Liquid phase hydroxylation of benzene to phenol over vanadium substituted Keggin anion supported on amine functionalized SBA-15

Article abstract of DOI:10.1016/j.molcata.2011.07.014

The Keggin type vanadium substituted molybdophosphoric acid (H ₅PMo₁₀V₂O₄₀) supported on amine functionalized SBA-15 was tested in direct hydroxylation of benzene to phenol in the liquid phase. The catalyst was characterized by various techniques such as X-ray diffraction, FT-IR spectroscopy, thermogravimetric analysis (TGA), N ₂ absorption-desorption surface area measurement and SEM. Gas chromatography of products showed high catalytic activity and selectivity to phenol. As we expected, the leaching of heteropoly acid from support during the reaction was negligible, because of strong interaction between heteropoly acid and amine groups of surface. The influence of reaction temperature, reaction time, amount of catalyst and H₂O₂ were investigated. Under optimized conditions 20% conversion of benzene and 95% selectivity to phenol was achieved.

Full text of DOI:10.1016/j.molcata.2011.07.014

Journal of Molecular Catalysis A: Chemical 348 (2011) 14–19
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Liquid phase hydroxylation of benzene to phenol over vanadium substituted
Keggin anion supported on amine functionalized SBA-15
Ali Nemati Kharat∗, Sayedreza Moosavikia, Bahareh Tamaddoni Jahromi, Alireza Badiei
School of chemistry, University College of science, University of Tehran, P.O. Box 13145-1357, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The Keggin type vanadium substituted molybdophosphoric acid (H5PMo10V2O40) supported on amine
functionalized SBA-15 was tested in direct hydroxylation of benzene to phenol in the liquid phase.
The catalyst was characterized by various techniques such as X-ray diffraction, FT-IR spectroscopy,
thermogravimetric analysis (TGA), N2 absorption–desorption surface area measurement and SEM. Gas
chromatography of products showed high catalytic activity and selectivity to phenol. As we expected, the
leaching of heteropoly acid from support during the reaction was negligible, because of strong interaction
between heteropoly acid and amine groups of surface. The influence of reaction temperature, reaction
time, amount of catalyst and H2O2 were investigated. Under optimized conditions 20% conversion of
benzene and 95% selectivity to phenol was achieved.
Received 29 January 2011
Received in revised form 24 July 2011
Accepted 26 July 2011
Available online 4 August 2011
Keywords:
Modified SBA-15
Molybdovanadophosphoric acid
Hydroxylation
© 2011 Elsevier B.V. All rights reserved.
Phenol
Benzene oxidation
1. Introduction
heteropoly and isopoly anions that the former is more active for cat-
alytic applications [12]. Molybdenum (VI) and tungsten (VI) are the
Phenol is one of the most useful intermediates in manufac-
ture of petrochemical, agrochemical and plastics [1–3]. It is also
an essential chemical to produce phenolic resins and bisphenol
A [4]. Currently, phenol is chiefly produced by three step Huck
process that suffers from many disadvantages such as low yield,
high energy consumption and production of acetone as byprod-
uct [5]. In two past decades many endeavors were carried out to
find one step procedure for the production of phenol by direct
hydroxylation of benzene in liquid or gas phase. Owing to envi-
ronmental and economical aspects, the use of molecular oxygen
and hydrogen peroxide is preferred to other oxidants [6,7]. Hydro-
gen peroxide in comparison with oxygen shows more activity and
better selectivity. In spite of its advantages, hydrogen peroxide
is inherently corrosive and tends to catalytic decomposition in
high concentration. In order to overcome these significant draw-
backs usually diluted aqueous solution is used, however the excess
water often produces a biphasic mixture and leads to separation
of organic substrate and active catalyst species [8,9]. It is worth
mentioning that the reaction of hydrogen peroxide with aromatic
compounds proceeds in the presence of transition metal ions of dif-
ferent oxidation states [10,11]. Polyoxometalates belong to a large
class of nano-sized metal–oxygen clusters. Generally there are two
kind of polyoxometalates based on their chemical composition:
best polyoxometalates formers as the result of their favorable com-
bination of ionic radius, charge and accessibility of empty d orbitals
for metal–oxygen ␲ bonding [13]. Heteropoly compounds with
their redox and acidic properties are active catalysts for various
oxidation reactions [14–17]. In addition, heteropoly compounds
have inherent stability toward oxygen donors such as molecular
oxygen and hydrogen peroxide, therefore they are useful catalysts
for liquid phase oxidation [18]. The main drawback for heteropoly
anions catalytic application is their low specific area, hence for
many applications, heteropoly compounds are dispersed on a high
surface area carriers [19,20]. The result of direct dispersion of HPAs
on a conventional supports such as silica, TiO2, carbon, or Al2O3
show that anchoring HPA on the surface of these supports is not
stable and the loaded HPAs can be easily released by polar sol-
vents resulting in the leaching of the active site from the supports.
