- Email: [email protected]
Thermal stability and specific heats of coordinating ionic liquids
Thermochimica Acta 684 (2020) 178482
Contents lists available at ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Thermal stability and specific heats of coordinating ionic liquids Bernardo Monteiro, Leonor Maria, Adelaide Cruz, José M. Carretas, Joaquim Marçalo, João P. Leal*
T
Centro de Ciências e Tecnologias Nucleares (CbTN) and Centro de Química Estrutural (CQE), Instituto Superior Técnico, Universidade de Lisboa, 2695-066, Bobadela, Portugal
ARTICLE INFO
ABSTRACT
Keywords: Ionic liquids Thermal stability Specific heats TGA DSC
Ionic liquids (ILs) have been revealed as promising solvents, namelly for the selective extraction of metals from aqueous solutions. ILs extractive properties can be trimmed but to do that in a programmed and educated way it is necessary to know their basic properties. The range of temperature in which they can be used as solvents as well as their specific heat, necessary to perform thermodynamic calculations with them, are fundamental properties needed to achieve this desideratum. Thus, the goal of this work has been the study of the thermal stability and specific heats of five ionic liquids that proved to be good extractive solvents.
1. Introduction
2. Experimental
Ionic liquids (ILs) are salts that possess a fusion temperature below 373 K [1]. They can be used in a lot of applications such as organic synthesis [2], pharmaceuticals [3], nuclear fuel reprocessing [4–6], solar thermal energy [7], batteries [8], cellulose processing [9], WEEE (Waste Electrical and Electronic Equipment) reprocessing [10] and reduction of carbon emission [11]. They can also be used as environmentally friendly solvents because of their favourable properties such as extremely low vapour pressure, low flammability, excellent thermal stability, and a wide temperature range in its liquid state. The use of ILs tends to improve the procedures used and make them more environmentally friendly. In a short/medium term they will become unavoidable in the separation and recovery of metals [10]. To have a clear idea of the adequacy of an ionic liquid for these goals, it is necessary to know in detail the physical properties of the ILs. Therefore, the aim of this work is the study of the thermal stability and behavior of three previously known ionic liquids (tetraoctylammonium oleate, [N8888][OL] – IL1 [12]; 1-butyl-3-methylimidazolium-methyloxalate, [C4mim][OX] – IL2 [13]; 1-butyl-3-methylimidazolium-di(2ethylhexyl)-oxamate, [C4mim][DEHOX] ̶ IL3 [13]) and two very recently synthesized ones (tetraoctylammonium dioctyl-diglycolamate, [N8888][DODGA] – IL4 [14]; tetraoctylammonium di(2-ethylhexyl)oxamate, [N8888][DEHOX] – IL5 [14]), all of them composed solely of C, H, O and N atoms (Fig. 1).
2.1. Chemicals
⁎
The ionic liquids used in this study; IL1 - [N8888][OL], IL2 – [C4mim][OX], IL3 – [C4mim][DEHOX], IL4 – [N8888][DODGA] and IL5 – [N8888][DEHOX], were prepared according to published procedures [12–14] and stored in a desiccator filled with silica gel. 2.2. Instrumentation and analysis Mass spectrometry analyses were done by electrospray ionization quadrupole ion trap mass spectrometry (ESI-QIT/MS), with a Bruker HCT, by direct infusion of IL solutions in acetonitrile. 1H and 13C NMR spectra of intermediate compounds and ionic liquids were recorded on a Bruker AVANCE II 300 or 400 MHz instruments. 1H and 13C chemical shifts were referenced to external SiMe4 using the residual proton or carbon of the solvents as an internal standard. CDCl3 and CD3CN were used as solvents for recording the NMR spectra. Before use, the water content in the ILs was determined by Karl Fischer titration (modelMetrohm 831 Karl Fischer coulometer) and taken into account in calculations. 2.3. Samples characterization All IL samples were characterized by 1H and 13C NMR spectroscopy (Supplementary information, Figs. S1 and S2 – IL1, Figs. S3 and S4 – IL2, Figs. S5 and S6 –IL3, Figs. S7 and S8 – IL4, Figs. S9 and S10 – IL5);
Corresponding author. E-mail address: [email protected] (J.P. Leal).
https://doi.org/10.1016/j.tca.2019.178482 Received 30 October 2019; Received in revised form 4 December 2019; Accepted 11 December 2019 Available online 12 December 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved.
Thermochimica Acta 684 (2020) 178482
B. Monteiro, et al.
Fig. 1. Schematic representation of the ionic liquids studied in this work: IL1 - [N8888][OL]; IL2 – [C4mim][OX]; IL3 – [C4mim][DEHOX]; IL4 – [N8888][DODGA] and IL5 – [N8888][DEHOX].
mass spectrometry (Fig. S11 – IL1, Fig. S12 – IL2, Fig. S13 –IL3, Fig. S14 – IL4, Fig. S15 – IL5) and water content under equilibrium conditions in the dessicator (2.66 % - IL1, 4.75 % - IL2, 1.89 % -IL3, 4.27 % - IL4, 3.37 % - IL5).
reproducibility of the two heating and cooling steps is a warranty of no decomposition of the sample as well as assures that all the residual water was eliminated during the second step. 3. Results and discussion
2.4. Thermal stability
3.1. Dynamic TGA
The TGA assays were carried out from 298.2 to 853.2 K, with a temperature precision of ± 0.1 K and a mass measurement precision of ± 10−3 mg, under a nitrogen flow, using a 10 K·min−1 heating rate, in a TGA Q500 thermogravimetric analyser from TA Instruments (New Castle, PA, USA), by using Platinum™ Software for TGA Q500, V20.13 Build 39.
