Tuning anion-functionalized ionic liquids for improved SO2 capture

Article abstract of DOI:10.1002/anie.201305234

You can have your cake and eat it too: A "dual-tuning" strategy for improving the capture of SO₂ was developed by introducing electron-withdrawing sites on the anions to produce several kinds of functionalized ionic liquids. Those functionalize

Full text of DOI:10.1002/anie.201305234

.
Angewandte
Communications
DOI: 10.1002/anie.201305234
Gas Separation
Tuning Anion-Functionalized Ionic Liquids for Improved SO2
Capture**
Guokai Cui, Junjie Zheng, Xiaoyan Luo, Wenjun Lin, Fang Ding, Haoran Li, and
Congmin Wang*
The increase in the concentration of gases such as SO2 and
CO2 in the atmosphere, which comes from the burning of
fossil fuels, threatens environment and human health.
Accordingly, the development of new materials and processes
for the efficient, reversible, and economical capture of these
gases is highly desired and of critical importance. The unique
properties of ionic liquids (ILs),[1] including extremely low
vapor pressures, wide liquid ranges, high stabilities, and
tunable properties, offer an opportunity to address this
challenge. Herein, we report a “dual-tuning” approach for
improving SO2 capture by several anion-functionalized ILs.
We show that both enhanced capacity and reduced enthalpy
can be achieved by introducing an electron-withdrawing
interaction site, such as halogen group,
whether both increasing the capacity for efficient absorption
and reducing the enthalpy for easy desorption by the tuning of
specific structures in an anion-functionalized IL can be
achieved.
Herein, we describe a dual-tuning approach for improving
SO2 capture by anion-functionalized ILs. The essence of our
strategy for increasing the capacity and reducing the enthalpy
is introducing an electron-withdrawing interaction site on the
anion. Thus, we designed and prepared several anion-
functionalized ILs, including ones based on benzoates,
acetates, and phenolates with a halogen group on the anion
(Scheme 1). Through a combination of absorption experi-
ments, quantum chemical calculations, and spectroscopic
onto the anion.
Gases such as SO2, CO2, BF3, and H2S
are expected to have a high solubility in
ILs, especially in functionalized ILs.[2]
Davis and co-workers reported the first
example for the chemical absorption of
CO2 that employs an amino-functional-
ized IL.[3] Subsequently, a great deal of
anion-functionalized ILs, including those
based on amino acids,[4] acetates,[5]
azoles,[6] and phenolates,[7] have been
used to capture these acid gases. Normally,
the chemisorption has a high capacity for
gas absorption along with a high absorp-
tion enthalpy, which results in difficult
desorption, as well as high energy demand Scheme 1. Structures of the cation and the anion in halogen-containing anion-functionalized
for regeneration.[8] Several methods,
ionic liquids for SO2 capture.
including reducing the basicity[9] and intro-
ducing an electron-withdrawing substitu-
ent on the anion,[10,6a,7b] have been developed for reducing the
absorption enthalpy. However, these methods often lead to
reduced capacity, owing to a decrease in the interaction
between the IL and the acid gas. The question remains
investigation, we show that both enhanced capacity and
reduced enthalpy for the capture of SO2 can be achieved
based on the dual role of the halogen group as both an added
interaction site and an electron-withdrawing group, which
produces an IL with a highly efficient and excellent reversible
process for SO2 capture.
These anion-functionalized ILs containing halogen groups
were easily prepared by acid–base neutralization between
substituted benzoic acids, substituted acetic acids, or substi-
tuted phenols and a solution of phosphonium hydroxide in
ethanol, which was obtained by an anion-exchange method.[11]
The structures of these halogen-containing ILs were verified
by NMR and IR spectroscopy (see the Supporting Informa-
tion). The physical properties, including the density and
viscosity, of these ILs were determined (Supporting Informa-
tion, Table S1). In general, halogen-containing ILs had higher
[*] G. Cui, J. Zheng, X. Luo, W. Lin, F. Ding, Prof. H. Li, Prof. C. Wang
Department of Chemistry, ZJU-NHU United R&D Center, Zhejiang
University, Hangzhou 310027 (P.R. China)
E-mail: chewcm@zju.edu.cn
[**] This work was supported by the National Natural Science
Foundation of China (21176205, 20976151, and 20990221), the
Zhejiang Provincial Natural Science Foundation of China
(LR12B06002), the Program for Zhejiang Leading Team of S&T
innovation (2011R50007), and the Fundamental Research Funds of
the Central Universities.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201305234.
