Heteropolytungstostannate as a homo- and heterogeneous catalyst for Knoevenagel condensations, selective oxidation of sulfides and oxidative amination of aldehydes

Article abstract of DOI:10.1039/c7ra06112a

Heteropolytungstostannate K₁₁H[P₂W₁₈O₆₈(HOSn^IVOH)₃]·20H₂O (1) is used as a homo- and heterogeneous catalyst for Knoevenagel condensations. New Fe₃O₄@SiO₂@CH₂(CH₂)₂NH₃⁺[P₂W₁₈O₆₈(HOSn^IVOH)₃]^11- nanocatalyst was fabricated through electrostatic grafting of (1) on amine-modified silica-coated magnetic nanoparticles. The new magnetically recoverable nanocatalyst was characterized using infrared spectroscopy (IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffractometry (XRD), energy-dispersive X-ray analysis (EDX) and alternating gradient force magnetometry (AGFM). The results indicate that the particles are mostly spherical in shape and have an average size of approximately <30 nm. The catalytic activity of the catalyst was investigated in the selective oxidation of sulfides to sulfoxides and oxidative amination of aldehydes in an aqueous medium. The catalyst was reused at least five times without significant decrease in catalytic activities, and the structure of the catalyst remained unchanged after recycling.

Full text of DOI:10.1039/c7ra06112a

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PAPER
Heteropolytungstostannate as a homo- and
heterogeneous catalyst for Knoevenagel
condensations, selective oxidation of sulfides and
oxidative amination of aldehydes
Cite this: RSC Adv., 2017, 7, 45495
Roushan Khoshnavazi, * Leila Bahrami and Manuchehr Rezaei
IV
Heteropolytungstostannate K11H[P
2
W
18
O
68(HOSn OH)
3 2
]$20H O (1) is used as a homo- and heterogeneous
+
3
catalyst for Knoevenagel
condensations.
New
3 4 2 2 2 2
Fe O @SiO @CH (CH ) NH
-
IV
11ꢀ
[
P
2
W
18
O
68(HOSn OH)
3
]
nanocatalyst was fabricated through electrostatic grafting of (1) on amine-
modified silica-coated magnetic nanoparticles. The new magnetically recoverable nanocatalyst was
characterized using infrared spectroscopy (IR), scanning electron microscopy (SEM), transmission electron
microscopy (TEM), X-ray powder diffractometry (XRD), energy-dispersive X-ray analysis (EDX) and
alternating gradient force magnetometry (AGFM). The results indicate that the particles are mostly spherical
in shape and have an average size of approximately <30 nm. The catalytic activity of the catalyst was
investigated in the selective oxidation of sulfides to sulfoxides and oxidative amination of aldehydes in an
aqueous medium. The catalyst was reused at least five times without significant decrease in catalytic
activities, and the structure of the catalyst remained unchanged after recycling.
Received 1st June 2017
Accepted 9th September 2017
DOI: 10.1039/c7ra06112a
rsc.li/rsc-advances
electrostatic interaction of external oxygen atoms of the POMs
with protonated hydroxyl groups of the surface.
Introduction
Polyoxometalates (POMs) are a large class of molecular metal-
oxide clusters with unmatched structural variety and versa- catalytic properties of sandwich-type POMs as heterogeneous
In comparison with the Keggin and Dawson-type POMs, the
14–17
tility properties. POMs due to diversity in structure and catalysts have not been extensively studied.
In this research,
composition have been used in many elds, including catalysis, we investigate the catalytic properties of tin-containing
IV
12ꢀ
1
–4
medicine and material science. Owing to their redox and sandwich-type polyoxometalates [P
W
O
68(HOSn OH)
(1) consists of two
34 anions linked by three tin(IV) cations leading to
]
.
2
18
12ꢀ
3
IV
acidic properties, POMs exhibit catalytic activity in both The structure of [P
homogeneous and heterogeneous catalysis. The application A-type a-PW
2
W
18
O
68(HOSn OH)
3
]
5,6
9
O
1
8
of POMs as catalysts is limited because they are solids with a low A-type sandwich POM (Fig. 1). Tin-containing sandwich-type
surface area (less than 10 m g ) and also have high solubility polyoxometalates are more hydrolytically stable than
in polar solvents. To overcome the limitations, POMs have been transition-metal-substituted sandwich-type polyoxometalates in
2
ꢀ1
18–20
supported on inert and high-surface-area materials. Self- aqueous solution
and thus are more suitable for oxidation
assembly techniques are being developed for designing cata- reactions. Encouraged by these results, we evaluated the inter-
lysts based on polyoxometalates such as self-assembly of action of 1 with already prepared amine-modied silica-coated
polyoxometalate with cations and positively charged nano- magnetite nanoparticles. The catalytic activity of the tin-
particles that could facilitate catalyst recycling via ltration or containing sandwich-type polyoxometalate and the prepared
7
–11
centrifugation.
have become promising candidates as high-surface-area mate- condensations, oxidation of suldes with H
