Thermophysical properties of dicationic imidazolium-based ionic compounds for thermal storage

Thermophysical properties of dicationic imidazolium-based ionic compounds for thermal storage

Accepted Manuscript Thermophysical properties of dicationic imidazolium-based ionic compounds for thermal storage Hang Zhang, Wei Xu, Jingru Liu, Min...

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Accepted Manuscript Thermophysical properties of dicationic imidazolium-based ionic compounds for thermal storage

Hang Zhang, Wei Xu, Jingru Liu, Mingtao Li, Bolun Yang PII: DOI: Reference:

S0167-7322(18)35692-7 https://doi.org/10.1016/j.molliq.2019.03.012 MOLLIQ 10553

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

4 November 2018 15 January 2019 4 March 2019

Please cite this article as: H. Zhang, W. Xu, J. Liu, et al., Thermophysical properties of dicationic imidazolium-based ionic compounds for thermal storage, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.03.012

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ACCEPTED MANUSCRIPT Thermophysical Properties of Dicationic Imidazolium-Based Ionic Compounds for Thermal Storage Hang Zhanga, 1, Wei Xub, 1, Jingru Liub, Mingtao Lia, Bolun Yanga,  a

Department of Chemical Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China State Key Laboratory of Chemical Safety and Control, Sinopec Research Institute of Safety

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b

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Engineering, Qingdao, Shandong 266071, China



Corresponding author Tel.: +86 29 82663189; fax: +86 29 82663189 E-mail address: [email protected] (B.L. Yang) 1 These authors contributed equally to this work. 1

ACCEPTED MANUSCRIPT ABSTRACT: In order to improve the thermal stability of ionic liquids and enhance the thermal storage density, a series of dicationic ionic compounds containing incorporated Br-, BF4-, PF6-, NTf2- anions were synthesized. The structures of these dicationic ionic compounds were confirmed by 1H NMR,

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C NMR and FT-IR. The

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dicationic ionic compounds’ thermo-physical properties were determined by

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thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The

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effect of molecular configuration on the heat of fusion was studied by changes alkyl side-chain, linkage chain, molecular structure symmetry, and anions. The quantum

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calculation was carried to validate the number of hydrogen bonds for the dicationic

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ionic compounds. The results showed that hydrogen bonds significantly increased compared to that of the monocationic ionic liquids (MILs). The thermal analysis

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results indicate that dicationic ionic compounds exhibit excellent thermal stability; the

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decomposition temperatures higher than 560 K; the melting points and heat of fusion value reached 481 K and 116.26 KJ·Kg-1, respectively. These values are higher than

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those of the MILs.

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KEYWORDS: Dicationic ionic compounds; Phase-change materials (PCMs); Heat of fusion; Thermal storage density.

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1. INTRODUCTION Recently, in the context of energy applications it is important to mention the role of ionic liquids (ILs) in addressing the growing need for phase-change materials (PCMs).1,2 PCMs is a material that melts and solidify at a specific temperature. This

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phase change process is often accompanied by endothermic or exothermic heat, that is

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phase change latent heat, this latent heat can be used as thermal energy storage.

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Traditional PCMs are classified into organic and inorganic PCMs. They have a low

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thermal storage density, undergo large changes during phase changes, often suffer from supercooling, phase separation or corrosive action, which can affect their

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thermal-storage capacity. To overcome shortcomings of organic salts, several studies have been conducted to improve the performance of latent heat storage. For example,

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R. Ghasemiasl et al.3 studied a numerical analysis of phase-change processes using a

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finite element analysis. Hoseinzadeh et al.4 studied a numerical analysis of performance of energy storage systems using two PCM with different nanoparticles.

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In addition, as for the heat of fusion for protic salts ILs, Vijayaraghavan et al.5

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reported the values determined by DSC. These protic salts exhibit a very high heats of fusion (guanidinium methane sulfonate △Hf=190 KJ·Kg-1). Their work has shown that the hydrogen bonding has been formed between six dissociable protons on a guanidinium cation and lone pairs of oxygen on three neighbouring sulfonate anions. Zhu et al.6 has studied a series of imidazolium-based ionic liquids. The results shown that [C16mim]Br and [C16mmim]Br have large heats of fusion (△Hf=152.56 KJ·Kg-1 and △Hf=126.62 KJ·Kg-1) and moderate melting temperatures (Tm=337.06 K and 3

ACCEPTED MANUSCRIPT Tm=368.15 K). These ILs have good potential for use as thermal energy storage media. Ionic liquids (ILs) are molten organic salt consisting of only organic cations and inorganic or organic anions.7 ILs possess many unique physicochemical properties,

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such as almost zero vapor pressures, non-flammability, good thermal stabilities, and

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high thermal storage density. These properties mean that ILs could be used as

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potential PCMs in thermal energy storage processes.8 Unlike solid salts, ILs can be customized by combining the appropriate anions and alkyl side chain groups attached

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to the cation.9-11 Thus, the design of new ILs as PCMs that exhibit high heat of fusion

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and high thermal storage density can be achieved.

Recently, our research team found12-14 that by properly adjusting the structure of

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the ILs, the anion-cation interaction energy can be enhanced, thereby improving the

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thermos-physical properties of the ILs. The anion-cation interactions of ILs mainly include hydrogen bonding energy and electrostatic energy. Generally, the hydrogen

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bond is formed between a hydrogen atom and an electronegativity atom. Therefore,

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introducing more H atoms in cations of ionic liquids and halide anions can form more hydrogen bonds to increase the hydrogen bond energy. In addition, the electrostatic energy of an ionic liquid depends on the charge distribution of ions. Thus, increasing the charge density of ionic liquids by increasing the number of cations can be considered an efficient approach to improve the electrostatic energy. Currently, the most extensive research on ILs heat storage is the study of ionic liquids based on imidazole compounds.12,15-17 Amir Sada Khan et al.18-20 has synthesized a series of

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ACCEPTED MANUSCRIPT dicationic ionic liquids, containing 1,1-Bis(3-methylimidazolium-1-yl) butylene ([C4(Mim)2]) cation with counter anions namely hydrogensulfate, methanesulfonate, trifluoromethanesulfonate, and paratoluene sulfonate. The present dicationic ionic liquids were found to be thermally stable in a wide range of temperature as reported

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for the most of the ILs.

