Microwave-Assisted hydroxylation of benzene to phenol over Fe(III)/TiO 2 catalysts at room temperature

Article abstract of DOI:10.14233/ajchem.2014.15514

Liquid phase hydroxylation of benzene to phenol with hydrogen peroxide over transition metals supported on TiO₂ catalysts prepared by impregnation method was studied under microwave irradiation. This paper investigated the catalytic performances of TiO₂ supports prepared by various calcined temperature and time. The catalytic activity of Co(II), Cu(II) and Fe(III) metal oxide support on TiO₂ for hydroxylation benzene to phenol was investigated. Coupled conventionally heated method gave phenol yield of 1.5 % and selectivity of 80 %, higher phenol yield of 7.8 % and selectivity of 98 % were obtained when the 5 % loading amounts of Fe(III) under microwave condition. Various reaction parameters, such as reaction time, amount of catalyst and hydrogen peroxide, were investigated to obtain an optimal reaction conditions for phenol formation.

Full text of DOI:10.14233/ajchem.2014.15514

Asian Journal of Chemistry; Vol. 27, No. 7 (2015), 2489-2491
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2015.17944
Low Temperature Hydroxylation of Benzene to Phenol Over
SiO2 as Supported Catalysts Under Microwave Irradiation
1,*
1
2
TONG LIU , HAO CHEN and JINHUI HOU
1School of Chemistry & Chemical Engineering, Xuzhou Institute of Technology, Xuzhou 221111, Jiangsu Province, P.R. China
2School of Food (Biology) Engineering, Xuzhou Institute of Technology, Xuzhou 221111, Jiangsu Province, P.R. China
*Corresponding author: E-mail: liutongcumt@126.com
Received: 5 May 2014;
Accepted: 30 June 2014;
Published online: 30 March 2015;
AJC-17063
This paper investigated the microwave assisted liquid phase hydroxylation of benzene to phenol with hydrogen peroxide over V(V)/SiO2
catalysts prepared by microwave impregnated method. The catalysts were characterized by XRD, N2 adsorption techniques. The results
manifested that the performance of the catalysts was significantly affected by the amount of V(V) loaded onto the SiO2 and microwave
irradiation time. The effects of various reaction parameters, such as the amount of catalyst used, the amount of hydrogen peroxide used
and reaction time on the yield of phenol was investigated at room temperature under microwave irradiation. Coupled conventionally
heated method gives phenol yield of 11.8 %, the V(V)/SiO2 prepared by microwave impregnated method gives higher phenol yield of
19.7 % and selectivity of 100 % under microwave irradiation.
Keywords: Microwave assisted, Hydroxylation, Benzene; V(V)/SiO2.
by the microwave impregnated method and indicated that the
performance of the catalysts was significantly affected by the
amount of V(V) loaded onto the SiO2 and microwave irradia-
tion time. In addition, various techniques were employed to
characterize the synthesized catalysts. Finally, we also explored
the optimal reaction conditions for phenol formation at room
temperature under microwave irradiation, obtained phenol
yield of 19.7 % and selectivity of 100 %.
INTRODUCTION
Phenol is one of the most important chemicals in industry
and widely used in the fields of resins, fibers and medicines1-3.
Currently, most of the worldwide production of phenol is now
based on the "cumene process", consisting of three main reac-
tion steps. However, this process has several disadvantages
i.e., low atom utilization, low phenol yield, high energy con-
sumption and the production of equal amount of acetone as
the by product4,5. Consequently, this situation has prompted
scientists to develop other methods for producing phenol from
benzene, preferably via a single-step and free of co-product(s)
reaction6,7. In recent years, microporous and mesoporous mate-
rials as the promising supports have been reported for the
liquid-phase hydroxylation process with clean oxidant H2O2,
such as the transition metal oxide catalysts supported on SBA-
EXPERIMENTAL
Materials and catalyst preparation: All the reagents,
including benzene, NH4VO3, FeCl3·6H2O, Cu(NO3)2·3H2O,
H2O2 (30 % by weight) and acetonitrile, were commercially
purchased and used without further purification.
