Hydroxylation of benzene to phenol over magnetic recyclable nanostructured CuFe mixed-oxide catalyst

Article abstract of DOI:10.1016/j.molcata.2014.11.023

A highly active and magnetically recyclable nanostructured copper-iron oxide (CuFe) catalyst has been synthesized for hydroxylation of benzene to phenol under mild reaction conditions. The obtained catalytic results were correlated with the catalyst structure, which was characterized by XRD, SEM, TEM, EDX, H₂-TPR and BET. The catalytic results indicated that the CuFe mixed oxide samples exhibited superior performance compared to its analogous single nano-oxide catalysts. The influence of the reaction condition variables, such as the solvent, reaction temperature, time, and amount of H₂O₂ oxidant, were investigated. Under optimized conditions, CuFe resulted in benzene conversion of 44% at a phenol selectivity of 91% and a corresponding combined hydroquinone/catechol/benzoquinone selectivity of 9%. In addition, the catalytic activity of the nano-oxide CuFe was significantly affected by the different calcination temperatures due to the induced catalyst structure, which was confirmed by the characterization results. This enhanced activity was due to the structural phases and redox modification resulting from the interface between the Cu and Fe metals, which was caused by varying calcination temperatures. The activity of CuFe does not require the formation of the typical favored CuFe₂O₄ spinel to achieve a highly active catalyst, which results from the enhanced redox potentials. CuFe is highly recyclable due to its magnetic nature, which results in an excellent ecofriendly catalyst for the direct synthesis of phenol from benzene hydroxylation.

Full text of DOI:10.1016/j.molcata.2014.11.023

Journal of Molecular Catalysis A: Chemical 398 (2015) 149–157
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Hydroxylation of benzene to phenol over magnetic recyclable
nanostructured CuFe mixed-oxide catalyst
Peter R. Makgwanea,∗, Suprakas Sinha Raya,b,∗∗
a DST-CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa
b Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly active and magnetically recyclable nanostructured copper–iron oxide (CuFe) catalyst has been
synthesized for hydroxylation of benzene to phenol under mild reaction conditions. The obtained cat-
alytic results were correlated with the catalyst structure, which was characterized by XRD, SEM, TEM,
EDX, H2-TPR and BET. The catalytic results indicated that the CuFe mixed oxide samples exhibited supe-
rior performance compared to its analogous single nano-oxide catalysts. The influence of the reaction
condition variables, such as the solvent, reaction temperature, time, and amount of H2O2 oxidant, were
investigated. Under optimized conditions, CuFe resulted in benzene conversion of 44% at a phenol selec-
tivity of 91% and a corresponding combined hydroquinone/catechol/benzoquinone selectivity of 9%. In
addition, the catalytic activity of the nano-oxide CuFe was significantly affected by the different calcina-
tion temperatures due to the induced catalyst structure, which was confirmed by the characterization
results. This enhanced activity was due to the structural phases and redox modification resulting from
the interface between the Cu and Fe metals, which was caused by varying calcination temperatures. The
activity of CuFe does not require the formation of the typical favored CuFe2O4 spinel to achieve a highly
active catalyst, which results from the enhanced redox potentials. CuFe is highly recyclable due to its
magnetic nature, which results in an excellent ecofriendly catalyst for the direct synthesis of phenol
from benzene hydroxylation.
Received 1 April 2014
Received in revised form
13 November 2014
Accepted 22 November 2014
Available online 5 December 2014
Keywords:
Copper
Iron
Nanostructured
Oxidation
Hydroxylation
Benzene
Phenol
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
process costs and a negative impact on the environment due to the
generated waste metal salt. In addition, the cumene conversion to
From an economic and environmental standpoint, the direct
synthesis of phenol from benzene hydroxylation has attracted
much research attention in recent years [1–3]. The current com-
mercial production of phenol involves a non-catalyzed chemical
reaction based on the Hock process of cumene liquid-phase oxi-
dation, which consists of the preparation of cumene from benzene
isopropylation and oxidation of cumene to cumene hydroperoxide
(CHP) followed by acid-cleavage decomposition of CHP into phenol
and acetone as a side product by H2SO4 [4,5]. The disadvantage of
this process is that the oxidation is carried out in a series of cascaded
bubble-column reactors operated at 5–10 bar and 100–130 ◦C in the
presence of an aqueous basic Na2CO3 salt [5], which results in high
CHP is limited to 25–30% to ensure low formation of by-products,
such as dimethylphenylcarbinol and acetophenone [4]. However,
the use of mineral acids, such as H2SO4, in the acid cleavage step
typically generates hazardous waste metal salts that are difficult to
dispose of. With the continuing demands for green and sustainable
chemical conversion protocols, the development of improved cat-
alytic processes utilizing environmentally friendly oxidants (e.g.,
H2O2, TBHP, and O2) are desirable.
Numerous heterogeneous catalysts based on iron (Fe) oxide
have been investigated for their catalytic activity to develop a ben-
zene hydroxylation process that would provide a direct synthetic
route to phenol [6–9]. The activity of the Fe catalyst stems from
its ability to operate as a Fenton reagent (Fe2+/Fe3+) with H2O2 by
mediating the catalyzed liquid-phase oxidation reactions to pro-
duce highly reactive free-radical oxygen species (i.e., •OH/HO2
)
•
∗
Corresponding author. Fax: +27 12 841 2229.
Corresponding author at: DST-CSIR National Centre for Nanostructured Materi-
∗∗
[10,11]. Bianchi et al. [12] developed a biphasic reaction system
using FeSO4 for the conversion of benzene in a hydroxylation pro-
cess that exhibited remarkable phenol selectivity of 97% but at low
substrate conversions. This was better when compared to those in
als, Council for Scientific and Industrial Research, Pretoria 0001, South Africa.
