Noble metal-free V2O5/g-C3N4 composites for selective oxidation of olefins using hydrogen peroxide as an oxidant

Article abstract of DOI:10.1039/c8dt02765j

Vanadium pentoxide modified graphitic carbon nitride (V₂O₅/g-C₃N₄) composites were prepared through a method of wet impregnation and calcination. The obtained samples were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, UV-Vis diffuse reflectance spectroscopy, photoluminescence, electron spin resonance and N₂ adsorption/desorption isotherms. Oxidation of olefins was employed to evaluate the catalytic and photocatalytic activities of the prepared V₂O₅/g-C₃N₄ composites. Different weight ratios (1%, 2%, 3%, 4% and 5%) of V₂O₅ loaded composites were prepared and a 3% loaded composite was found to show optimal catalytic performance for the reaction. This noble metal-free catalyst showed excellent performance in the oxidation of styrene to benzaldehyde with high conversion (98.7%) and selectivity (88.4%) under visible light irradiation. A plausible mechanism was proposed for this oxidation reaction with hydrogen peroxide as an oxidant. Other styrene substrates were also selectively transformed to their corresponding aldehydes with high yields (up to 92%), using such a noble metal-free catalytic system.

Full text of DOI:10.1039/c8dt02765j

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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Noble metal free V2O5/g-C3N4 composite for selective oxidation of
olefins using hydrogen peroxide as oxidant
Hui Zou,a Guansheng Xiao,a Kaihao Chena and Xinhua Peng*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Vanadium pentoxide modified graphitic carbon nitride (V2O5/g-C3N4) composites were prepared through a method of wet
impregnation and calcination. The obtained samples were characterized by Fourier transform infrared spectroscopy, X-ray
diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, scanning electron microscope, transmission
electron microscopy, UV-vis diffuse reflectance spectroscopy, photoluminescence, electron spin resonance and N2
adsorption/desorption isotherms. The oxidation of olefins was employed to evaluate the catalytic and photocatalytic
activity of the prepared V2O5/g-C3N4 composites. Different weight ratios (1%, 2%, 3%, 4% and 5%) of V2O5 loaded
composites were prepared and a 3% loaded composite was found to show optimal catalytic performance for the reaction.
This noble metal free catalyst showed excellent performance in the oxidation of styrene to benzaldehyde with high
conversion (98.7%) and selectivity (88.4%) under visible light irraiation. A plausible mechanism was proposed for this
oxidation reaction with hydrogen peroxide as oxidant. Other styrene substrates were also selectively transformed to the
corresponding aldehydes with high yield (up to 92%), using such noble metal free catalytic system.
www.rsc.org/
application.19-23 The g-C3N4 materials exhibit excellent performance
as promising catalysts due to the similarity to graphene, high
chemical, thermal stability and easily tailorable structure.24
1. Introduction
The partial oxidation of olefins to the corresponding aldehydes is
significant in organic synthesis and industrial chemistry.1, 2 In the
past several decades, tremendous efforts have been devoted to
achieving this transformation based on homogenous catalysts, such
as metalloporphyrins, transition metal-schiff bases and metal-
phthalocyanines.3-5 However, the majority of these catalysts are
suffering from difficulties in separation from the reaction mixture
and generating copious waste. In addition, most of these
homogenous catalysts become inactive after reaction, which is not
desirable in industrial application.6
Graphitic carbon nitride is widely used in oxidation,25
photocatalysis26 and photodegradation of pollutants.27 Thus far,
many g-C3N4 based catalysts have been reported to apply in the
oxidation of olefins. Most of the reported g-C3N4 catalysts are
employed to catalyse the substrates to relevant ketones or
epoxides with low conversion and selectivity.28-30 Few researches
focus on the transformation of olefins to the corresponding
aldehydes.
Herein, we have prepared vanadium pentoxide (V2O5) supported
on g-C3N4 by a mild wet impregnation and calcination method with
ammonium metavanadate (NH4VO3) and melamine as precursors.
