12-Molybdophosphoric acid anchored on aminopropylsilanized magnetic graphene oxide nanosheets (Fe3O4/GrOSi(CH2)3-NH2/H3PMo12O40): A novel magnetically recoverable solid catalyst for H2O2-mediated oxidation of benzylic alcohols under solvent-free conditions

Article abstract of DOI:10.1039/c8ra00312b

In this work, 12-molybdophosphoric acid (H₃PMo₁₂O₄₀, HPMo) was chemically anchored onto the surface of aminosilanized magnetic graphene oxide (Fe₃O₄/GrOSi(CH₂)₃-NH₂) and was characterized using different physicochemical techniques, such as powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, energy-dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), BET specific surface area analysis and magnetic measurements. The results demonstrated the successful loading of HPMo (～31.5 wt%) on the surface of magnetic aminosilanized graphene oxide. XRD patterns, N₂ adsorption-desorption isotherms and SEM images confirm the mesostructure of the sample. FT-IR and EDX spectra indicate the presence of the PMo₁₂O₄₀^3- polyanions in the nanocomposite. The as-prepared Fe₃O₄/GrOSi(CH₂)₃-NH₂/HPMo nanocomposite has a specific surface area of 76.36 m² g^-1 that is much higher than that of pure HPMo. The selective oxidation of benzyl alcohol to benzaldehyde was initially studied as a benchmark reaction to evaluate the catalytic performance of the Fe₃O₄/GrOSi(CH₂)₃-NH₂/HPMo catalyst. Then, the oxidation of a variety of substituted primary and secondary activated benzylic alcohols was evaluated with H₂O₂ under solvent-free conditions. Under the optimized conditions, all alcohols were converted into the corresponding aldehydes and ketones with very high selectivity (≥99%) in moderate to excellent yields (60-96%). The high catalytic performance of the nanocomposite was ascribed to its higher specific surface area and more efficient electron transfer, probably due to the presence of GrO nanosheets. The nanocomposite catalyst is readily recovered from the reaction mixture by a usual magnet and reused at least four times without any observable change in structure and catalytic activity.

Full text of DOI:10.1039/c8ra00312b

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12-Molybdophosphoric acid anchored on
aminopropylsilanized magnetic graphene oxide
Cite this: RSC Adv., 2018, 8, 6768
nanosheets (Fe O /GrOSi(CH ) –NH /
3
4
2 3
2
H PMo O ): a novel magnetically recoverable
3
12 40
solid catalyst for H O -mediated oxidation of
2
2
benzylic alcohols under solvent-free conditions
a
b
b
Saeed Farhadi, * Mohammad Hakimi and Mansoureh Maleki
3
In this work, 12-molybdophosphoric acid (H PMo12O40, HPMo) was chemically anchored onto the surface of
aminosilanized magnetic graphene oxide (Fe
3
O
4
/GrOSi(CH
2
)
3
–NH ) and was characterized using different
2
physicochemical techniques, such as powder X-ray diffraction (XRD), Fourier transform infrared
spectroscopy (FT-IR), Raman spectroscopy, energy-dispersive X-ray analysis (EDX), scanning electron
microscopy (SEM), BET specific surface area analysis and magnetic measurements. The results demonstrated
the successful loading of HPMo (ꢀ31.5 wt%) on the surface of magnetic aminosilanized graphene oxide.
XRD patterns, N
sample. FT-IR and EDX spectra indicate the presence of the PMo12
The as-prepared Fe /GrOSi(CH –NH /HPMo nanocomposite has a specific surface area of 76.36 m g
that is much higher than that of pure HPMo. The selective oxidation of benzyl alcohol to benzaldehyde was
initially studied as a benchmark reaction to evaluate the catalytic performance of the Fe /GrOSi(CH
NH /HPMo catalyst. Then, the oxidation of a variety of substituted primary and secondary activated benzylic
alcohols was evaluated with H under solvent-free conditions. Under the optimized conditions, all
2
adsorption–desorption isotherms and SEM images confirm the mesostructure of the
3ꢁ
O
40 polyanions in the nanocomposite.
2
ꢁ1
3 4
O
)
2 3
2
O
3 4
2 3
) –
2
2 2
O
alcohols were converted into the corresponding aldehydes and ketones with very high selectivity ($99%) in
moderate to excellent yields (60–96%). The high catalytic performance of the nanocomposite was ascribed
to its higher specific surface area and more efficient electron transfer, probably due to the presence of GrO
nanosheets. The nanocomposite catalyst is readily recovered from the reaction mixture by a usual magnet
and reused at least four times without any observable change in structure and catalytic activity.
Received 11th January 2018
Accepted 6th February 2018
DOI: 10.1039/c8ra00312b
rsc.li/rsc-advances
green process for selective oxidation of benzyl alcohol and
1
. Introduction
oxidants like molecular oxygen, hydrogen peroxide (H
2
O
2
) and
The oxidation of alcohols to the corresponding carbonyl TBHP are in the order as green options. Molecular oxygen as
compounds, i.e., benzyl alcohol to benzaldehyde, is one of the oxidant is not very active, while H is an active oxygen donor
4
O
2 2
most important and vital transformations in synthetic organic in catalytic oxygen-transfer reactions. However, most of the
1
chemistry. Typically, the chemoselective oxidation of alcohols catalyst systems using H
2
O
2
are based on noble metals such as
5
–7
to aldehydes or ketones without traceable over-oxidation prod- Pt, Pd, and Ru which are expensive and difficult to synthesize.
ucts (i.e., carboxylic acids) is attractive for the preparation of From the practical and environmental points of view, there is
ne chemicals owing to the nature of aldehydes or ketones as an a strong demand for the screening out efficient catalytic systems
intermediate for producing carbonyl products, which are of using inexpensive and environmentally benign metal catalysts

great importance and versatility for the chemical industry and and non-toxic H
O or O as the sole terminal oxidant. In this
2 2 2
2
pharmaceutical synthesis. Numerous efforts on developing context, polyoxometalates (POMs) are denitely an attractive
efficient catalysts towards these oxidations have been pro- alternative in terms of economic viability and easy-to manu-
3
gressed and reported. Solvent-free approach is an attractive facture alternative with a heterogeneous nature.
