A study on the liquid-phase oxidation of toluene in ionic liquids

Article abstract of DOI:10.1016/j.apcata.2012.06.026

The effect and mechanism of the hydrophilic ionic liquids (ILs) with cobalt naphthenate as catalyst on the liquid-phase oxidation of toluene were studied systematically. Five ILs with different hydrophilicities were employed as reaction media. The results

Full text of DOI:10.1016/j.apcata.2012.06.026

Applied Catalysis A: General 439–440 (2012) 1–7
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
A study on the liquid-phase oxidation of toluene in ionic liquids
∗
Yan Meng, Bin Liang, Shengwei Tang
Multi-phases Transfer and Reaction Engineering Laboratory, College of Chemical Engineering, Sichuan University, Chengdu 610065, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect and mechanism of the hydrophilic ionic liquids (ILs) with cobalt naphthenate as catalyst on
the liquid-phase oxidation of toluene were studied systematically. Five ILs with different hydrophilicities
were employed as reaction media. The results showed that toluene conversion increased with increasing
hydrophilicity of ILs. The toluene conversion and the corresponding selectivity of benzaldehyde reached
Received 18 January 2012
Received in revised form 16 June 2012
Accepted 18 June 2012
Available online 4 July 2012
1
9.6% and 19.5%, respectively, in 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF4). The solu-
bilities of toluene, benzaldehyde, benzyl alcohol and benzoic acid in [Emim]BF4 were further measured at
temperatures from 299.2 K to 343.2 K. The correlations between solubility and temperature were devel-
oped based on the critical point of phase separation or ideal solution model. The low solubility of toluene
in [Emim]BF4 indicated a heterogeneous oxidation mechanism for toluene. The improvement of toluene
conversion and product selectivity in [Emim]BF4 was attributed to the interactions between reaction and
extraction separation. The sensitivity study demonstrated that the increase in oxygen pressure was ben-
Keywords:
Ionic liquids
Liquid-phase oxidation
Solubility
Toluene
Benzaldehyde
Benzyl alcohol
Benzoic acid
◦
eficial to the reaction. A much lower reaction temperature of 130 C was achieved in [Emim]BF instead
4
◦
of a typical 160–170 C for current commercial plants. The high conversion, low reaction temperature
and high product selectivity for the liquid-phase oxidation of toluene in [Emim]BF4 makes it a green and
efficient hydrocarbon oxidation process.
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
catalyst in asymmetric epoxidations. So far, ILs have been con-
sidered as environmentally benign media for oxidation reactions
in many chemical processes [5], such as epoxidation of alkenes
[6,7], oxidation of alcohols [8,9], catalytic oxidation of alkanes
[10] and aromatics [11,12]. Seddon (2002) studied the oxidation
of toluene in 1-butyl-2,3-dimethylimidazolium tetrafluoroborate
Ionic liquids (ILs) are known as alternative “green” solvents
owing to their unique physical properties, such as good thermal
stability, low volatility, low flash point, and tunable polarity. There-
fore, there has been an increasing interest in the last decades to
study ILs as reaction media, reagents or catalysts [1].
([Bdmim]BF ) and 1-butyl-3-methylimidazolium tetrafluorobo-
4
The oxidation of toluene is an important hydrocarbon oxidation
process. The products, including benzoic acid, benzyl alcohol and
benzaldehyde, areimportantintermediatesin theorganicsynthesis
industry. This reaction is usually allowed to proceed in acetic acid
solvent with cobalt carboxylate as catalyst. During the reaction, a
significant mass loss of solvent is inevitable by the formation of CO
rate ([Bmim]BF ), respectively [13]. The results demonstrated that
4
the liquid-phase oxidation of toluene catalyzed by a transition
metal was feasible in the ILs, although the conversion of toluene
was rather low under the unoptimized conditions. However, to the
best of our knowledge, only few studies have been documented for
the oxidation of toluene in ILs [14–16]. It is therefore of great signif-
icance to systematically study the oxidation of toluene in ILs for an
alternative efficient and green process of hydrocarbon oxidation.
