Influence of temperature on hydroxylation of benzene to phenol using molecular oxygen catalyzed by V/SiO2

Article abstract of DOI:10.1016/j.catcom.2013.03.018

Effect of temperature on hydroxylation of benzene to phenol with molecular oxygen as an oxidant was studied over V/SiO₂ using different reductants. The V/SiO₂ with highly dispersed vanadium species was prepared by a sol-gel process a

Full text of DOI:10.1016/j.catcom.2013.03.018

Influence of temperature on hydroxylation of benzene to phenol using molec-
ular oxygen catalyzed by V/SiO2
Chao Guo, Weidong Du, Gang Chen, Lei Shi, Qi Sun
PII:
DOI:
Reference:
S1566-7367(13)00101-5
doi: 10.1016/j.catcom.2013.03.018
CATCOM 3443
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
18 January 2013
16 March 2013
20 March 2013
Please cite this article as: Chao Guo, Weidong Du, Gang Chen, Lei Shi, Qi Sun, Influence
of temperature on hydroxylation of benzene to phenol using molecular oxygen catalyzed
by V/SiO2, Catalysis Communications (2013), doi: 10.1016/j.catcom.2013.03.018
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ACCEPTED MANUSCRIPT
Influence of temperature on hydroxylation of benzene
to phenol using molecular oxygen catalyzed by V/SiO2
*
Chao Guo, Weidong Du, Gang Chen, Lei Shi, Qi Sun
Institute of Chemistry for Functionalized Materials, Faculty of Chemistry and
Chemical Engineering, Liaoning Normal University, Dalian 116029, Liaoning,
China
Tel: +86-0411-82159069
Fax: +86-0411-82156989
E-mail : sunqils@dl.cn (Qi Sun)
Abstract
Effect of temperature on hydroxylation of benzene to phenol with molecular
oxygen as an oxidant was studied over V/SiO using different reductants. The V/SiO2
2
with highly dispersed vanadium species was prepared by a sol-gel process and
characterized by diffuse reflectance UV-vis and ESR. This work shows that the onset
temperature range of benzene hydroxylation and the temperature reaching the
maximum phenol yield differ corresponding to each reductant. The reducing capacity
of reductant can be showed by changing temperature and affects, in turn, benzene
conversion, product selectivity, and the amount of leaching of V species.
1

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Keywords： Benzene; hydroxylation; molecular oxygen; reductant; phenol
1． Introduction
Direct oxidation of benzene to phenol without byproducts is an attractive
and challenging field from an economical and environmental point of view. A
variety of oxygen sources such as O [1], H O [2], and N O [3] are used for this
2
2
2
2
reaction. Among these oxygen sources, using the activation of O as an oxidant,
2
many catalysts can efficiently catalyze the oxidation of benzene to phenol, but
the reaction is truly catalytic only in the presence of a reductant able to recycle
the oxidized catalyst. In 1954, Udenfriend et al. [4] first reported the oxidation
of aromatics to produce phenol using the complex ferric sulfate-EDTA as a
catalyst with O and ascorbic acid as the oxidant and reductant, respectively.
2
From then on many reductants were tested to find a substitute for ascorbic acid.
o
Masumoto et al. [5] developed seven different reductants over V/Al O at 30 C
2
3
using O as oxidant, and concluded that all of reductants studied could not
2
efficiently substitute ascorbic acid. Similarly, Battistel et al. [6] tested seven
o
reductants at 50 C over VCl . Miyahara et al [7] studied eight reductants at 30
3
o
C over CuO-Al O . And they found that ascorbic acid was still the best
2
3
reductant. Thus，as reported in the current literature, the method for assessing
different reductants is at the identical temperature over a catalyst in
hydroxylation of benzene with O . One critical parameter that influences the
2
characteristic of a reductant should be reaction temperature. Therefore, we
2

