Tuning the physicochemical properties of diverse phenolic ionic liquids for equimolar CO2 capture by the substituent on the anion

Article abstract of DOI:10.1002/chem.201103092

Phenolic ionic liquids for the efficient and reversible capture of CO ₂ were designed and prepared from phosphonium hydroxide and substituted phenols. The electron-withdrawing or electron-donating ability, position, and number of the substituen

Full text of DOI:10.1002/chem.201103092

FULL PAPER
DOI: 10.1002/chem.201103092
Tuning the Physicochemical Properties of Diverse Phenolic Ionic Liquids for
Equimolar CO2 Capture by the Substituent on the Anion
Congmin Wang,*[a, b] Huimin Luo,[d] Haoran Li,[a] Xiang Zhu,[c] Bo Yu,[d] and
Sheng Dai*[b, c]
Abstract: Phenolic ionic liquids for the
efficient and reversible capture of CO2
were designed and prepared from
phosphonium hydroxide and substitut-
ed phenols. The electron-withdrawing
or electron-donating ability, position,
and number of the substituents on the
anion of these ionic liquids were corre-
lated with the physicochemical proper-
ties of the ionic liquids. The results
show that the stability, viscosity, and
CO2-capturing ability of these ionic liq-
uids were significantly affected by the
substituents. Furthermore, the relation-
ship between the decomposition tem-
perature, the CO2-absorption capacity,
and the basicity of these ionic liquids
was quantitatively correlated and fur-
ther rationalized by theoretical calcula-
tion. Indeed, these ionic liquids showed
good stability, high absorption capacity,
and low absorption enthalpy for CO2
capture. This method, which tunes the
physicochemical properties by making
use of substituent effects in the anion
of the ionic liquid, is important for the
design of highly efficient and reversible
methods for CO2-capture. This CO2
capture process using diverse phenolic
ionic liquids is a promising potential
method for CO2 absorption with both
high absorption capacity and good re-
versibility.
Keywords: absorption · carbon di-
oxide · ionic liquids · phenols · sub-
stituent effects
Introduction
deal of attention has been paid to the study of the effects of
cation structure on the physicochemical properties of ILs
because the structure of the cation is easy to modify. How-
ever, investigations of the role played by the anion structure
are few. In ILs, the anion generally plays a more-important
role in the reaction than the cation, thus significantly affect-
ing the conversion and selectivity of the reaction.[18,19] There-
fore, modifying the structure of the anion may be very im-
portant in improving the reaction or separation of the prod-
ucts. The famous example is the effect that different amino
acid anions have on physical properties such as ionic con-
ductivity and glass transition temperature, as well as on
chemical properties such as CO2 absorption.[20–22] Herein, we
report the effect of different substituents on phenol-based
anions on the physical and chemical properties of the ILs.
These results show that viscosity, thermal decomposition
temperature, and CO2 capture were easily tuned by the sub-
stituent on the anion, which originated from varying the
charge on the oxygen atom in the anion. Thus, we can ach-
ieve desired physicochemical properties such as high stabili-
ty, excellent absorption capacity, and low absorption enthal-
py for CO2 capture by conveniently tuning the anion struc-
ture of the ILs.
Recently, ionic liquids (ILs) have attracted considerable at-
tention as solvents,[1–3] catalysts,[4–7] sorbents,[8–10] materi-
als,[11–13] etc., owing to their remarkable properties, including
extremely low vapor pressure, high thermal stability, wide
temperature range as liquids, and high ionic conductivi-
ty.[14–17] ILs are also recognized as designable liquids, because
their physical and chemical properties are facilely tuned by
different combinations of the cation and the anion. A great
[a] Prof. C. Wang, Prof. H. Li
Department of Chemistry
Zhejiang University
Hangzhou, 310027 (China)
Fax : (+86)571-8795-1895
E-mail: chewcm@zju.edu.cn
[b] Prof. C. Wang, Dr. S. Dai
Chemical Sciences Divisions
Oak Ridge National Laboratory
Oak Ridge, TN, 37831 (USA)
Fax : (+1)865-576-5235
E-mail: sdai@ornl.gov
The emission of CO2—the most significant greenhouse
gas from the burning of fossil fuels—has received worldwide
attention because of its connection to climate change and
humanityꢀs growing energy needs. One of the most com-
monly used technologies in industry for the capture of CO2
is chemical absorption by an aqueous amine solution, with
its advantages of low cost, high reactivity, and good absorp-
tion capacity.[23] However, this process for the capture of
[c] X. Zhu, Dr. S. Dai
Department of Chemistry, University of Tennessee
Knoxville, TN 37966 (USA)
[d] Dr. H. Luo, Dr. B. Yu
Nuclear Science and Technology Division
Oak Ridge National Laboratory
Oak Ridge, TN, 37831 (USA)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103092.
Chem. Eur. J. 2012, 18, 2153 – 2160
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CO2 has some serious inherent drawbacks, including solvent
loss, corrosion, and high energy demands owing to the ther-
modynamic properties of water and high absorption enthal-
py.[24,25] Currently, the goal is to design industrially attractive
sorbent materials with high capacity and low absorption en-
thalpy for the capture of CO2.
Ionic liquids offer a new opportunity for addressing this
challenge to design and develop excellent CO2-capture sys-
tems. Many research groups have conducted outstanding re-
search on the physical absorption of CO2 in ILs using both
experimental and theoretical methods.[26–30] Their results
have shown that CO2 is very soluble in ILs, especially at
high pressures. However, the physical absorption of CO2 in
ILs has some way to go because of their low capacity under
Scheme 1. Structures of the ionic liquids [P66614]AHCNTUGERN[NUG SPhO] with various sub-
stituents. SPhO=substituted phenol.
The viscosity of an IL is an important physical property
that significantly affects its kinetics of gas absorption. We in-
vestigated the effects of different substituents on the phenol-
ic anion on viscosity (Table 1) and noticed several important
atmospheric pressure. The Davis group[31] reported
a
method for the chemisorption of CO2 that employs an
amino-functionalized IL, where 0.5 mole CO2 per mole IL
could be captured under ambient pressure over 3 h. Subse-
quently, some other amino-functionalized ILs, including sul-
fone anions with ammonium cations, and amino acid anions
with imidazolium or phosphonium cations, were reported
for CO2 capture.[32–34] Recently, a method was reported for
the capture of CO2 in a 1:1 manner by tunable azole-based
basic ILs that makes use of the reactivity of the anion.[35,36]
These investigations have made some significant advance-
ments; however, alternative IL systems that are able to ach-
ieve efficient, reversible capture of CO2 are still highly de-
sired.
