A magnetic particle-supported sulfonic acid catalyst: Tuning catalytic activity between homogeneous and heterogeneous catalysis

Article abstract of DOI:10.1002/adsc.201100352

Surface functionalization of magnetic particles is an elegant way to bridge the gap between heterogeneous and homogeneous catalysis. The introduction of magnetic particles (MPs) in a variety of solid matrices allows the combination of well-known procedures for catalyst heterogenization with techniques for magnetic separation. We have conveniently loaded sulfonic acid groups on magnetic particles supports in which chlorosulfonic acid is used as sulfonating agent. The main targets are room temperature, solvent-free conditions, rapid (immediately) and easy immobilization technique, and low cost precursors for the preparation of highly active and stable MPs with high densities of functional groups. The inorganic, magnetic, solid acid catalyst was characterized via Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermal gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), dynamic light scattering (DLS), vibrating sample magnetometer (VSM) and titration. The catalyst is active for the Hantzsch reaction and the products are isolated in high to excellent yields (90-98%). Supporting this acid catalyst on magnetic particles offers a simple and non-energy-intensive method for recovery and reuse of the catalyst by applying an external magnet. Isolated catalysts were reused for new rounds of reactions without significant loss of their catalytic activity. Copyright

Full text of DOI:10.1002/adsc.201100352

FULL PAPERS
DOI: 10.1002/adsc.201100352
A Magnetic Particle-Supported Sulfonic Acid Catalyst: Tuning
Catalytic Activity between Homogeneous and Heterogeneous
Catalysis
a,b,
a,
b,
Nadiya Koukabi, * Eskandar Kolvari, * Mohammad Ali Zolfigol, *
b,
c
b
Ardeshir Khazaei, * Behzad Shirmardi Shaghasemi, and Behzad Fasahati
a
Department of Organic Chemistry, Faculty of Chemistry, Semnan University, Semnan, Iran
Fax : (+98)-231-335-4057; phone: (+98)-231-336-6254; e-mail: n.koukabi@semnan.ac.ir or kolvari@semnan.ac.ir
Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran
b
Fax : (+98)-811-827-2046; phone: (+98)-811-828-2807; e-mail: zolfi@basu.ac.ir or Khzaei_1326@yahoo.com
Department of Chemistry, Payame Noor University, Hamedan, Iran
c
Received: May 5, 2011; Revised: February 3, 2012; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201100352.
Abstract: Surface functionalization of magnetic par- ric analysis (TGA), X-ray photoelectron spectrosco-
ticles is an elegant way to bridge the gap between py (XPS), transmission electron microscopy (TEM),
heterogeneous and homogeneous catalysis. The in- dynamic light scattering (DLS), vibrating sample
troduction of magnetic particles (MPs) in a variety magnetometer (VSM) and titration. The catalyst is
of solid matrices allows the combination of well- active for the Hantzsch reaction and the products
known procedures for catalyst heterogenization with are isolated in high to excellent yields (90–98%).
techniques for magnetic separation. We have conven- Supporting this acid catalyst on magnetic particles
iently loaded sulfonic acid groups on magnetic parti- offers a simple and non-energy-intensive method for
cles supports in which chlorosulfonic acid is used as recovery and reuse of the catalyst by applying an ex-
sulfonating agent. The main targets are room tem- ternal magnet. Isolated catalysts were reused for new
perature, solvent-free conditions, rapid (immediate- rounds of reactions without significant loss of their
ly) and easy immobilization technique, and low cost catalytic activity.
precursors for the preparation of highly active and
stable MPs with high densities of functional groups.
The inorganic, magnetic, solid acid catalyst was char- Keywords: Hantzsch reaction; heterogeneous cataly-
acterized via Fourier transform infrared spectroscopy sis; immobilization; magnetic nanoparticles; multi-
(
FT-IR), X-ray diffraction (XRD), thermal gravimet- component reactions; nanostructures
Introduction
acid, especially for economically and industrially at-
tractive issues such as using less toxic and readily
Sulfuric acid is an essential catalyst for the production available precursors as well as environmentally
of industrial chemicals. Over 15 million tons of sulfu- benign supports, simple and non-energy-intensive
[
1]
ric acid are annually consumed as “an unrecyclable methods for recovery and reuse of catalyst.
catalyst” – which requires costly and inefficient sepa- Tiny particles (the size range of nm to a few mm)
ration of the catalyst from homogeneous reaction have emerged as alternative form of the support ma-
[
2]
mixtures – for the production of industrially impor- terials. This is because when the size of the support
tant chemicals, thereby resulting in a huge waste of materials is decreased to the nanometer scale, the sur-
energy and large amounts of waste products. The face area of nanoparticles will increase dramatically.
