PdNPs@P2VP-Fe3O4 organic-inorganic hybrid microgels as a nanoreactor for selective aerobic oxidation of alcohols

Article abstract of DOI:10.1007/s10562-013-1107-2

The preparation of organic-inorganic hybrid microgels with functional Fe₃O₄ nanoparticles as the crosslinker and 2-vinyl pyridine (2VP) as the pH-sensitive monomer is reported. Magnetic Fe ₃O₄ nanoparticles were first synthesized via a redox reaction and then modified with 3-(trimethoxysilyl)propylmethacrylate (TMSPMA) via the silanization. The bonding of multiple TMSPMA monomers on the surface of Fe₃O₄ nanoparticles renders them as crosslinker. Radical polymerization of 2VP was then carried out with the presence of TMSPMA-modified Fe₃O₄ nanoparticles, leading to the formation of P2VPFe₃O₄ hybrid microgels crosslinked with Fe ₃O₄ nanoparticles. The synthesized hybrid microgels are able to trap metal ions such as Pd2? via complex formation of P2VP with metal ions. The reduction of trapped palladium ions by sodium borohydride led to immobilized-palladium nanoparticles. The synthesized PdNPs@P2VP-Fe ₃O₄ is employed as a new nanocatalyst toward oxidation reaction of alcohols. In addition, the catalytic activity of metal nanocomposite has been found that can be modulated by the volume transition of microgel structures by pH change for the oxidation reaction of benzyl alcohol. Transmission electron microscopy, thermogravimetric analysis, vibrating sample magnetometer, scanning electron microscopy, X-ray powder diffraction, Atomic absorption and Fourier transform Infrared were then used to characterize the resultant catalyst. Springer Science+Business Media New York 2013.

Full text of DOI:10.1007/s10562-013-1107-2

Catal Lett (2014) 144:355–363
DOI 10.1007/s10562-013-1107-2
PdNPs@P2VP-Fe3O4 Organic–Inorganic Hybrid Microgels
as a Nanoreactor for Selective Aerobic Oxidation of Alcohols
•
Mohammad Reza Nabid Yasamin Bide
•
•
Elham Aghaghafari Seyed Jamal Tabatabaei Rezaei
Received: 16 June 2013 / Accepted: 12 September 2013 / Published online: 12 October 2013
Ó Springer Science+Business Media New York 2013
Abstract The preparation of organic–inorganic hybrid
microgels with functional Fe3O4 nanoparticles as the
crosslinker and 2-vinyl pyridine (2VP) as the pH-sensitive
monomer is reported. Magnetic Fe3O4 nanoparticles were
first synthesized via a redox reaction and then modified
with 3-(trimethoxysilyl)propylmethacrylate (TMSPMA)
via the silanization. The bonding of multiple TMSPMA
monomers on the surface of Fe3O4 nanoparticles renders
them as crosslinker. Radical polymerization of 2VP was
then carried out with the presence of TMSPMA-modified
Fe3O4 nanoparticles, leading to the formation of P2VP-
Fe3O4 hybrid microgels crosslinked with Fe3O4 nanopar-
ticles. The synthesized hybrid microgels are able to trap
metal ions such as Pd2? via complex formation of P2VP
with metal ions. The reduction of trapped palladium ions
by sodium borohydride led to immobilized-palladium
nanoparticles. The synthesized PdNPs@P2VP-Fe3O4 is
employed as a new nanocatalyst toward oxidation reaction
of alcohols. In addition, the catalytic activity of metal
nanocomposite has been found that can be modulated by
the volume transition of microgel structures by pH change
for the oxidation reaction of benzyl alcohol. Transmission
electron microscopy, thermogravimetric analysis, vibrating
sample magnetometer, scanning electron microscopy,
X-ray powder diffraction, Atomic absorption and Fourier
transform Infrared were then used to characterize the
resultant catalyst.
Keywords Magnetic nanoparticles Á Organic–
inorganic hybrid microgels Á Palladium nanoparticles Á
Alcohol oxidation
1 Introduction
Metal nanoclusters which are soluble metal nanoparticles
in the size range 1–10 nm, are highly active catalysts.