Herein, we prepared diverse phenolic ILs by neutralizing
phosphonium hydroxide with various substituted phenols.
We found that the physical and chemical properties of the
ILs are facilely tunable by different substituents. Our inves-
tigation found that the electron-withdrawing or electron-do-
nating ability, position, and the number of the substituents
had significant effects on the capture of CO2, which was ex-
plained by density functional theory (DFT) methods. Based
on the relationship between CO2 capture and substituted
groups on the anion of the IL, a highly stable IL with high
capacity and good reversibility for CO2 capture was ach-
ieved.
Table 1. Physical properties of different phenolic ILs.
[a]
[a]
Ionic liquid
h [cPa][a] d [gcmÀ3
]
s [scmÀ1
]
Td [8C][b]
A
]
[4-Me-PhO]
392.7
253.4
246.7
376.5
286.4
984.3
223.2
378.3
472.5
672.1
335.1
415.5
334.1
371.3
388.2
585.8
374.8
512.1
0.894
0.919
0.896
0.949
0.968
0.958
0.931
0.925
0.966
1.009
0.893
0.900
0.901
0.909
0.906
0.887
0.937
0.916
0.929
0.930
2.561
4.210
3.001
5.680
3.800
4.700
3.940
1.825
2.179
1.560
2.951
2.408
1.653
2.552
1.612
0.812
3.840
0.891
1.019
1.010
238
217
245
277
259
292
270
299
302
341
221
243
205
189
250
255
258
233
294
284
AHCTNUERTG[GNNUN P66614][4-MeO-PhO]
R
]
]
N
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
E
]
]
T
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
R
]
]
G
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
E
]
]
1077
878.4
C
[a] Determined at 238C. [b] Determined by TGA under a N2 atmosphere.
trends: First, the presence of electron-withdrawing or elec-
tron-donating groups influenced the viscosity of the IL sig-
nificantly. For example, the viscosities of [P66614]AHCUTNGERN[NUG 4-Cl-PhO],
Results and Discussion
A series of phenol-based ILs were prepared by neutraliza-
tion of various substituted phenols, SPhOHs, with trihexyl(-
tetradecyl) phosphonium hydroxide, [P66614][OH], which was
obtained by the anion-exchange method.[20] Electron-with-
drawing groups, such as NO2, Cl, and CF3, and electron-do-
nating groups, such as CH3 and OCH3, were selected to in-
vestigate the electronic push-pull effect on the properties of
the ILs. ILs that had 2-Cl, 3-Cl, and 4-Cl groups, as well as
2,4-Cl and 2,4,6-Cl groups, on the phenolic anion were de-
signed to assess the influence of the position and number of
substituents on the properties of the ILs (Scheme 1). Phe-
nolic ILs from 1-naphthol and 2-naphthol were also investi-
gated.
[P66614][4-CF3-PhO], and [P66614][4-NO2-PhO] (376.5 cP,
286.4 cP, and 984.3 cP, respectively) were all higher than
[P66614]AHCUTNREGN[NUG PhO] (246.7 cP). The viscosity also changed a little
when a 4-MeO group was substituted on the anion. Second,
the changing the position of the substituent had an obvious
effect on viscosity. As shown in Table 1, the viscosity was re-
duced from 376.5 to 223.2 cP when [P66614]ACHTNGUTERNNU[G 4-Cl-PhO] was re-
placed by [P66614 [3-Cl-PhO]. Third, the number of substitu-
]
ACHTUNGTRENNUNG
ents also had a significant effect on viscosity. For example,
the viscosity of the IL increased with increasing amounts of
Cl groups on the anion. Table 1 also shows the influence of
the substituent on density. In general, the anions containing
the Cl or MeO groups had higher densities.
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Chem. Eur. J. 2012, 18, 2153 – 2160

Equimolar CO2 Capture by Phenolic Ionic Liquids
FULL PAPER
Table 2. The effect of the substituent at the 4-position of the phenolic
anion on CO2 capture.
The thermal-decomposition temperature of the ILs is an-
other key physical property for gas absorption. High stabili-
ties help ILs achieve good reversibility for CO2 capture. In
general, as shown in Table 1, placing electron-withdrawing
groups on the phenolic anion is useful for increasing the sta-
bilities of the ILs, whereas electron-donating groups de-
crease the stability. For example, the decomposition temper-
Ionic liquid
t [min][a]
CO2 absorption[b]
A
]
[4-Me-PhO]
30
30
30
30
30
20
0.91
0.92
0.85
0.82
0.61
0.30
AHCTNUERTG[GNNUN P66614][4-MeO-PhO]
R
]
]
E
AHCTUNGTRENNUNG
atures of [P66614
277 and 2928C, respectively, whereas that of [P66614][4-MeO-
PhO] was 2178C. In comparison with [P66614 [4-H-PhO], ILs
[P66614 [1-Naph] and [P66614 [1-Naph] showed higher stabili-
]ACHTUNGTRENNUNG[4-Cl-PhO] and [P66614][4-NO2-PhO] were
AHCTUNGTRENNUNG
[a] Determined at 308C. [b] Mole CO2 per mole of ionic liquid.
]ACHTUNGTRENNUNG
]N
]ACHTUNGTRENNUNG
ties. Furthermore, placing the substituents at different posi-
tions on the phenolic ring imparted different stabilities to
the ILs. A comparison of 2-Cl- and 3-Cl-substituted anions
showed that the anion of the 2-Cl substituent was more
stable. Finally, the more Cl groups present on the ring, the
higher was the stability of the ILs. We investigated whether
there was a quantitative relationship between the stability of
the IL and the substituents by considering the relationship
between the decomposition temperature and the basicity of
the anion. A roughly linear relationship between the pKa
value of the anion and the decomposition temperature was
afforded, as shown in Figure 1. Clearly, the stability of the
ed that the absorption capacity of CO2 was easily tuned by
electron-withdrawing and electron-donating substituents.
