Facile synthesis of α-Fe2O3 nanoparticles and their catalytic activity in oxidation of benzyl alcohols with periodic acid

Article abstract of DOI:10.1016/j.catcom.2015.05.017

α-Fe₂O₃ nanoparticles (NPs) were synthesized by homogeneous chemical precipitation followed by calcination. Size-tuning of NPs was achieved by using polyethylene glycol with a molecular weight of 400 and 4000 as surfactant. Synthesiz

Full text of DOI:10.1016/j.catcom.2015.05.017

Facile synthesis of α-Fe2O3 nanoparticles and their catalytic activity in
oxidation of benzyl alcohols with periodic acid
Bappi Paul, Bishal Bhuyan, Debraj Dhar Purkayastha, Siddhartha Sankar Dhar
PII:
DOI:
Reference:
S1566-7367(15)00215-0
doi: 10.1016/j.catcom.2015.05.017
CATCOM 4337
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Catalysis Communications
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Accepted date:
9 March 2015
30 April 2015
27 May 2015
Please cite this article as: Bappi Paul, Bishal Bhuyan, Debraj Dhar Purkayastha, Sid-
dhartha Sankar Dhar, Facile synthesis of α-Fe2O3 nanoparticles and their catalytic ac-
tivity in oxidation of benzyl alcohols with periodic acid, Catalysis Communications (2015),
doi: 10.1016/j.catcom.2015.05.017
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Facile synthesis of α-Fe2O3 nanoparticles and their catalytic activity in oxidation of benzyl
alcohols with periodic acid
Bappi Paul, Bishal Bhuyan, Debraj Dhar Purkayastha*, Siddhartha Sankar Dhar*
Department of Chemistry, National Institute of Technology Silchar, Silchar-788010, Assam,
India
*Corresponding authors. Tel: +91-03842-242915; fax: +91-03842-224797
Email: debrajdp@yahoo.com (D.D. Purkayastha), ssd_iitg@hotmail.com (S.S. Dhar).
Abstract
α-Fe2O3 nanoparticles (NPs) were synthesized by homogeneous chemical precipitation followed
by calcination. Size-tuning of NPs was achieved by using polyethylene glycol with a molecular
weight of 400 and 4000 as surfactant. Synthesized α-Fe2O3 NPs were characterized by powder
XRD, transmission electron microscopy, N2 adsorption-desorption isotherm and vibrating
sample magnetometry. These α-Fe2O3 NPs were used as magnetically recoverable catalyst in
oxidation of benzyl alcohols to aldehydes with periodic acid. The catalyst showed higher
efficiency with decrease in their sizes.
Keywords: α-Fe2O3 nanoparticles; polyethylene glycol; size-tuning; magnetically recoverable
catalyst.

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1. Introduction
Magnetic nanoparticles are explored extensively as catalysts or catalyst supported
materials owing to their high thermal and mechanical stability[1]. Their paramagnetic nature and
inherent insolubility in reaction solvents make them ideal for simple and efficient separation by
external magnetic field. Hematite (α-Fe2O3) is one of the most studied iron oxide nanomaterials
due to their variety of applications such as in catalysis[2], pigments[3], gas sensors[4], lithium
ion batteries[5] etc. Controlled growth of these NPs is critical for understanding and exploiting
their unique properties. One of the simplest and most efficient methods of preparation of iron
oxide NPs is homogeneous precipitation. The precipitation method is advantageous as it
produces large amount of materials; however, control on particle size is limited unless some
suitable surfactant is used. Polyethylene glycol (PEG) with uniform and ordered chain structure
can act as a very useful surfactant to control size and morphology of the nanomaterials[6]. The
chemical precipitation method often requires a base as hydrolyzing agent. Hexamine, a highly
water-soluble, non-ionic tertiary amine derivative has gained popularity as hydrolyzing agent
and plays a variety of roles in nanomaterial synthesis such as shape-inducing capping molecule,
reducing agent and also known to assist anisotropic growth of metal oxides including Fe3O4
NPs[7].
Benzaldehyde is the expected product from partial oxidation of benzyl alcohols. Earlier
carbon nanotube (CNT)-supported ruthenium and CNT-TiO2 nanohybrids were employed as
catalysts for the selective oxidation of benzyl alcohols with oxygen or air as oxidant[8,9]. Shi
et al. utilized bulk as well as nano Fe2O3 as catalysts for the selective oxidation of benzyl
alcohols with hydrogen peroxide as oxidant[2,10]. However, nano α-Fe2O3 has not been
explored extensively as catalyst, particularly in oxidation of benzyl alcohols with periodic acid.
Periodic acid is a cheap, non-toxic, efficient oxidant known to be used in catalytic oxidation of