In this work, the ILs were submited to thermogravimetric analyses from room temperature to 848.2 K with a heating rate of 10 K min−1. A composite picture of the TGA for the five ILs is shown in Fig. 2 and a composite picture of the derivatives of the TGA curves for all the ILs under study is presented in Fig. 3. Important results obtained from these measurements were listed in Table 1. The starting decomposition temperature is determined as usually (intersection of the slope before and after the decomposition starts). Also, the first derivatives maxima are presented in Table 1. The results show that all the ILs studied are quite resistant to temperature, with IL3 being the most temperature stable and IL2 the least one. As a curiosity, both have the C4mim cation in their composition with the differences arising from the anion moieties. This result stresses the importance that changes in the structure of the ILs (both the anion and the cation) can have in their properties, clearly showing that properties can be trimmed [15]. Nevertheless, all the studied ILs show
2.5. Specific heats DSC curves were obtained using a TA Instruments, model DSC Q2000. In general, a temperature range from 188.2 K to 293.2 K and a heating/cooling rate of 5 K min−1 was adopted. Samples weighted between 9.8 and 15.3 mg. Aluminium crucibles were used and in these conditions the measurements have a resolution of ± 0.2 μW. The calibration of the DSC equipment was made following the recommendations described in the manufacturer’s manual. The temperature and heat flow scales of the instrument were previously calibrated at the same heating rate by using indium (Aldrich; mass fraction 0.99999; Tfus =429.75 K, Δfusho = 28.45 J∙g−1). Two sapphires provided by the manufacturer were used in the specific calibration for the obtention of specific heat (Cp) values. For all the measurements Platinum™ Software for DSC Q2000 V24.11 Build 124 was used. In the determination of specific heats, the following procedure was used under gaseous nitrogen: (1) heating from 298.2 K to 393.2 K at 10 K min−1; (b) isothermal step at 393.2 K for 30 min to remove any thermal history of the samples; (c) cooling from 393.2 K to 188.2 K at 5 K min−1; (d) isothermal step at 188.2 K for 5 min; (e) heating from 188.2 K to 393.2 K at 5 K min-−1; (f) isothermal step at 393.2 K for 5 min; (g) cooling from 393.2 K to 188.2 K at 5 K min−1; (h) isothermal step at 188.2 K for 5 min; (i) heating from 188.2 K to 393.2 K at 5 K min−1 (For a graphical scheme see Fig. S16, Supplementary information). This procedure was done in order to check reproducibility and the second step was introduced with a sufficient duration to eliminate equilibrium water present in the ILs. Transitions and Cp calculations were done using (c), (e), (g) and (i) portions of the DSC curves. The
Fig. 2. Dynamic TGA thermograms for the studied ILs (heating rate: 10 K min−1). 2
Thermochimica Acta 684 (2020) 178482
B. Monteiro, et al.
Fig. 4. DSC curve of IL1. The two cooling cycles and the two heating cycles used for calculations are shown in the figure (exothermic processes – up).
Fig. 3. First derivative of dynamic TGA thermograms for the studied ILs (heating rate: 10 K min−1). Table 1 Dynamic TGA characteristic parameters obtained for the ILs. Compound
IL1
IL2
IL3
IL4
IL5
Cation Anion Starting decomposition temperature (K) Max 1st Derivative (K)
N8888 OL 488.6
C4mim OX 450.2
C4mim DEHOX 508.6
N8888 DODGA 479.0
N8888 DEHOX 473.8
543.2
473.6 514.8 609.8
543.2 585.1
532.8 582.1 700.1
527.2
good thermal resistance and have a large liquidus range in which they could be used without decomposition. The decomposition process could be in one, two or three steps as noticed by the analyses of the first derivative. The maxima of those steps could not be related with the ions that constitute each of the ILs under study. Probably, it is not the nature of the ions that constitute the main factor for their decomposition temperaturebut is instead the strength of the interactions between anion and cation that plays the most relevant role.
Fig. 5. DSC curve of IL2. The two cooling cycles and the two heating cycles used for calculations are shown in the figure (exothermic processes – up).
also known that a lower symmetry leads to weaker interactions and to a decrease on efficient packing in the crystal cell, resulting in lower melting temperatures. This can be seen when comparing the melting temperatures of IL1 and IL2 (IL1 is much less symmetric than IL2). An additional comment must be done on the exothermic event observed for IL1 on cooling. This event occurs at a Tmax of 221.5 K and had an enthalpic effect of 6.6 kJ mol−1. The cold crystallization observed on heating at Tmax = 216.6 K has a similar enthalpic value (6.7 kJ mol−1) and the second crystallizations observed at Tmax = 230.5 K had a smaller enthalpic effect (1.2 kJ mol−1). All together still had a maller effect than the fusion process (19.4 kJ mol−1). For the IL2 the cold crystallization observed on heating at Tmax = 260.0 K has an enthalpic effect (9.4 kJ mol−1) quite similar to the fusion process (9.0 kJ mol−1). The existence of these processes means that the specific heat cannot be calculated for IL1 and IL2 over the whole temperature range studied. Thus, values of experimental specific heats in the liquid phase were listed in Table 2 and graphically presented in Fig. 6 for the several ILs. Specific heats (Cp) were correlated against temperature (T) by a second order model (Eq. 1) and the fitted parameters are presented in Table 3.
3.2. Specific heats The measurement of specific heats for the five studied ILs was done from 273.2 to 383.2 K. As explained in the Experimental Part, two heating and two cooling runs were used for the calculation of the specific heats. A first comment goes to the very high reproducibility of the repeated runs, stressing that no decomposition occured during the study as well as that the samples on the second heating/cooling cycle are in the same physical state and structure as in the first one. For ILs 3, 4 and 5 no special comments on the measurements need to be done. The ILs were in the liquid phase in all the range (see the measurement register in Supplementary Information, Figs. S17–S19). For ILs 1 and 2 a fusion is noticed as well as one or two cold crystallization processes (Figs. 4 and 5). The existence of cold crystallization processes (two in IL1 and one in IL2) can raise the suspicion of the existence of polymorphism in the solid state. This is not new [16,17] and deserves a dedicated study in the near future. The fusion process seems faster in IL1 than in IL2. However, the fusion enthalpies are of the same magnitude ( f H (IL1) =19.4 kJ mol−1 with a fusion temperature of Tmax =252.4 K and f H (IL2) =9.0 kJ mol−1 with a fusion temperature of Tmax =330.2 K). These values, specially that for IL1, can be compared with 19.9 kJ mol−1 for [C4mim][PF6] or 22.4 kJ mol−1 for [C4mim][NTf2] [18]. In addition, a thermal event before T = 200 K (on heating) is detectable in Fig. 5 and could be compatible with a glass transition, however, the used equipment does not allow to go further deeper in temperature. It is
Cp = a + b. T + cT 2
(1)
4. Conclusions Thermal stability and specific heats of five ionic liquids have been studied in this work. All the ionic liquids studied present a large range of liquidus state thus confirming their suitability to be used as solvents. All of them present also a considerable resistance to thermal decomposition. 3
