Direct hydroxylation of benzene to phenol using H2O2 as an oxidant over vanadium-containing mesoporous carbon catalysts

Article abstract of DOI:10.1016/j.apcata.2014.10.056

A series of vanadium-containing mesoporous carbon catalysts were synthesized by the co-assembly method using phenolic resol, vanadyl acetylacetonate [VO(acac)₂], and Pluronic F127 as carbon source, metal source, and template, respectively. The

Full text of DOI:10.1016/j.apcata.2014.10.056

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Applied Catalysis A: General
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Direct hydroxylation of benzene to phenol using H2O2 as an oxidant
over vanadium-containing mesoporous carbon catalysts
Liya Hu, Cheng Wang, Lin Ye, Yanan Wu, Bin Yue∗, Xueying Chen, Heyong He∗
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for
Energy Materials, Fudan University, Shanghai 200433, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of vanadium-containing mesoporous carbon catalysts were synthesized by the co-assembly
method using phenolic resol, vanadyl acetylacetonate [VO(acac)2], and Pluronic F127 as carbon source,
metal source, and template, respectively. The effects of the calcination temperature on the transforma-
tion of vanadium species and the carbon framework were investigated. During the calcination treatment,
the nanoparticles of vanadium species mainly confined in the carbon matrix were formed along with the
further cross-linking and carbonization of the phenolic resol network. The obtained catalysts exhibited
the two-dimensional hexagonal mesostructures, high surface areas, and uniform pore sizes. Their cat-
alytic activity in the benzene hydroxylation reaction was tested and the 0.2-V-C-600 catalyst showed the
high performance with the benzene conversion of 31.8% and phenol selectivity of 94.9% as well as the
good stability in recycles.
Received 9 September 2014
Received in revised form 23 October 2014
Accepted 30 October 2014
Available online xxx
Keywords:
Benzene hydroxylation
Mesoporous carbon
Vanadium
Phenol
H2O2.
© 2014 Published by Elsevier B.V.
1. Introduction
such as VxOy@C [14] and multi-walled carbon nanotubes (MWC-
NTs) supporting catalysts Fe3O4/MWCNTs [15] and VO2/MWCNTs
Phenol, as an important intermediate, is produced mainly
through the three-step cumene process, which suffers from a low
one-pass yield of phenol, a low atomic efficiency, and an equimolar
amount of acetone byproduct [1,2]. An alternative way is the direct
hydroxylation of benzene to phenol under mild reaction conditions
using clean oxidants such as H2O2 [3–6]. Therefore, it is necessary
to probe the new catalysts with high activity and selectivity for phe-
nol. Recently, it was found that the benzene adsorption ability of the
catalysts had a great influence on phenol selectivity and yield [7,8].
By enhancing the affinity of the catalyst surface with non-polar
benzene and weakening that with polar phenol, an improvement
of phenol yield and selectivity could be achieved. Our previous
study also indicated that the strong benzene adsorption ability
over the catalyst could facilitate catalytic performance [9,10].
Porous carbon materials, such as activated carbon, carbon
microspheres, and carbon nanotubes, are widely used as adsor-
bents and catalyst supports due to their remarkable properties
such as high surface areas, large pore volumes, chemical inertness,
and good stability [11–13]. Recently, carbon supporting catalysts,
[16], have shown good performance in the direct hydroxylation
of benzene to phenol. However, since widely used porous car-
bon materials have mainly micropores or dominant disordered
mesopores, it is very attractive to synthesize carbon materials
with larger and more controlled pores to meet the requirement
for desired catalytic reactions. As a kind of novel carbon material,
ordered mesoporous carbons have been demonstrated as excel-
lent catalyst supports due to the uniform and tunable pore size,
regularly aligned pore architecture, high surface area, large pore
volume, and good thermal stability [17,18]. Thus, it is interest-
ing to integrate ordered mesoporous carbon with active metal
species to obtain the catalyst with highly accessible active sites
and good adsorption ability. Kong et al. reported a co-assembly
route to synthesize the MnO incorporated ordered mesoporous
carbon catalysts [19]. These MnO nanoparticles, which are stably
embedded or confined in the carbon matrix, showed remarkable
catalytic performance and good stability in wet oxidation of phe-
nol using H2O2 as the oxidant. As for the benzene hydroxylation
reaction, vanadium-based catalysts have been widely investigated
[20–23]. The hydrophobicity of the mesoporous carbon materials
is expected to be beneficial to the adsorption of non-polar benzene,
enhancing the catalytic activity according to our previous study
[9,10]. Therefore, it is anticipated that the integration of ordered
mesoporous carbons with vanadium species holds great promise
∗
Corresponding authors. Tel.: +86 21 65643916.
