Iron(III) supported g-Al2O3 catalyst for hydroxylation of benzene

Article abstract of DOI:10.14233/ajchem.2014.15884

The liquid-phase catalytic oxidation of benzene to directly produce phenol was studied under microwave irradiation using FeCl₃/g-Al ₂O₃ as the catalysts prepared by the microwave-impregnated method. The catalysts were char

Full text of DOI:10.14233/ajchem.2014.15884

Asian Journal of Chemistry; Vol. 26, No. 11 (2014), 3209-3212
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.15884
Iron(III) Supported γ-Al2O3 Catalyst for Hydroxylation of Benzene
2
T. LIU1,* and J.H. HOU
1School of Chemistry and 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: 8 June 2013;
Accepted: 16 August 2013;
Published online: 25 May 2014;
AJC-15214
The liquid-phase catalytic oxidation of benzene to directly produce phenol was studied under microwave irradiation using FeCl3/γ-Al2O3
as the catalysts prepared by the microwave-impregnated method. The catalysts were characterized by XRD and N2 adsorption techniques.
The results indicated that the performance of the catalysts was significantly affected by the amount of Fe(III) loaded onto the γ-Al2O3 and
microwave irradiation time. The influence of the reaction conditions, such as reaction temperature, the amount of catalyst, amount of
hydrogen peroxide on the yield of phenol was investigated under microwave irradiation. Coupled conventionally heated method gives
phenol yield of 9.8 %, the FeCl3/γ-Al2O3 prepared by the microwave- impregnated method gives higher phenol yield of 15.2 % and
selectivity of 100 % when irradiated with microwave energy.
Keywords: Benzene, Phenol, FeCl3/γ-Al2O3, Microwave-impregnated, Hydrogen peroxide.
INTRODUCTION
EXPERIMENTAL
Phenol is the most important intermediates for the manufac-
turing of petrochemicals, agrochemicals and polymers1,2. More
than 90 % of the world production is obtained by the cumene
process. However, this process has several disadvantages e.g.,
low one-pass yield of phenol, high energy cost and the produc-
tion of equal amount of acetone as the by product3. Therefore,
the direct hydroxylation of benzene to phenol with high atom
utilization and high selectivity has attracted much attention4,5.
In recent years, this interesting work has been extensively
studied in finding a suitable solid catalyst for the selective
oxidation of benzene to phenol under mild reaction conditions
with H2O2 as an oxidant6-10. The solvent is usually added to
the reaction system for dissolving hydrogen peroxide and
benzene into one phase for the oxidation of benzene to phenol.
Several solvents such as acetone, acetonitrile, acetic acid,
pyridine and dichloromethane have been employed. However,
there are only a few works comparing the effect of solvent
All the reagents, including benzene, γ-Al2O3 (200 mesh),
FeCl3·6H2O, H2O2 (30 % by weight) and acetonitrile, were
commercially purchased and used without further purification.
Catalyst preparation: The aqueous solution which
dissolved the prescribed amounts of FeCl3·6H2O and γ-Al2O3
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.
Catalytic tests: A typical oxidation reaction is as follows:
1 mL of benzene, 15 mL of acetonitrile, described amounts of
catalyst and H2O2 were added into a 50 mL CEM discover
microwave reactor with a magnetical stirrer. The reactor was
heated to an indicated temperature and kept at that temperature
for 10-40 min. Then the reactor was immediately cooled to
room temperature by purging compressed air. The reaction
mixture was analyzed by gas chromatography.
types11,12
.
In the present work, we prepared the FeCl3/γ-Al2O3 by the
microwave-impregnated method and indicated that the perfor-
mance of the catalysts was significantly affected by the amount
of Fe loaded onto the γ-Al2O3 and microwave irradiation time.
We also explored the optimal reaction conditions for phenol
formation under microwave irradiation, obtained phenol yield
of 15.2 % and selectivity of 100 %.
Characterization: X-ray diffraction measurements were
carried out with a D/max-RA X-ray diffractometer (Rigaku,
Japan), CuKα radiation 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).