The as-prepared V2O5/g-C3N4 catalysts are firstly employed to the
selective oxidation of olefins.
To circumvent these problems, heterogeneous catalysts have
been successfully developed for the oxidation of olefins.7-10 The
common strategy is to anchor active sites on various solid support,
such as molecular sieve, graphene oxide, reduced graphene oxide
and clay.11-15 Most of the reported catalysts for such oxidation
involve noble metals, for example, gold, silver, palladium, platinum
etc.7,16-18 Unfortunately, they are usually expensive, tedious and
environmentally unfriendly. The above disadvantages may hamper
their practical application in industries. Therefore, it’s imperative to
explore some new heterogeneous catalysts which are simple,
environmentally friendly, inexpensive and recyclable.
2. Experimental
2.1 Sample preparation
Vanadium pentoxide was prepared using NH4VO3 as precursor. In
detail, 1.17 g NH4VO3 and 5.76 g citric acid were dissolved in
deionized water and the resulting mixture was stirred at ambient
condition for 30 min. The obtained solution was heated to 100 oC to
o
remove water. The residue remained was calcined at 500 C for 2 h
Recently, graphitic carbon nitride (g-C3N4) has drawn
considerable attention in the field of catalysis and became a
continuous hot topic in both academic research and industrial
with ramping rate of 5 oC/min to prepare pure V2O5.
Graphitic carbon nitride was synthesized via thermal
polycondensation of melamine according to the previous report.19
Typically, 10 g melamine was put into an alumina crucible with
o
cover to avoid the sublimation of precursor, then heated to 550 C
under air atmosphere with a rate of 10 oC/min and kept at this
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vibration of C=N and aromatic C–N heterocycles.32 Three
strong absorption bands at 612, 827 and 1019 cm-1 are
detected in the V2O5 sample, the bands at 612 and 827 cm-1
are assigned to asymmetric stretching modes of the V–O–V
bond, and the other band at 1019 cm-1 is associated with the
stretching vibration of the V=O bond.33 As for the spectra of g-
C3N4 based catalysts, typical bands of at 1200 to 1700cm-1 and
the triazine units at 804 cm-1 are discernible in V2O5/g-C3N4
temperature for 4 h in a muffle furnace. After cooling to room
temperature, the obtained yellow product was ground into powder
for further use.
The V2O5/g-C3N4 composite catalysts with different amounts of
V2O5 were prepared according to the following process: 0.2 g bulk
g-C3N4 with different weight ratios of V2O5 (1%, 2%, 3%, 4% and 5%)
were added into 20 mL methanol, respectively. Then the ensuing
mixture was treated with ultrasonic for 30 min. Immediately after
the sonication, the resulting suspension was vigorously stirred at 80
oC to evaporate methanol. Then the remaining product was
annealed in a muffle furnace at 400 oC for 2 h with a heating rate of
5
oC/min. To be clear, the obtained catalysts with expected
different weight ratios of V2O5 (1%, 2%, 3%, 4% and 5%) were
collected as VC-1, VC-2, VC-3, VC-4 and VC-5, respectively. Bare
V2O5 and g-C3N4 were studied with the same procedure for
comparison.
2.2 Characterization
Fourier transform infrared (FT-IR) spectra were recorded on a
NicoletIS-10 spectrometer using KBr as the reference. The
structures of those prepared samples were analysed by a Bruker D8
Advance diffractometer with Cu Kα radiation (λ= 1.5406 Å). TG
analysis was performed on TGA/SDTA851 with a ramping rate of 10
composites.34 This demonstrates that the loading of V2O5
doesn’t change the structure of g-C3N4, which is also confirmed
by XRD analysis.
