Controlled immobilization of Keggin-type heteropoly acids on the surface of silica encapsulated γ-Fe2O3 nanoparticles and investigation of catalytic activity in the oxidative esterification of arylaldehydes with methanol

Article abstract of DOI:10.1016/j.molcata.2013.02.024

Preparation, leaching and solid acidity measurements of H ₃PW₁₂O₄₀, H₃PMo₁₂O ₄₀ and H₄SiW₁₂O₄₀ supported on silica-encapsulated γ-Fe₂O₃ nanoparticles were performed. The impregnating solvent and calcination temperature were optimized using various techniques. Catalytic activity of samples was examined by carrying out oxidative esterification of benzaldehyde with methanol under the same conditions. The acidity of the catalysts that was determined by NH ₃-temperature-programmed desorption technique and chemisorption of pyridine. The best catalyst was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy and laser particle size analyzer. Finally, a mild, simple and clean procedure was presented for the one-pot oxidative esterification of arylaldehydes with methanol using magnetically recoverable nanocatalyst and hydrogen peroxide as a green oxidant.

Full text of DOI:10.1016/j.molcata.2013.02.024

Journal of Molecular Catalysis A: Chemical 373 (2013) 30–37
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Controlled immobilization of Keggin-type heteropoly acids on the surface of
silica encapsulated ␥-Fe O nanoparticles and investigation of catalytic activity
2
3
in the oxidative esterification of arylaldehydes with methanol
∗
Ezzat Rafiee , Sara Eavani
Faculty of Chemistry, Razi University, Kermanshah, 67149, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Preparation, leaching and solid acidity measurements of H PW12O40, H PMo12O40 and H SiW12O40 sup-
3
3
4
Received 12 December 2012
Received in revised form 23 February 2013
Accepted 25 February 2013
Available online 5 March 2013
ported on silica-encapsulated ␥-Fe2O3 nanoparticles were performed. The impregnating solvent and
calcination temperature were optimized using various techniques. Catalytic activity of samples was
examined by carrying out oxidative esterification of benzaldehyde with methanol under the same condi-
tions. The acidity of the catalysts that was determined by NH3-temperature-programmed desorption
technique and chemisorption of pyridine. The best catalyst was characterized by Fourier transform
infrared spectroscopy, X-ray diffraction, scanning electron microscopy and laser particle size analyzer.
Finally, a mild, simple and clean procedure was presented for the one-pot oxidative esterification of
arylaldehydes with methanol using magnetically recoverable nanocatalyst and hydrogen peroxide as a
green oxidant.
Keywords:
Supported heteropoly acid
Maghemite iron oxides
Magnetically recoverable catalyst
Oxidative esterification
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
immobilized on the modified surface of magnetic NPs. Following
this line of research, we have previously reported interesting results
in the immobilization of heteropoly acids (HPAs) with Keggin anion
structure [11]. In our work, the primary motivation was to create
the novel magnetically recoverable nanocatalyst by immobilization
The development of new strategies for the recycling of catalysts
is a task of great economic and environmental importance in chemi-
cal and pharmaceutical industries. Immobilization of homogeneous
catalysts on various insoluble supports (especially porous materials
with high surface areas) can lead to simplify catalyst recycling via
filtration or centrifugation. However, a substantial decrease in the
activity of the immobilized catalyst is frequently observed due to
the loss of the catalyst in the separation processes and/or diffusion
factors [1]. At present, nanoparticles (NPs) are attractive candidates
as solid supports for the immobilization of well-defined homoge-
neous catalysts [2]. Because of the nanometer dimensions of NPs,
such catalysts have better reaction kinetics as compared to the
traditional heterogeneous catalysts on mesoporous solid supports.
Additionally, due to their large surface area, which can carry a high
payload of catalytically active species, these supported catalysts
exhibit very high activity under mild conditions.
of Keggin-structured H PW12O40 (PW) on silica-coated magnetic
3
NPs. This synthesized catalyst possesses both magnetic separation
and strong acidic sites. Moreover, the nano scale support materials
have the high surface area. Consequently, NPs could have higher
catalyst loading capacity and higher dispersion than many conven-
tional support matrices, leading to an improved catalytic activity
and high leaching stability of HPA species even in polar reaction
media.
However, the effects of various processing parameters, includ-
ing the concentration of impregnating solutions, the solvent used,
calcination temperature and the type of HPA have remained a
challenge. Literature survey showed that the above-mentioned
parameters are major factors contributing to the catalytic activ-
ity of HPA immobilized on conventional solid supports [12–14].
Nevertheless, there has been scant investigation of HPA catalyst
supported on magnetic NPs [15–20] and to the best of our knowl-
edge, the detailed studies of the effective parameters on catalyst
preparation have not been well developed.
In NPs-based heterogeneous catalysis, the use of silica coated
magnetic NPs can be of additional values: the magnetic nature
of these particles allows for facile recovery and recycling of cat-
alysts. Thus far, various catalytic species, including organometallic
[
3–5] and organic catalysts [6,7] and biocatalysts [8–10], were
Keeping in mind these statements, herein, we propose a simple
method for the synthesis of magnetically recoverable HPA-based
catalyst that relies on our pervious paper, but with an examination
and subsequently careful choice of processing parameters.