Since there is a significant difference between the polarity of
toluene and benzoic acid, the polarity of the reaction medium
plays a crucial role in the liquid-phase oxidation of toluene. In
order to investigate the effects of the IL’s hydrophilicity on the
liquid-phase oxidation of toluene, five types of ILs with differ-
ent hydrophilicities were used as reaction media with cobalt
naphthenate as catalyst and oxygen as oxidant. The solubilities
of toluene, benzaldehyde, benzyl alcohol, and benzoic acid in 1-
and CO [2]. The corrosion and over-oxidation are also major indus-
2
trial challenges. In particular, over-oxidation is difficult to control
in the hydrocarbon oxidation process.
The application of ILs for oxidation reactions has attracted
considerable attention in recent years. In 2000, Howarth [3]
reported the oxidation of aromatic aldehydes in 1-butyl-3-
methylimidazolium hexafluorophosphate ([Bmim]PF ), and Song
6
[
4] utilized [Bmim]PF6 to recycle Jacobsen’s chiral (salen) Mn
ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF ) were
measured to determine mechanism of the improved conversion of
toluene and product selectivity in [Emim]BF4.
4
∗ Corresponding author. Tel.: +86 28 85460557; fax: +86 28 85460557.
E-mail addresses: mengyan0370@163.com (Y. Meng), liangbin@scu.edu.cn
B. Liang), tangdynasty@scu.edu.cn (S. Tang).
(
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.06.026

2
Y. Meng et al. / Applied Catalysis A: General 439–440 (2012) 1–7
Fig. 1. Schematic diagram of the apparatus for the liquid-phase oxidation of toluene.
2
. Materials and methods
2.1. Materials
Chemical reagents N-methylimidazole (99%), 1-chlorohexane
Fig. 2. Schematic diagram of solubility measurement experimental apparatus.
(
(
99%), bromoethane (99%), sodium fluoborate (98%), lithiumbis
trifluoromethanesulfonimide) (99%), butyl-3-methylimidazolium
(GC) (Shanghai Precision & Scientific Instrument Co., Ltd., Shang-
tetrafluoroborate ([Bmim]BF ) (99%), [Bmim]PF6 (99%), toluene
4
hai, PRC) with a flame ionization detector (FID) and a split injector.
The chromatography column was an FFAP column, with dimen-
sions of 30 m × 0.25 mm × 0.33 ␮m (Lanzhou Institute of Chemical
Physics, Chinese Academy of Sciences, Lanzhou, China). The anal-
ysis methods and procedures were similar to those described in
detail previously [20].
(
AR), benzaldehyde (AR), benzoic acid (AR), benzyl alcohol (AR), o-
nitrotoluene (CR), benzyl benzoate (AR), dimethyl phthalate (AR),
and cobalt naphthenate (8% Co), were purchased commercially and
were used without further purification.
[
Bmim]BF4 (≥99%) and [Bmim]PF6 (≥99%) were obtained from
Henan Lihua Pharmaceutical Co. Ltd. (Henan, China). [Emim]BF ,
4
1
-hexyl-3-methylimidazolium tetrafluoroborate ([Hmim]BF ) and
4
1
-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
2.3. Determination of the solubilities of toluene, benzyl alcohol,
benzaldehyde and benzoic acid in [Emim]BF4
(
[Hmim]Nf T) were prepared using the procedures described pre-
2
viously [17–19]. The synthesized ILs were confirmed by FT-IR
1
(
Nicolet -170sx) and H NMR (Bruker, AVII).
The experimental scheme is shown in Fig. 2. A flat-bottom flask
with a magnetic bar was immersed in a thermostatic water bath.
The solubility measurement of toluene, benzaldehyde, benzyl alco-
hol in [Emim]BF4 was performed by the following procedure. A
predetermined amount of [Emim]BF4 was added into the flask.
After the flask was heated to the desired temperature with con-
tinuous stirring, the solute of toluene, benzaldehyde or benzyl
alcohol was separately added dropwise into ILs intermittently using
a microsyringe over an interval of 30 min. The solution was consid-
ered saturated when it remained turbid for 30 min. The solubility
was determined by the weight change of the solution before and
after the dissolution of solute. The solubility values reported are
the mean values from the measurements repeated three times.
