- Email: [email protected]
The effect of imidazolium and phosphonium ionic liquids on toluene absorption studied by a molecular simulation
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
22
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
24
separation.
25
Keywords: molecular simulation; density functional theory; volatile organic compounds; ionic liquids; toluene
26
1. Introduction
27
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
62
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.
86
2. Calculation method
87
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
97
3.1 The geometry properties
98
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
References
265
[1] ISO 16000-6: 2011 Indoor air — Part 6: Determination of volatile organic compounds in indoor and test chamber air by
266
active sampling on Tenax TA sorbent, thermal desorption and gas chromatography using MS or MS-FID.
267
https://www.iso.org/obp/ui/#iso:std:iso:16000:-6:ed-2:v1:en
268
[2] M. Schweigkofler, R. Niessner, Determination of Siloxanes and VOC in Landfill Gas and Sewage Gas by Canister
269
Sampling
and
GC-MS/AES
Analysis,
Environmental
14
Science
&
Technology
33(20)
(1999)
3680-3685.
270
https://doi.org/10.1021/es9902569
271
[3] D.M. Alzate-Sánchez, B.J. Smith, A. Alsbaiee, J.P. Hinestroza, W.R. Dichtel, Cotton Fabric Functionalized with a
272
β-Cyclodextrin Polymer Captures Organic Pollutants from Contaminated Air and Water, Chemistry of Materials 28(22) (2016)
273
8340-8346. https://doi.org/10.1021/acs.chemmater.6b03624
274
[4] H.J. Kim, H.R. Pant, N.J. Choi, C.S. Kim, Composite electrospun fly ash/polyurethane fibers for absorption of volatile
275
organic compounds from air, Chemical Engineering Journal 230 (2013) 244-250. https://doi.org/10.1016/j.cej.2013.06.090
276
[5] G. Zhang, Y. Liu, S. Zheng, Z. Hashisho, Adsorption of volatile organic compounds onto natural porous minerals, Journal
277
of Hazardous Materials 364 (2019) 317-324. https://doi.org/10.1016/j.jhazmat.2018.10.031
278
[6] Y. Guo, D.-P. Yang, M. Liu, X. Zhang, Y. Chen, J. Huang, Q. Li, R. Luque, Enhanced catalytic benzene oxidation over a
279
novel
280
https://doi.org/10.1039/C8TA10822F
281
[7] Y. Cheng, H. He, C. Yang, G. Zeng, X. Li, H. Chen, G. Yu, Challenges and solutions for biofiltration of hydrophobic
282
volatile
283
https://doi.org/10.1016/j.biotechadv.2016.06.007
284
[8] F. Feng, Y. Zheng, X. Shen, Q. Zheng, S. Dai, X. Zhang, Y. Huang, Z. Liu, K. Yan, Characteristics of Back Corona
285
Discharge in a Honeycomb Catalyst and Its Application for Treatment of Volatile Organic Compounds, Environmental Science
286
& Technology 49(11) (2015) 6831-6837. https://doi.org/10.1021/acs.est.5b00447
287
[9] Z. Shayegan, C.-S. Lee, F. Haghighat, TiO2 photocatalyst for removal of volatile organic compounds in gas phase – A
288
review, Chemical Engineering Journal 334 (2018) 2408-2439. https://doi.org/10.1016/j.cej.2017.09.153
289
[10] B. Wang, X. Xu, W. Xu, N. Wang, H. Xiao, Y. Sun, H. Huang, L. Yu, M. Fu, J. Wu, L. Chen, D. Ye, The Mechanism of
290
Non-thermal Plasma Catalysis on Volatile Organic Compounds Removal, Catalysis Surveys from Asia 22(2) (2018) 73-94.
291
https://doi.org/10.1007/s10563-018-9241-x
waste-derived
organic
Ag/eggshell
catalyst,
compounds,
Journal
of
Biotechnology
15
Materials
Chemistry
Advances
A
34(6)
7(15)
(2019)
(2016)
8832-8844.
1091-1102.