∗ Corresponding author. Tel.: +98 831 427 4559; fax: +98 831 427 4559.
E-mail addresses: ezzat rafiee@yahoo.com, e.rafiei@razi.ac.ir (E. Rafiee).
1
381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.02.024

E. Rafiee, S. Eavani / Journal of Molecular Catalysis A: Chemical 373 (2013) 30–37
31
The synthesis of silica coated magnetic NPs, covering a wide
range of compositions and tunable sizes, has made substantial
progress, especially over the past decade [21,22]. Most of these
researches focused on the generation of uniform particles, often of
distinct size, shape, and structure, with little consideration of the
expensive precursors used to prepare them or the cost of processes
to produce them. On the other hand, for the large-scale applica-
tion in industry, an important consideration will be to prepare NPs
economically using relatively low-grade reagents under aerobic
conditions. So, a simple ferric oxide, ␥-Fe O , was chosen as the
(water, MeOH and MeCN), calcination temperatures (100, 150, 200,
250 and 300 C) and HPA types (PW, SiW and PMo).
◦
All supported HPA catalysts were generally prepared by impreg-
nating 1.0 g of Fe@Si with an aqueous or organic solution of HPA
◦
(1.2 g in 50 mL of solvent) with stirring at about 60 C for 24–72 h.
The catalyst was dried using a rotary evaporator. Further calcina-
tion of the catalyst was carried out at corresponding temperature
in air for 2 h.
2.2. Catalyst characterization
2
3
magnetic material for its low price, air stability and non-toxicity.
It was prepared simply through chemical coprecipitation [23], and
subsequently was coated with silica shell by the ordinary Stöber
process [24] without expensive systems. After the surface coating
by silica, magnetic solid (designed as Fe@Si) was used as support
for immobilization of HPA. Preparation conditions including the
impregnating solvent used, calcination temperature and the type
of HPA have been optimized. Ultimately, the efficiencies of obtained
catalysts were evaluated in oxidative esterification of benzalde-
hyde with methanol as the test reaction. The ever-increasing
concern for the catalytic oxidative esterification of aldehydes as
an interesting and potentially valuable alternative transformation
The samples were characterized with a scanning electron
microscope (SEM) (Philips XL 30 and S-4160) with gold coating.
Transmission electron microscopy (TEM) image was obtained using
a TEM microscope (Philips CM 120 KV, The Netherlands). The size
distribution of the samples was obtained using a laser particle
size analyzer (HPPS 5001, Malvern, UK). X-ray diffraction (XRD)
measurements were performed using a Bruker axs Company, D8
ADVANCE diffractometer (Germany). Fourier transform infrared
(FTIR) spectra were recorded with KBr pellets using a FT-IR spec-
trometer ALPHA. UV–vis spectra were obtained with an Agilent
(8453) UV-vis diode-array spectrometer using quartz cells of 1 cm
optical path. The tungsten (W) content in the catalyst was mea-
sured by inductively coupled plasma atomic emission spectroscopy
[
25–29] and the wide application of aromatic esters in pharmaceut-
icals, agrochemicals, and food additives [30–32] encouraged us to
evaluate the scope and generality of the catalytic system by using
various substituted benzaldehyde. To the best of our knowledge,
the application of magnetically recoverable HPA-based catalyst in
the oxidative esterification of arylaldehydes has not yet been stud-
ied.
(ICP-AES) on a Spectro Ciros CCD spectrometer. NH -temperature
3
programmed desorption (NH -TPD) measurements were carried
3
out with 200 mg samples on a Micromeritics 2900 TPD/TPR appa-
ratus with a thermal conductivity detector (TCD).
The equilibrium adsorption of the support with different PW
solutions was performed by contacting 1.0 g of support with 50 mL
◦
of solution, at 60 C, under constant stirring during 72 h. At regular
intervals, the samples of this mixture were taken for their UV–vis
analyses.
2
. Experimental
To check the leaching stability of the catalysts, 0.02 g of each
catalyst was stirred in 5 mL methanol for 1 h. UV–vis spectra of the
diluted solutions were recorded after removal of solid. The con-
tent of HPA in solution was determined with the aid of calibration
curves.
2
.1. Catalyst preparation
FeCl ·4H O (99%) and FeCl ·6H O (99%), concentrated ammo-
2
2
3
2
nium hydroxide (25%), tetraethyl orthosilicate (TEOS, 98%), PW
(
>99%), H SiW12O40 (SiW, >99%), H PMo12O40 (PMo, >99%) and
The surface acidity of the catalysts was determined by
chemisorption of pyridine [34]. 1.0 g of solid was suspended in
hexane (100 mL). The suspension was titrated with a 0.1 M solu-
tion of pyridine in hexane. Equilibrium concentration of pyridine
in hexane solution between each addition was measured by UV–vis
spectroscopy based on the fact that pyridine molecule has an
absorbance at ꢀmax = 251 nm. For each measurement, 1 mL of solu-
tion was separated (1 mL of hexane was added into suspension in
order to maintain a constant volume). Then the absorption value
of this separated solution was measured, and the equilibrium con-
centration of pyridine can be calculated using calibration curves,
considering the diluted factor. Since the amount of pyridine added
was known, the amount of base adsorbed by the solid was calcu-
lated by difference.