“green” approach to chemical processes has stimulat- Thus, one could say that there is plenty of room on
ed the use of recyclable strong solid acids as replace- the surface of these small-sized particles for the heter-
ments for such unrecyclable “liquid acids” catalysts. ogenization of various homogeneous catalysts. Conse-
Although the technology has been widely explored quently, tiny particle supports could have higher cata-
and well-developed, there is still a need to discover lyst loading capacity than many conventional support
new methodologies for the immobilization of sulfonic matrices, leading to the improved catalytic activity of
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the tiny particle-supported catalysts. In addition, reac- reaction rate of up to overnight relative to the previ-
[
6]
tants in solution have easy access to the active sites ous system.
on the surface of tiny particles, avoiding the problems The supported sulfonic acid analogue was prepared
encountered in many heterogeneous support matrices by the concise route outlined in Scheme 1.
where a great portion of catalyst sites are present Maghemite (g-Fe O ) nanoparticles of 14 nm aver-
2
3
[
7]
deep inside the matrix backbones and reactants have age diameter were prepared by the coprecipitation
only limited access to the catalytic sites. Therefore, in- technique. Catalysts 1a and 1b have been prepared at
vestigating such small-sized particles will also assist in room temprature by two different procedures which
understanding the relationships between homogene-
ous and heterogeneous catalysis, and could efficiently
bridge the gap between these two traditional disci-
plines. However, the main difficulty is that such small
particles are almost impossible to separate by conven-
tional means, which can lead to the blocking of filters
and valves by the tiny particle catalyst. In such cases,
expensive ultracentrifugation is often the only way to
separate the product and catalyst. This drawback can
be overcome by using paramagnetic or superparamag-
[
3]
netic particles (the size range of nm to a few mm),
which can be easily removed from the reaction mix-
[
4]
ture by magnetic separation.
Results and Discussion
Herein, we report the synthesis of a magnetic parti-
cle-based solid acid with a high density of sulfonic
acid groups (SO H) and discuss its performance as
3
a novel strong and stable solid acid. The present con-
tribution is built on previous work with silica sulfuric
[5]
acid catalysts, and we were intrigued by the possibil-
ity of applying chlorosulfonic acid and nanotechnolo-
gy for the design of a novel, active, recyclable, and
magnetically recoverable sulfonic acid derivative for
the first time.
In contrast to the first report of the immobilized
magnetic nanoparticles with sulfonic acid groups in
[6]
2
007, the current work shows unique advantages
such as simplification of the catalyst system and its
synthesis, and the application of inexpensive and
available precursors. An attractive feature of this
method is that the use of a chlorosulfonic acid as the
sulfonating agent allows for solvent-free reaction con-
ditions and results in a significant enhancement in the
Figure 1. FT-IR spectra of (a) g-Fe O ; (b) g-Fe O @SiO ;
2
3
2
3
2
Scheme 1. Preparation of sulfonic acid supported g-Fe O .
(c) g-Fe O -SO H; (d) g-Fe O @SiO SO H.
2
3
2 3 3 2 3 2 3
2
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A Magnetic Particle-Supported Sulfonic Acid Catalyst: Tuning Catalytic Activity
Figure 2. X-ray diffraction patterns of uncoated g-Fe O nanoparticles (a) and g-Fe O -SO H (b).
2
3
2
3
3
[
7]
involve the reaction of chlorosulfonic acid with either the literature (Figure 2). The same peaks were ob-
[
8]
silica-coated magnetic nanoparticles which afforded served in both othe bare and sulfonic acid-loaded
g-Fe O @SiO -SO H (1a) or uncoated ones which magnetic particle (also see Supporting Information).
2
3
2
3
gave g-Fe O -SO H (1b). The loading process pro- the XRD patterns thus indicate retention of the crys-
2
3
3
ceeded cleanly and HCl evolved from the reaction talline inverse cubic spinel ferrite core structure
vessel immediately as the only by-product. It is note- during the sulfonation process. Seven peaks in
worthy that the second strategy was achieved in Figure 2 correspond to Miller indices value {hkl} of
a shorter step and sulfonation was the first and the {111}, {220}, {311}, {400}, {422}, {511}, and {440},
[
10]
only stage of the reaction. The total yield of the prod- respectively.
uct based on g-Fe O is about 100% by this method. The thermogravimetric (TG) analysis curve of the
The “inorganic magnetic solid acid catalysts” were g-Fe O -SO H is shown in Figure 3. The TG curve of
.