Tel.: +27 12 841 2388; fax: +27 12 841 2229.
E-mail addresses: pmakgwane@csir.co.za (P.R. Makgwane), rsuprakas@csir.co.za
(S.S. Ray).
http://dx.doi.org/10.1016/j.molcata.2014.11.023
1381-1169/© 2014 Elsevier B.V. All rights reserved.

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the single phase using other organic solvents. In other studies, Song
et al. [13] reported the activity of multi-walled carbon nanotube
(MWCNT) supported Fe3O4 nanoparticles as a highly active catalyst
that was better than Fe supported on conventional supports, such
as Al2O3 [14], MCM-41 [15], TiO2 [16], and ZSM-5 zeolites [17], for
benzene hydroxylation. In addition, the activity of Fe dispersed on
a MgO basic support was also investigated for benzene hydroxyla-
tion, and Fe/MgO exhibited nearly the same catalytic performance
as that over Fe/Al2O3 [14,18]. The most important aspect of these Fe
based catalysts is that they all allow the simple green catalytic con-
version of benzene to phenol. However, these catalysts primarily
afforded low benzene substrate conversions. The low conversions
obtained for some of the Fe catalysts may be due to the inactivity of
the supports, such as MgO or Al2O3. Therefore, the search for new
catalysts with improved conversion rates for the benzene hydrox-
ylation to phenol remains an important research challenge. In the
benzene hydroxylation reaction, the challenge is to achieve high
substrate conversion while maintaining phenol selectivity due to
its highly reactive nature that can result in other side products, such
as dihydroxybenzene compounds, that may eventually complicate
the product separation work-up.
The applications of multicomponent solid catalysts synthesized
at the nanosize atomic scale have recently emerged as promising
active materials for catalyzing various organic chemical transfor-
mations [19,20]. These mixed oxides possess distinct structural
properties with better activity than those of their individual coun-
terparts. The resulting enhanced activity has been attributed to
interactions that improve the electron charge transfer exchange
during chemical reactions [21,22]. In particular, the nano-structural
effects are induced by the tailored metal’s physico-chemical prop-
erties, such as size, morphology, pores, defects, compositions, and
interactions [22]. Because the high activity of Fe oxide for liquid-
phase oxidations is facilitated by its excellent exchangeable redox
cycle with the valence state of Fe2+/Fe3+, the stabilization and
mobility enhancement of such species would be important. In addi-
tion, one aspect that has been overlooked is the contribution effect
from the different Fe oxides phases (i.e., Fe2O3, Fe3O4, and FeO)
on its catalytic performance in liquid-phase oxidations [23,24].
Although Fe is active for oxidation reactions, its use in liquid-phase
oxidations has been primarily limited to soluble homogeneous
catalysts immobilized on various ligands [25–28]. However, the
problem with homogeneous designed catalyst systems is their
poor recyclability and rapid deactivation, which hinders their
ultimate application in industrial oxidation processes. However,
heterogeneous solid Fe catalysts exhibit enormous benefits related
to environmental sustainability and process economics due to
enhanced recyclability. Therefore, the hetero-mixing of Fe oxide
with another metal oxide that possesses an excellent redox cycle
would result in a highly active catalyst for liquid-phase oxidation
reactions. In fact, Fe would provide the catalytic activity as well
as the recyclability due to its magnetic recoverable properties in
the liquid phase. Catalysis using copper (Cu) oxides is well estab-
lished in the literature for liquid-phase oxidations of hydrocarbons
and alcohols [29–33]. In addition, Cu also possesses an interesting
redox cycle (Cu2+/Cu+) that is amenable to facilitating free-radical
oxidation reactions. Therefore, the combinations of Fe and Cu met-
als provide an opportunity to further explore their hetero-mixed
catalytic properties for the oxyfunctionalization of typical benzylic
hydrocarbons C H in liquid-phase oxidations.
versatile synthesis of the CuFe composite with a rod-like nanopar-
ticle morphology using a polyvinyl pyrrolidine (PVP) surfactant in
ethylene glycol (EG), which served as both the solvent and reduc-
tant. Kenfack and Langbein [36] reported that the formation of the
spinel structure properties of CuFe2O4 is temperature dependent
and the CuFe catalytic performance is sensitive to the structural
phase. Yan and Chen [37] demonstrated the enhanced catalytic per-
formance of nanostructured CuFe with different compositions for
the hydrogenation of biorenewable furfural and levulinic acid [38].
The activity of the CuFe composite was also evaluated for the liquid-
phase oxidation of toluene to benzaldehyde as the major reaction
product by Wang et al. [39]. The nanostructure CuFe composite
exhibited efficient catalytic activity and high magnetic recyclability
in the Friedel–Craft acylation for the organic chemical transfor-
mation in the synthesis of biologically active moieties [40] High
catalytic activity and excellent catalyst stability was reported for
Ullmann C O coupling reactions catalyzed by magnetic copper fer-
rite (CuFe) nanoparticles with high yields for the targeted chemical
products [41].
Despite the successful catalytic activity exhibited by CuFe based
catalysts in different liquid-phase reactions and environmental
cleaning, its application for benzene hydroxylation has not been
reported. Not only can nanostructured CuFe composite materials
provide a highly active catalyst for the activation of the C H bond
in benzene for selective oxygen introduction but the catalyst sys-
tem can be sustainable due to its less expensive metals and their
natural abundance. Herein, we report the synthesis of a nano-oxide
copper–iron (CuFe) composite material with interesting structure-
dependent catalytic activity performance, which was induced by
the calcination temperatures, in the hydroxylation reaction of ben-
zene for the direct synthesis of phenol. The synthesized CuFe
catalyst was catalytically active and magnetically recoverable in
this reaction. The structural features of the prepared catalysts were
investigated using X-ray diffraction (XRD), hydrogen-temperature
programmed reduction (H2-TPR), high-resolution transition elec-
tron microscopy (HRTEM) coupled to energy dispersive X-ray
(EDX), field emission scanning electron microscopy (FESEM), and
N2 physisorption.