Time dependent UV adsorption measurements were performed.
It was observed that the absorbance was constant for standing
times above 15 min. Therefore, the experiments were performed in
the time scale of 15 min between each addition of pyridine. All the
measurements were repeated at least three times and the reported
values are average of the individual runs.
4
3
other reagents and solvents used in this work were obtained from
Merck, Aldrich or Fluka and used without further purification.
Magnetic ␥-Fe O3 NPs were prepared through the chemical
2
co-precipitation method [23]. FeCl ·4H O (2.0 g) and FeCl ·6H O
2
2
3
2
(
5.4 g) were dissolved in water (20 mL) separately. These solutions
were being mixed together under vigorous stirring (1200 rpm). A
concentrated NH OH solution (25% w/w) was then added to the
stirring mixture at room temperature to maintain the pH between
4
1
(
1
1 and 12. The resulting black dispersion was continuously stirred
1200 rpm, 1 h) at room temperature and then heated to reflux for
h to yield a brown dispersion. The ␥-Fe O NPs were then purified
2
3
by a four times repeated centrifugation (3000–6000 rpm, 20 min),
decantation and redispersion cycle until a stable brown magnetic
dispersion was obtained.
Coating of a layer of silica on the surface of the ␥-Fe O3 NPs
2
was achieved by mixing a dispersion of the purified NPs (8.5%
w/w, 20 mL) obtained previously with ethanol (80 mL) for 1 h at
4
resulting mixture stirred at 40 C (800 rpm, 30 min). Subsequently,
TEOS (1.0 mL) was charged to the reaction mixture and the mixture
continuously stirred at 40 C (800 rpm, 24 h). The Fe@Si NPs were
◦
0 C. A concentrated ammonia solution (15 mL) was added and the
◦
◦
2.3. Catalytic experiments
collected using a permanent magnet, followed by washing three
times with ethanol, diethyl ether and drying in a vacuum for 24 h
The oxidative esterification was carried out as follows: catalyst
(25 mg), aldehyde (1 mmol) and alcohol (4 mL) were magnetically
[
33].
In order to study the effect of preparation conditions on leaching
stirred in the reaction flask. H O2 (6 mmol) was progressively
2
stability, acidity and activity of supported HPA catalysts, a series
of samples were prepared using different impregnating solvents
added to the reaction mixture using a syringe. The completion of
reaction was monitored by thin layer chromatography (TLC). At

3
2
E. Rafiee, S. Eavani / Journal of Molecular Catalysis A: Chemical 373 (2013) 30–37
Table 1
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
Effect of impregnation conditions on chemical composition of PW/Fe@Si catalyst.
Entry
Solvent
PW content (wt.%)a
Washed samples
Dried samples
1
2
3
MeOH
Water
MeCN
0
0
11
32
27
42
a
PW content in washed and dried samples was determined by ICP-AES analysis.
All catalysts were separated by centrifugation, washed with
excess solvent and their PW content was analyzed by ICP-AES tech-
nique. The results are listed in Table 1. A complete removal of PW
from support surface was observed in the samples that prepared in
MeOH and water solutions. In water, PW might be aggregated on
the exterior surface of support due to the high surface tension of
water and the secondary structure formed by aggregation of Keggin
units within PW multilayer. From the fact that only the first layer of
PW at the interphase with silica is significantly modified by interac-
tion with the support, the non-interacting aggregated Keggin units
are washed away by solvent. In MeOH, the formation of secondary
Water
MeOH
MeCN
0
10
20
30
40
50
60
70
80
Time (h)
+
+
Fig. 1. Time profiles for adsorption of PW on Fe@Si in different solvent.
3 3
n
structure involving CH OH2 or clusters (CH OH) H [36] can lead
to the aggregation of PW units as multilayer that are leached from
the support surface. In MeCN, PW prefers to form a monolayer of
isolated Keggin anions to the exterior surface of support. Therefore,
PW was strongly adsorbed on the surface of Fe@Si NPs in the MeCN
solution, leading to the relatively high PW content in washed and
dried samples.
When the samples were dried using a rotary evaporator without
washing, PW was retained in the surface of Fe@Si solid (Table 1).
Thus, the impregnation-evaporation technique was selected for the
preparation of HPA/Fe@Si catalyst. Among various solvent used, the
highest PW content was observed in the samples prepared in MeCN
the end of the reaction, the catalyst was separated from the prod-
uct solution using an external magnet, followed by decantation of
reaction mixture. The solvent was evaporated to generate the crude
product. The crude products were purified by column chromatog-
raphy on silica gel using hexane/ethyl acetate 4:1 as eluent.
The remaining catalyst was washed with diethyl ether to
remove the residual product, dried under vacuum and reused in
a subsequent reaction. More than 95% of the catalyst could usually
be recovered from each run.
