CHEMICAL ENGINEERING THERMODYNAMICS Chinese Journal of Chemical Engineering, 21(7) 766—769 (2013) DOI: 10.1016/S1004-9541(13)60518-2

Thermodynamic Properties of Caprolactam Ionic Liquids*

JIANG Lu (江璐), BAI Liguang (白立光), ZHU Jiqin (朱吉钦)** and CHEN Biaohua (陈标华) State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

Abstract A series of caprolactam ionic liquids (ILs) containing incorporated halide anions were synthesized. Their physical properties, such as melting points, heats of fusion and heat capacities, were measured by differential scanning calorimeter (DSC). The results indicate that these ionic liquids exhibit proper melting points, high value of heats of fusion, and satisfying heat capacities which are suitable for thermal energy storage applications. Keywords caprolactam ionic liquids, thermodynamic properties, thermal energy storage

1 INTRODUCTION

Ionic liquids (IL) are molten salts that are com-posed entirely of ions with melting points at ambient temperature [1]. They have been reckoned as envi-ronmentally benign alternatives to volatile organic solvents for reactions and separations because their excellent physical properties such as negligible vapor pressure, high thermal stability, low viscosity, large liquidus range and favorable solvation behavior [2-6]. The other benefits of ionic liquids, including high heat capacity, high thermal conductivity, non-flammability, and designable, suggest their potential use in heat transfer fluids and phase change materials (PCM).

Ionic liquids could be excellent liquid thermal storage media and heat transfer fluids in a solar thermal power plant [7-10]. Their superior physicochemical and thermal properties have been studied by the previous works. The calculated thermal storage density for 1-butyl- 3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C4MIM]+[Tf2N]− was greater than 180 MJ·m¯3 when the inlet and outlet field temperatures are 483.15 K and 663.15 K, respectively. The sensible heat storage density for 1-butyl-3-methylimidazolium tetrafluorobo-rate [C4MIM]+[BF4]−, 1-ethyl-3-methylimidazolium tetrafluoroborate [C2MIM]+ [BF4]− and 1-propyl-2,3- dimethylimidazolium bis(trifluoromethylsulfonyl)imide [C3MMIM]+ [Tf2N]− were 156.1, 192.1 and 176.3 MJ·m−3, respectively [11]. Comparing with the commercial heat transfer fluid Therminol®VP-1, an eutectic mixture of 26.5% (by mass) diphenyl and 73.5% (by mass) di-phenyl ether, the three ionic liquids are suited for use as heat transfer fluids. In our previous work [12], the melting points, heat of fusion and heat capacities for a series of imidazolium-based ionic liquids have been meas-ured. Among them [C16MIM]+Br− and [C16MMIM]+Br− have high heats of fusion (59.11 and 50.83 kJ·mol−1) and moderate melting temperatures (337.06 and 368.15 K) which indicate they are favorable candidates for PCMs.

At present, the most popular and extensive study on ionic liquids thermal storage is the study of ILs based on imidazolium. Compared with dialkylimidazolium

cation, lactam and its derivates are relatively cheaper, lower toxicity, and easily available in large amounts from industry. Ever since N-vinyl-N-alkylbutyrolactam ionic liquids was first prepared through two-step reac-tions in 2002 [13], caprolactam ILs was extensively investigated recently. Du et al. reported the prepara-tion and characterization of lactam-based ionic liquids containing [BF4]−, [CF3COO]−, [ClCH2COO]−, [NO3]−, and [H2PO4]− anions [14]. The heat storage densities of lactam-cation- based Brønsted acid ILs are more than 200 MJ·m−3, indicating that they would be more pref-erable to imidazolium- cation-based ILs. In their fur-ther research [15], ILs based on N-alkyl-ε-caprolactam as cations [Cn-CP]+ (CP is the abbreviation of caprolac-tam; Cn = alkyl with different number of C atoms, n = 6, 8, 10, 12, 16, or 18) containing toluene-p-sulfonate [TS]− and methanesulfonate [MS]− as anion were syn-thesized via a one-step atom-economic reaction. The results showed that they have higher transition enthal-pies (e.g., ΔH = 83.1 kJ·mol−1 for C18-CPTS), higher specific heat capacities (e.g., cp = 2.85 J·g−1·K−1 for C16-CPTS) and higher heat storage densities (e.g., sensible heat storage density Es=262.81 MJ·m−3 for C16-CPTS; latent heat storage density E1 = 146.0 MJ·m−3 for C18-CPTS). On the basis of their proper-ties, the caprolactam ionic liquids may have potential applications as thermal storage media.