2. Experimental section
2.1. Catalyst preparation procedure
The synthesis of the nanostructured CuO, Fe3O4, and CuFe2O4
catalyst materials were prepared by the polylol method using
ethylene glycol (EG) as the solvent and a poly (vinyl pyrrolidine)
(PVP) surfactant. Typically, Cu(NO3)2·6H2O (5 mol, Cu metal) and
5 mol of a mixture of Fe(III)Cl3·6H2O and Fe(II)SO4 in in a 1:1
molar ratio were dissolved in ethylene glycol (70 mL). In addition,
1.0 g of PVP was added as a structure directing templating agent.
The mixture was stirred vigorously for 30 min, and then, the pH
of the reaction was adjusted to 12 using ammonium hydroxide
(NH4OH, Aldrich). The materials were aged for 24 h under reflux.
Similarly, the respective individual oxides of Cu and Fe were pre-
pared following the same procedure. The prepared catalyst was
vacuum filtered and washed several times with ethanol prior to
drying in air for 12 h at 100 ◦C. The prepared CuFe catalyst was cal-
cined under oxidation conditions at different temperatures with
a heating rate of 10 ◦C/min, and the calcination temperature was
maintained for 4 h. The prepared catalysts are denoted CuFe-300,
CuFe-400, CuFe-500, and CuFe-600 depending on the calcination
temperature used. Based on the better catalytic results obtained for
CuFe-400, we also calcined the individual catalysts at 400 ◦C (Cu-
400 and Fe-400) for comparison of their activity with the CuFe-400
composite.
The challenge in the preparation of such nanostructured hetero-
mixed metal oxides is to ensure that the synthetic method produces
uniform nanoparticles with the desired interaction and exposed
active sites. The polylol synthetic method using a co-precipitation
approach for structuring more than two combined metals in the
presence of a polymer surfactant as a structure templating agent
has been extensively studied [34]. Yang et al. [35] reported the

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151
2.2. Catalyst characterization procedures
The total specific surface area as well as the micropores vol-
ume and areas of the catalysts were measured by nitrogen (N2)
physisorption at 77 K using a Micromeritics TRISTAR 3000 surface
area analyzer (USA). Prior to analysis, the samples were degassed
at 120 ◦C for 12 h under a continuous flow of N2 gas to remove
adsorbed contaminants. The XRD experiments were recorded on a
PAnalytical XPERT-PRO diffractometer using Ni filtered CuK␣ radi-
ation (ꢀ = 1.5406 Å) at 40 kV/50 mA. The diffraction measurements
were collected at room temperature in a Bragg–Brentano geome-
try with a scan range of 2ꢁ = 10–90◦ using continuous scanning at
0.02◦/s. TEM measurements were performed by a JEOL JEM 2100
operated at an accelerating voltage of 200 kV. The powder sam-
ples were sonicated for 5 min in ethanol and dispersed on a copper
grid for TEM analysis. The elemental analysis of the different metals
was simultaneously determined using EDX coupled to the TEM. H2-
TPR studies were performed on a Micromeritics Autochem AC2920
apparatus (USA). The reduction of the catalyst samples was car-
ried out under a gaseous mixture of 10% H2/Ar at a heating rate of
10 ◦C/min from 50 to 800 ◦C and a total gas flow rate of 50 ml/min.
Prior to starting the H2-TPR measurements, the samples were pre-
cleaned for approximately 1 h at 70 ◦C using an argon gas flow. The
Fig. 1. XRD patterns of the copper–iron based nanostructured catalysts.
of the CuO phase were determined to matched the (1 1 0), (0 0 2),
(1 1 1), (2 0 2), (0 2 0), (2 0 2), (1 1 3), (0 2 2), (2 0 0), (3 1 1), and (2 2 2)
plane reflections. Similarly, the formation of typical iron magnetite
Fe2O4 diffraction patterns at 2ꢁ = 30.3◦, 35.6◦, 43.2◦, 53.4◦, 57.2◦,
and 62.7◦ was confirmed by XRD analysis to correspond to the
(2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) reflection planes
of the face-centered cubic phase of Fe3O4 [42]. The CuFe catalyst
samples calcined at both 300 and 400 ◦C exhibited similar XRD
structural patterns. For CuFe-300 and CuFe-400, all of the XRD
peaks were indexed to the respective CuO and Fe3O4 phases. How-
ever, the peak intensities corresponding to CuO in the CuFe-300 and
CuFe-400 composite materials compared to the XRD of pure CuO
were low while those belonging to Fe3O4 were high and clearly
visible. Notably, as the CuFe calcination temperature increased to
500 and 600 ◦C, the formation of additional XRD peaks at 2ꢁ = 25.3◦,
34.7◦, 49.5◦ 58.8◦, 63.7◦, and 71.6◦ started to appear. These peaks
indicated the formation of the CuFe2O4 spinel structure phase. In
addition, the presence of XRD peaks at 2ꢁ = 34.7◦ and 35.9◦ repre-
sent the formation of the CuFe2O4 spinel structure with a tetragonal
phase [36]. The intensity of the peak at 2ꢁ = 34.7◦ increased as the
calcination temperature increased from 500 to 600 ◦C. There was
an additional peak that appeared at 41.9◦ in the XRD patterns for
the CuFe-500 and CuFe-600 catalysts that was not observed in the
CuFe-300 and CuFe-400 catalysts. This peak was attributed to the
formation of the ␣-Fe2O3 phase of the iron compounds associ-
ated with the CuFe2O4 spinel structure transformation. Another
noticeable effect of temperature on the structure of the CuFe com-
posite material was the gradual increase of the overlapping peaks
corresponding to both Cu and Fe at 2ꢁ = 35.9◦ as the calcination
temperature increased. This result indicated particle sintering or
ordering of the CuFe structure accompanied by the possible crys-
tallite size growth. Correspondingly, based on the values of the
full-width at half-maximum (FWHM) for the strong XRD peaks, the
crystallite size of the particles were estimated using the Scherrer
equation. As shown in Table 1 with the physical characterization
data of the nanostructured catalysts, the grain size of the nanopar-
ticles increased as the calcination temperature increased beginning
with the as-prepared CuFe composite material dried at 100 ◦C.