(Table 1, entry 3).
The nature of HPA species in impregnating solutions and after
3
. Results and discussion
drying was studied by UV–vis and FTIR spectroscopy. The UV–vis
spectra of PW in impregnating solutions after 72 h are shown in
Fig. 2. An intense peak centered at ꢀ = 265 nm is the characteristic
3.1. Immobilization characteristics
The presence of the Keggin anion on support surface is very
dependent upon the preparation conditions. Impregnation of PW
on support surface was examined by using different solvents
MeOH
Water
MeCN
(Fig. 1). The quantity of adsorbed PW is solvent dependent and
decreases in following order (after 72 h): MeOH > water > MeCN.
The adsorption profile of MeCN shows regular trends in compar-
ison with MeOH and water. It seems that 24 h were enough to
attain equilibrium adsorption of Fe@Si NPs with PW solutions. The
adsorption of PW on support surface is affected by various factors
including solvent polarity, PW-solvent, solvent-support, and PW-
support interactions. Methanol and water molecules are not only
adsorbed on the external surface of the support but also interact
with PW anions thus showing irregular trends.
The quantity of adsorbed PW depends on diffusion of PW
molecules from the solvent to the support surface. This factor is
not considerable effect in the case of conventional solid supports
[
13], but it is important in magnetic ␥-Fe O3 NPs because of their
2
nanometer dimensions and subsequently large surface area to vol-
ume ratio. The highest quantity of adsorbed PW was observed in
methanol due to its high polarity, hydrogen bond ability and low
surface tension that can be accelerate the diffusion of PW molecules
from the solvent to the support surface. However, in water and
MeCN, the quantity of adsorbed PW was decreased. These results
are probably ascribed to the high surface tension of water [35].
In the case of MeCN relatively low polarity of this solvent and no
hydrogen bond ability compare to water and MeOH decrease the
diffusion in this solvent.
2
20 240 260 280 300 320 340
Wavelength (nm)
Fig. 2. UV–vis absorbance spectra of PW in impregnating solutions after 72 h.

E. Rafiee, S. Eavani / Journal of Molecular Catalysis A: Chemical 373 (2013) 30–37
33
Water
MeOH
MeCN
6
4
2
0%
0%
0%
0
%
0
50 100 150 200 250 300 350
Calcination temperature (ºC)
Fig. 4. Effect of calcination temperature on the leaching stability of PW/Fe@Si cat-
alyst impregnated in different solutions.
acid sites in supported HPAs decreases in the following order:
PW/Fe@Si > SiW/Fe@Si > PMo/Fe@Si (Table 2, entries 6–8).
Leaching stability of supported HPA samples in methanol was
studied. The relatively same values were obtained in different sol-
vent (Table 2, entries 6–8).
The catalytic activities of samples were investigated in oxidative
methylesterification of benzaldehyde using H O as oxidant. With-
2
2
out any catalyst added, no ester was formed and only benzoic acid
was detected (Table 2, entries 1). Bulk HPAs as catalyst showed
lower activity in comparison with supported ones pointing out
the importance of the concentration of surface acid sites (Table 2,
entries 2–8). In most of the runs, methyl benzoate was a main prod-
uct at the end of the reaction. However, using PMo and PMo/Fe@Si
catalyst, the reaction gave benzoic acid and methylbenzoate with
comparable yields.
Fig. 3. FTIR spectra of PW/Fe@Si catalyst impregnated in (a) MeOH, (b) MeCN and
c) aqueous solutions.
(
charge transfer band of PW heteropoly anion [37]. It is observed
for MeOH and MeCN solutions. While, in the spectrum of aqueous
solution, a slight shifting of the adsorption maximum toward lower
wavelengths (about 250 nm) was observed. It may be due to the
hydrolysis reaction of PW.
The NH -TPD profiles for Fe@Si, PW/Fe@Si, SiW/Fe@Si and
3
PMo/Fe@Si samples are shown in Fig. 6. All supported samples
showed three desorption peaks occurred at different tempera-
ture ranges. It may be due to desorption of adsorbed ammonia
on weak and strong acid sites. The first and second desorption
The FTIR spectra of supported samples after drying are pre-
sented in Fig. 3. The typical bands for absorption of
P O
−1
−1
−1
O W (895 and 809 cm ) are
(
1080 cm ), W O (983 cm ), W
◦
peak at about 130–300 and 400–600 C could be assigned to weak
clearly displayed in the impregnation with MeOH. On the other
and strong acid sites respectively [38]. The remaining peaks in
−
1
hand, from water and MeCN solutions the band placed at 1080 cm
is masked by the Si Si absorption band in the Fe@Si NPs.
◦
the 700–800 C are probably due to dehydration rearrangements
O
of the Keggin unit [39]. The desorption temperature of NH3 on
Any leaching of the catalyst from the support would make the
catalyst unattractive for reusing. Therefore, it is necessary to inves-
tigate the stability of PW on Fe@Si NPs. Leaching experiments were
performed using dried and calcined catalyst samples. Methanol was
employed as the solvent since it further used in catalytic tests. The
results are shown in Fig. 4. Without calcination, significant leach-
ing of PW from Fe@Si surface in methanol was observed in all cases.
