A green approach for phenol synthesis over Fe3+/MgO catalysts using hydrogen peroxide

Article abstract of DOI:10.1016/j.molcata.2009.10.010

An efficient green route for the synthesis of phenol from benzene has been reported over Fe³⁺/MgO catalysts under mild conditions. Fe³⁺/MgO catalysts are prepared and well characterized using EDX, XRD, FTIR, surface area analysis pore volume determination and UV-vis diffuse reflectance studies. Activity of these systems towards single step oxidation of benzene to phenol at low temperature using H₂O₂ has been analyzed. The catalysts exhibited interesting oxidizing ability which has been attributed to Fe(III) dispersed in amorphous form as isolated units on MgO. Possibility of homogeneous nature of catalysis through leached Fe³⁺ ions has been ruled out by confirming the absence of Fe3⁺ in liquid phase, and the heterogeneous nature of catalysis has been confirmed. The decomposition of H₂O₂ by the catalysts has been studied. Reusability of the system is assured carrying out multiple runs using the recycled catalyst. The activity has been analysed towards reactants with various substituents in benzene ring. A metalloperoxide formation on the surface of MgO through heterolytic cleavage of H₂O₂ has also been suggested for the reaction.

Full text of DOI:10.1016/j.molcata.2009.10.010

Journal of Molecular Catalysis A: Chemical 316 (2010) 126–130
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
A green approach for phenol synthesis over Fe3+/MgO catalysts using
hydrogen peroxide
N.K. Renuka∗
Department of Chemistry, University of Calicut, Kerala, India
a r t i c l e i n f o
a b s t r a c t
An efficient green route for the synthesis of phenol from benzene has been reported over Fe3+/MgO
catalysts under mild conditions. Fe3+/MgO catalysts are prepared and well characterized using EDX, XRD,
FTIR, surface area analysis pore volume determination and UV–vis diffuse reflectance studies. Activity of
these systems towards single step oxidation of benzene to phenol at low temperature using H2O2 has
been analyzed. The catalysts exhibited interesting oxidizing ability which has been attributed to Fe(III)
dispersed in amorphous form as isolated units on MgO. Possibility of homogeneous nature of catalysis
through leached Fe3+ ions has been ruled out by confirming the absence of Fe3+ in liquid phase, and
the heterogeneous nature of catalysis has been confirmed. The decomposition of H2O2 by the catalysts
has been studied. Reusability of the system is assured carrying out multiple runs using the recycled
catalyst. The activity has been analysed towards reactants with various substituents in benzene ring. A
metalloperoxide formation on the surface of MgO through heterolytic cleavage of H2O2 has also been
suggested for the reaction.
Article history:
Received 10 August 2009
Received in revised form 6 October 2009
Accepted 6 October 2009
Available online 14 October 2009
Keywords:
Benzene hydroxylation
Phenol
Hydrogen peroxide
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
tive both economically and energetically. Heterogeneous catalytic
systems studied to this effect at elevated temperatures include
Attempts are going on in full swing to develop new efficient
oxygenation catalysts for functionalizing feedstock alkanes to raw
oxygen-containing chemicals. Recently, new environment friendly
catalytic processes using clean oxidants like molecular oxygen or
hydrogen peroxide are being explored exhaustively, which has
attracted much attention. In this direction, H2O2 in presence of
active metal oxide catalysts is extensively explored towards low
temperature oxidation reactions. However, in presence of transi-
tion metal ions, the catalytic applications of hydrogen peroxide
have been limited due to the self-decomposition of the oxidant.
But fine dispersion of transition metal ions on suitable supports
overcomes this demerit by site isolation of redox metal ions, and
has been proven as an excellent catalyst for a variety of oxidation
reaction [1,2]. In the present investigation, the oxidation activity of
Fe(III) dispersed MgO catalysts for direct hydroxylation of benzene
activated by H2O2 has been reported. Phenol, one of the valuable
intermediates for the synthesis of agrochemicals, petrochemicals
and plastics has been mainly manufactured by cumene process. But
the aforesaid reaction pathway is quite tiresome as it is marked by a
multistep process and production of byproduct. Hence synthesis of
phenol by direct hydroxylation of benzene in liquid phase is attrac-
ZSM-5 system using N2O [3,4]. Various solid systems under mild
conditions have been studied for the same which include Ti/MSM-
41 system [5], TS-1 [6] and supported metal oxides in various
solvents [7,8]. Heteropoly acids have been analysed for their oxida-
tion activity in the presence of peroxide [9,10]. Other related works
on the concerned reaction include supported transition metal oxide
systems [11], vanadia based systems [12], poly acids [13,14], Fe(III)
complexes [15] etc. Recently, Monfared et al have studied hydrogen
peroxide oxidation of benzene over Fe3+/Al2O3 systems and have
obtained considerable activity [16]. This prompted us to explore
Fe/MgO for the same reaction as these systems showed remarkable
activity towards various oxidation reactions [17,18]. Magnesium
oxide has been shown to stabilize the supported iron particles and
retards catalysts sintering. In this report, the nature of iron species
in the catalyst has been traced by different characterizations and
attempt has been made to relate the activity data and the nature of
iron species.