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Because the dicationic ionic liquids has more hydrogen bonds and higher

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electrostatic energy21-24, and the two anions in the molecule can form a strong electrostatic coupling with C2-H on the cation imidazole ring, the electrostatic energy

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between the anion and the cation can be further enhanced25,26. Reflected in the

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macroscopic nature, such compounds will exhibit a higher phase transition enthalpy. Therefore, this work firstly utilizes this basic characteristic of ionic liquids in an effort

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to design a dicationic ionic compound with a high heat storage density for use in a

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phase change heat storage process.

In addition, because the alkyl chain and anion structure changes will also affect

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the thermophysical properties of these ionic compounds27,28, this work will study the

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single crystal structure, molecular interaction and thermophysical properties through single crystal diffraction data analysis of the synthesized ionic compounds. The relationship between the electrostatic interactions and hydrogen bonding interactions in ionic compounds is verified by the DFT theory in quantum chemical calculations. Through these theoretical analysis and experimental work, it will undoubtedly provide new methods and new ideas for the development of thermal storage ionic compounds. 2. EXPERIMENTAL SECTION

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ACCEPTED MANUSCRIPT 2.1 Materials 1-Methylimidazole, 1-ethylimidazole, 1-propylimidazole, 1-butylimidazole, 1,2-dibromoethane, 1,3-dibromopropane, 1,4-dibromobutane, 1,5-dibromopentane, and 1,6-dibromohexane were purchased from the Alfa Aesar, with purities ≥99

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mass%. Methanol, ethanol, ethyl acetate were purchased from Energy Chemical, with

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purities ≥99 mass%. 2-Bromoethanol and bromoacetic acid (≥99 wt %) were

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purchased from the Sinopharm Chemical Reagent Co., Ltd.

2.2. Preparation of the Dicationic Imidazolium-Based Ionic Compounds 1.

Schematic

Representation

the

Synthesis

of

Dicationic

N

+

N R1

N

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N

D

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Imidazolium-Based Ionic Compounds

of

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Scheme

XR2X

Ethyl alcohol

N

N

N R2

R1= -CH3, -CH2CH3, -CH2CH2CH3, -CH2CH2CH2CH3 ;

N

2XR1

R2= -CH2CH2-,

X= Br

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-CH2CH2CH2-, -CH2CH2CH2CH2-, -CH2CH2CH2CH2CH2-, -CH2CH2CH2CH2CH2-;

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The dicationic imidazolium-based ionic compounds was synthesized, as shown in Scheme 1.29 Typically, the reaction flask was charged with 1-methylimidazole and 1,2-dibromoethane (molar ratio of 2:1) in ethanol (15 mL). Then the miscible liquids were heated to 343.15 K, stirred for 24 hours under dry nitrogen. Then, the volatile material is removed from the resulting solution under reduced pressure at 323.15 K. The water was then added. The aqueous phase was extracted three times with

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ACCEPTED MANUSCRIPT dichloromethane. The extract was recrystallized from ethyl acetate to obtain a white solid. The solid product was dried in vacuum for 24 hours in 313 K, and the ILs with a yield of about 90.4% was obtained.30, 31 The rest of the ionic compounds were synthesized in a similar way.

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2.3. Characterization

(13C)

NMR

analyses.

A

Thermo

Nicolet

AVATAR-360

FT-IR

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carbon

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A Bruker Avance III HD 400 MHz spectrometer was used for proton (1H) and

spectrophotometer was used for Fourier transform infrared (FT-IR) spectra by KBr

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pellets.23,32,33 NMR and FT-IR spectra of the dicationic ionic compounds are shown in

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supporting information. The characterization results of dicationic ionic compounds are consistent with the expected structure, indicating that the preparation methods are

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reliable. It can be claimed that the purities of the synthesized dicationic ionic

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compounds are very high and meet the requirements of further testing thermo-physical properties. The water content of each dicationic ionic compounds

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was determined by Karl Fischer titration (751 GPD Titrino, Metrohm, Switzerland).

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Water content is given in ppm. 2.4. Density Determination The densities (ρ) were measured with a densimeter (DSA 5000 M, Anton Paar) at a repeatability of 0.0001 g·cm-3 at the temperature of 298.15 K.1 2.5. Thermal Analysis A thermogravimetric analysis (NETZSCH STA449F5 analyzer) was used for thermal stability of the dicationic ionic compounds. Each sample was analyzed in an

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ACCEPTED MANUSCRIPT alumina crucible with N2 as the purge gas. The temperature was increased at 10 K/min over a temperature window of 303 K to 873 K. The melting points, heat of fusion, heat capacity, and thermal storage density of the dicationic ionic compounds were investigated by differential scanning calorimetry (DSC).34,35 The samples were heated

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at a scan rate of 5 K/min under a N2 atmosphere.

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2.6. X-ray Analysis

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A crystal of the C2(mim)2(Br)2 was grown. The diffraction data of C2(mim)2(Br)2 was measured using Mo Kα radiation on a Bruker APEX II CCD diffractometer

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equipped with a kappa geometry goniometer. The solution and refinement were

2.7. Computational Methods

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performed by SHELX.

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The quantum calculation was performed based on density functional theory (DFT)

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by Gaussian 09. All analyses for the ion pairs were performed using 6-31++G (d, p) basis set based on Becke’s three–parameter hybrid method with Lee-Yang-Parr

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correlation. Each optimized structure was checked as energy minima via frequency

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calculations that showed no negative frequencies.22,23,34 3. RESULTS AND DISCUSSION 3.1. Thermal Stability The thermal stability of the dicationic ionic compounds was measured by thermogravimetric analysis (TGA).36 The thermogravimetric curves of the dicationic ionic compounds are shown in Figure 1, and the decomposition temperatures (Td) of the dicationic ionic compounds provided in Table 1 lie mostly between 560 and 600 K.

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ACCEPTED MANUSCRIPT The molecular volumes (Vm) were calculated using the volume keyword of Gaussian 09 software at the B3LYP/6-31++G (d, p) level. In addition, the densities were measured with a densimeter at 298.15 K. As shown in Table 1, it is found that the density of the dicationic ionic compounds decreases with the increase of alkyl

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side-chain length.15 The melting points of the dicationic ionic compounds were

C3(Mim)2(Br)2

100

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obtained by DSC. 100

C3(Pim)2(Br)2

80

80

Mass Loss / %

60

40

60

40

C3(mim)2(Br)2 C4(mim)2(Br)2 C5(mim)2(Br)2 C6(mim)2(Br)2

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Mass Loss / %

C3(Bim)2(Br)2

C2(mim)2(Br)2

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C3(Eim)2(Br)2

20

20

0 300

400

500

600

700

Temperature / K

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0

800

900

300

400

500

600

700

800

Temperature / K

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Figure 1. TGA curves of the dicationic ionic compounds.

