way to estimate absorption capacity just by determining initial absorption rate for less than10min.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 carbondioxide (CO2), is the main cause of the global warming. Itthreatens the environment and the future of humankind. Captur-ing CO2 from fossil fuel combustion is a promising way to reduceCO2 emissions. Amine-based scrubbing was the traditionaltechnology used for the capture of CO2 in industry.1 But thisprocess suffered from inherent drawbacks, such as high energyconsumption, solvent loss, and corrosion.2 Ionic liquids (ILs)have received increasing attention as neoteric solvents in recentyears because they have unique properties, including low vaporpressure, high thermal stability, and tunable properties.3 They aredeemed promising candidates for the capture of CO2.

4,5 Thephysical absorption capacity of CO2 by ILs is limited, usually up toabout 3 mol % under atmospheric pressure.6 It can be enhancedby increasing alkyl chain length or adding the fluoroalkyl group.However, by these means, the CO2 solubility increases not morethan 10%.6,7 A promising strategy to enhance the absorptioncapacity of CO2 in ILs is based on chemisorption by task-specificILs, especially amine functionalization ILs8�10 or superbase-derived 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 absorp-tion capacity of ILs, while the absorption rate is another importantfactor in the evaluation of the industrial application of thistechnology.12 Some specific ILs, which can absorb CO2 quickly,have been synthesized and investigated.11,13,14 It took only 4 minfor poly-ILs to reach their 90% absorption capacities and about30min to reach their full capacities.13 CO2 absorption for superbase-derived protic ILs could be almost completed within 5 min.11

IL�amine solutions reached over their 90% absorption capacitywithin 15 min, and the reaction was completed after 25 min.14

However, it takes about 3 h to reach equilibrium for most other ILswhether by absorbing CO2 physically or by the amino-functio-nalization ILs reacting with CO2.

8,13 The absorption rate plays an

important role in CO2 capture for both common ILs and task-specific ILs.15

On the other hand, the properties of ILs, particularly theviscosity, are changed after CO2 absorption, which affects furtherCO2 absorption. The viscosity of some amino-functionalizedanion-tethered ILs increased by a factor of 2 when fully com-binedwithCO2.

16But the increasing viscosity of amine-functionalizedILs after absorbing CO2 was due to strong hydrogen-bondednetworks, which might be the hurdle preventing the applicationof ILs as the CO2 absorbent.

17 The basic ILs also suffered fromsimilar viscosity increment after CO2 uptake.

5 Interestingly, theviscosity of one basic ionic liquid was found to decrease aftercomplexing with CO2, a result that might be related to theabsence of hydrogen-bonded networks.5 The viscosity of notonly the functionalized ILs but also the common ILs could increaseor decrease aftermixing withCO2.

18Moreover, the thermodynamicfactor 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 characterizethe CO2 absorption is necessary. The volumetric method is thecommon way to measure CO2 solubility in ILs,9,10,16,19�23 butwith the volumetric method, the absorption kinetics are difficultto determine. In this regard, TGA seemed to be a fast and cheapway to measure the CO2 absorption capacity and rate simulta-neously. Usually, TGA is used to evaluate the thermal stability ofsubstances, but recently TGAwas used to desorb CO2 which wasabsorbed in ILs.5 TGA was also applied to absorb CO2 inaqueous amine solutions as a function of time. Taking a cuefrom the special application of the above-mentioned TGA andthe efficiency of TGA, we propose to measure the absorption