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alcohols including benzyl alcohols [11]. As continuation of our recent endeavor[12] on synthesis
and application of nanomaterials, we report herein, a new method of synthesis of α-Fe2O3 NPs
and their catalytic activity in oxidation of benzyl alcohols to aldehydes with periodic acid.
2. Experimental
2.1 Materials and physical measurements
Ferric nitrate, hexamine, PEG 400 and PEG 4000 were purchased from Merck India Ltd. Double
distilled water was used throughout the experiment. Thermogravimetric analysis (TGA) was
performed in air on a Netzsch STA449F3 thermal analyzer at a flow rate of 20 mL/min and a
heating rate of 10°C/min. FT-IR spectrum was recorded on KBr matrix with Bruker 3000
Hyperion Microscope with Vertex 80 FT-IR system. XRD measurements were carried out on a
Bruker AXS D8-Advance powder X-ray diffractometer with Cu-Kα radiation (λ=1.5418Å) with
a scan speed of 2°/min. Transmission electron microscopy images were obtained on a JEOL,
JEM2100 equipment. The sample powders were dispersed in ethanol under sonication and TEM
grids were prepared using a few drops of dispersion followed by drying in air. Nitrogen
adsorption-desorption isotherms were obtained with a Micromeritics ASAP 2010 surface area and
porosity analyzer. Magnetism of the material was studied using a Lakeshore VSM 7410 at
27°C.
2.2. Synthesis of α-Fe2O3 nanoparticles
Fe(NO)3.9H2O (0.80 g , 2 mmol) and hexamine (1.40 g , 10 mmol) were dissolved separately in
minimum volume of distilled water. The resultant solutions were then mixed together and stirred
magnetically for 4 h. The reddish suspension thus formed was collected and dried in a hot air
oven. The as-obtained precursor powder was then calcined in a muffle furnace for 4 h at 550°C
to obtain powder of α-Fe2O3.
2.3. Synthesis of α-Fe2O3 nanoparticles in presence of PEG

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Fe(NO)3.9H2O (0.80 g , 2 mmol) and hexamine (1.40 g , 10 mmol) were dissolved separately in
minimum volume of distilled water. The resultant solutions were then mixed together and stirred
magnetically followed by addition of 5 mmol of surfactant PEG 400 or PEG 4000. After stirring
for 4 h, the reddish suspension was collected and dried in a hot air oven. The as-obtained
precursor powder was then calcined in a muffle furnace for 4 h at 550°C to obtain powder of α-
Fe2O3.
2.4. Oxidation of benzyl alcohol
In a typical procedure, catalyst (4 mol%) and periodic acid (1.5 mmol/3 equiv) were stirred with
10 ml of water taken in a RB flask. The substrate (0.5 mmol/1 equiv) was added slowly to the
stirred solution/suspension. Stirring continued for required time at room temperature until
complete conversion was achieved. The catalyst was then separated magnetically from the
reaction mixture. The product was extracted with diethyl ether and purified by column
chromatography using n-hexane: ethanol (4:1) as eluent to give the aldehyde. The separated
catalyst was washed with ethanol and dried in a hot air oven.
3. Results and Discussion
In the present study, hematite (α-Fe2O3) nanoparticles have been synthesized by calcination of a
ferric hydroxide (Fe(OH)3) precursor obtained by a homogeneous chemical precipitation
method. Here, hydroxide anions are produced by hydration of hexamine, which cause a uniform
rise in pH of the solution till the solubility limit thereby preventing the occurrence of high local
supersaturation and allowing nucleation to occur homogeneously throughout the solution. The
reactions involved in the formation of α-Fe2O3 nanoparticles are believed to be as follows:
(1)
(2)
(3)
(4)
4NH3 + 6HCHO
(CH2)6N4 + 6H2O
NH3 + H2O
+
NH4 + OH-
Fe3+ + 3OH-
Fe(OH)3
Fe2O3 + 3H2O
2 Fe(OH)3