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Table 1. Acronyms, Molecular Weight, Molecular Volume, Density, Decomposition Temperatures and Melting Points of the Dicationic Ionic Compounds. Vm

ρs

(g·mol-1)

(cm3·mol-1)

(g·cm-3)

C2(mim)2(Br)2

352.09

214.13

C3(mim)2(Br)2

366.11

C3(eim)2(Br)2

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Mw

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Ionic Compounds

Acronym

Td (K)

Tm (K)

1.6547

595.81

481.47

233.22

1.6283

594.39

445.95

394.15

250.18

1.5186

589.18

422.07

C3(n-pim)2(Br)2

422.19

278.09

1.4785

576.66

419.37

C3(n-bim)2(Br)2

450.25

307.78

1.4168

575.08

411.99

C4(mim)2(Br)2

380.13

233.32

1.5039

591.48

431.50

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ACCEPTED MANUSCRIPT .2Br N

N

N

N

N

N

N

N

N

N

-

C5(mim)2(Br)2

394.16

249.99

1.5076

583.22

407.67

C6(mim)2(Br)2

408.19

261.53

1.4844

582.55

388.60

(mimC2eim)(Br)2

366.11

220.28

1.6780

576.12

456.90

(mimC2pim)(Br)2

380.14

232.01

1.4547

566.17

389.25

(mimC2bim)(Br)2

394.16

242.43

1.4063

564.38

366.45

5

.2Br N

N

-

6

.2Br N

N

-

2

.2Br N

N

-

2

N

N

-

N

2

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.2Br N

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As shown Figure 1, the Td values of C3(n-bim)2(Br)2 (575.08 K) are lower than

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that of C3(mim)2(Br)2 (594.39 K). This result is attributed to the molecular mass (Mw) and the steric hindrance. Steric effects arise from a fact that each atom within a

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molecule occupies a certain amount of space. If atoms are brought too close together,

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there is an associated cost in energy due to overlapping electron clouds, and this may affect the molecule's preferred shape (conformation). Along with an increase in the

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number of carbon atoms in the alkyl side chain, the molecular mass and the steric

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hindrance of imidazole dication also increase more, as shown in Table 1. This is in agreement with the general trends observed for different ionic liquid families: the

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stability of the ionic liquids decreases as the alkyl group attached to nitrogen increases.

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Reinert et al.37 synthesized five first generation ionic liquids (BMImCl, OMImCl, AMImCl, BPyBr and OPyBr). The experimental results indicated whatever the cation type, the thermal stability of the octyl ionic liquid was lower than the butyl one. Considering the propyl linkage chain dicationic ionic compounds, longer alkyl side-chains result in a lowering of the decomposition temperature, as shown in Figure 2. As the number of carbon atoms in the side chain alkyl group of the imidazole ring increases, the symmetry of the cations decreases and the effective stacking of crystals

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ACCEPTED MANUSCRIPT is hindered. Therefore, the decomposition temperature drops. Functionalized ionic liquids

as

thermal

storage

were

reported

by

Zhang

et

al.13

For

1-carboxyl-3-methylimidazolium bromide, the decomposition temperature (565 K) was higher than that of 1-carboxyl-3-butylimidazolium bromide (475 K) because it

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decreased gradually with the increasing length of the alkyl chain.

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580

570

560

550 300

350

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Decomposition Temperature / K

590

400

450

500

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-1 Mw / g·mol

Figure 2. The effect of the molecular weight on the decomposition temperature for propyl linkage

3.2. Melting Points

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dicationic ionic compounds.

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From this study, the melting points were affected by three main factors of the dicationic ionic compounds. These factors are (1) the molecular volume, (2) the

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linkage chain, and (3) the symmetry. Considering first the propyl linkage chain dicationic ionic compounds, the longer alkyl side chains have larger molecular volumes (Vm) and result in a lowering of the melting points (Tm).38 The Tm values of C3(n-bim)2(Br)2 (411.99 K) are lower than that of C3(mim)2(Br)2 (445.95 K). As shown Figure 3 the melting points decreases with the increase of alkyl side chain length. The larger molecular volume corresponds to a smaller density. Thus, the density shows an opposite trend in Table 1 compared to that of the molecular volume. 11

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Melting Temperature / K

440

430

420

210

240

270

300 3

Vm / cm ·mol

330

-1

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410

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dicationic ionic compounds.

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Figure 3 The effect of the molecular volume on the melting temperature for propyl linkage

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In addition to the effect of the different alkyl side-chain lengths, the length of the linkage chain separating the geminal dications also played a key role in determining

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the melting points. In Table 1, the length of the linkage chain gradually increases from two to six carbon. The C2(mim)2(Br)2 and C3(mim)2(Br)2 were salts with relatively

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high melting points. When the dications were connected by a hexane linkage chain, however, C6(mim)2(Br)2 was also salts with relatively low melting points. The

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dicationic ionic compounds with the longer linkage chains result in a lowering of the melting points. Anderson et al.29 has studied a series of 3-methylimidazolium-based

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dicationic ionic compounds. By connecting the 3-methylimidazolium dications with a nonane linkage chain, melting point of C9(mim)2(Br)2 sample was 279 K. When the dications were connected by a dodecane linkage chain, melting point of C12(mim)2(Br)2 sample was 256 K. It appears that longer alkyl linkage chains and long aliphatic substituents on the quaternary amine produce either low melting salts or room temperature ionic liquids. The symmetry of molecular structure also affected the melting points of these 12

ACCEPTED MANUSCRIPT dicationic ionic compounds. It is shown in Table 1 that the melting points are drastically reduced, when the symmetry of the molecules were lowered by lengthening one of the side chains in the dicationic ionic compounds. The substitution of longer alkyl side chains onto the imidazolium ring decreased the melting point

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significantly, possibly because the weak ion interaction reduces the effective stacking

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of crystals, the melting points of the dicationic ionic compounds decreases.