2. Experimental methods
The MgO carrier was prepared by heating alkaline magnesium
carbonate (Merck) at 500 ◦C. The catalyst was prepared by stir-
ring alcoholic solution of FeCl3 (Merck) and MgO for 24 h at room
temperature. The solvent was removed by filtration through a
fritted funnel. The solid obtained was subsequently washed with
∗
Tel.: +91 0494 2401144/414; fax: +91 494 2400269.
E-mail address: nkrenu@gmail.com.
1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.10.010

N.K. Renuka / Journal of Molecular Catalysis A: Chemical 316 (2010) 126–130
127
Table 1
Chemical composition and physical parameters of the catalysts.
Catalyst
Elemental composition (wt%)
Mg
Surface area (m2/g)
Pore volume (cm3/g)
Fe
0
0.98
1.90
3.80
MgO
100
91
136
161
165
124
111
0.12
0.19
0.22
0.20
0.18
0.16
1 Fe/MgO
2 Fe/MgO
4 Fe/MgO
8 Fe/MgO
12 Fe/MgO
99.02
98.10
96.20
92.31
88.26
7.69
11.74
methanol and dried overnight in air oven at 110 ◦C. The prepared
catalysts were named according to the wt% of iron in the system.
Thus, 1 Fe/MgO, 2 Fe/MgO, 4 Fe/MgO, 8 Fe/MgO and 12 Fe/MgO
stands respectively for systems containing 0, 1, 2, 4, 8, and 12 wt%
of Fe in MgO. The catalysts were used without any pretreatment.
The chemical composition of catalysts was obtained from Stere-
oscan 440 Cambridge, UK energy dispersive X-ray analyzer used
in conjunction with SEM. Powder X-ray diffraction pattern was
collected using Rigaku D-Max Ni fitted with Cu K␣ radiation.
Micromeritics Gemini Surface Area analyzer was used to deter-
mine the BET surface area and pore volume by N2 adsorption at
77 K. Fourier Transform Infrared spectra of the heat treated sam-
ples were obtained by KBr disk method over the wavenumber range
400–4000 cm−1 using Jasco FTIR-4100. DR UV–vis spectra were
taken in the range 200–800 nm with BaSO4 as reference using Jasco
V-550 spectrometer.
Commercially available benzene and acetonitrile of the highest
grade (Merck) were used without further purification. Hydrogen
peroxide (35% in water) was purchased from Fluka and its con-
centration was determined according to the standard procedure.
Activity studies were performed in a in a R.B. flask, fitted with
a reflex condenser. In a typical experiment, a solution of ben-
zene (1.46 mmol) containing catalyst (0.2 g) in acetonitrile (6.0 ml)
was allowed to attain the reaction temperature (60 ◦C). Required
amount of (8.76 mmol) 35% H2O2 was added, and the mixture
was stirred for 6 h. Catalyst was filtered and the filtrate was dried
over magnesium sulfate, and was analysed by Chemito 1000 GC
equipped with flame ionization detector containing BP-1 capil-
lary column. Analysis condition was programmed from 60 ◦C to
3 min, 20◦/min to 280 ◦C with injection and detection temperature
of 200 ◦C. Conversion and selectivity were calculated by comparing
with relative area of each of authentic samples, which is propor-
tional to the concentration of the substance. The selectivity for a
particular product = (yield of the particular compound/total prod-
uct yield) × 100. Each reaction was carried out in duplicate in order
to check reproducibility.
reflections at 42.9◦ and 62.1◦ (JCPDS 4-829). It is quite evident that
no signs were observed for the presence of Fe2O3 crystals in all the
catalysts. Contradictory to earlier reports, peaks due to MgFe2O4 of
spinel variety was also absent in the samples [19].
