The effect of imidazolium and phosphonium ionic liquids on toluene absorption studied by a molecular simulation

The effect of imidazolium and phosphonium ionic liquids on toluene absorption studied by a molecular simulation

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Journal Pre-proof The effect of imidazolium and phosphonium ionic liquids on toluene absorption studied by a molecular simulation Liang Tan, Jiamei Zhu, Min Zhou, Xiaodong He, Shuangquan Zhang PII:

S0167-7322(19)34814-7

DOI:

https://doi.org/10.1016/j.molliq.2019.112054

Reference:

MOLLIQ 112054

To appear in:

Journal of Molecular Liquids

Received Date: 28 August 2019 Revised Date:

21 October 2019

Accepted Date: 1 November 2019

Please cite this article as: L. Tan, J. Zhu, M. Zhou, X. He, S. Zhang, The effect of imidazolium and phosphonium ionic liquids on toluene absorption studied by a molecular simulation, Journal of Molecular Liquids (2019), doi: https://doi.org/10.1016/j.molliq.2019.112054. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

1

The effect of imidazolium and phosphonium ionic liquids on toluene absorption

2

studied by a molecular simulation

3

Liang Tan, Jiamei Zhu∗, Min Zhou, Xiaodong He, Shuangquan Zhang

4

School of Chemical Engineering and Technology, China University of Mining & Technology,

5

Xuzhou, Jiangsu 221116, P.R. China

6

Abstract

7

Toluene absorption by the ion pairs of 1-butyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide

8

([Bmim][TFSI]), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) and tributyl(propyl)phosphonium

9

tetrafluoroborate ([P4443][BF4]) is performed at the GGA/PW91 level using a density functional theory (DFT). The ion

10

pairs of [Bmim][TFSI], [Bmim][BF4] and [P4443][BF4] are stable with five, five and six hydrogen bonds between the

11

anions and cations, respectively. There is no hydrogen bond and just one C-H⋯π bond between toluene and

12

[Bmim][TFSI], while two hydrogen bonds and three C-H⋯π bonds are formed between absorbed toluene and

13

[Bmim][BF4] or [P4443][BF4]. That shows the interaction between the ion pairs of [Bmim][BF4] and [P4443][BF4] with

14

toluene is stronger than that of [Bmim][TFSI]. Moreover, [P4443]+ has greater effect on the charge transfer of toluene than

15

[Bmim]+ based on the higher electrostatic force with toluene, which indicates that [P4443][BF4] has better advantage on

16

toluene absorption than [Bmim][BF4]. The absorption energy (Eabs, in kJ·mol-1) between toluene and ion pairs of

17

[Bmim][TFSI], [Bmim][BF4] and [P4443][BF4] is -28.88 kJ·mol-1, -34.13 kJ·mol-1 and -39.38 kJ·mol-1, respectively. The

18

frontier molecular orbital (FMO) analysis indicates that absorption of toluene by ionic liquids is a physical process. The

19

HOMO and LUMO of [P4443][BF4]·C7H8 are all localized in toluene, but the LUMO of [Bmim][BF4]·C7H8 is composed

20

by the atoms of imidazolium ring in cation, which also shows that the interaction between toluene and [P4443][BF4] is

21

stronger. In the simulation study, the molecular insight into mechanism of toluene absorption by imidazolium and



Corresponding author. Tel: +86-516-83884079; E-mail address: [email protected] (J. Zhu). 1

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phosphonium ionic liquids is provided by the comprehension including hydrogen bond, C-H⋯π bonds, electrostatic

23

force and the frontier molecular orbital, which is fundamental for industrial application of ionic liquids in toluene

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separation.