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3.3. Phase Change Behavior

Phase change behavior determines the cyclicity of ionic liquids, which is an

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important parameter for economic efficiency. Four types of the dicationic ionic

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compounds with different thermal behavior were selected as examples, as illustrated in Figure 4.

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In Figure 4(a) C3(mim)2(Br)2 has a distinct melting point on the heating curve and

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a freezing point on the cooling curve; one is the endothermic peak at about 446 K, and the other is the exothermic peak at about 400 K. Although supercooling of ionic

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liquids is common, the particular dicationic ionic compounds C3(mim)2(Br)2 readily

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crystallizes and does not form glasses, which indicates that this material is a favorable candidate for PCMs. The broken lines indicate that crystal-crystal transitions occurred in the cooling process. There is a significant differences in the melting and crystallization behaviors between C3(mim)2(Br)2 and C3(n-bim)2(Br)2. The crystallization of C3(n-bim)2(Br)2 in Figure 4(b) is not observed in the DSC cooling curve due to the supercooling effect. This result indicate that the C3(n-bim)2(Br)2 is generally hard to crystallize as the

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ACCEPTED MANUSCRIPT longer alkyl side chains. This dicationic ionic compound could be a potential PCM if the supercooling problem was solved by adding special nucleating agents. In Figure 4(c) C4(mim)2(Br)2 has a distinct freezing point and a melting point. The broken lines indicate that sluggish crystal-crystal transitions occurred in the

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heating process.

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The phase change behavior for the asymmetric dicationic ionic compounds

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(mimC2bim)(Br)2 is illustrated in Figure 4(d). Only a glasslike transition temperature is observed; neither its melting point nor freezing point is detected. The (mimC2bim)2+

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cations have a high asymmetry around the nitrogen atom and form a glasslike

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transition. Therefore, longer alkyl side-chains and asymmetric molecular structures are detrimental to the phase change process of the dicationic ionic compounds. 0.4

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C3(mim)2(Br)2

Heat Flow Up (mw)

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0

-5

-10 300

350

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Heat Flow Up (mw)

5

400

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Heat Flow Up ( mw)

0.5

C3(bim)2(Br)2

0.0

-0.2

-0.4

Tm=412 K

Tm=446 K

-0.6

450

500

550

360

380

400

420

440

460

480

500

520

T (K)

T (K)

1.0

(b)

0.2

D

(a)

C4(mim)2(Br)2

(c)

0.0

-0.5

Tm =389 K

-1.0

-1.5 320

340

360

380

400

420

440

460

480

T (K)

Figure 4 Differential scanning calorimetric thermograms for (a) C3(mim)2(Br)2, (b)

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ACCEPTED MANUSCRIPT C3(n-bim)2(Br)2, (c) C4(mim)2(Br)2, and (d) (mimC2bim)2(Br)2.

3.4. Heat of Fusion Analysis 3.4.1. Effect of the Interaction Energy The heats of fusion of the dicationic ionic compounds was measured by

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differential scanning calorimetric (DSC). The DSC curves of the dicationic ionic

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compounds are shown in Figure 5. The single-peaked shape of the curve of four

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samples excludes the possibility of a solid-solid transition near melting. It can be seen from the Figure 5 that the four ionic compounds have obvious endothermic peaks,

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whose peak position is between 400 K and 450 K. This endothermic peak is caused by

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the transition of the ionic compounds from solid to liquid phase. The heat of fusion of the ionic compounds can be obtained by integrating the peak area.

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The heats of fusion values (ΔHm) of the dicationic ionic compounds provided in

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Table 2. In the case of sample C3(mim)2(Br)2, which consists of the methylimidazolium dications connected by a propane linkage chain with the Br- anion,

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the heat of fusion is decreased by nearly 15.43 KJ·Kg-1 by replacing the methyl

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groups with butyl groups to from sample C3(bim)2(Br)2. This result is attributed to the steric hindrance, because the chain length of butyl side-chain ionic compounds is longer than that of methyl and ethyl side-chain ionic compounds, it hinders the ions from being close; therefore, the distance between the ions may be beyond the range to form hydrogen bonds. Thus, the decreasing intermolecular forces and hydrogen bond energy leads to lower the heat of fusion values for these ionic compounds.

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Heat Flow Up /mW·mg-1

ACCEPTED MANUSCRIPT

C3(mim)2(Br)2 C3(eim)2(Br)2 C3(pim)2(Br)2 C3(bim)2(Br)2

350

400

450

500

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300

Temperature /K

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Figure 5 Melting curves of the dicationic ionic compounds.

ΔHm (KJ·Kg-1)

C3(mim)2(Br)2

115.82

C3(eim)2(Br)2

110.63

C3(n-pim)2(Br)2

108.41

ΔHm (KJ·Kg-1)

[C2MIM]Br

82.17 (ref 16)

[C3MIM]Br

69.72 (ref 9)

[C4MIM]Br

51.17 (ref 9)

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D

100.39

[C4MMIM]Br

66.98 (ref 16)

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C3(n-bim)2(Br)2

MILs

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Dicationic Ionic Compounds

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Table 2. Heat of Fusion of the Dicationic Ionic Compounds and MILs.

In addition, in Table 2, the dicationic ionic compounds possess higher heat of

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fusion than those of the MILs analogues. In general, the shorter alkyl side chains in imidazolium dications will centralize the positive charge, which increases the

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cation-anion interaction. The larger cation-anion interaction increases interaction energy among the ions and the heat of fusion indirectly. The interaction energy of an ion pair reflects the strength of the cation-anion interaction. The interaction energy (Ei) of an ion pair is defined as following Eq. (1):39,40

Ei (KJ  mol-1 )=2625.5[EAX (au)-(EA+ (au)+EX- (au))]

(1)

where EAX is the energy of the ionic system, and EA+ and EX- are the cationic and

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ACCEPTED MANUSCRIPT anionic energy. The interaction energy of the dicationic ionic compounds were calculated by Gaussian 09 at the B3LYP/6-31++G (d, p) level, as shown in Table 3. Table 3. Interaction Energies of the Dicationic Ionic Compounds and MILs. Ei (kJ·mol-1)

MILs

Ei (kJ·mol-1)

C3(mim)2(Br)2

-678.24

[mmim]Br

-395.52

C3(eim)2(Br)2

-559.00

[emim]Br

-372.82

C3(n-pim)2(Br)2

-503.73

[pmim]Br

C3(n-bim)2(Br)2

-449.69

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Dicationic Ionic Compounds

SC

-367.73

[bmim]Br

-362.14

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The calculated results in Table 3 show that the order of interaction energies

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decreases with the length of the alkyl side chain. It shows a trend, where the heat of fusion values of the dicationic ionic compounds decreases with the increase of chain

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length, and it is related to the reduction of the interaction energy. Thus, the shorter

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alkyl side chains benefit the cation-anion interaction energy and results in the higher heat of fusion values.

