Facile one-pot synthesis of VxOy@C catalysts using sucrose for the direct hydroxylation of benzene to phenol

Article abstract of DOI:10.1039/c3gc00084b

V_xO_y@C catalysts were prepared from sucrose and NH₄VO₃ by a one-pot hydrothermal method. They showed satisfactory catalytic performance for the hydroxylation of benzene to phenol in acetonitrile using oxygen as

Full text of DOI:10.1039/c3gc00084b

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Facile one-pot synthesis of V O @C catalysts using
x
y
sucrose for the direct hydroxylation of benzene
Cite this: DOI: 10.1039/c3gc00084b
Received 11th January 2013,
Accepted 13th March 2013
to phenol†
DOI: 10.1039/c3gc00084b
Weitao Wang, Guodong Ding, Tao Jiang,* Peng Zhang, Tianbin Wu and
Buxing Han*
www.rsc.org/greenchem
V
x y 4 3
O @C catalysts were prepared from sucrose and NH VO by a carbon has attractive potential as a catalytic material for the
one-pot hydrothermal method. They showed satisfactory catalytic title reaction.
performance for the hydroxylation of benzene to phenol in aceto-
nitrile using oxygen as the oxidant.
Utilization of renewable and abundant biomaterials is one
of the most sustainable approaches for fabricating functional
carbon materials. Hydrothermal carbonization of saccharides
is an effective and green route to prepare carbon materials in
Phenol is one of the most important industrial feedstocks that
is widely used as the precursor for the production of phenol
resins, fibers, dyestuffs, and medicine. From the viewpoints of
economy and the environment, the direct hydroxylation of
benzene to phenol in one step is very attractive compared to
the traditional three-step cumene process. The oxidants for
the direct hydroxylation of benzene are mostly focused on
catalysis, energy storage, CO sequestration, and water purifi-
2
21,22
cation.
In addition, the reducibility of saccharides makes
it possible to prepare metal–carbon catalytic materials via a
23–25
one-pot saccharides hydrothermal process.
There is no
doubt that developing highly efficient, economic and simple
catalytic systems for the direct hydroxylation of benzene to
phenol is highly desirable. In the present work, a novel
1
–3
4,5
6–8
H
2
O
2
,
O
2
and N
2
O.
2
Among these oxidants, O is the
most economical and environmentally benign. With O as the
2
vanadium–carbon composite catalyst (designated as V
x y
O @C)
oxidant and a sacrificial reductant, the highest phenol yield of
was prepared from sucrose and NH VO by a one-pot hydro-
4
3
9
,10
more than 20% could be achieved.
aqueous solution
Concentrated acetic acid
thermal carbonization process for the first time. The as-pre-
pared material exhibited satisfactory activity and selectivity for
the direct hydroxylation of benzene to phenol with oxygen in
acetonitrile.
9
–12
13,14
and acetonitrile
are often employed
as the solvents for the reaction. The reaction system with
acetic acid as the solvent usually results in corrosion and pol-
lution problems, although the yield of phenol is relatively
higher. The reaction in acetonitrile can avoid corrosion, but
the yield of the product is usually lower.
In this approach, sucrose was used as the carbon source
and reducing agent and NH VO as the metal precursor.
4
3
V O @C could be prepared using different amounts of sucrose
x
y
Catalysts using carbon as the supports have shown good
performance for the direct hydroxylation of benzene to
and NH
4 3
VO . The amount of the catalyst powder obtained
increased with an increased amount of NH VO when the
4
3
1
5
16
3 4 2 3 4
phenol, such as Fe O /MWCNTs, VO /MWCNTs, Fe O /
amount of sucrose was fixed (Fig. S1†). Little carbonaceous
powder was formed without NH VO , indicating that NH VO
was the key to the formation of V O @C. The vanadium
1
7
18
CMK-3 and activated carbon-supported ferric sulfate. In
addition, carbon treated with nitric acid shows catalytic
activity itself. For example, the MWCNTs treated with concen-
4
3
4
3
x
y
content in the V O @C prepared from different amounts of
x
y
trated HNO can catalyze the hydroxylation of benzene with
3
NH
4 3
VO gradually increased with an increase in the amount of
active oxygen which was generated on the defects of MWCNTs
NH VO used, which was determined by inductively coupled
4
3
1
9
with dangling incomplete bonding,
while the activated
plasma atomic emission spectroscopy (ICP-AES) (Table 2). This
implied that the NH VO was a reactant, which also acted as a
catalyst. During the carbonization, NH VO was exhausted
carbon treated with HNO and hydrogen peroxide shows good
3
4
3
2
0
catalytic activity for the hydroxylation of benzene. Therefore,
26
4
3
and the carbonization stopped. The functional groups on the
carbonaceous material can stabilize the vanadium affording
active and stable catalysts.