E-mail addresses: yuebin@fudan.edu.cn (B. Yue), heyonghe@fudan.edu.cn
(H. He).
http://dx.doi.org/10.1016/j.apcata.2014.10.056
0926-860X/© 2014 Published by Elsevier B.V.
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in developing the catalysts with highly accessible active sites
stably confined in the matrix, which probably leads to a further
improvement of benzene conversion and phenol yield.
yield of phenol were calculated as the molar ratio of the converted
benzene to its initial one, the formed phenol to the converted ben-
zene, and the formed phenol to the initial benzene, respectively.
Herein, we synthesized
a series of vanadium-containing
ordered mesoporous carbon catalysts by the co-assembly method.
The catalysts were characterized by various techniques and their
catalytic performances in the direct benzene hydroxylation using
H2O2 as an oxidant were investigated. The transformations of
vanadium species and the carbon framework during calcination
were also studied.
2.4. Catalyst characterization
The X-ray diffraction (XRD) patterns were recorded on a Bruker
D8 Advances X-ray diffractometer using Cu-K␣ radiation with a
voltage of 40 kV and a current of 40 mA. N2 adsorption–desorption
isotherms were obtained at −196 ◦C using a Micromeritics Tri-
star 3000 apparatus. The transmission electron microscopy (TEM)
images were obtained from a JEOL JEM2011 microscope oper-
ated at 200 kV. The high resolution TEM (HRTEM) measurements
were conducted on a FEI tecnai G2 F20 S-TWIN microscope
operated at 200 kV. The 13C solid state NMR experiments were
performed on a Bruker AVANCE III 400WB spectrometer by cross
polarization with total suppression of sidebands (CP-TOSS) at a
frequency of 100.7 MHz and a magic angle spinning (MAS) rate
of 7 kHz. The chemical shifts of 13C NMR spectra were referenced
to tetramethylsilane (TMS). The FT-IR spectra were recorded on a
Nicolet Nexus 470 infrared instrument using KBr discs. XPS spec-
tra were recorded on a Perkin-Elmer PHI 5000 C ESCA system
equipped with a dual X-ray source by using Mg K␣ (1253.6 eV)
anode and a hemispherical energy analyzer. All binding ener-
gies were calibrated with contaminant carbon (C1s = 284.6 eV)
as a reference. Elemental analysis was performed on a Thermo
2. Experimental
2.1. Synthesis of resol precursor
Soluble resol precursor was prepared by using phenol and
formaldehyde in a base-catalyzed process according to the proce-
dure early reported [24]. In a typical procedure, 10.0 g of phenol
was melted at 45 ◦C, and 2.13 g of 20 wt% NaOH aqueous solution
was added slowly under stirring. After 10 min, 17.7 g of forma-
lin solution (37 wt% formaldehyde) was added dropwise. Then the
reaction mixture was heated to 70 ◦C and stirred for another 60 min.
Upon cooling the mixture to room temperature, the pH value was
adjusted to about 6.0 using 2.0 M HCl solution. Then rotary evap-
oration was conducted to remove water, followed by drying the
product at 50 ◦C. At room temperature, the final product was dis-
solved in ethanol and NaCl was filtered out as a precipitate to give
the final soluble resol precursor (20 wt% ethanol solution).