Thermochimica Acta 684 (2020) 178482
B. Monteiro, et al.
Table 2 Measured specific heats (Cp) for the studied ILs.
Table 3 Adjustment parameters of specific heats for ionic liquids as a function of temperature by using Eq. (1).
Cp(J K−1 mol−1) T(K) 203.2 213.2 223.2 233.2 243.2 253.2 263.2 273.2 283.2 293.2 303.2 313.2 323.2 333.2 343.2 353.2 363.2 373.2 383.2 MW(g mol−1)
IL1
1434 1436 1454 1489 1548 1611 1662 1711 1754 1792 1819 1845 1873 748.36
IL2
IL3
IL4
IL5
367 354 351 352 358 356 354
711 673 663 673 684 695 707 720 739 756 786 826 863 899 928 954 975 997 1025 452.69
1245 1240 1231 1237 1244 1250 1258 1269 1275 1282 1305 1349 1395 1434 1467 1496 1520 1549 1582 637.97
826 835 835 850 866 877 889 899 906 917 943 985 1024 1054 1082 1103 1121 1130 1168 593.92
478 483 469 472 474 243.28
IL-1 IL-2 IL-3 IL-4 IL-5
a/J mol−1 K−1
b/J mol−1 K−2
c/J mol−1 K−3
Trange/K
9.70 × 102 3.03 × 102 1.30 × 103 1.14 × 103 1.95 × 103 1.01 × 103
4.09 × 10−4 3.95 × 10−1 −4.45 × 100 −4.68 × 100 −6.39 × 100 −2.45 × 100
6.40 × 10−3 −7.53 × 10-4 5.99 × 10−3 1.16 × 10−2 1.43 × 10−2 7.58 × 10−3
263.2–383.2 203.2–253.2 333.2–383.2 203.2–383.2 203.2–383.2 203.2–383.2
specific heats are quite high (partialy due to their high molecular masses) and literature values are not easily available for comparison. Among the ones studied here IL1 had the highest Cp values. A possible explanation could rely on a higher symmetry of the anion wich can lead to better packaging of ions in the structure. Funding sources This work was supported by Fundação para a Ciência e a Tecnologia (FCT) through the projects ENVIREE (ERA-MIN/0002/2014), REEuse (PTDC/QEQ-EPR/1249/2014) and UID/Multi/04349/2019, through RNEM – Portuguese Mass Spectrometry Network, ref. LISBOA-01-0145FEDER-022125, supported by FCT and the Lisboa Regional Operational Programme (Lisboa2020), under the PT2020 Partnership Agreement, through the European Regional Development Fund (ERDF); and by the PT2020 project RecValTR (03/SI/2017 – I&DT Entrepreneurial project in co-promotion nº 33576). B.M. and L.M. acknowledge FCT for financial support from contracts nº IST-ID/077/2018 and nº IST-ID/091/ 2018, respectively. The authors are also grateful to Dr. Luís Ferreira and Dr. Maria Helena Casimiro (C2TN) for providing access to TGA and DSC equipment. In order to facilitate the automatic retrieval of some of the values measured in this work by anyone looking for them by using a simple browser, CPBI codes [20] are included here: CPBI = XQJLZZBALCERJL-AZQMUMGESA-M/ MCPL,EXP,303.2(1547.51); CPBI = XQJLZZBALCERJL-AZQMUMGESA-M/MEPT,EXP,1(242.0); CPBI = LXMMYPYNEYKGQL-UHFFFAOYSA-M/ MCPL,EXP,343.2(448.71); CPBI = LXMMYPYNEYKGQL-UHFFFAOYSA-M/MEPT,EXP,1(303.3); CPBI = BCNQIESRQILLIV-UHFFFAOYSA-M/MCPL,EXP,303.2(756.24);
Concerning IL1 and IL2, their fusion enthalpies were also determined and compare quite well with the fusion enthalpies of other previously measured ionic liquids. The specific heats for all the studied compounds can be fitted by a second order equation and show very similar behaviour (a slight increase with the temperature) in all of them with exception of IL2. The fitted parameters for IL3, IL4 and IL5 (see Table 3) are all of the same order of magnitude. For IL2 the specific heat in the liquid phase is almost constant in the range of temperatures studied. Considering the ionic liquids with C4mim cations, the Cp value at 298.15 K of IL3 (780.0 J K−1 mol−1) can be compared with the compiled values by Pauluchka [19] of [C4mim]Cl (276−323 J K−1 mol−1), [C4mim]Br (296−317 J K−1 mol−1) and [C4mim]I (314 J K−1 mol−1). Being the anion in this case considerably larger, it is not a surprise that the value is clearly larger. The value for IL2 is not available for that temperature, but an extrapolation will lead to a value around 400 J K−1 mol−1, which is much closer to the ones listed in reference 19. For the ionic liquids with the N8888 cation the molar values of the
Fig. 6. Specific heat values for the several ILs studied (IL1 – green diamonds; IL2 – blue squares; IL3 – orange triangles; IL4 – magenta circles; IL5 – grey squares). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 4
Thermochimica Acta 684 (2020) 178482
B. Monteiro, et al.
CPBI = MDMNPKQGFXOGEU-UHFFFAOYSA-M/ MCPL,EXP,303.2(1304.61); CPBI = CRYFJBITZACPAY-UHFFFAOYSA-M/ MCPL,EXP,303.2(943.11).