oC/min under air atmosphere. XPS data was collected on
a
PHIQuantera II instrument with Al Kα as X-ray source. A scanning
electron microscopy (SEM) S-4800 was employed to examine the
morphology of the samples. The TEM images were also obtained
using a Tecnai 12 transmission electron microscope. UV-visible
diffuse reflectance spectra of the samples were studied by a
ThermoFisher Evolution 220 UV-vis spectrophotometer. The
photoluminescence (PL) measurements were carried out on a FL3-
TCSPC at room temperature. The electron spin resonance (ESR)
analysis was conducted with a JES-FA200 spectrometer. The N2
adsorption/desorption isotherms were obtained at 77 K using an
Autosorb IQ3 instrument. The high-resolution transmission electron
Fig. 1 FT-IR spectra of g-C3N4, VC-1, VC-2, VC-3, VC-4, VC-5 and
V2O5 samples.
The XRD patterns of the as-prepared g-C3N4, V2O5 and
V2O5/g-C3N4 samples are displayed in Fig. 2. The two distinct
diffraction peaks of bulk g-C3N4 at 13.0o and 27.5o are
attributed to the (100) and (002) planes of graphite-like carbon
nitride, respectively.35 All the characteristic peaks detected in
the pattern of V2O5 can be assigned to the orthorhombic V2O5
(JCPDS 65-0131),36 which reveals the excellent crystalline of
synthesized V2O5. For the V2O5/g-C3N4 composites, both the
distinct diffraction peaks of g-C3N4 and the orthorhombic
phase of V2O5 can be clearly observed and no other impurity is
found. In addition, the peak intensities of g-C3N4 decrease
gradually, while the characteristic peaks of V2O5 become
increasingly distinct with the increasing loading of V2O5. The
microscopy (HRTEM) image was obtained using
transmission electron microscope.
2.3 Oxidation of olefins
a JEM-2100
In a 50 mL two-necked round bottomed flask, olefin (1 mmol),
catalyst VC-3 (20 mg), and 5 mL of acetonitrile were added
together. After admixing, 30wt% H2O2 (6 mmol) was added into the
above mixture and the system was refluxed at 70 oC with
continuous stirring, along with simultaneous irradiation using 36
watt domestic bulb. Reaction was performed under the same
conditions without visible light irradiation. TLC was used to monitor
the progress of reaction until the substrate was completely
transformed. Then the catalyst was separated by centrifuge and
products were extracted with ethyl acetate. After drying over
sodium sulfate, products were analysed by gas chromatography
(GC).
3. Results and discussion
.
Fig. 1 shows the FT-IR spectra of the as-prepared g-C3N4,
V2O5 and V2O5/g-C3N4 composites. In the spectrum of g-C3N4,
characteristic band at 804 cm-1 is attributed to out-of plane
bending models of C-N heterocycles.31 Peaks at 1245, 1328,
1407, 1573 and 1637 cm-1 are associated with the stretching
results confirm the successful immobilization of V2O5 on g-
C3N4, which is further supported by TEM analysis.
Fig. 2 XRD patterns of g-C3N4, VC-1, VC-2, VC-3, VC-4, VC-5 and
V2O5 samples.
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Thermogravimetric (TG) analysis of V2O5, g-C3N4 and V2O5/g- spectrum of C 1s, the binding energy at 284.8 eV was selected
C3N4 composites are displayed in Fig. 3. As can be seen, pure as the reference for calibration. The strong peak at 288.2 eV of
V2O5 exhibits negligible mass loss even heated to 900 oC, while C 1s spectrum is attributed to the N-C-N coordination in g-C3N4.