The procedure for the preparation by the microwave
impregnated method was as follows: The aqueous solution
which dissolved the prescribed amounts of NH4VO3 and SiO2
along with a magnetical stirrer were placed into a 100 mL
CEM discover microwave reactor. The reactor was heated to
80 °C and kept at that temperature for 5-30 min. Then the
solution containing precipitates was filtrated and the solid
product obtained was further dried at 70 °C overnight in
vacuum oven. The catalysts were stored in a dessicator.
Catalytic tests: The liquid-phase hydroxylation of benzene
was carried out: 1 mL benzene, 35 °C reaction temperature,
described amounts of solvent, catalyst and H2O2 were added
16, Al2O3, TiO2, MCM-41, activated carbon8-10
.
Compared to conventional heating method, microwave
irradiation is well documented to significantly accelerate a
wide range of chemical reactions, especially in a polar system,
under mild conditions. Moreover, microwave irradiation has
many advantages, such as no direct contact between the energy
source and the reacting chemicals, increasing energy efficiency,
reducing heat transfer problems, easy automation and incident
power control11-13
.
In the present work, microwave assisted the hydroxylation
of benzene to phenol was investigated overV(V)/SiO2 prepared

2490 Liu et al.
Asian J. Chem.
TABLE-1
into a 50 mL CEM discover microwave reactor with a magne-
tical stirrer. The reactor was heated to 35 °C and kept at that
temperature for 5-20 min. The reaction mixture was analyzed
with a Hewlett-Packard 6890/5973 GC/MS equipped with a
capillary column coated with HP-5MS (cross-link 5 % PH
ME siloxane, 30 m × 0.32 mm i.d., 0.25 µm film thickness)
and a quadrupole analyzer and operated in electron impact
(70 eV) mode. The mass range scanned was from 30 to 500
amu. Data were acquired and processed using Chemstation
software. The compounds were identified by comparing mass
spectra with NIST05 library data.
The conversion of each substrate was calculated as molar
ratio of the substrate remained in the reaction mixture to that
added and the selectivity of each product was calculated as
molar ratio of the product to all the products.
X-ray diffraction measurements were carried out with a
D/max-RA X-ray diffractometer (Rigaku, Japan), CuKα radia-
tion at a scan rate of 3°/min. The tube voltage and current were
35 kV and 35 mA, respectively. The phases were identified by
comparing the diffraction patterns to standard powder XRD
cards compiled by the Joint Committee on Powder Diffraction
Standards (JCPDS).
EFFECT OF THE TRANSITION METAL SPECIES/SiO2
CATALYSTS PREPARED BY MICROWAVE IMPREGNATED
METHOD ON CATALYTIC ACTIVITY
Catalyst (1 mmoL)
NH4VO3/SiO2
FeCl3/SiO2
Conversion (%)
Selectivity (%)
19.7
15.2
7.5
100
100
95
Cu(NO3)2/SiO2
Reaction conditions: 1.5 g catalyst, 1 mL benzene, 1.5 mL H2O2, 15
mL acetonitrile, 35 °C, 15 min
TABLE-2
BET DATA FOR THE TRANSITION METAL SPECIES/SiO2
PREPARED BY MICROWAVE IMPREGNATED METHOD
Catalyst
(1 mmoL)
NH4VO3/SiO2
FeCl3/SiO2
Cu(NO3)2/SiO2
Surface area
(m2.g-1)
349.1
358.8
371.2
Total pore volume
(mLg-1)
0.65
0.68
0.71
suggesting that no bulk metal oxide was presented in all
catalysts, which could be due to the low metal loading or to
the oxide being in the amorphous state (not shown).
The results for the catalytic performances of V(V)/SiO2
catalysts prepared from the three amounts (e.g., 0.5 mmoL,
1 mmoL, 1.5 mmoL) of V(V) loading under microwave
irradiation are shown in Table-3. It is noticed that the benzene
conversion increased with increasing the amount of V(V)
loaded, however, the selectivity of phenol decreased slightly
when the amount ofV(V) loading added to 1.5 mmoL. It is because
phenol can be further reacted to give catechol, benzoquinone
and hydroquinone, which are undesirable byproducts. The
specific surface area of the catalysts decreased with increased
amount of metal loading (Table-4), indicating that some
micropore blockage by metal deposition on pore mouth. Thus, 1
mmoL is choosen as an optimum V(V) loading for all catalysts.
