Highly Selective Synthesis of Phenol from Benzene over a Vanadium-Doped Graphitic Carbon Nitride Catalyst

Article abstract of DOI:10.1002/cctc.201200502

Design and preparation of efficient and economical catalysts for direct hydroxylation of benzene to phenol is an important topic. In this work, a series of metal-doped graphitic carbon nitride catalyst (Cu-, Fe-, V-, Co-, and Ni-g-C₃N₄) were successfully synthesized by using urea as the precursor through a facile and efficient method. The catalysts were characterized systematically using N₂ adsorption-desorption, FTIR, thermogravimetric analysis, powder X-ray diffraction, and X-ray photoelectron spectroscopy techniques. It was found that the vanadium-doped graphitic carbon nitride catalyst V-g-C₃N₄ was the most efficient catalyst for the direct synthesis of phenol from benzene with hydrogen peroxide as the oxidant and it could be recycled at least 4times. The influence of reaction conditions such as the solvent, reaction temperature, reaction time, and the amounts of catalyst and hydrogen peroxide were investigated. Under optimized conditions, 18.2% yield of phenol was obtained with the selectivity to phenol as high as 100%.

Full text of DOI:10.1002/cctc.201200502

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FULL PAPERS
DOI: 10.1002/cctc.201200502
Highly Selective Synthesis of Phenol from Benzene over
a Vanadium-Doped Graphitic Carbon Nitride Catalyst
[
a]
Guodong Ding, Weitao Wang, Tao Jiang,* Buxing Han,* Honglei Fan, and Guanying Yang
Design and preparation of efficient and economical catalysts
for direct hydroxylation of benzene to phenol is an important
topic. In this work, a series of metal-doped graphitic carbon ni-
tride catalyst (Cu-, Fe-, V-, Co-, and Ni-g-C N ) were successfully
doped graphitic carbon nitride catalyst V-g-C N was the most
3 4
efficient catalyst for the direct synthesis of phenol from ben-
zene with hydrogen peroxide as the oxidant and it could be
recycled at least 4 times. The influence of reaction conditions
such as the solvent, reaction temperature, reaction time, and
the amounts of catalyst and hydrogen peroxide were investi-
gated. Under optimized conditions, 18.2% yield of phenol was
obtained with the selectivity to phenol as high as 100%.
3
4
synthesized by using urea as the precursor through a facile
and efficient method. The catalysts were characterized system-
atically using N adsorption–desorption, FTIR, thermogravimet-
2
ric analysis, powder X-ray diffraction, and X-ray photoelectron
spectroscopy techniques. It was found that the vanadium-
Introduction
Phenol is an important chemical intermediate in the manufac-
turing of phenol resins, caprolactam, adipic acid, fibers, and
using H O as the oxidant. Fe, Cu, and V are the most actively
2
2
[13,14]
studied transition metals for the hydroxylation of benzene.
[
1,2]
2+
[15]
many other chemicals.
Nowadays, more than 90% of the
The Fenton reagent (Fe -H O ), is traditionally used to cata-
2 2
phenol is still being produced industrially by the three-step
lyze benzene to phenol. Fe impregnated on functional carbon
[
3]
[16,17]
cumene process from benzene. This process suffers from sev-
eral drawbacks, such as high energy consumption, low yields
gave a phenol yield of approximately 20%.
Bianchi et al.
developed a FeSO /water/acetonitrile biphasic reaction system,
4
(
ꢀ5% based on the initial benzene quantity), and generation
for which the selectivity was enhanced markedly to 97% com-
pared with those in single phase using other organic sol-
of acetone as a byproduct. There are many other methods for
the synthesis of phenol, such as nucleophilic substitution of
aryl halides and chlorination/sulfonation processes of benzene
[18]
vents. Fe O supported on multi-walled carbon nanotubes
3
4
(MWCNTs)is superior to other supported Fe catalysts, such as
[
4,5]
etc.
However, the development and application of these
Fe supported on gamma (g) Al O and MCM-41, HY, and HZSM-
2
3
routes has been restrained as a result of the availability of
starting materials, the atomic economy, and energy efficiency.
5 zeolites. A 17% conversion and 95% selectivity toward
phenol was obtained over Fe/MWCNTs in the hydroxylation of
[
6,7]
[19]
[20]
The direct hydroxylation of benzene to phenol
has re-
benzene to phenol. The oxidation activity of Fe/MgO was
[21]
cently attracted much attention and been investigated exten-
sively as a potential route for phenol production from an eco-
nomic and environmental point of view. There are three main
pathways for direct hydroxylation of benzene to phenol in
almost the same as obtained over Fe/Al O , which gave 27%
2 3
yield of phenol, both catalysts being simple and “green”.
Parida and Rath investigated catalytic hydroxylation of ben-
zene to phenol over Cu/MCM-41, which showed 21% conver-
[
8,9]
[7,10]
terms of the oxidants. Nitrous oxide,
hydrogen peroxide,
sion and 94% selectivity towards phenol in acetic acid at room
[
11,12]
[22]
molecular oxygen,
and a mixture of oxygen and hydro-
temperature.