Table 3 shows the effect of the location and number of Cl
substituents on the capacity of CO2 capture. Clearly, placing
the Cl group at different positions around the phenol ring
Table 3. The effect of the position and number of substituents, tempera-
ture, and water on CO2 capture.
Ionic liquid
T [8C][a]
CO2 absorption[b]
A
]
]
]
G
30
30
30
30
30
50
70
30
30
30
30
30
0.82
0.72
0.67
0.48
0.07
0.65
0.50
0.89
0.88
0.94
0.89
0.86
T
T
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
R
]
]
]
]
E
T
E
ACHNUTRTGE[NNGUN P66614][3-NMe2-PhO]
E
]
]
G
[a] Determined for 30 min. [b] Mole CO2 per mole ionic liquid. [c] Addi-
tion of water (2 wt%) to the IL. [d] Addition of water (5 wt%) to the IL.
resulted in remarkable variations in the absorption capacity.
For example, the absorption capacity decreased from
0.82 mol CO2 per mole IL to 0.72 and 0.67 mol when [P66614]-
Figure 1. The relationship between the decomposition temperature (Td)
and the pKa value of the anion in the ionic liquids; experimental values
(*), fitted values (À).
AHCUTNERTGNNG[NU 3-Cl-PhO] and [P66614]ACHTUNRGTEGUN[NN 2-Cl-PhO] replaced [P66614]ACHTUNGTRENNUNG[4-Cl-
PhO], respectively. Furthermore, we investigated the effect
of the number of Cl substituents on CO2 capture (Table 3).
The CO2-absorption capacity decreased when a higher
number of Cl groups were substituted on the phenolic ring.
ILs increased as the pKa value of the anion decreased.
Thus, we can facilely tune the stability of the ILs by making
use of the basicity of the anion. After this discovery, we pre-
dominantly chose ILs with decomposition temperatures
above 2508C for our investigation of CO2 capture.
Next, we investigated the effect on CO2 capture when
substituents such as NO2, Cl, and Me were substituted at the
4-position on the phenolic anion (Table 2). The CO2-absorp-
tion capacity was significantly affected by placing these sub-
stituents on the anion; the CO2-absorption capacity de-
creased markedly from 0.85 mol CO2 per mole IL to 0.61
and 0.30 mole CO2 when CF3 and NO2 (both electron-with-
drawing groups) replaced the H group at the 4-position on
the anion, respectively. The stronger the electron-withdraw-
ing group, the lower the CO2-absorption capacity became.
Conversely, placing an electron-donating group on the anion
increased the CO2-absorption capacity. These results indicat-
Other phenolic ILs, such as [P66614]AHCUNTERGNN[UG 1-Naph] and [[P66614]ACHTUNGTRENNUNG[2-
Naph], also showed good CO2 absorption. Next, the addi-
tion of specific amounts of water on the efficiency of CO2
capture was investigated (Table 3). The results showed that
the CO2-absorption capacity increased when water was
added to the IL, presumably because bicarbonate formation
occurred. This result is in agreement with results obtained
for other anion-functionalized ILs.[19,21]
One important feature of the absorption by these phenol-
ic ILs is that the absorption rate is rapid. Figure 2 shows
some typical CO2 absorptions by several phenolic ILs, such
as [P66614]AHCNTUGERN[NUG 4-Me-PhO] and [P66614]ACTHUNGTRENN[UGN 4-Cl-PhO], as a function of
time. The absorption of CO2 was almost complete within the
first 10 min. This rapid absorption is presumably related to
the absence of strong hydrogen-bonded networks during the
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C. Wang, S. Dai et al.
bonate (C=O) stretches after the absorption of CO2 by
[P66614]ACHUTNGERNUNG
[4-Cl-PhO]. Furthermore, the 13C NMR spectrum of
the product showed a new band at d=161 ppm during the
absorption of CO2, thereby also indicating the formation of
carbonate salt. On the basis of previous reports[35,38–40] and
the observed reaction product, we present Scheme 2 as the
absorption mechanism of CO2 by these phenolic ILs. Thus,
equimolar CO2 capture can be achieved because these phe-
nolic ILs react with CO2 in a 1:1 manner.
Next, we considered why the CO2 capture was so signifi-
cantly affected by different substituents, and whether there
was a quantitative relationship between the CO2-absorption
capacity and the substituent. We investigated the relation-
ship between CO2-absorption capacity and the basicity of
the phenolic ILs. As shown in Figure 4, the CO2-absorption
Figure 2. CO2 absorption by three typical phenolic ionic liquids as a func-
tion of time at 308C: [P66614]ACHTUNGERTNN[UG 4-Me-PhO] (&), [P66614]ACHTUGNTREN[NUNG 4-Cl-PhO] (*), and
[P66614][4-NO2-PhO] (~).
absorption of CO2 by these phenolic ILs, which results in no
obvious change in viscosity.[35] However, on the contrary, for
traditional amino-functionalized ILs, molecular dynamics
simulations have revealed that the viscosity increased drasti-
cally because strong hydrogen-bonding networks formed
during the absorption of CO2 by these ILs, thereby resulting
in low absorption rates.[21,34,37]
To support our experimental results, the interaction of
these phenolic ILs and CO2 was investigated by Fourier
transform infrared (FTIR) and nuclear magnetic resonance
(NMR) spectroscopy. During the absorption of CO2 ob-
served in this study, the phenolic ILs reacted with CO2 to
form liquid carbonate salts. The formation of the carbonate
Figure 4. The relationship between CO2-absorption capacity and pKa
value of the anion in the ionic liquids.
salt was verified by FTIR spectra of fresh IL [P66614
]ACHTUNGTRENNUNG
PhO] and of the corresponding carbonate salt [P66614]ACHTNUGRTNEUNG
PhO]-CO2 (Figure 3). A new peak at 1617 cmÀ1 in the FTIR
spectrum of [P66614]ACHTUNGTRENNUNG[4-Cl-PhO]-CO2 can be assigned to car-
capacity decreased as the pKa value of the anion decreased.
This finding indicates that CO2 absorption can be facilely
tuned by varying the basicity of the ILs—a useful feature
for the future design of ILs for CO2 capture.
To further investigate the effect that different substituents
on the phenolic anion have on CO2 capture, theoretical cal-
culations were performed using the Gaussian 03 program.