2.2. Synthesis of mesoporous carbon composites
The vanadium-containing mesoporous carbon composites were
prepared via co-assembly of resol precursor, VO(acac)2, and
Pluronic F127. Typically, 1.0 g of Pluronic F127 and 14.0 g of ethanol
were stirred at 50 ◦C for 30 min to form a clear solution. Then a cer-
tain amount of VO(acac)2 (0.1, 0.2, and 0.3 g) and 7.5 g of the resol
precursor solution (20 wt% in ethanol) were added. After further
stirred for 1 h, the mixture was poured into a Petri dish to evapo-
rate ethanol at room temperature for 6 h, followed by heating in
an oven at 120 ◦C for 24 h. The obtained films were scraped off
and ground into powder. Calcination of the powder was carried
out in a tube furnace at different temperatures (300–600 ◦C) for 3 h
under nitrogen atmosphere with a flow rate of 100 mL min−1. The
calcination started from room temperature with a heating rate of
1 ◦C min−1. Before heating, the tube furnace was purged with nitro-
gen gas for 1 h to remove air. The final products were denoted as
x-V-C-y, wherein x and y referred to the amount of VO(acac)2 added
and the calcination temperature, respectively.
2.3. Benzene hydroxylation reaction
The catalytic performance of the catalysts was evaluated at 70 ◦C
by the reaction of benzene and H2O2 using 80 wt% acetic acid as a
solvent. A typical reaction was carried out as follows: 20 mg of cata-
lyst, 0.4 mL of benzene, and 5 mL of 80 wt% acetic acid were added in
a 25 mL of three-necked round bottom flask connected with a reflux
condenser. After stirring at 70 ◦C for 30 min, 1.4 mL of 30 wt% H2O2
was added to the flask dropwise. Then the reaction mixture was
stirred for another 3 h. After reaction, the catalyst was separated
by centrifugation. The content of liquid products was analyzed by
GC9560 gas chromatography (Shanghai Hua-Ai Chromatography
Analysis Co. Ltd.) equipped with a flame ionization detector and a
HP-5 capillary column (0.32 mm × 30 m × 0.25 ␮m, Agilent, USA).
The products were confirmed by comparing the retention time of
the standard samples. The quantitative analysis of the mixture was
determined by the calibration curves and using toluene as the inter-
nal standard. The conversion of benzene, the selectivity, and the
Fig. 1. Small-angle (A) and wide-angle (B) XRD patterns of (a) 0.1-V-C-600, (b) 0.2-
V-C-600, and (c) 0.3-V-C-600.
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Table 1
Structural and textural properties of x-V-C-600 samples.
b
c
d
Samples
Va (wt%)
ao (nm)
SBET (m2 g−1
)
Vpore (cm3 g−1
)
Dpore (nm)
WT (nm)
0.1-V-C-600
0.2-V-C-600
0.3-V-C-600
2.8
4.0
4.5
13.2
13.2
12.5
638
561
514
0.53
0.44
0.37
3.6
3.6
3.6
9.6
9.6
8.9
a
Vanadium content analyzed by ICP-AES.
Unit cell parameters calculated from small-angle XRD patterns.
Pore diameters derived from the desorption branches using BJH method.
Wall thickness values calculated from WT = a0 − Dpore
b
c
d
.
Elemental IRIS Intrepid ICP atomic emission spectrometer. The
thermal decomposition behavior of VO(acac)2 was determined by
a Perkin Elmer TGA7/DTA7 thermogravimetric instrument from
30 to 800 ◦C under nitrogen atmosphere with a heating rate of
to the (1 0 0) reflection of the 2D hexagonal symmetry despite the
absence of resolved higher order Bragg reflections [19]. The slightly
deteriorated hexagonal structures may be caused by the incor-
poration of vanadium species. Also, the peak intensity decreases
dramatically with a high vanadium amount, implying the further
deterioration of the ordered mesostructure. The unit cell parame-
ters are calculated to be 13.2, 13.2, and 12.5 nm for the 0.1-, 0.2-,
and 0.3-V-C-600 samples, respectively (Table 1). Additionally, the
wide-angle XRD patterns for all samples (Fig. 1B) exhibit no diffrac-
tion peak of any crystalline phase of vanadium species. The broad
diffraction peak at about 43◦ can be attributed to the diffraction of
carbon materials [17].