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3.4.2. Effect of the Linkage Chain

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The heat of fusion of the dicationic ionic compounds with different linkage chain were obtained by DSC test. The experimental results are shown in Table 4. Table 4 Effect of the Linkage Chain on the Heat of Fusion Values Dicationic Ionic Compounds

Heat of Fusion Value (KJ·Kg-1)

Ei (kJ·mol-1)

C2(mim)2(Br)2

116.26

-684.35

C3(mim)2(Br)2

115.82

-678.24

C4(mim)2(Br)2

115.69

-662.37

17

ACCEPTED MANUSCRIPT C5(mim)2(Br)2

114.48

-646.98

C6(mim)2(Br)2

113.70

-620.29

As shown in Table 4, the ion interaction decreases with the increase of the linkage chain length. The change of the heat of fusion value is very small. This is because the

PT

linkage chain is only a small fraction of the molecule compared to the larger

RI

imidazole moiety. The longer linkage chains hinder the ions from being close,

SC

therefore, the decreasing intermolecular forces and hydrogen bonding energy. General, weak ion interaction (such as suppressing hydrogen bonding) tends to reduce the

NU

crystal lattice energy of the salts, leads to lower heat of fusion value. So the sample

MA

C2(mim)2(Br)2 of the shorter linkage chain length have higher the heat of fusion. 3.4.3. Effect of the Molecular Structure Symmetry

D

The heat of fusion of the asymmetric dicationic ionic compounds were obtained

PT E

by DSC. The results are shown in Table 5.

Compounds

CE

Table 5 Heat of Fusion Values and Interaction Energies of Asymmetric Dicationic Ionic

Heat of Fusion Value (KJ·Kg-1)

Ei (kJ·mol-1)

(mimC2eim)(Br)2

85.21

-618.32

(mimC2pim)(Br)2

42.18

-590.72

(mimC2bim)(Br)2

39.75

-563.68

AC

Ionic Compounds

The structure symmetry of the cations also appears to affect the heat of fusion. As shown in Table 5, with the decrease of the cationic symmetry of the dicationic ionic compounds, the heat of fusion is reduced. As electrostatic interactions play a vital role 18

ACCEPTED MANUSCRIPT in the heat of fusion. In the case of (mimC2bim)(Br)2, the electrostatic interaction is weaker compared with that in C2(mim)2(Br)2, possibly due not only to steric hindrance but also to the weakly the hydrogen bond. Thus, it can be seen that the asymmetric molecular structure is an adverse impact on the thermal properties of the

PT

dicationic ionic compounds.

RI

3.4.4. Effect of the C2-H

SC

In Table 6, the melting point and the heat of fusion of the ionic liquid were significantly lowered when the hydrogen at the 2-position on the imidazole ring was

NU

replaced by a methyl group. For imidazole-based ionic liquids, the H−π bond exist in

MA

between the two cations, as observed in Figure 6. This is because the electron density of the C2-H is smaller than that of the C4-H and C5-H, and it is easier to accept

D

electrons, thereby forming an H-π bond. When the methyl group replaces the H atom

PT E

at the C2 position on the imidazole ring, the force field near the imidazole ring changes, and the corresponding H-π bond disappears, and the heat of fusion of phase

CE

change decreases.

AC

Table 6 Effect of the C2-H on the Heat of Fusion Values Ionic Compounds

Melting Temperature(K)

Heat of Fusion Value(KJ·Kg-1)

C2(mim)2(Br)2

461.47

116.26

C2(mmim)2(Br)2

383.27

17.86

C3(mim)2(Br)2

445.95

115.82

C3(mmim)2(Br)2

358.27

34.20

C4(mim)2(Br)2

388.60

115.69

19

ACCEPTED MANUSCRIPT

C4(mmim)2(Br)2

305.60

86.70

RI

PT

H-π bond

SC

H-π bond

NU

Figure 6 Structures of C2(mim)2Br2 and C2(mmim)2Br2 cations

3.4.5. Effect of the Anion on Thermal Properties

MA

The thermo-physical properties of the dicationic ionic compounds are strongly dependent on the anion and cation. The anions also play an important role in the

D

bo\nd

PT E

thermal properties of the dicationic ionic compounds. The thermal properties of the measured dicationic ionic compounds were those with an imidazolium dication

CE

containing incorporated Br-, BF4-, PF6-, NTf2- anions. The volume of the anions (V'm) were calculated using the volume keyword of Gaussian 09 software at the

AC

B3LYP/6-31++G (d, p) level. The decomposition temperatures (Td) was measured by thermogravimetric analysis (TGA). The melting points (Tm) and heats of fusion (ΔHm) of the dicationic ionic compounds was measured by differential scanning calorimetric (DSC). The V'm, Td, Tm and ΔHm are presented in Table 7. The order of anion volume (V'm) for the four dicationic ionic compounds agrees with the values expected from the sizes of the four anions. In different anion dicationic ionic compounds, the melting temperatures increased in the following order: Br->PF6->BF4->NTf2-. Considering 20

ACCEPTED MANUSCRIPT the ethyl linkage chain dicationic ionic compounds, larger anion volumes resulted in a lowering of the melting points. With an increase in the volume of the anions, the effective accumulation of the crystal was hindered. Therefore, the melting points decreases.

PT

In Table 7, it is found that when the anions are BF4-、PF6- and NTf2-, and the

RI

melting points and heat of fusion are lower. When the anion is a halogenated anion,

SC

the heat of fusion is higher. Structurally speaking, the anion volume increases, the interaction between the anions and the cations decreases, and the lattice energy

NU

decreases, thereby lowering the the melting points and latent heat.

