Preparation, characterization and heterogeneous catalytic applications of GO/Fe3O4/HPW nanocomposite in chemoselective and green oxidation of alcohols with aqueous H2O2

Article abstract of DOI:10.1002/aoc.4323

Graphene oxide-Fe₃O₄-NH₃ ⁺H₂PW₁₂O₄₀ ^- magnetic nanocomposite (GO/Fe₃O₄/HPW) was prepared by linking amino-functionalized Fe₃O₄ nanoparticles (Fe₃O₄-NH₂) on the graphene oxide (GO), and then grafting 12-tungstophosphoric acid (H₃PW₁₂O₄₀) on the graphene oxide-magnetite hybrid (GO-Fe₃O₄-NH₂). The obtained GO/Fe₃O₄/HPW nanocomposite was well characterized with different techniques such as FT-IR, TEM, SEM, XRD, EDX, TGA-DTA, AGFM, ICP and BET measurements. The used techniques showed that the graphene oxide layers were well prepared and the various stages of preparation of the GO/Fe₃O₄/HPW nanocomposites successfully completed. This new nanocomposite displayed excellent performance as a heterogeneous catalyst in the oxidation of alcohols with H₂O₂. The as-prepared GO/Fe₃O₄/HPW catalyst was more stable and recyclable at least five times without significantly reducing its catalytic activity.

Full text of DOI:10.1002/aoc.4323

Received: 5 December 2017
DOI: 10.1002/aoc.4323
Revised: 23 January 2018
Accepted: 23 January 2018
F U L L P A P E R
Preparation, characterization and heterogeneous catalytic
applications of GO/Fe O /HPW nanocomposite in
3
4
chemoselective and green oxidation of alcohols with
aqueous H O
2
2
Kamran Darvishi | Kamal Amani
| Manuchehr Rezaei
Department of Chemistry, Faculty of
+
‐
Graphene oxide‐Fe O ‐NH H PW O
magnetic nanocomposite (GO/
Sciences, University of Kurdistan, P.O. Box
617715175, Sanandaj, Iran
3
4
3
2
12 40
6
Fe O /HPW) was prepared by linking amino‐functionalized Fe O nanoparti-
3
4
3 4
cles (Fe O ‐NH ) on the graphene oxide (GO), and then grafting 12‐
3
4
2
Correspondence
Kamal Amani, Department of Chemistry,
Faculty of Sciences, University of
Kurdistan, P.O. Box 6617715175,
Sanandaj, Iran.
tungstophosphoric acid (H
PW12O40) on the graphene oxide‐magnetite hybrid
3
(
GO‐Fe O ‐NH ). The obtained GO/Fe O /HPW nanocomposite was well char-
3
4
2
3 4
acterized with different techniques such as FT‐IR, TEM, SEM, XRD, EDX,
TGA‐DTA, AGFM, ICP and BET measurements. The used techniques showed
that the graphene oxide layers were well prepared and the various stages of
preparation of the GO/Fe O /HPW nanocomposites successfully completed.
Email: amani_71454@yahoo.com
3
4
This new nanocomposite displayed excellent performance as a heterogeneous
catalyst in the oxidation of alcohols with H O . The as‐prepared GO/Fe O /
2
2
3 4
HPW catalyst was more stable and recyclable at least five times without signif-
icantly reducing its catalytic activity.
KEYWORDS
3 4 3 4 3 40
alcohol oxidation, Fe O nanoparticles, GO/Fe O /HPW nanocomposite, graphene oxide, H PW12O
1
| INTRODUCTION
recyclable in chemical reactions. Usually, they are used in
non‐polar solvents as a heterogeneous catalyst but in
these conditions they also have a low surface area
Over the last few decades, the use of heteropoly acids
HPAs) and polyoxometalates (POMs) as catalyst in
chemical reactions has become increasingly important
2
[1–5]
(
(<10 m /g).
To solve the above mentioned problems, two
heterogenization methods have been proposed: (i)
exchange of heteropoly acid protons with bulk organic
and inorganic cations which can reduce the solubility
and increase their surface area, (ii) immobilization of
heteropoly acids and polyoxometalates on suitable solid
[1–5]
and their application has been reviewed.
of many heteropoly acids are their use in catalyzing vari-
ous chemical reactions as Bronsted acid.
Polyoxometalates have reversible multi‐electron redox
behavior that makes them potential catalysts in oxidative
The benefits
[
2]
[3]
reactions in homogeneous or heterogeneous conditions.
supports with high surface area. It should be empha-
Moreover, by changing the POM structure, their chemical
properties can be customized. Keggin‐type heteropoly
acids such as H PW O , H PMo O or H SiW O
12 40
are the most popular and the most efficient. HPAs are sol-
uble in polar and oxidative media and therefore not easily
sized that by heterogenization of HPAs, their activity is
decreased relative to the initial catalyst and homogeneous
conditions and the catalyst is probably leached out from
the support surface in catalyst recycling. Therefore, the
covalent or ionic nature of the interaction between HPA
3
12 40
3
12 40
4
Appl Organometal Chem. 2018;e4323.
wileyonlinelibrary.com/journal/aoc
Copyright © 2018 John Wiley & Sons, Ltd.
1 of 14
https://doi.org/10.1002/aoc.4323