For each set of calculations, we calculated the geometry and
energy optimization of each free anion, free CO2, and each
anion–CO2 complex at the B3LYP/6-31+ +G
The optimized structures of some typical anion–CO2 com-
plexes, including [P66614 [4-Me-PhO]-CO2 and [P66614][4-NO2-
ACHTUNGTRENUN(NG p,d) level.
]
ACHTUNGTRENNUNG
PhO]-CO2, are shown in Figure 5. We also calculated the
gas-phase reaction energetics for CO2 capture by different
phenolic ILs according to Scheme 2. Table 4 shows that the
absorption enthalpy decreased when electron-withdrawing
groups (such as Cl) were present on the anion, and in-
creased when electron-donat-
Figure 3. IR spectra of CO2 capture by [P66614
formed from the desorption of CO2; middle: carbonate salt resulting
from the capture; bottom: fresh [P66614 [4-Cl-PhO].
]ACTHNGUTERN[UNG 4-Cl-PhO]. Top: IL
]ACHTUNGTRENNUNG
ing anions (such as Me) were
used. Therefore, we can tune
the absorption enthalpy of
CO2 by varying the electric
push-pull properties of the sub-
stituents, thereby lowering the
energy demand for regenera-
tion. Furthermore, for these
substituted phenolic ILs, the
Scheme 2. The CO2-absorption mechanism for the phenolic ionic liquids used in this study.
changes in the enthalpy of CO2
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Equimolar CO2 Capture by Phenolic Ionic Liquids
FULL PAPER
Figure 6. The relationship between CO2-absorption enthalpy and the
Mulliken charge on the oxygen atom for CO2 capture; experimental
values (*), fitted values (À).
Figure 5. The optimized structures of [P66614
NO2-PhO] with CO2. The calculations were carried out at the B3LYP/6-
31+ +G(p,d) level.
]ACHTUNGRETNNU[G 4-Me-PhO] and [P66614][4-
ACHTUNGTRENNUNG
ILs with lower energy demands during the desorption of
CO2.
Considering the high stability of the IL, the good absorp-
tion capacity, and the low absorption enthalpy, we believe
Table 4. The effect of the substituent on absorption enthalpy and charge
on the oxygen atom.
that [P66614
CO2. When we investigated the effect of temperature on
[P66614 [4-Cl-PhO] for CO2 capture, we found that the CO2-
]ACHTNGUTERN[UNG 4-Cl-PhO] is an ideal sorbent for the capture of
Ionic liquid[a]
Absorption
Charge on
O atom
enthalpy [kJmolÀ1
]
]
ACHTUNGTRENNUNG
A
]
[4-Me-PhO]
À49.2
À51.4
À46.8
À38.1
À22.4
À17.1
À31.4
À27.6
À29.5
À19.0
À42.9
À27.9
À32.3
À0.6932
À0.7062
À0.6873
À0.6833
À0.6577
À0.6187
À0.6678
À0.6599
À0.6386
À0.6270
À0.6784
À0.6665
À0.6735
absorption capacity decreased as the temperature increased
(Table 3). For example, the molar ratio of CO2/IL decreased
gradually from 0.82 to 0.50 as the temperature was increased
from 30 to 708C. This outcome indicates that captured CO2
is easy to release by heating the IL. We also investigated the
absorption/desorption process using [P66614]AHCUTNGREN[NUG 4-Cl-PhO]
(Figure 7). Good capacity and rapid absorption rate persist-
ed during the six recycles, thereby demonstrating the rever-
ACHTUNGTRENNUNG[P66614][4-MeO-PhO]
T
]
]
E
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
N
]ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
sibility of CO2 capture by [P66614]AHCUTNREGN[NUG 4-Cl-PhO].
T
]
]
A
[a] Performed at the B3LYP/6-31+ +GACTHUNRTGNE(GNU p,d) level.
absorption agree with the results of absorption capacity.
Next, we calculated the effect of the substituent on the
Mulliken atomic charge of the oxygen atom in the phenolic
anion (Table 4). The more-strongly electron-withdrawing the
substituent is, the weaker the charge on the oxygen atom is.
Conversely, more-electron-donating groups, such as MeO
groups, caused the charge on the oxygen atom to increase.
The charge on the oxygen atom in these phenolic ILs was
also significantly affected by the position and the number of
Cl groups. For example, the charge increased from À0.6833
in [4-Cl-PhO] to À0.6386 and À0.6270 with anions [2,4-Cl-
PhO] and [2,4,6-Cl-PhO], respectively.
Figure 7. Six absorption/desorption cycles of [P66614]AHCTUNGERN[NUG 4-Cl-PhO] during the
capture of CO2. The absorption of CO2 (containing ca. 3.2% H2O) was
carried out at 308C, and the desorption was performed at 808C under a
N2 atmosphere for 60 min.
Based on the results for oxygen charge, we investigated
the relationship between CO2-absorption enthalpy and the
Mulliken charge of the oxygen atom. Figure 6 shows a
roughly linear correlation between the CO2-absorption en-
thalpy of the ILs and the Mulliken charge of the oxygen
atom in the anion. The absorption enthalpy decreased with
decreasing charge on the oxygen atom[41] as a result of the
weaker interaction between the negative oxygen atom and
the acidic CO2. This relationship will be useful for designing
Conclusions
We have developed a method for capturing CO2 using di-
verse phenol-based ILs that were prepared from the neutral-
ization of phosphonium hydroxide and substituted phenols.