Received: October 7, 2011Revised: November 6, 2011

5811 dx.doi.org/10.1021/ef201519g |Energy Fuels 2011, 25, 5810–5815

Energy & Fuels ARTICLE

capacity and absorption rate of CO2 in ILs by TGA. In this work,11 ILs (Table 1) were screened to absorb CO2 concerning boththe capacity and rate. These ILs vary in anion, cation, chainlength, and C2 (the carbon between the two nitrogen atoms ofthe imidazolium ring) methylation. We intend to study the effectof 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 purchasedfrom Beijing Huayuan Gas Chemical Industry Co., Ltd. (Beijing China).All of the ILs with a purity over 99.9 wt % were purchased from LanzhouGreenchem ILs, LICP, CAS, China (Lanzhou, China). [BMIM][Ac],[BPy][Ac], and [DMIM][BF4] were dried at 50 �C under vacuumconditions for 96 h before use. The remaining ILs were dried at 60 �C for48 h under vacuum conditions. The reason for drying [BPy][Ac] and[DMIM][BF4] at the lower temperature 50 �C is that we witnessed acolor 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 anacetate anion and a long chain length octyl.24 Another reason for dryingthem so long is that ILs with the anion [Ac] were found to absorb watervery strongly;25 long-time drying for [BMIM][Ac] and [BPy][Ac] at arelatively low temperature must be ensured. After drying under vacuumconditions, the purity of all of the ILs was verified by 1H and 13C NMR.No impurities and no degradation products were detected in the NMRspectra. Furthermore, in order to exclude the water absorption from theair during the transfer process from the reagent bottle to the platinumsample pan of the TG apparatus, these ILs, particularly for the [Ac]anions, were purged with N2

26 at 50 �C in the TG apparatus until theweight change rate was below 10 � 10�6 mg/min. Since [BMIM][Ac]has a strong water-absorbing ability, its water content was immediatelydetermined by Karl Fischer titration25,27�30 (ZDJ-400S, Multifunctionaltitrator, Beijing Xianqu Weifeng Company, Beijing, China) from the

platinum sample pan. The water content was found to be lower than45 ppm. The other less hydroscopic ILs showed water contents less than34 ppm. Therefore, the mass gain in the TG curve (weight vs. time) canbe attributed to the CO2 absorption by ILs, regardless of the influence ofthe 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 TAInstruments Q50-TG. After the sample was loaded, the TG apparatuswas first purged with a N2 atmosphere26 (80 mL/min, 1 bar) at 50 �C inan isothermal mode to remove the possible water absorption from the airduring the transfer process. The high N2 flowing rate (80 mL/min) wasset to remove the volatile impurities in ILs faster.26 The impurity-removing process was not conducted at a higher temperature but at50 �C to avoid unexpected thermal degradation, because a color changeoccurred for [BPy][Ac] and [DMIM][BF4] at 60 �C after drying undervacuum conditions. After the weight�time curve remained nearlyhorizontal and constant (the mass loss rate is less than 10 � 10�6

mg/min), the sample was then purged in a CO2 atmosphere (5mL/min,1 bar) at 50 �C to monitor the CO2 absorption process. The very lowCO2 flowing rate (5 mL/min) ensured a minimum buoyancy influenceon the CO2 absorption. The volume of the TG column was estimated tobe about 30mL (radius = 1 cm, height = 10 cm). This little volume couldensure that the CO2 replaced the column space quickly (less than6 min), minimizing the effect on the initial absorption rate (r10). The ILswere spread in as well-distributed a manner as possible in the TGcrucible in every experiment to ensure an equal contact area for CO2

capture. It could minimize the effect of surface area of ILs on theabsorption capacity and rate. The mass precision of TG is(0.1 μg withthe weight ranging from 0 to 1 g. The temperature precision is(0.1 �C.All of the samples were averaged four times with these themicroscale TGstudy methods; the deviations were below (1.2%.