Generally, metal nanoclusters are formed from metal atom
precursors in the presence of a stabilizer capable to interact
with their surface, therefore preventing agglomeration and
controlling the size of nanoparticles. Moreover, from the
viewpoint of green chemistry, easy separation of the cat-
alyst, low temperature, and low leaching of heavy metal
into the product has become crucial for the catalysis
industry [1, 2]. Recently, size-controlled metal nanoclus-
ters that are stabilized by microgels have found hopeful
applications in the field of catalysis. Specially, microgel
systems because of thermodynamic transition that occurs
within the system can act as ‘‘active carriers’’ for the metal
nanoparticles, which permit us to modulate the catalytic
activity of nanoparticles [3]. The organic–inorganic hybrid
microgels have been reported in recent years [4–14]. The
organic–inorganic hybrid materials possess and combine
the properties of organic and inorganic components at the
same time, which give them more superior microstructures
and properties. Numerous attempts have been devoted to
the preparation and characterization of organic–inorganic
hybrid materials [4, 12, 13, 15–17].
M. R. Nabid (&) Á Y. Bide Á E. Aghaghafari Á S. J. T. Rezaei
Department of Polymer, Faculty of Chemistry, Shahid Beheshti
University, G.C., P.O. Box 1983963113, Tehran, Iran
e-mail: m-nabid@sbu.ac.ir
Among typical classes of organic–inorganic hybrid mi-
crogels, the easiest synthetic approach that does not need
any conventional chemical crosslinker is in situ formation
of hybrid microgels via covalent crosslinking with func-
tional inorganic nanoparticles which actually act as
S. J. T. Rezaei
Department of Chemistry, Zanjan University, P.O. Box
45195-313, Zanjan, Iran
123

356
M. R. Nabid et al.
solvents. Thermogravimetric analysis (TGA) was carried
out using STA 1500 instrument at a heating rate of
10 °C min-1 in air. The magnetization of the samples in a
variable magnetic field was measured using a vibrating
OH
O
PdNPs@P2VP-Fe3O4
water, O2, 80 oC
R1
R2
R1
R2
sample magnetometer (VSM) with
10-3 EMU and a magnetic field of up to 8 kOe. The mag-
netic field was changed uniformly at a rate of 66 Oe s-1
a sensitivity of
Scheme 1 PdNPs@P2VP-Fe3O4 as the catalyst for aerobic oxidation
of alcohols
.
chemical crosslinkers. Currently, just a few reports exist
about this class of organic–inorganic hybrid microgels.
Recently, the attention of many researchers has paid to
palladium nanoparticles supported on insoluble solids as a
new generation of heterogeneous catalysts in various scien-
tific fields because of their good stability, superior catalytic
performance, ease of separation and satisfactory reusability
[18–27]. There are several reports of palladium nanoparticles
catalyzed aerobic oxidation of alcohols [28, 29].
The scanning electron microscopic images were recorded
using Philips scanning electron microscope model XL 30.
X-ray powder diffraction (XRD) data were collected on an
XD-3A diffractometer using Cu Ka radiation.
2.3 Preparation and Surface Modification of Fe3O4
Magnetic Nanoparticles
Fe3O4 magnetic nanoparticles were prepared by coprecipi-
tating iron(II) and iron(III) in alkaline solution according to
literature method [35]. At first, 0.54 g FeCl3Á6H2O and
0.3 g FeSO4Á7H2O were taken in 40 mL Millipore water
under argon atmosphere. The aqueous ammonia solution
(1.5 M) was dropped into the reaction vessel with violent
stirring. Then, the reaction mixture was heated at 80 °C, and
the pH of the medium was maintained at 10 by the addition
of aqueous NH4OH (2.5 M) during the reaction. The
obtained magnetite was washed immediately with water
five times and with ethanol twice by magnetic separation.
After that, the resulting magnetic nanoparticles were dried
in a vacuum oven at 50 °C. Fe3O4 nanoparticles were red-
ispersed into 45 ml deionized water. Afterward, 0.20 ml of
3-(trimethoxysilyl)propylmethacrylate (TMSPMA) was
injected into the Fe3O4 nanoparticles solution under
mechanical stirring at 25 °C. The surface silanization of
Fe3O4 magnetic nanoparticles was allowed to proceed for
24 h. Surface modified Fe3O4 magnetic nanoparticles were
then collected and washed with deionized water for several
times with the help of a magnet.