10 ◦C min−1
.
3. Results and discussion
3.1. Characterization of x-V-C-600 samples
As shown in Fig. 1A, the small-angle XRD patterns of the x-V-C-
600 samples show one well-resolved peak, which can be indexed
Fig. 2. TEM images of (a) 0.1-V-C-600, (b) 0.2-V-C-600, and (c, d) 0.3-V-C-600.
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Table 2
Structural and textural properties of 0.2-V-C-y samples.
a
b
c
d
e
Samples
ao (nm)
SBET (m2 g−1
)
Vpore (cm3 g−1
)
d1 (nm)
d2 (nm)
Dpore (nm)
WT (nm)
0.2-V-C-300
0.2-V-C-400
0.2-V-C-500
0.2-V-C-600
14.8
14.2
13.9
13.2
241
420
540
561
0.34
0.45
0.51
0.44
3.8
3.8
3.6
3.6
5.6
4.8
5.1
5.6
3.8
3.6
3.6
9.2
10.4
10.3
9.6
a
c
Unit cell parameters calculated from the small-angle XRD patterns. b, Pore diameters of two modal pores. d Pore diameters derived from the desorption branches using
the BJH method. e Wall thickness values calculated from WT = a0 − Dpore
.
N2 adsorption–desorption isotherms of the x-V-C-600 samples
show representative type IV isotherms with pronounced H2 hys-
teresis loops and distinct capillary condensation steps at relative
pressures of 0.4–0.65, reflecting the typical mesoporous structures
with uniform pore sizes (Fig. S1A) [25]. The pore size distribution
curves derived from the desorption branches using the BJH model
exhibit a narrow peak centered at 3.6 nm in all samples (Fig. S1B).
The BET surface areas and pore volumes, which are in the range
of 638–514 m2 g−1 and 0.53–0.37 cm3 g−1, respectively, decrease
gradually with the increase of vanadium content (Table 1).
All x-V-C-600 samples show stripe-like TEM images, further
confirming the 2D hexagonal ordered mesostructures (Fig. 2). The
highly dispersed nanoparticles inside the carbon matrix are vana-
dium species (the dots in the red circles in Fig. 2a and the dark
dots in Fig. 2b–d), which are about 6–7 nm in size, regardless of the
vanadium content. Although the wide-angle XRD patterns exhibit
no crystalline peaks of vanadium species, the HRTEM images (Fig.
S2) show the poorly crystallized vanadium nanoparticles confined
in the carbon matrix. The low crystallization degree, the high dis-
persion of vanadium species, and the low vanadium content (less
than 5%) lead to the absence of XRD signals. Also, it should be
noted that the nanoparticle size is larger than the average pore
size (3.6 nm) but smaller than the carbon wall thickness (∼9 nm).
Considering that the amount of vanadium species located on the
external surface of the carbon framework is negligible, it is reason-
able to deduce that most of the nanoparticles partially penetrate
into the carbon walls and are embedded in carbon matrix to present
a semi-exposure state in the mesopores.
3.2. Effects of the calcination temperature on the structures of
0.2-V-C-y samples
To further elucidate the transformations of vanadium species
and carbon framework during calcination, an elaborate study was
carried out over 0.2-V-C-y samples calcined at different tempera-
tures.