Polyoxometalates (POMs) are a large family of bulky clusters
8
of transition metal oxide anions with structural diversity.
a
Chemistry Department, Lorestan University, Khorramabad 68151-44316, Iran. Among various POMs, Keggin-type heteropoly acids (e.g.
E-mail: sfarhadi1348@yahoo.com; Fax: +98-6633120618; Tel: +98-6633120611
b
3 3
H PMo12O40 and H PW12O40) have received much attention and
Chemistry Department, Payame Noor University, Tehran 19395-4697, Iran
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numerous organic transformations can be catalyzed by them, GrOSi(CH
2
)
3
–NH
2
) was synthesized by a facile method and used
not only due to their controllable and reversible multielectron as a novel magnetic GrO-based support. Due to relatively high
redox and acidic properties, but also due to their environmen- surface area and porosity, insolubility in water and easy
9
,10
tally benign behavior.
moderately high thermal stability in solid state, relatively appropriate solid support to anchor Keggin-type PMo12
simple synthesis procedure, and ability to form pseudo-liquid polyanion. The ternary magnetic nanocomposite material
Moreover, these compounds have magnetically separation, the Fe O /GrOSi(CH ) –NH is an
3 4 2 3 2
3
ꢁ
O
40
11
phases. In despite of the above advantages, there exist two (abbreviated as Fe
major drawbacks in the catalytic systems involving pristine by simple acid–base electrostatic interaction between
PMo12 40 and amino groups of the Fe /GrOSi(CH –NH
hindering accessibility of reactants to active sites and (ii) high support. The coupling of PMo O anion with Fe O /
3 4 2 3 2
O /GrOSi(CH ) –NH /HPMo) was prepared
a
2
ꢁ1
POMs: (i) low surface area in nonpolar solvents (<10 m g
)
H
3
O
3
ꢁ
O
4
2
)
3
2
3
1
2
40
3 4
solubility in polar solvents producing recovery and reuse GrOSi(CH ) –NH could improve the surface area and avoid the
2
3
2
problems. To overcome these obstacles, many researchers have dissolution of HPMo. This novel magnetically recyclable
tried to design heterogeneous catalysts by incorporating HPAs heterogeneous catalyst was used for selective oxidation of
12,13
14
into the structure of solid supports such as SiO
2
,
Al
zeolites, organic materials,
(ref. 21) and metal–organic frameworks
However, most of these supports have some limi- tions. The resulting Fe
2
O
3
,
alcohols with H
conditions. Our catalysts showed high catalytic performance in
-mediated alcohol oxidations under solvent-free condi-
/GrOSi(CH –NH /HPMo composite
2 2
O as a green oxidant under solvent free
15
16
17
18,19
2
activated carbon, TiO ,
2
0
clays, ZrO
(
2
–CeO
2
2 2
H O
2
2,23
MOFs).
3
O
4
2
)
3
2
tations, such as low loading of POM, high leaching of POM could be used as a magnetically separable and efficient catalyst
especially in polar medium and/or active sites that are unevenly for alcohol oxidation under solvent free conditions.
dispersed. Therefore, nding suitable solid supports to over-
come the drawbacks is important. Supports modied with
2
. Experimental
functional groups such as carboxylic groups, lactam, amide,
24,25
imide or amino-groups can allow solving this problem.
2.1 Materials
Especially, the amino-modied metal oxides supports are
commonly used for immobilizing POMs not only because of the
exceptionally high stability and surface area but also because
the amount and the basicity of anchored functional amino
groups can be expected to be important in determining the
1
2-Molybdophosphoric acid (H PMo O , 98%), graphite
3
12 40
powder (C, 99.95%) and 3-aminopropyltriethoxysilane (APTES,
9%) were purchased from Merck Chemical Co. All alcohols and
9
other chemicals were commercially purchased and used
without further purication.
26–30
guest–host interactions of the materials.
This approach
allows obtaining high dispersion of the POMs with minimal
leaching. Despite facile recovery, such heterogeneous POM
2.2 Preparation of aminosilanized magnetic graphene oxide
catalysts oen suffer from the poor accessibility of the H
2 2
O
Graphene oxide (GrO) was prepared by the modied Hummers
51,52
during the oxidation of alcohols with aqueous H due to the method through the oxidation of graphite powder.
2
O
2
Briey,
hydrophobicity of support. Hence, enhancing the accessibility graphite powder (2.0 g) and NaNO3 (1.0 g) were mixed with
of the oxidant is crucial to the heterogeneous catalyst in the 40 mL of concentrated H SO in a 500 mL ask and stirrer for 1
2
4
alcohols oxidation with H O .
hour in an ice bath. Then KMnO (6.0 g) was added into the
2
2
4
ꢂ
Among various carbon-based nanomaterials, graphene oxide vigorously stirred suspension slowly below 15 C. The ice bath
(GrO) has been proven as an effective support for the immobi- was then removed, and the mixture was stirred at room
lization of inorganic and organic materials owing to its large temperature until it slowly became a brownish slurry, and then
2
ꢁ1
theoretical specic surface area (ꢀ2630 m g ) and the pres- it was diluted with 100 mL of water. The reaction temperature
ꢂ
ence of numerous oxygen containing functional groups on its was rapidly increased to 98 C with effervescence, and the color
31–39
surface.
Due to these rich functional groups, GrO can be changed to brown. Aer that, 200 mL of water and 20 mL of
easily reacted with organic molecules to generate stable chem- H O (30 wt%) were added. For purication, the mixture was
2
2
40
ically functionalized GrO. In addition, in comparison with centrifuged and washed with 10% HCl and then deionized
other carbonaceous nanomaterials, GrO may be more environ- water several times to remove the residual metal ions and acid.
41
mental friendly and have better biocompatibility. However, it Aer centrifuging and drying at room temperature, GrO was
is difficult to separate GrO from aqueous solutions using obtained as a powder. To prepare Fe O /GrO, 0.25 g of GrO was
3
4
traditional ltration and centrifugation methods during and dispersed in 90 mL water by sonication for 1 hour. Then, 0.84 g
aer the process due to its hydrophilic nature and small particle of (NH Fe(SO and 2.08 g of (NH )Fe(SO were added to the
size which increase the cost of industrial application. The GrO dispersion and its pH was adjusted at 12 by adding
4
)
2
4
)
2
4
4 2
)
42
ꢁ
1
ꢂ
magnetic separation method is considered as a rapid and 1 mol L NaOH. The mixture was stirred at 50 C for 2 h,
effective technique for separating nanomaterials from aqueous ltered and washed with water and ethanol three times. The
43–45
solution.