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DARVISHI ET AL.
and the support surface is significant and affects the phys-
icochemical properties, catalytic activity, and leaching of
alcohols to its corresponding carbonyl compounds have
[
4,11–18,43–59]
been reported in the literature.
The mecha-
[6–20]
the active species.
Various insoluble supports have
nism of the effect of heteropoly acids and
polyoxometalates is well known in catalyzing of oxidation
reactions with hydrogen peroxide, and suggests the for-
mation of an active species: Ishii–Venturello complex
been used for immobilization of HPAs such as silica,
titana, alumina, zirconia, carbon, cellulose, zeolites and
clays, metal oxides, organic materials and metal‐organic
[2,3,5–19]
3– [60,61]
frameworks.
{PO [WO(O ) ] } .
4
2 2 4
Graphene oxide (GO) has a special single monolayer
structure with aromatic and aliphatic regions, and
various functional groups such as hydroxyl, epoxy,
carboxylic and carbonyl groups are located on the surface
of each layer. Usually, graphite oxide is prepared from
the oxidation of graphite and has extensive applications
Some previous reports have several defects as follows:
(i) the use of the catalyst in homogeneous conditions
which makes the process of separation and reuse of the
catalyst more difficult; (ii) the need for centrifuge or filter-
ing to separation of catalyst which are not simple pro-
cesses; (iii) over‐oxidation of alcohols to the
corresponding carboxylic acids; (iv) the use of volatile
and harmful organic solvents; (v) the need for rough con-
ditions to carry out oxidation reaction such as high tem-
perature, very long reaction time, high concentration of
H O ; (vi) the weak linking of HPAs and POMs onto car-
[20,21]
[22,23]
[24–29]
in biosensors,
electrocatalysis
imaging,
fields.
membrane,
and
the
[
22,24,30–32]
Recently,
functionalization of graphene oxide has attracted
considerable attention and various methods have been
used for this purpose in which organic groups are depos-
ited on the surface of graphite oxide through covalent
bonding usually by bonding with carboxylic acid groups,
forming amides and esters groups or by nucleophilic
2
2
rier which leads to leaching out of HPA and POM during
the oxidation process. We have attempted to develop and
introduce a new catalytic system based on H PW O to
3
12 40
[30,33–37]
attack on the epoxy rings.
In the recent past, in
eliminate these deficiencies.
order to utilize its high surface area, graphene and
graphene oxide have been used as a support and many
types of nanoparticles are immobilized or deposited
onto them. Some examples of this type of hybrid
Therefore, with regard to the cases cited above and in
the continuation of our activities in the synthesis of nano-
particles and nanocomposites and their applications in
[
9,62,63]
organic synthesis,
we planned to design and syn-
[20]
[38–40]
composite are GO/enzymes,
nanoparticle/ polyoxometalate/graphene nanohybrid,
GO/Fe O ,
Au‐
thesize GO/Fe O /HPW nanocomposite as a catalytic sys-
3
4
3 4
[
21]
tem with the hope that this system has unique catalytic
properties of heteropoly acids and high surface area to
have a heterogeneous condition which allows high stabil-
ity and easy separation from the reaction mixture
(Scheme 1). In continuation, we employed a facile and
efficient approach for chemoselective oxidation of alco-
hols to corresponding aldehydes or ketones catalyzed by
GO/Fe O /HPW nanocomposite with H O under
[41]
GO/polyoxometalate composite film,
and graphene/
[42]
HPA.
The decoration with magnetic nanoparticles is
particularly interesting because magnetic graphene oxide
is easily separated from the reaction medium using an
external magnet.
Magnetic nanoparticles (MNPs) are widely used as
support in catalytic systems due to easy separation and
reuse. The main disadvantage of MNPs is their low sur-
face area. Thus, to fix this deficiency, MNPs are often
placed in a composite structure alongside particles with
a high surface area. GO sheets with an excellent surface
area and easy connection are ideal options. Several
methods have been used to situate the MNPs on the sur-
face of GO sheets, such as (i) formation of the MNPs on
the surface of GO layers and (ii) linking of pre‐prepared
3
4
2 2
organic solvent‐free conditions.
2
| EXPERIMENTAL SECTION
.1 | Materials
2
All chemicals were purchased from Merck chemical com-
pany with no further purification. Fourier transforms
infrared spectra were recorded using FT‐IR Bruker (TEN-
SOR 27) spectrometer and KBr plates. Scanning electron
micrographs (SEM) and energy‐dispersive X‐ray spectros-
copy (EDX) of the samples were taken with FE‐SEM from
TSCAN Company. The magnetic properties of the studied
samples were measured by an alternating gradient force
magnetometer (AGFM) at room temperature. Thermal
analyses (TGA‐DTA) were performed using a Linseis
STA PT 1000 instrument at room temperature to 600 °C.
Melting points of solid samples were measured and
[21,38–42]
MNPs through covalent bonds to GO.
Oxidation of alcohols is one of the more important
and popular reactions in organic chemistry and so far
dozens of articles have been reported in this field. The oxi-
dation of alcohols in accordance with the principles of
green chemistry is limited to some environmentally
benign oxidants including hydrogen peroxide and molec-
ular oxygen. Various methods that use hydrogen peroxide
as oxidizing agents in the presence of heteropoly acids
and polyoxometalates to oxidize primary and secondary