292
[11] A.A. Assadi, S. Loganathan, P.N. Tri, S. Gharib-Abou Ghaida, A. Bouzaza, A.N. Tuan, D. Wolbert, Pilot scale degradation
293
of mono and multi volatile organic compounds by surface discharge plasma/TiO2 reactor: Investigation of competition and
294
synergism, Journal of Hazardous Materials 357 (2018) 305-313. https://doi.org/10.1016/j.jhazmat.2018.06.007
295
[12] E. Scholten, L. Bromberg, G.C. Rutledge, T.A. Hatton, Electrospun Polyurethane Fibers for Absorption of Volatile
296
Organic
297
https://doi.org/10.1021/am200748y
298
[13] P.-F. Biard, A. Couvert, S. Giraudet, Volatile organic compounds absorption in packed column: theoretical assessment of
299
water, DEHA and PDMS 50 as absorbents, Journal of Industrial and Engineering Chemistry 59 (2018) 70-78.
300
https://doi.org/10.1016/j.jiec.2017.10.008
301
[14] M.D. Vuong, A. Couvert, C. Couriol, A. Amrane, P. Le Cloirec, C. Renner, Determination of the Henry's constant and the
302
mass
303
https://doi.org/10.1016/j.cej.2009.01.027
304
[15] E. Dumont, G. Darracq, A. Couvert, C. Couriol, A. Amrane, D. Thomas, Y. Andrès, P. Le Cloirec, Determination of
305
partition coefficients of three volatile organic compounds (dimethylsulphide, dimethyldisulphide and toluene) in water/silicone
306
oil mixtures, Chemical Engineering Journal 162(3) (2010) 927-934. https://doi.org/10.1016/j.cej.2010.06.045
307
[16] W. Liu, L. Cheng, Y. Zhang, H. Wang, M. Yu, The physical properties of aqueous solution of room-temperature ionic
308
liquids based on imidazolium: Database and evaluation, Journal of Molecular Liquids 140(1) (2008) 68-72.
309
https://doi.org/10.1016/j.molliq.2008.01.008
310
[17]
311
https://pubs.rsc.org/en/content/ebook/9781847551610
312
[18] G. Quijano, A. Couvert, A. Amrane, G. Darracq, C. Couriol, P. Le Cloirec, L. Paquin, D. Carrié, Potential of ionic liquids
313
for VOC absorption and biodegradation in multiphase systems, Chemical Engineering Science 66(12) (2011) 2707-2712.
Compounds
transfer
M.
rate
Freemantle,
from
of
Air,
VOCs
An
ACS
in
Applied
solvents,
Introduction
to
Materials
Chemical
Ionic
Interfaces
Engineering
Liquids,
16
&
The
Journal
Royal
3(10)
(2011)
150(2)
Society
of
(2009)
3902-3909.
426-430.
Chemistry
2009.
314
https://doi.org/10.1016/j.ces.2011.01.047
315
[19]
316
1,3-dihexyloxymethyl-imidazolium based ionic liquids with alcohols, water, ketones and hydrocarbons: The effect of cation
317
and anion on solubility, Fluid Phase Equilibria 260(1) (2007) 9-18. https://doi.org/10.1016/j.fluid.2006.07.005
318
[20] J. García, J.S. Torrecilla, A. Fernández, M. Oliet, F. Rodríguez, (Liquid+liquid) equilibria in the binary systems (aliphatic,
319
or aromatic hydrocarbons+1-ethyl-3-methylimidazolium ethylsulfate, or 1-butyl-3-methylimidazolium methylsulfate ionic
320
liquids), The Journal of Chemical Thermodynamics 42(1) (2010) 144-150. https://doi.org/10.1016/j.jct.2009.07.023
321
[21] W. Wang, X. Ma, S. Grimes, H. Cai, M. Zhang, Study on the absorbability, regeneration characteristics and thermal
322
stability
323
https://doi.org/10.1016/j.cej.2017.06.178
324
[22] I. Cichowska-Kopczyńska, M. Joskowska, B. Debski, R. Aranowski, J. Hupka, Separation of toluene from gas phase
325
using
326
https://doi.org/10.1016/j.memsci.2018.08.058
327
[23] T. Gao, J.M. Andino, J.R. Alvarez-Idaboy, Computational and experimental study of the interactions between ionic liquids
328
and
329
https://doi.org/10.1039/C003386C
330
[24] V. Chaban, Polarizability versus mobility: atomistic force field for ionic liquids, Physical Chemistry Chemical Physics
331
13(35) (2011) 16055-16062. https://doi.org/10.1039/C1CP21379B
332
[25] J.J. Chen, W.W. Li, X.L. Li, H.Q. Yu, Carbon dioxide capture by aminoalkyl imidazolium-based ionic liquid: a
333
computational investigation, Phys Chem Chem Phys 14(13) (2012) 4589-96. http://dx.doi.org/10.1039/c2cp23642g
334
[26] F. Zhang, H. Feng, W. Sun, W. Zhang, J. Liu, Z. Ren, Selective Separation of Toluene/n-Heptane by Supported Ionic
335
Liquid
U.