The resistance to leaching improved significantly with increasing
◦
the strong acid site (400–600 C) decreases as the following order:
100
PW/Fe@Si
8
6
4
2
0
0
0
0
0
P Mo/Fe@Si
SiW/Fe@Si
Fe@Si
◦
calcination temperature to 250 C.
Based on above results, the best preparation achieved was to
immobilize HPA on Fe@Si NPs in MeCN solution followed by dry-
◦
ing and calcination at 250 C. Further investigations were carried
out using PW, SiW and PMo as the main, convenient, popular and
commercially available HPAs.
Adsorption of pyridine on Fe@Si NPs and PW, SiW and PMo
supported on Fe@Si NPs (designed as PW/Fe@Si, SiW/Fe@Si and
PMo/Fe@Si) from hexane was studied (Fig. 5 and Table 2). The
obtained L-curves with the plateau for supported catalysts and
Fe@Si NPs are shown in Fig. 5. The plateau level corresponds to
surface concentration of acidic sites of samples that are listed
in Table 2. Pyridine was adsorbed on Fe@Si NPs. Such adsorp-
tion is attributed to the presence of Si OH groups on the outer
surface of the support (Table 2, entry 5). The concentration of
0
0.25 0.5 0.75
1
1.25 1.5 1.75
2
Pyridine] (mmolL-1)
[
Fig. 5. Adsorption of HPA/Fe@Si catalysts reacted with pyridine in hexane.

3
4
E. Rafiee, S. Eavani / Journal of Molecular Catalysis A: Chemical 373 (2013) 30–37
Table 2
Surface acidity, leaching and catalytic activity of HPA and HPA/Fe@Si catalysts.
+
H /g of solid (mmol)a
Yield (%)b,c
Entry
Sample
Leaching (%)
Conversion (%)
Benzoic acid
Methyl benzoate
1
2
3
4
5
6
7
8
–
PW
SiW
PMo
–
–
–
–
4.1
94.3
46.9
39.7
–
–
–
–
6
36
17
32
14
75
67
63
6
9
4
25
14
15
12
52
–
27
13
7
Fe@Si
–
–
PW/Fe@Si
SiW/Fe@Si
PMo/Fe@Si
6.7
8.3
6.2
60
55
11
a
The number of acidic centers was determined by pyridine adsorption.
Conditions: catalyst (0.1 mol%), benzaldehyde (1 mmol); H2O2 (4 mmol); methanol (3 mL); Room temperature; 17 h.
The isolated yields were determined based on benzaldehyde.
b
c
Fig. 6. NH3-TPD profiles of (a) Fe@Si, (b) PW/Fe@Si, (c) SiW/Fe@Si and (d) PMo/Fe@Si
samples.
◦
Fig. 8. FTIR spectra of (a) PW/Fe@Si, (b) PW/Fe@Si calcined at 250 C and (c)
◦
PW/Fe@Si calcined at 250 C after reaction with 0.1 mol of pyridine per gram of
catalyst [34].
PW/Fe@Si > SiW/Fe@Si > PMo/Fe@Si. It reveal that the PW/Fe@Si
catalyst possess strongest acid sites in comparison with SiW/Fe@Si
and PMo/Fe@Si catalysts. The desorbed amount of NH3 from these
acid sites was calculated and summarized in Table 3. It is evident
that PW/Fe@Si catalyst has relatively higher number of acid sites
compared to SiW/Fe@Si and PMo/Fe@Si catalysts.
(b)
PW
PW
The TPD results are not similar to the acidity data measured by
pyridine chemisorption in liquid phase. This difference is mainly
Fe
Fe
Fe
Table 3
Fe
The acid strength distribution of the catalysts calculated by the results of NH3-TPD.
(
a)
Catalyst
Total acidity (mmol/g)
Aciditydistribution(mmol/g)
Weaka
Strongb
0
10
20
30
40
50
60
70
80
90
Fe@Si
1.33
3.31
2.09
1.95
0.25
0.74
0.62
0.62
1.08
2.57
1.47
1.33
PW/Fe@Si
SiW/Fe@Si
PMo/Fe@Si
2
θ ( )
◦
Fig. 7. XRD patterns of (a) PW/Fe@Si and (b) PW/Fe@Si calcined at 250 C.
a
◦
◦
Desorption temperature: 130–300 C.
b
Desorption temperature: 400–600 C.

E. Rafiee, S. Eavani / Journal of Molecular Catalysis A: Chemical 373 (2013) 30–37
35
◦
◦
Fig. 9. SEM images of (a) PW/Fe@Si and (b) PW/Fe@Si calcined at 250 C, (c) TEM image of PW/Fe@Si calcined at 250 C, (d) elemental maps of Fe, Si and W atoms of PW/Fe@Si
catalyst calcined at 250 C.
◦
that the decreasing of infrared absorption peak at 3450 cm 1 (cor-
responding to bridging hydroxyl groups) in calcined sample, might
result in the dehydroxylation between PW anions and the silanol
groups on the surface of Fe@Si support.