The results indicate that physical properties, such as decom-
position temperature and viscosity, were significantly affect-
ed by the substituents on the phenolic ILs. These phenolic
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2157

C. Wang, S. Dai et al.
phosphonium hydroxide, [P66614][OH], according to a literature proce-
dure. For example, in a typical experiment, a solution of [P66614][OH] in
EtOH was obtained from [P66614][Cl] using the anion-exchange resin, and
ILs reacted with CO2 to form a liquid carbonate salt, there-
by resulting in equimolar CO2-absorption capacity that was
verified by theoretical calculations and spectroscopic analy-
sis. CO2 absorption was heavily affected by the electron-
withdrawing or electron-donating ability, position, and quan-
tity of substituents on the anion. Furthermore, a quantitative
relationship existed between the stability, CO2-absorption
capacity, CO2-absorption enthalpy, and basicity of the anion,
which originated from variations in the Mulliken charge on
the oxygen atom in different phenolic ILs. Therefore, we
can achieve good absorption capacity as well as low absorp-
tion enthalpy by varying the substituents on the anion,
which is useful in the future design of ILs for efficient and
reversible CO2 capture. Considering its diversity and tuna-
bility, we believe that this efficient and reversible process
using phenolic ILs is a promising potential method for CO2
absorption with both high absorption capacity and good re-
versibility.
equimolar p-chlorophenol was added to the EtOH solution of [P66614
]
[OH]. Then, the mixture was stirred at RT for 16 h. Subsequently, EtOH
and water were distilled off at 708C under vacuum. The obtained ILs was
dried under a high vacuum at 608C for 24 h.
Trihexyl(tetradecyl)phosphonium
p-methylphenolate,
[P66614]ACHTUNGTRENNUNG[4-Me-
1
PhO]: H NMR (400 MHz, CDCl3): d=0.86 (m, 12H; 4ꢂCH3), 1.22–1.43
(m, 48H; 24ꢂCH2), 2.21 (s, 3H; C4H), 2.26 (m, 8H; 4ꢂPH2), 6.86 (d,
2H; C2H and C6H), 6.92 ppm (d, 2H; C3H and C5H); 13C NMR
(100 MHz, CDCl3): d=13.7, 18.4, 18.9, 20.2, 21.5, 22.1, 22.4, 29.1, 29.3,
29.4, 30.1, 30.2, 30.8, 31.7, 115.5, 126.6, 129.3, 156.0 ppm ; IR: n˜ =2956,
2924, 2855, 1600, 1559, 1497, 1466, 1412, 1378, 1296, 1161, 1101, 826, 750,
719 cmÀ1. MS (ESI): m/z: 483.5 [P66614], 107.1 [4-Me-PhO].
Trihexyl(tetradecyl)phosphonium p-methoxyphenolate ([P66614][4-MeO-
PhO]): 1H NMR (400 MHz, CDCl3): d=0.88 (m, 12H; 4ꢂCH3), 1.21–
1.44 (m, 48H; 24ꢂCH2), 2.22 (m, 8H; 4ꢂPH2), 3.67 (s, 3H; OCH3), 6.66
(d, 2H; C2H and C5H), 6.85 ppm (d, 2H; C3H and C4H); 13C NMR
(100 MHz, CDCl3): d=13.7, 13.9 18.4, 18.9, 21.5, 22.1, 28.7, 29.1, 29.3,
29.4, 30.1, 30.2, 30.8, 31.7, 55.6, 114.3, 116.4, 151.1, 153.6 ppm; IR: n˜ =
2956, 2924, 2855, 1597, 1559, 1491, 1466, 1438, 1378, 1295, 1249, 1220,
1176, 1096, 1046, 993, 827, 738, 721 cmÀ1
Trihexyl(tetradecyl)phosphonium phenolate
.
([P66614]AHCUTNREGN[NUG 4-H-PhO]):
1H NMR (400 MHz, CDCl3): d=0.88 (m, 12H; 4ꢂCH3), 1.22–1.46 (m,
48H; 24ꢂCH2), 2.29 (m, 8H; 4ꢂPH2), 6.71 (m, 1H; C4H), 7.01 (d, 2H;
C2H and C6H), 7.11 ppm (m, 2H; C3H and C5H); 13C NMR (100 MHz,
CDCl3): d=13.7, 13.9, 18.4, 19.1, 21.7, 22.2, 22.5, 28.8, 29.2, 29.5, 30.2,
30.3, 30.5, 30.9, 1.7, 115.9, 117.9 128.5, 158.5 ppm; IR: n˜ =2957, 2924,
2855, 2364, 2341, 1581, 1559, 1485, 1467, 1306, 1156, 1105, 1054, 983, 859,
823, 754, 720 cmÀ1. MS (ESI): m/z: 483.5 [P66614], 93.0 [4-H-PhO].
Experimental Section
General: Trihexyl(tetradecyl)phosphonium bromide, [P66614][Br], was pur-
chased from Sigma-Aldrich. Anion-exchange resin Dowex Monosphere
550 A (OH) was obtained from Dow Chemical Company. Substituted
phenols, including phenol (PhOH), p-methylphenol (4-Me-PhOH), p-me-
thoxyphenol (4-MeO-PhOH), p-nitrophenol (4-NO2-PhOH), p-trifluoro-
methylphenol (4-CF3-PhOH), p-chlorophenol (4-Cl-PhOH), m-chloro-
phenol (3-Cl-PhOH), o-chlorophenol (2-Cl-PhOH), 2,4-dichlorophenol
(2,4-Cl-PhOH), 2,4,6-trichlorophenol (2,4,6-Cl-PhOH), 2,4-dimethylphe-
nol (2,4-Me-PhOH), 2,6-dimethylphenol (2,6-Me-PhOH), p-isopropyl-
phenol (4-iPr-PhOH), p-tert-butylphenol (4-tBu-PhOH), m-methylphenol
(3-Me-PhOH), o-methylphenol (2-Me-PhOH), and m-methoxyphenol (3-
MeO-PhOH), m-dimethylaminophenol (3-NMe2-PhOH), 1-naphthol (1-
Naph), and 2-naphthol (2-Naph) were purchased from Sigma-Aldrich.
All chemicals were obtained in the highest purity grade possible and
used as received unless otherwise stated. All ILs were dried under high
vacuum at 608C for 24 h to remove possible traces of water. The water
content of the IL was determined by Karl Fischer titration, and was
found to be less than 0.1 wt%. The residual chloride content was deter-
mined by a semi-quantitative Nessler cylinder method, which showed
that the halide content was lower than 0.15 wt%.