The following observation can be gathered from the FTIR spectra
of the thermally treated samples provided in Fig. 2. The broad band
at 1500–1600 cm−1 justifies the presence of physisorbed water
in the samples. In the case of pure Magnesium oxide, hydrogen
bonded OH appeared ∼3440 cm−1 region and a sharp band was
noticed at 3697 cm−1 region, indicating the existence of isolated OH
groups without mutual hydrogen bonding interaction. The inten-
sity of the latter diminished with increase in percentage of Fe in the
catalysts. The participation of surface hydroxyl groups in bond for-
mation with the dispersed, active surface species has already been
established. Both hydrogen bonded and isolated hydroxyl groups
may be participating in the creation of Fe–O bond formation. The
aforesaid bond gives rise to the broad band seen around 1070 cm−1
[20,21]. Another band characteristic of iron species appeared at
470 cm−1 in the case of impregnated samples corresponding to the
stretching vibration of Fe–O which arises from tetrahedrally coor-
dinated iron atom [22]. Hence, XRD data and FTIR spectral analysis
results when considered together hints to the possibility of Fe(III)
existing as isolated amorphous tetrahedral species in the catalysts.
Further confirmation about the nature of dispersed iron was
attained from Diffuse Reflectance UV spectral studies (Fig. 3). Pure
3. Results
The catalyst after loading with Fe was slight yellowish and the
colour was intensified as the weight percentage of iron increases
in the supported systems. Chemical composition and the physical
characterization data is presented in Table 1. It can be seen that
the surface area showed an initial increase followed by a drop at
higher amounts of Fe(III). The pore volume values also followed the
same trend. The similar trend of both the parameters may be due to
the concomitant effect of the structural variation of MgO as hinted
by the XRD pattern, and the fine dispersion of iron species on the
support.
The XRD pattern of pure MgO and ferric ion impregnated MgO
are provided in Fig. 1. The diffraction profile of magnesium oxide
was rather amorphous, and the addition of Fe(III) increased the
crystallinity of the sample as evident from the XRD pattern. Only
peaks characteristic of MgO were observed, which gave prominent
Fig. 1. X-ray diffraction patterns of support and Fe dispersed systems.

128
N.K. Renuka / Journal of Molecular Catalysis A: Chemical 316 (2010) 126–130
Fig. 2. FTIR spectral pattern of the catalysts.
Fig. 3. Diffuse reflectance UV spectra of the various systems.
MgO support gave no reflectance peaks in the selected spectral
region of 200–800 nm. Occurrence of isolated tetrahedral Fe3+ in
the iron loaded magnesium oxide is quite evident from the band
observed at 260 nm, since Fe2O3 crystals if present will show
absorbance at higher wavelength region [23]. Figure reveals that
bulk Fe2O3 gives rise to a broad band in the range 320–700 nm, hav-
ing strong maxima at 560 nm and at 660 nm. Hence it is confirmed
that Fe3+ is well dispersed over magnesium oxide up to 12 Fe/MgO
sample. The position of this band exhibited slight red shifts as the
iron content in the catalysts increased. All the above mentioned
analytical techniques pave way to the remark that isolated Fe(III)
in tetrahedral geometry is present in the impregnated analogues
up to 12 wt% Fe incorporation in MgO.
Mild condition of the reaction assures the absence of over oxi-
dation products to a large extent. Temperature above 80 ◦C causes
decomposition of H2O2 and hence it is always desirable to select
low reaction temperature. Screening experiments were performed
at various conditions to optimize the reaction parameters. Temper-
ature range 40–80 ◦C were tried out for the reaction. Best results
were obtained at 60 ◦C. Obviously, the data presented in Table 2
show that Fe3+/MgO systems are efficient for the oxidation of ben-
zene in presence of H2O2 and 100% phenol selectivity was noticed
over these systems. Even though the reaction temperature is higher
than the normal decomposition temperature of hydrogen perox-
ide, no over oxidation product has been noticed over the catalysts.