2
3
2
3
3
characterized via FT-IR, XRD, TGA, XPS, TEM, ti- synthesized sulfonic acid-functionalized MPs indicat-
tration, VSM and DLS.
ed two weight loss transitions at ~1208C and ~2008C.
Figure 1 shows the FT-IR results of the synthesized The initial weight loss from the sulfonic acid-function-
particles with and without SO H loading, respectively. alized MPs up to 1208C was probably due to the re-
3
For the bare magnetic nanoparticle (Figure 1, a), the moval of surface hydroxy groups and/or surface ad-
À1
vibration band at 557 cm is the typical IR absorb- sorbed water while the weight loss at higher tempera-
[
9]
ance induced by structure FeÀO vibration. In the
case of g-Fe O @SiO nanoparticles the band at
2
3
2
À1
1
079 cm is corresponding to SiÀOÀSi antisymmetric
stretching vibrations, being indicative of the existence
of SiO in the nanoparticles. In the sulfonic acid-func-
2
tionalized MPs (Figure 1, b), three new bands ap-
À1
peared at 1200–1250, 1010–1100 and 650 cm , which
are attributed to the O=S=O asymmetric and sym-
metric stretching vibration and SÀO stretching vibra-
tion of the sulfonic groups (ÀSO H), respectively. The
3
increase in the intensities of the band at 3000–
À1
3
500 cm (I 3360) suggests that there are more OH
groups under the magnetic nanoparticle surface after
the sulfonation. On the other hand, the band at
À1
3
360 cm became much broader. All these observa-
tions confirm that the sulfonic groups have functional-
ized the surface of the magnetic particles (MPs).
X-ray diffraction (XRD) of the bare magnetic
nanoparticles displayed patterns consistent with the Figure 3. Weight loss analysis from TG curves of sulfonic
patterns of inverse cubic spinel ferrites described in acid coated g-Fe O particles.
2
3
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Figure 4. XPS spectrum of g-Fe O -SO H.
2
3
3
ture could be mainly attributed to the evaporation (Figure 5). The hydrodynamic diameter of the parti-
and subsequent decomposition of SO H groups. cles calculated from DLS was 10 times larger than
3
[
14]
Figure 4 shows X-ray photoelectron spectroscopy that from TEM, which is quite normal. This is due
(
XPS) for the g-Fe O -SO H sample (also see Sup- to the fact that the hydrodynamic nanoparticle diame-
2
3
3
[
11]
porting Information). The XPS spectra, after sulfo- ter is measured in their dispersion state in water, in
nation, exhibit a single S 2p peak attributable to contrast with TEM which analyzes the particle core.
SO H groups at 168 eV. After sulfonation, the iron Moreover, DLS provides the z-average of the diame-
3
material, therefore, contains SO H, and all S atoms in ter, which is strongly affected by the presence of large
3
[
12]
the iron material are contained in SO H groups.
particles, while TEM gives the number average.
3
Figure 5 displays TEM images of g-Fe O @SiO , g- Moreover, the formation of iron oxide aggregates in
2
3
2
Fe O @SiO -SO H, g-Fe O @SO H which revealed water cannot be ruled out. The particle size distribu-
2
3
2
3
2
3
3
that all the samples were spherical-like particles (also tions for g-Fe O @SiO , g-Fe O @SiO -SO H, and g-
2
3
2
2
3
2
3
see Supporting Information). Compared to those of g- Fe O -SO H, are in the range of 164–342 nm, 142–
2
3
3
Fe O @SiO -SO H, the spheres formed from g-Fe O - 396 nm and 142–531 nm with peak maxima arround
2
3
2
3
2
3
SO H showed a little tendency of fusing (Figure 5, c). 220 nm, 190 nm and 220 nm, respectively.
3
g-Fe O @SiO -SO H, which has a silica shell on the
The amounts of sulfonic acid groups on silica-
2
3
2
3
preformed nanoparticles, gave rise to fine spherical coated and uncoated supports are very similar and
nanostructures. Upon undergoing direct sulfonation the total acid capacity was found to be in the range of
À1
on the backbone of preformed nanoparticles, spheres 2.50–2.52 mmolg , which was determined through
much bigger than those of g-Fe O @SiO -SO H were the neutralization titration. This finding means that
2
3
2
3
generated exclusively (Figure 5, c). The SO H groups the effective density of the acid sites in the samples is
3
loading on magnetic nanoparticles appeared to have about five times as large as that of nafion, which is
been aggregated before loading. Thus, they were a highly active and stable perfluorosulfonated ionom-
loaded as aggregates, leading to irregularly spherical- er. These results indicate that these materials are g-
[
6,13]
shaped particles in size from 50 nm up to 1 mm.