Trihexyl(tetradecyl)phosphonium p-chlorophenolate ([P66614]AHCUTNGERN[NUG 4-Cl-PhO]):
1H NMR (400 MHz, CDCl3): d=0.89 (m, 12H; 4ꢂCH3), 1.26–1.48 (m,
48H; 24ꢂCH2), 2.29 (m, 8H; 4ꢂPH2), 6.88 (d, 2H; C2H and C6H),
7.01 ppm (d, 2H; C3H and C5H); 13C NMR (100 MHz, CDCl3): d=13.8,
18.8, 19.1, 21.7, 22.2, 28.8, 29.2, 29.4, 29.5, 30.2, 30.4, 30.9, 31.7, 117.5,
127.9, 128.7, 158.8 ppm; IR (): n˜ =2955, 2923, 2854, 2362, 2340, 1576,
1481, 1466, 1314, 1153, 1111, 1084, 985, 830, 720 cmÀ1. MS (ESI): m/z:
483.5 [P66614], 127.0 [4-Cl-PhO].
Trihexyl(tetradecyl)phosphonium p-trifluorophenolate ([P66614][4-CF3-
PhO]): 1H NMR (400 MHz, CDCl3): d=0.88 (m, 12H; 4ꢂCH3), 1.23–
1.47 (m, 48H; 24ꢂCH2), 2.32 (m, 8H; 4ꢂPH2), 6.81 (d, 2H; C2H and
C6H), 7.22 ppm (d, 2H; C3H and C5H); 13C NMR (100 MHz, CDCl3):
d=13.7, 13.9, 18.6, 19.0, 21.6, 22.1, 28.7, 29.1, 29.4, 30.1, 30.3, 30.5, 30.6,
30.8, 57.4, 116.1, 126.3, 163.8 ppm; IR: n˜ =2957, 2925, 2856, 1591, 1514,
1467, 1358, 1314, 1267, 1161, 1143, 1087, 1054, 880, 840, 750, 721 cmÀ1
.
1H and 13C NMR spectra were recorded on a Bruker MSL-400 NMR
spectrometer in CDCl3 using tetramethylsilane as the standard. FTIR
data were obtained using a Bio-Rad Excalibur FTS-3000 spectrometer.
Mass spectra (MS) were determined using a QSTAR elite mass spec-
trometer. Ionic conductivity was measured by an EC meter model 3084
from Amber Science Inc, and viscosity was determined by a Brookfield
DV pro viscometer. Decomposition temperature was measured with a
MS (ESI): m/z: 161.0 [4-CF3-PhO].
Trihexyl(tetradecyl)phosphonium
p-nitrophenolate
([P66614][4-NO2-
PhO]): 1H NMR (400 MHz, CDCl3): d=0.87 (m, 12H; 4ꢂCH3), 1.26–
1.46 (m, 48H; 24ꢂCH2), 2.17 (m, 8H; 4ꢂPH2), 6.34 (d, 2H; C2H and
C5H), 7.96 ppm (d, 2H; C3H and C4H); 13C NMR (100 MHz, CDCl3):
d=13.8, 14.0, 18.4, 18.9, 21.5, 22.2, 28.7, 29.1, 29.2, 29.4, 29.5, 30.2, 30.3,
30.5, 30.8, 31.8, 119.1, 127.7, 131.0, 179.4; IR: n˜ =2955, 2924, 2854, 1581,
1543, 1512, 1466, 1424, 1378, 1262, 1214, 1161, 1106, 1009, 986, 849, 819,
761, 718 cmÀ1. MS (ESI): m/z: 138.0 [4-NO2-PhO].
TGA 2950 under a N2 atmosphere at a ramp rate of 108C minÀ1
.
Absorption of CO2: In a typical absorption of CO2, CO2 at atmospheric
pressure was bubbled through about 1.0 g of IL in a glass container with
an inner diameter of 10 mm, where the flow rate of CO2 was
60 mLminÀ1. The glass container was partially immersed in a water bath
at the desired temperature. The amount of CO2 captured was obtained at
regular intervals by an electronic balance with an accuracy of (Æ0.1) mg.
Desorption of CO2: In a typical desorption of CO2, N2 at atmospheric
pressure was bubbled through about 1.0 g of captured IL in a glass con-
tainer that was partially immersed in a circulation oil bath at the desira-
ble temperature; the flow rate was about 60 mLminÀ1. The release of
CO2 was determined at regular intervals by an electronic balance.
Trihexyl(tetradecyl)phosphonium o-chlorophenolate ([P66614]AHCUTNGERN[NUG 2-Cl-PhO]):
1H NMR (400 MHz, CDCl3): d=0.88 (m, 12H; 4ꢂCH3), 1.21–1.44 (m,
48H; 24ꢂCH2), 2.23 (m, 8H; 4ꢂPH2), 6.21(d, 1H; C6H), 6.81 (m, 1H;
C5H), 6.88 (m, 1H; C4H), 7.13 ppm (d, 1H; C3H); 13C NMR (100 MHz,
CDCl3): d=13.8, 14.0, 18.5, 19.0, 21.7, 22.2, 28.9, 29.2, 29.4, 29.5, 30.2,
30.3, 31.0, 33.9 111.9, 119.7, 127.5, 129.0, 162.6 ppm; IR : n˜ =2956, 2924,
2855, 1573, 1475, 1435, 1378, 1337, 1279, 1105, 1053, 1019, 880, 848, 739,
717 cmÀ1. MS (ESI): m/z: 127.0 [2-Cl-PhO].
Trihexyl(tetradecyl)phosphonium 2,4-dichlorophenolate ([P66614][2,4-Cl-
PhO]): 1H NMR (400 MHz, CDCl3): d=0.88 (m, 12H; 4ꢂCH3), 1.22–
1.43 (m, 48H; 24ꢂCH2), 2.20 (m, 8H; 4ꢂPH2), 6.55 (m, 1H; C6H), 6.77,
(m, 1H; C5H), 7.07 ppm (d, 1H; C3H); 13C NMR (100 MHz, CDCl3):
Synthesis of phenolic ILs: These phenolic ILs were prepared from the
neutralization of various substituted phenols with trihexyl(tetradecyl)-
2158
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 2153 – 2160

Equimolar CO2 Capture by Phenolic Ionic Liquids
FULL PAPER
d=13.8, 18.5, 18.9, 21.7, 22.2, 29.2, 29.4, 29.5, 30.2, 30.3, 30.5, 30.9, 120.2,
122.7, 127.3, 128.2, 163.4 ppm; IR: n˜ =2956, 2924, 2855, 1569, 1493, 1476,
1394, 1338, 1245, 1100, 1032, 862, 809, 772, 715 cmÀ1. MS (ESI): m/z:
160.9 [2,4-Cl-PhO].