3. RESULTS AND DISCUSSION

Capturing CO2 with ILs involves the capacity and absorptionrate.12,15 The absorption rate varies with the time, and it isassociated with the potential of ILs to absorb CO2 and the abilityof ILs to reach phase equilibrium. Thus, we use the initialabsorption rate (r10) and degree of difficulty to reach phaseequilibrium (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 absorptionrate (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 themixtures with the time.The purpose of multiplying the mole fraction by 100 here is onlyfor easy description. The initial absorption rate (r10) refers toCO2 absorption capacity during the first 10 min. We find that theCO2 absorption capacity vs time curve is near linear in the first10 min, consistent with the previous finding;31 thus the absorptioncapacity in the first 10 min could represent the initial absorptionrate. The degree of difficulty to reach phase equilibrium (t0.9) isexpressed as the time when 90% of the CO2 absorption capacityis reached. Phase equilibrium of CO2 capture could be hard toreach due to the viscosity of ILs, especially when large quantitiesof ILs are used in the application, a way that could causediffusivity difficulty. Moreover, the equilibrium state is not easyto determine and is unnecessary to arrive at in practice. Thus, anequilibrium time at which 90% of CO2 absorption capacity isreached could be deemed a choice to represent the degree ofdifficulty to reach phase equilibrium. Actually, 90% of the CO2

Table 1. Structure, Name, and Abbreviation of ILs



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 inFigures 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 C2substitution) have nearly the same tendency; i.e., ILs with highercapacity have a higher initial absorption rate (Figure 1). In particular,ILs with the [Ac] anion involving chemical absorption20,29,30 havethe advantage in both absorption capacity and the initial absorp-tion rate over the other ILs absorbing CO2 physically. In this way,we may preliminarily estimate the absorption capacity using theinitial absorption rate, which can be easily determined by TGA.However, the tendency between the absorption capacity (x) anddegree of difficulty to reach phase equilibrium (t0.9) for these 11ILs is complicated (Figure 2). This may be caused by a differentproperty change, especially viscosity,18 for different ILs after theyabsorb 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 timewasmeasured byTGA at50 �C and atmospheric pressure (Figure 3). All of the ILs have thesame cation [BMIM], but the anions span awide range of chemicaltypes and basicity. The absorption capacity and the initial absorp-tion 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 adifferent 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 micro-balance measurement, with the deviation ranging from 8.04% to13.3% (Table 2). The sample weight difference (micro for TGAbutmacro for the latter) and theCO2 flowing state (5mL/min forTGA but usually static for the latter) might be reason for thosedeviations. Even though, the maximum deviationmeasured in thesame way can reach 20% by different reports,32,34 indicating aproper deviation value for our results. However, a remarkablediscrepancy 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

Table 2. CO2 Absorption Rate and Capacity Value at 50�Cand atmospheric pressure by TGA

xc

ILs r10a t0.9

b/min TGA exptl.

[BMIM][Ac] 5.207 43.3 9.500 20.430

[BMIM][TFO] 0.466 92.9 1.840

[BMIM][PF6] 0.441 69.4 1.133 1.2,32 1.0,34 1.0133

[BMIM][BF4] 0.425 83.0 1.090 1.232

[BMIM][NO3] 0.279 82.5 0.921

[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 0.986aCO2 absorption capacity during the first 10 min, indicating the initialabsorption rate. bThe time when 90% of CO2 absorption capacity isreached, indicating the degree of difficulty to reach phase equilibrium.c 100 mol fraction of CO2 in ILs, indicating the CO2 absorption capacity.

Figure 1. Initial absorption rate r10 and absorption capacity x of CO2 inILs at 50 �C and atmospheric pressure by TGA.

Figure 2. Practical phase equilibrium time at 90% capacity t0.9 andabsorption capacity x of CO2 in ILs at 50 �C and atmospheric pressureby TGA.