Domańska,
of
ionic
supported
volatile
A.
Marciniak,
liquids
for
imidazolium
organic
Membranes
VOCs
ionic
compounds,
with
liquid
Phase
removal,
Journal
Chemistry
Chemical
of
Chemical
membrane,
Physical
[Bmim][BF4],
behaviour
1-hexyloxymethyl-3-methyl-imidazolium
Engineering
of
17
Membrane
Chemical
Engineering
Journal
&
Physics
328
Science
566
12(33)
Technology
(2017)
38(2)
(2018)
(2010)
(2015)
and
353-359.
367-373.
9830-9838.
355-361.
336
https://doi.org/10.1002/ceat.201400160
337
[27] G. García, M. Atilhan, S. Aparicio, A density functional theory insight towards the rational design of ionic liquids for SO2
338
capture, Physical Chemistry Chemical Physics 17(20) (2015) 13559-13574. http://dx.doi.org/10.1039/C5CP00076A
339
[28] V. Chaban, Hydrogen fluoride capture by imidazolium acetate ionic liquid, Chemical Physics Letters 625 (2015) 110-115.
340
https://doi.org/10.1016/j.cplett.2015.02.041
341
[29] V. Chaban, Acetone as a polar cosolvent for pyridinium-based ionic liquids, Rsc Advances 6(11) (2016) 8906-8912.
342
https://doi.org/10.1039/c5ra19511j
343
[30] C.J. Bradaric, A. Downard, C. Kennedy, A.J. Robertson, Y. Zhou, Industrial Preparation of Phosphonium Ionic Liquids,
344
Ionic Liquids as Green Solvents, American Chemical Society2003, pp. 41-56. https://doi.org/10.1021/bk-2003-0856.ch004
345
[31] M. Chen, B.T. White, C.R. Kasprzak, T.E. Long, Advances in phosphonium-based ionic liquids and poly(ionic liquid)s as
346
conductive materials, European Polymer Journal 108 (2018) 28-37. https://doi.org/10.1016/j.eurpolymj.2018.08.015
347
[32] R.P. Morco, A.Y. Musa, J.C. Wren, The molecular structures and the relationships between the calculated molecular and
348
observed bulk phase properties of phosphonium-based ionic liquids, Solid State Ionics 258 (2014) 74-81.