1205, 1147, 1112, 1044, 987, 842, 753, 720 cmÀ1. MS (ESI): m/z: 123.0 [3-
MeO-PhO].
Trihexyl(tetradecyl)phosphonium m-dimethylaminophenolate ([P66614][3-
NMe2-PhO]): 1H NMR (400 MHz, CDCl3): d=0.87 (m, 12H; 4ꢂCH3),
1.25–1.41 (m, 48H; 24ꢂCH2), 2.21 (m, 8H; 4ꢂPH2), 2.84 (s, 6H; N-
Trihexyl(tetradecyl)phosphonium 2,4,6-trichlorophenolate ([P66614][2,4,6-
1
AHCTUNGTREN(GNUN CH3)2), 6.35 (m, 3H; C2H, C4H, and C6H), 6.94 (m, 1H; C5H);
Cl-PhO]): H NMR (400 MHz, CDCl3): d=0.89 (m, 12H; 4ꢂCH3), 1.29–
13C NMR (100 MHz, CDCl3): d=13.8, 18.4, 18.9, 21.6, 22.2, 22.5, 28.0,
28.8, 29.0, 29.2, 29.5, 30.1, 30.3, 30.5, 30.6, 30.8, 31.1, 31.7, 40.6, 101.3,
102.5, 105.5, 129.1,152.0, 160.5 ppm; IR: n˜ =2954, 2922, 2853, 1577, 1457,
1.44 (m, 48H; 24ꢂCH2), 2.24 (m, 8H; 4ꢂPH2), 7.03 ppm (s, 2H; C3H
and C5H); 13C NMR (100 MHz, CDCl3): d=13.8, 14.0, 18.4, 18.9, 21.7,
22.2, 22.6, 29.2, 29.4, 29.5, 30.2, 30.4, 30.5, 30.9, 108.9, 123.7, 126.9,
160.4 ppm; IR: n˜ =2956, 2924, 2855, 2362, 2337, 1563, 1487, 1475, 1379,
1240, 1155, 855, 794, 774, 728 cmÀ1. MS (ESI): m/z: 194.9 [2,4,6-Cl-PhO].
1347, 1247, 1147, 1161, 1057, 978, 834, 749, 719 cmÀ1
Trihexyl(tetradecyl)phosphonium 1-naphtholate
.
([P66614]ACHTUNGTRENNUNG[1-Naph]):
1H NMR (400 MHz, CDCl3): d=0.87 (m, 12H; 4ꢂCH3), 1.18–1.27 (m,
48H; 24ꢂCH2), 1.75 (m, 8H; 4ꢂPH2), 6.78 (m, 2H; C2H and C3H), 7.55
(d, 1H; C5H), 7.13 (m, 2H; C4H and C6H), 7.24 (m, 1H; C7H), 8.35 (d,
1H; C8H); 13C NMR (100 MHz, CDCl3): d=13.8, 17.9 18.4, 18.9, 21.3,
22.2, 22.5, 28.8, 29.1, 29.2, 29.4, 29.5, 30.0, 30.1, 30.3, 30.4, 30.9, 31.8,
109.4, 112.3, 122.2, 123.9, 124.9, 126.9, 127.6, 128.2, 135.4, 161.2 ppm; IR:
n˜ =2956, 2923, 2854, 1554, 1443, 1410, 1315, 1080, 1012, 963, 885,
767 cmÀ1. MS (ESI): m/z: 143.0 [1-Naph].
Trihexyl(tetradecyl)phosphonium 2-naphtholate [2-Naph]): 1H NMR
(400 MHz, CDCl3): d=0.86 (m, 12H; 4ꢂCH3), 1.24–1.34 (m, 48H; 24ꢂ
CH2), 2.10 (m, 8H; 4ꢂPH2), 7.07 (m, 1H; C3H), 7.10 (m, 2H; C1H and
C6H), 7.46 (m, 2H; C7H and C8H), 7.60 (m, 2H; C4H and C5H);
13C NMR (100 MHz, CDCl3): d=13.8, 18.4, 18.8, 21.5, 22.2, 22.5, 28.1,
28.8, 29.1, 29.2, 29.4, 29.5, 30.1, 30.2, 30.4, 30.6, 30.9, 31.2, 31.8, 109.4,
120.3, 120.8, 124.8, 126.7, 127.3, 128.6, 135.6, 160.1 ppm; IR: n˜ =2955,
2923, 2854, 1611, 1584, 1551, 1490, 1434, 1341, 1279, 1257, 1221, 1175,
1109, 1015, 981, 834, 811, 741, 724 cmÀ1. MS (ESI): m/z: 143.0 [2-Naph].
Trihexyl(tetradecyl)phosphonium m-methylphenolate ([P66614]AHCUTNEGRN[NUG 3-Me-
PhO]): 1H NMR (400 MHz, CDCl3): d=0.88 (m, 12H; 4ꢂCH3), 1.21–
1.43 (m, 48H; 24ꢂCH2), 2.22 (s, 3H; CH3), 2.25 (m, 8H; 4ꢂPH2), 6.67
(m, 1H; C4H), 6.94, (m, 1H; C5H), 7.01 (d, 1H; C6H), 7.20 ppm (d, 1H;
C3H); 13C NMR (100 MHz, CDCl3): d=13.6, 13.8, 18.4, 18.8, 21.4, 22.0,
22.4, 28.7, 29.0, 29.2, 29.4, 30.0, 30.2, 30.8, 112.8, 116.4, 118.7, 128.6, 138.6,
158.3; IR: n˜ =2956, 2924, 2855, 1581, 1559, 1468, 1416, 1378, 1319, 1303,
1255, 1164, 1150, 1104, 1054, 986, 860, 804, 767, 720 cmÀ1. MS (ESI): m/z:
107.0 [3-Me-PhO].