Figure 3. Effect of anion of ILs on the CO2 absorption rate and capacityat 50 �C and atmospheric pressure by TGA.



interact.28 But even when we dried the IL in a vacuum drying ovenat 50 �C for another 48 h and removed volatile impurities withN2

26

at 50 �C in TG until the weight�time curve remained nearlyhorizontal, there was no substantial CO2 absorption capacitychange. It cannot be ascribed to some extent of decompositionoccurring after the drying process, because no decompositionproducts were detected in 1H and 13C NMR. The possibility ofN2 solubility in [BMIM][Ac] was also excluded, for there was nosign ofmass gain in theweight�time curves after theN2 purge. Theabnormal deviation for CO2 capture by [BMIM][Ac] awaitsfurther investigation. After all, measuring the absorption kineticsby TGA is convenient and efficient. In addition to the absorptioncapacity, it could also reflect the initial absorption rate and thedegree of difficulty to reach phase equilibrium.The IL with the [Ac] anion has a considerably higher absorp-

tion capacity and initial absorption rate for CO2 than any otherfour ILs (Figures 1 and 3). Furthermore, its degree of difficulty inreaching phase equilibrium is the shortest (Figure 2), indicatingthe easiest phase equilibrium when capturing CO2. The lowviscosity25 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 benegligible at atmospheric pressure. The IL containing fluoroalkylgroup [TFO] has the second highest absorption capacity andinitial absorption rate (Figures 1 and 3). This might be due tofavorable interactions between CO2 and the fluoroalkyl substi-tuent on the anion.21 Interestingly, we found that the phaseequilibrium of [BMIM][TFO] and CO2 is the most difficult toreach (Figure 2); one possible explanation is that it suffers fromsome 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 fluorinatedanions, 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 phaseequilibrium with CO2 is observed for [BMIM][PF6] than for[BMIM][BF4] (Figure 2). The absorption capacity and the initialabsorption rate of CO2 in the IL with nonfluorinated anion[BMIM][NO3] are the least, as is expected.6,21 The degree ofdifficulty to reach phase equilibrium is nearly the same with[BMIM][BF4].

3.2. Effect of Cation. Like the absorption capacity and initialabsorption rate of the ILs varying in the anion, these properties ofthe ILs with different cations have the same tendency (Figure 1).Table 2 shows that the solubilities of CO2 in imidazolium andpyridinium [BF4] compounds at 50 �C and atmospheric pressureare virtually identical. The low-pressure solubility of CO2 for thecorresponding [Tf2N] salts was also reported to be very similar.

35

In these ILs with the physical absorption anion, the primaryinteractions of the CO2 appear to be with the anions, and thecation plays a less important role. Very unexpectedly, when weinvestigated 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 alsoplay an important role in determining the CO2 solubility for ILsinvolving a chemical absorption anion. One possible explanationis that a stable adduct of imidazolium carboxylate forms accom-panied by a loss of acetic acid when CO2 interacts with the C2position of the imidazolium ring, but there is no such interactionfor the pyridinium ring.30 Also, it is reported that the cation couldplay a more important role than the anion in CO2 absorption forthe poly-ILs.13 Clearly, one can even design ILs directly inter-acting 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). Thismay be due to the same stable imidazolium carboxylate.30 But forILs [BMIM][BF4] and [BPy][BF4] with the physical absorptionanion, the difference in the difficulty to reach phase equilibrium isnot so substantial, probably because of the lack of such a stableimidazolium carboxylate formation.30

3.3. Effect of Alkyl Chain Length. To study the influence ofcation alkyl chain length, [BMIM][BF4], [HMIM][BF4],[OMIM][BF4], and [DMIM][BF4] were chosen as the subjectsof investigation. The solubility increases slightly as the chainlength increases from the butyl to octyl (Figures 2 and 5). Theresults are consistent with the ILs with [BF4] anions when thealkyl chain length was increased from butyl to octyl.22 Resultsalso show that the solubility of CO2 in [PF6]-based ILs

36,23 and[Tf2N]-based ILs21 increased when the alkyl chain length wasincreased from ethyl to hexyl and from butyl to octyl, respec-tively. Higher solubility in ILs with longer cation alkyl chains(from butyl to octyl) can be ascribed to the ILs with longer alkylchains having greater free volume.21 However, the solubilitydecreases slightly as the chain length increases from octyl todocanyl (Figures 1 and 5). Therefore, an optimum chain length

Figure 4. Effect of cation of ILs on theCO2 absorption rate and capacityat 50 �C and atmospheric pressure by TGA.