DARVISHI ET AL.
3 of 14
3 4
SCHEME 1 The strategy for the synthesis of the GO/Fe O /HPW nanocomposite
determined with an electrothermal 9100 apparatus. X‐ray
diffraction (XRD) pattern of the product was carried out
with a X’Pert Pro Philips X‐ray diffractometer with Cu
Kα radiation, and the scan range (2 h) was from 10° to
respectively. The amount of Fe, Si, and W elements was
obtained using the ICP‐OES (Varian, 730‐ES). All the
reported yields for the reaction products are the isolated
yields.
8
0°. The transmission electron microscopy (TEM) images
were recorded by a Philips microscope (EM 208, Tokyo,
Japan) operating at 100 kV. The Brunauer‐Emmett‐Teller
2.2 | Preparation of graphene oxide (GO)
(
BET) surface area was obtained by applying the BET
Graphene oxide (GO) was made from graphite by the
modified Hummers method.
[
64,65]
equation and P/P0 = 0.305 to the adsorption data,

4
of 14
DARVISHI ET AL.
2.3 | Preparation of acid chlorides of
3 | RESULTS AND DISCUSSION
graphene oxide (GOCl)
As can be seen in Scheme 1 and in the ‘Experimental
Section’, our magnetic catalytic system was prepared in
several steps. The structure of the synthesized catalyst
was then characterized by different physicochemical tech-
niques such as FT‐IR, XRD, EDX, FE‐SEM, TEM, TGA,
BET, ICP and AGFM.
GOCl was prepared according to the previously reported
[66]
method.
2
.4 | Preparation of silica‐coated Fe O4
3
MNPs (Fe O @SiO )
3
4
2
Fe O @SiO nanoparticles were prepared following a
3
4
2
[9]
method reported previously in the literature.
3
.1 | Fourier Transform Infrared
Spectroscopy (FT‐IR)
2
.5 | Preparation of NH ‐functionalized
2
Figure 1 shows FT‐IR spectra for graphite, GO, GOCl,
Fe O , Fe O @SiO , Fe O ‐NH , GO‐Fe O , H PW O
Fe O @SiO MNPs (Fe O ‐NH )
3
4
2
3
4
2
3
4
3
4
2
3
4
2
3
4
3
12 40
and GO/Fe O /HPW. In the FT‐IR spectra of GO, the
Fe O ‐NH nanoparticles were prepared with surface
3 4
3
4
2
‐1
[
67]
peaks at 1729 and 1615 cm is related to stretching vibra-
tion of C=O and C=C bonds, respectively, and a very
broad peak in the range of 3500‐2500 cm‐ is attributed
to the stretching vibrations of O‐H bonds of acidic and
alcoholic groups (Figure 1, b). The peaks at 1220, 1051
and 991 cm‐ correspond to asymmetric and symmetric
vibrations of the epoxy ring and the stretching vibration
modification of Fe O @SiO using APTES in ethanol.
3
4
2
1
2
.6 | Preparation of the graphene oxide‐
Fe O MNPs (GO‐Fe O )
3
4
3 4
1
GO‐Fe O4 was prepared according to the method
3
[66]
described in the literature.
[
68]
of C‐O in the order mentioned.
In the curve of Fe O NPs (Figure 1, d), the peak at
3
4
‐1
[69]
2
.7 | Preparation of GO‐Fe O ‐
3
4
about 570 cm is ascribed to Fe‐O stretching vibration.
+ ‐
NH H PW O nanocomposite (GO/
3
2
12 40
In FT‐IR spectrum of silica‐coated MNPs (Figure 1, e), the
peaks intensity of Fe‐O bond is decreased and the sharp
Fe O /HPW)
3
4
‐
1
band at about 1063 cm is attributed to Si‐O‐Si asymmet-
1
00 mg of GO‐Fe O MNPs was dispersed in 5 ml of
3
4
ric stretching vibration indicating the survival of a SiO
2
deionized water and ultrasonicated for 30 min. Then, a
solution of H PW O .nH O (200 mg) dissolved in 2 ml
[
70]
layer around the Fe O nanoparticles.
3
4
3
12 40
2
‐1
In f and g curves, the peak of about 3430 cm is attrib-
uted to N‐H stretching, but this peak can interfere with a
peak of O‐H stretching absorbance. Furthermore, the peaks
deionized water was added dropwise into the solution
and ultrasonicated for a further 30 min and the mixture
stirred for 24 h at room temperature. Finally, the formed
GO/Fe O /HPW nanocomposite was magnetically sepa-
‐1
of N‐H bending at 1629 cm for Fe O ‐NH is clearly
3
4
2
3
4
[70]
observed (Figure 1, f and g).
In the FT‐IR spectrum of
rated and washed twice with water and dried at 60 °C.
GO‐Fe O hybrid (Figure 1, g), two peaks at around 1648
3
4
‐1
and 1600 cm are related to C=O bond of the amide group
and C=C bond in the graphene layer, respectively. The peak
2.8 | Oxidation of alcohols to the
‐1
corresponding aldehydes or ketones by
H O
at 3430 cm is concern the stretching vibration of O‐H and
[
66]
2
2
N‐H bonds that overlap.
In FT‐IR spectrum of
H PW O four characteristic vibration peaks at 1081 (P‐
3
12 40
2
ml of H O (10%, about 5 mmol) with 1 mmol of the
2 2
‐1
O), 985 (W=O), 890 and 795 (W‐O‐W) cm areshown. Com-
alcohol and 20 mg of the catalyst was charged into a
round‐bottomed flask. The resultant mixture was stirred
at 70 °C. The progress of the reaction was followed by
thin‐layer chromatography (TLC). After the completion
‐1
pared to the starting Keggin unit, the peak at 1081 cm over-
lap with Si‐O‐Si stretching vibration but new peaks at 979,
‐1
8
93 and 810 cm are observed on FT‐IR spectrum of GO/
Fe O /HPW (Figure 1, i) indicating that H PW O was
3
4
3
12 40
[71]
of the reaction, the GO/Fe O /HPW was separated using
3
4
anchored to the GO‐Fe O hybrid successfully.
3
4
the magnet and the reaction mixture was extracted with
ether (3×5 ml). After drying the organic phase and purify-
ing the products by chromatographic methods, the prod-
ucts were identified by various spectroscopic methods.
The solid catalyst was washed three times with ethanol
and then dried under vacuum for subsequent use.
3.2 | Thermogravimetric analysis (TGA)
The thermal stability of GO, GO‐Fe O , and the GO/
3
4
Fe O /HPW nanocomposite were analyzed by TGA.
3
4