Hence, magnetic graphene-based composites with resulting solid was Fe O/GrO. To prepare amino functionalized
3
large specic surface area and magnetic separation have begun magnetic graphene oxide, 0.40 g of the as-synthesized Fe O /
3
4
46–50
to be used in the eld of organic transformations.
GrO dispersed in 50 mL of water, 150 mL ethanol and 5 mL of 3-
On the basis of the above discussions, in this work, amino- aminopropyltriethoxysilane (APTES) were added to a round
organosilane functionalized magnetic graphene oxide (Fe
3
O
4
/
bottom ask. The mixture was stirred for 30 min in room
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ꢂ
temperature and then was reuxed at about 80 C for 24 h. Aer were recorded on a Rigaku D-max C III X-ray diffractometer
ꢀ
the reaction, the solid was separated by a magnet and washed using Ni-ltered Cu Ka radiation (l ¼ 1.54184 A). The
with ethanol to remove the unreacted APTES. The nal product morphology of samples was studied by MIRA3 TESCAN scan-
ꢂ
was dried at 80 C in vacuum for 12 h to obtain the Fe O / ning electron microscope equipped with energy dispersive X-ray
3
4
GrOSi(CH
2
)
3
–NH
2
.
analyzer (EDX) for the elemental analysis. Optical adsorption
spectra were obtained from a Cary 100 Varian UV-Vis spectro-
photometer in a wavelength range of 200–800 nm. The Bru-
2.3 Preparation of the Fe
3 4 2 3 2
O /GrOSi(CH ) –NH /HPMo
nauer–Emmett–Teller (BET) surface area was measured by N
2
nanocomposite catalyst
adsorption measurements at 77 K using a Nova 2000 instru-
ment. The concentration of Mo in the composite was deter-
mined by inductively coupled plasma atomic emission
spectroscopy (ICP-AES, model OEC-730). The content of HPMo
in the nanocomposite was determined by inductively coupled
plasma atomic emission spectroscopy (ICP-AES, model OEC-
1
8
g the as-prepared Fe
0 mL water and sonicated for 1 h. To the above suspension, 1 g
3 4 2 3 2
O /GrOSi(CH ) –NH was dispersed in
HPMo in 200 mL ethanol was added and sonicated for another
1
2
ethanol three times to remove the unreacted HPMo. The nal
product was dried at 60 C in open air to obtain magnetic Fe O /
h. The resulting mixture was stirred at room temperature for
4 h, and then ltered, washed with deionized water and
730). Magnetic measurements were carried out at room
ꢂ
3
4
temperature using a vibrating sample magnetometer (VSM,
Magnetic Daneshpajoh Kashan Co., Iran) with a maximum
magnetic eld of 10 kOe. Raman spectra were obtained using
a Raman microscope (SENTERRA-2009, Germany) with Laser
wavenumber of 785 nm at 785 nm. Thin-layer chromatography
GrOSi(CH ) –NH /HPMo hybrid nanomaterial. According to the
2
3
2
elemental analysis (ICP-AES) results and molecular weight of
PMo12 40, the loading amount (wt%) of HPMo in Fe
GrOSi(CH –NH /HPMo was estimated to be 31.5%.
H
3
O
3 4
O /
2
)
3
2
(
TLC) was conducted on glass plates coated with silica gel
2
.4 General procedure for oxidation of benzylic alcohols
with H over the Fe /GrOSi(CH –NH /HPMo
nanocomposite
Benzylic alcohol (10 mmol) and catalyst Fe
NH /HPMo (0.2 g) were added to a 25 mL ask. Under reuxing
GF254. GC-MS analysis was carried out on a Shimadzu QP 5050
GC-MS instrument with 5973 MS detector at an ionization
voltage of 70 eV and equipped with a HP-INNOWAX capillary
column (internal diameter ¼ 0.25 mm, length ¼ 30 m).
2
O
2
3
O
4
2
)
3
2
3 4 2 3
O /GrOSi(CH ) –
2
conditions, vigorous stirring, and the heating temperature of
3
. Results and discussion
ꢂ
1
00 C, the aqueous H O (30 wt%, 15 mmol) was added into the
2
2
3.1 Characterization of the hybrid nanocatalyst
above mixture within 5 min, then the reaction mixture was
stirred for 4 h. The progress of the reaction was monitored by In this work, Keggin-type H PMo O (HPMo) was chemically
3
12 40
TLC and/or GC. Aer reaction, the solid catalyst was removed by immobilized onto 3-aminopropyl functionalized graphene
an external magnet, and the liquid was analyzed using a gas oxide nanosheets decorated with magnetic Fe nanoparticles.
chromatography (GC SP-6890) equipped with an FID detector The preparation process of Fe /GrOSi(CH –NH /HPMo
3 4
O
3
O
4
2
)
3
2
and a capillary column (SE-54; internal diameter ¼ 0.32 mm, hybrid is illustrated in Fig. 1. These suggest that attaching
length ¼ 30 m) using He as the carrier gas. In the GC experi- organic bases on GrO might afford an efficient, reusable and
ments, n-decane was used as internal standard and the yields environmentally benign base support for heteropoly acid such
were determined by using peak area. The isolated yield was as HPMo. This novel magnetically recyclable hybrid nano-
obtained using silica gel plate or column chromatography with material was constructed by protonating amino groups
a mixture of ethyl acetate/n-hexane as an eluent, and the prod- anchored on the magnetic GrO nanosheets with Keggin-type
ucts was characterized by GC-MS. The recovered catalyst for heteropoly acid HPMo. The surface of GrOSi(CH
2 3 2
) –NH is
recycling tests was obtained by magnetic separation, washing positively charged by the protonation of –NH groups by react-
2
with ethanol and dried.