Cu-substituted aluminophosphate molecular
[
6]
[23]
[24]
gen have been adopted by numerous researchers. Amongst
the oxidants, hydrogen peroxide has a clear advantage be-
cause water is its only by-product.
sieves (CuAPO-11), Cu/Al O , and Cu-containing ternary hy-
2 3
[25]
drotalcite have also been reported for this reaction. Other
metals, including Co, Mn, Ni, Cr, and Mo, have shown activity
[25,26,27]
In recent years, there has been growing interest in develop-
ing highly efficient, “green”, heterogeneous catalysts for the
hydroxylation of benzene to phenol under mild conditions
in catalysis of hydroxylation of benzene.
For example,
a phenol yield of 29% was obtained over Mn-amine-functional-
ized mesoporous silica SBA-15 using hydrogen peroxide in the
[26]
presence of O atmosphere.
2
More importantly, the development of compounds of vana-
dium in oxidation catalysis is significant because of its highly
[
a] G. Ding, W. Wang, Prof. Dr. T. Jiang, Prof. Dr. B. Han, Dr. H. Fan, G. Yang
Beijing National Laboratory for Molecular Sciences (BNLMS)
Centre for Molecular Science, Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190 (P.R. China)
Fax: (+86)10-62562821
[28]
feasible activity towards the hydrocarbon oxidation. Studies
on the direct oxidation of benzene to phenol with V-based cat-
alysts on various supports have been reported, such as VOx
species anchored on amorphous siliceous microporous mixed
E-mail: jiangt@iccas.ac.cn
[29–32]
hanbx@iccas.ac.cn
oxides and MCM zeolite catalysts,
VO species supported
x
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Results and Discussion
[
33]
[34]
[35]
[36]
on SBA-15, SBA-16, clay, and MWCNTs, vanadium sub-
[
37,38]
[39]
stituted polyoxometalates
and phosphomolybdates, and
[
40]
2+
sodium metavanadate. VO ions have been immobilized on
mesoporous silica MCM-41 organofunctionalized by 3-amino-
propyltrimethoxysilane, which were employed to study the
catalytic oxidation of benzene with hydrogen peroxide giving
Catalytic performance of g-C N doped with different metals
3
4
We initially conducted the liquid-phase direct hydroxylation of
benzene using hydrogen peroxide with g-C N doped with dif-
3
4
5
8.6% conversion and 18.5% phenol selectivity in acetoni-
ferent metals. The results are summarized in Table 1. Without
[
32]
trile. A benzene conversion of 20% and phenol selectivity of
5% were achieved over Keggin type vanadium substituted
molybdophosphoric acid supported on amine functionalized
doping, g-C N4 displays no activity for the hydroxylation of
3
9
benzene with hydrogen peroxide (Table 1, Entries 1). Ni- and
Co-doped g-C N did not show any activity for the hydroxyl-
3
4
[
33]
SBA-15. The leaching of heteropoly acid from the support
was negligible because of the strong interaction between the
heteropoly acid and amine groups of the surface in this
system. Gao and Xu prepared a vanadium oxide catalyst using
a cheap and readily available clay for the hydroxylation of ben-
zene to phenol and showed 14% conversion of benzene and
ation of benzene with hydrogen peroxide (Table 1, Entries 2, 3).
Table 1. Catalytic activity of various metal-based g-C
hydroxylation.
3
N
4
for benzene
[
a]
Entry
Catalysts
Metal content in
catalyst [mmolg ]
Yield
[%]
Selectivity to
phenol [%]
[
35]
[
b]
ꢁ1
94% selectivity to phenol. MWCNTs with defects have been
proven to be a suitable support for VO . The as-constructed
2
1
2
3
4
5
6
g-C
Ni-g-C N
Co-g-C
V-g-C N
Cu-g-C
Fe-g-C
3
N
4
0
0
0
9.6
3.3
2.3
VO defects/MWCNT catalysts presented high catalytic activity
2.1
2.0
2.0
2.0
2.1
2
3
4
and selectivity to phenol in the hydroxylation of benzene ex-
3
N
4
4
[
36]
100
100
100
3
4
hibiting 24.5% conversion 93.7% selectivity. More recently,
Zhao et al. synthesized a periodic mesoporous organosilica
embedded with a vanadyl (IV) acetylacetonate complex, which
is an excellent new catalyst for direct hydroxylation of
3
N
3
N
4
[
(
a] Reaction conditions: benzene (1.0 mL, 11.3 mmol), hydrogen peroxide
30 wt%, 3.0 mL, 29.6 mmol), acetonitrile (3.0 mL), catalyst (0.1 mmol
[
41]
benzene.
metal), t=6 h, T=333 K; [b] Determined by using ICP-AES.