349
https://doi.org/10.1016/j.ssi.2014.02.004
350
[33] C. Yan-Hong, C. Yan-Fei, D. Dong-Shun, A. Ning, Z. Yong, Difference for the absorption of SO2 and CO2 on
351
[Pnnnm][Tetz] (n=1, m=2, and 4) ionic liquids: A density functional theory investigation, Journal of Molecular Liquids 199
352
(2014) 7-14. https://doi.org/10.1016/j.molliq.2014.06.023
353
[34] M. Nishio, CH/π-Hydrogen Bonds in Crystals, ChemInform 35(41) (2004). https://doi.org/10.1002/chin.200441276
354
[35] K. Dong, S. Zhang, J. Wang, Understanding the hydrogen bonds in ionic liquids and their roles in properties and reactions,
355
Chemical Communications 52(41) (2016) 6744-6764. https://doi.org/10.1039/C5CC10120D
356
[36] B. Delley, An all‐electron numerical method for solving the local density functional for polyatomic molecules, The
357
Journal of Chemical Physics 92(1) (1990) 508-517. https://doi.org/10.1063/1.458452
18
358
[37] B. Delley, From molecules to solids with the DMol3 approach, The Journal of Chemical Physics 113(18) (2000)
359
7756-7764. https://doi.org/10.1063/1.1316015
360
[38] K. Dong, S. Zhang, D. Wang, X. Yao, Hydrogen Bonds in Imidazolium Ionic Liquids, The Journal of Physical Chemistry
361
A 110(31) (2006) 9775-9782. https://doi.org/10.1021/jp054054c
362
[39] H.-C. Weiss, D. Bläser, R. Boese, B. M. Doughan, M. M. Haley, C–H···π interactions in ethynylbenzenes: the crystal
363
structures of ethynylbenzene and 1,3,5-triethynylbenzene, and a redetermination of the structure of 1,4-diethynylbenzene,
364
Chemical Communications (18) (1997) 1703-1704. https://doi.org/10.1039/A704070I
365
[40] H. Suezawa, T. Yoshida, Y. Umezawa, S. Tsuboyama, M. Nishio, CH/π Interactions Implicated in the Crystal Structure of
366
Transition Metal Compounds − A Database Study, European Journal of Inorganic Chemistry 2002 (2002) 3148-3155.
367
https://doi.org/10.1002/1099-0682(200212)2002:12<3148::AID-EJIC3148>3.0.CO;2-X
368
[41] R. Lü, J. Lin, Z. Qu, Theoretical study on interactions between thiophene/dibenzothiophene/cyclohexane/toluene and
369
1-methyl-3-octylimidazolium
370
https://doi.org/10.1007/s11224-012-0106-z
tetrafluoroborate,
Structural
371
19
Chemistry
24(2)
(2013)
507-515.
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.
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
22
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
24
separation.
25
Keywords: molecular simulation; density functional theory; volatile organic compounds; ionic liquids; toluene
26
1. Introduction
27
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
62
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.
86
2. Calculation method
87
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
97
3.1 The geometry properties
98
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
References
265
[1] ISO 16000-6: 2011 Indoor air — Part 6: Determination of volatile organic compounds in indoor and test chamber air by
266
active sampling on Tenax TA sorbent, thermal desorption and gas chromatography using MS or MS-FID.
267
https://www.iso.org/obp/ui/#iso:std:iso:16000:-6:ed-2:v1:en
268
[2] M. Schweigkofler, R. Niessner, Determination of Siloxanes and VOC in Landfill Gas and Sewage Gas by Canister
269
Sampling
and
GC-MS/AES
Analysis,
Environmental
14
Science
&
Technology
33(20)
(1999)
3680-3685.
