Thermal Stability of Ionic Liquids for Their Application as New

Oct 14, 2013 - absorption heat pumps and refrigeration cycles. First, the influence of experimental conditions such as sample mass, heating rate,...
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Thermal Stability of Ionic Liquids for Their Application as New Absorbents María Villanueva,† Alberto Coronas,§ Josefa García,# and Josefa Salgado*,† †

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Thermophysical Properties of Fluids and Biomaterials Group, Applied Physics Department, Faculty of Physics, University of Santiago de Compostela, Santiago de Compostela 15782, Spain § CREVER-Mechanical Engineering Department, Universitat Rovira i Virgili, Av. Paı̈sos Catalans 26, 43007 Tarragona, Spain # Department of Applied Physics, University of Vigo, Lagoas-Marcosende, 36310 Vigo, Spain ABSTRACT: The thermal stability of three ionic liquids (ILs) with the same anion and different lengths of imadazolium cations was investigated using thermogravimetric analysis to consider their usefulness for their potential application as absorbents in absorption heat pumps and refrigeration cycles. First, the influence of experimental conditions such as sample mass, heating rate, and atmosphere was analyzed for one of these ILs. Afterward, dynamic and isothermal studies were performed for the three ILs. From dynamic results, onset temperature has been often selected to define the thermal stability. Nevertheless, to obtain a more precise value, long-term isothermal studies at temperatures lower than the onset one were required, showing that the thermal stability decreases slightly as the length of the cations increases. Using the Arrhenius relationship, the kinetics of isothermal decomposition was also evaluated, and an upper limit for the “working” time of these ILs as potential absorbents was estimated.

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INTRODUCTION Over the past decade, room temperature ionic liquids (ILs) have been attracting attention for their wide variety of potential applications due to their interesting properties such as high thermal stability and conductivity and low melting point. For example, several books, more than 14000 articles, and many patents involving ILs and their impact in energy technologies have been published. Thus, ILs play an important role in energy technology, for example, as thermal fluids, as potential absorbents in absorption heat pumps and refrigeration cycles, in capture and conversion of solar energy into electricity, in the conversion of raw biomass or fossil resources into cleaner fuels, and in enabling advanced nuclear energy technologies. On the other hand, although devices are still in many cases in the research and development stage, commercial applications of ILs continue.1−6 To take advantage of their potential favorable properties, in particular, the development of ILs as absorbents with better performance and higher working maximum temperature than those corresponding to the conventional ones (lithium bromide for water or water for ammonia) is suggested. From 2006, companies such as BASF, DuPont, and Evonic have patented new combinations of ILs with water, ammonia, carbon dioxide, alcohols, fluorinated alcohols, and hydrofluorocarbons as refrigerants.7 Many of these investigations are focused on developing new mixtures of H2O plus ILs to replace LiBr in absorption chillers8−14 or to replace water in the ammonia plus water system.15−17 Other researchers propose new mixtures of ILs with CO2,18−20 alcohols such as 2,2,2-trifluoroethanol or methanol, and hydrofluorocarbons such as HFC134a. In addition, Nakata et al.21 introduced the use of ILs as working fluids for heat pipes determining different thermal properties (as thermal stability of ILs using a TG device in nonisothermal mode (10 °C min−1)) for binary mixtures of water and five ionic liquids. © 2013 American Chemical Society

Nevertheless, investigations are still very incipient and are focused, for example, on the selection of ILs, compatibility of materials, and measurement and modeling of thermophysical properties, such as thermal stability, density, heat capacity, enthalpy of mixing, solubility, surface tension, and viscosity because they play an important role in determining the performance of absorption chillers and heat pumps and simulating absorption cycles. Particularly, the thermal stability of ionic liquids is a key property for industrial applications, namely, for absorption refrigeration cycles. Low thermal stability may limit and decrease the performance in this industrial application; furthermore, in the case of formation of corrosive and toxic products, operational safety can also be reduced. In addition, the employment of ILs as absorbents often requires extended operations at elevated temperatures; therefore, it is essential to determine also the long-term thermal stabilities of the ILs and also the time of degradation at working temperatures. Several research studies22−24 have concluded that the thermal stability of ILs depends strongly on the anion, increasing with their size, [Tf2N]− being one of the most stable, whereas the cation seems to have lower influence on this property. Furthermore, the highly fluorinated anions are also very interesting as absorbents owing to their hydrophobicity, conductivity, and low viscosity because they can offer more efficient mass transport and heat transfer.25 Thermogravimetric (TG) analysis is the most used technique to determine the thermal stability of a substance, commonly, through a single linear heating rate over a wide interval of temperatures. It is well known that the experimental conditions Received: Revised: Accepted: Published: 15718