To minimize this problem, modification by silanization has been
explored to bind heteropoly acids on the surface of support and
an acid–base interaction made has been invoked to explain their
interaction [21].
Here, in this work we synthesized the vanadium-substituted
heteropoly acid (H5PMo10V2O40·nH2O) and loaded this HPA on
amine functionalized SBA-15. This catalyst was tested in direct
hydroxylation of benzene to phenol and the optimum conditions
were investigated. In order to show the influence of silanization, the
activity of this catalyst was compared with loaded HPAs according
to impregnation method over conventional supports such as NaY
and MCM-41.
∗
Corresponding author. Tel.: +98 21 61112499; fax: +98 21 66495291.
E-mail address: alnema@khayam.ut.ac.ir (A. Nemati Kharat).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.07.014

A. Nemati Kharat et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 14–19
15
2. Experimental
O
OH
2.1. Materials
H2O2
H2O2
The polyoxometalates used in this work, H3PMo12O40 and
H5PMo10V2O40, were synthesized in our laboratory with lit-
erature methods [12,22]. Triblock copolymer poly(ethylene
oxide)–poly(propylene oxide)–poly(ethylene oxide) (P123),
tetraethyl orthosilicate (TEOS), hydrochloric acid (37%), 3-
aminopropyltriethoxysilane (APTES), molybdenum trioxide,
vanadium pentoxide, phosphoric acid 85%, acetonitrile, benzene,
hydrogen peroxide (30%) and toluene were purchased from Merck.
Catalyst
Catalyst
O
Scheme 1. The hydroxylation reaction of benzene.
room temperature. The solid product obtained after evaporation
of solvent dried at 110 ◦C for 12 h.
2.2. Synthesis of SBA-15
The synthesis of SBA-15 was carried out according to zhao’s
method [23]. Typically, 4 g of triblock copolymer P123 was dis-
solved in 30 g deionized water and 120 ml of HCl 2 M solution with
constant stirring while temperature was kept at 40 ◦C. After that,
8.5 g of TEOS was added during 30 min and the resulting mixture
was stirred at 40 ◦C for 24 h. In the last step, the mixture was aged
for 48 h at 100 ◦C under static condition. The white solid was fil-
tered, washed with warm water, dried in room temperature, and
calcined in air at 550 ◦C for 5 h.
2.6. Catalytic test
All reactions were carried out in three neck 50 ml flask equipped
with a magnetic stirring bar and reflux condenser. Typically 0.1 g
catalyst was dispersed in 6 ml acetonitrile. When reaction temper-
ature was fixed, 1 ml benzene and desired amount of hydrogen
peroxide 30% was added to the suspension. The mixture was stirred
constantly at the required temperature for 1–8 h. Conversions and
selectivities were calculated by gas chromatography using toluene
as internal standard. Under this condition in addition to phenol
as main product, 1,4-benzoquinone was detected as byproduct
(Scheme 1)
2.3. Preparation of amino-functionalized SBA-15
Amine modification of SBA-15 was performed according to
reported procedure [24]. In a typical preparation 2 g SBA-15 was
heated up to 110 ◦C under vacuum and immediately was dispersed
in 50 ml dried toluene and was refluxed for 4 h to remove the occu-
pied moisture. 1 g APTES was dissolved in 10 ml toluene and was
added dropwise to above suspension and was refluxed for another
4 h. The mixture was cooled to room temperature and the solid was
filtered. The resulting solid was washed three times with warm
ethanol to remove unreacted APTES and then was dried at room
temperature. The product is designated as NH2-SBA-15.