270
https://doi.org/10.1021/es9902569
271
[3] D.M. Alzate-Sánchez, B.J. Smith, A. Alsbaiee, J.P. Hinestroza, W.R. Dichtel, Cotton Fabric Functionalized with a
272
β-Cyclodextrin Polymer Captures Organic Pollutants from Contaminated Air and Water, Chemistry of Materials 28(22) (2016)
273
8340-8346. https://doi.org/10.1021/acs.chemmater.6b03624
274
[4] H.J. Kim, H.R. Pant, N.J. Choi, C.S. Kim, Composite electrospun fly ash/polyurethane fibers for absorption of volatile
275
organic compounds from air, Chemical Engineering Journal 230 (2013) 244-250. https://doi.org/10.1016/j.cej.2013.06.090
276
[5] G. Zhang, Y. Liu, S. Zheng, Z. Hashisho, Adsorption of volatile organic compounds onto natural porous minerals, Journal
277
of Hazardous Materials 364 (2019) 317-324. https://doi.org/10.1016/j.jhazmat.2018.10.031
278
[6] Y. Guo, D.-P. Yang, M. Liu, X. Zhang, Y. Chen, J. Huang, Q. Li, R. Luque, Enhanced catalytic benzene oxidation over a
279
novel
280
https://doi.org/10.1039/C8TA10822F
281
[7] Y. Cheng, H. He, C. Yang, G. Zeng, X. Li, H. Chen, G. Yu, Challenges and solutions for biofiltration of hydrophobic
282
volatile
283
https://doi.org/10.1016/j.biotechadv.2016.06.007
284
[8] F. Feng, Y. Zheng, X. Shen, Q. Zheng, S. Dai, X. Zhang, Y. Huang, Z. Liu, K. Yan, Characteristics of Back Corona
285
Discharge in a Honeycomb Catalyst and Its Application for Treatment of Volatile Organic Compounds, Environmental Science
286
& Technology 49(11) (2015) 6831-6837. https://doi.org/10.1021/acs.est.5b00447
287
[9] Z. Shayegan, C.-S. Lee, F. Haghighat, TiO2 photocatalyst for removal of volatile organic compounds in gas phase – A
288
review, Chemical Engineering Journal 334 (2018) 2408-2439. https://doi.org/10.1016/j.cej.2017.09.153
289
[10] B. Wang, X. Xu, W. Xu, N. Wang, H. Xiao, Y. Sun, H. Huang, L. Yu, M. Fu, J. Wu, L. Chen, D. Ye, The Mechanism of
290
Non-thermal Plasma Catalysis on Volatile Organic Compounds Removal, Catalysis Surveys from Asia 22(2) (2018) 73-94.
291
https://doi.org/10.1007/s10563-018-9241-x
waste-derived
organic
Ag/eggshell
catalyst,
compounds,
Journal
of
Biotechnology
15
Materials
Chemistry
Advances
A
34(6)
7(15)
(2019)
(2016)
8832-8844.
1091-1102.
292
[11] A.A. Assadi, S. Loganathan, P.N. Tri, S. Gharib-Abou Ghaida, A. Bouzaza, A.N. Tuan, D. Wolbert, Pilot scale degradation
293
of mono and multi volatile organic compounds by surface discharge plasma/TiO2 reactor: Investigation of competition and
294
synergism, Journal of Hazardous Materials 357 (2018) 305-313. https://doi.org/10.1016/j.jhazmat.2018.06.007
295
[12] E. Scholten, L. Bromberg, G.C. Rutledge, T.A. Hatton, Electrospun Polyurethane Fibers for Absorption of Volatile
296
Organic
297
https://doi.org/10.1021/am200748y
298
[13] P.-F. Biard, A. Couvert, S. Giraudet, Volatile organic compounds absorption in packed column: theoretical assessment of
299
water, DEHA and PDMS 50 as absorbents, Journal of Industrial and Engineering Chemistry 59 (2018) 70-78.
300
https://doi.org/10.1016/j.jiec.2017.10.008
301
[14] M.D. Vuong, A. Couvert, C. Couriol, A. Amrane, P. Le Cloirec, C. Renner, Determination of the Henry's constant and the
302
mass
303
https://doi.org/10.1016/j.cej.2009.01.027
304
[15] E. Dumont, G. Darracq, A. Couvert, C. Couriol, A. Amrane, D. Thomas, Y. Andrès, P. Le Cloirec, Determination of
305
partition coefficients of three volatile organic compounds (dimethylsulphide, dimethyldisulphide and toluene) in water/silicone
306
oil mixtures, Chemical Engineering Journal 162(3) (2010) 927-934. https://doi.org/10.1016/j.cej.2010.06.045
307
[16] W. Liu, L. Cheng, Y. Zhang, H. Wang, M. Yu, The physical properties of aqueous solution of room-temperature ionic
308
liquids based on imidazolium: Database and evaluation, Journal of Molecular Liquids 140(1) (2008) 68-72.