The solubility of benzoic acid in ILs was determined as follows:
a predetermined amount of benzoic acid was weighed and placed
in the flask. When the flask was heated to the desired tempera-
ture with continuous stirring, the solvent [Emim]BF4 was added
dropwise into the flask intermittently by the microsyringe over an
interval of 60 min. When the solute benzoic acid was dissolved in ILs
completely and the solution became clear, the solution was consid-
ered saturated with benzoic acid. The solubility was calculated by
the weight change of the solution. Each measurement was repeated
thrice to reduce the experimental errors.
2
.2. Liquid-phase oxidation of toluene in ionic liquids (ILs)
The reaction was performed in a batch reactor as shown
schematically in Fig. 1. A stainless steel autoclave with a height
of 100 mm and an inner diameter of 37 mm was used as the reac-
tor. The reactor was equipped with an electrical heater, a pressure
gauge, a thermocouple, a magnetic stirrer and a pressure relief
valve. The reaction temperature was controlled and monitored with
a Heating and Control System.
All experiments were performed in batch mode. Briefly, the
toluene, the cobalt naphthenate catalyst, ionic liquids (ILs) and the
benzaldehyde initiator were added into the reactor. The reactor
was subsequently sealed and oxygen was charged to the desired
pressure. After the oxygen inlet valve was closed and the stir-
ring started, the reactor was heated to the desired temperature.
The time when the reactor reached the desired temperature was
recorded as the start time of the reaction. The pressure was kept
constant with rapid oxygen makeup. After a predetermined reac-
tion time, the reaction was stopped by cooling the reactor with
tap water to room temperature. The gas in the reactor was slowly
released and the reaction liquid was then transferred to a separa-
tion funnel. The mixture was extracted thrice by 50 ml of diethyl
ether to separate toluene and reaction products from ionic liquids.
Diethyl ether was evaporated in a rotary evaporator. The result-
ing liquid samples were analyzed offline by gas chromatography
The solubility of benzoic acid in water at 30, 40, 50, 60 and
◦
70 C was measured to evaluate the accuracy and reliability of the
measurement method. In comparison with the literature data [21],
the maximum error was 3.47% and the average error was only

Y. Meng et al. / Applied Catalysis A: General 439–440 (2012) 1–7
3
Table 1
The toluene conversion and product selectivity with/without [Emim]BF4 as reaction
medium.
IL
Toluene
conversion (%)
Selectivity (%)
Benzyl alcohol
Benzaldehyde
Benzoic acid
None
10.0
13.1
13.7
4.9
23.3
19.7
55.6
69.0
[
Emim]BF4
◦
Reaction conditions: temperature 130 C, pressure 2.0 MPa, molar ratio of cobalt
napthenate/toluene 0.125%, mass ratio of [Emim]BF4/toluene 1.6, mass ratio of ben-
zaldehyde/toluene 0.75%, toluene 7.5 g and reaction time of 1 h.
continuously extracted by ILs from the toluene phase. Thus, a
coupling effect of reaction and separation is associated with the
improved conversion of toluene in the liquid-phase oxidation pro-
cess.
It has been reported that the liquid-phase oxidation of toluene
is a sequential reaction process [22]. As an intermediate product
of the reaction, benzaldehyde can be further oxidized to benzoic
acid. According to industrial data from Shijiazhuang Fiber Ltd. Co.
China, the selectivity of benzaldehyde is only about 5–8% with a
conversion of toluene of 15% [22]. However, the selectivity of ben-
zaldehyde is higher than 10% in this study, as shown in Fig. 3b.