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The surfactant PEG was added to the precursor solution to control the particle size as well as
size distribution of the nanoparticles. The synthesized ferric hydroxide precursor was
characterized by TGA and FTIR. α-Fe2O3 nanoparticles were characterized by powder XRD,
TEM, N2 adsorption-desorption isotherm and VSM. The TGA of the precursor synthesized in
presence of PEG 400 was carried out as a representative case. The TGA curve (Fig.1 (a)) shows
a two step decomposition pathway. The weight loss of 8.04% in the first step (30-100°C) is due
to the removal of physically adsorbed moisture in the precursor. The weight loss of 79.67% in
the second step (280-520°C) indicated removal of surfactant PEG 400 and subsequent
conversion of Fe(OH)3 to α-Fe2O3. About 7% of the sample escaped without pyrolysis. FT-IR
spectrum (Fig.1(b)) showed O-H stretching at around 3400 cm-1. Band at 2830 cm-1 arose due
to stretching vibrations of methylene group and strong bands in the range of 1000-1245 cm-1
may be attributed to C-O and C-C stretching vibrations[13]. Band at 1105 cm-1 corresponds to
C-O-C stretching vibrational mode[14]. This confirms the presence of surfactant PEG on the
surface of Fe(OH)3 precursor.
The powder XRD patterns (Fig. 1(c)) were recorded for the identification of phases. The
diffraction peaks matches well with the reported data of α-Fe2O3 (JCPDS File no. 87-1166). α-
Fe2O3 possess a rhombohedrally centered hexagonal structure of the corundum type with a
close-packed oxygen lattice in which two thirds of the octahedral sites are occupied by Fe(III)
ions[15].

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Fig.1.
The TEM image of α-Fe2O3 nanoparticles (Fig.S1 in ESI) synthesized without surfactant
showed some irregular shaped particles of size 50-100 nm. The nanoparticles synthesized in
presence of PEG 400 (Fig.2) showed some quasi-spherical particles of size 10-50 nm. The
lattice fringes are separated by 0.252 nm, which corresponds to the (110) plane of α-Fe2O3.
Electron diffraction (ED) pattern indicated polycrystalline nature. The nanoparticles synthesized
in presence of PEG 4000 (Fig.3) also showed some quasi-spherical particles of size 5-20 nm.
The lattice fringes are separated by 0.251 nm, which corresponds to the (110) plane of α-Fe2O3.
ED pattern indicated single crystalline nature. Thus surfactant PEG played a crucial role in
controlling the particle size as well as size distribution. Moreover, when PEG of high molecular
weight i.e. PEG 4000 was used nanoparticles so formed are comparatively well dispersed with

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much smaller size. The TEM image of the nanoparticles recovered after five cycles (Fig.S2 in
ESI) did not show much difference in sizes, shapes and morphology.
Fig.2.

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Fig.3.
N2 adsorption-desorption measurements were conducted to determine the Brunauer
Emmett Teller (BET) specific surface area of α-Fe2O3 nanoparticles synthesized in presence of
PEG 400 and PEG 4000. Physical parameters of the materials obtained by means of N2
adsorption-desorption study are summarized in Table 1. From Table 1 it can be noted that,
α-Fe2O3 nanoparticles synthesized in presence of PEG 4000 possess high specific surface area
and pore volume. The amount of N2 adsorbed rose gradually at lower relative pressures and then
increased sharply at higher relative pressures (Fig.4(a) and (b)). This follows the characteristics

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of a type IV isotherm with a type H3 hysteresis loop associated with aggregates of plate-like
particles forming slit-like pores[16]. The hysteresis loop shifts to a higher relative pressure on
approaching P/P0=1, suggesting the presence of macropores and a low degree of
aggregation[16], which was also confirmed by TEM images (Fig.2 and Fig.3). The Barrett
Joyner Halenda (BJH) pore size distribution indicated that most of the pores range from 3 to 50
nm (inset of Fig.4(a) and (b)), which further confirmed the loose structures and a low degree of
aggregation.
Fig.4.

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Table 1.
Catalysts
BET specific surface area
Pore Volume
Pore size
(nm)
9.74
(m2/g)
244.8
(cm3/g)
0.596
α-Fe2O3 with PEG 400
α-Fe2O3 with PEG 4000
276.2
0.609
8.83
The magnetic hysteresis measurement (Fig.4(c)) of α-Fe2O3 nanoparticles synthesized in
presence of PEG 4000 was done at 27°C with the applied magnetic field sweeping from -20000
to +20000 G. The magnetic hysteresis loops showed a ferromagnetic behavior with a remnant
magnetization (Mr) of 8.63 emu g-1 and coercivity (Hc) of 312.07 G.
The catalytic potential of as-synthesized α-Fe2O3 NPs was investigated in the oxidation of a few
chosen substituted benzyl alcohols with periodic acid. For this purpose, benzyl alcohol was
chosen as the model substrate. α-Fe2O3 nanoparticles synthesized in presence of PEG 4000
showed greatest catalytic efficiency in terms of yield and reaction time (Fig 5(a)). The results
are summarized in Table 2.
Fig.5.
Best results were obtained when catalyst amount is 4 mol% and substrate to oxidant ratio is 1:3.
The yield of aldehyde in uncatalyzed reaction is very poor after a long reaction time. After every
single catalytic run, the ferromagnetic α-Fe2O3 catalyst was magnetically separated, recovered