The BET surface area, total pore volume and pore size
distribution of the fresh catalysts were determined via N2
physisorption at the normal boiling point of N2 (-196 °C), using
an Autosorb-1 MP instrument (Quanta chrome, USA).
The terms of reaction performance were defined as follows:
Mole of phenol produced
Yield of phenol =
Initial mole of benzene
Mole of phenol produced
Selectivity of phenol =
Mole of benzene reacted
RESULTS AND DISCUSSION
TABLE-3
Evaluation of performances of catalysts preparation conditions
EFFECT OF THE DIFFERENT LOADING CONTENTS IN
NH4VO3/SiO2 CATALYSTS PREPARED BY MICROWAVE
IMPREGNATED METHOD ON THE YIELD OF PHENOL
Effects of transition metal species: The transition metal/
SiO2 catalysts were prepared by microwave impregnated
method from three precursors i.e., NH4VO3, FeCl3, Cu(NO3)2.
The influence of the catalytic activity of the different transition
metal species supported SiO2 on the benzene conversion and
phenol selectivity under microwave irradiation are summarized
in Table-1. The results reveal that the activity of different transi-
tion metal catalysts followed the orderV(V) > Fe(III) > Cu(II).
However, BET surface area and pore volume followed the
opposite order (as shown in Table-2). The results indicate that
the metal V(V) was located larger amount on the surface of
SiO2 than Fe(III) and Cu(II). It is known that the catalytic
activity of the transition metal species is dependent on the
outer d electron density. The electron density ofV(V) is lesser
than Fe(III) and Cu(II). So, the capacity of V(V) to activate
oxidant species is greater. Meanwhile, the accessibility of metal
species into the catalyst pores followed the orderV(V) > Fe(III)
> Cu(II). Therefore,V(V) is considered as a suitable precursor.
Effect of theV(V) loading: The XRD patterns of samples
with various amounts (e.g., 0.5 mmoL, 1 mmoL, 1.5 mmoL)
of V(V)/SiO2 are determined. However, no obvious peaks of
V2O5 crystalline phases were found in the XRD patterns,
NH4VO3/SiO2 (mmoL)
Conversion (%)
Selectivity (%)
0.5
1.0
1.5
11.7
19.7
21.5
100
100
83
Reaction conditions: 1.5 g catalyst, 1 mL benzene, 1.5 mL H2O2, 15
mL acetonitrile, 35 °C, 15 min
TABLE-4
BET DATA FOR THE DIFFERENT LOADING CONTENTS
IN NH4VO3/SiO2 CATALYSTS PREPARED BY
MICROWAVE IMPREGNATED METHOD
NH4VO3/SiO2
(mmoL)
Surface area
(m2 g-1)
Total pore volume
(mLg-1)
0.5
1.0
1.5
360.8
349.1
339.2
0.68
0.65
0.61
Effect of the microwave irradiation time: The effect of
the microwave irradiation time over V(V)/SiO2 catalysts
prepared by microwave impregnated method at 80 °C on the
conversion of benzene was investigated and the results is shown
in Table-5. Increasing of microwave irradiation time from 5

Vol. 27, No. 7 (2015)
Low Temperature Hydroxylation of Benzene to Phenol Over SiO2 as Supported Catalysts 2491
TABLE-7
TABLE-5
EFFECT OF THE MICROWAVE
EFFECT OF THE H2O2 AMOUNT ON THE YIELD OF PHENOL
IRRADIATION TIME ON CATALYTIC ACTIVITY
H2O2 (moL)
Conversion (%)
Selectivity (%)
Time (min)
Conversion (%)
Selectivity (%)
0.5
1.0
1.5
2.0
5.2
12.3
19.7
22.5
100
100
100
77
5
10
20
30
7.8
13.2
19.7
15.0
100
100
100
100
Reaction conditions: 1.5 g catalyst, 1 mL benzene, 15 mL acetonitrile,
35 °C, 15 min
Reaction conditions: 1.5 g catalyst, 1 mL benzene, 1.5 mL H2O2, 15
mL acetonitrile, 35 °C, 15 min
TABLE-8
EFFECT OF THE REACTION TIME ON THE YIELD OF PHENOL
to 20 min, the conversion of benzene was found to increase
from 7.8 to 19.7 %. However, further increased to 0.5 h, the
conversion of benzene decreased significantly. Thus, 20 min
is considered as a suitable irradiation time.