In recent years our group has been working on the
application of catalysts for the development of useful new
synthetic methodologies [30–33], and also the synthesis of
microgel for controlled drug release [34]. Herein we report
the synthesis and characterization of P2VP-Fe3O4 organic–
inorganic hybrid microgels and its application for stabi-
lizing palladium nanoparticles. Palladium nanoparticles
immobilized on P2VP-Fe3O4 (PdNPs@P2VP-Fe3O4) was
employed as a nanoreactor for aerobic oxidation of alco-
hols to corresponding aldehydes or ketones (Scheme 1).
2 Experimental
2.1 Materials
FeCl3Á6H2O and FeSO4Á7H2O were procured from Merck.
NH4OH were received from Merck. 3-(Trimethoxysilyl)
propylmethacrylate (TMSPMA: 98 %) were purchased
from Acros Organics. Toluene was dried from sodium.
Palladium(II) chloride was purchased from Merck. All
other solvents and reagents were purchased from Aldrich or
Merck and used without further purification unless other-
wise stated.
2.4 Synthesis of P2VP-Fe3O4 Organic–Inorganic
Hybrid Microgels
Surface modified Fe3O4 magnetic nanoparticles (0.15 g)
and 60 ml of toluene were charged into the 100 ml flask
then exposed the mixture in ultrasonication for 10 min.
After dispersion, 2.5 g of 2-vinylpyridine monomer was
dissolved in the mixture. AIBN (0.025 g, 1 wt% to
2-vinylpyridine) was injected into the flask and the reaction
vessel was heated to 70 °C to initiate solution polymeri-
zation for 24 h with stirring under N2 atmosphere. After
polymerization, the solution was washed with acetone once
and free poly(2-vinylpyridine) (P2VP) was extracted out in
Soxhlet apparatus using boiling methanol for 36 h. Finally,
P2VP-Fe3O4 organic–inorganic hybrid microgels were
prepared and characterized (Scheme 1).
2.2 Instruments and Characterization
IR spectra were recorded on a Bomem MB-Series Fourier
transform Infrared (FT-IR) spectrophotometer. Transmis-
sion electron microscopy (TEM) analyzes were performed
by LEO 912AB electron microscope. Catalysis products
were analyzed using a Varian 3900 GC (GC conversions
were obtained using n-decane as an internal standard based
on the amount of alcohol employed relative to authentic
standard product). Ultrasonic bath (EUROSONICÒ 4D
ultrasound cleaner with a frequency of 50 kHz and an
output power of 350 W) was used to disperse materials in
123

A Nanoreactor for Selective Aerobic Oxidation of Alcohols
2.5 Preparation of PdNPs@P2VP-Fe3O4
357
CH2Cl2 (6.0 mL). Then the organic phase was combined
and removed the solvent under vacuum to give the pure
product (Scheme 1).
H2PdCl4 (0.15 M) aqueous solution was first prepared by
mixing PdCl2 and HCl (mole ratio was 1:2) in water at
50 °C with a vigorous stir. Aqueous solution of H2PdCl4
(0.01 mL, 0.15 M) and P2VP-Fe3O4 (0.1 g in 10 mL) were
mixed and placed in an ultrasonic bath (50 kHz) for 10 min
to well disperse metal ions in the hybrid material. Mixture
was stirred at room temperature for 8 h and the reduction
was carry out by the addition 0.5 mL of aqueous solution
of NaBH4 (0.01 M) to the mixture and stirring at room
temperature for 1 h. It was filtered under vacuum, washed
well with ethanol and water (2 9 20 mL) and dried under
vacuum at 50 °C for 4 h (Scheme 2). The atomic absorp-
tion spectroscopy (AAS) and TGA were used to determine
the amount of palladium in the synthesized catalyst.
For the blank test with the support and without Pd, the
oxidation reaction of benzyl alcohol was accomplished
with 0.0013 g P2VP-Fe3O4 instead of PdNPs@P2VP-
Fe3O4.
3 Results and Discussion
3.1 Synthesis and Characterization of the Supported
Catalyst
The synthesis procedure for the PdNPs@P2VP-Fe3O4
organic–inorganic hybrid microgels consists of the fol-
lowing four steps as shown schematically in Schemes 1
and 3: (i) synthesis of magnetic nanoparticles by co-pre-
cipitation from Fe2? and Fe3? using NH4OH solution; (2)
the surface silanization of Fe3O4 magnetic nanoparticles by
using TMSPMA; (3) the synthesis of P2VP-Fe3O4 hybrid
microgels by using TMSPMA-modified Fe3O4 nanoparti-
cles as the crosslinker and 2VP as the monomer by simple
solution polymerization (Scheme 1); (4) introducing Pd2?
ions into the P2VP-Fe3O4 organic–inorganic hybrid mi-
crogels and subsequently reducing by sodium borohydride
to obtain PdNPs@P2VP-Fe3O4 (Scheme 3).