The hydrothermal temperature was important in the prepa-
x y
ration of V O @C. The amount of the obtained catalysts
Beijing National Laboratory for Molecular Sciences (BNLMS), Centre for Molecular
Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
E-mail: Jiangt@iccas.ac.cn, Hanbx@iccas.ac.cn; Tel: +86 10 62562821
27
†
Electronic supplementary information (ESI) available: Fig. S1–S11 and Tables
S1–S2. See DOI: 10.1039/c3gc00084b
increased with increasing hydrothermal temperature in the
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range of 120–180 °C (Fig. S1†), but the amount of the catalyst
obtained reduced with a further increase in temperature. Sac-
charides usually carbonized at a relatively high temperature
2
8–31
(160–190 °C).
However, when some metal salts were added
to the system, the carbonization temperature could be
decreased. For example, glucose could be carbonized at 80 °C
in the presence of iron nitrate, which was attributed to the
2
4
ability of iron nitrate to catalyze the dehydration of glucose.
In this work, the sucrose and NH
above 120 °C.
4 3
VO system could carbonize
The liquid phase of the hydrothermal reaction solution was
1
3
analyzed using C NMR (Fig. S2†). The signals of glucose and
x y
Scheme 1 Schematic illustration of the synthesis of V O @C by the hydro-
fructose were observed, which demonstrated that sucrose was
3
2,33
thermal process.
hydrolyzed to glucose and fructose.
Furthermore, the
signals of HMF (hydroxymethylfurfual) were also observed in
1
3
the C NMR spectrum, suggesting that glucose and fructose more uniform when less NH VO was employed. NH VO or
4
3
4
3
were partially transformed to HMF under the hydrothermal vanadium oxides could not be observed in the XRD patterns
3
4,35
conditions.
(Fig. S5†), which was due to the high dispersion of the
Fourier transform infrared (FT-IR) spectra of all the pre- vanadium oxides. The BET surface area of the
pared catalyst samples were nearly identical (Fig. S3†). The V O @C-0.195–120 determined by nitrogen sorption–deso-
x
y
−
1
−1
2
−1
O–H stretching bands at 3400 cm and 2930 cm indicated rption measurement was 33.1 m g (Fig. S7†). The N content
3
6
the presence of aliphatic hydrocarbons (–C–H). The bands at on V O @C-0.195–120 was about 1.5% according to the XPS
x
y
1
−1
4+
1
710 cm− and 1617 cm were attributed to CvO and CvC analysis. This may be the adsorbed NH on the surface which
vibrations, respectively, indicating the aromatization of balanced the charge of the carboxyl groups.
2
5
glucose during the hydrothermal treatment. The bands in
x y
The catalytic performances of the V O @C for the hydroxy-
−
1
the range 1000–1300 cm were assigned to the C–OH stretch- lation of benzene to phenol with molecular oxygen were inves-
ing and OH bending vibrations, which showed the existence of tigated. Acetonitrile was used as the solvent and ascorbic acid
3
0,37
residual hydroxy groups.
Furthermore, the surface oxygen as the sacrificial reductant, which proved to be essential for
was characterized by XPS (Fig. S4†). Carbonyl groups, alcohol the hydroxylation reaction (Table 1, entry 6). In the reaction,
and/or ether groups, and carboxyl and/or ester groups were phenol and hydroquinone were produced without observing
2
0
observed at 531.6, 532.5 and 533.5 eV, respectively, which the formation of catechol and benzoquinone. The yields of
was consistent with the FT-IR spectra.
phenol were very low in water or without solvent (Table 1,
The mechanism of the hydrothermal process was proposed entries 4, 5). It is known that the catalysts were hydrophilic. As
(
Scheme 1) on the basis of the above characterizations and shown in Fig. S8,† the catalyst V O @C-0.195–120 could be dis-
x
y
existing knowledge. NH VO catalyzed the hydrolysis of persed in the water phase in the mixture of water and benzene,
4
3
sucrose to produce glucose and fructose, both of which were but could not be stably dispersed in benzene. It could be well
then partially transformed into HMF. Nuclei-oligomers were dispersed in the mixture of benzene and acetonitrile,
formed by the dehydration of glucose or polymerization of suggesting that acetonitrile improved the dispersion of the
HMF in the initial stage. Simultaneously, NH
4
VO
3
was partially catalyst in benzene.
transformed into the vanadium oxides, which were stabilized
When water was the solvent, the water around the catalyst
by the functional groups (OH and CvO) on the surface of the formed a stagnant film which depressed the adsorption of
oligomers. With further polymerization and carbonization, the benzene and lowered the activity. When acetonitrile was
amorphous vanadium–carbon (XRD patterns in Fig. S5†) cata-
lysts were formed. This was a hydrolysis–polymerization–
carbonization process.