H2 consumption was monitored by a calibrated thermal conductiv-
ity detector (TCD). The microstructure surface morphologies of the
catalysts were characterized by FESEM (JEOL). Prior to analysis, the
samples were sputter coated with gold to minimize their possible
charging.
2.3. Catalytic testing
The catalytic reactions were performed in a 50 ml two-necked
round-bottom flask equipped with a reflux condenser, mag-
netic stirrer, and thermometer. The catalyst (50 mg) and benzene
(10 mmol) in solvent (5 ml) were combined and maintained under
stirring at 70 ◦C for 5 min prior to slowly adding the aqueous 50%
H2O2 (30 mmol) oxidant. The reaction was continued for 4 h at a
constant temperature, which was controlled using an oil bath. After
3 h, the reaction was terminated and cooled prior to analyzing the
products on an Agilent Gas Chromatograph (GC) A6890 equipped
with a split inlet (250 ◦C, split ratio of 100) using SPB 20 capillary
column (30 m × 25 ␮m × 0.25 mm ID) at a constant flow of carrier
gas coupled to a flame ionization detector (FID). The elution tem-
perature of the oven components was ramped from 80 ◦C (3 min,
hold-up time) to 220 ◦C at a heating rate of 10 ◦C/min. Nitrobenzene
was used as the internal standard for quantification of the compo-
nents, which were identified using authentic standards purchased
from Aldrich. For the CuFe catalyst recyclability studies, after each
reaction cycle, the catalyst was separated using an external mag-
net, and the new reaction liquids were added to start the next
recyclability reaction cycle.
3. Results and discussion
3.1. Catalyst characterization
3.1.1. XRD crystalline structure characterization
To understand the structural characteristics of the nano-
synthesized CuFe composite, wide-angle powder XRD was used
to identify the phases and crystallinity of the catalyst samples.
The obtained XRD patterns of the CuFe catalysts calcined at differ-
ent temperatures including the individual nano-oxides of Cu and
Fe are shown in Fig. 1. For copper oxide nanostructure, the XRD
diffraction peaks 2ꢁ = 32.4◦, 35.5◦, 38.7◦, 48.7◦, 53.2◦, 57.9◦, 61.3◦,
66.1◦, 68.0◦, 71.9◦, and 74.8◦ corresponding to the crystal structure
3.1.2. N2 physisorption characterization
The physical characterization data of the catalysts are reported
in Table 1. The CuO and Fe3O4 catalysts calcined at 400 ◦C possessed
a surface area of 58.0 and 23.4 m2/g, respectively, and the CuFe

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P.R. Makgwane, S.S. Ray / Journal of Molecular Catalysis A: Chemical 398 (2015) 149–157
Table 1
Physical characterization data of the nanostructured Cu and Fe based catalysts.
Entry
Catalyst
BET surface area
(m2/g)
Micropores volume
(cm3/g)
Micropores area
(m2/g)
Particles size (nm)a
1
2
3
4
5
6
Cu-400
Fe-400
CuFe-300
CuFe-400
CuFe-500
CuFe-600
23.4
58.0
44.4
33.7
18.1
5.1
0.36
0.41
0.35
0.33
0.20
0.07
17.6
21.3
14.6
10.5
8.3
14.4
13.6
15.9
17.4
19.3
23.4
2.3
a
Estimated from XRD results using the Scherrer equation.
mixed oxide, which was also calcined at 400 ◦C, possessed a sur-
face area of 33.7 m2/g, which is in between the two values for the
CuO and Fe3O4 catalysts. The calcination of CuFe at different tem-
peratures over a range of 300–600 ◦C induced significant changes in
the BET surface area of the materials, which ranged from 44.4 m2/g
(300 ◦C) to 5.1 m2/g (600 ◦C). This relative drop in the CuFe surface
area with temperature indicated possible grain size growth lead-
ing to the structural morphology of the nanoparticles becoming
stacked together decreasing their surface area, as discussed later
with the FESEM results (Fig. 2). The catalyst micropore volumes
and areas corresponded to the obtained changes in the samples
surface areas, which indicated that the structural modification of
the physicochemical properties of the catalysts resulted from the
interaction of CuO and Fe3O4 as well as the effect of the calcination
temperature. The low micropore volume of the CuFe-600 sample
compared to other CuFe catalysts may be related to its different
porosity nature, as shown by the FESEM images (Fig. 3a–g)
3.1.3. TEM and SEM morphology characterization
To investigate the calcination temperature-induced morpholog-
ical evolution of the CuFe mixed oxides catalyst, FESEM and TEM
were used to analyze the structural morphological development of
the catalyst during a series of thermal treatments (i.e., 300–600 ◦C).