2.6 General Procedure for the Aerobic Oxidation
of Alcohols
A
mixture of PdNPs@P2VP-Fe3O4 (0.2 mol% Pd,
0.0013 g) in water (3.0 mL) was placed into a two-necked
flask equipped with a magnetic stirrer. The flask was
evacuated and refilled with pure oxygen (balloon filled) for
4 times. Then the benzyl alcohol (0.108 g, 1.0 mmol) in
water (3.0 mL) was added into the solution with a syringe.
The pH of the solution was adjusted in 5, and the resulting
mixture was vigorously stirred at 80 °C under O2 atmo-
sphere for 4 h. After the reaction, the catalyst was removed
via an external magnetic field and washed twice with
The palladium amount of synthesized PdNPs@P2VP-
Fe3O4 was determined by AAS. The data showed that the
weight percent of palladium nanoparticles on P2VP-Fe3O4
is 16.9 wt%.
Scheme 2 Synthesis route of
PdNPs@P2VP-Fe3O4 hybrid
microgels
123

358
M. R. Nabid et al.
FT-IR of Fe3O4 magnetic nanoparticles, TMSPMA-
nanoparticles via the silanization reaction [36–38]. More-
over, the chemical grafting of P2VP chains on Fe3O4
nanoparticles is verified by means of FT-IR analyses. In the
spectrum of P2VP- Fe3O4 hybrid microgels, bands at 1,412
and 1,592 cm-1 are characteristics peaks of pyridine ring.
Characteristic absorption bands of Fe3O4 and TMSPMA
were also observed. The absorption bands of vinyl groups
at 1,632 cm-1 was not observed anymore for P2VP-Fe3O4
hybrid microgels, indicating that all of the carbon–carbon
double bonds (C=C) have taken part in the free radical
polymerization. FT-IR spectrum of PdNPs@P2VP-Fe3O4,
apart from characteristic absorption bands of Fe3O4,
TMSPMA and P2VP shows a peak at 520 cm-1 which is
attributed to the Pd–N stretching [39].
modified Fe3O4 nanoparticles, P2VP-Fe3O4 hybrid micro-
gels, and PdNPs@P2VP-Fe3O4 catalyst are shown in
Fig. 1a–d, respectively.
In both spectra of Fe3O4 and TMSPMA-modified Fe3O4
nanoparticles, the characteristic absorption band of Fe3O4
appears at 586 cm-1. The broad band appeared in the
region 3,200–3,600 cm-1, is attributed to the OH stretch-
ing vibration. In aqueous environment, the surfaces of
Fe3O4 nanoparticles were readily covered with hydroxyl
groups. Most of the peaks in the spectrum of TMSPMA-
modified Fe3O4 nanoparticles are the characteristic
absorption bands of TMSPMA. For example, bands at
2,923 and 2,854 cm-1 are due to the stretching vibrations
of –CH2 group, band at 1,044 cm-1 is attributed to the
stretching vibration of Si–O bond, a strong absorption band
at 1,719 is due to the stretching vibration of the –CO–
groups. The band at 1,632 cm-1, corresponding to the
vinyl groups of TMSPMA units on TMSPMA-modified
Fe3O4, represents that the carbon–carbon double bonds
(C=C) have been successfully introduced onto the Fe3O4
nanoparticles. So, the FT-IR results confirm that TMSPMA
was successfully grafted onto the surface of Fe3O4
The results from the VSM measurements show that the
nanoparticles are super-paramagnetic (Fig. 2) with zero
values for both remanence and coercivity in the hysteresis
loop.
The TEM observation indicates that the P2VP-Fe3O4
hybrid microgels contained many small black spheres,
which were Fe3O4 nanoparticles, as shown in Fig. 3.