The small-angle XRD patterns of the 0.2-V-C-y samples show a
well-resolved (1 0 0) diffraction peak (Fig. 3A), indicating the exist-
ence of the ordered mesostructure. The peak shifts to the higher
angle range as the calcination temperature increases. The unit
cell parameters are calculated to be 14.8, 14.2, 13.9, and 13.2 nm
for the 0.2-V-C-300, -400, -500, and -600 samples, respectively
(Table 2), implying the contraction of the framework [17]. Simi-
larly, all the 0.2-V-C-y samples display no diffraction peak of any
crystalline phase of vanadium species in the wide-angle XRD pat-
terns (Fig. 3B). The N2 adsorption–desorption results of 0.2-V-C-y
samples are shown in Fig. S3 and the related textural properties
are summarized in Table 2. All the samples exhibit the similar type
IV isotherms with the H2 hysteresis loops (Fig. S3A), indicating the
ordered mesoporous structures. The difference among these sam-
ples is that the capillary condensation step shifts to a lower relative
pressure with the increase of calcination temperature, which is
related to the reduction of pore size. Indeed, the primary pore size
of 0.2-V-C-y samples calculated from the BJH model decreases from
5.6 to 3.6 nm as the calcination temperature increases from 300 to
Fig. 3. Small-angle (A) and wide-angle (B) XRD patterns of (a) 0.2-V-C-300, (b) 0.2-
V-C-400, (c) 0.2-V-C-500, and (d) 0.2-V-C-600.
600 ◦C (Table 2). In addition, the 0.2-V-C-y samples calcined lower
than 600 ◦C exhibit the bimodal pore size distribution curves (Fig.
S3B), which is probably caused by the incomplete template decom-
position and the phenolic resol carbonization. On the other hand,
the BET surface area and the pore volume of 0.2-V-C-y samples,
which are in the ranges of 241–561 m2 g−1 and 0.34–0.51 cm3 g−1
(Table 2), respectively, both generally increase with the rise of the
calcination temperature. These results suggest the decomposition
of the template and the carbonization of the framework during the
calcination treatment.
As displayed in Fig. 4, TEM images of 0.2-V-C-y samples show
stripe-like pores, further confirming the ordered mesostructures. It
can be observed that there are no visible nanoparticles of vanadium
species in 0.2-V-C-300 while very small nanoparticles (the dots in
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Fig. 4. TEM images of (a) 0.2-V-C-300, (b) 0.2-V-C-400, (c) 0.2-V-C-500, and (d) 0.2-V-C-600.
the red circles) appear in the carbon matrix in 0.2-V-C-400. In the
0.2-V-C-500 and 0.2-V-C-600 samples calcined at higher temper-
atures, highly dispersed nanoparticles of vanadium species about
6–7 nm in size are formed. The HRTEM images (Fig. S4) display that
the vanadium nanoparticles are poorly crystallized in the 0.2-V-C-
500 and 0.2-V-C-600 samples while no crystallization is obtained
in the case of 0.2-V-C-300 and 0.2-V-C-400. These results reveal
that the vanadium species are in the highly dispersed state at a low
calcination temperature while the gradual migration, agglomera-
tion, and crystallization occur to form nanoparticles about 6–7 nm
in size with the further increase of temperature.
phenolic resins and copolymer F127. In the 0.2-V-C-400 sample, the
signal at 70 ppm disappears while the one at 16 ppm still exists. In
this case, the signal of methyl groups comes from cresol and xylenol
in the pyrolyzed phenolic resol rather than F127 [29]. Therefore,
the template F127 can be completely decomposed at 400 ◦C. Mean-
while, a shoulder peak at around 135 ppm appears, which can be
attributed to the substituted ortho- and para-aromatic carbons
[26]. As the calcination temperature rises, the intensity of the peak
at 135 ppm increases, while those of the peaks at 150, 36, and
16 ppm gradually decrease and they become barely visible after cal-
cination at 600 ◦C. These results imply that the cross-linking degree
of phenolic resins along with the carbonization degree improves
during the calcination treatment.