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Keywords: molecular simulation; density functional theory; volatile organic compounds; ionic liquids; toluene

26

1. Introduction

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Volatile organic compounds (VOCs) were defined by the World Health Organization as a general term of organic

28

compounds with melting points below room temperature and boiling points between 50 ℃ and 260 ℃ [1], which mainly

29

include ketones, hydrocarbons, aromatic hydrocarbons, acids, alcohols, lipids, amines and organic acids, etc. VOCs have

30

drawn great attention over the past decades because photochemical pollution caused partly by VOCs has threaten the

31

human health and global environment [2, 3]. As a sort of VOCs, aromatic hydrocarbons are danger than others due to its

32

complex and stable molecular structure, so it is a serious challenge to eliminate the emission of VOCs, especially

33

aromatic hydrocarbons in industrial processes. However, there is little improvement in the governance of VOCs pollution

34

compared with other atmospheric pollutants like sulfur dioxide, nitrogen oxides, particulate matter and so on. At present,

35

techniques such as thermal destruction, condensation, membrane separation, adsorption and absorption have been

36

investigated for the removal of VOCs [4-6]. There are also lots of new technologies that have been developed, including

37

biofiltration [7], corona discharge [8], photodecomposition [9], plasma decomposition [10, 11], etc. Among these

38

techniques for VOCs removal, absorption has been believed as a feasible great solution [12, 13]. Unfortunately, there are

39

several weak points about current absorbents in the absorption processes, such as secondary pollution and lower

40

absorption capacity [14, 15]. In this case, it is essential to develop a more economical and effective VOCs absorbent.

41

Ionic liquids (ILs) are molten salts that are completely composed of cations and anions. ILs have been attracted many

42

attentions owing to their remarkable advantages such as great solubility, low vapor pressure, thermostability, pH

43

designability, etc [16-18]. The use of the bulk ILs or supported IL phase (SILP) for VOCs removal has become a 2

44

research hotspot in recent years. Domańska and Marciniak [19] investigated the solubility of alcohol on

45

imidazolium-based ILs with different anion and the concluded that anions have a great effect on VOCs absorption, which

46

the solubility of alcohol in bis(trifluoromethylsulfonyl)imide anion ([TFSI]-) is larger than that in tetrafluoroboron anion

47

([BF4]-). García et al. [20] discussed the solubility of n-heptane, n-hexane, benzene, toluene, o-xylene, m-xylene and

48

p-xylene

49

methanesulfonate ([Bmim][MeSO4]) respectively, and the solubility order follows the rule: benzene > toluene > o-xylene

50

> p-xylene > m-xylene > n-hexane > n-heptane. Wang et al. [21] performed their experiment to test absorption ability of

51

1-butyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide ([Bmim][TFSI]) to toluene as a model VOC, and

52

Cichowska-Kopczyńska

53

bis(trifluoromethylsulfonyl)imide and trifluoromethanesulfone to separate toluene from binary mixed gas phase

54

(toluene/N2). Thus, it indicates that ILs have huge potential ability in absorption aliphatic and aromatic hydrocarbons.

in

1-ethyl-3-methylimidazolium

et

al.

[22]

ethylsulfate

used

supported

([Emim][EtSO4])

ionic

liquid

and

1-butyl-3-methylimidazolium

membrane

based

imidazolium

55

ILs present tunable property and can be designed to have different chemical and physical properties by modifying

56

cations or anions. Looking for regular pattern of gas absorption only by experiments will waste time and energy. It is

57

essential to study the inertial structure properties and interaction by the computer simulations in this process [23-29].

58

Chen et al. [25] investigated aminoalkyl imidazolium-based IL for CO2 capture using quantum chemistry and molecular

59

dynamics (MD) approaches and the results showed that the anions have a much less capacity of capture CO2 based on

60

physical absorption. Chaban [28] studied HF capture by imidazolium acetate ILs using molecular dynamics and density

61

functional theory (DFT) simulation and reported that acetate anion plays a key role in HF absorption and

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imidazolium-based cation only maintains acetate in the liquid state. Gao et al. [23] calculated the interactions between

63

imidazolium-based ILs (anion is Cl-, Br- and BF4-) and VOCs (including methanol, ethanol, 2-methyl-1-propanol,

64

3-methyl-1-butanol, formaldehyde, acetaldehyde, butanal, acetone, ethane, propane, ethylene, cis-2-butene, acetylene,