3210 Liu et al.
Asian J. Chem.
The BET surface area, total pore volume and pore size
BET surface area and pore volume followed the opposite order
(as shown in Table-2). The results indicate that the Fe(III) was
located more at the surface of γ-Al2O3 than other metal species.
In addition, it is clear that the type of precursor influenced the
metal distribution in the catalysts. As known, the catalytic
activity of the transition metal species is dependent on the
outer d electron density. Lesser the electron density, greater is
the capacity of these transition metals to activate oxidant
species like hydrogen peroxide. From the point of view of
electron density the catalyst activity should be Fe(III) > Cu(II)
> Co(II). So, Fe(III) is considered as a suitable precursor.
distribution of the fresh catalysts were determined via N2
physisorption at the normal boiling point of N2 (-196 °C), using
an Autosorb-1MP 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
TABLE-1
RESULTS AND DISCUSSION
EFFECT OF THE TRANSITION METAL SPECIES/γ-Al2O3
CATALYSTS PREPARED BY MICROWAVE IMPREGNATED
METHOD ON CATALYTIC ACTIVITY
Catalyst characterization: The XRD patterns of samples
with the different loaded amount of FeCl3/γ-Al2O3 catalysts
prepared by the microwave-impregnated method was shown
in Fig. 1. The broad diffraction peaks are observed over a range
of 20° to 70°, which is in agreement with of γ-Al2O3 structure
reported by the literature13. This indicates that the γ-Al2O3
structure was retained in the catalysts modified with promoters.
In addition, no characteristic peaks of crystalline Fe2O3 phases
were found in high-angle XRD patterns, 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.
Catalyst (5 wt %)
FeCl3/γ-Al2O3
Conversion (%)
Selectivity (%)
15.2
5.2
100
100
95
Cu(NO3)2/γ-Al2O3
Co(NO3)2/γ-Al2O3
4.5
Reaction conditions: 0.4 catalyst, 1 mL benzene, 3 mL H2O2, 15 mL
acetonitrile, 70 °C, 15 min
TABLE-2
BET DATA FOR THE TRANSITION METAL SPECIES/γ-Al2O3
PREPARED BY MICROWAVE IMPREGNATED METHOD
Catalyst
(5 wt. %)
Surface
Total pore
Average pore
area (m2 g-1) volume (mL g-1) diameter (nm)
256.4
281.1
0.44
0.51
5.74
6.48
FeCl3/γ-Al2O3
Cu(NO3)2/
γ-Al2O3
a:3 wt% Fe/ -Al O
γ
2
3
b:5 wt% Fe/
-Al O
2
γ
3
Co(NO3)2/
γ-Al2O3
292.3
0.56
6.58
c: 7 wt% Fe/
-Al O
γ
2
3
Effect of the Fe(III) loading: The effect of the amount
(e.g., 3, 5, and 7 %) of Fe loaded onto the surface of the FeCl3/
γ-Al2O3 catalysts prepared by microwave impregnation method
on the yield of phenol was shown in Table-3. The benzene
conversion increased with increasing the amount of Fe loaded,
while the selectivity decreased slightly. This situation because
phenol can be further reacted to give catechol, benzoquinone
and hydroquinone, which are undesirable by products. Thus,
5 % is considered as a suitable amount.
c
b
a
TABLE-3
EFFECT OF DIFFERENT LOADED AMOUNT IN FECL3/γ-Al2O3
CATALYSTS PREPARED BY MICROWAVE IMPREGNATED
METHOD ON CATALYTIC ACTIVITY
10
20
30
40
50
60
70
80
2θ (°)
Conversion (%)
Selectivity (%)
FeCl3/γ-Al2O3 (wt. %)
Fig. 1. XRD patterns of FeCl3/γ-Al2O3 prepared by microwave impregnated
method
3
5
7
11.2
15.2
17.2
100
100
84
Evaluation of performances of the catalysts preparation
conditions by microwave impregnation method
Reaction conditions: 0.4 g catalyst, 1 mL benzene, 3 mL H2O2, 15 mL
acetonitrile, 70 °C, 15 min
Effects of transition metal species: The transition metal/
γ-Al2O3 catalysts were prepared from three precursors: Fe(III)
trichloride, Cu(II) nitrate, Co(II) nitrate. The effects of the
transition metal species on the benzene conversion, phenol
selectivity and yields for different catalysts are summarized
in Table-1. It was noted that the activity of transition metal
catalysts followed the order Fe(III) > Cu(II) > Co(II). However,
Effect of the microwave irradiation time: The results
for the effect of the microwave irradiation time over 5 % (w/t)
FeCl3/γ-Al2O3 catalysts prepared by microwave- impregnation
method on the yield of phenol was investigated at 80 °C are
shown in Table-4. The phenol yield was found to increase with
the increase of microwave irradiation time from 5 to 20 min.