DARVISHI ET AL.
5 of 14
3 4 3 4 2 3 4 2 3 4 3
FIGURE 1 FT‐IR spectra of (a) G, (b) GO, (c) GOCl, (d) Fe O , (e) Fe O @SiO , (f) Fe O ‐NH , (g) GO‐Fe O hybrid, (h) H PW12O40, (i)
fresh GO/Fe O /HPW, and (j) GO/Fe O /HPW after recycling in run 5
3 4 3 4
Samples were heated under an atmosphere of N at rt. to
approximately 6.5% weight loss at 600 °C. This demon-
strates that the GO/Fe O /HPW nanocomposite has a
high thermal stability.
2
6
00 °C at a heating rate of 10 °C/min (Figure 2). Graphene
oxide was thermally unstable and even at less than
00 °C, it began to lose moisture and thus the mass. Los-
ing approximately 12% of the weight below 150 °C and
5% in the temperature range of 150‐266 °C indicates
extensive oxidation of graphene oxide. GO‐Fe O also
3
4
1
3.3 | Magnetic properties
6
The magnetic properties were measured at room temper-
ature as illustrated in Figure 3. The saturation magnetiza-
tions for Fe O , Fe O @SiO , Fe O ‐NH nanoparticles,
3
4
had a weight loss of approximately 3.4% at 200 °C and
0.5% in the range of 200‐600 °C. This illustrates that
GO‐Fe O is more stable than GO. The TGA curve for
1
3
4
3
4
2
3
4
2
GO‐Fe O hybrid, and GO/Fe O /HPW nanocomposite
3
4
3
4
3 4
GO/Fe O /HPW
nanocomposite
shows
only
are 0.437, 0.191, 0.189, 0.175 and 0.112 M (a.u.), respec-
3
4
tively. The magnetic property of the GO/Fe O /HPW
3
4
nanocomposite was so high that it took only a few sec-
onds to separate with the external magnet from the aque-
ous solution, which made it possible to recycle and reuse
the catalyst.
3.4 | Scanning electron micrograph (SEM)
The SEM technique was used to study the morphology of
GO/Fe O /HPW nanocomposite (Figure 4). The SEM
3
4
images confirm the successful attachment of Fe O ‐NH
3
4
2
and H PW O nanoparticles on GO surface. Regarding
3
12 40
Figures b and c, it can be observed that graphene oxide
sheets are distributed between Fe O ‐NH and
GO/Fe O /HPW
3
4
2
FIGURE 2 TGA analysis of GO, GO‐Fe
3
O
4
, and GO/Fe
3
O
4
/HPW
H PW O
nanoparticles
and
3
12 40
3 4