65

and toluene) and the thermodynamic data by using the Hartree–Fock (HF) and DFT methods in Gaussian 03. Conclusion 3

66

showed that 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) is more likely to interact with alcohol than other ILs

67

([Bmim]Br and [Bmim][BF4]). Zhang et al. [26] reported supported IL membranes to separate toluene/n-heptane and the

68

interaction energy of [Bmim][BF4] with toluene used quantum chemical calculation was 34.028 kJ·mol-1, which was

69

much larger than that of [Bmim][BF4] with n-heptane (-5.211 kJ·mol-1). It is obvious that imidazolium ILs are mainly

70

used to absorb acid gas or VOCs both in the experiments and computer simulations.

71

Recently phosphonium ILs having higher productivity and lower cost than nitrogen-based ILs, have some

72

applications in gas separation [30, 31]. There have been some researches on quantum chemical calculations including

73

CO2 or SO2 absorption [32, 33]. Cui et al. [33] designed the highly efficient phosphonium ILs with the anion tetrazole

74

[Tetz]- to perform multiple sites absorption of SO2 and CO2 using first-principle DFT calculation in the model DMol3 of

75

Materials Studio. The structure of the aromatic compounds makes the interaction of ILs between aromatic ring and CO2

76

or SO2 different and understanding the gas sorption behavior in ILs would be interesting.

77

Hydrogen bond and C-H⋯π bond [34, 35] are extensively applied to understand the mechanism of VOCs absorption

78

by ILs and they play a great important role in absorption of aromatic organic compounds. In order to further investigate

79

the effect of imidazolium and phosphonium-based ILs on aromatic hydrocarbons absorption, several ILs are designed to

80

conduct the quantum chemical calculation with toluene as a model compound in this work. The absorption of toluene by

81

the ILs is calculated at GGA/PW91 level. The microstructures of the isolated [Bmim]+, [P4443]+, [BF4]-, [TFSI]- and the

82

ion pairs of [Bmim]TFSI, [Bmim]BF4, [P4443]BF4 are analyzed. The geometries of toluene absorption by the ion pairs are

83

optimized, and the interaction between toluene and ILs is expounded. The work gives the molecule explanation of the

84

processes of toluene absorption by imidazolium and phosphonium ILs, and it provides the theoretical basis for the

85

rational design of using ILs for VOCs absorption.

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2. Calculation method

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The DFT calculations are performed by the Accelrys DMol3 DFT module in Materials Studio (MS). The DMol3 [36, 4

88

37] method is widely used to deal with gas, solution and surface molecules by numerical orbital basis set. The research

89

fields mainly cover electronic structures and orbital electron occupations. The GGA/PW91 is used to describe the

90

exchange and correlation energy. It is augmented with polarization p-function in the double numerical basis set. The

91

convergence tolerance of the system total energy is set to l.0×10-5 Ha, and the force field is set to 0.002 Ha/Å. The

92

maximum offset tolerance is set to 0.005 Å. Self-consistent field iteration (SCF) is set to l.0×10-6.

93

About 20 different initial position configurations are designed for each set of simulation calculations to more

94

accurately and comprehensively find the absorption site between toluene and ILs since the uncertainty of the position of

95

ILs to absorb toluene. The graphical displays are generated with Materials Studio.

96

3. Result and discussion

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3.1 The geometry properties

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3.1.1 The isolated [Bmim]+, [P4443]+, [BF4]- and [TFSI]-

99

There is the most stable structure of the isolated [Bmim]+, [P4443]+, [BF4]- and [TFSI]- at the GGA/PW91 level, which

100

are shown in Fig.1. Some optimized structure parameters are listed in Table 1. The length of single and double bonds of

101

imidazolium ring in the [Bmim]+ tends to average, which infers that imidazolium ring is a five-membered aromatic ring

102

formed by SP2 hybrid of atoms [38]. The lengths between P1 and C2, C5, C9, C13 are 1.821 Å、1.824 Å、1.825 Å and

103 5

104 105

Figure 1. The stable configurations of isolated [Bmim]+ cation(a), [P4443]+ cation(b), [BF4]- anion(c) and [TFSI]- anion (d).