Fe O with attached SO H groups functioning as
2 3 3
The average diameter determined from TEM images a strong solid acid with a high density of acid sites. To
are 16Æ5 and 20Æ5 nm for g-Fe O @SiO and g- determine whether the particles exhibit magnetic
2
3
2
Fe O @SiO -SO H, respectively.
properties, the sulfonic acid-loaded magnetic particles
2
3
2
3
Synthesized particles with and without SO H load- were dispersed in water, resulting in a dark disper-
3
ing, were also investigated by dynamic light scattering sion, as shown in Figure 6, a. Within about 10 sec, in
4
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A Magnetic Particle-Supported Sulfonic Acid Catalyst: Tuning Catalytic Activity
Figure 5. TEM images and DLS size distributions of (a) g-Fe O @SiO ; (b) g-Fe O @SiO -SO H; and (c) g-Fe O -SO H.
2
3
2
2
3
2
3
2
3
3
the presence of a magnetic stirrer bar or external
magnet, sulfonic acid functionalized MPs were com-
pletely gathered onto the stirrer bar or one side of
the cuvette wall steadily and the dispersion became
clear and transparent (Figure 6, b and c).
The magnetization curves of the g-Fe O and g-
2
3
Fe O -SO H were further recorded at room tempera-
2
3
3
ture (Figure 7). The magnetizations were expressed in
units of emu per gram of powder. The two measured
samples display a super paramagnetic behaviour, as
evidenced by a zero coercivity and remanence on the Figure 6. Photographs of aqueous suspension of g-Fe O -
2
3
magnetization loop. The saturation magnetization SO H before (a) and after (b, c) magnetic capture.
3
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Table 1. Influence of the amount of g-Fe O -SO H on the
2
[
3
3
a]
Hantzsch synthesis of 1,4-dihydropyridines.
[b]
Entry
g-Fe O -SO H [g]
Time [min]
Yield [%]
2
3
3
1
2
3
4
0.1
0.05
0.025
0.0125
140
105
75
82
91
95
95
225
[a]
Benzaldehyde-ethyl acetoacetate-NH OAc =1:2:1.5, sol-
4
vent-free, 808C.
Refers to isolated yield.
[b]
Table 2. Effect of temperature on the g-Fe O -SO H cata-
2
3
3
[a]
lyzed synthesis of 1,4-dihydropyridines.
Figure 7. Room-temperature magnetization curves of the
SO H coated and uncoated g-Fe O particles.
[b]
3
2
3
Entry
Temperature [8C]
Time [min]
Yield [%]
1
2
3
4
70
80
90
100
95
75
60
60
92
95
98
98
À1
value of the g-Fe O -SO H is 28 emug . The number
2
3
3
is lower than that of uncoated magnetic particles
about 60 emug ). Xu et al. have pointed out that
coating with non-magnetic materials influences the
magnitude of magnetization of the coated magnetic
materials due to quenching of surface moments.
À1
(
[a]
Benzaldehyde-ethyl acetoacetate-NH OAc =1:2:1.5, sol-
vent-free, 0.025 g of g-Fe O -SO H.
Refers to isolated yield.
4
2
3
3
[
15]
[b]
Since the magnetite nanoparticles were coated with
non-magnetic sulfonic acid groups in the present
work, a similar mechanism could be considered for
By screening different amounts of the catalyst
the decrease in saturation magnetization after being (Table 1), we found that the product 4a could be ob-
coated in the present work. Nonetheless, the magneti- tained in yields ranging from 82 to 95%. As little as
zation value is enough for common magnetic separa- 0.025 g of g-Fe O -SO H can catalyze the reaction to
2
3
3
tion. Comparisons of catalysts containing silica-coated the same extent but it needs a longer reaction time (~
and uncoated supports identified no significant differ- 4 h).
ences in catalytic activity (Table 3, entries 2, 5, 12),
We next investigated the effect of temperature on
and considering forgoing explanation, between g- the reaction rate as well as the yields of products. The
Fe O @SiO -SO H and g-Fe O -SO H catalysts, we variation of the temperature (908C) revealed an in-
2
3
2
3
2
3
3
have chosen catalyst g-Fe O -SO H as the desirable creased conversion of reactants and a product yield of
2
3
3
one.