.5 Recyclability test of the catalyst
ing with HPMo, which is benet for anchoring polyanion via an
electrostatic interaction. To conrm the successful construction
of Fe
3
O
4
/GrOSi(CH
2
)
3
–NH
2
/HPMo hybrid, element analysis was
/GrOSi(CH –NH /HPMo
2
employed. It reveals that the Fe
3
O
4
2
)
3
2
The activity of the recovered catalyst provides useful informa-
tion about its stability during the catalytic cycle. In order to
recover, the magnetic catalyst was separated from the reaction
mixture by a magnet and washed three times with distilled
water and ethanol. It was further dried at 100 C for 2 h. The
recovered catalyst was then used in the reaction with a fresh
contains 36.5% Mo, indicating that HPMo was anchored on
GrO nanosheets. Along with the anchoring of HPMo on Fe O /
3
4
GrOSi(CH
2
)
3
–NH
2 2
, the –NH groups can be protonated to form
+
the –NH
electrostatic (ionic) attraction interaction. And due to the
3
groups, which absorbs the HPMo polyanions via the
ꢂ
53
+
electrostatic attraction, the hydrogen bonds between –NH
HPMo polyanions molecules are reinforced.
3
and
reaction mixture and products were analyzed aer the reaction.
54,55
The structure
and composition of the hybrid nanomaterial was further char-
acterized by XRD, FT-IR, Raman spectra, EDX, SEM, VSM and
2.6 Characterization techniques
The infrared spectra were recorded at room temperature on BET surface area analyses.
a Schimadzu FT-IR 160 spectrophotometer in the 4000– The FT-IR spectra of synthesized materials are shown in
00 cm region using KBr pellets. The XRD patterns of powder Fig. 2. It is well known that Keggin-type polyoxometalate
ꢁ1
4
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3 4 2 3 2
Fig. 1 Preparation process of the magnetic Fe O /GrOSi(CH ) –NH /HPMo catalyst.
3
ꢁ
(
PMo O
) contains a cluster of Mo(VI) ions linked by oxygen Mo–O –Mo (O corresponds to oxygen atom bridging the two
b b
1
2
40
3
ꢁ
atoms with a tetrahedral phosphate group. PMo O
2 40
characteristic vibration bands as shown in Fig. 2(a): 1066 cm
for asymmetric stretch vibration of P–O (O corresponds to structure).
oxygen atom of tetrahedral phosphate group), 966 cm for shown in Fig. 2(b). The characteristic absorption bands at 1053,
asymmetric stretch vibration of Mo]O
terminal oxygen atoms), 870 cm for bending vibration of
ꢁ1
has four tungsten atoms), and 786 cm for bending vibration of Mo–Oc–
1
ꢁ1
Mo (O represents oxygen atom at the corners of the Keggin
c
56,57
a
a
The FT-IR spectrum of the composite sample is
ꢁ1
ꢁ
1
t
(O
t
corresponds to the 945, 875 and 798 cm corresponding to the P–O
–Mo and Mo–O –Mo band vibrations conrm the presence of
cluster. The graphitic C]C stretching band at
a t
, Mo]O , Mo–
ꢁ
1
O
b
c
3
ꢁ
PMo12O
40
ꢁ
1
ꢁ1
about 1505 cm and a strong band at about 588 cm assigned
to the grapheme nanosheets and Fe–O stretching vibration of
58–60
the spinel-type Fe O structure, respectively.
And the pres-
3
4
ence of the anchored APTES chain to the GrO sheets was
conrmed by stretching vibrations of C–H bond (2926 and
ꢁ
1
2
(
(
850 cm ), the stretching and bending modes of –NH
2
bonds
ꢁ1
3430 and 1630 cm ) and the C–N stretching vibration
ꢁ
1
61
ꢁ1
1223 cm ). The Si–O–C stretching vibration (1116 cm
)
shows the successful graing of APTES onto magnetic GrO
nanosheet through covalent bonds. All bands appeared the IR
spectrum of the hybrid nanomaterial demonstrate the coexis-
3
ꢁ
tence of PMo12
nanomaterial. The shi of some peaks of HPMo in the Fe
GrOSi(CH –NH /HPMo compared to the parent HPMo can be
attributed to hydrogen bonding and strong electrostatic
3 4 2 3 2
O40 , Fe O and GrOSi(CH ) –NH in the hybrid
3 4
O /
2
)
3
2
3
ꢁ
attraction between negatively charged PMo12
O
40
and posi-
+
62,63
tively charged Fe O /GrOSi(CH ) –NH surface.
3
4
2 3
3
Fig. 3 displays the XRD patterns for Fe O and Fe O /
3
4
3 4
GrOSi(CH ) –NH /HPMo samples. All of the diffraction patterns
2
3
2
3 4
in Fig. 3(a) and (b) are similar and can be indexed to the Fe O
phase (JCPDS no. 41-1488). No characteristic diffraction peaks of
the HPMo appeared which presumably was due to the low
content incorporation of HPMo. Also, this result implies that the
Keggin unit homogeneously disperses into the GrO nanosheets,
which will be benet to enhance the catalytic activity of the
hybrid nanomaterial. Moreover, no characteristic diffraction
peaks for GrO are observed in the pattern indicating that the GrO
nanosheets do not stack during the synthesis process. The reason
Fig. 2 FT-IR spectra of (a) pure HPMo and (d) Fe
NH /HPMo.
3 4 2 3
O /GrOSi(CH ) –
2
3 4
can be attributed to that the Fe O nanoparticles, aminopropyl
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3 4 2 3 2
Fig. 4 Raman spectra of (a) GO and (b) Fe O /GrOSi(CH ) –NH /
HPMo.
3 4 3 4 2 3
Fig. 3 XRD patterns of (a) pure Fe O and (b) Fe O /GrOSi(CH ) –
NH /HPMo.
2
Fig. 5. SEM image of pure GO in Fig. 5(a) shows highly porous
groups and HPMo anchored on the surfaces of GrO prevent the and layered structure of GO having large stacks, possibly con-
64
exfoliated GrO nanosheets from restacking. However, a broad sisting of hundreds of grapheme oxide nanosheets. It should also
ꢂ
peak at about 23.5 corresponding to the reduced GrO was be noted that the surfaces of the GO sheets are quite at and
appeared, indicating that GrO nanosheets were reduced to gra- smooth. The SEM image of Fe
phene during the functionalization process.