106 107

Table 1 Some optimized bond lengths of isolated cations, ionic pairs, and toluene absorption of ionic pairs: bond length (in Å) B2-F1 [BF4]

-

B2-F3

B2-F4

B2-F5 +

P1-C2

P1-C5

P1-C9

P1-C13

1.821

1.824

1.825

1.825

1.426

1.426

1.426

1.426

[P4443]

[P4443][BF4]

1.436

1.436

1.438

1.386

[P4443][BF4]

1.816

1.818

1.819

1.832

[P4443][BF4]·C7H8

1.428

1.437

1.441

1.389

[P4443][BF4]·C7H8

1.817

1.819

1.820

1.831

108

1.825 Å in the [P4443]+, respectively. The bond length of P1-C2 is slightly shorter than that of other three bonds because

109

the length of side chain in C2 is a little short. The single bonds in [BF4]- have the equal length with 1.426 Å through

110

computation as shown in Table 1.

111

3.1.2 The ion pairs of [Bmim][BF4], [Bmim][TFSI] and [P4443][BF4]

112

Fig.2 shows the most stable ion pairs of [Bmim][BF4], [Bmim][TFSI] and [P4443][BF4]. The lengths of hydrogen bonds

113

and some structure parameters are shown in Table 2. There are five hydrogen bonds between the anion and cation formed

114

in [Bmim][BF4], five hydrogen bonds formed in [Bmim][TFSI] and six hydrogen bonds in [P4443][BF4] as shown in Fig.

115

2. For [Bmim][BF4] and [Bmim][TFSI], the anions are nearby H13 atom of imidazolium ring, where some hydrogen

116

bonds are formed and structures tend to be stable. The data in Table 2 show that the lowest energy value of the ion pairs

117

of [Bmim][BF4], [Bmim][TFSI], and [P4443][BF4] is -2.23×106 kJ·mol-1, -5.91×106 kJ·mol-1, -3.57×106 kJ·mol-1,

118

respectively. The investigation of hydrogen bonds of [Bmim][BF4] and [Bmim][TFSI] was in similarity with the results

119

obtained by Dong [38].

6

120 121 122 123

Figure 2. The stable configurations of [Bmim][BF4] (a), [Bmim][TFSI] (b) and [P4443][BF4] (c). Table 2 The main optimized structure parameters of the ionic pairs and toluene absorption of ionic pairs: bond length (in Å), the energy (E, in kJ·mol-1) and the absorption energy (Eabs, in kJ·mol-1)

124 125

For [P4443][BF4], two hydrogen bonds are formed in each F atom except F53. Compared with the isolated ions, some

126

structural parameters have changed due to the function of hydrogen bonds and the interaction between the anion and

127

cation. Combined with Table 1, the lengths of the three F-B bonds involved in the formation of hydrogen bonds in the

128

ion pair increase from 1.426 Å to 1.436 Å, 1.436 Å and 1.438 Å, while the length of the F-B bond that is not drawn into

129

the formation of hydrogen bonds decreases to 1.386 Å. The bond lengths of P1-C2, P1-C5. P1-C9 are shorter than those

130

in the isolated [P4443]+, but the P1-C13 bond length is 1.832 Å, which is a little longer than that in the isolated cation.

131

3.1.3 The absorption of toluene in the ion pairs

132

The most stable structure of toluene and toluene absorption by the ion pairs is shown in Fig.3 and main structure

7

133

parameters are also listed in Table 2. The C-C bonds in benzene ring have almost same length. Bond angles of benzene

134

ring are all around 120° and the dihedral angles are very close to 0°. Therefore, it can be seen that the presence of a

135

methyl group on benzene ring has no effect on aromatic structure of toluene. Based on those stable configurations and

136

structure parameters, the absorption of toluene by the ion pairs will be explored and optimized.

137 138 139

Figure 3. The stable configurations of toluene(a), [Bmim][BF4]·C7H8(b), [Bmim][TFSI]·C7H8 (c) and [P4443][BF4]·C7H8(d).