ACCEPTED MANUSCRIPT
decided to carry out a study on the effect of temperature on hydroxylation of
benzene with different reductants, with the aim of providing a method to select
an effective reductant for hydroxylation of benzene using O as oxidant.
2
Furthermore, vanadium compounds have been reported to be an active catalyst
for phenol formation by liquid-phase catalyzed benzene oxidation using O as
2
the oxidant [5, 6, 8]. Herein, a V/SiO xerogel was used to screen a suitable
2
reductant.
2
. Experimental
2
.1. Catalysts preparation
3
The preparation of V/SiO was as follows: 18.61 cm of tetraethoxysilane
2
3
(Tianjin Kermel Chemical Reagent Co. Ltd, AR grade) was mixed with 19.44 cm
ethanol under stirring to form solution A. Solution B was prepared by dissolving
NH VO (0.5740g) (Tianjin Tianhe Chemical Reagent Factory, AR grade) and citric
4
3
acid monohydrate (4.0114g) (Tianjin Beifang Tianyi Chemical Reagent Factory, AR
3
grade) in 24 cm deionized water. Solution B was then added dropwise into solution A
o
o
under stirring at room temperature followed by aging at 50 C, drying at 110 C for 20
o
h and calcining at 500 C for 4 h. The vanadium loading per unit mass of the SiO2
matrix was 0.98mmol/g.
2.2. Catalyst Characterization
Diffuse reflectance UV-vis spectra were recorded in the range of 200-900 nm on
3

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a JASCO Corp V-550 spectrophotometer. The ESR spectrometer was recorded at
room temperature with a Bruker A 200 spectrometer. The UV-vis spectra of H O
2
2
formed in the reaction solution were measured by a Shimazu Model UV-240
spectrophotometer. Vanadium content was determined by inductively coupled plasma
(
ICP) on a Perkin Elmer Plasma 2000.
2
.3. Catalytic Tests
The catalytic experiments were carried out in a 75 ml steel autoclave. The
3
optimized reaction conditions were as follows: the catalyst (0.1 g), 10 cm of
3
aqueous acetic acid solvent, 1 cm of benzene (11.3 m mol) and reductant (3
mmol) were added to the reactor and stirred by a magnetic stirrer for 10 h under
1
MPa of O . After the reaction, the products were analyzed using Agilent-6890
2
gas chromatograph equipped with an FID using HP-5 capillary column (30
m×0.32 mm×0.25 μ m) and N as the carrier gas.
2
3
. Results and Discussion
3
.1. Influence of temperature on hydroxylation of benzene
We assessed the reductants at different reaction temperatures with V/SiO2
as a catalyst, and the results are summarized in Table 1. Each reductant
corresponds to two temperatures. “a” is the temperature of reaching the
maximum phenol yield, and “b” is the onset temperature range of hydroxylation
o
of benzene. Beginning at 20 C, ascorbic acid was the only one found to function
4

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as an effective reductant, and the phenol yield was 6.2% with selectivity of
o
1
00%. At the same temperature (20 C), no activity was observed with other
o
reductants investigated. When increasing the temperature from 20 to 40 C, the
onset of the catalytic reaction was observed using hydroquinone as the reductant.
However, benzoquinone was found to be the only product and 2.3% benzene
o
o
conversion was detected at 40 C. Also, with benzoquinone at 40 C, similar
o
phenomenon was observed. Further increasing temperature from 40 to 60 C,
with pyrocatechol as the reductant the formation of phenol was observed. The
phenol selectivity was 67.5% and remaining products were benzoquinone
o
(
5.5%), hydroquinone (7.8%) and other products (19.2%) at 60 C. Using oxalic
acid or glucose as the reductant, the direct oxygenation of benzene began in
higher temperature range. The yields of phenol were 3.3% using oxalic acid at
o
o
9
0 C and 4.9% using glucose at 100 C, respectively. Hence, ascorbic acid is the
best reductant at lower temperatures.
With each reductant the phenol yield gradually increased with increasing
temperature. The maximum phenol yields were reached at different temperatures
(Table1). The maximum phenol yield decreased in the following order: hydroquinone
o
o
o
o
(
120 C) > ascorbic acid (90 C) > oxalic acid (140 C) > benzoquinone (120 C) >
o
o
pyrocatechol (120 C) > glucose (120 C). Thus, hydroquinone is more effective
reductant at higher reaction temperatures. This suggests that elevated reaction
temperature is presumably required to show the reducing capacity of reductant.
Without the reductant the V/SiO showed no activity for phenol formation, indicating
2
5