Time (min)
Conversion (%)
Selectivity (%)
5
10
15
20
6.8
13.7
19.7
21.5
100
100
100
81
Effect of the reaction conditions on the yield of phenol under
microwave irradiation
Reaction conditions: 1.5 g catalyst, 1 mL benzene, 1.5 mL H2O2, 15
mL acetonitrile, 35 °C
Effect of the catalyst amount on the yield of phenol:
The results for the effect of the amount of catalyst on the yield
of phenol investigated overV(V)/SiO2 prepared by microwave
impregnated method at room temperature under microwave
irradiation is shown in Table-6. It was indicated that the catalyst
amount increased from 0.5 to 1.5 g, the conversion of benzene
increased sharply from 7.7 to 19.7 %, but a further increase in
the amount of catalyst inversely caused a decrease in the selec-
tivity of phenol, which result from the further oxidation of the
phenol formed, the benzoquinone was formed. Therefore, 1.5 g
V(V)/SiO2 is considered as a suitable amount in this reaction.
be caused by the further oxidation of the phenol. So, 15 min is
chosen as a suitable reaction time in this reaction.
Conclusion
The liquid phase hydroxylation of benzene to phenol
with hydrogen peroxide catalyzed byV(V) supported on TiO2
catalysts prepared microwave impregnated method under
microwave irradiation at room temperature was investigated.
The optimum preparation condition forV(V)/SiO2 is 1 m moL
loading amount and 20 microwave irradition time. The activity
of transition metals supported on SiO2 catalyst in production
of phenol was V(V) > Fe (III) > Cu(II). Compared to conven-
tionally heated method gives phenol yield of 11.8 %, theV(V)/
SiO2 prepared by the microwave impregnated method gives
higher phenol yield of 19.7 % and selectivity of 100 % at the
optimum reaction conditions: l mL benzene, 15 mL acetoni-
trile, 1.5 g catalyst, 1.5 mL 30 % aqueous solution of H2O2, 15
min reaction time, 35 °C reaction temperature.
TABLE-6
EFFECT OF THE CATALYST
AMOUNT ON THE YIELD OF PHENOL
NH4VO3/SiO2 (g)
Conversion (%)
Selectivity (%)
0.5
1.0
1.5
2.0
7.7
12.1
19.7
22.5
100
100
100
81
Reaction conditions: 1 mL benzene, 1.5 mL H2O2, 15 mL acetonitrile,
35°C, 15 min
ACKNOWLEDGEMENTS
The authors are grateful to the Natural Science Foundation
of China (Projects 51374205) for the financial support.
Effect of the amount of H2O2 for the hydroxylation of
benzene: The effect of the amount of H2O2 for the hydroxyla-
tion of benzene on the yield of phenol overV(V)/SiO2 prepared
by microwave impregnated method at room temperature under
microwave irradiation is shown in Table-7. When the amount
of H2O2 increased from 0.5 to 1.5 mL, the yield of phenol
increased from 5.2 to 19.7 %. However, when the amount of
H2O2 was further up to 2 mL, the yield of phenol dropped
quickly. It can be explained that further oxidation of phenol to
hydroquinone or benzoquinone can be occurred with the
presence of excessive amount of H2O2 oxidant. So, 1.5 mL is
considered as a suitable amount in this reaction.
Effect of the reaction time on the yield of phenol: The
effect of reaction time on the yield of phenol over V(V)/SiO2
prpeared by microwave impregnated method at room tempe-
rature under microwave irradiation is shown in Table-8. It can
be observed that the conversion of benzene increased signi-
ficantly from 6.8 to 19.7 % when the reaction time increased
from 5 to 15 min and after that, the yield decreased sharply
with the further increase of reaction time to 20 min. This may
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