140

As seen the stable absorption structure and parameters of [Bmim][BF4]·C7H8 as shown in Fig.3 and Table 2. The

141

interaction between the absorbed toluene and ion pair of [Bmim][BF4] leads to appearance of the C-H⋯π bonds [34, 39,

142

40] formed between [Bmim]+ and toluene, besides, the hydrogen bonds are formed between [BF4]- and toluene. The

143

lengths of three C-H⋯π bonds are 2.682 Å, 3.587 Å and 3.909 Å, and the lengths of hydrogen bonds H38⋯F28 and

144

H44⋯F27 are 2.505 Å and 2.617 Å, respectively. In addition, the interaction between the anion and cation of ILs

145

becomes difference as addition of toluene, which is concretely reflected in the change of length and site of hydrogen

146

bond. All the position and length of the hydrogen bonds between anion and cation in [Bmim][BF4]·C7H8 have changed

8

147

after absorption as shown in Fig. 3 and Table 2, but the number of the hydrogen bonds is still five. The hydrogen bond

148

has the great important role in maintaining the stable system. It is believed that ordinary hydrogen bond is the interaction

149

between hard acid and hard base, while the interaction of C-H⋯π bond is considered as the interaction between soft acid

150

and soft base, which has the properties of weak hydrogen bond. The C-H⋯π bond plays an important role in various

151

fields of chemistry such as molecular conformation and self-assembly, though it is weak. In the unconventional hydrogen

152

bond, the enthalpy of one C-H⋯π bond is the smallest (about 1 kcal/mol). A distinctive property of this interaction is that

153

many C-H groups can collaboratively participate in the interaction of π area, resulting in molecules or molecular

154

aggregates stable and flexible [34]. Besides, the changes of the structure parameters such as bond length and bond angle

155

in [Bmim][BF4]·C7H8 are very little, which is compared with the ion pair of [Bmim][BF4]. It indicates that the

156

absorption of toluene using [Bmim][BF4] is a physical process, which corresponds to the reported research [41].

157

The most stable configurations of [Bmim][TFSI]·C7H8 are also shown in Fig.3. There are one C-H⋯π bond between

158

[Bmim]+ and single toluene, and the distance is 2.729 Å. There is no hydrogen bond between [TFSI]- anion and the

159

toluene. It is notable that the stability of [Bmim][TFSI] is slightly disturbed by toluene absorption due to the number of

160

the hydrogen bonds between ion pairs reducing from 5 to 3. And the lengths in [Bmim][TFSI]·C7H8 with 2.245 Å, 1.997

161

Å, 2.156 Å in order are shorter than that in [Bmim][TFSI]. However, the structures of [Bmim][TFSI] and toluene do not

162

change obviously, and toluene still retains aromatic structure of six-membered ring, which demonstrate that the

163

absorption of toluene on [Bmim][TFSI] is also a physical process.

164

For [P4443][BF4]·C7H8, three C-H⋯π bonds are formed between [P4443]+ and toluene, and the lengths are 2.856 Å,

165

3.500 Å and 3.925 Å in order. There are two hydrogen bonds composed by F54, F55 in [BF4]- and H70, H65 in the

166

methyl of toluene in [P4443][BF4]·C7H8. The lengths of the hydrogen bonds H65⋯F55 and H70⋯F54 are 2.382 Å and

167

2.507 Å. Furthermore, the interaction between the anion and cation in the ion pair of [P4443][BF4] has obviously changed

168

after toluene absorption. The number of hydrogen bonds in the ion pair is still 6, but the hydrogen bond H33⋯F51 in 9

169

[P4443][BF4]·C7H8 disappears because the distance between H33 and F51 becomes longer, then the new hydrogen bond is

170

formed between H36 and F51. The lengths of hydrogen bonds in [P4443][BF4]·C7H8 are 2.205Å, 2.371 Å, 2.565 Å, 2.083

171

Å, 2.262 Å and 2.525 Å, respectively. There is merely alteration of the structure parameters in [P4443][BF4] and

172

[P4443][BF4]·C7H8, thus the results show that this process is still a physical absorption.