In this paper, a series of caprolactam halide ionic liquids were synthesized as candidate PCMs for further screening. The thermodynamic properties of caprolac-tam halide ionic liquids such as melting temperature, heat of fusion, and heat capacity were measured.

2 EXPERIMENTAL

2.1 Materials

1,2-Dichloromethane, 1,2-dichloroethane and 1,2-dibromoethane were purchased from Tianjin Fuchen Chemical Reagent Factory. 1,4-dibromobutane, 1,4-dichlorobutane, 1-bromohexane and 1-bromobutane were purchased from Beijing Yili Fine Chemical Co.

Received 2012-03-10, accepted 2012-11-06.

* Supported by the National Natural Science Foundation of China (21176010, 20706005). ** To whom correspondence should be addressed. E-mail: zhujq@mail.buct.edu.cn

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Caprolactam and toluene were purchased from Beijing Chemical Plant. Their mass fraction purities were bet-ter than 98%.

2.2 Synthesis of ILs

Ionic liquids synthesized in this work are listed in Table 1. General synthetic routes to ionic liquids in this study are shown in Fig. 1. Representative synthe-sis of caprolactam ionic liquids are list below.

Butyl-caprolactam bromide: caprolactam (11.3 g, 0.1 mol) was dissolved in 30 ml of acetonitrile in a 250 ml three-neck round-bottomed flask, and then a slight excess of butylbromide (13.7 g, 0.1 mol) was added. The solution was stirred continuously under refluxing conditions at 353.15 K for 72 h. After it was cooled to room temperature, the resulted yellowish mixture was filtered and recrystallized from ethyl

acetate and ethanol successively. The product was obtained as fine white crystals by vacuum filtration.

1,1-bis(caprolactam-1-yl) methane chloride: caprolactam (22.6 g, 0.2 mol) was dissolved in 60 ml of acetonitrile in a 250 ml three-neck round-bottomed flask, and then 1,2-dichloromethane (8.5 g, 0.1 mol) was added. The solution was stirred continuously un-der refluxing conditions at 353 K for 96 h. After it was cooled to room temperature, the resulted yellowish mixture was filtered and recrystallized from ethyl acetate and ethanol in turn. The product was obtained as fine white crystals by vacuum filtration. All prepared ionic liquids were characterized by infra-red spectrum analysis (FI-IR) and X-ray Diffraction (XRD).

2.3 DSC measurements

Measurements of melting temperature, heat of

Table 1 List of room temperature ionic liquids prepared in this study

Compound Abbreviation Structure Molecular mass /g·mol−1

Water content/g·m−3

butyl-caprolactam bromide [BC]+Br−

250.18 334

hexyl-caprolactam bromide [HC] +Br−

278.23 323

1,1-bis(caprolactam-1-yl) methane chloride [DCM] +Cl−

327.29 457

1,2-bis(caprolactam-1-yl) ethane chloride [DCE] +Cl−

341.32 426

1,4-bis(caprolactam-1-yl) butane bromide [DCB] +Br−

442.23 361

1,4-bis(caprolactam-1-yl) butane chloride [DCB]+Cl−

353.33 419

R = H, or methyl; n = 1, 2, or 4; X= Cl, or Br

Figure 1 Synthetic routes for the caprolactam ionic liquids

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fusion and heat capacity were done with a Pyris I dif-ferential scanning calorimeter, produced by Perki-nElmer. The samples were weighted typically 6 mg to 7 mg and exposed to a flowing N2 atmosphere inside the DSC furnace. The standard heating rate of the present DSC measurement was set 2 K·min−1. The pure samples were weighed using a balance with an uncertainty of ±0.0001 g. The temperature was con-trolled using a precision thermometer to determine the temperature with an uncertainty of ±0.01 K. For the heat of fusion and heat capacity, the instrument uncer-tainty verified in this study by the measurement of the pure water was 5%. The heat capacity of liquid water is about 4.2 J·g−1·K−1 [16, 17].

Because ionic liquids tend to pick up moisture from the environment and the water contents of ionic liquid have crucial influence on their properties, care was taken during the preservation and measurements. As mentioned above, the ionic liquids were put into ground glass stoppered flasks. The ionic liquids were purified by vacuum evaporation for 48 h at 373 K be-fore use. During the procedure, taking the ionic liquids must be as quick as possible to reduce the exposure time and minimize the absorption of moisture. The water contents in the ionic liquids were determined by Karl Fischer method. The water contents in all the samples were less than 500 g·m−3.