Fig. S5 shows the FT-IR spectra of the 0.2-V-C-y samples. In
the 0.2-V-C-300 sample, the broad band at around 3437 cm−1 can
be assigned to the stretching of phenolic –OH groups [29,30]. The
bands at 2922, 2866, and 1468 cm−1 can be attributed to the C–H
stretching of methyl and methylene groups [29], mainly arising
from copolymer F127 and the phenolic resins. The band at about
1708 cm−1 is caused by the stretching of hydrogen-bonded car-
bonyl groups from VO(acac)2 [30,31], which almost disappears at
13C CP-TOSS MAS NMR spectra of the 0.2-V-C-y samples are
presented in Fig. 5. The 0.2-V-C-300 sample shows two peaks at
around 153 and 128 ppm, which are assigned to the –OH sub-
stituted carbons in phenol and non-substituted meta-aromatic
carbons, respectively [26,27]. The intense peak at about 70 ppm
is attributed to the methylene and methine carbons in F127,
while the one at around 16 ppm is assigned to the methyl car-
bons in F127 [28]. The broad overlapping signals at around 36 ppm
can be ascribed to methylene linkages between phenolic rings
[26,27]. These results reveal that the 0.2-V-C-300 sample consists of
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Table 3
6
Catalytic performances of vanadium-containing mesoporous carbon catalysts.
Catalysts
Benzene
conversion (%)
Phenol
selectivity (%)
Phenol yield
(%)
a
–
0
0
0
0
0
0
0-V-C-600
VO(acac)2
20.5
28.5
31.8
26.1
39.1
38.9
35.9
2.8
77.3
94.1
94.9
94.8
92.8
94.0
93.9
∼100
15.8
26.8
30.2
24.7
36.3
36.6
33.7
2.8
0.1-V-C-600
0.2-V-C-600
0.3-V-C-600
0.2-V-C-300
0.2-V-C-400
0.2-V-C-500
0.2-V-C-600b
Reaction conditions: 20 mg of catalyst, 0.4 mL of benzene, 5 mL of 80 wt% acetic acid,
1.4 mL of 30 wt% H2O2, 70 ◦C for 3 h.
a
The blank experiment without catalyst.
1.6 mL of absolute ethanol was added into the reaction system while other
b
conditions kept the same.
crystallized at a temperature higher than 400 ◦C. Also, the TG-DTA
curves of VO(acac)2 conducted at nitrogen atmosphere (Fig. S6) fur-
ther confirms that the complete decomposition of VO(acac)2 occurs
at the temperature higher than 450 ◦C.
Fig. 5. 13C CP-TOSS MAS NMR spectra of (a) 0.2-V-C-300, (b) 0.2-V-C-400, (c) 0.2-
V-C-500, and (d) 0.2-V-C-600.
a calcination temperature higher than 400 ◦C. The band at 1602
and 1205 cm−1 can be attributed to the stretching of benzene ring
and the C–O stretching of phenol, respectively [29], implying the
existence of phenolic resol network. As the calcination temper-
ature rises, the intensities of all characteristic bands mentioned
above decrease and they become barely distinct after calcination
at 600 ◦C. The variation of the band intensities clearly indicates
that the decomposition of the template, the further cross-link of
phenolic resol, and the carbonization of the framework, which is
consistent with the NMR results.
The V 2p3/2 XPS spectra of the 0.2-V-C-y samples are shown
in Fig. 6. The peak at 516.0 eV in 0.2-V-C-300 and the peak at
515.7 eV in 0.2-V-C-400 can be attributed to the V4+ species, while
the peaks at 517.0 and 517.1 eV correspond to the V5+ species
[32–34]. For the 0.2-V-C-500 and 0.2-V-C-600 samples, the peaks
at 517.4 and 517.2 eV both correspond to the V5+ species [32–34].