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that the presence of the reductant is essential.
Benzoquinone is also an effective reductant (Figure 1A). The controlled
5
+
experiment indicated that benzoquinone can react with V from the V/SiO form
2
4
+
o
hydroquinone and V at > 80 C. Benzoquinone and hydroquinone may be
transformed into each other in the catalyst system (Figure 1B). Similar phenomenon
was also observed in other catalyst systems [9]. With either of reductants the catalyst
o
system showed the similar activity and 100% phenol selectivity at 120 C.
The percentage of leaching of V was obviously different with different
reductants (Table 2). For example, with hydroquinone as the reductant the
o
o
percentage of the leaching of V decreased from 88.7% at 40 C to 32.5% at 120 C.
The difference in leaching of V may be attributed to the difference in the nature of
reductant, such as coordination to the vanadium center and the reducing capacity.
Effect of the leaching of vanadium species on the yield of phenol was
investigated using the filtrate and the spend catalyst, respectively, at optimization
temperature corresponding to each reductant. With the filtrates, yields of phenol
were 1.8% for hydroquinone, 2.5% for ascorbic acid, 1.9% for oxalic acid, 1.3% for
pyrocatechol and 2.4% for glucose, respectively. Furthermore, unsupported V O5
2
(
NH VO ) was also used as the catalyst. Yields of phenol were 1.5%-5% (5% for
4 3
ascorbic acid) using the five reductants mentioned above. Using the spend V/SiO2,
yields of phenol using hydroquinone, ascorbic acid, oxalic acid, pyrocatechol and
glucose as the reductant were 9.6%, 7.5%, 5.2%, 7.8% and 6.1%, respectively. The
decrease of phenol yields may be due to the V leaching. Obviously, the
6

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heterogeneous catalysis plays a major role in the reaction.
3
.2. Characterization of catalyst
The colorimetric reaction was performed in a glass flask. A mixture of V/SiO2,
reductant, benzene, and aqueous acetic acid was heated under a N atmosphere and
2
o
the temperature was increased from 20 to 150 C. With ascorbic acid as the reductant,
o
the surface of the V/SiO in the solution at 20 C gave a green color, corresponding to
2
4
+
V species [10]. With other reductants the surface of the V/SiO also showed green
2
o
color at higher temperatures, i.e. 30, 60, 80 and 100 C with hydroquinone,
pyrocatechol, oxalic acid and glucose, respectively. The change in color caused by
variation in temperature accords with the onset temperature range corresponding to
each reductant.
4
+
The presence of V species in the V/SiO can be confirmed by the diffuse
2
reflectance UV-vis and ESR spectra. Taking hydroquinone as example, the results of
the characterization of the fresh and hydroquinone-reduced V/SiO are shown in
2
Figure 2 and 3. In Figure 2, for fresh V/SiO , the bands in the range of 250-300 nm
2
should be assigned to charge transfer transitions involving oxygen and vanadium (V)
in tetrahedral coordination, present as isolated species [11]. The absorption band in
the region of 333-500 nm is associated with the lower-energy charge transfer of
5
+
O-V electron transfer for octahedral coordination [12]. When fresh V/SiO was
2
o
reduced in hydroquinone at 40 C for 2h, two bands at around 250 and 282 nm are
5
+
observed. The band at 250 nm was assigned to an isolated tetrahedral V [11]. The
7