173

Moreover, the absorption energies (Eabs, Eabs = Etot – (EIL + Etol), in kJ·mol-1) of unimolecular toluene in the most

174

stable ILs are listed in Table 2. Eabs of [Bmim][TFSI]·C7H8, [Bmim][BF4]·C7H8 and [P4443][BF4]·C7H8 is -28.88 kJ·mol-1,

175

-34.13 kJ·mol-1 and -39.38 kJ·mol-1, respectively, indicating the interaction between [P4443][BF4] and toluene is the

176

largest in these processes of absorption. It is noted that Eabs and the number of hydrogen and C-H⋯π bonds formed

177

between ILs and toluene are non-linear relations. The Eabs of [Bmim][TFSI]·C7H8 having only one C-H⋯π bond and

178

zero hydrogen bond between toluene and IL is 5.25 kJ·mol-1 lower than that of [Bmim][BF4]·C7H8. [Bmim][BF4]·C7H8

179

has the same number of hydrogen and C-H⋯π bonds between toluene and ILs as [P4443][BF4]·C7H8, but its Eabs is also

180

5.25 kJ·mol-1 lower than that of [P4443][BF4]·C7H8. That means the existence of other interaction between IL and toluene

181

except hydrogen and C-H⋯π bond. It is essential to make the charge analysis to further study the absorption of toluene

182

by ion pairs.

183

3.2 Charge analysis

184

The Mulliken charge (in e) of [Bmim]+, [Bmim][BF4], [Bmim][BF4]·C7H8, [Bmim][TFSI], [Bmim][TFSI]·C7H8,

185

[P4443]+, [P4443][BF4] and [P4443][BF4]·C7H8 is shown in Table S1 (placed at supporting information) and the atomic label

186

is the same as that in Figure 3. The anion has a great effect on the charge distribution of the cation in ion pairs as can be

187

seen from Table S1. There are charge transfer between cations and anions in ion pairs, which can successfully interpret

188

that hydrogen bonds are formed as discussed before. The positive changes of several H atoms in the cations increase

189

remarkably and these H atoms form hydrogen bonds with the negative charge of F, N or O atoms in the anion. There are

190

some examples to illustrate the interaction. In Table S1, compared with the isolated [Bmim]+ action, the positive charge 10

191

values of H13, H16 and H23 in [Bmim][BF4] increase from 0.132 e, 0.097 e and 0.125 e to 0.221 e, 0.165 e and 0.193 e,

192

respectively. H38 and H44 also have the higher positive charge by absorbed toluene in [Bmim][BF4]·C7H8. These H

193

atoms can be found in the hydrogen bonds.

194

Similarly, the positive changes of H13, H15, H16 and H24 in [Bmim][TFSI] increase to 0.212 e, 0.168 e, 0.147 e and

195

0.207 e, respectively. Meanwhile, the negative charges of F36, N28, O34 and O31 atoms have the highest

196

electronegativity in [Bmim][TFSI] (-0.300 e, -0.396 e, -0.407 e and -0.455 e, respectively). The negative charges of N3

197

and O9 furtherly increase to -0.438 e and -0.428 e in [Bmim][TFSI]·C7H8, and that corresponds to the hydrogen bonds

198

mentioned above. However, the positive charge value of H31 in [Bmim][TFSI]·C7H8 decreases from 0.147 e in the ion

199

pair of [Bmim][TFSI] to 0.111 e, which causes the hydrogen bond cannot be formed. For [P4443][BF4], the charge transfer

200

still exists because of formation of the hydrogen bonds, notably embodied in H17, H24 and H33 atoms, which have

201

almost 55 percent increase of the charge compared with the isolated cation [P4443]+.