MA

In addition, it is found that interaction between the anions and the cations decreases with increasing F atom. The reason of the reduced intermolecular forces is

D

the tightly outer layer electron cloud of F atom decreasing inductive effect of the

PT E

anion. Meanwhile, F atoms have a strong electronegativity, the negative charge on the anion is not localized, which decreases the interaction force among the ions. The

CE

smaller interaction force decreases the binding energy and the heat of fusion indirectly.

AC

Therefore, it is necessary to fully consider the F atom effect in the anion, which provides important guiding significance for the design of new ionic liquid phase change materials.

Table 7. Effects of the Anion on Decomposition Temperatures, Melting Points, and Heat of Fusion. Ionic Compounds

V'm (cm3·mol-1)

Td (K)

Tm (K)

ΔHm (KJ·Kg-1)

C2(mim)2(Br)2

35.44

557.75

481.47

116.26

21

ACCEPTED MANUSCRIPT

C2(mim)2(BF4)2

43.25

629.09

417.97

94.41

C2(mim)2(PF6)2

59.53

661.88

464.49

109.41

C2(mim)2(NTf2)2

145.20

725.56

412.78

73.99

3.5. Heat Capacity

PT

In order to calculate the heat storage density of the dicationic ionic compounds,

RI

the heat capacity (Cp) must be known. Cp of the dicationic ionic compounds were

SC

determined by DSC. Data were obtained at atmospheric pressure and within 333.15 K to 453.15 K for the ionic liquids in steps of 1 K. The heat capacities of these

NU

dicationic ionic compounds in a solid state was higher than 2.85 KJ·Kg-1·K-1 and

MA

increased with increasing temperature. This heat capacity is higher compared to MILs and performed good sensible heat storage in Table 8. Compared with the MILs and

D

commercial PCM, the heat capacity of the dicationic ionic compounds is larger. The

PT E

number of translation, vibration, and rotational energy storage modes in ionic compound molecule largely determines the size of the heat capacity. Therefore, when

CE

the dicationic ionic compounds contains more atoms in a molecule, the molecule will

AC

have more energy modes. Thus, these dicationic ionic compounds exhibit larger heat capacity. In addition, the dicationic ionic compounds have additional hydrogen bonds. These hydrogen bonds provide additional vibration mode, when the temperature rises can absorb more heat. This means that the dicationic ionic compounds can be better stored thermal after the solid-liquid phase changes.41 Table 8. Heat Capacities of the Dicationic Ionic Compounds and Other PCMs. Ionic Compounds

Cp (KJ·Kg-1·K-1)

Other PCMs

Cp (KJ·Kg-1·K-1)

22

ACCEPTED MANUSCRIPT

2.82

[C2MIM]BF4

1.12 (ref 42)

C3(mim)2(Br)2

2.85

[C2MIM]PF6

1.00 (ref 42)

C3(eim)2(Br)2

1.97

CH3NH3Cl

1.35 (ref 42)

C4(mim)2(Br)2

2.82

[C4MIM]Cl

1.58 (ref 42)

C5(mim)2(Br)2

2.78

[C4MIM][NTf2]

1.05 (ref 42)

C6(mim)2(Br)2

2.80

Thermal oil

PT

C2(mim)2(Br)2

RI

1.69 (ref 9)

SC

3.6. Thermal Storage Density

On the basis of the density and heat capacity values above, the thermal storage

NU

density (E) can be calculated by Eq. (2) when inlet (Tin) and outlet (Tout) temperatures

MA

are 297.15 K and 397.15 K. For comparison purposes, we chose the temperature change value 100 K commonly used in solar energy applications as the calculation

D

temperature.1,13

E   C p (Tout  Tin )

PT E

(2)

For the calculated results listed in Table 9, these values are higher than those of

CE

the MILs. The temperature change of the dicationic ionic compounds is much greater

AC

than 100 K, so the realistic thermal storage density of the dicationic ionic compounds may be higher. Ours data show that there is a possibility that current latent thermal energy storage media such as organic PCMs can be replaced with dicationic ionic compounds. Table 9. Thermal Storage Densities (E, MJ/m3) of Various Dicationic Ionic Compounds at 100 K. Acronym

E (MJ/m3)

MILs

E (MJ/m3)

C2(mim)2(Br)2

>466.15

[C4MIM]BF4

>200 (ref 13)

23

ACCEPTED MANUSCRIPT >463.27

[C4MIM][NTf2]

>180 (ref 13)

C3(eim)2(Br)2

>314.30

[C6MIM]Cl

>170 (ref 9)

C4(mim)2(Br)2

>423.38

[C6MIM]PF6

>210 (ref 9)

C5(mim)2(Br)2

>409.95

[N444H]Cl

>63 (ref 43)

C6(mim)2(Br)2

>425.00

Mineral oil

>104 (ref 43)

PT

C3(mim)2(Br)2

single

crystal

of

C2(mim)2(Br)2

was

formed

in

the

SC

Large

RI

3.7. Crystal Structure of C2(mim)2(Br)2

dichloromethane-methanol system [V(dichloromethane):V(methanol)=1:2] at room

NU

temperature. The large single crystal is stable under the air, which resulted in

MA

colourless crystals with sizes of 0.51×0.46×0.40 mm3. The crystal structure of C2(mim)2(Br)2 was determined by single crystal X-ray diffraction. Crystal data and

D

refinement results are given in Table 10.

PT E

Table 10 Summary of the Crystal Data for C2(mim)2(Br)2

CE

Compound

C2(mim)2(Br)2

C10H16Br2N4

Crystal system

triclinic

Space group (Z)

P1

a (Å)

5.1342(2)

b (Å)

11.3628(6)

c (Å)

12.988(4)

AC

Empirical formula

24

ACCEPTED MANUSCRIPT

75.652(1)

β (degrees)

79.540(6)

γ (degrees)

83.062(4)

Volume (Å3)

683.2(5)

PT

α (degrees)

Z/density (calcd.) (mg/m3)

RI

2

5.223

NU

Absorption coefficient (mm-1)

325

MA

F (000) (e)

1.50-28.01

R indices (all data) a

R1= 0.0420, wR2b= 0.1128

D

θ Range for data collection (degrees)

   F  F   /    F  

PT E

R1   Fo  Fc /  Fo .

b

wR2 

2 o

2 2 c

2 2 o

1/2

.

AC

CE

a

1.731

SC

D (calad) (Mg/m3)

Figure 7 Structure of C2(mim)2(Br)2 in the solid state.