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Chemie
Table 1: The effect of different halogen-containing ionic liquids on SO2
absorption capacities, interaction enthalpies, and the Mulliken atomic
charges of oxygen and halogen atoms.
and 3.74 to 1.60 and 1.48 molSO per molIL, respectively, when
the temperature increased from 20 to 1208C.
2
The desorption of SO2 by these halogen-containing ILs
was investigated, and is shown in Figure 1 (see also Fig-
ure S2). As can be seen, the release of SO2 by halogen-
containing ILs such as [P66614][4-BrC6H4COO] at 1208C is
complete, whereas the residual capacity for SO2 desorption by
Anions
Absorption capacities[a]
DH[b,c] Mulliken charges[c]
[kJmolÀ1
]
O
X
[PhCOO]
3.74
4.12
3.93
3.96
3.48
3.89
3.02
3.22
3.35
3.17
3.25
3.10
3.31
3.15
–
À0.624
–
[4-BrC6H4COO]
[4-ClC6H4COO]
[4-FC6H4COO]
[CH3COO]
[BrCH2COO]
[PhO]
[4-BrC6H4O]
[2-BrC6H4O]
[3-BrC6H4O]
[4-CF3C6H4O]
[4-CH3C6H4O]
[PhSO3]
À26.6 À0.620 À0.021
À16.3 À0.618
À16.8 À0.625 À0.367
À0.627
À35.9 À0.583 À0.237
0.132
–
–
–
À0.687
–
À50.5 À0.681 À0.096
À51.0 À0.628 À0.104
À40.3 À0.668 À0.087
À24.7 À0.658 À0.329
–
–
À0.693
À0.638
–
–
[4-BrC6H4SO3]
À20.6 À0.626
0.003
[a] SO2 (1 bar) was absorbed at 208C for 30 min; values given in units of:
Figure 1. Effect of the halogen group in benzoate-based ionic liquids
on SO2 absorption and desorption as a function of time. SO2
absorption was carried out at 208C, and desorption was performed at
molSO per molIL. [b] Interaction enthalpies of the complexes with
2
a halogen group on the anion with the closest SO2 molecule. [c] Carried
out at the B3LYP/6-31+ +G(d,p) level of theory.
^
1208C under N2. [P66614][4-BrC6H4COO]: absorption ( ), desorption
~
~
^
( ); [P66614][4-ClC6H4COO]: absorption ( ), desorption ( ); [P66614
]
&
[PhCOO]: absorption ( ), desorption (&).
densities and viscosities than the corresponding non-halo-
genated analogues.
The effect of different ILs with halogen groups on the
absorption of SO2 was investigated, and is shown in Table 1. It
[P66614][PhCOO] is about 0.35 molSO per molIL, thus indicating
2
was found that the SO2 absorption capacities of [P66614
[4-BrC6H4COO], [P66614][4-ClC6H4COO], and [P66614
]
]
that the desorption of SO2 improved considerably owing to
the presence of the electron-withdrawing bromine group.
These halo-containing ILs were compared with other anion-
functionalized ILs (Table S2). It was seen that the halogen-
containing IL [P66614][4-BrC6H4COO] exhibited the highest
available capacity of SO2 absorption (up to 4.12 mole per
molIL) owing to its high absorption and facile desorption.
Multiple SO2 absorption/desorption cycles were investigated
for [P66614][4-BrC6H4COO] (Figure S3). It can be seen that
[P66614][4-BrC6H4COO] could be recycled more than six times
without a loss of absorption capability, indicating that the
process of SO2 absorption by these halogen-containing
functionalized ILs is highly reversible.