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determination of the activation energy using the Arrhenius equation to analyze the degradation process. (4) Estimation of the useful time of [C2MIM][Tf2N], [C3MIM][Tf2N], and [C4MIM][Tf2N] absorbents at different working temperatures.

can affect considerably the results obtained from thermal analysis techniques. In particular, the atmosphere, the scanning rate, and the sample mass can shift the temperatures associated with a thermal process. Thus, as an example, Fernandez et al.26 found that the peak in the derivate thermogravimetric (DTG) curve of the decomposition of [C2 MIM][EtSO4 ] and [C4MIM][MeSO4] increased 50 °C when the heating rate went from 2 to 16 °C min−1. Kosmulski et al.27 observed an increase up to 50 °C when the mass of the sample doubled in studies of 1-alkyl-3-methylimidazolium phosphates and 1-decyl3-methylimidazolium triflate. Another aspect to take into account is that the onset and peak decomposition temperatures obtained from a single temperature ramp experiment, even at low scanning rates, are often overestimations of the long-term thermal stabilities of the ILs.27,28 Certainly, isothermal studies have shown that the ionic liquids exhibit appreciable decomposition at temperatures significantly lower than those indicated by the peak or onset decomposition temperatures determined from scanning TG experiments.27−29 Zhang et al.30 have investigated long-term isothermal stabilities of 1-butyl-3 methylimadazolium bis(trifluoromethylsulfonil)imide ([C4MIM][Tf2N]), observing that this IL decomposes at temperatures significantly lower than the onset temperature, traditionally determined as the temperature at which the loss weight starts, the degradation rate being faster with increasing rate. Additionally, Baranyai et al.31 have reported that ionic liquids yield volatile products at temperatures essentially lower than the onset decomposition temperature. Recently, for 1-ethyl-3 methylimadazolium bis(trifluoromethylsulfonil)imide ([C 2 MIM][Tf 2 N]) and [C4MIM][Tf2N], Heym et al.32 have concluded that both ILs are relatively volatile; that is, depending on the conditions (e.g., heating rate) evaporation and/or decomposition may determine the mass loss even at ambient pressure. In the present work, the aim is to give a first step in the study of the thermal stability of three ILs containing imidiazolium cations and the [Tf2N]− anion for their application in absorption heat pumps, taking into account the operational temperatures in absorption and refrigeration cycles. We have analyzed 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonil)imide ([CnMIM][Tf2N], n = 2, 3, 4). To our knowledge, the thermal stability of 1-propyl-3 methylimidazolium bis(trifluoromethylsulfonil)imide ([C3MIM][Tf2N]) has not been reported. Thus, the scarcity of reliable long-term thermal stabilities of IL references and the unavailability of studies for estimation of the useful time at different working temperatures lead to the current research. A deep study of the thermal stabilities of these ILs was performed, through the following sequence: (1) Analysis of the influence of the atmosphere, the heating rate, and the sample mass on the thermal stability of one of these ILs. (2) Isothermal study of [C2MIM][Tf2N], [C3MIM][Tf2N], and [C4MIM][Tf2N] ILs at six different temperatures lower than the onset temperature determined through a dynamic scan at 10 °C min−1 under N2 atmosphere. The selection of N2 and air atmospheres to perform this study is a previous step to more specific studies with other atmospheres related to absorption heat pumps and refrigeration cycles. Inert and air atmospheres are extreme conditions in the cited application, because in absorption and refrigeration cycles, atmospheres of refrigerants (such as ammonia, water, and carbon dioxide) are involved. (3) Kinetic study of the degradation process of [C2MIM][Tf2N], [C3MIM][Tf2N], and [C4MIM][Tf2N] ILs and