mmol phenol + mmol benzoquinone
Conversion of benzene =
mmol initial benzene
Selectivity to phenol (1, 4-benzoquinone)
mmol phenol (1, 4-benzoquinone)
mmol phenol + mmol benzoquinone
=
2.7. Apparatus and instruments
2.4. Synthesis of di-vanadium substituted
molybdovanadophosphoric acid
X-ray pattern was obtained on a Bruker AXSD8 diffractome-
ter at 2ꢀ = 1–10◦ with Ni filtered Cu K␣ radiation (ꢁ = 1.5418 A)
˚
operated at 40 kV and 30 mA. N2 adsorption–desorption isotherms
were obtained using BELSORP-mini II at 77 K. SBA-15, NH2-
SBA-15 and NH2-SBA-15 were degassed at 200 ◦C, 100 ◦C and
100 ◦C respectively for 3 h. The specific surface areas were
calculated with Brunauer–Emmett–Teller (BET) equation and
Barret–Joyner–Halenda (BJH) method was used for computing the
pore size distribution and total pore volume. V and Mo contents
were determined by ICP using varian VISTA-MPX model. The FT-
IR spectra for the powdered samples were obtained over a range
of 600–4000 cm−1 on a EQUINOX 55BRUKER. Thermal gravimet-
ric analysis (TGA) was conducted on a TGA Q50 V6.3. Sample
sizes of 11.929 and 4.626 mg were used for HPMoV, and NH2-
SBA-15, respectively. The SEM images of SBA-15, NH2-SBA-15 and
HPMoV/NH2-SBA-15 were obtained using Philips XL30.
Vanadium
substituted
molybdophosphoric
acid
(H5PMo10V2O40·xH2O) was synthesized at the P/V/Mo molar
ratio of 1/2/10 using MoO3, V2O5 and H3PO4 85% [22]. 9.6 g,
66.7 mmol MoO3, and 1.214 g, 6.67 mmol V2O5 were dispersed
in 150 ml deionized water in a 500-ml three-necked glass flask
equipped with a condenser and magnetic bar. The suspension was
heated to reflux temperature using an oil bath. 1.15 g, 6.67 mmol
85% H3PO4, was added dropwise to the suspension. The above
mixture was maintained at 120 ◦C and stirred for 24 h, dark
suspension turned to a clear orange–red solution. The solvent was
evaporated to yield a solid product.
2.5. Anchoring of molybdovanadophosphoric acid onto
NH2-SBA-15
0.1 g of molybdovanadophosphoric acid (HPMoV) was dissolved
in 50 ml acetonitrile and then 0.9 g NH2-SBA-15 was dispersed
in above solution and refluxed for 4 h. The product was filtered,
washed with acetonitrile and then dried at 100 ◦C under vacuum.
The final product is designated as HPMoV/NH2-SBA-15. This cata-
lyst was used in hydroxylation of benzene to phenol and its activity
was compared with impregnated HPAs on HY and MCM-41. For
impregnation, typically 0.1 g HPMoV dissolved in 1:1 deionized
water–ethanol solution and then 0.9 g support (HY or MCM-41)
was added and the resulting suspension was stirred for 24 h at
3. Result and discussion
3.1. Catalyst characterization
The XRD patterns for SBA-15 and HPMoV/NH2-SBA-15 were
illustrated in Fig. 1. Three reflections, one strong in 2ꢀ less than
1 and two in 2ꢀ 1.6 and 1.8 with lower intensity attributed to
(1 0 0), (1 1 0) and (2 0 0) planes are all well consistent with hexag-
onal pore arrays. The diffraction patterns indicate that the primary
structure of SBA-15 are maintained after modification with APTES

16
A. Nemati Kharat et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 14–19
Fig. 1. The XRD patterns of SBA-15 and NH2-SBA-15–HPMoV.