The prepared magnetic particle-supported SO H
98% (Table 2, entry 3).
In summary, the optimal conditions for the g-
3
groups have been tested as catalysts in the Hantzsch Fe O -SO H-catalyzed Hantzsch reaction involved
2
3
3
reaction. For initial optimization of the reaction con- a combination of g-Fe O -SO H (0.025 g), benzalde-
2
3
3
ditions and the identification of the best amount of hyde 1a (1 mmol), ethyl acetoacetate 2a (2 mmol),
the catalyst, benzaldehyde 1a, ethyl acetoacetate 2a ammonium acetate 3 (1.5 mmol) at 908C under sol-
and ammonium acetate 3 were chosen as model sub- vent-free conditions. In view of these results, we then
strates (Scheme 2).
selected the optimized reaction conditions to deter-
mine the scope of this g-Fe O -SO H-catalyzed reac-
2
3
3
tion. A wide range of aromatic, aliphatic and hetero-
aromatic aldehydes were subjected to react with 2a
and b in the presence of ammonium acetate 3 and
0
.025 g of g-Fe O -SO H to generate 4 and 5
2 3 3
(
Scheme 1) the results of which are summarized in
Table 3.
The aryl group substitution with different groups
and with the same groups located at different posi-
tions of the aromatic ring has been shown not to have
much effect on the formation of the final product and
Scheme 2. Synthesis of Hantzsch 1,4-dihydropyridines cata- afford the expected products 4 and 5 in good to high
lyzed by g-Fe O -SO H. yields. The products were characterized by IR,
2
3
3
6
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A Magnetic Particle-Supported Sulfonic Acid Catalyst: Tuning Catalytic Activity
Table 3. g-Fe O -SO H-catalyzed Hantzsch synthesis of 1,4- presence of a magnetic stirrer bar, g-Fe O -SO H
2
3
3
2
3
3
[a]
dihydropyridines 4.
moved onto the stirrer bar steadily and the reaction
mixture turned clear within 10 s. The catalyst can be
isolated by simple decantation. After being washed
with acetone and dried in air, the g-Fe O -SO H can
be directly reused without any deactivation even after
five rounds of synthesis of product 4a (Table 4).
1
2
R
R
Product Time
min]
Yield
[%]
[b]
[
2
3
3
Ph
Ph
OEt 4a
OEt 4a
OEt 4b
OEt 4c
OEt 4c
OEt 4d
OEt 4f
OEt 4g
OEt 4h
OEt 4i
OEt 4j
OEt 4j
OEt 4k
OEt 4l
60
60
120
62
60
20
50
60
60
50
50
50
65
70
98
[
[
c]
98
93
99
97
95
91
98
95
97
92
94
96
90
p-Me-C H4
p-Cl-C H4
p-Cl-C H4
p-HO-C H4
p- O N-C H
p-F-C H4
p-NC-C H4
p-Br-C H4
6
6
c]
6
Conclusions
6
2
6
4
In conclusion, we have developed the current impor-
tant areas of heterogenization of sulfonic acid, which
is a rapidly developing research area. The main objec-
tives are room temperature, solvent-free conditions,
a rapid (immediately) and easy immobilization tech-
nique, and low cost precursors for the preparation of
highly active and stable MPs with high densities of
functional groups. Furthermore, applying the young
area of magnetite particles which are intrinsically not
magnetic, but can readily be magnetized by an exter-
nal magnet, can have a positive effect on high activity
on one hand and separation and recycling on the
other hand.