Although some catalysts with adequate activity have been
developed, it is necessary to probe and synthesize new hy-
droxylation catalysts with high activity and selectivity. It is still
a challenge to obtain high selectivity to phenol at high conver-
sion of benzene because phenol could be more easily oxidized
V-, Fe-, and Cu-doped g-C N catalysts were found to be active
3
4
in the direct synthesis of phenol from benzene (Table 1, En-
tries 4–6). The selectivity toward phenol over V-, Fe-, and Cu-
[30]
than benzene producing over-oxygenated byproducts. Mes-
oporous graphitic (mpg) carbon nitride has recently attracted
much attention because of its extreme chemical and thermal
stability and easy accessibility. The use of mpg-C N as a catalyt-
doped g-C N4 was 100% in all cases. Phenol was the only
3
product detected in the hydroxylation of benzene. Amongst
the metal-doped g-C N , V-g-C N showed the highest activity,
3
4
3
4
[14]
which is in accordance with the reported results.
3
4
ic material is a relatively new area with outstanding potential.
Recent research shows that it can be chemically shaped to a va-
riety of nanostructures, and can be directly used for photoca-
Vanadium content in V-g-C N4
3
[
42,43]
[44–46]
talysis.
Metal modified mpg-C N4
exhibited attractive
Next we studied the catalytic performances of V-g-C N cata-
3 4
3
[
44,45]
potential in photo catalyzed oxidation of benzene
al-
lysts with different vanadium contents for the direct hydroxyl-
ation of benzene with hydrogen peroxide. The results are
listed in Table 2. From the table, we can see that all vanadium-
containing catalysts are catalytically active in the hydroxylation
of benzene. Across the range of catalysts with low vanadium
content, the phenol yield increased with the vanadium content
(Table 2, Entries 1–5). High phenol yields could be obtained if
the amount of ammonium metavanadate used was approxi-
mately 0.4–0.6 g (Table 2, Entries 6–8). If more ammonium met-
avanadate was used in the synthesis of the catalysts, the yields
of phenol decreased gradually (Table 2, Entries 9–12). During
the experiments, we observed that the reactions proceeded
very rapidly at the start (in the first 5 min), coinciding with in
a sharp decrease in product yields. This phenomenon might
be caused by self-decomposition of hydrogen peroxide in-
duced by the high-vanadium catalysts and/or self-scavenging
though the conversion was relatively low. Pd@mpg-C N4
3
showed excellent performance in phenol hydrogenation to cy-
[
47]
clohexanone with 100% conversion and selectivity.
Design and preparation of highly efficient, “green”, economi-
cal catalytic systems for direct hydroxylation of benzene to
phenol is an interesting topic in “green” chemistry. In the pres-
ent work, we synthesized a series of metal-doped graphitic
carbon nitride catalysts (metal-g-C N ) by using cheap urea as
3
4
the precursor through a facile and efficient method. The cata-
lysts were applied in the selective oxidation of benzene to
phenol with hydrogen peroxide. The catalysts were character-
ized systematically using N adsorption–desorption, FTIR, ther-
2
mogravimetric analysis (TGA), powder X-ray diffraction (XRD),
and X-ray photoelectron spectroscopy (XPS) techniques. It was
found that vanadium-doped graphitic carbon nitride (V-g-C N )
3
4
was an effective catalyst for the direct synthesis of phenol
from benzene with hydrogen peroxide as oxidant.
of excess reactive intermediates (OH radical and O ). Notably,
2
the selectivity to phenol remained at 100% across different va-
nadium contents in the V-g-C N catalysts. If V O was used as
3
4
2
5
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[
a]
Table 2. Catalytic performances of V-g-C
dium contents for the hydroxylation of benzene.
3
N
4
catalysts with different vana-
Table 3. Solvent effect on the direct synthesis of phenol from benzene.
[
a]
Entry
Acetonitrile/
acetic acid
Yield of
phenol [%]
Selectivity to
phenol [%]
Entry
Catalysts
0.1-V-g-C
Yield of
phenol [%]
Selectivity to
phenol [%]
1
2
3
4
5
6
7
8
1:0
10:1
7:1
5:1
4:1
3:1
2:1
1:1
1:2
1:3
1:5
0:1
11.6
14.9
15.0
15.2
15.8
15.5
14.5
15.2
13.5
13.1
12.2
11.9
100
100
100
100
100
100
100
100
100
100
100
100
1
2
3
4
5
6
7
8
9
3
N
4
12.7
13.1
14.1
14.3
14.3
15.2
15.0
15.3
11.1
6.2
100
100
100
100
100
100
100
100
100
100
100
100
0.15-V-g-C
0.2-V-g-C
0.25-V-g-C
3
N
4
4
3
N
4
3
N
0.3-V-g-C
0.4-V-g-C
0.5-V-g-C
0.6-V-g-C
0.7-V-g-C
1.0-V-g-C
1.2-V-g-C
1.5-V-g-C
3
3
3
3
3
3
3
3
N
N
N
N
N
N
N
N
4
4
4
4
4
4
4
4
9
10
11
12
10
1
1
6.3
4.6
4.1
1
1
2
3
[
b]
[a] Reaction conditions: benzene (1.0 mL, 11.3 mmol), hydrogen peroxide
30 wt%, 3.0 mL, 29.6 mmol), catalyst 0.4-V-g-C (0.05 g, 0.18 mmolV),
V
2
O
5
–
(
3 4
N
[
(
a] Reaction conditions: benzene (1.0 mL, 11.3 mmol), hydrogen peroxide
30 wt%, 3.0 mL, 29.6 mmol), acetonitrile (3.0 mL), acetic acid (3.0 mL),
solvent total volume (6.0 mL), T=333 K, t=6 h.
catalyst (0.05 g), T=333 K, t=6 h; [b] Hydroquinone was also detected
not quantified).