The TEM images confirmed that the TMSPMA-Fe3O4
nanoparticles actually act as crosslinkers. The covalent
Fe3O4
Fe3O4
Fe3
O4
Fe3O4
1) H2PdCl4
2) NaBH4
Fe3O4
Fe3O4
Fe3O4
Fe3O4
Fe3O4
Fe3O4
Fe3O4
Fe3O4
Scheme 3 Synthesis of P2VP-Fe3O4 organic–inorganic hybrid microgels
Fig. 1 FT-IR spectra of Fe3O4 magnetic nanoparticles, TMSPMA-modified Fe3O4 nanoparticles, P2VP-Fe3O4 hybrid microgels, and
PdNPs@P2VP-Fe3O4, A–D, respectively
123

A Nanoreactor for Selective Aerobic Oxidation of Alcohols
359
bonds of TMSPMA participated in the radical polymeri-
zation of 2VP. In other word, the TMSPMA modified
Fe3O4 nanoparticles are like chemical crosslinkers with
multiple function groups, leading to the successful forma-
tion of hybrid microgels. Also, TEM experiment indicates
that palladium nanoparticles were relatively uniformly
dispersed on P2VP-Fe3O4 which the main average size of
palladium nanoparticles is around 2–4 nm. This is because
of the existence of a large number of nitrogen containing
pyridine groups, as anchoring sites on the surface of Fe3O4.
XRD pattern of P2VP-Fe3O4 was shown in Fig. 4a. The
six characteristic peaks occur at 2h of 30.2, 35.6, 43.3,
53.7, 57.2 and 62.8, which correspond to indices (2 2 0), (3
1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) of Fe3O4 MNPs,
respectively. This reveals that after being coated with
P2VP, the crystal structure of Fe3O4 is well maintained.
The XRD pattern of PdNPs@P2VP-Fe3O4 (Fig. 4b) has the
same characteristic peaks of P2VP-Fe3O4 with additional
peaks at 2h values of about 40.02, 46.59, and 68.03, cor-
responding to the planes (1 1 1), (2 0 0) and (2 2 0),
respectively which are characteristic of face centered cubic
(fcc) crystalline Pd. The size of Pd nanoparticles was cal-
culated by Scherrer equation from XRD pattern data, to be
3.1 nm [40]. The calculated size approximately matches to
the size observed from TEM image. Also the crystalline
size of Fe3O4 was estimated from XRD which is 12.1 nm.
TGA was performed for the quantitative analysis of
functionalization of Fe3O4 nanoparticles. Prior to TGA
measurements, freely adsorbed P2VP molecules were
removed by Soxhlet extraction using boiling methanol for
36 h. The weight loss curves of modified Fe3O4 nanopar-
ticles where 2VP monomer was incorporated in the in situ
polymerization process are shown in Fig. 5a.
Fig. 2 VSM curve of PdNPs@P2VP-Fe3O4
Fig. 3 TEM images of PdNPs@P2VP-Fe3O4
Fig. 4 XRD patterns of P2VP-
Fe3O4 (a), PdNPs@P2VP-
Fe3O4 (b)
Pd
B
Pd
Fe3O4
Fe3O4
Pd
Fe3O4
Fe3O4
Fe3O4
Fe3O4
Fe3O4
A
Fe3O4
Fe3O4
Fe3O4
123

360
M. R. Nabid et al.
For P2VP-grafted Fe3O4 nanoparticles, the main weight
the most important reactions in organic synthesis [41–47].
With the P2VP-Fe3O4 hybrid microgels supported palla-
dium catalyst in hand, we effort to evaluate its catalytic
activity for the aerobic oxidation of alcohols, and some
examples are summarized in Table 1.
loss is obtained for the decomposition of grafted P2VP in
the temperature range of 300–420 °C. Furthermore, the
TGA results showed that the amount of Fe3O4 nanoparti-
cles inside the P2VP-Fe3O4 hybrid microgels was ca.
32.2 wt%, as shown in Fig. 5a. The thermal properties of
PdNPs@P2VP-Fe3O4 were characterized by TGA tech-
nique that was shown in Fig. 5b. Comparing the two curves
demonstrates that the weight percent of palladium in syn-
thesized catalyst is about 17 % which is in good agreement
with AAS data.
Initially, the oxidation reaction of the benzyl alcohol as
the model substrate was selected, and was carried out in
Table 1 The aerobic oxidation of alcohols with PdNPs@P2VP-
Fe3O4 as the catalyst
O
OH
PdNPs@P2VP-Fe3O4 (x mol% Pd)
R1
R2
SEM image of the catalyst before and after the reaction
were shown in Fig. 6a, b respectively, which the results
presented the retained morphology of synthesized catalyst
after employing it in the reaction.