pure g-C3N4 and V2O5/g-C3N4 samples show significant weight In the N 1s spectrum, peak at 398.7 eV is associated with the
loss at the same temperature. For pure g-C3N4, the weight loss sp2 hybridization of nitrogen atoms (C=N-C). As for the
is occurred at approximately 600 oC and it shows an entire spectrum of V 2p, two peaks at 524.5 and 517.0 eV are
weight loss at roughly 700 oC. As for the prepared V2O5/g-C3N4 detected, which are assigned to V 2p1/2 and V 2p3/2 levels,
composites with increasing amount of V2O5, the beginning respectively. An obvious peak centered at 532.5 eV is found in
temperature of the mass loss is lower than pure g-C3N4. The the spectrum of O 1s, which corresponds to the oxygen atom
weight loss of g-C3N4 based materials is mainly attribute to the in V2O5. The above results convince the presence of V2O5 and
thermal decomposition of g-C3N4 and the remained residue is g-C3N4 in the VC-3 sample.
associated with the content of V2O5. The loss of weight is
98.6%, 98.1%, 96.5%, 96.1% and 95.4%, respectively, which is
similar to the expected amount of V2O5.
The morphologies of g-C3N4 and VC-3 sample were
investigated by scanning electron microscope (SEM). As shown
in Fig. 5, pure g-C3N4 exhibits obvious stacked-up flakes with
rough surface. In the case of the VC-3 sample, there is no
visible difference between g-C3N4 and VC-3 sample, which
indicates that the loading of V2O5 doesn’t change the
morphology of g-C3N4.
Fig. 5 SEM images of (a) g-C3N4 and (b) VC-3 samples.
Transmission electron microscopy (TEM) was used to further
analyse the morphologies and microstructures of g-C3N4 and
VC-3 sample. It’s clearly seen from Fig. 6 that pure g-C3N4
appears a nanosheet like structure. As for VC-3 sample, it can
be seen that a few small V2O5 nanoparticles are wrapped by
the thin layers of g-C3N4.
Fig. 3 TG analysis of g-C3N4, VC-1, VC-2, VC-3, VC-4, VC-5 and
V2O5 samples.
Fig. 6 TEM images of (a) g-C3N4 and (b) VC-3 sample.
The HRTEM image in Fig. 7 shows the lattice spaces of VC-3
sample. The lattice fringe spacing is 0.289 nm, which belongs
to the (301) planes of V2O5.
Fig. 4 XPS spectra of VC-3 sample.
XPS measurement was employed to investigate the surface
compositions and chemical states of the as-prepared VC-3
sample. Fig. 4 shows the XPS spectrum of VC-3 sample and
high-resolution spectra of C 1s, N 1s, V 2p and O 1s. In the
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Fig. 7 HRTEM image of VC-3 sample.
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energy of g-C3N4 and V2O5 are obtained from the plots of
Fig. 8 shows the N2 adsorption-desorption isotherms and the (αhv)1/2 vs. hv and (αhv)2 vs. hv, respectively.37 As shown in Fig.
BJH pore size distribution curves of g-C3N4 and VC-3 samples. 9(b), the band gap energy of g-C3N4 is estimated to be 2.61 eV
Both samples exhibit type IV adsorption isotherm, indicating and V2O5 is found to be 2.21eV, which are similar to the
the presence of mesopores. The BET surface area of g-C3N4 previous reports.38,39 Furthermore, the band gap energy of g-
and VC-3 samples are 28.7 and 56.9 m2 g-1, respectively. In C3N4 based composites is estimated to be 2.56, 2.31, 2.29, 2.26
addition, the pore size distribution curves of both samples and 2.23 eV, respectively.
show a range of pore diameter from 2 to 16 nm.
Fig. 9 (a) UV–vis diffuse reflection spectra of as-prepared
samples; (b) Plots of the (αhv)1/2 vs. hv for g-C3N4 and other g-
C3N4 based samples, plots of the (αhv)2 vs. hv for V2O5.
The photoluminescence measurement was employed to
estimate the efficiency of charge carriers separation and
transfer ability of the as-prepared samples. Fig. 10 describes
the PL spectra of g-C3N4 and V2O5/g-C3N4 composites under
o
390 nm excitation at 25 C. It’s well known that the lower PL
intensity has inherent affinity for the slow recombination of
charge carriers, which may be favourable for improving the
photocatalytic performance.40 A broad visible emission peak
centered at around 460 nm is detected for pure g-C3N4, which
can be ascribed to the inner band gap energy of g-C3N4. After
loading V2O5, the PL intensity of V2O5/g-C3N4 samples
decreases rapidly, implying that the recombination rate of
charge carriers is much lower compared with pure g-C3N4. This
is beneficial for the enhancement of photocatalytic process.