Table 1 Catalytic performances of V
x
O
y
@C-0.195–120 for the hydroxylation of
a
benzene to phenol
x y
The XPS spectra of the V O @C catalysts showed the
binding energy of V 2P3/2 in the range of 515.5–516.9 eV
Time
[h]
Phenol
selectivity [%]
Phenol
yield [%]
b
4
+
5+
Entry
Solvent
(
Table S1†), indicating the existence of V and V in the cata-
38,39
lysts.
It was known that saccharides could act as reduc-
1
2
3
4
5
6
Acetonitrile
Acetonitrile
Acetonitrile
—
Water
Acetonitrile
5
10
24
10
10
10
95.9
93.8
93.8
100
100
100
9.1
9.2
12.2
0.5
0.8
0.03
2
4,25
tants for many reactions.
this work could reduce V in NH
Therefore, the sucrose used in
5
+
4+
4 3
VO to V , which has been
proved to be the active species for the hydroxylation of
4
0,41
c
benzene.
(
It can be observed from the SEM images
VO did not
Fig. S6†) that the variation in the amount of NH
4
3
a
Catalyst V
x
O
y
@C-0.195–120, 25 mg; ascorbic acid, 0.8 g; acetonitrile,
pressure, 3.0 MPa; temperature, 80 °C.
remarkably influence the morphology of the catalysts prepared
at 120 °C. However, at 180 °C the particles of the catalysts were
2
.0 g; benzene, 1.0 mL; O
2
b
c
Hydroquinone was detected. Without ascorbic acid.
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Table 2 Catalytic performances of the
V
x
O
y
@C for the hydroxylation of phenol did not vary considerably with the hydrothermal temp-
erature (Table 2, entries 5–8).
a
benzene to phenol
The effect of the vanadium precursors used for preparing
the catalysts was also investigated. Vanadium(IV) oxide bis(2,4-
pentanedionate) and vanadium(III) chloride were employed to
V%
(ICP)
Phenol
selectivity [%]
Phenol
yield [%]
Entry
Catalysts
1
2
3
4
5
6
7
8
V
x
V
x
V
x
V
x
V
x
V
x
V
x
V
x
O
y
O
y
O
y
O
y
O
y
O
y
O
y
O
y
@C-0.293–120
@C-0.195–120
@C-0.098–120
@C-0.050–120
@C-0.025–120
@C-0.195–150
@C-0.195–180
@C-0.195–210
5.85
4.38
3.62
3.09
—
3.73
3.80
4.55
91.0
92.1
93.3
90.3
91.3
93.5
92.4
90.8
8.4
9.2
9.4
9.7
9.5
9.5
9.4
9.2
x y x y
synthesize the V O @C-A and V O @C-Cl catalysts, respecti-
vely. The vanadium content (Table S2,† 4.19 wt%) and
vanadium species (Table S2,† 516.7 eV) of the V O @C-A
x
y
detected from the XPS data were similar to those of the
@C-0.195–120 (Table 2, entry 2, 4.38 wt%; Table S1,† entry
2, 516.9 eV). Therefore, the yield of phenol over the V O @C-A
x y
V O
x
y
a
(Table S2,† 8.9%) was similar to that over the
@C-0.195–120 (Table 2, entry 2, 9.2%). The vanadium
content of the V O @C-Cl was only 0.91 wt% (Table S2†),
Reaction conditions: catalyst, 25 mg; ascorbic acid, 0.8 g; acetonitrile,
x y
V O
2
1
.0 g; benzene, 1.0 mL; O
0 h.
2
pressure, 3.0 MPa; temperature, 80 °C, time,
x
y
which was the main reason that V
x y
O @C-Cl gave a low yield of
phenol (Table S2,† 5.9%). The precursors might affect the
employed as the solvent, the substrate benzene was miscible process of forming the catalysts and, subsequently, influenced
with acetonitrile, and the catalyst could be well dispersed. The the activity of the catalysts.
excellent dispersion of the catalyst in the mixture of aceto-
x y
In conclusion, V O @C can be prepared using sucrose as
nitrile and benzene favored the adsorption of benzene the carbon source by a facile one-pot hydrothermal method,
affording a high yield of phenol in acetonitrile (Table 1, which can catalyze the direct hydroxylation of benzene to
entries 1–3).
phenol with molecular oxygen in acetonitrile efficiently.
The yield of phenol first increased with reaction temp-
erature and then decreased with a threshold of 80 °C (Fig. S9†).