6
of 14
DARVISHI ET AL.
FIGURE 3 Magnetization curves of Fe
NH , GO‐Fe and GO/Fe /HPW
3 4 3 4 2 3 4
O , Fe O @SiO , Fe O ‐
2
3
O
4
3
O
4
FIGURE 5 Energy dispersive X‐ray analysis (EDX) of GO/Fe O /
3
4
HPW
nanocomposite are well‐formed with a large amount of
free space. The Fe O ‐NH nanoparticles are anchored
3
4
2
catalyst loaded. The results showed that the amount of
HPA loaded on the support is 19.76%wt.
uniformly and with a high density on the wrinkled GO
layers. In particular, the pleat structure of GO may pre-
vent the accumulation and agglomeration of Fe O ‐NH
3
4
2
and H PW O particles and contribute to their better
distribution.
3
12 40
3.6 | X‐ray diffraction (XRD)
The structures of GO, Fe O ‐NH , H PW O , and GO/
3
4
2
3
12 40
Fe O /HPW nanocomposite were identified by XRD tech-
3
4
nique shown in Figure 6. GO has two distinct peaks at 2θ
11.5° and 42.3°, and the peak removal of 2θ = 26.4°
indicates successful oxidation of graphite to graphene
3
.5 | Energy dispersive X‐ray analysis (EDX)
=
Energy dispersive X‐ray analysis (EDX) of GO/Fe O /
3
4
[
21,66,68]
HPW nanocomposite is shown in Figure 5. The elemental
analysis by the EDX recorded peaks is related to C, O, N,
Fe, W, P, Si elements. Therefore, the EDX analysis con-
firmed the presences of expected elements in the structure
of the synthesized GO/Fe O /HPW nanocomposite. The
oxide by the Hummer method.
Diffraction peaks
with 2θ = 30.3, 35.8, 43.4, 53.8, 57.3, 62.8, 71.3 and 74.3
properly confirm the structure of Fe O ‐NH . The peak
3
4
2
positions and relative intensities are consistent with stan-
dard XRD data for magnetite, which indicates good for-
3
4
[
9]
ICP analysis was used to determine the quantity of
mation of Fe O NPs.
3 4
3 4
FIGURE 4 SEM images of GO (a), and GO/Fe O /HPW (b and c)

DARVISHI ET AL.
7 of 14
3 4 2 3 3 4
FIGURE 6 XRD patterns of GO, Fe O ‐NH , H PW12O40 and GO/Fe O /HPW
The characteristic and sharp diffraction peaks of
H PW O which confirm its crystalline structure is not
close to the specific surface area reported for the graphene
oxide. It is a fact that having such high surface area is a
major advantage for the catalytic activity of this
nanocomposite.
3
12 40
visible in GO/Fe O /HPW nanocomposite indicating the
3
4
non‐crystalline structure of the H PW O immobilized
3
12 40
on the support. The GO/Fe O /HPW nanocomposite has
3
4
inherited Fe O diffraction peaks with the same position
3
4
3
.9 | Catalytic activity studies
[15,17,72]
but with a slight decrease in their intensities.
fraction peaks with 2θ = 26.7, 30.3, 35.7, 43.5, 53.7, 57.3,
3.0 and 74.4 are registered in the XRD pattern of GO/
Fe O /HPW nanocatalyst. The average size calculated
Dif-
First, to evaluate the efficiency of the catalyst, the oxida-
tion of benzyl alcohol to benzaldehyde by hydrogen per-
oxide in the presence of the catalytic amount of GO/
Fe O /HPW was selected as the probe reaction (Scheme 2)
6
3
4
for the magnetic nanoparticles from the XRD data accord-
ing to the Scherrer equation are about 15.56 and 28.37 nm
for the Fe O ‐NH nanoparticles and GO/Fe O /HPW
3
4
and the reaction conditions were optimized.
3
4
2
3 4
nanocomposite, respectively.
3.9.1 | Influence of reaction temperature
and reaction time
3.7 | Transmission electron micrographs
At 25, 30, 40 °C, even after 24 hours, the reaction did not
progress significantly. With the increase of the reaction
temperature from 50 to 70 °C, the conversion of benzyl
alcohol increased from 10% to up to 99% and the reaction
time was drastically reduced to 3 hours. Increasing the
reaction temperature to 80 °C would not change much
in the yield of the product, but with increment the reac-
tion temperature to 90 and 100 °C, due to over‐oxidation
to benzoic acid, selectivity was decreased (Figure 8).
(
TEM)
The morphology and particle size of the prepared GO/
Fe O /HPW nanocomposites were further analyzed using
TEM technique and is shown in Figure 7. The images
clearly indicate the accumulation of Fe O and Fe O –
NH nanoparticles on the wrinkled surface of GO. A large
3
4
3
4
3 4
2
number of nanoparticles are distributed randomly on the
surface and edges of the graphene sheets.
[
40,73]
3.8 | Brunauer Emmet Teller (BET)
3.9.2 | Influence of the amount of H O
2
2
The specific surface area of H PW O , graphite powder,
In the presence of various amounts of 10% hydrogen perox-
ide and by considering the above‐mentioned conditions,
the oxidation reaction of 1 mmol benzyl alcohol to benzal-
dehyde occurred (Figure 9). The results revealed that the
yield of benzaldehyde was positively dependent on the
amounts of H O . By increasing the amount of 10% H O
2 2
3
12 40
GO, and Fe O ‐NH nanoparticles measured were
3
4
2
2
[74–76]
approximately 5, 8.5, 492, and 162 m /g, respectively.
The specific surface area of the GO/Fe O /HPW nano-
3
4
2
−1
composite measured was approximately 478 m g ,
which is remarkably high. As expected, the specific
surface area for the GO/Fe O /HPW nanocomposite was
2
2
to 5 mmol, the conversion increased gently to 98%.
3
4