3 RESULTS AND DISCUSSION

3.1 Melting temperatures and heats of fusion

The physicochemical properties of caprolactam ionic liquids were obtained by differential scanning calorimetry (DSC) according to reported procedures and methods. The melting temperature and heat of fusion of six caprolactam ionic liquids were shown in Fig. 2 and listed in Table 2. According to the DSC curves (Fig. 2), six carprolactam ILs remain stable up to 425 K. For each IL, only one endothermic peak in the range of 330-350 K corresponds to fusion of the sample. There is no phase transition in the solid-phase zone and no association or decomposition in the researched liquid-phase zone. As shown in Table 2, all the samples possess large heats of fusion (123.01-141.87 J·g−1).

Figure 2 DSC curves of caprolactam ionic liquids 1—[DCM]+Cl−; 2—[DCE]+Cl−; 3—[DCB]+Cl−; 4—[DCB]+Br−; 5—[BC]+Br−; 6—[HC]+Br−

Table 2 Melting temperature and heat of fusion for the studied ionic liquids

Ionic liquid Melting temperature/K Heats of fusion/J·g−1

[DCM]+Cl− 343.39±0.01 123.01±6.15

[DCM]+Cl− 345.78±0.01 139.05±6.95

[DCB]+Cl− 344.75±0.01 140.61±7.03

[DCB]+Br− 336.40±0.01 126.35±6.32

[BC]+Br− 346.17±0.01 141.87±7.09

[HC]+Br− 345.24±0.01 137.02±6.85

3.2 Heat capacity

Heat capacity (cp) is the measurable physical quantity that characterizes the amount of heat required to change the temperature of a compound by a given amount. cp is related to the number of translational, vibrational, and rotational energy storage modes in the molecule [18]. So, a molecule containing more atoms would have more energy modes and thus a higher heat capacity. Data have been obtained at atmospheric pres-sure and within 298.15 K to 383.15 K for ionic liquids in steps of 1 K. Fig. 3 shows the heat capacities of IL samples varying with temperature. For simplicity, only values at some temperatures are shown in Table 3.

Figure 3 Heat capacities of caprolactam ionic liquids ■ [DCM]+Cl−; ● [DCE]+Cl− ; ▲ [DCB]+Cl−; ▼ [DCB]+Br−; ◆ [BC]+Br−; ★ [HC]+Br−

The heat capacities of these caprolactam ionic liquids are lower than those of imidazolium ionic liquids (e.g., cp = 2.85 J·g−1·K−1 for C16-CPTS). The reason is that the caprolactam ionic liquids do not have the long alkyl chain as [C16MIM]+Br− and [C16MMIM]+Br− which presents the high number of vibrational degrees.

4 CONCLUSIONS

A series of novel caprolactam ionic liquids were synthesized and characterized. Their thermodynamic properties, such as melting temperatures, heats of fu-sion and heat capacities were determined. The samples possess relatively moderate melting temperatures and large heats of fusion which are benefit to heat storage.

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Table 3 Experimental heat capacities cp for the studied ILs

cp/J·g−1·K−1 T/K

[DCM]+Cl− [DCE]+Cl− [DCB]+Cl− [DCB]+Br− [BC]+Br− [HC]+Br−

solid 298.15 1.44±0.07 1.33±0.07 1.62±0.08 1.46±0.07 1.59±0.08 1.44±0.07

303.15 1.72±0.09 1.54±0.08 1.95±0.10 2.12±0.11 1.91±0.10 1.73±0.09

308.15 1.76±0.09 1.60±0.08 1.99±0.10 2.40±0.12 1.96±0.10 1.76±0.09

313.15 1.81±0.09 1.62±0.08 2.03±0.10 1.99±0.10 1.79±0.09

318.15 1.90±0.10 1.75±0.09 2.08±0.10 2.02±0.10 1.81±0.09

323.15 2.07±0.10 1.76±0.09 2.18±0.11 2.05±0.10 1.85±0.09

328.15 1.77±0.09 2.10±0.11 1.94±0.10

333.15 1.83±0.09 2.22±0.11

liquid 353.15 1.99±0.10 2.10±0.11 2.25±0.11 2.45±0.12 2.48±0.12 2.14±0.11

358.15 1.99±0.10 2.10±0.11 2.22±0.11 2.44±0.12 2.49±0.12 2.14±0.11

363.15 2.00±0.10 2.10±0.11 2.20±0.11 2.43±0.12 2.49±0.12 2.15±0.11

368.15 2.00±0.10 2.11±0.11 2.20±0.11 2.44±0.12 2.49±0.12 2.16±0.11

373.15 2.00±0.10 2.11±0.11 2.20±0.11 2.44±0.12 2.49±0.12 2.18±0.11

378.15 2.01±0.10 2.12±0.11 2.22±0.11 2.44±0.12 2.50±0.13 2.18±0.11

383.15 2.00±0.10 2.13±0.11 2.25±0.11 2.44±0.12 2.50±0.13 2.20±0.11