Fig. 8 (a) N2 adsorption–desorption isotherms for g-C3N4 and
VC-3 samples; (b) pore size distribution for g-C3N4 and VC-3
samples.
The UV-vis diffuse reflectance spectra of V2O5, g-C3N4 and
V2O5/g-C3N4 composites are shown in Fig. 9. As can be seen in
the Fig. 9 (a), the absorption edge of g-C3N4 is around 460 nm
while V2O5 exhibits an absorption edge at 580 nm. As for
V2O5/g-C3N4 composites, obvious red-shifts are observed
among these materials compared with pure g-C3N4. The results
imply that the loading of V2O5 on g-C3N4 can highly broaden
the absorption edge of g-C3N4 based materials. No distinct
difference in photoabsorption performance is discovered
among the above V2O5/g-C3N4 samples with different amounts
of V2O5. In addition, the band gap energy of the as-prepared
materials can be calculated according to the following formula:
αhv
=
A
(hv-Eg)n/2
(1)
where α, h, v, Eg and A are absorption coefficient, Planck
constant, light frequency, band gap energy and a constant,
respectively. Besides, the value of n is different with different
types of optical transition of a semiconductor (n = 1 for direct
transition and n = 4 for indirect transition). The band gap
Fig. 10 PL spectra of g-C3N4, VC-1, VC-2, VC-3, VC-4 and VC-5
samples.
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Transition metals such as vanadium, cobalt and of benzaldehyde is 72.4% and 76.7% (entry 8), respectively.
molybdenum, have gained a lot of attention as catalysts for The results indicate that independent V2O5 or g-C3N4 cannot
Entry Catalyst hv Time Con. Sel. (%)b
effectively promote the transformation of styrene into
benzaldehyde. It’s the combination of V2O5 and g-C3N4 that
(h)
24
24
1
(%)b BzAc
19.2 31.3
10.4 21.5
38.4 92.7
89.5 87.6
98.7 88.4
98.8 82.6
99.4 78.4
72.4 76.7
71.5 74.2
77.5 86.8
53.7 89.4
99.3 85.4
99.1 82.6
PhEDd BzAce
rapidly increases the activity of the catalyst and the affinity
of g-C3N4 toward visible light also has a positive impact on
the reaction. There is little difference when bulk V2O5 is
employed in such reaction under visible-light irradiation
compared with the reaction carried out in dark (entries 8,
1f
2g
3
g-C3N4
g-C3N4
VC-1
VC-2
VC-3
VC-4
VC-5
V2O5
V2O5
VC-3
VC-3
VC-3
VC-3
+
-
trace
trace
3.80
4.90
4.50
5.30
4.40
1.60
2.40
2.50
4.80
4.00
4.10
trace
trace
3.50
7.50
7.10
12.1
17.2
21.7
23.4
10.7
5.80
10.6
13.3
+
+
+
+
+
+
-
4
1
5
1
6
1
7
1
8
6
9
6
10
11h
12i
13j
a
-
1
+
+
+
1
1
1
Reaction conditions: styrene (1 mmol), acetonitrile (5 mL),
b
c
catalyst (20 mg), H2O2 (6 mmol), 70 oC. Determined by GC.
d
e
f
Benzaldehyde. Phenyl-1,2-ethanediol. Benzoic acid. The
selectivity of styrene oxide was 68.7%. g The selectivity of styrene
h
oxide was 78.5%. Catalyst (10 mg). Catalyst (30 mg). Catalyst
i
j
(40 mg).