202

The sum of the Mulliken charge of each part of the ionic pairs [Bmim][BF4], [Bmim][TFSI], [P4443][BF4],

203

[Bmim][BF4]·C7H8, [Bmim][TFSI]·C7H8 and [P4443][BF4]·C7H8 is shown in Table 3. The charge transfers between the

204

cation and anion in the ionic pairs cause positive charge of the cation or negative charge of the anion decreases a little,

205

comparing with that of the isolated cation or anion. Likewise, the charge transfers between toluene and ILs make the sum

206

of the toluene charge change. As shown in Table 3, the total charge of toluene in [P4443][BF4]·C7H8 is -0.014 e, which is

207

the highest. The next is 0.008 e in [Bmim][TFSI]·C7H8 and the minimum is -0.005 e in [Bmim][BF4]·C7H8. The value

208

generally reflects the strength of the electrostatic force between toluene and ILs, which obviously influences their

209

interaction, especially Eabs discussed before. For example, [Bmim][BF4]·C7H8 has the same number of hydrogen and

210

C-H⋯π bonds between toluene and ILs as [P4443][BF4]·C7H8, but the charge of toluene is 0.009 e lower than that in

211

[P4443][BF4]·C7H8, which leads to a lower Eabs of [Bmim][BF4]·C7H8.

212 213

Table 3 The sum of the Mulliken charge (in e) of each parts in [Bmim][BF4], [Bmim][TFSI], [P4443][BF4] and [Bmim][BF4]·C7H8, [Bmim][TFSI]·C7H8, [P4443][BF4]·C7H8, calculated at the GGA/PW91. 11

[Bmim]+ [BF4]

-

[Bmim][BF4]

[Bmim][BF4]·C7H8

[Bmim][TFSI]

[Bmim][TFSI]·C7H8

[P4443][BF4]

[P4443][BF4]·C7H8

0.887

0.883

0.913

0.884

-

-

-0.889

-0.878

-

-

-0.898

-0.888

[TFSI]-

-

-

-0.911

-0.891

-

-

+

-

-

-

-

0.895

0.900

toluene

-

-0.005

-

0.008

-

-0.014

[P4443]

214

In contrast with the isolated toluene and ILs, the charge of each part in [Bmim][BF4]·C7H8 transfers a little from [BF4]-

215

to [Bmim]+ and toluene. For [Bmim][TFSI]·C7H8, the charge transfers from [TFSI]- and toluene to [Bmim]+ and the

216

charge transfers from [P4443]+ and [BF4]- to toluene for [P4443][BF4]·C7H8. In terms of contribution to the total charge of

217

toluene, the anion [BF4]- of imidazolium ILs plays the more important role in absorption of toluene. As for

218

[Bmim][TFSI]·C7H8, total charge of toluene is only caused by [Bmim]+ cation, and toluene carries positive charge that

219

results in much weak interaction between toluene and [Bmim]+. It is also noted that [Bmim]+ in [Bmim][BF4]·C7H8 has

220

no contribution which is opposite to [P4443]+ in [P4443][BF4]·C7H8, which means [P4443]+ has more advantage in

221

absorption of toluene than [Bmim]+.

222

3.3 Molecular orbital properties

223

To better expound the mechanism of the absorption of toluene by the ion pairs, the frontier molecular orbital (FMO)

224

analysis is also considered. Before the process of toluene absorption, the highest occupied molecular orbitals (HOMO) of

225

the ion pairs are mainly concentrated on the anions ([BF4]-, [TFSI]-) and the lowest unoccupied molecular orbitals

226

(LUMO) are mainly composed by the atoms in the cations ([Bmim]+, [P4443]+) as shown in Fig.4. There is no overlap

227

between the cations and anions in the frontier molecular orbital of the ILs, and their interaction is mainly composed by

228

the hydrogen bonds which is consistent with the geometry analysis.