3
O
4
/GrOSi(CH
Fig. 5(b) and (c) clearly shows graphene oxide nanosheets were
Fig. 4(a) and (b) presents the Raman spectra of pure GrO and successfully decorated with agglomerated Fe and HPMo
Fe O /GrOSi(CH ) –NH /HPMo nanocomposite. Two samples particles that completely covered the surfaces of large graphene
2 3 2
) –NH /HPMo in
65
3 4
O
3
4
2 3
2
3
display the characteristic D (sp carbon atoms of disorders and sheets. From images, it can be clearly seen that the Fe O4
defects) and G (sp carbon atoms in graphitic sheets) bands of nanoparticles with a size of about 15–20 nm were well deposited
GrO structure. As compared with the D and G bands of pure on GO nanosheets which were a exible interleaved structure.
GrO (D, 1318 cm ; G, 1590 cm ), the two characteristic bands Some wrinkles are found on the surface, which may be important
3
2
66
ꢁ
1
ꢁ1
of GrO in the Fe
shi to D, 1311 cm
3
O
4
/GrOSi(CH
2
)
3
–NH
2
/HPMo nanocomposite for preventing aggregation of GO nanosheets and maintaining
ꢁ
1
ꢁ1
and G, 1592 cm , suggesting the high surface area. All the micrographs of the nanocomposite
successful functionalization of GrO with APTES and HPMo. clearly indicated that the surface properties of modied GO
These shis in Raman peaks can be attributed to strong inter- product were strongly affected. In opposite of pure GO, the
action between HPMo polyanion and positively charged Fe O / surfaces of GO nanosheets in the nanocomposite are rough, and
3
4
+
GrOSi(CH ) –NH
in the Fe O /GrOSi(CH ) –NH /HPMo the edges are highly crumpled. Thus, the Fe O /GrOSi(CH ) –
3 4 2 3 2 3 4 2 3
2
3
3
67
hybrid, which is consistent with the FI-IR analysis. Comparing NH
with pristine GrO, the ratio of D and G peaks of Fe porosity for catalytic uses. The results are in good agreement with
GrOSi(CH –NH /HPMo becomes higher, suggesting a higher BET results. The composition of the as-prepared Fe /GrO–NH
level of disorder of the graphene layers during the functionali- HPMo hybrid nanomaterial was investigated by energy dispersive
2
/HPMo could provide a rough and coarse surface with high
3 4
O /
2
)
3
2
O
3 4
2
/
68
zation process. The peaks of Keggin structure have not been X-ray spectroscopy (EDX). Fig. 5(d) shows EDX spectrum of the
appeared, which indicated that HPMo species on Fe O / hybrid nanocomposite. The EDX elemental spectrum shows the
3
4
GrOSi(CH ) –NH sample were in a high-dispersed state. This existence of C, N, Si, O, Fe, P, and Mo elements in the composite.
2
3
2
result was identical to the XRD results.
SEM images indicating the microstructural features of GO and conrm that the HPMo particles have been successfully sup-
/GrOSi(CH –NH /HPMo nanocomposite are shown in ported on the surface of the Fe /GrOSi(CH –NH
The elements of P and Mo are from HPMo and the results further
Fe O
3 4
)
2 3
2
O
3 4
)
2 3
2
.
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Fig. 5 SEM images of (a) GO, (b) and (c) Fe
O
3 4
/GrOSi(CH
2
)
3 2
–NH /HPMo nanocomposite at different magnification and (d) EDX spectrum of the
3 4 2
Fe O /GrO–NH /HPMo nanocomposite.
The magnetic properties of the Fe O and Fe O /
3
4
3 4
GrOSi(CH ) –NH /HPMo samples were investigated by
2
3
2
vibrating sample magnetometry (VSM) at room temperature.
The magnetic hysteresis loops are depicted in Fig. 6. It is clear
that both Fe
3
O
4
3 4 2 3 2
and the Fe O /GrOSi(CH ) –NH /HPMo
composite are so-magnetic materials due to their coercivity
69
c
(H ) of zero. Moreover, they are also superparamagnetic
materials, as their magnetic hysteresis loops passed through
the origin of the coordinates. The saturation magnetization
values of Fe O and Fe O /GrOSi(CH ) –NH /HPMo are 27.50
3
4
ꢁ
3
4
2 3
2
1
and 9.15 emu g , respectively. The saturation magnetization of
the magnetic nanocomposite decreases by approximately 75%
3 4
compared with that of pure Fe O , which can be attributed to
the nano-magnetic components (GrO and HPMo) in the
composite sample. However, the saturation magnetization of
the composite could satisfy the requirements of easy separation
in the suspension solution using an extra magnet aer reaction
Fig.
GrOSi(CH
/HPMo composite can be easily separated using a magnetic behavior of the nanocomposite under an external magnetic field.
2
6
Magnetic hysteresis loop of (a) Fe
3 4 3 4
O and (b) Fe O /
as shown in the inset of Fig. 6. Thus, the Fe O /GrOSi(CH ) –
3
4
2 3
2
)
3
–NH /HPMo at room temperature. The inset shows the
2
NH
separation process aer being used for the reaction.
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H
2
O
2
(30%, 15 mmol) and a catalytic amount of Fe
3
O
4
/
ꢂ
GrOSi(CH ) –NH /HPMo (0.2 g) was heated for 4 h at 100 C, the
2
3
2
benzaldehyde was formed as the only product in 90% yield
Scheme 1). In control experiments, when pure Fe O and GrO
(
3
4
support were used as heterogeneous catalysts for the oxidation
of benzyl alcohol, the results showed that about 15% and 18%
of benzaldehyde was formed, respectively, aer 4 h heating at
ꢂ
1
00 C under the same reaction conditions, suggesting that the
3 4 2 3 2
catalytic activity of the Fe O /GrOSi(CH ) –NH /HPMo system is
mainly due to HPMo heteropoly acid. Using pure and unsup-
ported HPMo, we found that 45% of benzyl alcohol was oxidized
to benzaldehyde within 4 h under the same conditions. These

3 4 2 3 2
ndings conrm that the activity of Fe O /GrOSi(CH ) –NH /
HPMo nanocomposite is higher than that of the pure HPMo
cluster. This is due to (i) higher specic surface area of
composite than that of the starting POM cluster and (ii) the
synergistic effect of the HPMo unit and the GO nanosheets
support. No noticeable oxidation products were observed when
blank experiments were run in the in the absence of Fe O /
3
4
Fig. 7
2 3 4 2 3
N adsorption–desorption isotherm of Fe O /GrOSi(CH ) –
GrOSi(CH ) –NH /HPMo or H O .