309
https://doi.org/10.1016/j.molliq.2008.01.008
310
[17]
311
https://pubs.rsc.org/en/content/ebook/9781847551610
312
[18] G. Quijano, A. Couvert, A. Amrane, G. Darracq, C. Couriol, P. Le Cloirec, L. Paquin, D. Carrié, Potential of ionic liquids
313
for VOC absorption and biodegradation in multiphase systems, Chemical Engineering Science 66(12) (2011) 2707-2712.
Compounds
transfer
M.
rate
Freemantle,
from
of
Air,
VOCs
An
ACS
in
Applied
solvents,
Introduction
to
Materials
Chemical
Ionic
Interfaces
Engineering
Liquids,
16
&
The
Journal
Royal
3(10)
(2011)
150(2)
Society
of
(2009)
3902-3909.
426-430.
Chemistry
2009.
314
https://doi.org/10.1016/j.ces.2011.01.047
315
[19]
316
1,3-dihexyloxymethyl-imidazolium based ionic liquids with alcohols, water, ketones and hydrocarbons: The effect of cation
317
and anion on solubility, Fluid Phase Equilibria 260(1) (2007) 9-18. https://doi.org/10.1016/j.fluid.2006.07.005
318
[20] J. García, J.S. Torrecilla, A. Fernández, M. Oliet, F. Rodríguez, (Liquid+liquid) equilibria in the binary systems (aliphatic,
319
or aromatic hydrocarbons+1-ethyl-3-methylimidazolium ethylsulfate, or 1-butyl-3-methylimidazolium methylsulfate ionic
320
liquids), The Journal of Chemical Thermodynamics 42(1) (2010) 144-150. https://doi.org/10.1016/j.jct.2009.07.023
321
[21] W. Wang, X. Ma, S. Grimes, H. Cai, M. Zhang, Study on the absorbability, regeneration characteristics and thermal
322
stability
323
https://doi.org/10.1016/j.cej.2017.06.178
324
[22] I. Cichowska-Kopczyńska, M. Joskowska, B. Debski, R. Aranowski, J. Hupka, Separation of toluene from gas phase
325
using
326
https://doi.org/10.1016/j.memsci.2018.08.058
327
[23] T. Gao, J.M. Andino, J.R. Alvarez-Idaboy, Computational and experimental study of the interactions between ionic liquids
328
and
329
https://doi.org/10.1039/C003386C
330
[24] V. Chaban, Polarizability versus mobility: atomistic force field for ionic liquids, Physical Chemistry Chemical Physics
331
13(35) (2011) 16055-16062. https://doi.org/10.1039/C1CP21379B
332
[25] J.J. Chen, W.W. Li, X.L. Li, H.Q. Yu, Carbon dioxide capture by aminoalkyl imidazolium-based ionic liquid: a
333
computational investigation, Phys Chem Chem Phys 14(13) (2012) 4589-96. http://dx.doi.org/10.1039/c2cp23642g
334
[26] F. Zhang, H. Feng, W. Sun, W. Zhang, J. Liu, Z. Ren, Selective Separation of Toluene/n-Heptane by Supported Ionic
335
Liquid
U.
Domańska,
of
ionic
supported
volatile
A.
Marciniak,
liquids
for
imidazolium
organic
Membranes
VOCs
ionic
compounds,
with
liquid
Phase
removal,
Journal
Chemistry
Chemical
of
Chemical
membrane,
Physical
[Bmim][BF4],
behaviour
1-hexyloxymethyl-3-methyl-imidazolium
Engineering
of
17
Membrane
Chemical
Engineering
Journal
&
Physics
328
Science
566
12(33)
Technology
(2017)
38(2)
(2018)
(2010)
(2015)
and
353-359.
367-373.
9830-9838.
355-361.
336
https://doi.org/10.1002/ceat.201400160
337
[27] G. García, M. Atilhan, S. Aparicio, A density functional theory insight towards the rational design of ionic liquids for SO2
338
capture, Physical Chemistry Chemical Physics 17(20) (2015) 13559-13574. http://dx.doi.org/10.1039/C5CP00076A
339
[28] V. Chaban, Hydrogen fluoride capture by imidazolium acetate ionic liquid, Chemical Physics Letters 625 (2015) 110-115.