the oxidation reactions.41-44 Especially, vanadium is
widespreadly applied in the oxidation of olefins with excellent
catalytic performance.45-47 It inspires us to exam the possibility
of combing V2O5 with g-C3N4, which will highly facilitate the
oxidation process. The next crucial step is to find the
appropriate ratio of V2O5 loading that is optimum for the
oxidation reaction. Different amounts of V2O5 were loaded on
the surface of g-C3N4 by a mild method of wet-impregnation
and calcination. Here, the catalytic activity of g-C3N4 based
catalysts was examined in the selective oxidation of styrene
using H2O2 as co-oxidant and results are summarized in Table
1. The major products of the transformation are benzaldehyde,
phenyl-1, 2-ethanediol and benzoic acid. As shown in the
table, the weight ratio of V2O5 is important for the
transformation of styrene to benzaldehyde. Without loading
V2O5, the conversion of styrene is very low either in the dark or
under visible light (entries 1, 2).Owing to these results, it can
be speculated that pure g-C3N4 is not active enough to catalyse
the oxidation of styrene. The conversion of styrene increases
to 38.4% with 92.7% selectivity of benzaldehyde using VC-1 as
catalyst (entry 3). The increase weight ratio of V2O5 loading to
2% rapidly improves the conversion to 89.5% along with
benzaldehyde selectivity of 87.6% (entry 4). It's worth noting
that styrene was completely transformed under visible light
and the selectivity of benzaldehyde reached 88.4% when the
amount of V2O5 increased to 3% (entry 5).However, no positive
effect on the reaction is detected with the further increase
amount of V2O5, there is an evident decline in the selectivity of
benzaldehyde accompanied by the increase amount of benzoic
(entries 6, 7). This can be attributed to the excessive oxidation
9). The enhanced catalytic performance of the catalyst is
possibly due to the synergic effect between V2O5 and g-C3N4.
After confirming the optimal loading of V2O5, it’s necessary to
determine the optimum amount of catalyst. Therefore, a
series of experiments were performed using 10 mg, 20 mg, 30
mg and 40 mg of catalyst. Styrene is not entirely transformed
when 10 mg of catalyst is applied to the reaction (entry 11). 20
mg of VC-3 catalyst is found to possess the most outstanding
activity in the oxidation of styrene as it affords 98.7%
conversion of styrene and 88.4% selectivity of benzaldehyde.
Further increase in the weight of catalyst does not show any
significant change in the conversion and selectivity (entries 12,
13).
Table 1 Catalytic performance of prepared samples for the
oxidation of styrene with (+) and without (-) visible lighta
It’s well recognized that the
decomposition of H2O2, has
·
OH radical obtained from the
a
strong ability to oxidize
substrates into desired products.48-50 For the present reaction
system, in order to investigate whether the
·OH is involved in
the reaction process, 10 mol% isopropanol (a well-known
·
OH
scavenger) was added into the reaction under optimal
condition, and the conversion of styrene decreased a lot.
of styrene to corresponding carboxylic acid.
A control
experiment was carried out under similar condition using pure
V2O5 as catalyst, the conversion of styrene and the selectivity
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These results demonstrates that the oxidation reaction is the conversion and selectivity (entries 2, 4). However, the
possibly carried out through the OH radical pathway. The conversion of substrate decreases dramatically when the
·
electron spin resonance (ESR) and DMPO (5, 5-dimethyl-1- substituent group is at ortho-position (entry 3). It is worth
pyrroline N-oxide) spin trap technique were used to further Table 2 Extension of substratesa
Entry
Substrate
Product
Time
(h)
Yieldb
(%)
confirm the presence of
the four characteristic peaks of the DPMO-
the relative intensity ratio of 1:2:2:1 are observed. It indicates
that OH radicals are produced in current catalytic system.
·
OH radicals. As shown in the Fig. 11,
·OH adducts with
1
1
87
80
46
78
83
76
88
89
43
25
trace
92
·
Fig. 11 DMPO spin-trapping ESR spectrum for DMPO-
under visible light irradiation with VC-3 catalyst.