The phenomenon is probably associated with the partial extrac-
tion of benzaldehyde by the ILs, and thus leading to the dilution
of benzaldehyde in the toluene phase. Since the further oxidation
of benzaldehyde is restrained, the selectivity of benzaldehyde is
therefore significantly improved. This result is consistent with the
previous findings by Seddon [13]. When the reaction was perform
◦
in [Bmim]BF at 80 C and 10 atm for 24 h with oxygen as oxidant, a
4
benzaldehyde yield of 4.5% is achieved [13]. Obviously, the reaction
temperature of this work was much higher than that of Seddon. It
was not surprising, therefore, to obtain a lower benzaldehyde yield.
The above results confirm that ILs are potential reaction media
for the liquid-phase oxidation of toluene. Because of the higher con-
version of toluene and better selectivity of benzaldehyde observed
Fig. 3. The results of liquid phase oxidation of toluene in different ILs: (a) the con-
version of toluene and (b) the selectivity of reaction products.
◦
Reaction conditions: toluene 1.9 g, temperature 150 C, pressure 2.0 MPa, molar ratio
in [Emim]BF , [Emim]BF4 was chosen as a representative reaction
4
of cobalt naphthenate/toluene 2.0%, mass ratio of ILs/toluene 200%, mass ratio of
benzaldehyde/toluene 2.0%, reaction time 6.0 h.
medium for the subsequent investigation.
3
.2. Liquid-phase oxidation of toluene with or without [Emim]BF4
1
.97%, indicating that the method was reliable. The relative error of
as reaction medium
repeated experiment is less than 5%, indicating the reproducibility
is acceptable.
To determine the role of ILs as reaction medium in the oxidation
reaction, the liquid-phase oxidation of toluene was conducted with
or without [Emim]BF . The results are summarized in Table 1 for
comparison purpose.
3
. Results and discussion
4
As compared to the typical industrial process without ILs, the
conversion of toluene and selectivity of benzoic acid are much
3.1. Liquid-phase oxidation of toluene in ILs
higher in [Emim] BF , suggesting that the oxidation reaction of
toluene can be improved using ILs as reaction medium. However,
the selectivities of benzaldehyde and benzyl alcohol in [Emim]BF4
are less than those without [Emim]BF4 as reaction medium. The
To determine the effect of ILs on the liquid-phase oxidation of
4
toluene, five ILs with different levels of hydrophilicity were used as
reaction media. The liquid-phase oxidation of toluene in different
◦
ILs was allowed to proceed in a batch reactor at 150 C and 2.0 MPa
result is probably attributed to the increase in toluene conversion.
It has been reported that the selectivity of intermediates decreased
upon prolonged reaction time or increased substrate conversion in
sequential reaction [22].
with cobalt naphthenate as catalyst and benzaldehyde as initiator.
As shown in Fig. 3a, an interesting result observed was that
the conversion of toluene in hydrophilic ILs was higher than
that in hydrophobic ILs. Regarding the structures of [Emim]BF4,
[
Bmim]BF4 and [Hmim]BF , the hydrophobicity of ILs increases
4
with the increase in side-chain length, while [Emim]BF4 and
Bmim]BF4 are both hydrophilic. The highest conversion ratio of
3.3. Effects of reaction parameters on the liquid-phase oxidation
of toluene in [Emim]BF4
[
toluene is obtained in [Emim]BF . When the reaction is performed
4
in hydrophobic ILs, toluene will be dissolved well in the ILs. The
reactant is therefore diluted by ILs and the conversion of toluene is
low. On the contrary, when the reaction is performed in hydrophilic
ILs, toluene is only partially miscible with the hydrophilic ILs
3.3.1. Effects of reaction pressure
In general, the oxygen concentration can control the reaction
rate and product selectivity in an oxidation reaction. In this study,
the oxygen concentration was changed by varying the operating
pressure to evaluate its effect on the liquid-phase oxidation of
(
Section 3.4). Benzoic acid has
a much larger solubility in
[
Emim]BF than toluene. As the reaction proceeds, the products are
toluene in [Emim]BF . Fig. 4 shows the conversion and product
4
4

4
Y. Meng et al. / Applied Catalysis A: General 439–440 (2012) 1–7
Fig. 4. Effects of oxygen pressure on the liquid phase oxidation of toluene in
[
Emim]BF4: (a) the conversion of toluene and (b) the selectivity of reaction products.