Trihexyl(tetradecyl)phosphonium
o-methylphenolate
([P66614]ACHTGNURTEN[NUNG 2-Me-
PhO]): 1H NMR (400 MHz, CDCl3): d=0.88 (m, 12H; 4ꢂCH3), 1.21–
1.44 (m, 48H; 24ꢂCH2), 2.20 (s, 3H; CH3), 2.24 (m, 8H; 4ꢂPH2), 6.52
(d, 1H; C6H), 6.78, (d, 1H; C4H), 6.80, (s, 1H; C2H), 7.20 ppm (m, 1H;
C5H); 13C NMR (100 MHz, CDCl3): d=13.7, 13.9, 18.6, 19.0, 21.6, 22.1,
22.5, 29.2, 29.3, 29.5, 30.1, 30.3, 30.9, 31.7, 115.3, 118.3, 123.9, 126.3, 130.2,
155.8 ppm; IR: n˜ =2956, 2924, 2855, 1585, 1468, 1378, 1299, 1104, 1054,
1038, 978, 879, 857, 751, 718 cmÀ1. MS (ESI): m/z: 107.0 [2-Me-PhO].
Trihexyl(tetradecyl)phosphonium 2,4-dimethylphenolate ([P66614][2,4-Me-
PhO]): 1H NMR (400 MHz, CDCl3): d=0.87 (m, 12H; 4ꢂCH3), 1.21–
1.46 (m, 48H; 24ꢂCH2), 2.16 (s, 6H; 2ꢂCH3), 2.36 (m, 8H; 4ꢂPH2),
6.74 (d, 1H; C6H), 6.83, (s, 1H; C3H), 7.02 ppm (d, 1H; C5H);
13C NMR (100 MHz, CDCl3): d=13.8, 14.0, 18.7, 19.2, 21.7, 22.2, 22.5,
28.8, 29.2, 29.4, 29.5, 30.2, 30.3, 30.9, 57.7, 115.7, 126.7, 130.9, 153.5 ppm;
IR: n˜ =2957, 2924, 2855, 1602, 1479, 1467, 1408, 1378, 1301, 1288, 1104,
1054, 873, 819, 785, 712 cmÀ1. MS (ESI): m/z: 121.1 [2,4-Me-PhO].
Acknowledgements
This work was supported by the National Natural Science Foundation of
China (No. 20976151, 21176205, and 20704035) and by the Fundamental
Research Funds of the Central Universities. The authors also gratefully
acknowledge the support of the division of Chemical Sciences, Geoscien-
ces, and Biosciences, Office of Basic Energy Sciences, U.S. Department
of Energy.
Trihexyl(tetradecyl)phosphonium 2,6-dimethylphenolate ([P66614][2,6-Me-
PhO]): 1H NMR (400 MHz, CDCl3): d=0.89 (m, 12H; 4ꢂCH3), 1.24–
1.51 (m, 48H; 24ꢂCH2), 2.28 (s, 6H; 2ꢂCH3), 2.43 (m, 8H; 4ꢂPH2),
6.72 (d, 1H; C4H), 6.95 ppm (d, 2H; C3H and C5H); 13C NMR
(100 MHz, CDCl3): d=13.8, 18.8, 19.3, 21.7, 22.2, 28.8, 29.2, 29.4, 29.5,
30.3, 30.4, 31.0, 31.8, 57.8, 119.4, 126.5, 128.3 ppm; IR: n˜ =957, 2924,
2855, 2364, 2339, 1584, 1559, 1462, 1420, 1369, 1328, 1294, 1102, 1086,
1053, 881, 843, 743, 720 cmÀ1. MS (ESI): m/z: 121.1 [2,6-Me-PhO].
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2008, 47, 1127–1129.
Trihexyl(tetradecyl)phosphonium p-isopropylphenolate ([P66614][4-iPr-
PhO]): 1H NMR (400 MHz, CDCl3): d=0.88 (m, 12H; 4ꢂCH3), 1.19 (d,
6H; 2ꢂCH3), 1.28–1.45 (m, 48H; 24ꢂCH2), 2.29 (m, 8H; 4ꢂPH2), 2.85
(m, 1H; CH), 6.91 (d, 2H; C2H and C6H), 6.97 ppm (d, 2H; C3H and
C5H); 13C NMR (100 MHz, CDCl3): d=13.7, 18.4, 18.9, 21.5, 22.1, 22.4,
24.1, 29.1, 29.4, 30.0, 30.2, 30.8, 31.6, 32.9, 115.3, 126.6, 138.2, 155.8 ppm;
IR: n˜ =2955, 2924, 2855, 1595, 1559, 1499, 1458, 1379, 1300, 1165, 1110,
1049, 833, 720 cmÀ1. MS (ESI): m/z: 135.1 [4-iPr-PhO].
Trihexyl(tetradecyl)phosphonium p-tertbutylphenolate ([P66614][4-tBu-
PhO]): 1H NMR (400 MHz, CDCl3): d=0.89 (m, 12H; 4ꢂCH3), 1.21–
1.46 (m, 48H; 24ꢂCH2), 1.23 (s, 9H; 3ꢂCH3), 2.34 (m, 8H; 4ꢂPH2),
6.93 (d, 2H; C2H and C6H), 7.14 ppm (d, 2H; C3H and C5H); 13C NMR
(100 MHz, CDCl3): d=13.7, 18.6, 19.0, 21.6, 22.1, 29.1, 29.3, 29.5, 30.1,
30.3, 30.8, 31.4, 114.9, 125.6, 140.9, 155.1 ppm; IR: n˜ =2956, 2924, 2855,
1595, 1501, 1460, 1413, 1390, 1360, 1301, 1264, 1173, 1105, 1054, 995, 833,
720 cmÀ1. MS (ESI): m/z: 149.1 [4-tBu-PhO].
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Trihexyl(tetradecyl)phosphonium m-methoxyphenolate ([P66614][3-MeO-
PhO]): 1H NMR (400 MHz, CDCl3): d=0.87 (m, 12H; 4ꢂCH3), 1.25–
1.42 (m, 48H; 24ꢂCH2), 2.20 (m, 8H; 4ꢂPH2), 3.68 (s, 3H; OCH3), 6.19
(m, 1H; C6H), 6.46 (s, 1H; C2H), 6.47 (m, 1H; C4H), 6.95 (m, 1H;
C5H); 13C NMR (100 MHz, CDCl3): d=13.7, 14.0, 18.4, 18.9, 21.6, 22.2,
28.8, 29.2, 29.4, 30.1, 30.3, 30.9, 31.7, 51.7, 102.2, 109.5, 129.2, 160.7,
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Received: October 3, 2011
Published online: January 13, 2012
2160
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