characterization of uranium oxide deposit, Electrochim. Acta 52 (2007) 3006–3012. [5] M. Jayakumar, K.A. Venkatesan, T.G. Srinivasan, Electrochemical behavior of fission palladium in 1-butyl-3-methylimidazolium chloride, Electrochim. Acta 52 (2007) 7121–7127, https://doi.org/10.1016/j.electacta.2007.05.049. [6] M. Jayakumar, K.A. Venkatesan, T.G. Srinivasan, P.R.V. Rao, ExtractionElectrodeposition (EX-EL) process for the recovery of fission palladium from highlevel liquid waste, J. Appl. Electrochem. 39 (2009) 1955–1962, https://doi.org/10. 1007/s10800-009-9905-3. [7] S. Ghosh, T. Singh, Role of ionic liquids in organic-inorganic metal halide perovskite solar cells efficiency and stability, Nano Energy 2019 (63) (2019), https:// doi.org/10.1016/j.nanoen.2019.06.024 UNSP 103828. [8] M. Armand, F. Endres, D.R. MacFarlane, H. Ohno, B. Scrosati, Ionic-liquid materials for the electrochemical challenges of the future, Nat. Mater. 8 (2009) 621–629. [9] R.P. Swatloski, S.K. Spear, J.D. Holbrey, R.D. Rogers, Dissolution of cellulose with ionic liquids, J. Am. Chem. Soc. 124 (2002) 4974–4975. [10] M. Sethurajan, E.D. van Hullebusch, D. Fontana, A. Akcil, H. Deveci, B. Batinic, J.P. Leal, T.A. Gasche, M.A. Kucuker, K. Kuchta, I.F.F. Neto, H.M.V.M. Soares, A. Chmielarz, Recent advances on hydrometallurgical recovery of critical and precious elements from end of life electronic wastes-a review, Crit. Rev. Environ. Sci. Technol. 49 (2019) 212–275. [11] F. Endres, S. Zein El Abedin, Air and water stable ionic liquids in physical chemistry, Phys. Chem. Chem. Phys. 8 (2006) 2101–2116. [12] D. Parmentier, S.J. Metz, M.C. Kroon, Tetraalkylammonium oleate and linoleate based ionic liquids: promising extractants for metal salts, Green Chem. 205–209 (2013) 15, https://doi.org/10.1039/c2gc36458a. [13] A. Braam, Synthesis of Alkyl Oxalate and Oxamate Based Ionic Liquids as Extractors for the Separation of Rare Earths, PhD thesis Philipps-Universitat Marburg, 2015. [14] L. Maria, A. Cruz, J.M. Carretas, B. Monteiro, C. Galinha, S.S. Gomes, M.F. Araújo, I. Paiva, J. Marçalo, J.P. Leal, Improving the separation of lanthanides by using functionalized ionic liquid extractants, Sep. Purif. Technol. (2019), https://doi.org/ 10.1016/j.seppur.2019.116354. [15] J. Dupont, From molten salts to ionic liquids: a "nano" journey, Acc. Chem. Res. 44 (2011) 1223–1231, https://doi.org/10.1021/ar2000937. [16] T. Endo, K. Fujii, K. Nishikawa, Crystal polymorphism of 1-butyl-3-methylimidazolium hexafluorophosphate: phase diagram, structure, and dynamics, Aust. J. Chem. 72 (2019) 11–20 doi:10.1071/CH18422. [17] H. Abe, E. Kohki, A. Nakada, H. Kishimura, Phase behavior in quaternary ammonium ionic liquid-propanol solutions: hydrophobicity, molecular conformations, and isomer effects, Chem. Phys. 491 (2017) 136–142, https://doi.org/10.1016/j. chemphys.2017.05.012. [18] J. Troncoso, C.A. Cerdeiriña, Y.A. Sanmamed, L. Romaní, L.P.N. Rebelo, Thermodynamic properties of imidazolium-based ionic liquids: densities, heat capacities, and enthalpies of fusion of [bmim][PF6] and [bmim][NTf2], J. Chem. Eng. Data 51 (2006) 1856–1859, https://doi.org/10.1021/je060222y. [19] Y.U. Paulechka, Heat capacity of room-temperature ionic liquids: a critical review, J. Phys. Chem. Reference Data 39 (2010) 033108, , https://doi.org/10.1063/1. 3463478. [20] J.P. Leal, R.C. Santos, Unveiling literature data: chemical, physical and biological properties identifier (CPBI), Rasāyan J. Chem. 10 (2017) 1–5, https://doi.org/10. 7324/RJC.2017.1011527 http://www.cpbi.ctn.tecnico.ulisboa.pt.
Credits Bernardo Monteiro is involved in Thermal Analysis and writing of the paper. Leonor Maria is involved in IL synthesis, NMR determination and writing of the paper. Adelaide Cruz is involved in synthesis and purification of ILs. José Manuel Carretas is involved in Mass Spectra determination and writing of the paper. Joaquim Marçalo is involved in Mass Spectra determination and writing of the paper. João P. Leal is involved in Funding, Thermal Analysys, and coordination of paper writing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.tca.2019.178482. References [1] R.D. Rogers, K.R. Seddon, Ionic liquids–solvents of the future? Science 302 (2003) 792–793. [2] G.W. Meindersma, M. Maase, A.B. Haan, "Ionic Liquids" in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2007, https://doi.org/10.1002/ 14356007.l14_l01. [3] F. Postleb, D. Stefanik, H. Seifert, R. Giernoth, B.I. Onic, Liquids: imidazolium-based ionic liquids with antimicrobial activity, Zeitschrift für Naturforschung B 68b (2013) 1123–1128, https://doi.org/10.5560/ZNB.2013-3150. [4] P. Giridhar, K.A. Venkatesan, T.G. Srinivasan, P.R.V. Rao, Electrochemical behavior of uranium(VI) in 1-butyl-3-methylimidazolium chloride and thermal
5
Contents lists available at ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Thermal stability and specific heats of coordinating ionic liquids Bernardo Monteiro, Leonor Maria, Adelaide Cruz, José M. Carretas, Joaquim Marçalo, João P. Leal*
T
Centro de Ciências e Tecnologias Nucleares (CbTN) and Centro de Química Estrutural (CQE), Instituto Superior Técnico, Universidade de Lisboa, 2695-066, Bobadela, Portugal
ARTICLE INFO
ABSTRACT
Keywords: Ionic liquids Thermal stability Specific heats TGA DSC
Ionic liquids (ILs) have been revealed as promising solvents, namelly for the selective extraction of metals from aqueous solutions. ILs extractive properties can be trimmed but to do that in a programmed and educated way it is necessary to know their basic properties. The range of temperature in which they can be used as solvents as well as their specific heat, necessary to perform thermodynamic calculations with them, are fundamental properties needed to achieve this desideratum. Thus, the goal of this work has been the study of the thermal stability and specific heats of five ionic liquids that proved to be good extractive solvents.