◦
Reaction conditions: temperature 140 C, mole ratio of cobalt naphthenate/toluene
Fig. 5. Effects of reaction temperature on the liquid phase oxidation of toluene in
Emim]BF4: (a) the conversion of toluene and (b) the selectivity of reaction products.
Reaction conditions: pressure 2.0 MPa, mass ratio of ILs/toluene 1.6, molar ratio of
cobalt naphthenate/toluene 1%, mass ratio of benzaldehyde/toluene 0.75%, toluene
1
7
%, mass ratio of ILs/toluene 1.6, mass ratio of benzaldehyde/toluene 0.75%, toluene
.5 g and reaction time 2 h.
[
7.5 g and reaction time 2 h.
selectivity of the liquid-phase oxidation of toluene under different
oxygen pressures.
reaction temperature. However, the toluene conversion decreases
It is clear that the conversion of toluene increases gradually with
increasing oxygen pressure. Since the oxygen solubility in toluene
is proportional to the oxygen pressure in the vapor space [23],
it is readily concluded that the oxygen concentration in reaction
mixtures increases upon increasing operating pressure. It has been
reported that the reaction of liquid-phase oxidation of toluene was
a first-order reaction with the oxygen concentration in the liquid
phase [20]. Therefore, the reaction rate increases linearly with oper-
ating pressure. The selectivities of benzaldehyde and benzyl alcohol
both undergo a slight decrease and the selectivity of benzoic acid
increases slightly with the increase in oxygen pressure. This is a
typical characteristic of a sequential reaction.
◦
◦
with the increase in reaction temperature from 140 C to 160 C.
The decrease of the toluene conversion could be explained by
the decrease in oxygen solubility with the increase in reaction tem-
perature. Another possible reason is the slight decomposition of
ILs at higher temperature. The selectivity of benzoic acid increases
with reaction temperature, while the selectivities of benzaldehyde
and benzyl alcohol decrease upon increasing reaction temperature.
Liquid-phase oxidation of hydrocarbon is a thermodynamically
favorable process. Since benzaldehyde and benzyl alcohol are read-
ily further oxidized to benzoic acid, increasing temperature is
favorable for them to be further oxidized, and thus resulting in the
decrease in the selectivity of benzaldehyde and benzyl alcohol.
3.3.2. Effects of reaction temperature
3.3.3. Effects of the concentration of ILs
It has been widely recognized that reaction temperature has a
Fig. 6 shows the conversion of toluene and product selectivity
of the liquid-phase oxidation of toluene under different concen-
crucial effect on the reaction rate and product selectivity. In this
study, the effects of reaction temperature on the liquid-phase oxi-
dation of toluene in [Emim]BF4 was investigated, and the results
are shown in Fig. 5.
trations of [Emim]BF . The conversion of toluene remains almost
4
constant with the increase in mass ratio of [Emim]BF /toluene from
4
1.6 to 2.9. The selectivity of benzaldehyde remains in the range of
22–25%. Under the experimental conditions, the oxidation reaction
is consistent with a heterogeneous mechanism. Toluene is con-
sumed in the toluene phase, while the products are extracted from
◦
◦
In a temperature range from 110 C to 140 C, the conversion
of toluene increases gradually with increasing reaction tempera-
ture, indicating that the reaction rate is positively correlated with

Y. Meng et al. / Applied Catalysis A: General 439–440 (2012) 1–7
5
Fig. 7. Effects of catalyst dosage on the liquid-phase oxidation of toluene: (a) the
Fig. 6. Effects of the concentration of [Emim]BF4 on the liquid-phase oxidation of
conversion of toluene and (b) the selectivity of reaction products.
toluene: (a) the conversion of toluene and (b) the selectivity of reaction products.
◦
◦
Reaction conditions: temperature 130 C, pressure 2.0 MPa, toluene 7.5 g, mass ratio
Reaction conditions: temperature 130 C, pressure 2.0 MPa, toluene 7.5 g, molar
of ILs/toluene 1.6, mass ratio of benzaldehyde/toluene 0.75% and reaction time 2 h.
ratio of cobalt naphthenate/toluene 1%, mass ratio of ILs/toluene 1.6, mass ratio
of benzaldehyde/toluene 0.75% and reaction time 2 h.