The symmetric unit contains two anions and one dication. The molecular structure of C2(mim)2(Br)2 with the zigzag ethyl linkage chain is also shown in Figure 7. Two anions are ordered into two positions in which the bromine anions between the C2-H of imidazole rings and the ethyl linkage chain. The methyl side chains are

25

ACCEPTED MANUSCRIPT turned almost perpendicular to the respective imidazolium ring. The [C2(mim)2]2+ dication has a highly twisted configuration with two aromatic rings almost perpendicular

to

the

C2

plane

(angles

of

C5C6-N1C3N2C4C5

and

C5C6-C9C8N4C7N3 are 93.2(5) and 105.6(7)°, respectively). The imidazolium cation

PT

ring has a typical planar conformation, and all of the ring C-C and C-N distances and

RI

angles are within the normal range. The C-C bond of ethyl moiety is almost

SC

perpendicular to the plane of the aromatic ring, which is also typical for (N)CC fragments of alkylimidazolium crystal structures.44 The geometrical parameters of the

NU

aromatic rings for both structures are in good accord with each other except that the

MA

angle in C2(mim)2(Br)2 between the two methylimidazolium rings is much smaller. Several hydrogen bonds are found in this structure and these bonds influence the

D

unique packing within the solid state. The crystal structure of C2(mim)2(Br)2 consists

PT E

of zigzag bands of molecules along the crystallographic c-axis. Those bands are formed by separated stacks of isolated dications and bromide anions joined with

CE

hydrogen bonds typical for dicationic ionic compound.

AC

3.8. Hydrogen-Bond Network Analysis One of the crucial molecular level interactions affecting the thermo-physical properties of the dicationic ionic compounds is hydrogen bonding.44, 45,46 Quantum chemical calculations can provide some information about the relationship between the structures and properties of the dicationic ionic compounds. Therefore, the hydrogen bonds of the dicationic ionic compounds were investigated by density functional theory (DFT) using Gaussian 09 at the B3LYP/6-31++G (d, p) level.47,48

26

ACCEPTED MANUSCRIPT Table 11. Bond Lengths and Angles of H-Bonds in C2(mim)2(Br)2. Distance(Å)

Hydrogen bond

Bond angles (°)

Br31…H6

2.45

∠ Br31…H6—C2

145.14

Br31…H14

2.87

∠ Br31…H14—C13

119.68

Br31…H17

2.83

∠ Br31…H17—C16

123.17

Br31…H25

2.67

∠ Br31…H24—C23

144.24

Br32…H7

2.67

∠ Br32…H7—C4

Br32…H15

2.83

∠ Br32…H7—C13

123.17

Br32…H18

2.87

∠ Br32…H7—C16

119.68

Br32…H24

2.45

∠ Br32…H7—C20

145.14

144.24

MA

NU

SC

RI

PT

Hydrogen bond

Table 11 lists the bond lengths and angles of hydrogen bonds. The C-C and C-N

D

bond lengths on the imidazole ring are between 1.369-1.535 Å and 1.349-1.469 Å,

PT E

respectively, which are shorter than that of normal C-C bond (1.540 Å) and C-N bond (1.470 Å), but longer than that of normal C=C bond (1.340 Å) and C=N bond (1.270

CE

Å). In other words, the bond length on the imidazole ring has a tendency to

AC

equalization between single and double bonds. Besides, it is found that the dihedral angle (θC5N1C2N3 and θN1C2N3C4) are -0.447° and 0.954°, respectively. It shows that the atoms on the imidazole ring are all sp2 hybrid valence states. Each atom is connected to each other by a hybrid orbital to form a planar structure. The remaining 2pz orbitals are parallel to each other to form a closed conjugated large π bond. It is indicated that the cationic imidazole ring has aromatic structural features. Therefore, the unit positive charge is dispersed, so that the imidazolium cation tends to be stable. 27

ACCEPTED MANUSCRIPT The calculated results are in good agreement with experimental crystal data. Figure 8 depicts the hydrogen-bond network of the dicationic ionic compounds C2(mim)2(Br)2. The shows that the C2(mim)2(Br)2 has eight hydrogen bonds within one dication and two Br - anions. A three-dimensional stable hydrogen-bonded

PT

network structure is built by the interaction of these hydrogen bonds in dicationic

MA

NU

SC

RI

ionic compounds.

Figure 8 Hydrogen-bond network for C2(mim)2(Br)2.

D

4. CONCLUSIONS

PT E

A series of the dicationic ionic compounds containing different alkyl side chains were synthesized with anions (i.e., Br-, PF6-, BF4- and NTf2-). All experimental

CE

decomposition temperatures exhibited a decrease with an increase the alkyl side-chain

AC

or molecular asymmetry. When longer alkyl side chains are introduced into the imidazolium ring, the melting points of the dicationic ionic compounds was observed to decrease. In addition, the heat of fusion is greatly dependent on the structure of the ionic compounds and will increase with increasing interaction energy. Obtained by a simple calculation, the heat capacities of the dicationic ionic compounds are greater than those of most traditional MILs. The interaction energies of the dicationic ionic compounds are larger than MILs, and resulting in high latent heat and thermal storage

28

ACCEPTED MANUSCRIPT density. The quantum calculation indicate that dicationic ionic compounds can form the complicated hydrogen bonding network. Moreover, the dicationic ionic compounds contain more hydrogen bonds compared with those of the MILs. More hydrogen bonds make these dicationic ionic compounds exhibit preferable thermal

PT

performances. This new overall understanding opens opportunities for the design of

RI

PCMs based on dicationic ionic compounds in thermal energy storage.

SC

AUTHOR INFORMATION Corresponding Author

NU

*E-mail: [email protected] (B.L. Yang)

MA

ACKNOWLEDGMENTS

This work was supported by National Natural Science Foundation of China

D

(U1662117) and State Key Laboratory of Chemical Safety and Control (Sinopec

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PT E

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Liquid

on

Electrochemical

and

Pseudocapacitance Performance of Conductive Polymer Electroactive Film. J. Colloid

PT

Interf. Sci. 2017, 505, 1158-1164. DOI: 10.1016/j.jcis.2017.07.001.

RI

31. Patil, R. A.; Talebi, M.; Xu, C.; Bhawal, S. S.; Armstrong, D. W. Synthesis of

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Thermally Stable Geminal Dicationic Ionic Liquids and Related Ionic Compounds: An Examination of Physicochemical Properties by Structural Modification. Chem.