·OH
2
1
Based on the above results and discussions, a plausible
mechanism for the selective oxidation of styrene to
benzaldehyde is proposed as followings:
3
2.5
1.5
1.5
2
As illustrated in Fig. 12, under visible light irradiation, both
g-C3N4 and V2O5 can be excited to generate electron-hole
pairs. After loading V2O5 particles, the probability of electron-
hole recombination decreases sharply which is confirmed by
the PL analysis (Fig. 10). The photogenerated electrons in the
conduction band (CB) of g-C3N4 tend to transform to V2O5,
while the holes in the valence band (VB) of V2O5 have the
opposite transform tendency. And then, H2O2 molecules can
4
5
6
be decomposed into active oxygen species (
presence of V2O5/g-C3N4 composite. The obtained
will react with styrene and form radical . Immediately, radical
is further combined with the radical OH and generates the
·OH) in the
·OH radicals
A
7
1
A
·
intermediate 1-phenyl-1, 2-ethanediol. Finally, benzaldehyde is
prepared from 1-Phenyl-1, 2-ethanediol.
8
1
Fig. 12 A plausible mechanism for the reaction.
9
12
12
12
0.5
The recyclability of catalyst is an important parameter to
evaluate the performance of catalytic system. Therefore, a
series of experiments were performed for the selective
oxidation of styrene using VC-3 as catalyst under optimal
conditions. After reaction, catalyst was separated from the
mixture by centrifugation, washed with ethanol and deionized
10
11
12
o
water, then dried at 70 C overnight before applying to a new
reaction. It can be seen from the results in Fig. 13a that VC-3
can be reused for seven times without obvious decrease of
activity. The XRD pattern of VC-3 catalyst after seventh cycle
shows little difference compared with the fresh one (Fig. 13b).
a Reaction conditions: substrate (1 mmol), acetonitrile (5 mL), VC-
3 (20 mg), H2O2 (6 mmol), 36 watt domestic bulb, 70 C. b Isolated
o
yield.
noting that properties of substituent groups have an important
effect on the formation of desired products. An electron
withdrawing group either on the aromatic ring or on the
exocyclic double bond will inhibit the oxidation of substrate
and
cause low yield of products (entries 9, 10). Besides of these,
the conjugation of the aromatic ring with the exocyclic double
bond is beneficial to the catalytic system (entry 12). Little
product is obtained when substrate is not conjugated (entry
11).
Fig. 13 (a) Recycle of catalyst; (b) XRD patterns of VC-3 sample:
fresh and after seventh cycle.
The oxidation of a wide range of substituted styrene as well
as relevant aromatic olefins was carried out under optimal
conditions and results are summarized in Table 2. It can be
seen from the results that the position of substituted groups
(meta and para) does not show any significant difference in
4. Conclusion
In conclusion, we have prepared V2O5/g-C3N4 composites
through a mild method of wet impregnation and calcination,
using ammonium metavanadate and melamine as precursors.
6 | J. Name., 2012, 00, 1-3
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22 (a) J. Zhang, Y. Wang, J. Jin, J. Zhang, Z. Lin, F. Huang and J.
Yu, ACS Appl Mater Interfaces, 2013, , 10317-10324; (b) K.
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The structure and composition of the as-prepared composites
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optimal efficiency and give 98.7% conversion of styrene with
88.4% selectivity for benzaldehyde under visible light
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A plausible
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the corresponding aldehydes with high yield (up to 92%). And
the substituent at ortho-position will hamper the oxidation,
while electron-donating group and the conjugation of the
aromatic ring with the exocyclic double bond are beneficial for
this transformation.
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Conflicts of interest
There are no conflicts to declare.
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A noble metal free catalytic system for the oxidation of olefins to the corresponding aldehydes
using H2O2 as oxidant.