8
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DARVISHI ET AL.
3 4
FIGURE 7 Representative TEM images of GO/Fe O /HPW
SCHEME 2 The oxidation reaction of benzyl alcohol to
benzaldehyde
3.9.3 | Influence of the amount of catalyst
Under the above‐optimized conditions, the oxidation of
benzyl alcohol in the presence of 10 and 15 mg GO/
Fe O /HPW nanocatalyst only resulted in benzaldehyde
3
4
FIGURE 9 Optimization of the amount of H
2
O
2
(mmol) in the
oxidation of 1 mmol benzyl alcohol
in 20% and 63% yields, respectively. Further increasing
the amount of catalyst up to 20 mg afforded the desired
product in excellent yields of 98% under the same reaction
conditions (Figure 10). Increasing the catalyst amount by
more than 20 mg led to a slight reduction in selectivity.
The oxidation of benzyl alcohol in the presence of HPW
and GO‐Fe O were also performed. The results of Table
FIGURE 8 Optimization of the reaction temperature and the
3 4
reaction time in the oxidation reaction of benzyl alcohol
ESI‐1 show that under homogeneous conditions and in

DARVISHI ET AL.
9 of 14
the presence of 20 mg HPW, within 2 h, the reaction yield
is 97%, while in the presence of GO‐Fe O , within 8 h, no
3
4
product is obtained. This indicates the need for acid to cat-
alyze the oxidation reaction.
Therefore, the ideal conditions for the selective oxida-
tion of benzyl alcohol to benzaldehyde in the present
work is as follows: the oxidation of 1 mmol benzyl alcohol
was carried out with 20 mg of GO/Fe O /HPW catalyst in
3
4
the presence of 10% H O (5 mmol) at 70 °C and within
2
2
3
h under organic solvent‐free conditions to meet the
requirements of a green oxidation reaction. Interestingly,
under this condition, benzyl alcohol was selectively trans-
formed into the corresponding benzaldehyde without any
detectable overoxidation to benzoic acid.
For obtaining the optimal reaction conditions, we
decided to investigate the scope of GO/Fe O /HPW in
3
4
the oxidation of various primary and secondary alcohols.
The experimental results indicate that the catalytic perfor-
mance of the GO/Fe O /HPW nanocomposite in the
3 4
FIGURE 10 Optimization of the amount of GO/Fe O /HPW
(
mg) in the oxidation of 1 mmol benzyl alcohol
3
4
TABLE 1 Catalytic performance of the GO/Fe
reaction conditions
3
O
4
/HPW nanocatalyst for the selective oxidation of alcohols with H
2
O
2
under optimal
a
a
b
c
Entry
Alcohol
Product
Time (h)
Yield (%)
Selectivity (%)
1
3
99
100
100
100
100
2
3
4
1.30
2
100
99
4
94
5
9
70
100
6
7
7
6
90
85
100
100
8
9
7
70
50
45
99
99
100
d
24
24
1
85
e
1
1
1
0
1
2
76
100
100
1
a
2 2
Reaction conditions: alcohol (1 mmol), catalyst (20 mg), H O (5 mmol), 70 °C.
b
Yields are referring to isolated yields.
c
Selectivity was based on the corresponding aldehydes or ketones.
d
The byproduct is phenylacetic acid.
e
The byproduct is 3‐Phenylpropanoic acid.

10 of 14
DARVISHI ET AL.
oxidation of alcohols to corresponding aldehydes or
ketones differs drastically.
In this context, we found that substituted benzyl alco-
hols bearing either electron‐releasing or electron‐with-
drawing groups were also oxidized selectively and
afforded the corresponding aldehydes in excellent to good
yields under the optimized reaction conditions (Table 1,
entries 1‐7). For instance, 4‐methoxybenzyl alcohol was
converted into its corresponding aldehyde with 100% yield
after 1.5 h while for 4‐nitrobenzyl alcohol only 90% of its
corresponding aldehyde was produced after 7 h (Table 1,
entries 2, 6) without any over‐oxidation to the correspond-
ing carboxylic acid. 2‐nitrobenzyl alcohol, a relatively hin-
dered substrate was also converted to the corresponding
aldehyde in good yields (70%) (Table 1, entry 5). The selec-
tive oxidation of 2‐phenylethanol and 2‐phenyl‐1‐
propanol as models for challenging primary aliphatic
alcohols was achieved in moderate yields (50% and 45%,
respectively) and byproducts being their corresponding
carboxylic acids. Secondary aromatic alcohols, such as 1‐
phenylethanol, and 1‐phenyl‐1‐propanol were also con-
verted to the corresponding ketones under the described
reaction conditions with excellent yield (Table 1, entries
SCHEME 3 Chemoselective oxidation of an equimolar mixture
of alcohols in the presence of GO/Fe /HPW nanocomposite
3
O
4
[
Reaction conditions: Cat. (20 mg), H O (10%, 5 mmol), 70 °C, 3 h]
2 2
1
1
0 and 11). All our efforts for homoselective oxidation of
,4‐phenylenedimethanol failed and a mixture of mono‐
oxidation of alcohol to the corresponding aldehyde,
[
60,61]
and dicarbonyl products were obtained (Table 1, entry 8).
To verify the chemoselectivity, a series of competition
experiments were performed. An equimolar mixture of
two or three types of alcohols was subjected to oxidation
in the presence of GO/Fe O /HPW nanocomposite under
ketone or carboxylic acid.
The catalytic activity of GO/Fe O /HPW in the benzyl
3
4
alcohol oxidation with hydrogen peroxide was compared
with a few previous similar catalytic systems and summa-
rized in Table 2. The results show that this new catalytic
system is comparable or superior to some previous reports
and has higher yield, shorter reaction time, milder reac-
tion conditions, uses retrievable and reusable catalyst
and avoids the use of toxic and volatile organic solvents.
3
4
optimized reaction conditions. Compared with primary
o
o
aliphatic alcohols, the results revealed that 1 and 2 ben-
zylic alcohols are more rapidly oxidized. When a 1:1 mix-
ture of benzyl alcohol and 3‐phenylpropanol was
subjected to oxidation, benzaldehyde was produced in
9
9.2% yield and only 5.9% of 3‐phenylpropanaldehyde
3
.9.4 | Recyclability of the catalyst
o
was detected. Cyclohexanol as a 2 aliphatic alcohol
remained intact in the course of oxidation so that when
a mixture of cyclohexanol and benzyl alcohol was sub-
jected to oxidation, only benzaldehyde was detected in
The recyclability of the heterogeneous catalyst is of para-
mount importance and has a critical advantage over
homogeneous counterparts. Therefore, the recyclability
of GO/Fe O /HPW nanocomposite was studied for the
1
00% yield. Other competitive reactions were also per-
3
4
formed and the results are summarized in Scheme 3.
These results clearly indicate that this method can be used
to oxidize benzyl alcohol in the presence of primary and
secondary alcohols.
oxidation of benzyl alcohol under optimized reaction con-
ditions (Figure 11). After each run, the catalyst was iso-
lated from the reaction mixture by an external magnet,
and then washed twice with ethanol and subsequently
dried under vacuum. GO/Fe O /HPW can be reused five
This high catalytic activity of GO/Fe O /HPW exhib-
3
4
3 4
ited in the oxidation of alcohols can be attributed to high
Bronsted acidity and acid content, synergetic catalytic
effect, excellent surface activity and good solubility in
times without significant loss in conversion and
selectivity.
The quantitative leaching out of active species of HPW
from the support, before and after the reaction was evalu-
ated by ICP analysis. According to the amount of Fe, Si,
and W elements, it was found that the amounts of HPW
washed out after the fifth run of the reaction was just
[5,43,45]
water.
H O was activated by the GO/Fe O /HPW
2 2 3 4
nanocomposite
and
then
reacted
with
the
heteropolyanion to generate active peroxo intermediate,
and this active intermediate was responsible for the