(
Catalyst amount
To investigate the effect of the amount of catalyst 0.4-V-g-C N
3
4
the catalyst, phenol and hydroquinone were detected and the
yield of phenol was only 4.1% (Table 2, Entry 13). Finally, the
catalyst 0.4-V-g-C N was chosen for subsequent investigation.
on the direct synthesis of phenol from benzene, several experi-
ments were performed and the experimental results are de-
picted in Figure 1. As the catalyst amount increased from 0.01
to 0.06 g, the yield of phenol increased from 13.2 to 16.4%. As
illustrated in Figure 1, on a further increase in the amount of
catalyst, the yield of phenol decreased, resulting probably
from the self-decomposition of hydrogen peroxide induced by
the presence of more vanadium catalyst. We noticed that the
selectivity to phenol remained at 100% in the presence of dif-
ferent amounts of 0.4-V-g-C N .
3
4
Solvent effects
It is well-known that the nature of the solvent is very impor-
tant in a liquid phase reaction, even more so in a two-liquid
3
4
[
18]
phase reaction. It has been reported that the addition of an
acidic component strongly affects the catalytic activity of hy-
[
48]
droxylation with hydrogen peroxide as the oxidant. With the
above research results, we then focused our attention on the
solvent effect in the direct synthesis of phenol from benzene.
Acetonitrile and acetic acid were used as the solvent. The ef-
fects of volume ratios of acetonitrile to acetic acid were investi-
gated. The research results are summarized in Table 3. As can
be seen, the yields of the phenol were low if using pure aceto-
nitrile or pure acetic acid as the solvent (11.6% and 11.9% re-
spectively). Interestingly, in the mixed solvent of acetonitrile
and acetic acid, the yield of phenol was enhanced to some
extent. This is clearly caused by the involvement of acetic acid
which turned the original two liquid phases into one liquid
phase. Thus the contact frequency of the reactants was greatly
enhanced. On the other hand, the acidic environment can in-
fluence the decomposition of hydrogen peroxide. In weak
acidity, the hydrogen peroxide was relatively stable. In con-
trast, in the strong acidity environment, the self-decomposition
3 4
Figure 1. Effect of the amount of the catalyst 0.4-V-g-C N on the hydro-
hexylation of benzene. Reaction conditions: benzene (1.0 mL, 11.3 mmol),
hydrogen peroxide (30 wt%, 3.0 mL, 29.6 mmol), acetonitrile (4.8 mL), acetic
acid (1.2 mL), T=333 K, t=6 h.
[
49,50]
rate of hydrogen peroxide is accelerated,
which leads to
Reaction temperature
decreased activity and the gradual decrease in the yield of
phenol. The two opposite effects of acetic acid resulted in the
phenomenon that the yield increased first and then declined
with increasing acetic acid (Table 3). The optimized volume
ratio of acetonitrile/acetic acid was 4:1 with the maximum
phenol yield of 15.8% (Table 3, Entry 5).
The effect of reaction temperature on the hydroxylation of
benzene is shown in Figure 2. The yield of phenol increased
quickly from 3.1 to 16.4% as the reaction temperature was
increased from 303 to 323 K. This might be because H O de-
2
2
composition increases to produce active species with the in-
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drogen peroxide addition was studied by adding it dropwise
with intervals instead of all at once. The results are also listed
in Table 4. It is shown that the method of hydrogen peroxide
addition affects the yield of phenol. If the amount of hydrogen
peroxide was less than 3 mL, the method of hydrogen perox-
ide addition did not affect the yield of phenol notably (Entry 1
vs. 2, entry 3 vs. 4, Entry 5 vs. 6). If the amount of used hydro-
gen peroxide was more than 4 mL, the yield of phenol was in-
creased slightly by adding hydrogen peroxide dropwise in sev-
eral portions (Entry 7 vs. 8, Entry 9 vs. 10). The yield of phenol
was 18.2% if 5 mL of hydrogen peroxide was added dropwise.
Reaction time
Figure 2. Effect of reaction temperature on the hydroxylation of benzene.
Reaction conditions: benzene, 1.0 mL (11.3 mmol), hydrogen peroxide
The influence of reaction time on the direct synthesis of
phenol from benzene at a reaction temperature of 333 K is
shown in Figure 3. It can be observed that the yield of phenol
increased greatly over a reaction time of up to 6 h, then
stayed at a constant level over the prolonged reaction time.