Considering both enhanced absorption and easy desorp-
tion by these ILs, halogen groups on the anion play a dual-
tuning role for improving the capture of SO2. Generally, an
electron-withdrawing substituent on the anion would reduce
the interaction between the anion and acid gas, resulting in
decreased capacity. Why do these halogen-containing anion-
functionalized ILs exhibit such a different behavior for SO2
capture? We believe that the different behavior of halogen
groups in these ILs may be contributed to by the following
two factors: 1) Halogen–sulfur interaction between the hal-
ogen group on the anion and SO2, which leads to an increase
in the absorption capacity. 2) The halogen group is an
electron-withdrawing group, which disperses the negative
charge of the O atoms on the anion and decreases the
enthalpy for SO2 absorption, resulting in the improved
desorption. Therefore, the halogen group should play a dual
role, both as an added interaction site and as an electron-
withdrawing group, which improves the capture of SO2
considerably.
[4-FC6H4COO] are 4.12, 3.93, and 3.96 molSO per molIL,
2
respectively, whereas that of [P66614][PhCOO] is 3.74 molSO
per molIL. It can be seen that, compared to non-halogen-
containing benzoate-based ILs such as [P66614][PhCOO],
benzoate-based ILs with a halogen group, such as [P66614
[4-BrC6H4COO], exhibited higher SO2 absorption capacities.
Similarly, the acetate-based ILs and phenolate-based ILs that
2
]
include bromine groups, [P66614][BrCH2COO] and [P66614
]
[4-BrC6H4O], also exhibited enhanced absorption of SO2 in
comparison with that of [P66614][CH3COO] and [P66614][PhO].
The effect of the position of the halogen group in the
phenolate anion on the capture of SO2 was investigated, and is
also shown in Table 1. As can be seen, the capacity varied in
the range of 3.17–3.32 molSO per molIL as the position of the
2
Br group on the phenolate anion changed. Furthermore, the
capture of SO2 by [4-CH3C6H4O], which contains an electron-
donating group, was also investigated (Table 1). It was seen
that the capacity increased somewhat owing to the presence
of the electron-donating CH3 group, which is in agreement
with the results by phenolate ILs for CO2 capture.[7b]
Figure S1 shows the effect of pressure and temperature on
SO2 absorption by [P66614][4-BrC6H4COO] and [P66614
]
[PhCOO]. It was seen that the molar ratios of SO2 to IL for
[P66614][4-BrC6H4COO] and [P66614][PhCOO] decreased from
4.12 and 3.74 to 1.66 and 1.60, respectively, as the SO2 partial
pressure decreased from 1.0 bar to 0.1 bar (Figure S1a). The
temperature dependence of SO2 absorption by [P66614
[4-BrC6H4COO] and [P66614][PhCOO] at 1 bar is shown in
Figure S1b. SO2 absorption capacities by [P66614
[4-BrC6H4COO] and [P66614][PhCOO] decreased from 4.12
]
]
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To investigate the dual role of the halogen group on the
anion in these halogen-containing anion-functionalized ILs,
we calculated the Mulliken atomic charge of the oxygen and
halogen atoms in the anion using the Gaussian 03 program.
As can be seen in Table 1, compared with the non-halogen
counterpart, the Mulliken atomic charge of the oxygen atom
in the halogen-containing anion decreased because of its
electron-withdrawing nature, resulting in reduced interaction.
For example, the Mulliken atomic charge of the oxygen atom
in [4-BrC6H4COO] is À0.620, whereas that in [PhCOO] is
À0.624. On the other hand, the halogen group shares the
negative charge of the oxygen atom in the anion, which
enhances halogen–sulfur interaction between the halogen
group and SO2,[12] leading to an increase in the absorption
capacity. For example, the Mulliken atomic charge of the Br
atom in [4-BrC6H4COO] is À0.021, whereas that in bromo-
benzene is 0.041.