Fig. 2. Adsorption–desorption isotherms of SBA-15, NH2-SBA-15 and HPMoV/NH2-
SBA-15.
and anchoring with HPMoV. As we can see the reflections of modi-
fied sample is reduced in intensity and as a result we can conclude
that the modification of SBA-15 with APTES and anchoring with
HPMoV occurred in the pores of SBA-15.
Nitrogen adsorption–desorption for SBA-15, NH2-SBA-15 and
HPMoV/NH2-SBA-15 were performed by BET method at 77 K and
resultant isotherms are depicted in Fig. 2. Isotherm curves for all
samples exhibit a type IV isotherms which indicate all samples have
well ordered mesoporous structure. The textural properties of sam-
ples including specific surface area, pore diameter and total pore
volume are given in Table 1. Because of modification of SBA-15 with
APTES, a reduction in specific surface area from 554 to 281 m2/g can
be seen, and also loading this catalyst with heteropoly acid leads
Fig. 3. SEM images of the (A) SBA-15, (B) NH2-SBA-15 and (C) HPMoV/NH2-SBA-15.
to more reduction in specific surface area. Similarly same behavior
are expected for pore diameter and total pore volume.
The SEM images for SBA-15, NH2-SBA-15 and HPMoV/NH2-
SBA-15 are shown in Fig. 3. All samples show robe-like structure
which is well consistent with known SBA-15 structure [25,26]. SEM
image also provides evidence for the structural integrity of SBA-15,
NH2-SBA-15 and HPMoV/NH2SBA-15. We can infer that function-
alization with APTES and anchoring with HPMoV have no strong
effect on pure SBA-15.
Table 1
The specific surface area (SBET), pore diameter and total pore volume of SBA-15,
NH2-SBA-15 and HPMoV/NH2-SBA-15.
Samples
SBET (m2/g)
Pore diameter
(nm)
Total pore
volume (cm3/g)
SBA-15
NH2-SBA-15
HPMoV/NH2-SBA-15
554.03
281.32
237.43
6.18
6.18
5.42
0.8586
0.4884
0.4097
Fig. 4 shows the FT-IR spectroscopy of H5PMo10V2O40, NH2-
SBA-15 and HPMoV/NH2-SBA-15. The Keggin type heteropoly acid

A. Nemati Kharat et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 14–19
17
3.2. Catalyst performance
The catalytic activity of HPMoV supported on Amine function-
alized SBA-15 was tested in direct hydroxylation of benzene in
acetonitrile as solvent and the results were shown in Table 2. The
activity of supported HPMoV were compared with unsupported
HPMoV and HPMo. Also in order to show the effect of modification
on the leaching supports the activity of HPMoV/NH2-SBA-15 was
compared with HPMoV/HY and HPMoV/MCM-41.
As can be seen, pure HY, MCM-41 and SBA-15 show no activity
in conversion of benzene to phenol and also it is clear that HPMo
has no activity in this particular reaction (see run 1–4). By com-
parison of run 4 with run 5, we concluded that vanadium species
are active species in this reaction. Comparison 6 with 7 and 8
with 9 lead us that molybdovanadophosphoric acid leached from
HY and MCM-41 but the values of benzene conversion in run 10
and 11 expressed that no leaching has occurred when functional-
ized support is used. Therefore it was chosen as desirable catalyst
and reaction conditions, such as reaction temperature, reaction
time, amount of catalyst and H2O2 were investigated for this
catalyst.