Vol. 26, No. 11 (2014)
Iron(III) Supported γ-Al2O3 Catalyst for Hydroxylation of Benzene 3211
However, further increased to 30 min, the yield of phenol
decreased slightly. Thus, 20 min is considered as a suitable
irradiation time.
of H2O2 inversely caused a decrease in the yield of phenol.
The yield of phenol had a maximum value at the amount of
H2O2 around 3 mL.
16
12
8
TABLE-4
EFFECT OF THE MICROWAVE IMPREGNATED
TIME ON CATALYTIC ACTIVITY
Time (min)
Conversion (%)
Selectivity (%)
5
7.8
100
100
100
100
10
20
30
11.2
15.2
14.0
Reaction conditions: 0.4 g catalyst, 1 mL benzene, 3 mL H2O2, 15 mL
acetonitrile, 70 °C, 15 min
4
Effect of the reaction conditions on the yield of phenol
under microwave irradiation
Effect of the reaction time: The influence of reaction
time on yield of phenol over 5 % (w/t) FeCl3/γ-Al2O3 prepared
by the microwave-impregnated method is shown in Fig. 2. It
is obvious that the yield of phenol increase remarkably when
the reaction time increased from 5 to 15 min and after that the
yield decreased with further increase of reaction time. The
decreased of yield may be caused by the further oxidation of
the product. So, 15 min is chosen as a suitable reaction time.
0
0.1
0.2
0.3
0.4
0.5
Amount of catalyst (g)
Fig. 3. Effect of catalyst amount on the yield of phenol, Reaction
conditions: 1 mL benzene, 3 mL H2O2, 15 mL acetonitrile, 70 °C,
15 min
16
12
16
8
4
12
8
0
1
2
3
4
Amount of H2O2 (mL)
4
Fig. 4. Effect of H2O2 amount on the yield of phenol, Reaction conditions:
0.4 g catalyst, 1 mL benzene, 15 mL acetonitrile, 70 °C, 15 min
A classic Fenton-type system for benzene oxidation to
phenol catalyzed by Fe(III)/γ-Al2O3 in the presence of H2O2
under microwave irridiation. The mechanism of Fenton system
is widely accepted as follow:
0
5
10
15
20
Reaction time (min)
Fig. 2. Effect of reaction time on the yield of phenol, Reaction conditions:
0.4 g catalyst, 1 mL benzene, 3 mL H2O2, 15 mL acetonitrile, 70 °C
Fe(II) + H2O2 → Fe(III) + OH• + OH–
(1)
OH• +
→ CHD
(2)
Effect of the amount of catalyst:As is indicated in Fig. 3,
when the amount of catalyst increased from 0.1 to 0.4 g, the
yield of phenol increased sharply from 6.2 to 15.2 %. On the
other hand, a further increase in the amount of catalyst caused
a decreased in the yield of phenol. This may be due to the
oxidation of benzene by the use of catalyst. Therefore, 0.4 g,
5 % (w/t) FeCl3/γ-Al2O3 is chosen as an optimum amount in
this reaction.
OH
Fe(III) + CHD → Fe(II) +
+ H+
(3)
Both Fe(III) and Fe(II) play important roles in the reaction
system. Fe(III) reacts with a cyclohexadienyl (CHD) radical
intermediate to form phenol, while Fe(II) promotes the decom-
position of hydrogen peroxide to a hydroxyl free radical. The
amount of generated hydroxyl radicals which affects the phenol
formation could be controlled by the amounts of H2O2 and metal
catalyst added.
Effect of the amount of H2O2: The effect of the amount
of H2O2 on the yield of phenol is shown in Fig. 4. When the
amount of H2O2 increased from 0.5 to 3 mL, the yield of phenol
increased from 5.2 to 15.2 %. But a further increase in amount

3212 Liu et al.
Asian J. Chem.
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Conclusions
• The optional preparation conditons of FeCl3/γ-Al2O3 by
the microwave-impregnated method as follows: 0.5 % of loading
amount and 15 min microwave irridiation time .
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• The FeCl3/γ-Al2O3 prepared under optional preparation
conditons exhibits the high activity for the hydroxylation of
benzene with 15.2 % phenol yield and 100 % selectivity of
phenol at the optimum reaction conditions: l mL benzene,
15 mL acetonitrile, 0.4 g catalyst, 3 mL 30 % aqueous solution
of H2O2, 15 min reaction time, 70 °C reaction temperature.
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ACKNOWLEDGEMENTS
The authors are grateful to the Scientific Research Item
of Xuzhou Institute of Technology (grant No. XKY2012304)
for the financial support.
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