À96.9 kJmolÀ1 and À55.5 kJmolÀ1 to À92.2 kJmolÀ1 and
À53.7 kJmolÀ1, respectively, when the bromine group is
present on the anion, thus leading to easy desorption. For
comparison, the interaction between the halogen group in
a halogen-containing anion such as [4-BrC6H4O] and CO2 was
also investigated (Figure S4). The interaction in
[4-BrC6H4O]–CO2 and [4-BrC6H4COO]–CO2 were lower
than that in [4-BrC6H4O]–SO2 and [4-BrC6H4COO]–SO2,
thus leading to reduced capacity for CO2 capture by the
halogen-containing anion.[7b]
The interaction between halogen-containing ILs and SO2
was further investigated by FTIR and NMR spectroscopy to
support the experimental and theoretical results (Figure 3).
As can be seen, compared with neat SO2 at 1150 cmÀ1 in the
To further investigate the dual role of the halogen group
in the capture of SO2, we calculated the geometry optimiza-
tion for the free halogen-containing anions, the free SO2, and
the complex of the halogen with the closest SO2 at the B3LYP/
6-31 ++ G(d,p) level of theory.[13] The optimized structures
and the energetics, which reflect the interaction between
halogen group and SO2, are shown in Figure 2a (see also
Figure 2. Optimized structures of [4-BrC6H4COO]–SO2 complexes at
the B3LYP/6-31+ +G(d,p) level of theory. a) The Br atom in a
[4-BrC6H4COO] anion with the closest SO2 molecule, DH=
À26.6 kJmolÀ1. b–d) Multiple-site interactions between a
Figure 3. The FTIR (a) 1H NMR (b) and 13C NMR (c) spectra of [P66614
[4-BrC6H4COO] before and after the absorption of SO2.
]
[4-BrC6H4COO] anion and a SO2 molecule: b) [4-BrC6H4COO]–SO2,
DH=À92.2 kJmolÀ1; c) [4-BrC6H4COO]–2SO2, DH=À53.7 kJmolÀ1
;
d) [4-BrC6H4COO]–3SO2, DH=À19.6 kJmolÀ1
.
FTIR spectra,[15] [P66614][4-BrC6H4COO]–SO2 adducts exhibit
À
a red shift of the symmetric S O stretching mode, indicating
2
À
Table S3). It can be seen that the intermolecular C(sp ) Br···S
distance in [4-BrC6H4COO]–SO2 is predicted to be 3.267 ꢀ,
which corresponds to a reduction of approximately 10.5% of
the presence of a halogen–sulfur interaction between the
anion and SO2.[16] Furthermore, compared with the IR
spectrum of fresh IL [P66614][4-BrC6H4COO], new absorption
bands at 1326 and 938 cmÀ1, which are attributable to the
the sum of the van der Waals radii of the two interacting
2
[6a]
atoms,[14] whereas the calculated C(sp ) Br···S angle amounts
sulfate S O stretch, are produced
as a result of the
À
À
2
À
to 88.28. The absorption enthalpy for C(sp ) Br···S interac-
absorption of SO2. Similarly, compared with the fresh IL
[P66614][4-BrC6H4COO] in the 13C NMR spectrum, the peak of
carbon bonded to bromine in the anion moved downfield
from 121.9 ppm to 126.3 ppm, as a result of the interaction
between the anion and SO2. Based on previous reports[6a,b]
and the observed product, the mechanism of SO2 absorption
by [P66614][4-BrC6H4COO] can be proposed as in Figure 2,
which posits two kinds of interaction sites between the
tion of the [4-BrC6H4COO]–SO2 complex was found to be
À26.6 kJmolÀ1, which indicates that the halogen–sulfur
interaction between [4-BrC6H4COO] and SO2 is strong, and
results in an increase in the absorption capacity (Table 1).
Furthermore, the interaction between the O atoms and SO2
was also investigated (Figure 2b–d; see also Table S4). It can
be seen that the absorption capacities decreased from
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an electronic balance with an accuracy of Æ 0.1 mg.[6,9] The standard
electronegative oxygen atoms and the halogen atom in the
anion with SO2.
deviation of the absorption loadings under 1.0 bar is 0.05 molSO per
2
molIL. The ILs were regenerated by heating or bubbling through with
nitrogen. In a typical SO2 desorption experiment, atmospheric
pressure N2 was bubbled through about 1.0 g captured ILs in a glass
container, which was partly immersed in a circulation oil bath of
desired temperature, with a flow rate of about 60 mLminÀ1 at 1208C.