The selectivity toward phenol was 100% across the whole
reaction time.
(30 wt%, 3.0 mL, 29.6 mmol), acetonitrile (4.8 mL), acetic acid (1.2 mL),
catalyst 0.4-V-g-C (0.06 g, 0.22 mmolV), t=6 h.
3
N
4
creasing temperature. As the reaction temperature was in-
creased further to 363 K, the yield of phenol gradually dropped
to 8.3%, mostly as a result of the more drastic self-decomposi-
tion of hydrogen peroxide at higher temperatures. The most
suitable reaction temperature was between 323 and 333 K.
Hydrogen peroxide amount
The impact of the amount of hydrogen peroxide on the direct
synthesis of phenol from benzene was investigated and the re-
sults are summarized in Table 4. The yield of phenol increased
sharply from 3.7 to 16.4% as the amount of hydrogen perox-
ide was increased from 1 to 3 mL (Entries 1, 3, 5), and in-
creased slightly if more hydrogen peroxide was used (Entries 7
and 9). As the self-decomposition of hydrogen peroxide is un-
[40]
avoidable in the direct synthesis of phenol from benzene,
more than the stoichiometric amount of hydrogen peroxide
was found appropriate to this reaction although the stoichio-
metric ratio of hydrogen peroxide to benzene for the oxidation
of benzene is 1:1. Meanwhile, the effect of the method of hy-
Figure 3. Effect of the reaction time on the hydroxylation of benzene. Reac-
tion conditions: benzene (1.0 mL, 11.3 mmol), hydrogen peroxide (30 wt%,
5.0 mL, 49.4 mmol), catalyst 0.4-V-g-C
3 4
N (0.06 g, 0.22 mmolV), acetonitrile
(
4.8 mL), acetic acid (1.2 mL), T=333 K.
Table 4. The effect of the method of hydrogen peroxide addition on the
hydroxylation of benzene.
Comparison with literature results and the stability of
[
a]
catalyst 0.4-V-g-C N4
3
Entry
H
2
O
2
Yield of
phenol [%]
Selectivity to
phenol [%]
The results of direct hydroxylation of benzene over the catalyst
[
mL]/[mmol]
0
.4-V-g-C N in this work are compared with some of the re-
3 4
1
1/9.9
1/9.9
3.7
3.5
9.6
100
100
100
100
100
100
100
100
100
100
sults using V-based catalysts on different types of supports,
such as MCM-41, SBA-15, MWCNT, and clay. As shown in
Table 5, compared to other supported V catalysts for the hy-
droxylation of benzene with hydrogen peroxide, the catalyst
[
[
[
[
a]
a]
a]
a]
[
2
3
4
5
6
7
8
9
1
2/19.8
2/19.8
3/29.6
3/29.6
4/39.5
4/39.5
5/49.4
5/49.4
9.4
16.4
16.5
16.7
17.2
17.7
18.2
0
.4-V-g-C N4 exhibited comparable catalytic performances at
3
similar reaction conditions. The catalyst 0.4-V-g-C N has the
3
4
greatest selectivity to phenol of the reported V catalysts in lit-
erature. To test the catalyst stability, we also investigated the
a]
0
possibility to reuse the catalyst 0.4-V-g-C N . The used catalyst
[
a] Hydrogen peroxide added dropwise over 2 h (0.2 mL for Entries 2, 4,
and 6; 0.4 mL for Entries 8 and 10). Reaction conditions: benzene (1.0 mL,
1.3 mmol), catalyst 0.4-V-g-C (0.06 g, 0.22 mmolV), acetonitrile
4.8 mL), acetic acid (1.2 mL), T=333 K, t=6 h.
3
4
was washed with distilled water (3ꢁ10 mL), and re-examined
with a new reactant mixture at conditions similar to those in
Figure 3. As shown in Table 5, the yields of phenol over the
1
(
3 4
N
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+
H
interaction with the NH groups. Therefore, very high selec-
2
Table 5. Comparison with literature results for the hydroxylation of
benzene and recycle of the catalyst 0.4-V-g-C N .
3 4
tivity to phenol was achieved.
Entry
Catalyst
Conversion of
benzene [%]
Selectivity to Reference
phenol [%]
Catalyst characterization
[
[
[
[
[
a]
b]
c]
d]
e]
2+
1
2
3
4
5
VO /MCM-41
58.6
18.5
95.0
92.0
93.7
100
100
100
100
[32]
[33]
[35]
[36]
The FTIR spectra of g-C N4 and the catalyst 0.4-V-g-C N is
3
3
4
HPMoV/SBA-15 20.0
ꢁ
1
shown in Figure 4a. Several bands in the 1200–1650 cm
V/clay
VO /MWCNT
0.4-V-g-C
11.7
24.5
region corresponding to the typical stretching modes of CN
heterocycles are clear for the two samples. The characteristic
2
3
N
4
17.7 (1st recycle)
this work
1
1
1
6.3 (2nd recycle)
6.1 (3rd recycle)
5.5 (4th recycle)
ꢁ1
mode of the triazine units (C N ) at 803 cm is also ob-
6
7
[53]
served, indicating the presence of typical structure of g-C N .