R1
Entry R1
R2
water, O2, 80 o
C
R2
x
t (h) Yielda (%) Selectivity (%)
1
Ph
H
H
H
0.15
0.2
5
4
89
96
[99
[99
[99
[99
[99
[99
[99
[99
[99
[99
[99
[99
[99
[99
[99
[99
2
3b
Ph
Ph
Ph
Ph
Ph
3.2 The Catalytic Activity of PdNPs@P2VP-Fe3O4
Toward the Oxidation Reactions of Alcohols
0.2
5.5 92
4
Me 0.3
Et 0.3
Ph 0.3
13
95
5
14.5 94
The selective oxidation of alcohols, especially employing
oxygen as the terminal oxidant, has been believed one of
6
18
94
7
4-MePh
4-MePh
4-MeOPh
4-FPh
H
0.2
3.5 97
8
Me 0.3
7
3
93
96
9
H
H
H
H
H
H
H
0.2
0.3
0.2
0.2
0.2
0.3
0.3
10
11
12
13
14
15
16
7.5 93
93
4.5 95
4-ClPh
6
4-BrPh
2-BrPh
5
6
9
7
74
66
95
95
2,4-diClPh
4-NO2Ph
PhCO
Ph 0.3
All the reactions were conducted with alcohol (1.0 mmol) in water
(3.0 mL) and PdNPs@P2VP-Fe3O4 (x mol% Pd) as catalyst in pH 5
at 80 °C under an oxygen atmosphere unless otherwise noted
a
Fig. 5 TGA curves of P2VP-Fe3O4 nanoparticles (a) and
PdNPs@P2VP-Fe3O4 hybrid microgels (b)
Isolated yield
b
Air was used instead of oxygen
Fig. 6 SEM images of PdNPs@P2VP-Fe3O4 before (a) and after the aerobic oxidation of benzyl alcohol (b)
123

A Nanoreactor for Selective Aerobic Oxidation of Alcohols
361
Scheme 4 Plausible pathways
for the oxidation of alcohols
over PdNPs@P2VP-Fe3O4
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Table 2 Effect of pH on the aerobic oxidation of benzyl alcohol with
PdNPs@P2VP-Fe3O4 as the catalyst
Entry
pH
Yield
1
2
3
4
5
6
7
8
96
68
32
10
All the reactions were conducted with benzyl alcohol (1.0 mmol) in
water (3.0 mL) and PdNPs@P2VP-Fe3O4 (0.2 mol% Pd) as catalyst
in at 80 °C
Fig. 7 Effect of recycling on the catalytic efficiency of
PdNs@P2VP-Fe3O4
water at 80 °C in pH 5. Within 4 h and with 0.2 mol% Pd,
96 % isolated yield of the benzaldehyde was obtained and
the selectivity was more than 99 % (Table 1, entry 2). In
another reaction, 0.15 mol% Pd was used and it causes
reaction to occur in longer time with decreased yield
(Table 1, entry 1). Therefore, the optimized amount of
catalyst was determined 0.2 mol% for model reaction.
Moreover, the starting material could also completely react
under air atmosphere although the reaction was prolonged
slightly (Table 1, entries 3). The oxidation of 2-substitute
benzyl alcohol was obtained in a moderate yield (Table 1,
entries 14, 15), which may be attributed to the steric hin-
drance by the surface of the palladium sites. The catalyst
also showed satisfactory performance for secondary alco-
hols, and the desired products were achieved in moderate
to excellent yields. In addition, the catalyst PdNPs@P2VP-
Fe3O4 showed good activity in the oxidation of other pri-
mary benzyl alcohols with substituents such as 4-Me,
4-MeO, 4-F, 4-Cl, 4-Br and 4-NO2 groups. No carboxylic
acids or esters were observed for all substrates investigated.
To verify the real catalytic species and to exclude the
possible role of the iron in the catalytic cycle, the oxidation
reaction of benzyl alcohol as a blank test with the support
and without Pd was carried out as a model reaction which
the reaction yield was 17 %. So, the catalytic activity of the
P2VP functionalized Fe3O4 is very low.