340
https://doi.org/10.1016/j.cplett.2015.02.041
341
[29] V. Chaban, Acetone as a polar cosolvent for pyridinium-based ionic liquids, Rsc Advances 6(11) (2016) 8906-8912.
342
https://doi.org/10.1039/c5ra19511j
343
[30] C.J. Bradaric, A. Downard, C. Kennedy, A.J. Robertson, Y. Zhou, Industrial Preparation of Phosphonium Ionic Liquids,
344
Ionic Liquids as Green Solvents, American Chemical Society2003, pp. 41-56. https://doi.org/10.1021/bk-2003-0856.ch004
345
[31] M. Chen, B.T. White, C.R. Kasprzak, T.E. Long, Advances in phosphonium-based ionic liquids and poly(ionic liquid)s as
346
conductive materials, European Polymer Journal 108 (2018) 28-37. https://doi.org/10.1016/j.eurpolymj.2018.08.015
347
[32] R.P. Morco, A.Y. Musa, J.C. Wren, The molecular structures and the relationships between the calculated molecular and
348
observed bulk phase properties of phosphonium-based ionic liquids, Solid State Ionics 258 (2014) 74-81.
349
https://doi.org/10.1016/j.ssi.2014.02.004
350
[33] C. Yan-Hong, C. Yan-Fei, D. Dong-Shun, A. Ning, Z. Yong, Difference for the absorption of SO2 and CO2 on
351
[Pnnnm][Tetz] (n=1, m=2, and 4) ionic liquids: A density functional theory investigation, Journal of Molecular Liquids 199
352
(2014) 7-14. https://doi.org/10.1016/j.molliq.2014.06.023
353
[34] M. Nishio, CH/π-Hydrogen Bonds in Crystals, ChemInform 35(41) (2004). https://doi.org/10.1002/chin.200441276
354
[35] K. Dong, S. Zhang, J. Wang, Understanding the hydrogen bonds in ionic liquids and their roles in properties and reactions,
355
Chemical Communications 52(41) (2016) 6744-6764. https://doi.org/10.1039/C5CC10120D
356
[36] B. Delley, An all‐electron numerical method for solving the local density functional for polyatomic molecules, The
357
Journal of Chemical Physics 92(1) (1990) 508-517. https://doi.org/10.1063/1.458452
18
358
[37] B. Delley, From molecules to solids with the DMol3 approach, The Journal of Chemical Physics 113(18) (2000)
359
7756-7764. https://doi.org/10.1063/1.1316015
360
[38] K. Dong, S. Zhang, D. Wang, X. Yao, Hydrogen Bonds in Imidazolium Ionic Liquids, The Journal of Physical Chemistry
361
A 110(31) (2006) 9775-9782. https://doi.org/10.1021/jp054054c
362
[39] H.-C. Weiss, D. Bläser, R. Boese, B. M. Doughan, M. M. Haley, C–H···π interactions in ethynylbenzenes: the crystal
363
structures of ethynylbenzene and 1,3,5-triethynylbenzene, and a redetermination of the structure of 1,4-diethynylbenzene,
364
Chemical Communications (18) (1997) 1703-1704. https://doi.org/10.1039/A704070I
365
[40] H. Suezawa, T. Yoshida, Y. Umezawa, S. Tsuboyama, M. Nishio, CH/π Interactions Implicated in the Crystal Structure of
366
Transition Metal Compounds − A Database Study, European Journal of Inorganic Chemistry 2002 (2002) 3148-3155.
367
https://doi.org/10.1002/1099-0682(200212)2002:12<3148::AID-EJIC3148>3.0.CO;2-X
368
[41] R. Lü, J. Lin, Z. Qu, Theoretical study on interactions between thiophene/dibenzothiophene/cyclohexane/toluene and
369
1-methyl-3-octylimidazolium
370
https://doi.org/10.1007/s11224-012-0106-z
tetrafluoroborate,
Structural
371
19
Chemistry
24(2)
(2013)
507-515.
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.