2
3
2
2 2
NH /HPMo. The inset shows the pore size distribution plot.
2
3.2.1 Effect of catalyst dosage. The effect of Fe
3
O
4
/
GrOSi(CH –NH
2
)
3
2
/HPMo catalyst dosage on the progress of
benzyl alcohol oxidation is illustrated in Fig. 8. With the
increase of the catalyst amount from 0.05 g to 0.3 g, the benzyl
alcohol conversion increased from 35% to 98%, but the selec-
tivity towards benzaldehyde reduced slightly from 99% to 75%.
This may be due to the increase in active sites resulting from
higher amount of catalyst which facilitate the further oxidation
of benzaldehyde to benzoic acid.
3.2.2 Effect of reaction time. Fig. 9 shows the inuence of
reaction time on the conversion of benzyl alcohol and product
3 4 2 3 2
selectivity over the Fe O /GrOSi(CH ) –NH /HPMo catalyst (0.2
N adsorption/desorption measurements were performed to
2
investigate specic surface area and the pore size distribution of
the Fe /GrOSi(CH –NH /HPMo. As can be seen in Fig. 7, the
nitrogen adsorption isotherm is a typical IV type curve with an
3
O
4
2
)
3
2

0
ne H1-type hysteresis loop in the range of ca. 0.8–1.0p/p ,
7
0,71
indicating the existence of mesoporous structure.
The BET
2
ꢁ1
surface area is measured to be 76.36 m g that is much higher
than the value of pure HPMo (#10 m g ). In addition, the
total pore volume is 0.01 cm g and according to the corre-
sponding Barrett–Joyner–Halenda (BJH) pore size distribution
curve in the inset of Fig. 7, the pore size distribution of the
2
ꢁ1 72
3
ꢁ1
g). As can be seen in Fig. 9, with increase of the reaction time
from 1 h to 4 h, the conversion marginally increased from 45%
to 90%, and the selectivity towards benzaldehyde remains
Fe
2
3
O
4
/GrOSi(CH
2
)
3
–NH
2
/HPMo shows peak centered at around
/GrOSi(CH –NH /HPMo
.28 nm. Such porosity of Fe
3
O
4
2
)
3
2
100%. However, when the reaction time was increased from 4 h
composite can improve the catalytic performance. It can be
concluded that immobilization of HPMo on the surface of
magnetic Fe O /GrOSi(CH ) –NH increases its surface area
to 5 h and then 6 h, the conversion of benzyl alcohol increased
3
4
2 3
2
and porosity which they are useful factors for improving the
catalytic performance.
3
.2 Catalytic oxidation of alcohols over Fe O /GrOSi(CH ) –
3 4 2 3
NH /HPMo nanocomposite
2
The oxidation of benzyl alcohol to obtain benzaldehyde was
used as a typical benchmark reaction to verify the catalytic
activity of the as-prepared magnetic Fe O /GrOSi(CH ) –NH /
3 4 2 3 2
HPMo nanocomposite (see Scheme 1). In an initial experiment,
a heterogeneous mixture of benzyl alcohol (10 mmol), aqueous
Fig. 8 The conversion of benzyl alcohol and the selectivity to benz-
aldehyde at different catalyst dosage. Conditions: benzyl alcohol (10
mmol), H
100 C for 4 h.
2
O
2
(15 mmol, 30%), catalyst (0.20 g) at reflux temperature
ꢂ
Scheme 1
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40%, resulted by the excess unreacted H
2
O
2
in reaction mixture,
as compared to that obtained with 1.5 mole ratio of H O to
2
2
benzyl alcohol. Based on the results, the mole ratio of 1.5 was
chosen as the optimal ratio of H O /benzyl alcohol.
2
2
3
.2.4 Effect of temperature. The oxidation reaction was
ꢂ
investigated at ve temperatures, 55, 70, 85, 100 and 115 C,
while other parameters are kept xed (Fig. 11). The conversion%
increased with increasing temperature from 55 to 115 C, but the
ꢂ
ꢂ
selectivity towards benzaldehyde drastically decreased at 115 C.
This might be due to the self-decomposition of H O at higher
2
2
temperatures and/or further oxidation of benzaldehyde to ben-
zoic acid at elevated temperatures.
Thus, optimized conditions for oxidation of benzyl alcohol
(
90% conversion, 100% selectivity of benzaldehyde) as a model
Fig. 9 The conversion of benzyl alcohol and the selectivity to benz-
aldehyde at different reaction time over Fe O /GrOSi(CH ) –NH /
3 4 2 3 2
reaction are as follows: amount of the catalyst (0.2 g), temper-
ature (100 C), H
time (4 h).
ꢂ
2 2
O oxidant to alcohol ratio (1.5) and reaction
HPMo as the catalyst. Conditions: benzyl alcohol (10 mmol), H
2
O
2
ꢂ
(
15 mmol, 30%), catalyst (0.20 g) at 100 C.
3.2.5 Oxidation of various alcohols. Under the optimized
reaction conditions for oxidation of benzyl alcohol, we explored
the substrate scope of this catalytic oxidation system. The
oxidation of a series of primary and secondary benzylic alcohols
too, but the selectivity towards benzaldehyde declined due to
further oxidation of benzaldehyde to benzoic acid. Thus,
optimum reaction time was found to be 4 h, when catalyst gave
the highest conversion and selectivity, i.e. 90% conversion of
benzyl alcohol and 100% selectivity towards benzaldehyde.