229

In the process of absorption, some of the FMOs have changed. The distributions of HOMOs of [Bmim][BF4]·C7H8

230

and [P4443][BF4]·C7H8 are concentrated in the toluene molecule, which indicates that [BF4]- has the great effect on the

231

absorption of toluene. The result is consistent with the analysis of charge transfer. However, the HOMO-1 of

232

[Bmim][TFSI]·C7H8 is mainly concentrated on the [TFSI]-, while the HOMO is transferred to the toluene molecule. The 12

233

geometry analysis indicates that the [TFSI]- anion cannot form the interaction with toluene, and the direction of charge

234

transfer is from the toluene to [Bmim]+, which means the transfer of the HOMO of [Bmim][TFSI]·C7H8 is unrelated with

235

the [TFSI]- and interrelated with the internal charge transfer of toluene caused by the C-H⋯π bond. Therefore, the

236

absorption capability of the [BF4]- for toluene is much larger compared with the [TFSI]-. The LUMOs of

237

[Bmim][BF4]·C7H8 and [Bmim][TFSI]·C7H8 are almost the same as [Bmim][BF4] and [Bmim][TFSI], mainly composed

238

by the atoms in the imidazolium ring of the cation [Bmim]+.

239 240 241

Figure 4 The important frontier molecular orbitals of [Bmim][BF4], [Bmim][BF4]·C7H8, [Bmim][TFSI], [Bmim][TFSI]·C7H8, [P4443][BF4], and [P4443][BF4]·C7H8

242

But in the process of toluene absorption in [P4443][BF4], the LUMOs transfer from the [P4443]+ in [P4443][BF4] to

243

toluene in [P4443][BF4]·C7H8, which illustrates that the [P4443]+ has more contribution to the absorption of toluene than the

244

[Bmim]+. In consequence, absorption capacity of toluene is in the descending order of [P4443][BF4], [Bmim][BF4] and

245

[Bmim][TFSI] and that corresponds the analysis of geometry properties and charge.

246

4. Conclusion

247

The ion pairs of [Bmim][BF4], [Bmim][TFSI] and [P4443][BF4] are stable with some hydrogen bonds between the 13

248

anion and cation. The absorption of toluene by ILs is a physical process, and the C-H⋯π bonds are also formed between

249

the cation and toluene. Due to the difference of the stable geometry properties of ILs in absorption, the hydrogen bonds

250

and C-H⋯π bonds are different. The amount of hydrogen bonds between toluene and ion pairs is zero in

251

[Bmim][TFSI]·C7H8, but two in [Bmim][BF4]·C7H8. The amount of the C-H⋯π bonds is just one in [Bmim][TFSI]·C7H8,

252

but three in [Bmim][BF4]·C7H8. That means there is a stronger interaction between IL and toluene in [Bmim][BF4]·C7H8

253

than that in [Bmim][TFSI]·C7H8. [Bmim][BF4]·C7H8 and [P4443][BF4]·C7H8 have the same amount of hydrogen bonds

254

and C-H⋯π bonds, but charge transfer between toluene and [P4443][BF4] is higher than that between toluene and

255

[Bmim][BF4], which means [P4443][BF4] has the higher electrostatic force with toluene. The Eabs of [Bmim][TFSI]·C7H8,

256

[Bmim][BF4]·C7H8 and [P4443][BF4]·C7H8 is -28.88 kJ·mol-1, -34.13 kJ·mol-1 and -39.38 kJ·mol-1 respectively, which

257

further verifies the above conclusion. In the frontier molecular orbital, the HOMO and LUMO of the [P4443][BF4]·C7H8

258

are all composed by the atoms of toluene, while the LUMO of the [Bmim][BF4]·C7H8 is concentrated on the

259

imidazolium ring of cation, which also means [P4443][BF4] behaves better in toluene absorption.

260

Acknowledgements

261

The authors gratefully acknowledge the financial supports provided by the Fundamental Research Funds for the

262

Central Universities (NO. 2019XKQYMS14). Calculations were performed on the ScGrid/CNGrid of Supercomputing

263

Environment of Chinese Academy of Sciences.

264

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Highlights  [P4443]+ charge transfer to toluene is larger than [Bmim]+.  HOMO and LUMO of [P4443][BF4]·C7H8 are all concentrated in toluene.  Hydrogen bond and C-H⋯π bond are all present in ILs absorption of toluene.

Conflict of interest statement

Manuscript entitled: The effect of imidazolium and phosphonium ionic liquids on toluene absorption studied by a molecular simulation

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. We have no financial and personal relationships with other people or organizations that can inappropriately influence our work.