−
due to differences between measurements in gas- and liquid-phase.
In addition, pyridine and ammonia have different size, basicity
and adsorption capability, and comparison is not straight. How-
ever, it can be conclude, independent of the measurement method,
PW/Fe@Si showed strongest acidity and highest number of acid
sites among the others. As a result, PW/Fe@Si showed better results
in terms of strength and number of acid sites, and leaching stabil-
ity than any others and chosen as the suitable catalyst for more
investigations.
Brönsted or Lewis acidity of the PW/Fe@Si catalyst was distin-
guished by studying the FTIR spectrum of the catalyst after reaction
with pyridine (Fig. 8). Pyridine molecules were adsorbed on Lewis
−
1
acid sites (1610 and 1450 cm ) and formed the pyridinium ion
−
1
by interaction with Brönsted acid sites (1640 and 1540 cm ) [42].
−
1
Both types of adsorbed species contribute to the band at 1490 cm
42]. FTIR spectrum of the catalyst shows contribution of pyri-
dine adducts in the region 1400–1700 cm . The formation of
pyridinium ion was observed by absorptions at 1488, 1542 and
[
3.2. Characterization of PW/Fe@Si catalyst
−
1
Stability of the PW anion upon calcination was investigated
−
1
1
648 cm (expanding region in Fig. 8). It should be mentioned that
by XRD and FTIR spectroscopic analysis. The XRD pattern for the
PW/Fe@Si catalyst is shown in Fig. 7. Silica shows a broad band cen-
−
1
absorption band at 1648 cm is not totally conclusive although
considered as pyridinium ion because of contributions of pyri-
dinium ion and moisture, since the spectrum was taken at ambient
conditions.
◦
◦
tered at 2Â = 25 . ␥-Fe O displays typical sharp peaks at 2Â = 30.4 ,
2
◦
3
◦
◦
◦
3
2
5.5 , 55.7 , and 62.5 and PW displays reflections at 2Â = 9.7 and
6.3 [40,41]. The presence of the intense peaks in calcined sample
◦
The morphological features of the PW/Fe@Si catalyst were
investigated by SEM and TEM techniques. Fig. 9(a) shows the
SEM micrograph of the catalyst with different magnifications. The
particles were almost spherical, regular in shape and dispersed
uniformly. However, some particle aggregations were clearly
indicates that there is no significant change in the structure of the
PW and its crystalline character after calcination.
The four fingerprint peaks at approximately 1080, 983, 895 and
−
1
8
09 cm
assigned to Keggin anions of PW are clearly observed
◦
when the sample was calcined at 250 C (Fig. 8). It should be noted

3
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E. Rafiee, S. Eavani / Journal of Molecular Catalysis A: Chemical 373 (2013) 30–37
Table 4
CHO
COOH
COOMe
PW/Fe@Si
H O , MeOH, r.t.
+
2
2
Optimization of the oxidative esterification of benzaldehyde with methanola
.
Entry
Catalyst (mol%)
MeOH (mL)
H2O2 (mmol)
Time (h)
Conversion (%)
Ester selectivity (%)
1
2
3
4
5
6
7
8
9
0.1
0.1
0.1
0.15
0.20
0.25
0.30
0.25
0.25
0.25
3
3
3
3
3
3
3
2
4
5
4
6
8
6
6
6
6
6
6
6
17
17
17
17
15
12
12
12
8
75
98
98
98
98
98
98
70
98
98
80
71
74
81
95
100
100
95
100 (96, 91, 87)b
91
1
0
8
a
The isolated yields were determined based on benzaldehyde.
Results in parentheses are second; third and fourth runs. The conversion remained constant during four successive runs.
b
◦
observed. After calcination at 250 C, the relatively smooth surface
could be easily noted and it was attributed partial collapse of the
NPs as well as sintering of the particles (Fig. 9(b)).
methanol were chosen as test substrates to react in the presence
of hydrogen peroxide as oxidant. The conversion was improved by
increasing the amount of oxidant from 4 to 6 mmol, but selectivity
TEM observation indicated that PW/Fe@Si NPs have well-
defined core-shell structure (Fig. 9(c)). Wavelength dispersive
X-ray (WDX) image studied that are map of individual elements
of iron, silica and tungsten elements in cross-section were shown
in Fig. 9(d). It showed an excellent uniform distribution of PW on
Fe@Si NPs. According to WDX analysis, the atomic Si/Fe and W/Fe
ratios in PW/Fe@Si NPs were 0.795 and 5.86 respectively, which are
remained relatively constant. Further increasing of H O2 amount
2
to 8 mmol had no effect on the selectivity (Table 4, entries 1–3). The
selectivity to ester product increased with increasing the amount
of the catalyst and only ester product was observed using 25 mg
of the catalyst in relatively short reaction time (Table 4, entries 2,
4–7). The yield of the ester was decreased to 70% by using 2 mL
of methanol as the solvent (Table 3, entry 8). In contrast, 98% of
methyl benzoate (with 100% selectivity) was achieved with 4 mL of
◦
close to the values obtained for PW/Fe@Si calcined at 250 C (Si/Fe:
0.722 and W/Fe: 5.78).