2 2 3 4 2 3 2
with H O in the presence of Fe O /GrOSi(CH ) –NH /HPMo
catalyst was studied. We found that these substrates are effi-
ciently oxidized to the corresponding carbonyl compounds in
high yield without observable over-oxidation of the benzalde-
hydes to the carboxylic acids. The results were presented in
Table 1. As can be seen in Table 1, H O -mediated oxidation
3
.2.3 Effect of oxidant H O amount. The amount of
2 2
hydrogen peroxide is another important parameter inuencing
the results of this oxidation reaction. According to stoichiom-
2
2
a variety of ring-substituted benzyl alcohols having various
electron-withdrawing and -donating substituents e.g. iso-Pr–,
etry, one mole of H
alcohol to one mole of benzaldehyde. Effect of mole ratio of
/benzyl alcohol was investigated in range of 1.0–2.0 and
2 2
O is required to convert one mole of benzyl
–
OMe, –NO
2
, and halogens (–Br and –Cl) was investigated over
–NH /HPMo catalyst. The results showed
2 2
H O
the Fe /GrOSi(CH
3
O
4
2
)
3
2
the results are given in Fig. 10, from which a lower conversion of
benzyl alcohol was observed with 1.0 mole ratio of H O /benzyl
that all primary benzyl alcohols were selectively converted to
their corresponding aldehydes in moderate to high yields
without over-oxidation to the carboxylic acids (Table 1, entries
2
2
alcohol. Although a higher conversion of benzyl alcohol was
obtained when the mole ratio of H /benzyl alcohol is 2.0, the
percentage selectivity to benzaldehyde was reduced around
2 2
O
1–10). The yields were found to be affected by the substituent
Fig. 10 The conversion of benzyl alcohol and the selectivity to Fig. 11 Effect of temperature on the conversion of benzyl alcohol
benzaldehyde over the Fe
3
O
4
/GrOSi(CH
2
)
3
–NH
2
/HPMo catalyst with using the Fe
3
O
4
/GrOSi(CH
2
)
3
–NH
2
/HPMo catalyst. Conditions: benzyl
ꢂ
different amount of H (30%). Conditions: benzyl alcohol (10 mmol), alcohol (10 mmol), H
2
O
2
ꢂ
O
2 2
(15 mmol, 30%), catalyst (0.20 g) at 100 C for
catalyst (0.20 g) at 100 C for 4 h.
4 h.
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a
2 2 3 4 2 3 2
Table 1 Results of various alcohol oxidation with H O catalyzed by the Fe O /GrOSi(CH ) –NH /HPMo catalyst
b
Entry
1
Substrate
Product
Yield (%)
90
96
94
86
2
3
4
5
6
60
82
7
8
92
94
9
82
90
1
0
1
1
1
2
90
90
1
3
90
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Table 1 (Contd.)
b
Entry
Substrate
Product
Yield (%)
1
4
5
88
65
1
1
1
6
7
65
60
1
8
68
a
ꢂ
b
Reaction conditions: alcohol (10 mmol), catalyst (0.2 g), H
2
O
2
(15 mmol, 30 wt%), 100 C under solvent-free conditions for 4 h. Yields were
determined by GC analysis with n-decane as internal standard.
groups on the phenyl ring of benzylic alcohols somewhat. corresponding unsaturated aldehyde in high yield without
Generally, alcohols having stronger electron withdrawing oxidation of carbon–carbon double bond (Table 1; entry 14). In
groups gave products with lower yields compared with those contrast to benzylic and allylic alcohols which were oxidized in
having electron donating groups on the phenyl rings and benzyl a highly efficient way, non-benzylic alcohols such as and
alcohol (Table 1, entries 1–6). These results indicated that 3-phenyl-1-propanol, 1-heptanol, and 2-octanol were oxidized
electron density on the aromatic ring played an important role into the corresponding carbonyl compounds with much lower
in the reactivity of the oxidation reaction. On the other hand, efficiency under the same reaction conditions (Table 1; entries
attachment of halogen groups to the para-position of the 15–17). Also, alicyclic alcohol such as cyclohexanol was selec-
aromatic ring increased the efficiency of the oxidation reaction tively oxidized to the corresponding cyclic ketone in moderate
(Table 1, entries 7 and 8) while ortho-substituted derivative gave yield (Table 1; entry 18). The GC yields of these conversions were
lower yield (Table 1, entries 9). The slightly improvement of in the range of 60–68% aer reaction time of 4 h. From the
yield in 4-Br- and 4-Cl-benzyl alcohols is probably due to posi- above ndings, it is clear that an extended pi structure conju-
tive resonance effect (+R) of halide groups at para-position, gated with the aldehyde increase the yield.
activating of the benzene ring. Steric hindrance is another
The experiments, usually performed on a 10 mmol scale, can
important factor that affects the reactivity as electron with- be scaled up to 100 mmol without difficulties. A 50 mmol
drawing –NO and –Cl groups attached to the aromatic ring at reaction of 4-methoxybenzyl alcohol provided the correspond-
2
ortho-position decreased the efficiency of the reaction. In ing aldehyde in a 92% yield and a 100 mmol reaction of
addition, a 2,4-dichloro substitution did not reduce or increase 1-phenylethanol gave acetophenone in an 88% yield.
the yield (Table 1, entry 10), conrming that the catalytic
performance is dependent on the electronic (inductive and reusability of the catalysts are very important for commercial
resonance) effect and steric effect of the substituents present on applications. The reusability of Fe /GrOSi(CH –NH /HPMo
the phenyl ring. From the above ndings, it is clear that an catalyst in the selective oxidation of benzyl alcohol to benzal-
extended pi structure conjugated with the aldehyde and dehyde with 30% H was investigated, the results were
3.2.6 Stability and reusability of the catalyst. Stability and
3
O
4
2
)
3
2
2 2
O
aromatic rings with high electron density increase the yield. summarized in Fig. 12. Aer completion of the reaction, the
Various secondary benzylic alcohols were also converted with catalyst was easily recovered from the reaction mixture by an
high selectivity ($99%) to their corresponding ketones without external magnet and then used for the subsequent catalytic runs
any side reactions (Table 1, entries 11–13). Cinnamyl alcohol, as without further activation. Interestingly, the catalyst could be
an allylic alcohol, was selectively converted into the quantitatively recovered from the reaction mixture and reused
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experiment when the catalyst separated from the reaction
mixture shortly (1 h) aer the beginning the reaction and the

ltrate was further heated under our reaction condition, no
extra formation of benzaldehyde was observed via GC analysis
even aer 4 h and the oxidation was completely stopped by the
removal of the catalyst. These observations conrm that the
reaction catalyzed by the Fe
3 4 2 3 2
O /GrOSi(CH ) –NH /HPMo
nanocomposite is heterogeneous in nature.