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and reused up to five times without any significant loss of activity (Fig.5(b)). It was observed
that although selectivity remains almost constant but yield of the products decrease slightly with
the number of recycles.
Table 2.
Entrya
1
Substrates
Products
Yield (%)b
92
Time (min)c
60
Selectivity (%)d
> 99
O
OH
OH
OH
H
O
2
3
98
94
55
50
> 99
> 99
H
Cl
Cl
O
H
O2N
O2N
O
4
5
92
86
50
62
> 99
> 98
OH
H
O
HO
HO
OH
H3C
N
H
H3C
N
CH3
CH3
O
6
7
8
96
92
86
60
45
55
> 95
> 99
> 99
OH
H
H3C
H3C
O
O
OH
H
H3CO
H3CO
OH
OH
OH
H
Cl
Cl
O
9
85
82
60
60
> 99
> 97
H
NO2
NO2
O
10
H
CH3
CH3
a substrate:oxidant:catalyst: 1:3:.04; b isolated yield; c time correspond to complete conversion; d selectivity based
on alcohol conversion

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The oxidation reaction possibly followed the M-v K type mechanism [17]. The catalytic path
involves two successive steps. At first α-Fe2O3 is reduced by benzyl alcohols, leading to the
formation of aldehydes and reduced catalysts. This step includes the transfer of lattice oxygen
atoms and the reduction of α-Fe2O3. Subsequently, the reduced catalysts are re-oxidized by
oxygen produced from decomposition of periodic acid, replenishing oxygen back to the lattice.
The plausible reaction mechanism is shown in Fig.S3 in ESI.
4. Conclusion
In summary, a new approach to synthesis of hematite, α-Fe2O3 NPs is demonstrated. Calcination
of hydroxylated iron(III) with PEG 4000 resulted in NPs with smallest size. These α-Fe2O3 NPs
with or without PEG modification are all catalytically active and magnetically recoverable.
However, PEG 4000 modified nano α-Fe2O3 exhibited highest efficiency for the oxidation of
benzyl alcohols with periodic acid. The present catalytic protocol is simple and efficient with
advantage of easy recovery and recyclability of the catalyst.
Acknowledgement
The authors thank SAIF, IIT Madras, SAIF, IIT Bombay and SAIF, North Eastern Hill
University for analysis. The Director NIT Silchar is acknowledged for financial support.
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Figure and table captions
Fig.1. (a) TGA curve of Fe(OH)3 with PEG 400 (b) FT-IR spectra of Fe(OH)3 with (i) PEG 400
(ii) PEG 4000 and (c) powder XRD patterns of α-Fe2O3 nanoparticles.
Fig.2. (a, b) N2 adsorption-desorption isotherms of α-Fe2O3 nanoparticles with PEG 400 and
PEG 4000 (c) VSM of α-Fe2O3 nanoparticles with PEG 4000.
Fig.3. (a, b) TEM images (c) HRTEM image and (d) ED pattern of α-Fe2O3 nanoparticles with
PEG 400.
Fig.4. (a, b) TEM images (c) HRTEM image and (d) ED pattern of α-Fe2O3 nanoparticles with
PEG 4000.

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Fig.5. (a) Percentage yield of benzaldehydes versus time with all three types of α-Fe2O3
nanoparticles (b) Catalytic activity of PEG 4000 modified α-Fe2O3 nanoparticles against
number of cycles.
Table 1. Physical parameters of the materials obtained by means of N2 adsorption-desorption
analysis.
Table 2. Catalytic oxidation of substituted benzyl alcohols with periodic acid in presence of α-Fe2O3
nanoparticles with PEG 4000.

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Graphical abstract

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Highlights
Homogeneous chemical precipitation followed by calcination effectively resulted in α-
Fe2O3 nanoparticles.
Nanoparticles size was tuned using surfactant polyethylene glycol with a molecular
weight of 400 and 4000.
The α-Fe2O3 nanoparticles showed rhombohedrally centered hexagonal structure.
Nanoparticles synthesised in presence of PEG 4000 are quasi-spherical in shape with a
size of 5-20 nm.
α-Fe2O3 nanoparticles synthesized in presence of PEG 4000 showed highest catalytic
efficiency.