3
4
Notably, not all the as-synthesized samples in this paper show
the two-region spectra perfectly (Figure 4b). This is probably
Reaction conditions: [a] Benzene (10 mmol), catalyst (0.1 g), acetonitrile
6 mL), H (35 wt%, 10 mmol), T=333 K, t=1 h; [b] Benzene (1.0 mL),
catalyst (0.1 g), acetonitrile (6 mL), H (30 wt%, 3 mL), T=333 K, t=6 h;
c] Benzene (56 mmol), catalyst (1.0 g), acetic acid (30 mL), H (30 wt%,
8 mmol), T=313 K, t=4 h; [d] Benzene (11.3 mmol), catalyst (0.05 g),
(30 wt%, 13 mmol), T=333 K, t=15 min; [e] Ben-
zene (1.0 mL), catalyst (0.06 g) acetonitrile/acetic acid (v/v 4:1, 6 mL), H
30 wt%, 5.0 mL), T=333 K, t=6 h.
(
2 2
O
because the formation process of the g-C N was affected by
2
O
2
3
4
[
9
2
O
2
the amount of added vanadium species to some extent, which
is also demonstrated by XRD measurements of the samples
2 2
acetonitrile (8 mL), H O
(
Figure 5b). If the amount of ammonium metavanadate used
2
O
2
to prepare V-g-C N exceeded 0.5 g, the characteristic bands in
(
3
4
ꢁ
1
the 1200–1650 cm region became weak.
catalyst 0.4-V-g-C N in the second, third, and fourth recycle
3
4
were 16.3, 16.1, and 15.5%, respectively, suggesting that the
catalyst 0.4-V-g-C N can be recycled at least 4 times without
3
4
considerable decrease in activity.
Inductively coupled plasma atomic emission spectroscopy
(
ICP-AES) analysis of the reaction solution after the first run in-
ꢁ
1
dicated that the content of V in the solution was 1.3 mmolL ,
which is approximately 3% of the V in the catalyst. The small
percentage of V leached during the reaction is consistent with
the good recyclability. The stability of the catalyst might be as-
cribed to its special properties and structure. The g-C N has
3
4
basic NH and NH groups on the surface, high surface area,
2
and large pore volume. The interaction between the NH and
NH groups and V species could mitigate the leaching of V. An
2
acid-base interaction has been invoked as the reason to pre-
vent the leaching of the active sites from the supports by sila-
[51]
nization to bind heteropoly acids on the surface of support.
Kharat et al. demonstrated that the leaching of vanadium sub-
stituted molybdophosphoric acid from support during the hy-
droxylation of benzene was negligible because of the strong
interaction between heteropoly acid and amine groups on the
[
33]
surface of functionalized SBA-15.
The selectivity toward phenol remained 100% across all
tested reaction conditions. The selective production of phenol
might be closely related to the special properties and structure
of the catalyst. g-C N is a defect-rich, N-bridged poly(tri-s-tria-
3
4
zine) and the s-triazine ring is aromatic. In the reaction, the
benzene was first adsorbed on the g-C N support with the p
3 4 3 4
Figure 4. FTIR spectra of the g-C N catalyst and V-g-C N catalysts.
3
4
[
52]
electron of carbon nitride. The benzene was hydroxylated to
phenol by hydrogen peroxide over the V species which was
stabilized by the NH and NH groups on the support. After the
XRD patterns of g-C N4 and the catalyst 0.4-V-g-C N are
3 3 4
2
benzene was converted to phenol, the aromatic ring altered
shown in Figure 5a. Both the samples showed similar diffrac-
tion peaks, though the diffraction peaks of catalyst 0.4-V-g-
C N are not as strong as that of pure g-C N The strong peak
[
52]
the symmetry of the molecular orbital, resulting in the non-
planar adsorption fashion of phenol over the surface of g-
3
4
3
4.
[
47]
C N . In the acetonitrile/acetic acid/H O solution, the desorp-
at 27.58 represents the stacking of the conjugated interlayers,
3
4
2
tion of the generated phenol was more easier because of the
which is indexed for graphitic materials as the (002) peak. An-
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[57]
cies is formed. Therefore, at least two kinds of vanadium
species are present on the surface of the catalyst 0.4-V-g-C N .
3
4
The high resolution O1s XPS spectrum is shown in Figure 6e.
The presence of both V O and VO species on the surface
2
5
2
of g-C N might be the origins of the high catalytic activity of
3
4
the V-g-C N4 catalyst. It is known that the hydroxylation of
3
benzene over vanadium species with hydrogen peroxide as
the oxidant proceeds by a radical mechanism, as shown in the
[
36]
IV
followed equations. Equation (1) shows that V species are
V
more favorable for the formation of ·OH than V species. Thus,
the catalytic activity of VO for the hydroxylation of benzene
2
by hydrogen peroxide is superior to that of V O . Furthermore,
2
5
V
IV
V species could be converted to V species by hydrogen per-
[36]
oxide [Eq. (2)].
active.