3
.4. Solubilities of toluene, benzaldehyde, benzyl alcohol and
the toluene phase by the IL phase. The reaction rate of the liquid-
phase oxidation is controlled by the reaction rate in the toluene
phase. On the other hand, the products are separated from the
toluene phase nearly instantly once generated, thus their further
oxidation is restrained. The above results indicate that the selectiv-
benzoic acid in [Emim]BF4
It is readily concluded from the above results that [Emim]BF4
used as reaction medium can improve the conversion and product
selectivity of the liquid-phase oxidation of toluene. To probe fur-
ther into the mechanism of these improvements, the solubilities of
benzoic acid, benzaldehyde, benzyl alcohol and toluene were inves-
tigated in a temperature range from 299.2 K to 343.2 K. The results
are shown in Fig. 8.
ity of benzaldehyde can probably be improved by using [Emim]BF4
as reaction medium.
3.3.4. Effects of catalyst dosage
Cobalt naphthenate was employed as catalyst for the liquid-
The solubility of benzoic acid in [Emim]BF4 is almost twice that
of benzaldehyde, benzyl alcohol or toluene. The polarity of ben-
zoic acid is much stronger than other products. It is not difficult to
phase oxidation of toluene. Fig. 7 shows the conversion of toluene
and product selectivity with different dosages of cobalt naph-
thenate. When no catalyst is loaded in the reaction system, the
conversion of toluene is very low. When the catalyst is added with
a catalyst/toluene molar ratio of 0.00125, the conversion of toluene
dramatically increases from 7.1% to 18.9%. Upon increasing the
molar ratio of catalyst/toluene from 0.00125 to 0.0050, the con-
version of toluene increases slightly. The maximum conversion of
toluene is achieved at a catalyst/toluene molar ratio of 0.0050. How-
ever, the selectivities of benzaldehyde and benzyl alcohol show
decreasing trends with the increase in catalyst dosage. The selec-
tivity of benzoic acid increases upon increasing catalyst dosage,
indicating that the catalyst dosage plays a crucial role in converting
toluene to benzoic acid.
explain the big solubility of benzoic acid in [Emim]BF based on the
4
rule of like dissolves like.
The solubilities of benzoic acid, toluene and benzyl alco-
hol increase with increasing temperature with a similar slope.
Although the solubility of benzaldehyde also increases with
increasing temperature, the slope is much smaller than those of
other products.
In this study, the mole fraction of toluene in the reaction sys-
tem is between 25% and 40%, indicating that toluene could not
be completely dissolved in [Emim]BF . Therefore, the liquid-phase
4
oxidation of toluene is a heterogeneous reaction. The products,
including benzoic acid, benzaldehyde and benzyl alcohol, are partly

6
Y. Meng et al. / Applied Catalysis A: General 439–440 (2012) 1–7
For benzaldehyde,
x (1 − x ) = 1.26416 × 10 5T2 − 0.0069T + 1.03085
−
(6)
1
1
The basic equation for predicting the saturation mole fraction of
a solid in liquids is as follows [24]:
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
Hfus (Tm)
RT
T
ꢀCp
R
Tm
T
ln x ꢁ = −
1 −
−
1 − T + ln
(7)
1
1
Tm
Tm
It is assumed that ꢀCp is negligible and the solution is consid-
ered an ideal solution, so Eq. (7) becomes,
ꢀ
ꢁ
ꢀ
Hfus (Tm)
RT
T
ln x1 = −
1 −
(8)
Tm
Define,
ꢀ
Hfus (Tm)
RTm
m =
ꢀ
Hfus (Tm)
R
Fig. 8. Solubilities of benzaldehyde, toluene, benzyl alcohol and benzoic acid in
n =
[Emim]BF4.