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EXPERIMENTAL SECTION Materials. The three ILs selected were 1-ethyl-3 methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2MIM][Tf2N]) (CAS Registry No. 174899-82-2), 1-propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C3MIM][Tf2N]) (CAS Registry No. 216299-72-8), and 1-buthyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4MIM][Tf2N]) (CAS Registry No. 174899-83-3). All of them were provided by Sigma-Aldrich with purity >97%, although chromatography of ionic exchange, HPLC, or electrophoresis confirmed that the purities were >99%. All ILs were studied as received (not dried), and the water content, measured with a Karl Fischer coulometer (Mettler Toledo) was 300 °C in all cases, but degradation temperatures increased with the heating rate, revealing that the onset temperature increased by approximately 100 °C when the rate grew from 1 to 20 °C min−1. Tpeak, Tendset, T2%, T5%, and T10% show the same behavior as Tonset; they increase with heating rate. According to Hatakeyama and Quinn,42 the cause of this behavior is that at low heating rates the sample temperature is more uniform and diffusion of product gases can occur within the sample, lowering the decomposition temperature. Furthermore, the discrepancy between the true sample temperature and the programmed temperature increases at high heating rates. As can be observed in Table 3, the degradation decomposition interval is narrower as the heating rate decreases. This is also in concordance with Hatakeyama and Quinn.42 Although linear correlations, as used by Fernandez et al.,26 fit quite well, second-degree polynomial functions present better results, as can be seen in Figure 4 and Table 4. In this equation the T0 parameter represents the characteristic temperature at which the degradation process is almost null (v ≈ 0 °C min−1). As can be seen, the T0 value corresponding to Tonset is >350 °C, so, according to that, the hypothesis that the IL [C2MIM]-

between mass and Tonset, Tendset, and Tpeak were observed (R2 ∼ 0.99, p < 0.05) for the studied sample mass. Influence of Scanning Rate. It is well known that the scanning rate can have a strong influence on the nonisothermal thermogravimetric results,39−41 and this aspect has caused discussions between scientific community experts in this field since the end of the 1980s.42 However, despite the great number of references about the thermal stability of ILs, very few present an in-depth study addressing the influence of this parameter; some of them analyzed the kinetics of decomposition from the TG curves obtained at different heating rates,33,41 others consider it, joined with other techniques, to make analyses of vaporization,32,38 and only, up to now, Fernandez et al.26 established a linear relationship between the decomposition temperature and the scanning rate in the TG study of [C2MIM][EtSO4] and [C4MIM][MeSO4]. Additionally, Seeberger et al.43 concluded that to know the real thermal stability, dynamic scans are not enough and isothermal studies must be done. TG and DTG curves of [C2MIM][Tf2N] obtained at six different heating rates, 1, 3, 5, 10, 15, and 20 °C min−1, in air atmosphere and with similar mass, 2.5 ± 0.2 mg, are presented in Figure 3. Onset and endset temperatures and those corresponding to the loss of 2, 5, and 10% of mass were

Table 2. Characteristic Temperatures and Mass Loss at Tonset (Δmonset) of [C2MIM][Tf2N] with Different Sample Masses Obtained in Air Atmosphere mass/mg

Tonset/°C

Tendset/°C

Tpeak/°C

T2%/°C

T5%/°C

T10%/°C

Δmonset/%

1.443 3.594 5.687 7.622

431 439 451 458

485 494 506 512

478 485 494 498

375 367 377 388

392 392 404 415

409 414 436 436

23 23 22 21

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Table 3. Characteristic Temperatures and Mass Loss at Tonset (Δmonset) of [C2MIM][Tf2N] at Different Heating Rates, under Air Atmosphere rate/ °C min−1

Tonset/°C

Tendset/°C

Tpeak/°C

T2%/°C

T5%/°C

T10%/°C

Δmonset/%

1 3 5 10 15 20

354 384 395 425 440 452

395 430 443 477 494 507

390 424 435 468 484 494

316 325 330 363 367 375

328 343 349 384 392 404

338 358 367 402 414 425

23 25 26 23 23 24

Figure 4. Correlations between Tonset, Tendset, Tpeak, T2%, T5%, and T10% and the heating rate. Dark lines represent the fits to a second-degree polynomial function.