Therefore, the catalyst V-g-C N4 was very
3
4
þ
þ
5þ
ꢁ
V
V
þ H O ! V þ HO þ HOC
ð1Þ
ð2Þ
2
2
5
4þ
þ
þ H O ! V þ H þ HOO
2
2
The N2 adsorption–desorption isotherm is shown in Fig-
ure 7a. The catalyst 0.4-V-g-C N had a BET surface area of
3
4
2
ꢁ1
3
ꢁ1
4
4.5 m g , and a pore volume of 0.084 cm g . The BET sur-
2
ꢁ1 [43]
face area of pure g-C N was 96.6 m g . The decrease in the
3
4
BET surface area of 0.4-V-g-C N should be attributed to the
3
4
loading of vanadium. Figure 7b gives the pore size distribution
of the 0.4-V-g-C N catalyst calculated by using original DFT,
3
4
which shows a main peak at approximately 4–5 nm.
Figure 5. XRD patterns of the g-C
CPS=counts per second.
3 4 3 4
N catalyst and V-g-C N catalysts.
TGA of 0.4-V-g-C N4 in Figure 8 showed a weight loss of
3
64% at 1000 K. The weight loss of the sample at high tempera-
ture is mainly because of the decomposition of carbon nitride
matrix, whereas the amount of inorganic residue is directly re-
lated to the metal content. Results from TGA revealed that the
amount of vanadium in the catalyst 0.4-V-g-C N was approxi-
other peak at 12.98 corresponds to in-plane ordering of tri-s-tri-
azine units, which form 1D melon strands. The two diffraction
peaks are in good agreement with that of g-C N reported in
3
4
3
4
[
43]
ꢁ1
literature. No diffraction peaks of vanadium species are ob-
served in Figure 5, even in the sample with the highest vanadi-
um content (Figure 5b). The presence of the vanadium species
in V-g-C N could be confirmed by XPS analysis.
mately 3.6 mmolg . However, the value calculated from the
yield of pure g-C N4 and the vanadium added was
3
ꢁ1
3.2 mmolg ; this discrepancy could be because some of the
precursor was blown away by nitrogen gas during the catalyst
preparation process.
3
4
The XPS spectrum of the catalyst 0.4-V-g-C N is shown in
3
4
Figure 6. As indicated in Figure 6, the main elements on the
surface of the sample are C, N, O and V. The photoelectron
peaks of these elements appear at binding energies of 288.0
Conclusions
(
C1s), 399.0 (N1s), 530.9 (O1s), and 517.1 eV (V2p), respective-
A series of metal-doped graphitic carbon nitride catalyst
(metal-g-C N ) have been synthesized by using urea as the pre-
ly. Figure 6b shows the high resolution C1s XPS spectrum of
the sample: the XPS peak with binding energy of 288.0 eV is
attributed to the carbon atoms bonded to three nitrogen
3
4
cursor through a facile and efficient method. Cu-C N , Fe-C N ,
3
4
3
4
and V-g-C N are active for catalyzing the hydroxylation of ben-
3
4
[
54]
atoms in the g-C N lattice.
The high resolution N1s XPS
zene with hydrogen peroxide as the oxidant under mild condi-
tions. V-g-C N is the most effective among these catalysts. The
3
4
spectrum (Figure 6c) shows the special nitrogen species: the
two peaks at 401.5 and 399.8 eV can be assigned to tertiary ni-
trogen and the handful of amino functional groups, respective-
3
4
vanadium content in V-g-C N , the reaction temperature, and
3
4
the amount of catalyst have significant effects on the yield of
phenol. A phenol yield of 18.2% can be obtained with a selec-
tivity of 100% to phenol under the optimized conditions. The
catalyst is also stable and can be recycled at least 4 times with
a slight decrease in yield. Direct hydroxylation of benzene with
this easily prepared and cheap catalyst provides a valuable al-
ternative to the multi-step methods for the synthesis of
phenol.
[
55]
ly. The peak at 398.6 eV is typically attributed to nitrogen
[
56]
bonded to two carbon atoms (C-N-C). The high resolution
V2p XPS spectrum is shown in Figure 6d: two main peaks lo-
cated at approximately 524.5 and 517.6 eV corresponds to
V2p1/2 and V2p3/2, which confirms that the V species is V O .
2
5
The other two peaks located at approximately 523.4 and
14.9 eV correspond to V2p1/2 and V2p3/2, indicating VO spe-
5
2
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3 4
Figure 6. XPS patterns of the catalyst 0.4-V-g-C N .