The stability of this material is further discussed. As shown
in Fig. 13(a)–(c), the XRD pattern, FTIR and Raman spectrum of
the recovered catalyst aer fourth run are consistent with those
of the fresh catalyst (see Fig. 2(b), 3(b) and 4(b)). These obser-
vations conrm that the structure of the Fe
3 4 2 3
O /GrOSi(CH ) –
NH /HPMo catalyst is stable under the reaction conditions and
2
is not affected by the reactants. The morphology of the recycled
catalyst particles was also analyzed. Fig. 13(d) shows catalyst
almost kept initial size and morphology even aer four runs
(see Fig. 5(b) and (c)). The surface of GO nanosheets is still
3 4 2 3 2
Fig. 12 Recyclability of the Fe O /GrOSi(CH ) –NH /HPMo catalyst.
Conditions: benzyl alcohol (10 mmol), H
2
O
2
(15 mmol, 30%), catalyst decorated with spherical Fe O and white HPMo particles
3
4
ꢂ
(
0.20 g) at 100 C for 4 h. Yields are for isolated pure benzaldehyde.
revealing the strong binding between the Fe O nanoparticles
3
4
and HPMo with amino functionalized GO nanosheets.
Considering the above mentioned experiment results, we can
conclude that the structure of the catalyst remains intact, which
further conrms its excellent stability and recyclability.
for several times without signicant loss in conversion and
selectivity (Fig. 12). As is shown in Fig. 12, a slight decrease in
the yield of benzaldehyde from 90% with fresh catalyst to about
8
7% in the fourth run was observed. The molybdenum content
4
. Conclusion
in aqueous phase aer reaction was analyzed by ICP-AES. On
the other hand, no detectable leaching of Mo was observed in In conclusion, the Fe O /GrOSi(CH ) –NH /HPMo nano-
3
4
2 3
2
the rst as well as the four run of the reaction. Also, in an composite was prepared and tested as a novel magnetically
3 4 2 3 2
Fig. 13 (a) XRD pattern, (b) FT-IR spectrum, (c) Raman spectrum and (d) SEM image of the recovered Fe O /GrOSi(CH ) –NH /HPMo nano-
composite after fourth run.
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separable oxidation catalyst under solvent free conditions. 12 H. Zhao, L. Zeng, Y. Li, C. Liu, B. Hou, D. Wu, N. Feng,
Selective oxidation of benzyl alcohol to benzaldehyde was used
as a benchmark reaction to evaluate the catalytic performance
A. Zheng, X. Xie and S. Su, Microporous Mesoporous Mater.,
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of the as-prepared catalyst. Under the optimized conditions, 13 Y. Chen, F. Zhang, Y. Fang, X. Zhu, W. Zhen, R. Wang and
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desired carbonyl compounds in moderate to excellent yields. 14 J. L. Garc ´ı a-Guti ´e rrez, G. A. Fuentes, M. E. Hern ´a ndez-Ter ´a n,
2 2
H O
This reaction is highly selective ($99%) for benzylic alcohols. In
the oxidation of benzylic alcohols, strong electron-withdrawing
F. Murrieta, J. Navarrete and F. Jim ´e nez-Cruz, Appl. Catal., A,
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that carbocation-type intermediates are involved in the catalytic Appl. Energy, 2014, 113, 78–85.
oxidation reaction due to hydride elimination from the beta 16 A. Micek-Ilnicka, E. Biela n´ ska, L. Lity n´ ska-Dobrzy n´ ska and
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A. Biela n´ ski, Appl. Catal., A, 2012, 421, 91–98.
external magnet and reuse the catalyst for another catalytic 17 K. Zhu, J. Hu, X. She, J. Liu, Z. Nie, Y. Wang, C. H. Peden and
recycling. The catalytic performances of the re-used catalyst, J. H. Kwak, J. Am. Chem. Soc., 2009, 131, 9715–9721.
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dehyde production in industry. Also, the present work provides 19 G. Morales, R. Van Grieken, A. Mart ´ı n and F. Mart ´ı nez,
a new type of heterogeneous catalytic materials for selective Chem. Eng. J., 2010, 161, 388–396.
organic transformations. Work on the detailed mechanism of 20 S. K. Bhorodwaj and D. K. Dutta, Appl. Catal., A, 2010, 378,
this catalytic reaction is under investigation in our laboratory.
221–226.
21 S. Mallick, S. Rana and K. Parida, Ind. Eng. Chem. Res., 2012,
51, 7859–7866.
Conflicts of interest
22 N. V. Maksimchuk, K. A. Kovalenko, S. S. Arzumanov,
Y. A. Chesalov, M. S. Melgunov, A. G. Stepanov, V. P. Fedin
and O. A. Kholdeeva, Inorg. Chem., 2010, 49, 2920–2930.
There are no conicts of interest to declare.
23 A. Dhakshinamoorthy, A. M. Asiri and H. Garcia, Chem.
Commun., 2017, 53, 10851–10869.
Acknowledgements
24 M. Kooti and E. Nasiri, J. Mol. Catal. A: Chem., 2015, 406,
1
68–177.
Financial supports from the Lorestan University Research
Council and Iran Nanotechnology Initiative Council (INIC) are
gratefully acknowledged.
2
2
5 M. Kooti and M. Afshari, Mater. Res. Bull., 2012, 47, 3473–3478.
6 M. Masteri-Farahani, J. Movassagh, F. Taghavi, P. Eghbali
and F. Salimi, Chem. Eng. J., 2012, 184, 342–346.
2
7 S. Li, S. R. Zhai, J. M. Zhang, Z. Y. Xiao, Q. D. An, M. H. Li and
X. W. Song, Eur. J. Inorg. Chem., 2013, 31, 5428–5435.
8 J. Zhang, S. Zhai, S. Li, Z. Xiao, Y. Song, Q. An and G. Tian,
Chem. Eng. J., 2013, 215, 461–471.
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