The size distribution of the ␥-Fe O , Fe@Si and PW/Fe@Si NPs
2
3
derived from a laser particle size analyzer, illustrated in Fig. 10.
-Fe O NPs had a mean diameter of 70.43 nm and a wide size
␥
2
3
distribution with a polydispersity of 0.719. In this case, the mag-
netostatic interactions between particles can cause agglomeration.
Fe@Si NPs showed a mean diameter equal to 91.28 nm with poly-
dispersity of 0.352. The relatively narrow size distribution implied
that the Fe@Si NPs were highly stable in suspension, with the sil-
ica coating. PW functionalized Fe@Si NPs yielded a hydrodynamic
diameters of 91.77 nm (polydispersity of 0.48), which corresponds
to a size increase of about 21 nm in comparison with ␥-Fe O3 NPs.
2
3.3. Catalytic activity of PW/Fe@Si catalyst
In order to examine the effects of catalyst on the direct trans-
formation of aldehydes to corresponding esters, benzaldehyde and
Table 5
a
PW/Fe@Si catalyzed oxidative esterification of aldehydes with methanol
.
CHO
PW/Fe@Si
COOH
COOMe
+
H O , MeOH,
2
2
R
8
h, r.t.
R
R
Entry
Aldehyde
Conversion (%)b
Ester selectivity (%)b
1
2
3
4
5
6
7
8
9
H
98
90
90
95
98
98
98
95
90
85
100
100
94
100
96
100
100
95
4-NO2
3-NO2
3-OH
4-OH
4-CH3
4-OCH3
4-Cl
4-Br
3,4-OCH3
94
98
1
0
a
Conditions: aldehyde (1 mmol); H2O2 (6 mmol); catalyst (0.25 mol%); methanol
(
4 mL); Room temperature; 8 h.
Conversion and selectivity were calculated from the yield of isolated products.
b
Fig. 10. Grain size distribution of (a) ␥-Fe O , (b) Fe@Si and (c) PW/Fe@Si NPs.
2
3

E. Rafiee, S. Eavani / Journal of Molecular Catalysis A: Chemical 373 (2013) 30–37
37
methanol as the solvent in shorter reaction time (Table 4, entry 9).
References
Reusability of PW/Fe@Si catalyst was investigated under the opti-
mized reaction conditions (Table 4, entry 9). The catalyst could be
facilely recycled through magnetic separation and no significant
loss of activity and selectivity was found even after four subsequent
runs.
With the best reaction conditions in our hand (Table 4,
entry 9), we decided to test the generality and efficiency of
this methodology (Table 5). In general, the corresponding aro-
matic esters were obtained in good to excellent yields regardless
of the electron-donating or electron-withdrawing group in the
benzene ring.
[
[
1] H.F. Rase, Handbook of Commercial Catalysts: Heterogeneous Catalysts, CRC
Press, New York, 2000.
2] C.W. Lim, I.S. Lee, Nano Today 5 (2010) 412–434.
[3] A. Schätz, M. Hager, O. Reiser, Adv. Funct. Mater. 19 (2009) 2109–2115.
[4] Y. Zheng, P.D. Stevens, Y. Gao, J. Org. Chem. 71 (2006) 537–542.
[5] C. Che, W. Li, S. Lin, J. Chen, J. Zheng, J. Wu, Chem. Commun. (2009) 5990–5992.
[6] C.S. Gill, B.A. Price, C.W. Jones, J. Catal. 251 (2007) 145–152.
[7] D. Guin, B. Baruwati, S.V. Manorama, Org. Lett. 9 (2007) 1419–1421.
[
8] K.S. Lee, M.H. Woo, H.S. Kim, E.Y. Lee, I.S. Lee, Chem. Commun. (2009)
780–3782.
3
[
9] L. Gao, J. Wu, S. Lyle, K. Zehr, L. Cao, D. Gao, J. Phys. Chem. C 112 (2008)
17357–17367.
[10] L. Chien, C. Lee, Biotechnol. Bioeng. 100 (2008) 223–230.
11] E. Rafiee, S. Eavani, Green Chem. 13 (2011) 2116–2122.
12] L.R. Pizzio, C.V. Cáceres, M.N. Blanco, Appl. Surf. Sci. 151 (1999) 91–101.
[13] E. Caliman, J.A. Dias, S.C.L. Dias, A.G.S. Prado, Catal. Today 107–108 (2005)
16–825.
14] D.P. Sawant, A. Vinub, N.E. Jacob, F. Lefebvre, S.B. Halligudi, J. Catal. 235 (2005)
41–352.
[
[
4
. Conclusions
8
[
The catalyst Fe@Si-PW was prepared using different solvents
3
by the impregnation-evaporation technique. The results clearly
indicated the dependence of acidity, activity and leaching stabil-
ity of supported catalyst on different basic points, as which are
the impregnating solutions, calcination temperatures and the type
of HPA. The best preparation achieved was to impregnate PW on
Fe@Si NPs in MeCN solution, followed by evaporation of solvent
[15] M. Masteri-Farahani, J. Movassagh, F. Taghavi, P. Eghbali, F. Salimi, Chem. Eng.