123

362
M. R. Nabid et al.
A hypothesis of the possible mechanism of catalytic
easy separation of the catalyst, using water as solvent and
relatively low temperature. These organic inorganic hybrid
microgels were found to be efficient and reusable catalysts
in oxidation reactions of primary and secondary benzyl
alcohols in short reaction time and good to excellent yields.
oxidation of alcohols over PdNPs@P2VP-Fe3O4 was pre-
sented in Scheme 4. Deprotonation of alcohol was promoted
at the basic sites of pyridine to from alkoxide on the Pd
surfaces. Pd catalyzes the b-hydride elimination to produce
corresponding aldehyde or ketone along with the formation
of O2 and H2O.
Acknowledgments We are grateful to Shahid Beheshti University
Research Council for partial financial support of this work.
We designed and synthesized the pH-sensitive microgel
system that allows us to modulate the catalytic activity of
microgel-stabilized nanoparticles by a thermodynamic
transition that takes place within the carrier system. At low
pH metallic nanoparticles embedded in such a network are
completely reachable by the reactants. But above the tran-
sition, the diffusion of the reactants within the network is
slowing down which is result of the marked shrinking of the
network. Consequently, the rate of reactions catalyzed by the
nanoparticles is decreased considerably. In this way, the
network acts as a nanoreactor which can be opened or closed
to a certain extent. To investigate the effect of pH, we
employed oxidation reaction of benzyl alcohol as model
reaction in different pHs (Table 2). The results indicated that
as expected, reaction was occurred efficiently at pH 5. But at
larger pH, catalyst efficacy was decreased significantly.
References
1. Anastas PT, Kirchhoff MM (2002) Acc Chem Res 35:686–694
2. Hashmi ASK, Hutchings GJ (2006) Angew Chem Int Ed Engl
45:7896–7936
3. Mei Y, Lu Y, Polzer F, Ballauff M, Drechsler M (2007) Chem
Mater 19:1062–1069
4. Karg M, Hellweg T (2009) J Mater Chem 19:8714–8727
5. Pich AZ, Adler HJP (2007) Polym Int 56:291–307
6. Ballauff M, Lu Y (2007) Polymer 48:1815–1823
7. Das M, Zhang H, Kumacheva E (2006) Annu Rev Mater Res
36:117–142
8. Nayak S, Lyon LA (2005) Angew Chem Int Ed Engl 44:
7686–7708
9. Agrawal M, Rubio-Retama J, Zafeiropoulos NE, Gaponik N,
Gupta S, Cimrova V, Lesnyak V, Lopez-Cabarcos E, Tzavalas S,
Rojas-Reyna R, Eychmuller A, Stamm M (2008) Langmuir
24:9820–9824
10. Gong YJ, Gao MY, Wang DY, Mohwald H (2005) Chem Mater
17:2648–2653
3.3 Recycling PdNPs@P2VP-Fe3O4
11. Rubio-Retama J, Zafeiropoulos NE, Frick B, Seydel T, Lopez-
Cabarcos E (2010) Langmuir 26:7101–7106
12. Hoffmann F, Cornelius M, Morell J, Froba M (2006) Angew
Chem Int Ed Engl 45:3216–3251
13. Sanchez C, Lebeau B, Chaput F, Boilot JP (2003) Adv Mater
15:1969–1994
14. Sanchez C, Soler-Illia G, Ribot F, Lalot T, Mayer CR, Cabuil V
(2001) Chem Mater 13:3061–3083
15. Palioura D, Armes SP, Anastasiadis SH, Vamvakaki M (2007)
Langmuir 23:5761–5768
The recyclability and reusability of the PdNPs@P2VP-
Fe3O4 catalyst was tested for the aerobic oxidation reaction
of benzyl alcohol up to ten cycles. The results shown in
Fig. 7 demonstrate that after every run, the yield of product
does not change indicating the fair stability of catalyst
under experimental conditions. In addition, the magnetic
extraction precludes the need for filtration or centrifugation
steps and the workup of the final reaction mixture to
recover the catalyst.