The recorded low and high magnification FESEM images of the cat-
alyst morphologies are shown in Fig. 2a–i. For the as-prepared CuFe
and calcined CuFe at 300 ◦C, the formation of disconnected particles
were observed compared to the slightly agglomerated morphology
for the CuFe catalysts calcined at 500 ◦C and 600 ◦C. Both the high
magnification images of the as-prepared CuFe and that calcined
at 300 ◦C indicated the formation of uniform nanosized particles
with an average size of 30–50 nm (Fig. 2b and d). With a further
increase in the calcination temperature to 600 ◦C, a completely dif-
ferent morphology was observed compared to the other calcination
temperatures (Fig. 2e–g). The disconnected particle morphology
obtained for CuFe calcined at 300–500 ◦C became stacked together
Fig. 2. SEM images for (a) the as-prepared CuFe, (b) high magnification of the as-prepared CuFe, (c) CuFe-300 (d) high magnification of CuFe-300, (e) CuFe-400, (f) CuFe-500,
(g) CuFe-600, (h) Cu-400 and (i) Fe-400. High magnification: ×140 000 at 30 nm scale bar for (b) and (d).

P.R. Makgwane, S.S. Ray / Journal of Molecular Catalysis A: Chemical 398 (2015) 149–157
153
Fig. 3. TEM image (a) and SAED (b) of CuFe-400, (c) enlarged CuFe-400 from (a), high resolution TEM images of (d) CuFe-400, (e) Fe-400, (f) Cu-400 and (g) EDX for CuFe in
Fig. 3c.
forming a continuous rough surface at 600 ◦C. This drastic morpho-
logical change in CuFe as the temperature increased to 600 ◦C was
confirmed by the XRD phase changes where the separate CuO and
Fe3O4 phases at 300 ◦C and 400 ◦C were converted into the spinel
CuFe2O4 structure phase for CuFe calcined at 500 ◦C and 600 ◦C.
For individual CuO and Fe3O4 nano-oxides calcined at 400 ◦C, both
nano-oxides formed distinctive particle shape morphologies that
were porous compared to the CuFe catalysts.
The representative morphology observed in the TEM images
of the CuO and Fe3O4 nano-oxides as well as selected CuFe com-
posite samples calcined at 400 ◦C are shown in Fig. 3a–g. The
TEM and HRTEM images of CuFe-400 indicated the formation of
a uniform nanosized particle shape (Fig. 3a and b). The selected
area electron diffraction (SAED) pattern of the synthesized nanos-
tructured CuFe-400 composite catalyst is shown in Fig. 4c. The
well-defined concentric rings displayed in the SAED patterns sup-
port the nanocrystalline nature of the synthesized material, which
correlates with the obtained XRD results (Fig. 1). For the CuFe
composite, the formation of particles with a morphology that was
similar to the individual catalyst oxide samples was observed
(Fig. 3d). According to the HRTEM images, the pure Fe3O4 catalysts
were comprised of irregular nanospherical shaped particles with an
estimated nanosize of approximately 10–30 nm. In addition, CuO
was composed of a spherical and octahedral shaped morphology
(Fig. 3e and 3f). The EDX spectra of the CuFe-400 catalyst shown in
Fig. 3g confirmed the presence of Cu (44%) and Fe (56%) metal in
the composite catalyst sample with.
CuFe catalysts calcined at different temperatures exhibited distinct
reduction peak patterns that were categorized by three different
reduction temperatures at 50–250 ◦C, 250–450 ◦C, and 450–800 ◦C.
First, at a reduction temperature below 300 ◦C, CuFe-400 exhib-
ited high reactivity to the surface oxygen removal initiated by the
reduction of CuO to Cu compared to the other CuFe mixed oxide
catalysts calcined at different temperatures. Therefore, a shift in
the reduction temperature of Cu to a low region based on the
calcination temperature effect decreased in the following order:
CuFe-400 (Tmax = 176.9 ◦C) < CuFe-300 (Tmax = 182.4 ◦C) < CuFe500
(Tmax = 260.5 ◦C) < CuFe-600 (Tmax = 225.3 ◦C). It is important to note
that the difference in the peak reduction intensities corresponding
to CuO at 300 ◦C was poor for CuFe-600 compared to the other nano-
oxides CuFe catalysts. In addition, this low reduction peak also
exhibited a slight shift in reduction temperature accompanied by
a multiple-stage reduction compared to the single reduction peak
of CuO observed in the other CuFe catalysts that were calcined at
300, 400 and 500 ◦C. By comparing the H2-TPR data for a reduction
temperature of less than 300 ◦C to the FESEM morphologies of the
CuFe catalysts, it is reasonable to suggest that the particle agglom-
eration observed in CuFe-600 could have resulted in the coverage
of the copper species by iron oxide, which could make their reduc-
0.6
CuFe300
CuFe500
Fe-400
CuFe400
CeFe600
Cu-400
0.5
0.4
0.3
0.2
0.1
0
3.1.4. H2-TPR chemisorption characterization
The reactivity of the mixed and single oxide CuFe catalyst sur-
faces was examined by studying oxygen removal under hydrogen
(H2) reduction conditions. Fig. 4 shows the redox properties of
the catalysts characterized by H2-TPR. The H2-TPR curves of CuO
indicated a one-step reduction of Cu+2 → Cu0 centered at 244 ◦C.
The Fe nano-oxide exhibited two reduction peaks in the tem-
perature range of 200–300 and 350–800 ◦C, which indicated that
the reduction of Fe3O4 involved a multiple-step reduction to the
metallic Fe phase. The initial peak was sharp and centered at
315 ◦C, and the high temperature reduction peak was broad and
stretched over a wide temperature range. The TPR profiles of the
50 150 250 350 450 550 650 750 850
Temperature/ºC
Fig. 4. H2-TPR profiles of the different copper–iron based nanostructured catalysts.

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Table 2
Catalytic performance of the Cu and Fe based catalysts for benzene hydroxylation.