Thus, two kinds of vanadium species are present on the surface
of catalysts calcined at low temperatures. At higher temperatures,
the V4+ species transform to the V5+ species. This is consistent
with the FT-IR and HRTEM results, where the stretching band
caused by the VO(acac)2 disappears and the vanadium species are
3.3. Benzene hydroxylation reaction
The catalytic activity of various catalysts in the benzene hydrox-
ylation reaction is summarized in Table 3. The blank experiment
in the absence of catalyst reveals no measurable benzene con-
version. The bare mesoporous carbon without vanadium species
(0-V-C-600) also shows no catalytic activity. As for the VO(acac)2,
a benzene conversion of 20.5% and a phenol selectivity of 77.3%
are obtained. The relatively low benzene conversion and the poor
phenol selectivity may be caused by the fast H2O2 decomposition
in the homogeneous catalysis system. For the x-V-C-600 catalysts,
the benzene conversion increases from 28.5% over 0.1-V-C-600 to
31.8% over 0.2-V-C-600 and decreases to 26.1% over 0.3-V-C-600,
while the phenol selectivity remains almost constant. The decrease
of catalytic activity over 0.3-V-C-600 could be attributed to the
deterioration of the mesoporous structure as displayed in Fig. 1c.
For the 0.2-V-C-y catalysts, with the increase of the calcination tem-
perature, the benzene conversion decreases gradually from 39.1 to
31.8%, whereas the phenol selectivity increases slightly from 92.8 to
94.9%. To ascertain the precision of catalytic results, the carbon bal-
ance closure shown in Table S1 was calculated based on the analysis
of products. The high benzene conversion over 0.2-V-C-300 cata-
lyst may be attributed to the high dispersion of vanadium species
(Fig. 4a), which provides more active centers for reactants. Also,
considering that both the V4+ and V5+ species exist in the 0.2-V-C-
300 catalyst, V4+ species are more favorable than the V5+ ones for
the formation of ·OH radicals [35], which readily attack phenol to
obtain the over-oxidized products, leading to the lower selectiv-
ity of phenol. As the calcination temperature rises, the dispersion
of vanadium species decreases and the V4+ species are converted
to the V5+ ones, leading to the decrease of active centers as well
as the amount of ·OH radicals. Therefore, the benzene conversion
over 0.2-V-C-y catalysts is reduced while the phenol selectivity is
slightly enhanced with the increase of the calcination temperature.
The benzene hydroxylation reaction over vanadium species
with H2O2 as an oxidant may follow a radical mechanism. In order
to investigate the reaction mechanism, ethanol, known as a radical
scavenger, was added in the reaction system to evaluate its effects
on the phenol yield and selectivity. As shown in Table 3, the phenol
yield decreases dramatically to 2.8% while the phenol selectivity
increases to nearly 100% over 0.2-V-C-600 catalyst after the addi-
tion of excess ethanol. According to these experimental results and
Fig. 6. V 2p3/2 XPS spectra of (a) 0.2-V-C-300, (b) 0.2-V-C-400, (c) 0.2-V-C-500, and
(d) 0.2-V-C-600.
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Table 4
Reusability of vanadium-containing mesoporous carbon catalysts.
Catalysts
Cycle
Benzene conversion (%)
Phenol selectivity (%)
Phenol yield (%)
0.2-V-C-300
1
2
3
4
5
39.1
36.4
31.0
26.8
26.5
92.8
92.3
92.8
92.3
92.0
36.3
33.6
28.8
24.7
24.4
0.2-V-C-600
1
2
3
4
5
31.8
29.6
29.5
29.5
29.6
94.9
95.5
95.2
94.9
94.6
30.2
28.3
28.1
28.0
28.0
those reported in the literature [36–38], a possible reaction mecha-
nism of benzene hydroxylation reaction is proposed in Scheme S1.