Similar reports exist in literature in the concerned reaction reported
over metal oxide systems [7,16]. This may be ascribed to the decom-
position of H2O2 by catalysts, which rules out the possibility of
excess oxidant. The results of preliminary experiments showed
that the oxidising moiety has not been generated from hydrogen
peroxide in the presence of MgO or Fe2O3 alone. No phenol yield
has been obtained when the reaction was carried over iron dis-
persed catalyst, and hydrogen peroxide in the absence of catalyst
also was inactive for the reaction. The reaction was carried out in
different solvents with an aim to study the influence of polarity
of solvents on the reaction; acetonitrile; ε = 36, dichloromethane;
ε = 8.93, and carbon tetra chloride; ε = 2.2 were the solvents used (ε:
dielectric constant). Out of the there solvents selected, the activ-
ity was maximum in the most polar solvent, acetonitrile. Probably
the reaction may be taking place through an ionic intermediate,
which is most stabilized in a solvent of highest polarity. Thus ace-
tonitrile has been fixed as the solvent medium for the present
study.
Table 2
Benzene hydroxylation data over Fe(III) doped catalysts.
Catalyst
Benzene conversion (%)
Phenol (%)
Fe1
Fe2
Fe4
Fe8
Fe12
Fe20
18
24
27
31
36
28
100
100
100
100
100
100
Catalyst: 0.2 g; acetonitrile: 6 ml; benzene: 1.46 mmol; temperature: 60 ◦C; time:
6 h.

N.K. Renuka / Journal of Molecular Catalysis A: Chemical 316 (2010) 126–130
129
Table 3
Activity of the catalyst for various reactants.
Reactant
Conversion (%)
Products (%)
Toluene
Benzene
Chlorobenzene
41
36
28
o-Cresol (50%)
p-Cresol (50%)
Phenol (100%)
o-Chlorophenol (56%)
p-Chlorophenol (44%)
Reaction conditions: 12 Fe/MgO: 0.2 g; acetonitrile: 6 ml; benzene: 1.46 mmol; H2O2: 8.76 mmol; temperature: 60 ◦C; time: 6 h.
4. Discussion
ruled out by this observation, which was again confirmed through
the control experiment as described below. The catalytic system
in acetonitrile was filtered so that supported iron was removed
from the liquid phase. Benzene and H2O2 were added and the reac-
tion was carried out to yield nil conversion of benzene. Hence the
possibility of homogeneous catalysis is completely ruled out here.
Finally, the oxidation activity of iron dispersed MgO system
towards reactants carrying different side chains in the benzene ring
has been examined under similar experimental conditions over 12
Fe/MgO system, which was observed as the most active among the
series. Toluene and chlorobenzene constitute the two reactants that
are selected. From the data provided in Table 3 it can be inferred
that, the system has enhanced activity towards the reactant when
weak electron-donating groups exist in the ring. It was difficult to
oxidize the side chain in the case of toluene. The methyl substituent
is a weakly electron-donating group which, when attached to a ben-
zene ring, renders it more susceptible to an electrophilic attack at
the ortho and para-positions. Such a result suggests that hydroxy-
lation of the aromatic ring occurs according to an electrophilic-like
mechanism [24]. Weakly electron withdrawing chloro substituent
lead to reduced activity for ring hydroxylation justifying the above
statement.
As the aromatic C–H bonds are stronger than aliphatic C–H,
the reaction pathway cannot be via H-abstraction by the active
oxidizing moiety. In oxidation reactions using hydrogen perox-
ide, the cleavage of the oxygen–oxygen bond in peroxides takes
either of two distinct pathways, heterolytic or homolytic. In the
first case a molecule of H2O2 undergoes a heterolytic scission lead-
ing to ionic mechanism. In the other, a homolytic cleavage of the
oxidant molecule occurs leading to radical pathway [25]. For the
oxidation of aromatics with hydrogen peroxide over transition
metal substituted oxides, it has been proposed that the hydrox-
ylation of the aromatic ring occurs via the heterolytic mechanism,
involving the formation of a metalloperoxide species [24,26,27].
Although no detailed mechanistic studies have been carried out
in the present work, hydroxylation of the ring indicates the pos-
sibility of heterolytic cleavage in the present case. In accordance
with earlier reports, a MgO–Fe(III) ꢀ1-hydroperoxide species stabi-
lized by hydrogen bonding between the hydroperoxide ion and the
oxygen atom of support has been proposed [28]. The route can be
shown as S-Fe–OOH + C6H6 → C6H5-OH + S-Fe–OH (S representing
the support oxide).