6
6
6
o-MeO-C H4
o-MeO-C H4
6
[
c]
6
6
m-Cl-C H4
4
-HO-3-MeO-
C H3
6
n-Pr
-furyl
Ph
OEt 4m
OEt 4n
OMe 5a
OMe 5b
OMe 5c
OMe 5d
OMe 5f
OMe 5g
OMe 5h
OMe 5i
OMe 5j
OMe 5k
OMe 5l
20
20
55
45
40
100
60
15
70
45
30
15
110
94
98
97
96
90
97
95
94
91
93
90
93
97
2
o-MeO-C H4
6
p-O N-C H
p-Me-C H4
p-Cl-C H4
p-HO-C H4
2
6
4
6
6
6
6
p-F-C H4
Experimental Section
p-NC-C H4
p-Br-C H4
o-Cl-C H4
6
6
General Remarks
6
4
-HO-3-MeO-
All chemicals were purchased from Merck, Fluka or Acros
C H3
n-Pr
and used without any further purification. Nano-g-Fe O
6
2
3
[7]
OMe 5m
OMe 5n
OMe 5o
15
30
45
96
90
92
was prepared with the reported method. NMR spectra
were recorded with a Bruker Avance 300 spectrometer
2
-furyl
m-O N-C H
4
1
13
( H NMR 300 MHz and C NMR 75 MHz) in pure deuteri-
ated chloroform, acetone and dimethyl sulfoxide with tetra-
methylsilane (TMS) as the internal standard. Transmission
electron microscope, TEM (Philips CM10) was also used to
obtain TEM images. The crystal structure of synthesized
materials was determined by an X-ray diffractometer
2
6
[
a]
1
All products were characterized by IR, H NMR and
C NMR spectroscopic data, and melting points.
Yields refer to isolated products.
The use of 0.025 g of g-Fe O @SiO -SO H instead of g-
1
3
[
[
b]
c]
2
3
2
3
Fe O -SO H gave similar reactions.
2
3
3
(
XRD) [38066 RIVA, d/G. via M. Misone, 11/D (TN), Italy]
at ambient temperature. XPS spectra were acquired using
a Kratos Axis ULTRA “DLD” X-ray photoelectron spec-
trometer equipped with a monochromatic Al-KR X-ray
source. The pass energies were 160 eV for the survey scans
1
13
H NMR and C NMR spectroscopy, and also by
comparison with authentic samples.
Importantly, note that the ferromagnetic property and 20 eV for the high resolution scans. The instrument was
of g-Fe O -SO H made the isolation and reuse of this operated in the electrostatic mode as the samples were mag-
2
3
3
catalyst very easy. After completion of the reaction, netic. The magnetic measurement was carried out in a vibrat-
ing sample magnetometer (VSM) (4 inch, Daghigh Megh-
the mixture was triturated with ethyl acetate. In the
natis Kashan Co., Kashan, Iran) at room temperature. Ther-
mal analysis was done by using a thermogravimetric ana-
Table 4. Reuse of g-Fe O -SO H in the Hantzsch synthesis
of 1,4-dihydropyridine.
lyzer (TGA, Mettler system TA 4000) in N at a heating
rate of 108Cmin .
2
2
3
3
[
a]
À1
Run
1
2
3
4
5
Preparation of the g-Fe O -SO H and g-Fe O @SiO -
2
3
3
2
3
2
[b]
Yield [%]
98
98
98
96
96 SO H
3
[
a]
Benzaldehyde-ethyl acetoacetate-NH OAc =1:2:1.5, sol-
A suction flask equipped with a constant-pressure dropping
funnel and a gas inlet tube for conducting HCl gas over an
adsorbing solution (i.e., water) was used, charged with the
4
vent-free, 908C, 0.025 g of g-Fe O -SO H.
2
3
3
[
b]
Isolated yields.
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[
8]
g-Fe O3 or g-Fe O @SiO
2
nanoparticles (3 g). Chlorosul-
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2
3
fonic acid (1.1636 g, 0.0099 mol) was added dropwise over
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1
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(
2 mmol), ammonium acetate (1.5 mmol) and g-Fe O -SO H
2 3 3
(
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2
010, 49, 7786–7789; g) A. Schatz, O. Reiser, W. J.
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3
3
moved onto the stirrer bar steadily and the reaction mixture
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2
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2
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Acknowledgements
1
The authors acknowledge Semnan University Research
Council, Bu-Ali Sina University Research Council (Grant
Number 32-1716), Center of Excellence in Development of
Chemical Methods (CEDCM), and National Foundation of
Elites (BMN) for support of this work. We also highly appre-
ciate the valuable help of Dr. Hormozi-Nezhad for the DLS.
Also we acknowledge to Professor A. R. Massah for his gen-
erous support.
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ÝÝ
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FULL PAPERS
A Magnetic Particle-Supported Sulfonic Acid Catalyst:
Tuning Catalytic Activity between Homogeneous and
Heterogeneous Catalysis
9
Adv. Synth. Catal. 2012, 354, 1 – 9
Nadiya Koukabi,* Eskandar Kolvari,*
Mohammad Ali Zolfigol,* Ardeshir Khazaei,*
Behzad Shirmardi Shaghasemi, Behzad Fasahati
Adv. Synth. Catal. 0000, 000, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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ÞÞ
These are not the final page numbers!