Entry
Benzene
conversion
(%)
Selectivity (%)
Phenol
H2O2
efficiency
(%)
Catalyst
HQa
CATb
BQc
Others
1
2
3
4
5
6
7
Blank
Cu-400
Fe-400
CuFe-300
CuFe-400
CuFe-500
CuFe-600
CuFe-400
–
4.3
–
–
–
–
–
–
75.4
77.2
89.3
92.4
81.4
72.4
94.3
7.8
5.6
4.5
3.6
12.7
15.3
2.5
6.5
4.1
3.2
2.6
3.1
2.3
2.0
6.0
10.1
3.0
1.4
2.8
5.4
1.2
4.3
3.0
–
–
–
87.6
89.6
91.5
93.0
88.9
86.7
92.3
12.6
29.2
38.4
23.0
20.0
26.4
4.6
–
8d
Reaction conditions: Benzene (10 mmol); catalyst (50 mg), 50% H2O2 (30 mmol), acetonitrile solvent (5 ml), reaction temperature (70 ◦C) and reaction time (4 h).
a
HQ – hydroquinone.
CAT – catechol.
BQ – benzoquinone.
The control reaction was carried out with 30% H2O2 oxidant.
b
c
d
tion slightly difficult or incomplete. This result was also confirmed
by the multiple-stage reduction of the CuFe-600 catalyst sample
compared to the other CuFe catalysts calcined at different temper-
atures. It is possible that this result indicates the presence of Cu
particles with different sizes that will require different reduction
temperatures. An additional impact of the calcination temperature
on the reduction of the different CuFe catalysts was clearly visible in
the reduction temperature shift of the peaks at 500–850 ◦C for all of
the CuFe mixed oxide catalysts. The CuFe catalysts calcined at 300
and 400 ◦C exhibited slightly low reduction temperatures for the
reduction of the FeO phase to metallic Fe compared to CuFe-500 and
CuFe-600. The observed variations in the CuFe reduction patterns
resulting from different calcination temperatures can have a sig-
nificant influence on the catalytic performance of the catalyst. The
shift in the reduction temperatures according to the different cal-
cination temperature of mixed CuFe oxide catalyst was previously
observed by Xiao et al. [43].
8). It can be seen that the comparison of CuFe-400 with 50% H2O2
exhibited high catalytic performance when compared to 30% H2O2.
This could be ascribed to the possible effect of saturated H2O2 with
little water effect of 50% compared to 70% in the H2O2 that might
create competitive adsorption of benzene and water at the catalyst
active sites, thus slightly lowering the catalyst activity. This issue
is also discussed later on the effect of solvent, where water was
used as the solvent (Section 3.2.2). The selectivity with respect to
H2O2 efficiency to product formation was found to be slightly bet-
ter for the CuFe-300 and CuFe-400 catalysts. As shown in Table 2,
the catalytic conversion of the CuFe catalysts was initially high for
calcination temperatures of up to 400 ◦C and the decreased signifi-
cantly at 500 and 600 ◦C. These observations may be related to the
mixed oxide catalyst structural characteristics, such as redox prop-
erties (H2-TPR Fig. 4), and the respective particles morphological
and phase transformation exhibited by the CuFe composite cat-
alysts as the calcination temperature increased (XRD, Fig. 1 and
FESEM, Fig. 2). According to the SEM images (Fig. 2), the CuFe
catalysts calcined at 500 and 600 ◦C exhibited morphology with
highly agglomerated particles with a continuous smooth surface.
This transformation in catalyst’s morphology was accompanied by
significant changes in porosity and particles size. As results, it is
possible that this type of morphology for CuFe-500 and CuFe-600
could hinder the active catalyst sites, which could make it diffi-
cult for the reaction species to access them compared to the high
catalytic performance exhibited by the CuFe-300 and CuFe-400
catalysts, which have loose particles that are more porous. As pre-
viously mentioned, the best performing catalyst was composed of
Fe3O4 and CuO phases rather than the CuFe2O4 spinel. Therefore, it
is reasonable to suggest that in the current study, the spinel struc-
tural formation of the CuFe2O4 phase is not the only requirement
for achieving an active catalyst capable of oxidizing the typical
inert benzene C H bond to oxygenates. It is also important to note
that the catalytic activity of CuFe calcined at various temperatures
exhibited different H2-TPR surface oxygen reactivity, which indi-
cated the significant structural modification effect on the composite
catalyst redox cycles of the two metals. This result is consistent with
the low reduction temperatures (i.e., H2-TPR) of the CuFe-300 and
CuFe-400 catalysts samples exhibited high catalytic performance
compared to CuFe-500 and CuFe-600, which exhibited a slight high
H2-TPR temperature shift. Therefore, the correlation of the catalytic
and characterization results clearly demonstrated the significant
influence of the calcination temperature on the CuFe mixed-oxide
for achieving a highly active catalyst with enhanced exposed cat-
alytic sites. In addition to the better benzene conversions achieved
with the CuFe-300 and CuFe-400 catalysts, these catalysts afforded
good phenol selectivity compared to the CuFe-500 and CuFe-600
catalysts (Table 2). To investigate if the redox cycle structure of
3.2. Catalytic activity evaluation
3.2.1. Catalysts activity screening
The direct hydroxylation of benzene into phenol and the cor-
responding dihydroxybenzene derivatives constitute one of the
important chemical reaction processes in liquid-phase oxida-
tions. Therefore, we carried out the benzene oxidation reaction
using aqueous H2O2 as the terminal oxidant to evaluate the cat-
alytic activity of the nanostructured CuFe based catalysts. Table 2
lists the obtained catalytic results for the different CuFe cata-
lysts. The non-catalyzed reaction resulted in no benzene substrate
conversion after 4 h irrespective of the addition of the highly reac-
tive H2O2 oxidant. However, for catalyzed reactions using the
CuFe catalysts, benzene substrate conversions of up to 38% were
achieved under similar reaction conditions. Among the evaluated
catalysts, CuFe calcined at 400 ◦C exhibited the highest benzene
conversion. This conversion for the CuFe-400 catalyst was better
than that obtained with the individual CuO and Fe2O4 catalysts
under the same reaction conditions. Overall, in terms of the ben-
zene conversion efficiency, t an increasing order of performance
was observed as follows: CuFe-400 > CuFe-300 > CuFe-500 > CuFe-
600 > Fe-400 > Cu-400 (Table 2). This trend indicated the significant
catalytic effect exerted by the CuFe mixed oxide catalyst depends
on the calcination temperatures due to the various induced struc-
tural characteristics. For example, CuFe-400, which exhibited the
best performance, was composed of the Fe3O4 phase, and CuFe-600
consisted of a mixture of Fe3O4 and Fe2O3 iron phases in addition
to the spinel CuFe2O4 structure phase (Fig. 1). Representative catal-
ysed benzene hydroxylation reaction with typical industrial used
aqueous H2O2 with content of 30% was carried out (Table 2, entry

P.R. Makgwane, S.S. Ray / Journal of Molecular Catalysis A: Chemical 398 (2015) 149–157
155
Table 3
Effect of different solvents on the catalytic performance of the Cu and Fe based catalysts.