J. 184 (2012) 342–346.
[
[
16] X. Cui, D. Yao, H. Li, J. Yang, D. Hu, J. Hazard. Mater. 205–206 (2012) 17–23.
17] Z. Zhang, F. Zhang, Q. Zhu, W. Zhao, B. Ma, Y. Ding, J. Colloid Interface Sci. 360
(2011) 189–194.
[18] M. Kooti, M. Afshari, Mater. Res. Bull. 47 (2012) 3473–3478.
[19] Y.L. Shi, W. Qiu, Y. Zheng, J. Phys. Chem. Solids 67 (2006) 2409–2418.
[20] J. Yuan, P. Yue, L. Wang, Powder Technol. 202 (2010) 190–193.
◦
and calcination at 250 C. The Fe@Si-PW NPs are mostly spher-
[21] U. Jeong, X. Teng, Y. Wang, H. Yang, Y. Xia, Adv. Mater. 19 (2007) 33–60.
[
22] S. Laurent, D. Forge, M. Port, A. Roch, C. Robic, L.V. Elst, R.N. Muller, Chem. Rev.
08 (2008) 2064–2110.
23] E. Darezereshki, M. Ranjbar, F. Bakhtiari, J. Alloys Compd. 502 (2010) 257–260.
ical in shape and have an average size of approximately 92 nm.
Experiments assessing the stability of supported HPA catalysts
indicated their resistance to leaching by methanol as polar sol-
1
[
[24] W. Stöber, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62–69.
[25] L.A. Parreira, N. Bogdanchikova, A. Pestryakov, T.A. Zepeda, I. Tuzovskaya, M.H.
Farías, E.V. Gusevskaya, Appl. Catal. A: Gen. 397 (2011) 145–152.
vent. The acidity of the samples was determined by NH -TPD
3
and chemisorption of pyridine. The strength and dispersion of the
protons on PW/Fe@Si NPs was considerably high and active sur-
face protons became more available for reactant. Catalytic activity
of HPA/Fe@Si and pure HPA catalysts was examined in oxida-
tive esterification of benzaldehyde with methanol. The Fe@Si-PW
NPs was found as an effective magnetically recoverable hetero-
geneous catalyst for the liquid-phase oxidative esterification of
a wide range of arylaldehydes with hydrogen peroxide as green
oxidant.
[
[
26] C. Noonan, L. Baragwanath, S.J. Connon, Tetrahedron Lett. 49 (2008) 4003–4006.
27] S. Kiyooka, M. Ueno, E. Ishii, Tetrahedron Lett. 46 (2005) 4639–4642.
[28] N.N. Karade, V.H. Budhewar, A.N. Katkar, G.B. Tiwari, ARKIVOC xi (2006)
62–167.
[
1
29] R.K. Sharma, S. Gulati, J. Mol. Catal. A: Chem. 363–364 (2012) 291–303.
30] Y. Diao, R. Yan, S. Zhang, P. Yang, Z. Li, L. Wang, H. Dong, J. Mol. Catal. A: Chem.
303 (2009) 35–42.
[
[
31] J. Otera, Esterification: Methods Reactions and Applications, Wiley, New York,
2003.
[
32] R.C. Larock, Comprehensive Organic Transformations, VCH, New York, NY,
1989.
[
[
33] Y. Deng, D. Qi, C. Deng, X. Zhang, D. Zhao, J. Am. Chem. Soc. 130 (2008) 28–29.
34] J.A. Dias, E. Caliman, S.C.L. Dias, M. Paulo, A.T.C.P. de Souza, Catal. Today 85
Acknowledgments
(
2003) 39–48.
[
[
[
35] T.V. Nguyen, K.J. Kim, O.B. Yang, J. Photochem. Photobiol. A 173 (2005) 56–63.
36] J.G. Highfield, J.B. Moffat, J. Catal. 95 (1985) 108–119.
37] M.T. Pope, Heteropoly and Isopoly Oxometalates, Springer-Verlag, Berlin, 1983.
We thank the Razi University Research Council and Iran National
Science Foundation (INSF) for support of this work.
[38] R. Rinaldi, F.Y. Fujiwara, W. Hölderich, U. Schuchardt, J. Catal. 244 (2006)
2–101.
9
[
[
39] C. Pazé, S. Bordiga, A. Zecchina, Langmuir 16 (2000) 8139–8144.
40] E. Rafiee, S. Eavani, S. Rashidzadeh, M. Joshaghani, Inorg. Chim. Acta 362 (2009)
Appendix A. Supplementary data
3
555–3562.
41] E. Zhang, Y. Tang, K. Peng, C. Guo, Y. Zhang, Solid State Commun. 148 (2008)
96–500.
[42] D.P. Sawant, A. Vinu, N.E. Jacob, F. Lefebvre, S.B. Halligudi, J. Catal. 235 (2005)
341–352.
[
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
4
2
013.02.024.