1. Introduction
2. Experimental
Ionic liquids (ILs) are salts that possess a fusion temperature below 373 K [1]. They can be used in a lot of applications such as organic synthesis [2], pharmaceuticals [3], nuclear fuel reprocessing [4–6], solar thermal energy [7], batteries [8], cellulose processing [9], WEEE (Waste Electrical and Electronic Equipment) reprocessing [10] and reduction of carbon emission [11]. They can also be used as environmentally friendly solvents because of their favourable properties such as extremely low vapour pressure, low flammability, excellent thermal stability, and a wide temperature range in its liquid state. The use of ILs tends to improve the procedures used and make them more environmentally friendly. In a short/medium term they will become unavoidable in the separation and recovery of metals [10]. To have a clear idea of the adequacy of an ionic liquid for these goals, it is necessary to know in detail the physical properties of the ILs. Therefore, the aim of this work is the study of the thermal stability and behavior of three previously known ionic liquids (tetraoctylammonium oleate, [N8888][OL] – IL1 [12]; 1-butyl-3-methylimidazolium-methyloxalate, [C4mim][OX] – IL2 [13]; 1-butyl-3-methylimidazolium-di(2ethylhexyl)-oxamate, [C4mim][DEHOX] ̶ IL3 [13]) and two very recently synthesized ones (tetraoctylammonium dioctyl-diglycolamate, [N8888][DODGA] – IL4 [14]; tetraoctylammonium di(2-ethylhexyl)oxamate, [N8888][DEHOX] – IL5 [14]), all of them composed solely of C, H, O and N atoms (Fig. 1).
2.1. Chemicals
⁎
The ionic liquids used in this study; IL1 - [N8888][OL], IL2 – [C4mim][OX], IL3 – [C4mim][DEHOX], IL4 – [N8888][DODGA] and IL5 – [N8888][DEHOX], were prepared according to published procedures [12–14] and stored in a desiccator filled with silica gel. 2.2. Instrumentation and analysis Mass spectrometry analyses were done by electrospray ionization quadrupole ion trap mass spectrometry (ESI-QIT/MS), with a Bruker HCT, by direct infusion of IL solutions in acetonitrile. 1H and 13C NMR spectra of intermediate compounds and ionic liquids were recorded on a Bruker AVANCE II 300 or 400 MHz instruments. 1H and 13C chemical shifts were referenced to external SiMe4 using the residual proton or carbon of the solvents as an internal standard. CDCl3 and CD3CN were used as solvents for recording the NMR spectra. Before use, the water content in the ILs was determined by Karl Fischer titration (modelMetrohm 831 Karl Fischer coulometer) and taken into account in calculations. 2.3. Samples characterization All IL samples were characterized by 1H and 13C NMR spectroscopy (Supplementary information, Figs. S1 and S2 – IL1, Figs. S3 and S4 – IL2, Figs. S5 and S6 –IL3, Figs. S7 and S8 – IL4, Figs. S9 and S10 – IL5);
Corresponding author. E-mail address: [email protected] (J.P. Leal).
https://doi.org/10.1016/j.tca.2019.178482 Received 30 October 2019; Received in revised form 4 December 2019; Accepted 11 December 2019 Available online 12 December 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved.
Thermochimica Acta 684 (2020) 178482
B. Monteiro, et al.
Fig. 1. Schematic representation of the ionic liquids studied in this work: IL1 - [N8888][OL]; IL2 – [C4mim][OX]; IL3 – [C4mim][DEHOX]; IL4 – [N8888][DODGA] and IL5 – [N8888][DEHOX].
mass spectrometry (Fig. S11 – IL1, Fig. S12 – IL2, Fig. S13 –IL3, Fig. S14 – IL4, Fig. S15 – IL5) and water content under equilibrium conditions in the dessicator (2.66 % - IL1, 4.75 % - IL2, 1.89 % -IL3, 4.27 % - IL4, 3.37 % - IL5).
reproducibility of the two heating and cooling steps is a warranty of no decomposition of the sample as well as assures that all the residual water was eliminated during the second step. 3. Results and discussion
2.4. Thermal stability
3.1. Dynamic TGA
The TGA assays were carried out from 298.2 to 853.2 K, with a temperature precision of ± 0.1 K and a mass measurement precision of ± 10−3 mg, under a nitrogen flow, using a 10 K·min−1 heating rate, in a TGA Q500 thermogravimetric analyser from TA Instruments (New Castle, PA, USA), by using Platinum™ Software for TGA Q500, V20.13 Build 39.