The release of SO2 was determined at regular intervals by electronic
balance.
In summary, a new method for SO2 capture through the
addition of an interaction site on the anion to both enhance
the absorption capacity and decrease the absorption enthalpy
has been developed. Several halogen-containing anion-func-
tionalized ILs exhibited both enhanced capacity and easier
desorption than non-halogen-containing ILs, resulting in
highly efficient and excellent reversible SO2 capture. Spec-
troscopic investigation and quantum-mechanical calculations
show that the capacity was increased because of the presence
of a halogen–sulfur interaction between the halogen group
Received: June 18, 2013
Published online: August 12, 2013
Keywords: dual tuning · halogens · ionic liquids · sulfur ·
and SO , whereas the desorption properties were improved
.
2
SO2 capture
due to the role of the halogen as an electron-withdrawing
group. Aside from these halogen groups, some other electron-
withdrawing groups, such as nitrile or aldehyde groups, could
also play a dual-tuning role in the capture of gas. The
approach developed in this work provides new insight into gas
capture, which opens the door to achieving high capacity as
well as excellent reversibility, and to capture other gases such
as H2S and CO2 with ILs. Considering its diversity and
tunability, we believe that this highly efficient and reversible
process is promising in the field of gas separation.
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Trihexyl(tetradecyl)phosphonium chloride ([P66614][Cl]), 4-bromo-
benzoic acid (4-BrC6H4COOH), 4-chlorobenzoic acid (4-
ClC6H4COOH), 4-fluorobenzoic acid (4-FC6H4COOH), benzoic
acid (C6H5COOH), acetic acid, bromoacetic acid, 4-bromophenol
(4-Br-PhOH), 3-bromophenol (3-BrC6H4OH), 2-bromophenol (2-
BrC6H4OH), 4-methylphenol (4-CH3C6H4OH), 4-trifluoromethyl-
phenol (4-CF3C6H4OH), phenol (PhOH), benzenesulfonic acid
(PhSO3H) and 4-bromobenzenesulfonic acid (4-BrC6H4SO3H) were
all purchased from Sigma–Aldrich. All chemicals were obtained in
the highest purity grade possible, and were used as received unless
1
otherwise stated. H NMR and 13C NMR spectra were recorded on
a Bruker spectrometer (500 MHz) in [D6]DMSO with tetramethylsi-
lane as the standard. FTIR spectra were recorded on a Nicolet 470
FTIR spectrometer, and in-site IR spectra were recorded on a Bruker
MATRIX-MF, respectively. Viscosity was determined by a Brookfield
DV II pro viscometer at 258C. Decomposition temperatures were
measured with a TGA 2100 series of TA Instrument with a heating
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rate of 108CminÀ1
Preparation of ILs: In
.
a
typical synthesis of [P66614]
[4-BrC6H4COO], an equimolar amount of 4-BrC6H4COOH was
added to an ethanol solution of [P66614][OH], which was prepared
from [P66614][Cl] by anion-exchange. The mixture was then stirred at
RT for 24 h. Subsequently, ethanol and water were distilled off at
608C under reduced pressure. The ILs obtained were dried under
high vacuum for 24 h at 608C to reduce possible traces of water. The
structures of these ILs were confirmed by NMR and IR spectroscopy;
no impurities were found by NMR. The water content of these ILs
was determined with a Karl Fisher titration and found to be less than
0.1 wt%. The residual chloride content of these ILs was determined
by a semiquantitative Nessler cylinder method, which showed that the
bromine content was lower than 0.15 wt%.
Absorption and desorption of SO2: In a typical SO2 absorption
experiment, atmospheric pressure SO2 was bubbled through IL (ca.
1.0 g) in a glass container with an inner diameter of 10 mm, with
a flow rate of about 60 mLminÀ1 under 208C. The glass container was
partly immersed in a circulation water bath of desired temperature.
The amount of SO2 absorbed was determined at regular intervals by
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