The ability to initiate oxidation of benzene in the presence of
oxidizing agent increases with increase of Fe in the catalysts as
obvious from Table 2. The species dispersed on MgO are con-
firmed to be isolated Fe(III) by various techniques and the increase
of isolated species upon increase in iron loading also has been
established. Hence the active centers for catalytic oxygen trans-
fer attributable are isolated Fe3+ species, which is in agreement
with the reports of Monfared and Amouei [16]. Analogous cases of
activity for isolated metal species for oxidation of aromatic hydro-
carbons in presence of hydrogen peroxide have been established
in literature [24]. For further verification, the percentage of iron
species in MgO was enhanced to 20 (designated as 20 Fe/MgO)
where crystals of Fe2O3 appeared in the catalyst and the activity
study was carried out. Reduced oxidation activity was observed as
anticipated (Table 2), since bulk metal oxide was inactive for the
purpose. Diffuse reflectance UV spectra of 20 Fe/MgO provided in
Fig. 3 clearly offer confirmation for the presence of crystalline iron
oxide via the reflectance band at 560 nm region, which was charac-
teristic for aggregated Fe2O3. This conclusion was also confirmed by
the XRD pattern of the sample which showed peaks characteristic
of Fe2O3 crystals (not included in the text). The catalytic activity in
terms of turn over number increased as the wt% of Fe(III) increased
in the catalysts. The turn over numbers were 7.4, 12.2, 18.0, 21.6,
27.5 and 17.8 for 1 Fe/MgO, 2 Fe/MgO, 4 Fe/MgO, 8 Fe/MgO, 12
Fe/MgO and 20 Fe/MgO respectively, which again establishes the
upper hand of isolated Fe(III) species in catalyzing the reaction. A
comparison attempted with results reported in literature revealed
that the catalytic activity of the present system was almost the
same as obtained over Fe/Al2O3 system [16] under similar experi-
mental conditions (about 27% conversion with catalyst containing
∼0.04 mmol Fe3+).
As it was observed that the activity was enhanced with an
increase in the amount of peroxide in the reaction mixture, the per-
centage conversion of hydrogen peroxide over the catalysts was
studied by analyzing non-productive decomposition as reported
earlier [24]. Hydrogen peroxide conversion gradually increased
with the amount of iron in the catalysts and the conversion per-
centages were 50, 66, 75, 80, 83 and 92 respectively for 1, 2, 4, 8,
12 and 20 wt% Fe(III) loaded samples. Non-productive decomposi-
tion of major portion of the peroxide is evident as the conversion of
hydrogen peroxide exceeded significantly that of benzene. Higher
hydrogen peroxide decomposition over 20 Fe/MgO can also be fac-
tor for the observed reduced activity of the system, which also
indirectly hints to the presence of clustered Fe(III) in the sample.
Hence gathering evidences from all these, the conclusion can be
made that the fraction of iron which is present as isolated species
has a decisive role in the catalytic activity.
The most important and attractive feature of heterogeneous cat-
alytic system is the reusability of the catalysts. Fe/MgO system has
been checked to this effect, after filtering and washing the catalysts
by methanol. The conversion percentages were 18, 23, 26, 30, 36
and 27 for 1 Fe/MgO, 2 Fe/MgO, 4 Fe/MgO, 8 Fe/MgO, 12 Fe/MgO and
20 Fe/MgO systems respectively. It is evident that no much deacti-
vation was noticed as the catalyst maintained the almost the same
activity. The possibility of leaching of Fe3+ ions to liquid phase is
5. Conclusions
Fe(III) dispersed MgO catalyst systems were prepared and were
characterized with various instrumental techniques. Impregnation
of MgO with Fe3+ titrated the hydroxyl groups of the support
and hence the anchoring on the surface is supposed to take place
through Mg–O–Fe bond. Isolated Fe(III) dispersed on the MgO
was found to be efficient for the conversion of benzene to phe-
nol by direct hydroxylation using hydrogen peroxide. The active
species are concluded to be heterogeneous supported Fe3+ ions.
Clustered iron species were inactive for oxidation in presence of
hydrogen peroxide. The catalyst system was more active towards
reactants with electron-donating groups on the ring and less activ-
ity was observed for arene with electron withdrawing substituent.

130
N.K. Renuka / Journal of Molecular Catalysis A: Chemical 316 (2010) 126–130
A metal hydroperoxide formation on the surface through a het-
erolytic cleavage of H2O2 has been suggested for the reaction.
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The author gratefully acknowledges the help received from Dr.
Anas K., Sud-Chemie India Ltd.
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