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



(C8) for maximum CO2 capture occurs. The viscosity increaseswith increasing alkyl chain length37,24,3 while the IL becomes tooviscous with the longest chain length, which might prevent thefree volume space of the long alkyl chain length from holdingmuch CO2. Likewise, the investigation for the CO2 solubility influorocarbons also states that there will be an optimum numberof fluorine atoms for maximum CO2-philicity, and then con-tinued increases in fluorine may lead to reduced CO2-philicity.

38

So, continually increasing the chain length does not necessarilylead to a proportionate increase in CO2 solubility, which isimportant 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). Butinterestingly, the initial absorption rate of [HMIM][BF4] is theslowest for the four ILs varying in alkyl chain length (Figures 1and 5). It is probably due to ILs with a moderate chain lengthmaybe 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 para-meters (the absorption capacity, the initial absorption rate, and thedegree of difficulty to reach phase equilibrium) than that with[BMIM][BF4] (Figures 1, 2, and 6). The acidic C2 hydrogen inthe imidazolium ring has the potential to interact with the oxygenatoms on theCO2.Methylation of the C2 position could eliminatethe C2�H/O interactions,39 thus making the CO2 solubilityin [BMMIM][BF4] 9.5% lower than that in [BMIM][BF4](Table 2). The difference in solubility may become moreapparent at a higher pressure.21 Even though [BMMIM][BF4]with C2 methylation has a slightly lower absorption capacity andinitial absorption rate than [BMIM][BF4], it can reach phaseequilibrium with CO2 quickly (Table 2). This indicates that theC2�H in the imidazolium ring can increase the CO2 affinity, andtherefore the initial absorption rate, but could not make its phaseequilibrium time shorter.

4. CONCLUSION

A cheap and fast way to determine the absorption kinetics andcapacity of CO2 in ILs by TGA was provided. Three parameterswere proposed to indicate the efficiency of CO2 capture by 11ILs, including the absorption capacity, the initial absorption rate,and the degree of difficulty to reach phase equilibrium. Theabsorption capacity and the initial absorption rate of CO2 hadalmost the same tendency, indicating a convenient way to estimate

the CO2 absorption capacity just by the r10 value. However, therelationship for the absorption capacity and the degree of difficultyto reach the phase equilibrium was complicated.

ILs with the chemical absorption anion [Ac] could captureCO2 faster and have a higher absorption capacity than other ILsinvestigated. [BMIM][TFO] with the fluoroalkyl group couldabsorb CO2 with a higher capacity and bigger initial absorptionrate than ILs with inorganic fluorinated anions ([BF4], [PF6])and nonfluorinated anions ([NO3]), but these had the mostdifficulty reaching phase equilibrium, which was probably due tosome property changes upon uptaking CO2. The chemicalabsorption anion [Ac] could also play an important role in theCO2 solubility and the initial absorption rate. On the contrary,the physical absorption anion [BF4] determined the CO2 ab-sorption capacity and the initial absorption rate, with the cationtype (imidazolium and pyridinium) showing a minor effect.Moreover, octyl was the optimal cation alkyl chain length forthe maximum CO2 absorption capacity and for the easiest phaseequilibrium with CO2, but ILs with a hexyl chain length had themaximum initial absorption rate for CO2. Finally, methylation atthe C2 position in the imidazolium ring slightly decreased the CO2

absorption capacity and the absorption rate but slightly increasedthe degree of difficulty to reach phase equilibrium.

’AUTHOR INFORMATION

Corresponding Author*Tel.: +86-10-62514925. Fax: +86-10-62516444. E-mail:[email protected].

’ACKNOWLEDGMENT

This work was funded by the Natural Science Foundation ofChina (21173267).

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determination of absorption rate and capacity of co 2 in ionic liquids...

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