Characterization
Experimental Section
Chemicals
FTIR spectra were recorded on Bruker Tensor 27 spectrometer with
ꢁ
1
a resolution of 1 cm and 32 scans. Powder XRD patterns were re-
All chemicals were analytical grade and used as purchased without
further purification. Urea was purchased from Shantou Xilong
Chemical Factory (Guangdong, China). Cu(NO ) ·3H O, FeCl were
corded on Rigaku D/max-2500 X-ray diffractometer using CuK ra-
a
diation (l=0.15406 nm). The tube voltage was 40 kV and current
was 200 mA. BET surface areas and pore volumes were measured
on a Micromeritics ASAP 2020 sorptometer by using nitrogen ad-
sorption at 77 K. Thermogravimetrical analysis (TGA) was studied
3
2
2
3
obtained from Sinopharm Chemical Reagent Co. Ltd. Benzene, am-
monium metavanadate (NH VO ), acetic acid (99.5%), hydrogen
4
3
ꢁ
1
peroxide (30 wt%) were purchase from Beijing Chemical Reagent
Company. Acetonitrile was purchased from Guangdong Guanghua
Sci-Tech Co. Ltd (Guangdong, China). NiCl ·6H O was purchased
using Netzsch STA 409 PC under an air flow of 60 mLmin at
ꢁ1
a heating rate of 10 Kmin up to 1173 K. X-ray photoelectron
spectroscopy data (XPS) were obtained with an ESCALab220i-XL
2
2
from Beijing Shuanghuan Chemical Reagent Company (Beijing,
electron spectrometer from VG Scientific using 300 W AlK radia-
tion. The base pressure was about 3ꢁ10 mbar. The binding ener-
gies were referenced to the C1s line at 284.8 eV from adventitious
a
ꢁ
9
China). CoCl ·6H O was purchased from Tianjin Zonghengxing In-
2
2
dustrial and Trading Co. Ltd. (Tianjin, China).
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Synthesis of V-g-C N4
3
In a typical preparation, 12.0 g of urea dissolved in 100 mL of dis-
tilled water was heated with desired amount of ammonium meta-
vanadate (NH VO ), and stirred at 353 K for 1 h. The mixed solution
4
3
was then heated at 373 K to remove water. The resulting mixtures
ꢁ
1
were then heated at a rate of 3.0 Kmin to reach a temperature
of 573 K, and tempered at this temperature for 2 h in a flowing-ni-
trogen atmosphere. The mixture was then continuously heated to
8
23 K over 1.5 h, and tempered at this temperature for 4 h. This
was followed by cooling the sample naturally to room temperature
with nitrogen gas. The final powder was collected and labeled as
V-g-C N . The catalysts with different vanadium contents were la-
3
4
beled as x-V-g-C N , in which x represents the mass (in g) of ammo-
3
4
nium metavanadate added if using 12.0 g of urea. The amount of
ammonium metavanadate used was varied in the range 0.1–1.5 g.
Other metal doped g-C N , including Ni-g-C N , Cu-g-C N , Fe-g-
3
4
3
4
3
4
C N , and Co-g-C N , was synthesized similarly. The metal precur-
3
4
3
4
sors used for preparing these catalysts were NiCl ·6H O, Cu-
2
2
(
NO ) ·3H O, FeCl , and CoCl ·6H O. The amount of these chemicals
3 2 2 3 2 2
used was all 2.1 mmol (0.50 g, 0.51 g, 0.34 g, and 0.50 g for
NiCl ·6H O, Cu(NO ) ·3H O, FeCl , and CoCl ·6H O, respectively).
2
2
3 2
2
3
2
2
Catalytic tests
The direct synthesis of phenol from benzene was carried out in
a 50 mL single-necked round bottom flask equipped with a mag-
netic stirrer, and a water-cooled condenser. In a typical reaction,
the catalyst (0.06 g) and benzene (1.0 mL) were added to 6.0 mL of
acetonitrile and acetic acid (volume ratio is 4:1) mixed solvent.
After the temperature reached 333 K, 3.0 mL of aqueous solution
of 30 wt% hydrogen peroxide was added into the mixture. The re-
action was conducted for 6 h. After centrifuged, the reaction
mother liquid was analyzed by gas chromatography (Agilent 6820)
equipped with a FID detector and a PEG-20m capillary column
Figure 7. Nitrogen adsorption–desorption isotherm and the pore size distri-
bution of 0.4-V-g-C N .
3 4
(30 mꢁ0.25 mmꢁ0.25 mm). An internal standard 1, 4-dioxane was
used to quantify the generated phenol. Under the employed con-
ditions, no biphenyl, catechol, hydroquinone, or benzoquinone
were detected by GC. Phenol was the confirmed product with
1
00% selectivity. The yield of phenol was defined as the molar
ratio of phenol produced to the initially added benzene.
Acknowledgements
The authors thank the National Natural Science Foundation of
China (21273253, 20932002, 20903103, 21021003) and Chinese
Academy of Sciences (KJCX2.YW.H30).
3 4
Figure 8. TGA of 0.4-V-g-C N under air flow.
Keywords: hydroxylation · nitride · phenol · porous materials ·
vanadium
carbon. The content of metal in the catalysts was determined by
ICP-AES (VISTAMPX).
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