DARVISHI ET AL.
11 of 14
TABLE 2 Comparison of the catalytic activity of GO/Fe
3
O
4
/HPW with some previous catalytic systems in the benzyl alcohol oxidation with
2 2
H O
Entry
Catalyst
11[Pr(PW11
Reaction conditions
Yield (%)
Ref.
1
K
O
39
)
2
].xH
2
O
Cat. (0.24 mol%), alcohol (2 mmol), H
2
O
2
70
4
(
4 mmol), H
Cat. (50 mg, 0.004 mmol), alcohol (0.5 mmol),
(5 mmol), CH CN (3 mL), reflux, 4 h
2
O (0.4 ml), 90 °C, 3 h
5
‐
2
[PZnMo
MWCNT = multi‐wall carbon nanotubes
APIB = 1‐(3‐aminopropyl)‐3‐butylimidazolium
2
W
9
O
39] @APIB‐MWCNT
95
11
H
2
O
2
3
3
4
5
H
3
PW12
O
40‐IL‐SBA‐15
Cat. (100 mg), alcohol (5 mmol), H
(15 mmol), H O (5.0 ml), 100 °C, 6 h
2
O
2
98
98
93
12
13
15
IL = 1‐(3‐silylpropyl)‐3‐methylimidazolium
2
Fe @SiO /NH‐PW10
3
O
4
2
V
2
O
40
Cat. (20 mol%, 0.02 g), alcohol (1.0 mmol),
(2 ml), toluene (2 ml), 80 °C, 8 h
2 2
H O
2
‐
[DPyAM] HPW12
O
40
2 2
Cat. (0.1 g), alcohol (10 mmol), H O (30%,
DPyAM=N,N'‐bis‐2‐aminoethyl‐4,4'‐
6 mmol), 90 °C, 0.5 h
bipyridinium
6
[TMGHA]2.4
H0.6PW12
O
40
Cat. (0.03 mmol), alcohol (10 mmol), H
2
O
2
91
17
TMGHA = N″‐(3‐amino‐2‐hydroxypropyl)‐N,N,
N′,N′‐tetramethylguanidinium
(30%, 15 mmol), H O (6 ml), 90 °C, 6 h
2
a
7
8
9
[n‐C16
H
33N(CH
3
)
3
]
3
PW12
O
40
Cat. (25 μmol), alcohol (15 mmol), H
27.5%, 10 mmol), 90 °C, 6 h
2
O
2
95
47
94
43
45
47
(
[(C18
H
37
)
2
(CH
3
)
2
N]10[SiW
9
O
34
]
Cat. (12 μmol), alcohol (2 mmol), H
2 2
O
(
2 mmol), 65 °C, 6 h
S
4
[H
4
SiW12
O
40
]
Cat. (0.05 mmol), alcohol (30 mmol), H
(45 mmol), 70 °C, 4 h
2
O
O
2
2
S=N‐dodecyl‐N,N‐dimethyl‐3‐ammonio‐1‐
propanesulfonate
10
11
12
13
14
[(N‐butylpyridinium]
4
W
10
O
32
Cat. (0.06 mmol), alcohol (50 mmol), H
2
87
49
51
52
53
54
(
30%, 125 mmol), 80 °C, 8 h.
Cat. (0.05 mmol), alcohol (1 mmol), H
10 mmol), CH
Cat. (0.02 mmol), alcohol (1 mmol), H
mmol), t‐BuOH (3 ml), 80 °C, 12 h
[bmim]
5
[PW11ZnO39].3H
2
O
2
O
2
2
100
(
3
CN (3 ml), reflux, 1.25 h
a
Polypyridinium phosphotungstate
2
O
(30%,
98
6
b
PW‐NH
2
‐IL‐SBA‐15
Cat. (0.1 g), alcohol (10 mmol), 100 °C, H
alcohol (3:1), 6 h
2
O
2
/
92
b
Dendritic phosphotungstate hybrid
PW–PAMAM‐G2)
Cat. (1.5 mol%), alcohol (10 mmol), H
(1.5 ml), H (30 wt.%, 15 mmol), 100 °C,
2
O
89
(
2 2
O
6
h
b
c
a
15
16
17
18
19
20
(PMo11Co)
Co–substituted Keggin phosphomolybdate
Cat. (20 mg), H
2
O
2
/alcohol (3:1), 90 °C, 24 h
56 (91)
55
56
57
58
59
‐
mesoporous Cr
2
O
3
–PWA composite
Cat. (0.008 mmol), alcohol (0.08 mmol), H
2
O
2
83
(
30%, 4 equiv.), CH CN (2 ml), 50 °C, 1 h
3
b
c
Cu1.00Fe1.00SiW11
with graphite calcined at 350 °C
Cat. (10 mg), alcohol (0.43 g), H /alcohol
(5:1), toluene (5 ml), 80 °C, 1 h
2
O
2
98 (98.3)
b
c
c
c
VHPW/MCM‐41/NH
2
Cat. (0.05 g), alcohol (4 mmol), H
2
O
O
2
2
/alcohol
93 (100)
(
5:1), toluene (5 ml), 80 °C, 8 h.
3
‐
GO/Im‐PW12
Im = 1‐(3‐silylpropyl)‐3‐methylimidazolium
O
40
Cat. (0.6 g), alcohol (40 mmol), H
(100 mmol), 90 °C, 7 h
2
90 (99)
GO/Fe /HPW
3
O
4
Cat. (20 mg), alcohol (1 mmol), H
mmol), 70 °C, 3 h
2
O
2
(10%,
99 (100)
5
a
1
‐Phenylethanol;
b
Conversion (%);
c
Selectivity (%).