ACCEPTED MANUSCRIPT
4
+
band at 282 nm is attributed to the lower-energy change transfer of O-V electron
transfer [13]. For the fresh V/SiO , the eight-line spectrum with very low intensity
2
IV
1
was observed (Figure 3), which is attributed to vanadyl V O (d ) species [6, 14].
After reduction, the signal becomes well-resolved and the intensity of the signal
4+
increases, which indicates the V complexes appear to be isolated and highly
dispersed [15]. Thus, the vanadium species on the surface of the V/SiO are highly
2
dispersed.
To gain insight into the reaction mechanism, the formation of H O was directly
2
2
measured via UV-vis spectroscopy of the reaction solution with different reductants.
The results in Figure 4 indicated that the appearance of an absorption peak at ca.220
nm with each reductant (curves b-f), this absorption peak based on H O [16]. Without
2
2
reductant, the absorption peak at 220 nm was silent (curve g). Therefore, the present
V/SiO catalytic system using both O and reductant is considered to proceed via a
2
2
5+
reaction scheme similar to a Fenton type reaction [16-18]: V species in the V/SiO
2
4
+
was reduced to V species by reductant. H O was formed by the reaction of the
2
2
4
+
V species with O and a proton. Hydroxyl radicals produced by the reaction of H O
2
2
2
4
+
with the V species and protons are considered to participate in the phenol formation.
4
. Conclusions
In spite of the differences in the reduction properties, all the tested reductants
were found to exhibit excellent reactivity for hydroxylation of benzene at the
selective temperature. It was suggested that screening reductant should be carried out
8

ACCEPTED MANUSCRIPT
at different temperatures. With each reductant the selectivity to phenol can be up to
1
00% corresponding to the maximum benzene conversion. Also, benzoquinone is also
an effective reductant, and benzoquinone and hydroquinone may be transformed into
each other. With each reductant, H O was formed and hydroxylation scheme was
2
2
similar to a Fenton type reaction.
Acknowledgement
This work was supported by the National Natural Science Foundation of China
21173110)
(
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List of Tables
Table 1 Effect of various reductants on hydroxylation of benzene over V/SiO2
a
the temperature of reaching the maximum phenol yield
b
the onset temperature range of hydroxylation of benzene
1
0

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c
benzoquinone selectivity is 100%
Reaction conditions: 0.1 g V/SiO , 1 ml benzene, 3 m mol reductant, 7 ml acetic
2
acid, 3 ml H O, 1 MPa O , and 10 h. All of oxidation reactions were carried out
2
2
with fresh catalyst each time.
Table 2 Influence of reaction temperature on the percentage of V leaching
Reaction conditions are the same as table 1
The percentage of V leaching equals the difference of V contents between fresh and
spent V/SiO divided by V content of fresh V/SiO .
2
2
List of Figures
Figure 1 Effect of benzoquinone (A) or hydroquinone (B) as the reductant on the
oxidation of benzene to phenol over V/SiO2
a. Selectivity to phenol (%)
b. Conversion of benzene (%)
c. Selectivity to benzoquinone (%)
Reaction conditions are the same as table 1
11

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Figure 2 Diffuse reflectance UV-vis spectra of (a) fresh V/SiO ； (b)
2
hydroquinone–reduced V/SiO2
Figure 3 ESR spectra of (a) fresh V/SiO ；(b) hydroquinone–reduced V/SiO2
2
Figure 4 UV-vis spectra of H O formed from the reaction solution with
2
2
different reductants
a. aqueous solution of H O (8×10 mol/l)
-6
2
2
b. glucose ; c. pyrocatechol ; d. hydroquinone； e. ascorbic acid;
f. oxalic acid ;
g. without reductant
Reaction conditions are the same as table 1
1
2