Table 4. Fitting Parameters to the T (°C) = T0+ bv + cv2 Equation (v, Heating Rate) for Tonset, Tendest, Tpeak, T2%, T5%, and T10% Temperatures of [C2MIM][Tf2N] T0/°C Tonset Tpeak Tendset T2% T5% T10%

350.1 385.1 390.1 307.1 320.1 330.2

± ± ± ± ± ±

5.5 6.1 6.4. 5.3 4.6 3.4

b/min 10.1 11.3 17.7 11.3 7.6 9.0

± ± ± ± ± ±

1.4 1.6 1.7 1.4 1.2 0.8

c/(min2/°C) −0.253 −0.299 −0.300 −0.159 −0.172 −0.217

± ± ± ± ± ±

0.066 0.074 0.077 0.065 0.055 0.041

R2 0.9863 0.9856 0.9860 0.9724 0.9864 0.9940

[Tf2N] would be thermostable up to 350 °C can be established. To confirm that, isothermal scans at temperatures lower than these values were preformed and are presented in the next section. Thermal Degradation of [CnMIM][Tf2N]. Dynamic Scans. Figure 5 presents the dynamic TG and DTG plots for [CnMIM][Tf2N], n = 2, 3, 4, in N2 atmosphere, with a scanning rate of 10 °C min−1 and a sample weight of 7.5 ± 0.5 mg. The thermal degradation behavior is similar in all of them, characterized by a significant loss of weight after 350 °C in a single step. The rate of degradation increases between 400 and 480 °C approximately, when the maximum is raised (>15% loss mass per degree) and from this temperature decreases until 550 °C, at which the thermal degradation is almost completed. Although dynamic thermal stability is similar in the three ILs, small differences can be found. Table 5 shows calculated values for onset temperatures (Tonset), percentage of loss mass at these onset temperatures (Δmonset), peak temperatures in DTG curves (Tpeak), temperatures at 5% mass loss (T5%), and char yields (%) at 550 °C. For all TG analyses, the mean of, at least, two replicate measurements is reported in this table. The temperatures have uncertainties of σ = ±3 °C. The values of Tonset and Tpeak for the [C2MIM]+ cation are slightly higher (8 and 11 °C, respectively) than those corresponding to the other two cations, which present no

Figure 5. TG and DTG curves of the [CnMIM][Tf2N] (n = 2, 3, 4) ILs in N2 atmosphere, with scanning rate of 10 °C min−1 and sample weight of 7.5 ± 0.5 mg.

significant differences. However, the values of the temperature at 5% loss do not show differences with regard to the cations, as well as the mass loss at Tonset that rose to values of 20%. Almost all of the sample is practically disappeared at 550 °C, with a residue around 1% of the initial mass. The trend followed for the cations, according to the dynamic study, is [C2MIM]+ > [C3MIM]+ ≥ [C4MIM]+. This trend 15722

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Table 5. Characteristic Temperatures (Tonset, Tpeak, T5%), Mass Loss at Tonset (Δmonset), and Residue at 550 °C for the Dynamic Degradation Process at 10 °C min−1 in N2 Atmosphere IL

Tonset/°C

Δmonset/%

Tpeak/°C

T5%/°C

char (%)

[C2MIM][Tf2N] [C3MIM][Tf2N] [C4MIM][Tf2N]

446 438 438

21.0 20.0 19.6

487 479 476

405 406 401

0.6 0.9 0.8

overestimation of the operation level of an IL. Nevertheless, this characteristic temperature can be useful as a comparative parameter. Isothermal Scans. Previous works22,27,33,43 indicate that to predict the long-term thermal stability, thermal decomposition mechanisms and kinetics of these ILs must be investigated using isothermal TG. Figure 6 shows the isothermal scans of the ILs. The temperature selection was done by taking into account the following items: (1) The onset temperature overrates the thermal stability; thus, the isothermal scans must be done at temperatures lower than the onset one. In this sense, it is important to note that, despite the fact that the heating rate has a strong influence on this value, actually the dynamic scan can be performed to

agrees with that of Tokuda et al.,44 who found that the onset temperature in TG experiments of [C2MIM][Tf2N] is 12 °C higher than that corresponding to [C4MIM][Tf2N], whereas in our work the differences are 8 °C. Table 6 shows onset temperatures reported by other authors. Although these temperatures have a strong dependence on the Table 6. Previously Published Onset Temperature Data IL

Tonset/°C

ref

[C2MIM] [Tf2N]