12 of 14
DARVISHI ET AL.
layers, and at the same time, the amino groups have the
capability to maintain HPA particles by creating suitable
ionic bonds. The presence of HPA on the magnetic
graphene oxide results in an active and stable catalytic
system with a high surface area with the capability of eas-
ily separating and being reused. The catalytic activity was
observed only in the presence of HPA and the magnetic
graphene oxide layers were designed for heterogenization
of HPA and improvement of the retrievability and reus-
ability of the catalyst.
This magnetically modified nanohybrid displayed
excellent performance as a heterogeneous catalyst in the
selective oxidation of alcohols with aqueous H O . Our
2
2
new catalytic system was found to be very effective towards
the oxidation of primary and secondary benzylic alcohols
to the corresponding aldehydes or ketones with almost
FIGURE 11 Recyclability of GO/Fe
the oxidation reaction of benzyl alcohol
3 4
O /HPW nanocomposite in
75‐99% conversions, without any over‐oxidation to acid
1
.6 %. Furthermore, in order to qualitatively test for
(100% selectivity). The oxidation reaction was also per-
formed for two types of primary aliphatic alcohol to the cor-
responding aldehyde. The product efficiency was less than
that of benzylic alcohols (45 and 50%) due to over‐oxidation
to the corresponding carboxylic acid. TGA analysis showed
that the thermal stability of the catalyst was much higher
than graphene oxide. To verify the chemoselectivity, a
series of competition experiments were also undertaken.
leaching and heterogeneity, the oxidation reaction of ben-
zyl alcohol to benzaldehyde was performed in the pres-
ence of GO/Fe O /HPW under optimized reaction
3
4
conditions. After 1.5 h (t/2), the catalyst was removed by
using a magnet. The GC results showed that in the
absence of the catalyst, the reaction was completely
stopped. Therefore, it can be concluded that the active
component of the catalyst was not leached out and catal-
ysis is heterogeneous in nature. In other procedures, the
five times reused catalyst was also studied by FT‐IR spec-
troscopy (Figure 1, j) and EDX analysis (Figure ESI‐19).
The results of the EDX analysis confirmed the presence
of the desired elements and the FT‐IR spectrum showed
the characteristic peaks of the primary catalyst. This con-
firmed that the prepared catalyst was remarkably stable
under reaction conditions and in practice, HPW particles
were only slightly leached out from the support.
ACKNOWLEDGEMENTS
The authors are grateful for the financial support of the
University of Kurdistan and the Iranian Nanotechnology
Initiative Council.
ORCID
Kamal Amani http://orcid.org/0000-0001-8717-7893
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