ꢀ
, ♦, ꢀ, ꢀ: Experimental solubilities of benzoic acid, benzaldehyde, toluene and
Then, Eq. (8) becomes,
benzyl alcohol respectively. —, - - -, ·······, –· –· –·: Fitting lines of the experimental
n
solubility of benzoic acid, benzaldehyde, toluene and benzyl alcohol respectively.
ln x1 = m − T
(9)
Eq. (9) is used to fit the experimental solubility of benzoic acid in
Emim]BF . From Fig. 8, it is seen that Eq. (9) fits the experimental
data very well. The values of m and n were obtained by fitting Eq.
extracted from the toluene phase to the [Emim]BF4 phase. More
benzoic acid can migrate from the toluene phase to the [Emim]BF4
[
4
phase due to its large solubility in the [Emim]BF . When the bulk
4
(9) to the experimental data to obtain.
temperature is higher than 333.2 K, the solubility of benzoic acid
in [Emim]BF4 is larger than 34.7 mol%. The extraction separation
leads to a decrease of the product concentrations in the toluene
phase, which is favorable for the forward chemical reactions and
increases the final extent of reactions. Since the reaction products
can be efficiently separated from the toluene phase, the over oxida-
tion of the products are reduced to some extent. The liquid-phase
oxidation of toluene in [Emim]BF4 is therefore a coupling process
between catalytic reaction and extraction separation.
In this study, thermodynamic criteria for phase equilibrium in
multi-component liquid systems were used to correlate the sol-
ubility with temperature. The critical point of phase separation
was used as a criterion for the measurement of the solubilities
of toluene, benzaldehyde and benzyl alcohol. The one-parameter
Margules model was used to describe the simulation. Therefore, at
the critical point of phase separation, the solubility and tempera-
ture should satisfy the following equation [24]:
1072.91021
ln x1 = 2.15133 −
(10)
T
4. Conclusions
Five ionic liquids (ILs) were used as reaction media for the liquid-
phase oxidation of toluene. The ionic liquids with strong polarity
substantially improved the reaction performance by increasing
both toluene conversion and benzaldehyde selectivity. On the con-
trary, the conversion of toluene was low when hydrophobic ILs such
as [Bmim]PF6 were used as reaction medium. The low solubility of
toluene in [Emim]BF4 indicates that the liquid-phase oxidation of
toluene in [Emim]BF4 is a heterogeneous reaction. The solubility
of reaction products, especially benzoic acid, is much higher than
that of toluene in [Emim]BF . The improvement of toluene con-
4
version and high product selectivity was attributed to the coupling
effect of catalytic reaction and extraction separation. The sensitivity
study revealed that increasing oxygen pressure was favorable for
RT = 2Ax (1 − x )
(1)
1
1
◦
the reaction, and the reaction performance at 130 C, which was
where A is the parameter; R the gas constant; T the temperature
◦
much lower than the typical reaction temperature of 160–170 C
(
K) and x1 is the mole fraction of solute in [Emim]BF4.
for commercial plants. The results confirm that [Emim]BF4 offers
a green and highly efficient approach to perform the liquid-phase
oxidation of toluene with high conversion of toluene, low reaction
temperature and high product selectivity.
The parameter A is a function of temperature. Simply, it is
assumed,
RT
A =
(2)
2
(aT2 + bT + c)
So,
Acknowledgments
2
x (1 − x ) = aT + bT + c
(3)
1
1
We would like to thank the Joint Funds of the National Science
Foundation of China and China National Petroleum Corporation
The correlation (3) is used to fit the solubilities of toluene, ben-
zaldehyde and benzyl alcohol. The results are shown in Fig. 8. Eq.
3) fits the experiment data well, and the different coefficients are
obtained for different solutes. The correlations are as follows:
For toluene,
(
Grant No. 20736009) and the Research Fund of Young Scholars
for the Doctoral Program of Higher Education of China (Grant No.
0070610128) for financial support.
(
2
Appendix A. Supplementary data
x (1 − x ) = −6.41186 × 10 6T2 + 0.00711T − 1.50964
−
(4)
(5)
1
1
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.apcata.2012.06.026.