Fig. 4. FT-IR spectrum of (A) HPMoV, (B) NH2-SBA-15 and (C) HPMoV/NH2-SBA-15.
3.3. Effect of reaction temperature
has four characteristic peaks in range of 750–1100 cm−1 [27]. It
can be seen that peaks appeared at 734 cm−1 and 875 cm−1 corre-
spond to Mo–O–Mo and at 948 cm−1 and 1049 cm−1 are related to
In order to investigate the effect of temperature on the cat-
alytic activity, several discrete reactions was carried out under the
same reaction conditions and the results are depicted in Fig. 6.
In low temperatures phenol was detected as the only product.
Increasing in reaction temperature leads to decrease in selec-
tivity toward phenol. This reduction in selectivity is related to
further oxidation of phenol at higher temperature. As illustrated
in Fig. 6 increase in the conversion of benzene was observed along
with increase in temperature up to 60 ◦C, and then, decrease in
benzene conversion, with further increase in reaction temper-
ature was observed, this decrease is probably because of two
reasons, first, the spontaneous decomposition of H2O2 to O2 at
high temperatures [28] and second, an alteration in active species
[29–32].
Previous reports manifestly explained that vanadic oxide
specious (VO2+) detach from the Keggin unit at high temperatures.
The presence of these species were well proved by carrying out
51V NMR, 31P NMR and FT-IR spectroscopy for catalysts before and
after reaction and have proved that this specious are responsible
for the deactivation of recovered HPMoV for the phenol formation
[31]. Both of two mentioned factors are responsible for reduction in
benzene conversion in high temperatures. According to what men-
M
O and P–O that explicitly indicates the neat structure of Keg-
gin type heteropoly acid. In FT IR spectrum of HPMoV/NH2-SBA-15
these peaks overlaps with NH2-SBA-15, therefore these informa-
tion cannot confirm that loading of heteropoly acid was occurred
over NH2-SBA-15. However due to appearance of two new peaks in
FT-IR spectrum of HPMoV/NH2-SBA-15 in range of 850–950 cm−1
in comparison with NH2-SBA-15 it can induce that HPMoV actually
exists in the pores of NH2-SBA-15.
Purity of synthesized heteropoly acid was confirmed by
measuring Mo/V molar ratio by ICP. Fig. 5(A) represents the ther-
mogravimetric analysis of HPMoV that shows mass loss up to 150 ◦C
which is related to physically absorbed water. This amount of mass
loss, 10.38%, is related to 11.12 H2O per Keggin unit. The mass
loss from 180 to 450 ◦C is related to decomposition of HPMoV
to corresponding oxides. Also in order to determine the value of
reacted APTES, thermal gravimetric analysis was applied. Fig. 5(B)
shows the TGA profile for NH2-SBA-15. The mass loss in range of
100–600 ◦C is attributed to removal of carbon chain and NH2 group.
The mass loss due to organic compounds was 6.8867% of the initial
mass, therefore the amount of NH2 was determined 1.2 mmol.
Fig. 5. Thermal gravimetric analysis of (A) HPMoV and (B) NH2-SBA-15.

18
A. Nemati Kharat et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 14–19
Table 2
Direct oxidation of benzene to phenol with aqueous hydrogen peroxide using different catalysts.a
Selective conversion
of H2O2
Selectivity to
banzoquinone
Selectivity to
phenol
Benzene
conversion%
Catalyst
Run
–
–
–
–
10.3
9.00
2.00
8.16
4.80
8.03
7.63
–
–
–
–
8
7
5
7
7
5
5
–
–
–
–
92
93
95
93
93
95
95
0
0
0
HY
SBA-15
MCM-41
H5PMo12O40
1
2
3
4
5
6
7
8
9
0
25.4
22
5
20
11
20
19
(HPMoV) H5PMo10V2O40
HY/HPMoV
HYb/HPMoV
MCM-41/HPMoV
MCM-41b/HPMoV
NH2-SBA-15/HPMoV
NH2-SBA-15b/HPMoV
10
11
a
Reaction conditions: 1 ml benzene, 3 ml 30% H2O2, 6 ml acetonitrile, 0.1 g. HPMoV/NH2-SBA-15, temperature 60 ◦C, reaction time 6 h.