455 419 391 417 455,c 453d 452 399 452,c 453d

23 32a 54b 55 22 23 54b 22

422 439 427 461 409 403 450 450

24 23 44 29 32a 54b 56 57e

[C3MIM] [Tf2N]

[C4MIM][Tf2N]

a

Tonset is calculated as the temperature at which 1% mass loss occurred. The temperature of decomposition is calculated as the temperature with 10% mass loss. cAluminum sample pan. dAlumina sample pan. e Dynamic scan was performed under argon atmosphere. b

experimental conditions, as was demonstrated in the previous section of this work, the average relative deviation between our results and those of other authors was 20% of mean deviations in our case, the maximum temperature must be lower than that calculated previously. Wooster et al.34 suggest that this maximum operating temperature should be at least 10 °C lower than the calculated one. Although Heym et al.38 established that for ILs such as [C2MIM][Tf2N] and [C4MIM][Tf2N] in permanent contact with a gas flow mass losses by evaporation can happen, the maximum operation temperature depends on both decomposition and evaporation limits. For a typical single-effect absorption cycle the higher temperature is the generator temperature, about 90−100 °C.53 From the results obtained in this paper, the “working time” for [C2MIM][Tf2N] as absorbent can vary from almost 1 year to 5 years, depending on the mass loss considered (2, 5, or 10%). Thus, at this temperature, this IL shows good thermal stability. Finally, the results obtained in this work for the three selected ILs would allow using the ILs during long periods of time at temperatures slightly superior to the ones used now.

of 2, 5, and 10% can be estimated for each IL. Figure 10 shows these estimated values against temperature. An exponential

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CONCLUSIONS In this work, the influence of the experimental conditions, namely, atmosphere, sample mass, and scanning rate, on the study of thermal stability is demonstrated. Onset temperature determined in nitrogen atmosphere is higher than that obtained under air atmosphere, and it increases with sample mass and scanning rate. Onset decomposition temperatures obtained using TG dynamic experiments under nitrogen atmosphere are >400 °C for the three ILs studied. These values are overrated, and true stability temperatures are lower than those; they should be evaluated only by isothermal scans. Even the estimated T2% temperature for null heating rate is too high to ensure that the liquid preserves its properties at these temperatures. [C2MIM][Tf2N] presents the highest stability from both dynamic and isothermal scans. The reason is that, although the anion presents a major influence on the thermal stability, this property is also affected by the cation; as the cation chain is shorter, thermal stability is higher. In addition, it was demonstrated that at the lowest temperature, the mass shows practically no variation in experiments during >10 h and is independent of the atmosphere. The Arrhenius equation allows the determination of the activation energy of the thermal degradation process, which takes similar values for the three ILs analyzed, of around 120 kJ mol−1 in the three ILs. The thermal stability of these ILs is higher than that of conventional absorbents in absorption and refrigeration cycles (lithium bromide or water). This fact, together with the suitable values of other thermophysical properties such as density, viscosity, vapor pressure, and heat capacity, makes them possible as new absorbents. Nevertheless, deeper analyses of

Figure 10. Exponential decay (R2 > 0.9 and p < 0.05) of time for 2, 5, and 10% degradation against temperature.

decay following the next mathematical function with temperature can be observed: td = a + b e−cT

(5)

td (in minutes) is the time for a defined mass loss, T is temperature in °C, and a, b, and c are constants. [C2MIM][Tf2N] and [C4MIM][Tf2N] presented the best fits (R2 > 0.99 and p < 0.001), whereas the results for [C3MIM][Tf2N] were worse (R2 > 0.90 and p < 0.05). The three ILs showed a complete degradation at 380 °C, which is lower than Tonset and T0, making it evident, once again, 15725

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the properties of the possible compatible refrigerants and of the refrigerant−absorbent solution should be performed. In addition, as future work, analysis in other different atmospheres, depending on the used refrigerant, and study of mass losses due to evaporation or decomposition of ILs will be performed.

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AUTHOR INFORMATION

Corresponding Author

*(J.S.) Phone: +34881814110. E-mail: j.salgado.carballo@usc. es. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Xunta de Galicia (PGIDIT07PPXIB314132PR).

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REFERENCES

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dx.doi.org/10.1021/ie401656e | Ind. Eng. Chem. Res. 2013, 52, 15718−15727