NU

Mater. 2016, 28, 4315-4323. DOI: 10.1021/acs.chemmater.6b01247.

MA

32. Zhang, Z.; Zhang, Z.; Hao, B. N.; Zhang, H.; Wang, M.; Liu, Y. D. Fabrication of Imidazolium-Based Poly(Ionic Liquid) Microspheres and their Electrorheological

D

Responses. J. Mater. Sci. 2017, 52, 5778-5787. DOI: 10.1007/s10853-017-0812-4.

PT E

33. Aboudzadeh, M. A.; Munoz, M. E.; Santamaria, A.; Mecerreyes, D. New Supramolecular Ionic Networks Based on Citric Acid and Geminal Dicationic Ionic

CE

Liquids. Rsc Adv. 2013, 3, 8677-8682. DOI: 10.1039/c3ra40629f.

the

AC

34. Sun, H.; Zhang, D.; Liu, C.; Zhang, C. Geometrical and Electronic Structures of Dication

and

Ion

Pair

in

the

Geminal

Dicationic

Ionic

Liquid

1,3-Bis[3-Methylimidazolium-Yl]Propane Bromide. Journal of Molecular Structure: THEOCHEM. 2009, 900, 37-43. DOI: 10.1016/j.theochem.2008.12.024. 35. Dong, K.; Zhang, S. Hydrogen Bonds: A Structural Insight into Ionic Liquids. Chem.-Eur. J. 2012, 18, 2748-2761. DOI: 10.1002/chem.201101645. 36. Xiong, Y.; Wang, H.; Wu, C.; Wang, R. Preparation and Characterization of

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ACCEPTED MANUSCRIPT Conductive Chitosan-Ionic Liquid Composite Membranes. Polym. Advan. Technol. 2012, 23, 1429-1434. DOI: 10.1002/pat.2061. 37. Reinert, L.; Batouche, K.; Leveque, J.; Muller, F.; Beny, J.; Kebabi, B.; Duclaux, L. Adsorption of Imidazolium and Pyridinium Ionic Liquids onto Montmorillonite:

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Characterisation and Thermodynamic Calculations. Chem. Eng. J. 2012, 209, 13-19.

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38. Greaves, T. L.; Drummond, C. J. Protic Ionic Liquids: Properties and Applications. Chem. Rev. 2008, 108, 206-237. DOI: 10.1021/cr068040u.

NU

39. Zhang, S.; Qi, X.; Ma, X.; Lu, L.; Zhang, Q.; Deng, Y. Investigation of

MA

Cation-Anion Interaction in 1-(2-Hydroxyethyl)-3-Methylimidazolium-Based Ion Pairs by Density Functional Theory Calculations and Experiments. J. Phys. Org.

D

Chem. 2012, 25, 248-257. DOI: 10.1002/poc.1901.

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40. Wang, Y.; Li, H. R.; Han, S. J. Structure and Conformation Properties of 1-Alkyl-3-Methylimidazolium Halide Ionic Liquids: A Density-Functional Theory

CE

Study. J. Chem. Phys. 2005, 123, 17450117. DOI: 10.1063/1.1979478.

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41. Paul, T. C.; Morshed, A. K. M. M.; Fox, E. B.; Visser, A. E.; Bridges, N. J.; Khan, J. A. Thermal Performance of Ionic Liquids for Solar Thermal Applications. Exp. Therm. Fluid Sci. 2014, 59, 88-95. DOI: 10.1016/j.expthermflusci.2014.08.002. 42. Holbrey, J. D.; Reichert, W. M.; Reddy, R. G.; Rogers, R. D. Heat Capacities of Ionic Liquids and their Applications as Thermal Fluids. ACS Symposium Series; American Chemical Society: Washington, DC. 2003, 856, 121-133. DOI: 10.1021/bk-2003-0856.ch011.

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ACCEPTED MANUSCRIPT 43. Lamas, A.; Brito, I.; Salazar, F.; Graber, T. A. Synthesis and Characterization of Physical, Thermal and Thermodynamic Properties of Ionic Liquids Based on [C(12)Mim] and [N-444H] Cations for Thermal Energy Storage. J. Mol. Liq. 2016, 224, 999-1007. DOI: 10.1016/j.molliq.2016.10.103.

Anion.

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2009,

48,

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Difluorophosphate

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10.1021/ic9008009.

45. Ding, Y.; Zha, M.; Zhang, J.; Wang, S. Synthesis, Characterization and Properties

NU

of Geminal Imidazolium Ionic Liquids. Colloid. Surface. A 2007, 298, 201-205. DOI:

MA

10.1016/j.colsurfa.2006.10.063.

46. Frizzo, C. P.; Bender, C. R.; Tier, A. Z.; Gindri, I. M.; Salbego, P. R. S.; Meyer,

D

A. R.; Martins, M. A. P. Energetic and Topological Insights into the Supramolecular

PT E

Structure of Dicationic Ionic Liquids. Crystengcomm 2015, 17, 2996-3004. DOI: 10.1039/c5ce00073d.

CE

47. Dong, K.; Song, Y.; Liu, X.; Cheng, W.; Yao, X.; Zhang, S. Understanding

AC

Structures and Hydrogen Bonds of Ionic Liquids at the Electronic Level. J. Phys. Chem. B 2012, 116, 1007-1017. DOI: 10.1021/jp205435u. 48. Dong, K.; Zhang, S.; Wang, D.; Yao, X. Hydrogen Bonds in Imidazolium Ionic liquids. J. Phys. Chem. A 2006, 9775-9782. DOI: 10.1021/jp054054c.

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ACCEPTED MANUSCRIPT Graphical abstract Br H

N

H

C

N

C

H N

H

H

H

H

H

N H

CH3

PT

H H3C

RI

Br

AC

CE

PT E

D

MA

NU

SC

Figure Hydrogen bonding structure schematic of C2(mim)2(Br)2.

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ACCEPTED MANUSCRIPT Highlight 1. Ionic liquids are investigated as the new high-efficiency thermal storage material. 2. Ionic liquids’ properties were excellent during thermal storage process.

AC

CE

PT E

D

MA

NU

SC

RI

PT

3. More hydrogen bonds make ionic liquids exhibit preferable thermal performances.

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