Reuse of the catalyst after separation from the reaction mixture.
b
tioned above and (Fig. 6), 60 ◦C is considered as a optimum reaction
temperature.
3.4. Effects of reaction time
The influence of reaction time in hydroxylation of benzene
in 60 ◦C was investigated by several separate reaction under the
same reaction conditions and results are depicted in Fig. 7. It can
be observed that the conversion of benzene increases along with
expansion the reaction time up to 8 h, Benzene conversion was not
affected by further increasing in temperature and reached a plateau
in the reaction period of 8–9 h; however, the selectivity to phenol
decreased with reaction time due to further oxidation of phenol.
The suitable reaction time for oxidation of benzene, was found to
be 6 h.
3.5. Effect of the amount of H2O2
Fig. 7. Effect of reaction time on the conversion of benzene and selectivity to phenol
using HPMoV/NH2-SBA-15a. aReaction conditions: 1 ml benzene, 3 ml 30% H2O2,
6 ml acetonitrile, HPMoV/NH2-SBA-15, reaction temperature 60 ◦C.
The impact of the amount of H2O2 on the hydroxylation of ben-
zene was investigated and results are portrayed in Fig. 8. It is seen
that the conversion of benzene increased while the amount of H2O2
increased and also it is observed that the selectivity toward phenol
decreased when more than 3 ml H2O2 (molar ratio H2O2 to ben-
zene equals to 2.6) was employed which is due to further oxidation
of phenol. Stoichiometrically in oxidation of benzene to phenol,
the molar ratio of H2O2 to benzene is 1:1, but because of sponta-
neous decomposition of hydrogen peroxide, in practice, we needed
2.6 times more stoichiometric volume. In fact, in the reaction of
benzene hydroxylation, the amount of consumed hydrogen perox-
ide by self-decomposition is much more than that of consumed in
the hydroxylation reaction [29] and this may provide some under-
Fig. 8. Effect of the amount of hydrogen peroxide on the conversion of benzene
and selectivity to phenol using HPMoV/NH2-SBA-15a. aReaction conditions: 1 ml
benzene, 6 ml acetonitrile, 0.1 g HPMoV/NH2-SBA-15, reaction temperature 333 K,
reaction time 6 h.
Fig. 6. Effect of reaction temperature on the conversion of benzene and selectivity
to phenol using HPMoV/NH2-SBA-15a. aReaction conditions: 1 ml benzene, 3 ml 30%
H2O2, 6 ml acetonitrile, 0.1 g. HPMoV/NH2-SBA-15, reaction time 6 h.

A. Nemati Kharat et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 14–19
19
4. Conclusion
In this work, we prepared molybdophosphoric acid contain-
ing vanadium supported on amine functionalized SBA-15 and
employed this catalyst for direct hydroxylation of benzene to phe-
nol in acetonitrile medium. This catalyst shows good activity in
this reaction, in addition owning to strong acid–base interaction
between heteropoly anions and amine group it exhibits good sta-
bility against leaching. It is found that vanadium species are active
sites in this certain reaction.
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Chem. Res. 44 (2005) 1–7.
In order to investigation of catalyst stability, vanadium substi-
tuted Keggin anion supported on amine functionalized SBA-15, was
reused several times. After each run, the catalyst was separated by
centrifuge, washed with acetonitrile and used for the next run. After
the fifth run, the conversion of benzene decreased to 15% (equal
+
to 75% activity of the fresh catalyst). As mentioned VO2 specious
[32] N.A. Alekar, et al., J. Mol. Catal. A: Chem. 164 (2000) 181–189.
can be segregate from Keggin unit and led to reduction in catalyst
activity.