For benzyl alcohol,
x (1 − x ) = −5.5047 × 10 6T2 + 0.00703T − 1.5802
−
1
1

Y. Meng et al. / Applied Catalysis A: General 439–440 (2012) 1–7
7
References
[13] K.R. Seddon, A. Stark, Green Chem. 4 (2002) 119–123.
[
[
[
[
14] M.J. Earle, K.S. Prabhakar, WO 02/30862 A1 (2002).
15] J. Tang, F. Yang, H. Zhu, G. Zou, M. He, CN1528726A (2003).
16] Y.H. Liu, P. Cui, J. Sun, F. Yang, J. Tang, Chem. J. Chinese U. 27 (2006) 2314–2318.
17] J.M. Crosthwaite, S.N.V.K. Aki, E.J. Maginn, J.F. Brennecke, Fluid Phase Equilibr.
[
[
[
[
[
1] H. Olivier-Bourbigou, L. Magna, D. Morvan, Appl. Catal. A 373 (2010) 1–56.
2] W. Partenheimer, Appl. Catal. A 409–410 (2011) 48–54.
3] J. Howarth, Tetrahedron Lett. 41 (2000) 6627–6629.
4] C.E. Song, E.J. Roh, Chem. Commun. (2000) 837–838.
228-229 (2005) 303–309.
[
18] J.G. Huddleston, A.E. Visser, W.M. Reichert, H.D. Willauer, G.A. Broker, R.D.
Rogers, Green Chem. 3 (2001) 156–164.
5] D. Betz, P. Altmann, M. Cokoja, W.A. Herrmann, F.E. Kühn, Coord. Chem. Rev.
2
55 (2011) 1518–1540.
6] H. Li, Z.S. Hou, Y.X. Qiao, B. Feng, Y. Hu, X.R. Wang, X.G. Zhao, Catal. Commun.
1 (2010) 470–475.
7] S. Gago, S.S. Balula, S. Figueiredo, A.D. Lopes, A.A. Valente, M. Pillinger, I.S.
Gon c¸ alves, Appl. Catal. A 372 (2010) 67–72.
[19] H.P. Zhu, F. Yang, J. Tang, M.Y. He, Green Chem. 5 (2003) 38–39.
[20] S. Tang, B. Liang, Ind. Eng. Chem. Res. 46 (2007) 6442–6448.
[21] G.Q. Liu, L.X. Ma, J. Liu, Hua Xue Hua Gong Wu Xing Shu Ju, Shou Ce, You Ji
Juan, Handbook of Physical and Chemical Properties for Chemistry and Chem-
ical Engineering, Organic Section, Chemical Industry Press, Beijing, 2002 (In
Chinese).
[
[
[
[
1
8] Y. Oda, K. Hirano, T. Satoh, S. Kuwabata, M. Miura, Tetrahedron Lett. 52 (2011)
5
392–5394.
[
22] S. Tang, B. Liang, P. Wu, Q. Zhang, C. Liu, Sichuan Daxue Xuebao (Gongcheng
Kexue Ban), J. Sichuan Univ. (Engineering Science Edition) 37 (2005) 76–81 (In
Chinese).
9] D. Ramakrishna, B.R. Bhat, R. Karvembu, Catal. Commun. 11 (2010) 498–501.
[10] J.Y. Wang, F.Y. Zhao, R.J. Liu, Y.Q. Hu, J. Mol. Catal. A: Chem. 279 (2008)
1
53–158.
[
[
23] A. Li, S. Tang, P. Tan, C. Liu, B. Liang, J. Chem. Eng. Data 52 (2007) 2339–2344.
24] S.I. Sandler, Chemical and Engineering Thermodynamics, 3rd ed., John Wiley &
Sons, New York, 1999.
[11] I. Yavari, E. Karimi, Synth. Commun. 39 (2009) 3420–3427.
[12] X.H. Zhao, A.G. Kong, C.C. Shan, P. Wang, X.H. Zhang, Y.K. Shan, Catal. Lett. 131
(
2009) 526–529.