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1
00
100
80
60
40
20
0
A
8
6
4
2
0
0
0
0
0
a
b
4
0
60
80
100
120
o
Temperature ( C)
1
00
100
B
8
6
4
2
0
0
0
0
0
80
60
40
20
0
c
a
b
4
0
60
80
100
120
140
o
Temperature ( C)
Figure1 Effect of benzoquinone (A) or hydroquinone (B) as the reductant on the
oxidation of benzene to phenol over V/SiO2
a. Selectivity to phenol (%);
b. Conversion of benzene (%)；
c. Selectivity to benzoquinone (%)
Reaction conditions are the same as table 1
1
3

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1
0
0
0
0
0
. 0
. 8
. 6
. 4
. 2
. 0
(
a )
(
b )
200
300
400
500
600
700
800
900
W a v e l e n g t h ( n m )
Figure 2 Diffuse reflectance UV-vis spectra of (a) fresh V/SiO ；(b)
2
hydroquinone-reduced V/SiO2.
1
4

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(a)
(
b)
2000
3000
Gauss
4000
5000
Figure 3 ESR spectra of (a) fresh V/SiO ；(b) hydroquinone-reduced
2
V/SiO2.
1
5

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a
b
c
g
d
e
f
2
00
250
300
350
400
wavelength(nm)
Figure 4 UV-vis spectra of H O formed from the reaction
2
2
solution with different reductants
-6
a. aqueous solution of H O (8×10 mol/L)
2
2
b. glucose ; c. pyrocatechol ; d. hydroquinone
e. ascorbic acid; f. oxalic acid ; g. without reductant
Reaction conditions are the same as table 1
1
6

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Table 1
Effect of various reductants on hydroxylation of benzene over V/SiO2
Reaction
Benzene
conversion
(%)
Phenol
selectivity
(%)
Phenol
yield
(%)
6.2
Reductant
temperature
o
(
C)
2
9
0
0
6.2
13.7
100
Ascorbic acid
a
100
13.7
0
c
2
.3
0.0
4
0
b
Hydroquinone
(20-40)
a
1
20
14.3
11.8
100
14.3
0
c
4
0
0.0
b
Benzoquinone
Pyrocatechol
Oxalic acid
(20-40)
a
1
20
10.0
13.4
100
67.5
10.0
9.0
6
0
b
(40-60)
a
1
20
9.7
3.3
100
100
9.7
3.3
9
0
b
(60-90)
a
1
40
00
(90-100)
10.1
4.9
100
100
10.1
4.9
1
b
Glucose
a
1
20
9.5
100
9.5
a
the temperature of reaching the maximum phenol yield
the onset temperature range of hydroxylation of benzene
benzoquinone selectivity is 100%
b
c
Reaction conditions: 0.1 g V/SiO (5 wt%), 1 ml benzene, 3 m mol reducing
2
agent, 7 ml acetic acid, 3 ml H O, 1 MPa O , and 10 h. All of oxidation
2
2
reactions were carried out with fresh catalyst each time.
1
7

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Table 2
Influence of reaction temperature on the percentage of V leaching
o
Reductant
Reaction temperature ( C)
Leaching of V (%)
77.4
2
9
0
0
Ascorbic acid
63.4
88.7
32.5
51.0
19.5
89.3
30.6
41.6
87.1
4
0
Hydroquinone
Pyrocatechol
Oxalic acid
Glucose
1
20
6
0
1
20
9
0
1
40
00
20
Reaction conditions are the same as table 1.
1
1
The percentage of V leaching equals the difference of V contents between fresh and
spent V/SiO divided by V content of fresh V/SiO .
2
2
1
8

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Graphical abstract
1
9

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Highlights
1.
V/SiO with highly dispersed vanadium species was prepared by sol-gel process
2
2
3
4
5
. Studying the effect of temperature on benzene oxidation with O over V/SiO2
2
. Reducing capacity of reductant can be showed by changing temperature
. Both of reductants benzoquinone and hydroquinone can be interconvertible
. Presenting 100% selectivity to phenol with each reductant
2
0