In this work, the ILs were submited to thermogravimetric analyses from room temperature to 848.2 K with a heating rate of 10 K min−1. A composite picture of the TGA for the five ILs is shown in Fig. 2 and a composite picture of the derivatives of the TGA curves for all the ILs under study is presented in Fig. 3. Important results obtained from these measurements were listed in Table 1. The starting decomposition temperature is determined as usually (intersection of the slope before and after the decomposition starts). Also, the first derivatives maxima are presented in Table 1. The results show that all the ILs studied are quite resistant to temperature, with IL3 being the most temperature stable and IL2 the least one. As a curiosity, both have the C4mim cation in their composition with the differences arising from the anion moieties. This result stresses the importance that changes in the structure of the ILs (both the anion and the cation) can have in their properties, clearly showing that properties can be trimmed [15]. Nevertheless, all the studied ILs show
2.5. Specific heats DSC curves were obtained using a TA Instruments, model DSC Q2000. In general, a temperature range from 188.2 K to 293.2 K and a heating/cooling rate of 5 K min−1 was adopted. Samples weighted between 9.8 and 15.3 mg. Aluminium crucibles were used and in these conditions the measurements have a resolution of ± 0.2 μW. The calibration of the DSC equipment was made following the recommendations described in the manufacturer’s manual. The temperature and heat flow scales of the instrument were previously calibrated at the same heating rate by using indium (Aldrich; mass fraction 0.99999; Tfus =429.75 K, Δfusho = 28.45 J∙g−1). Two sapphires provided by the manufacturer were used in the specific calibration for the obtention of specific heat (Cp) values. For all the measurements Platinum™ Software for DSC Q2000 V24.11 Build 124 was used. In the determination of specific heats, the following procedure was used under gaseous nitrogen: (1) heating from 298.2 K to 393.2 K at 10 K min−1; (b) isothermal step at 393.2 K for 30 min to remove any thermal history of the samples; (c) cooling from 393.2 K to 188.2 K at 5 K min−1; (d) isothermal step at 188.2 K for 5 min; (e) heating from 188.2 K to 393.2 K at 5 K min-−1; (f) isothermal step at 393.2 K for 5 min; (g) cooling from 393.2 K to 188.2 K at 5 K min−1; (h) isothermal step at 188.2 K for 5 min; (i) heating from 188.2 K to 393.2 K at 5 K min−1 (For a graphical scheme see Fig. S16, Supplementary information). This procedure was done in order to check reproducibility and the second step was introduced with a sufficient duration to eliminate equilibrium water present in the ILs. Transitions and Cp calculations were done using (c), (e), (g) and (i) portions of the DSC curves. The
Fig. 2. Dynamic TGA thermograms for the studied ILs (heating rate: 10 K min−1). 2
Thermochimica Acta 684 (2020) 178482
B. Monteiro, et al.
Fig. 4. DSC curve of IL1. The two cooling cycles and the two heating cycles used for calculations are shown in the figure (exothermic processes – up).
Fig. 3. First derivative of dynamic TGA thermograms for the studied ILs (heating rate: 10 K min−1). Table 1 Dynamic TGA characteristic parameters obtained for the ILs. Compound
IL1
IL2
IL3
IL4
IL5
Cation Anion Starting decomposition temperature (K) Max 1st Derivative (K)
N8888 OL 488.6
C4mim OX 450.2
C4mim DEHOX 508.6
N8888 DODGA 479.0
N8888 DEHOX 473.8
543.2
473.6 514.8 609.8
543.2 585.1
532.8 582.1 700.1
527.2
good thermal resistance and have a large liquidus range in which they could be used without decomposition. The decomposition process could be in one, two or three steps as noticed by the analyses of the first derivative. The maxima of those steps could not be related with the ions that constitute each of the ILs under study. Probably, it is not the nature of the ions that constitute the main factor for their decomposition temperaturebut is instead the strength of the interactions between anion and cation that plays the most relevant role.
Fig. 5. DSC curve of IL2. The two cooling cycles and the two heating cycles used for calculations are shown in the figure (exothermic processes – up).
also known that a lower symmetry leads to weaker interactions and to a decrease on efficient packing in the crystal cell, resulting in lower melting temperatures. This can be seen when comparing the melting temperatures of IL1 and IL2 (IL1 is much less symmetric than IL2). An additional comment must be done on the exothermic event observed for IL1 on cooling. This event occurs at a Tmax of 221.5 K and had an enthalpic effect of 6.6 kJ mol−1. The cold crystallization observed on heating at Tmax = 216.6 K has a similar enthalpic value (6.7 kJ mol−1) and the second crystallizations observed at Tmax = 230.5 K had a smaller enthalpic effect (1.2 kJ mol−1). All together still had a maller effect than the fusion process (19.4 kJ mol−1). For the IL2 the cold crystallization observed on heating at Tmax = 260.0 K has an enthalpic effect (9.4 kJ mol−1) quite similar to the fusion process (9.0 kJ mol−1). The existence of these processes means that the specific heat cannot be calculated for IL1 and IL2 over the whole temperature range studied. Thus, values of experimental specific heats in the liquid phase were listed in Table 2 and graphically presented in Fig. 6 for the several ILs. Specific heats (Cp) were correlated against temperature (T) by a second order model (Eq. 1) and the fitted parameters are presented in Table 3.
3.2. Specific heats The measurement of specific heats for the five studied ILs was done from 273.2 to 383.2 K. As explained in the Experimental Part, two heating and two cooling runs were used for the calculation of the specific heats. A first comment goes to the very high reproducibility of the repeated runs, stressing that no decomposition occured during the study as well as that the samples on the second heating/cooling cycle are in the same physical state and structure as in the first one. For ILs 3, 4 and 5 no special comments on the measurements need to be done. The ILs were in the liquid phase in all the range (see the measurement register in Supplementary Information, Figs. S17–S19). For ILs 1 and 2 a fusion is noticed as well as one or two cold crystallization processes (Figs. 4 and 5). The existence of cold crystallization processes (two in IL1 and one in IL2) can raise the suspicion of the existence of polymorphism in the solid state. This is not new [16,17] and deserves a dedicated study in the near future. The fusion process seems faster in IL1 than in IL2. However, the fusion enthalpies are of the same magnitude ( f H (IL1) =19.4 kJ mol−1 with a fusion temperature of Tmax =252.4 K and f H (IL2) =9.0 kJ mol−1 with a fusion temperature of Tmax =330.2 K). These values, specially that for IL1, can be compared with 19.9 kJ mol−1 for [C4mim][PF6] or 22.4 kJ mol−1 for [C4mim][NTf2] [18]. In addition, a thermal event before T = 200 K (on heating) is detectable in Fig. 5 and could be compatible with a glass transition, however, the used equipment does not allow to go further deeper in temperature. It is
Cp = a + b. T + cT 2
(1)
4. Conclusions Thermal stability and specific heats of five ionic liquids have been studied in this work. All the ionic liquids studied present a large range of liquidus state thus confirming their suitability to be used as solvents. All of them present also a considerable resistance to thermal decomposition. 3
Thermochimica Acta 684 (2020) 178482
B. Monteiro, et al.
Table 2 Measured specific heats (Cp) for the studied ILs.