Vanadium oxide species contact with H2O2 to form peroxovana-
date species after the release of H2O, followed by the cleavage of
V–O bond to generate VOO· species accompanied with the reduc-
tion of V5+ to V4+. The VOO· species attack the benzene ring to
generate an intermediate, followed by the cleavage of O–O bond
in the intermediate to form the final product phenol along with
the oxidation of V4+ to V5+ to complete the catalytic cycle. Addi-
tionally, the vanadium oxides may also contact with H2O2 to form
·OH radicals [35], which will further react with phenol to gener-
ate byproducts. The addition of excess ethanol may scavenge most
VOO· radicals to quench the benzene conversion, leading to the low
phenol yield. However, the scavenging of ·OH radicals reduces the
contact between phenol and ·OH radicals, which is beneficial to the
phenol selectivity.
in this work, the rough comparison is still feasible and shows the
great potential of vanadium-containing mesoporous carbon cata-
lysts in benzene hydroxylation reaction. The high catalytic activity
and stability of 0.2-V-C-600 are mainly due to the highly dispersed
and tightly confined active centers as well as the hydrophobicity
of the support and the ordered mesostructure, which gives us a
sight in the future design of the catalyst with excellent catalytic
performance and stability.
4. Conclusions
A series of vanadium-containing mesoporous carbon catalysts
were synthesized by a co-assembly method and evaluated by the
benzene hydroxylation reaction using H2O2 as an oxidant. With the
calcination treatment, the vanadium species gradually migrated to
form nanoparticles of 6–7 nm in size, which were mainly confined
in the carbon matrix. The catalysts possessed ordered mesoporous
structures, large surface areas, uniform pore sizes, and moderate
pore volumes, which facilitated the accessibility of reactants to the
active centers. Also, the hydrophobicity of the carbon support was
beneficial to the adsorption of non-polar benzene. These features
endowed the 0.2-V-C-600 catalyst with a high catalytic activity as
well as a good stability in the titled reaction.
3.4. Reusability of the catalysts
Two catalysts, 0.2-V-C-300 and 0.2-V-C-600, were chosen to test
the reusability in benzene hydroxylation as they showed the best
catalytic activity in the corresponding series of 0.2-V-C-y and x-V-
C-600, respectively. After each reaction, the catalyst was separated
by centrifugation and washed thoroughly with ethanol and dried at
120 ◦C. The recovered catalyst was used in the reaction with a fresh
reaction mixture. As shown in Table 4, the phenol yield decreases
from 36.3 to 24.4% after five recycles over the 0.2-V-C-300 catalyst,
while the 0.2-V-C-600 catalyst shows no significant decrease of the
catalytic activity after five times of recycles. According to the above
characterization results, there are no observable vanadium crystals
or particles in the 0.2-V-C-300 and vanadium species are in a high
dispersion and low oxidation state. These vanadium species should
be responsible for the poor reusability of the 0.2-V-C-300 catalyst.
As for the 0.2-V-C-600 catalyst calcined at a higher temperature, the
vanadium nanocrystals are formed and tightly confined or embed-
ded in the carbon walls, which improves the stability of the active
centers and leads to the good catalytic reusability.
In our previous study, we found that the catalytic activity of
SBA-15 supporting V catalyst may be improved by silylation, due to
the enhanced surface hydrophobicity leading to better adsorption
ability of non-polar benzene [10]. However, the silylation degree is
limited due to the poor activity of silanol groups on the surface of
silica materials. Therefore, using the hydrophobic carbon materials
is an alternative way. The results in this work show that the high
catalytic performance of 0.2-V-C-600 is comparable with that of
V-containing SBA-15 silylated with dimethyldichlorosilane while
the reusability of 0.2-V-C-600 is much better [10]. Also, various
catalysts (such as TS-1B [1], HPMoV/NH2-SBA-15 [3], CuAPO-11(d)
[39], Co-SBA-16 [40], and V-SBA-16 [41]) have been used in the
titled reaction, giving the phenol yield from 8.6 to 30.4% and the
phenol selectivity from 94 to ∼100%. Although the reaction condi-
tions over those reported catalysts may be quite different from that
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (21173050, 21371035). L.Y. Hu would like to thank
Changyang Jiang and Professor Wuzong Zhou at University of St.
Andrews for help in HRTEM analysis and the Sino-UK Higher Educa-
tion Research Partnerships Project supported by China Scholarship
Council and British Council.
Appendix A. Supplementary data
Supplementary material related to this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.
2014.10.056.
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