16. Pich A, Karak A, Lu Y, Ghosh AK, Adler HJP (2006) J Nanosci
Nanotechnol 6:3763–3769
17. Lu Y, Proch S, Schrinner M, Drechsler M, Kempe R, Ballauff M
(2009) J Mater Chem 19:3955–3961
18. Lamblin M, Nassar-Hardy L, Hierso JC, Fouquet E, Felpin FX
(2010) Adv Synth Catal 352:33–79
4 Conclusion
19. Kim JY, Jo Y, Kook SK, Lee S, Choi HC (2010) J Mol Catal A
323:28–32
20. Patel SA, Patel KN, Sinha S, Kamath BV, Bedekar AV (2010) J
Mol Catal A 332:70–75
21. Makhubela BCE, Jardine A, Smith GS (2011) Appl Catal A
393:231–241
22. Tamami B, Allahyari H, Ghasemi S, Farjadian F (2011) J Or-
ganomet Chem 696:594–599
The pH-sensitive organic–inorganic P2VP-Fe3O4 hybrid
microgels were successfully prepared via radical poly-
merization by using 2VP as the pH-sensitive monomer and
TMSPMA-modified magnetic Fe3O4 nanoparticles as the
crosslinker. The TMSPMA-modified Fe3O4 nanoparticles
were confirmed to act as crosslinkers, causing to the for-
mation of P2VP-Fe3O4 hybrid microgels which confirmed
with FT-IR and TGA analysis. The synthesized hybrid
microgels were effectively employed as substrate for in situ
generation of Pd nanoparticles. The encapsulation of Pd
nanoparticles on P2VP-Fe3O4 was confirmatively studied
by AAS, TGA, and TEM. The prepared PdNPs@P2VP-
Fe3O4 was used as a new nanocatalyst toward oxidation
reaction of benzyl alcohols under green conditions such as
23. Zhao Y, Yang X, Tian J, Wang F, Zhan L (2010) Mater Sci Eng
B 171:109–115
24. Guerra J, Herrero MA (2010) Nanoscale 2:1390–1400
25. He Y, Cai C (2011) Catal Commun 12:678–683
26. Islam RU, Witcomb MJ, Lingen E, Scurrell MS, Otterlo WV,
Mallick K (2011) J Organomet Chem 696:2206–2210
27. Cirtiu CM, Dunlop-Briere AF, Moores A (2011) Green Chem
13:288–291
28. Nielsen RJ, Goddard WA (2006) J Am Chem Soc 128(33):
9651–9660
123

A Nanoreactor for Selective Aerobic Oxidation of Alcohols
363
29. Stoltz BM (2004) Chem Lett 33:362–367
39. Davidi I, Semionov A, Eisenberg D, Goobes G, Shenhar R (2012)
Soft Mater 8:7393–7401
40. Hsieh CT, Lin JY, Wei JL (2009) Int J Hydrogen Energy
34:685–693
30. Nabid MR, Sedghi R, Jamaat PR, Safari N, Entezami AA (2007)
Appl Catal A 328:52–57
31. Nabid MR, Rezaei SJT (2009) Appl Catal A 366:108–113
32. Nabid MR, Bide Y, Rezaei SJT (2011) Appl Catal
406:124–132
33. Ahmar H, Fakhari AR, Nabid MR, Rezaei SJT, Bide Y (2012)
Sens Actuators B 171–172:611–618
34. Abandansari HS, Aghaghafari E, Nabid MR, Niknejad H (2013)
Polymer 54:1329–1340
A
41. Sheldon RA, Kochi JK (1981) Metal-catalysed oxidation of
organic compounds. Academic Press, New York
42. Hudlicky M (1990) Oxidations in organic chemistry, ACS
monograph 186. American Chemical Society, Washington, DC
43. Punniyamurthy T, Velusamy S, Iqbal J (2005) Chem Rev
105:2329–2363
35. Sumanta KS, Arindam C, Dipsikha B, Sudip KG, Panchanan P
(2011) J Nanopart Res 13:2475–2484
44. Karimi B, Zamani A, Clark JH (2005) Organometallics
24:4695–4698
36. Du BY, Mei AX, Tao PJ, Zhao B, Cao Z, Nie JJ, Xu JT, Fan ZQ
(2009) J Phys Chem C 113:10090–10096
45. Karimi B, Abedi S, Clark JH, Budarin V (2006) Angew Chem Int
Ed Engl 45:4776–4779
37. Narain R, Gonzales M, Hoffman AS, Stayton PS, Krishnan KM
(2007) Langmuir 23:6299–6304
46. Matsumoto T, Ueno M, Wang N, Kobayashi S (2008) Chem
Asian J 3:196–214
38. De Palma R, Peeters S, Van Bael MJ, Van den Rul H, Bonroy K,
Laureyn W, Mullens J, Borghs G, Maes G (2007) Chem Mater
19:1821–1831
47. Vinod CP, Wilson K, Lee AF (2011) J Chem Technol Biotechnol
86:161–171
123