Table 3 Adjustment parameters of specific heats for ionic liquids as a function of temperature by using Eq. (1).
Cp(J K−1 mol−1) T(K) 203.2 213.2 223.2 233.2 243.2 253.2 263.2 273.2 283.2 293.2 303.2 313.2 323.2 333.2 343.2 353.2 363.2 373.2 383.2 MW(g mol−1)
IL1
1434 1436 1454 1489 1548 1611 1662 1711 1754 1792 1819 1845 1873 748.36
IL2
IL3
IL4
IL5
367 354 351 352 358 356 354
711 673 663 673 684 695 707 720 739 756 786 826 863 899 928 954 975 997 1025 452.69
1245 1240 1231 1237 1244 1250 1258 1269 1275 1282 1305 1349 1395 1434 1467 1496 1520 1549 1582 637.97
826 835 835 850 866 877 889 899 906 917 943 985 1024 1054 1082 1103 1121 1130 1168 593.92
478 483 469 472 474 243.28
IL-1 IL-2 IL-3 IL-4 IL-5
a/J mol−1 K−1
b/J mol−1 K−2
c/J mol−1 K−3
Trange/K
9.70 × 102 3.03 × 102 1.30 × 103 1.14 × 103 1.95 × 103 1.01 × 103
4.09 × 10−4 3.95 × 10−1 −4.45 × 100 −4.68 × 100 −6.39 × 100 −2.45 × 100
6.40 × 10−3 −7.53 × 10-4 5.99 × 10−3 1.16 × 10−2 1.43 × 10−2 7.58 × 10−3
263.2–383.2 203.2–253.2 333.2–383.2 203.2–383.2 203.2–383.2 203.2–383.2
specific heats are quite high (partialy due to their high molecular masses) and literature values are not easily available for comparison. Among the ones studied here IL1 had the highest Cp values. A possible explanation could rely on a higher symmetry of the anion wich can lead to better packaging of ions in the structure. Funding sources This work was supported by Fundação para a Ciência e a Tecnologia (FCT) through the projects ENVIREE (ERA-MIN/0002/2014), REEuse (PTDC/QEQ-EPR/1249/2014) and UID/Multi/04349/2019, through RNEM – Portuguese Mass Spectrometry Network, ref. LISBOA-01-0145FEDER-022125, supported by FCT and the Lisboa Regional Operational Programme (Lisboa2020), under the PT2020 Partnership Agreement, through the European Regional Development Fund (ERDF); and by the PT2020 project RecValTR (03/SI/2017 – I&DT Entrepreneurial project in co-promotion nº 33576). B.M. and L.M. acknowledge FCT for financial support from contracts nº IST-ID/077/2018 and nº IST-ID/091/ 2018, respectively. The authors are also grateful to Dr. Luís Ferreira and Dr. Maria Helena Casimiro (C2TN) for providing access to TGA and DSC equipment. In order to facilitate the automatic retrieval of some of the values measured in this work by anyone looking for them by using a simple browser, CPBI codes [20] are included here: CPBI = XQJLZZBALCERJL-AZQMUMGESA-M/ MCPL,EXP,303.2(1547.51); CPBI = XQJLZZBALCERJL-AZQMUMGESA-M/MEPT,EXP,1(242.0); CPBI = LXMMYPYNEYKGQL-UHFFFAOYSA-M/ MCPL,EXP,343.2(448.71); CPBI = LXMMYPYNEYKGQL-UHFFFAOYSA-M/MEPT,EXP,1(303.3); CPBI = BCNQIESRQILLIV-UHFFFAOYSA-M/MCPL,EXP,303.2(756.24);
Concerning IL1 and IL2, their fusion enthalpies were also determined and compare quite well with the fusion enthalpies of other previously measured ionic liquids. The specific heats for all the studied compounds can be fitted by a second order equation and show very similar behaviour (a slight increase with the temperature) in all of them with exception of IL2. The fitted parameters for IL3, IL4 and IL5 (see Table 3) are all of the same order of magnitude. For IL2 the specific heat in the liquid phase is almost constant in the range of temperatures studied. Considering the ionic liquids with C4mim cations, the Cp value at 298.15 K of IL3 (780.0 J K−1 mol−1) can be compared with the compiled values by Pauluchka [19] of [C4mim]Cl (276−323 J K−1 mol−1), [C4mim]Br (296−317 J K−1 mol−1) and [C4mim]I (314 J K−1 mol−1). Being the anion in this case considerably larger, it is not a surprise that the value is clearly larger. The value for IL2 is not available for that temperature, but an extrapolation will lead to a value around 400 J K−1 mol−1, which is much closer to the ones listed in reference 19. For the ionic liquids with the N8888 cation the molar values of the
Fig. 6. Specific heat values for the several ILs studied (IL1 – green diamonds; IL2 – blue squares; IL3 – orange triangles; IL4 – magenta circles; IL5 – grey squares). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 4
Thermochimica Acta 684 (2020) 178482
B. Monteiro, et al.
CPBI = MDMNPKQGFXOGEU-UHFFFAOYSA-M/ MCPL,EXP,303.2(1304.61); CPBI = CRYFJBITZACPAY-UHFFFAOYSA-M/ MCPL,EXP,303.2(943.11).