Entry
Benzene
conversion (%)
Selectivity (%)
Phenol
Solvent
BQa
HQb
CATc
Others
1
2
3
4
5
6
7
Acetonitrile
Water
Methanol
Acetic acid
Hexane
38.4
7.4
45.3
12.4
–
91.0
97.3
92.8
90.1
–
3.1
0.3
4.0
5.8
–
2.5
0.4
1.2
2.6
–
2.4
1.0
2.0
1.5
–
–
–
–
–
–
–
–
Ethanol
Propanol
34.3
31.7
93.3
95.2
3.1
2.8
2.2
1.5
1.4
0.5
Reaction conditions: Benzene (10 mmol); catalyst (50 mg), 50% H2O2 (30 mmol) acetonitrile solvent (5 ml), temperature (70 ◦C) and time (3 h).
a
HQ – hydroquinone.
BQ – benzoquinone.
CAT – catechol.
b
c
oxidized CuFe-400 (i.e., Fe+3/+2 and Cu+2/+1) was the requirement
for the high catalytic performance of the catalyst, we tested the
activity of reduced CuFe-400 under H2 together with Fe-400. The
details of the reduction conditions and characterisation results are
summarized in the supporting information (Fig. S1a and b). The
reduced CuFe-400 exhibited low catalytic activity of 5% benzene
conversion with 100% phenol selectivity. For reduced Fe-400, no
benzene conversion was observed. It is noteworthy to mention that
under the 4 h reaction time the oxidized CuFe-400 afforded higher
conversion compared to reduced CuFe-400 of less than 1%. The 5%
conversion was only achieved after prolonged reaction time of up
to 12 h. Furthermore, to exclude any possible self-catalytic effect of
benzene/H2O2, the blank reaction carried out with both substrates
showed no conversion after 12 h. The XRD correlation of fresh and
used CuFe-400 (reduced) exhibited significant structural changes
of the used catalyst (Fig. S1a and b). We assume that these struc-
tural changes after prolonging the reaction time to up to 12 h was
accompanied by the formation of the active phase of CuFe-400 from
its reduced form that facilitated the operation of efficient redox
Fe+2/Fe+3 (Fenton-like) for the active catalyst. However, this obser-
vation requires further studies to probe the actual influence of the
different phases and oxidation state of mixed CuFe oxide catalyst
activity.
an active role in the catalyst by accelerating the initiation of the
reaction, as observed for the catalytic activity of pure CuO in Table 2.
The selectivity of the formed HO/HO2 hydroxyl species from the
Fenton chemistry decomposition has been questioned in the litera-
ture, especially the HO2 species, which is believed to be unselective.
Therefore, the potential quantification of such hydroxyl species in
the H2O2 catalyzed oxidation reactions would allow for the devel-
opment of Fenton-like oxidation catalysis that would allow for
optimization of the selectivity and tuning of the catalyst redox
cycles.
3.2.2. Effect of reaction variables on CuFe-400 catalytic
performance
After the initial high catalytic activity displayed by the CuFe
catalyst calcined at 400 ◦C during the catalyst screening, we fur-
ther investigated the influence of the different reaction variables
(i.e., solvent, temperature, time and re-usability) on the catalyst
performance in the benzene hydroxylation reaction.
Table 3 summarizes the results for the effect of the type of sol-
vent on the catalytic performance for the CuFe-400 catalyst sample.
The effect of the solvent in the liquid-phase reactions is important
for facilitating the solubility of the reacting species, which enhances
the mass transport of their interaction. Therefore, different solvents
(i.e., water, methanol, acetonitrile, ethanol, propanol, acetic acid
and hexane) were investigated to determine their effect on the cat-
alytic performance of the CuFe-400 catalyst for the hydroxylation
of benzene. The nature of the solvent plays a major role in chem-
ical reactions to enhance on reactants solubility, thus improving
the mass transfer of reactants contact. It can also increase on con-
version and tailor products selectivity significantly. The conversion
of benzene as well as the selectivity to phenol varied significantly
with the different solvents. Water as protic and immiscible sol-
vent to benzene yielded poor conversion but excellent selectivity
for phenol compared to acetonitrile, which exhibited high conver-
sion with a slightly lower phenol selectivity. We believe that water
was strongly adsorbed on the catalyst surface active sites, which
competed with benzene and H2O2, thus lowering the substrate
conversion. On the other hand, methanol performed exceptionally
well among the evaluated solvents. When the type of alcohol was
varied from methanol to propanol, the alcohol with more carbon
atoms performed relatively poorly for substrate conversion com-
pared to the C1 alcohol but the selectivity for phenol improved.