The decrease of the oxygen solubility with an increase of temp-
Experimental
Preparation of the catalysts
4
2
erature was one of the main reasons for this phenomenon.
The self-oxidation of ascorbic acid as the sacrificial reductant
4
3,44
The V O @C catalyst was prepared by a hydrothermal method.
prolonged the reaction,
which caused the decline in the
x y
Typically, sucrose (3.40 g) and the desired amount of NH
4 3
VO
yield of phenol. The influence of oxygen pressure was investi-
gated (Fig. S10†). The yield of phenol had a maximum at an
oxygen pressure of 3.0 MPa. In the low pressure range, the
yield enhancement with increasing pressure could be attribu-
ted to the larger solubility of oxygen at the higher oxygen
(
for example 0.195 g) was dissolved in distilled water (30 mL).
The solution was stirred for several minutes, and transferred
into a 50 mL Teflon-lined stainless steel autoclave, which was
then sealed and heated at 120 °C for 12 h. The resulting pro-
ducts were filtered, washed several times with distilled water
and finally dried at 100 °C overnight. The obtained catalyst
4
2
pressure. However, too much oxygen might cause the over-
oxidation of phenol, which resulted in a decrease in the yield
of phenol. When the reaction time was prolonged, the yield of
phenol was increased to 12.2% (Table 1, entries 1–3), which
was higher than that reported in the literature with acetonitrile
x y x y
was denoted as V O @C-0.195–120. V O @C prepared under
different conditions is denoted as V O @C-c-t, in which c rep-
x
y
4 3
resents the amount of NH VO used (with the unit of g), and t
1
4,45
is the hydrothermal temperature (with the unit of °C). The cata-
lysts prepared from vanadium precursors of vanadium(IV)
oxide bis(2,4-pentanedionate) and vanadium(III) chloride were
as the solvent and oxygen as the oxidant.
The performance of the catalysts prepared under different
conditions were evaluated (Table 2). The catalysts prepared at
x y x y
denoted as V O @C-A and V O @C-Cl, respectively. The
1
4 3
20 °C with different amounts of NH VO gave similar phenol
amounts of the two vanadium precursors used were 0.441 g
and 0.265 g, respectively, which were equal to the moles of
yields (Table 2, entries 1–4), although V content in the catalysts
decreased gradually with the decrease of the amount of
NH
4 3 x y
VO for preparing the V O @C-0.195–120.
4 3
NH VO used for preparing the catalysts. This, however, is
understandable considering that the binding energy of V 2P3/2
decreased with the decreased V content (Table S1†), indicating
Characterization of the catalysts
4
+
5+
4+
that ratio of V /V increased in the catalysts, and V has The catalysts were characterized by scan electron microscopy
4
0,41
been proved to be more active for activating oxygen.
the hydrothermal temperature for preparing the catalysts was electron spectroscopy (XPS) techniques. The SEM examination
increased, the reaction efficiency of sucrose and NH VO was was conducted on a Hitachi-s4300 electron microscope oper-
When (SEM), powder X-ray diffraction (XRD), FT-IR and X-ray photo-
4
3
improved, therefore, the vanadium content in the catalysts ated at 15 kV. The samples were spray-coated with a thin layer
was increased. However, the carbonization was enhanced with of platinum before observation. The XRD was performed on a
the increase of the hydrothermal temperature and more active X’PERT SW X-ray diffractometer operated at 30 kV and 100 mA
sites were embedded in the carbonaceous powder. The compe- with Cu Kα radiation. The XPS data were obtained with an
tition of the opposite effects led to the result that the yield of ESCALab220i-XL electron spectrometer from VG Scientific
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1
1
1
1
1
2
2
2
2
Catalytic tests
1
The hydroxylation of benzene was performed in a Teflon-lined
stainless-steel reactor (20 mL) that was equipped with a
pressure gauge, thermocouple, gas-inlet valve, magnetic
stirrer, and an electric heater with a controller. In a typical
experiment, 0.025 g of catalyst, 0.80 g of ascorbic acid, and
1
.0 mL of benzene were added to 2.0 g of acetonitrile succes-
sively. After the system was charged with 3.0 MPa of O at
2
room temperature, the hydroxylation reaction was conducted
at 80 °C for the desired time under vigorous stirring. After the
reaction, toluene was added into the product mixture as an
internal standard for product analysis. The mixture was
analyzed using a gas chromatograph (GC) of Agilent 6820
equipped with a flame ionization detector (FID). The products
of phenol and hydroquinone were detected using GC in this
reaction system. The yield of phenol was calculated as: (mole
of formed phenol)/(mole of initial benzene) × 100%.
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