characterization of uranium oxide deposit, Electrochim. Acta 52 (2007) 3006–3012. [5] M. Jayakumar, K.A. Venkatesan, T.G. Srinivasan, Electrochemical behavior of fission palladium in 1-butyl-3-methylimidazolium chloride, Electrochim. Acta 52 (2007) 7121–7127, https://doi.org/10.1016/j.electacta.2007.05.049. [6] M. Jayakumar, K.A. Venkatesan, T.G. Srinivasan, P.R.V. Rao, ExtractionElectrodeposition (EX-EL) process for the recovery of fission palladium from highlevel liquid waste, J. Appl. Electrochem. 39 (2009) 1955–1962, https://doi.org/10. 1007/s10800-009-9905-3. [7] S. Ghosh, T. Singh, Role of ionic liquids in organic-inorganic metal halide perovskite solar cells efficiency and stability, Nano Energy 2019 (63) (2019), https:// doi.org/10.1016/j.nanoen.2019.06.024 UNSP 103828. [8] M. Armand, F. Endres, D.R. MacFarlane, H. Ohno, B. Scrosati, Ionic-liquid materials for the electrochemical challenges of the future, Nat. Mater. 8 (2009) 621–629. [9] R.P. Swatloski, S.K. Spear, J.D. Holbrey, R.D. Rogers, Dissolution of cellulose with ionic liquids, J. Am. Chem. Soc. 124 (2002) 4974–4975. [10] M. Sethurajan, E.D. van Hullebusch, D. Fontana, A. Akcil, H. Deveci, B. Batinic, J.P. Leal, T.A. Gasche, M.A. Kucuker, K. Kuchta, I.F.F. Neto, H.M.V.M. Soares, A. Chmielarz, Recent advances on hydrometallurgical recovery of critical and precious elements from end of life electronic wastes-a review, Crit. Rev. Environ. Sci. Technol. 49 (2019) 212–275. [11] F. Endres, S. Zein El Abedin, Air and water stable ionic liquids in physical chemistry, Phys. Chem. Chem. Phys. 8 (2006) 2101–2116. [12] D. Parmentier, S.J. Metz, M.C. Kroon, Tetraalkylammonium oleate and linoleate based ionic liquids: promising extractants for metal salts, Green Chem. 205–209 (2013) 15, https://doi.org/10.1039/c2gc36458a. [13] A. Braam, Synthesis of Alkyl Oxalate and Oxamate Based Ionic Liquids as Extractors for the Separation of Rare Earths, PhD thesis Philipps-Universitat Marburg, 2015. [14] L. Maria, A. Cruz, J.M. Carretas, B. Monteiro, C. Galinha, S.S. Gomes, M.F. Araújo, I. Paiva, J. Marçalo, J.P. Leal, Improving the separation of lanthanides by using functionalized ionic liquid extractants, Sep. Purif. Technol. (2019), https://doi.org/ 10.1016/j.seppur.2019.116354. [15] J. Dupont, From molten salts to ionic liquids: a "nano" journey, Acc. Chem. Res. 44 (2011) 1223–1231, https://doi.org/10.1021/ar2000937. [16] T. Endo, K. Fujii, K. Nishikawa, Crystal polymorphism of 1-butyl-3-methylimidazolium hexafluorophosphate: phase diagram, structure, and dynamics, Aust. J. Chem. 72 (2019) 11–20 doi:10.1071/CH18422. [17] H. Abe, E. Kohki, A. Nakada, H. Kishimura, Phase behavior in quaternary ammonium ionic liquid-propanol solutions: hydrophobicity, molecular conformations, and isomer effects, Chem. Phys. 491 (2017) 136–142, https://doi.org/10.1016/j. chemphys.2017.05.012. [18] J. Troncoso, C.A. Cerdeiriña, Y.A. Sanmamed, L. Romaní, L.P.N. Rebelo, Thermodynamic properties of imidazolium-based ionic liquids: densities, heat capacities, and enthalpies of fusion of [bmim][PF6] and [bmim][NTf2], J. Chem. Eng. Data 51 (2006) 1856–1859, https://doi.org/10.1021/je060222y. [19] Y.U. Paulechka, Heat capacity of room-temperature ionic liquids: a critical review, J. Phys. Chem. Reference Data 39 (2010) 033108, , https://doi.org/10.1063/1. 3463478. [20] J.P. Leal, R.C. Santos, Unveiling literature data: chemical, physical and biological properties identifier (CPBI), Rasāyan J. Chem. 10 (2017) 1–5, https://doi.org/10. 7324/RJC.2017.1011527 http://www.cpbi.ctn.tecnico.ulisboa.pt.
Credits Bernardo Monteiro is involved in Thermal Analysis and writing of the paper. Leonor Maria is involved in IL synthesis, NMR determination and writing of the paper. Adelaide Cruz is involved in synthesis and purification of ILs. José Manuel Carretas is involved in Mass Spectra determination and writing of the paper. Joaquim Marçalo is involved in Mass Spectra determination and writing of the paper. João P. Leal is involved in Funding, Thermal Analysys, and coordination of paper writing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.tca.2019.178482. References [1] R.D. Rogers, K.R. Seddon, Ionic liquids–solvents of the future? Science 302 (2003) 792–793. [2] G.W. Meindersma, M. Maase, A.B. Haan, "Ionic Liquids" in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2007, https://doi.org/10.1002/ 14356007.l14_l01. [3] F. Postleb, D. Stefanik, H. Seifert, R. Giernoth, B.I. Onic, Liquids: imidazolium-based ionic liquids with antimicrobial activity, Zeitschrift für Naturforschung B 68b (2013) 1123–1128, https://doi.org/10.5560/ZNB.2013-3150. [4] P. Giridhar, K.A. Venkatesan, T.G. Srinivasan, P.R.V. Rao, Electrochemical behavior of uranium(VI) in 1-butyl-3-methylimidazolium chloride and thermal
5