The high performance of methanol was ascribed to its high polar
protic nature and high dielectric constant compared to other alco-
hols. Under acetic acid conditions, the reaction also exhibited poor
substrate conversion, which is most likely due to acceleration of the
self-decomposition rate of hydrogen peroxide assisted by the acidic
medium catalysis that can lead to a decrease in activity. Water
exhibited the lowest catalytic activity but a good phenol selectiv-
ity of approximately 97.3%. This result may be due to the reaction
The effect of the classical Fenton-like catalytic chemistry in the
oxidation reactions involving H2O2 produces hydroxyl species by
the ferrous ion (Fe2+) according to the proposed redox cycle mech-
anism described below [11].
Fe2+ + H2O2 → Fe3+ + HO• + OH−
Fe3+ + H2O2 → Fe2+ + HOO• + H+
Fe2+ + HO• → Fe3+ + OH−
(1)
(2)
(3)
In the initial step, H2O2 is decomposed by iron to form highly
reactive hydroxyl species, which oscillates between Fe+2/Fe+3 dur-
ing the reaction according to Eqs. (1) and (2). These hydroxyl species
are capable of attacking the benzene ring C H bond to form ben-
zylic free radicals that subsequently trap the oxygen species from
the available hydroxyl source. Similarly, copper is capable of oper-
ating in the same manner as iron in the H2O2 catalyzed liquid-phase
oxidations according to the equations described below and as indi-
cated by the catalytic results in Table 2.
Cu+ + H2O2 → Cu2+ + HO• + OH−
Cu2+ + H2O2 → Cu+ + HOO• + H+
Cu+ + HO• → Cu2+ + OH−
(1)
(2)
(3)
It has been proposed that the presence of copper in such a CuFe
composite for Fenton-like operation facilitates an enhanced and
efficient redox exchange cycle between the Fe+2/Fe+3 oxidation
states for oxidation reactions. In addition, the copper may also play

156
P.R. Makgwane, S.S. Ray / Journal of Molecular Catalysis A: Chemical 398 (2015) 149–157
Fig. 5. Catalytic performance of the CuFe-400 catalyst under different reaction conditions. (a) Effect of temperature, (b) effect benzene to H2O2 molar ratio, (c) effect of
reaction time, and (d) catalytic recyclability test performance. Other reaction conditions: Benzene (10 mmol); catalyst (50 mg), solvent (5 ml).
occurring in a biphasic reaction medium where the products are
separated from the active catalyst by the organic and aqueous phase
medium. Therefore, catalytic decomposition of the formed phenol
is less severe. Bianchi et al. [12] demonstrated that this biphasic
strategy is beneficial for improving the selectivity and yield of phe-
nol in the benzene hydroxylation process.
Fig. 5c. The conversion of benzene gradually increased from the
initiation of the reaction to 6 h, and then plateaued at 6–8 h with
a maximum conversion of 62%. At 62% benzene conversion, the
phenol selectivity was 61%, and the other over-oxidation products
that accumulated constituted 39%. The nano-CuFe-400 catalyst also
exhibited excellent recyclability performance for 3 reaction cycles
(Fig. 5d). The benzene conversion stabilized at 43–45% after each
reaction cycles consisting of 4 h in length. In addition, the selectivity
towards phenol did not change during the catalyst re-use period.
More importantly, the easy recyclability of the nano-oxide CuFe
composite using the external magnet makes the catalyst attractive
for mediating catalyzed benzene hydroxylation for the synthesis of
phenol.
Next, we varied the reaction temperature (i.e., 60–80 ◦C) while
keeping all of the other reaction variables constant. As shown in
Fig. 5a, the increase in reaction temperature was beneficial for
increasing the oxidation reaction rates as well as the benzene
substrate conversion. However, the negative impact of increasing
reaction temperature was noticeable on the selectivity to phenol at
80 ◦C, which decreased to approximately 73%. The over oxidation
products, which substantially increased, were identified as hydro-
quinone, benzoquinone and catechol. Therefore, a temperature of
70 ◦C was regarded as the optimum required for reasonable oxi-
dation rates with appreciable preservation of a phenol selectivity
of approximately 90%. Fig. 5b shows the effect of varying the con-
centration of the H2O2 oxidant in the range of 2–4 molar ratios to
benzene on the conversion and product selectivity in the hydrox-
ylation of benzene. According to the obtained results, a high H2O2
concentration resulted in a decrease in the phenol selectivity, which
promoted the formation of the other dihydroxybenzene derivatives
due to overoxidation of phenol. Based on these results, a molar ratio
of 1:3 (benzene to H2O2) afforded efficient CuFe-400 catalytic per-
formance. The influence of the reaction time on the conversion and
product selectivity over the CuFe-400 catalyst sample is shown in
4. Conclusions
In conclusion, we demonstrated that the mixed oxides of Cu
and Fe result in a highly active catalyst for benzene hydroxyla-
tion for the synthesis of phenol. In addition, the activity of the
CuFe catalyst depended on the calcination temperature, which sig-
nificantly influenced the evolution of its structural characteristics.
The correlation of the catalyst performance with the characteriza-
tion results demonstrated that the formation of a CuFe2O4 spinel
structure phase was not the primary requirement for obtaining a
highly active and selective catalyst. However, the CuFe calcined at
400 ◦C with this spinel phase exhibited the best performance. Under
optimized reaction conditions, the CuFe-400 catalyst afforded a

P.R. Makgwane, S.S. Ray / Journal of Molecular Catalysis A: Chemical 398 (2015) 149–157
157
benzene conversion of 44% at a combined phenol selectivity of
91%. The enhanced activity of CuFe-400 along with a H2O2 oxidant
provides a green synthetic route for benzene hydroxylation.
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Appendix A. Supplementary data
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