Recently, magnetic nanoparticles (MNPs) nanocomposite was examined in reactions such as Knoevenagel
and oxidative
2 2
O
12,13
rials for immobilization of homogeneous catalysts.
Owing to amination of aldehydes in a green medium.
their exceptionally high surface areas, MNPs exhibit high
activity under mild conditions. Most examples of immobiliza-
tion of POMs on surfaces have been limited to non-covalent Experimental section
interactions between Keggin heteropolyacids through
Materials
All reagents were commercially obtained from Merck chemical
company, Acr ¯o s Organics and Sigma-Aldrich and used without
Department of Chemistry, University of Kurdistan, P.O. Box 66135-416, Sanandaj,
Iran. E-mail: r.khoshnavazi@uok.ac.ir; khoshnavazi@yahoo.com; Fax: +98 8733
2 18
further purication. Heteropolytungstostannate K11H[P W -
IV
6
24 133; Tel: +98 8733 624 133
3 2
O68(HOSn OH) ]$20H O was prepared according to the
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Synthesis
Magnetic nanoparticle-supported polyoxometalate
+
IV
11ꢀ
Fe
3
O
4
@SiO
2
@CH
(CH
2
(CH NH
2
)
2
3
[P
2
W
18
O
68(HOSn OH)
3
]
(1a).
Fe
3
O
4
@SiO CH
2
2
2 2
)
NH (0.1 g) was dispersed in 20 mL of
2
deionized water and ultrasonicated for 30 min. Then, a solution
of 1 (0.1 g in 5 mL deionized water) was added dropwise into the
solution, ultrasonicated for 30 min and the mixture was stirred
under reux condition for 24 h. The obtained solid was then
magnetically collected from the solution and washed abundantly
ꢁ
with water/ethanol and dried at 60 C.
Typical procedures for catalytic experiments
The Knoevenagel condensations. In a typical experiment, alde-
hyde (1 mmol) and malononitrile (1.2 mmol), 0.010 g of 1 as
catalyst and water or ethanol (1 mL) were placed in a 20 mL glass
ꢁ
bottle at 25 C. The reaction mixture was constantly stirred
vigorously. The progress was monitored through TLC (eluent: n-
hexane/EtOAc, 2 : 1). Aer completion of the reaction, the reac-
tion mixture was transferred to a separating funnel and EtOAc (3
ꢂ 5 mL) was added and then the products were separated. Aer
evaporation of EtOAc, the obtained products were recrystallized
from EtOH and pure products were obtained in 85–95% yield.
The oxidation of suldes to sulfoxides. The catalytic activity of
the catalyst was investigated in the selective oxidation of
suldes to sulfoxides using 30% H O as a green oxidant. A
2
2
mixture containing a sulde (1 mmol), H O (1.25 mL), 30%
2
H O (6 mmol) and catalyst 1a (0.030 g) was stirred at room
2
2
temperature for the time specied. The progress of the reaction
was monitored through TLC (eluent: n-hexane/EtOAc, 2 : 1).
Aer completion of the reaction, the catalyst was separated
from the product using an external magnet. The product was
IV
12ꢀ
.
Fig. 1 Polyhedral representation of [P
W
2 18
O
68(HOSn OH)
3
]
extracted with CH
2
Cl
2
(3 ꢂ 5 mL) and the combined organics
were washed with brine (10 mL) and dried over anhydrous
Na SO . The solvent was removed under reduced pressure to
1
8
2
4
literature. The Fe
nanoparticles (Fe
3
O
4
nanoparticles, silica-coated magnetite
afford the corresponding pure sulfoxide in most cases.
The oxidative amination of aldehydes. While stirring aldehyde
3
O
4
@SiO ) and Fe @SiO CH (CH NH
2
3
O
4
2
2
2
)
2
2
21–23
were prepared according to the literature.
(1 mmol), hydroxylamine hydrochloride (1.2 mmol) and
NaHCO
3
(1.2 mmol) in water (1 mL), 1a nanocatalyst (0.020 g)
ꢁ
was added to the mixture at a temperature of 90 C. Aer
completion of the reaction and separation of the magnetic
nanocatalyst using an external magnet, the reaction mixture
was extracted with ethyl acetate and washed with brine and then
dried with Na SO . For further purication, the products were
Characterization
The crystal structure of the synthesized nanocomposite was
characterized by a Philips (X'Pert MPD) diffractometer in 40 kV
˚
and 30 mA with a CuKa radiation (l ¼ 1.5418 A). Infrared
2
4
spectra were recorded on a Bruker Vector 22 FT-IR using KBr
pellet. The morphology of nanocomposite was revealed using
a scanning electron microscope (FESEM-TESCAN MIRA3 from
Czech Republic) and a transmission electron microscope (TEM,
Zeiss-EM10C 80 KV).
recrystallized from hexane and dichloromethane mixture to
afford the desired products. The obtained products were iden-
tied on comparison of the physical properties and spectral
data with pure samples and those reported in the literature.
The elements in the nanocomposite sample were probed
using energy-dispersive X-ray (EDX) spectroscopy accessory to
the FESEM-TESCAN MIRA3 scanning electron microscopy.
The magnetic properties were investigated using a home-made
Results and discussion
Synthesis and characterization
3 4
alternative gradient force magnetometer (AGFM) in the Magnetic Fe O nanoparticles were prepared by chemical co-
2
+
3+
magnetic eld range of ꢀ5000 to 5000 Oe at room tempera- precipitation of Fe and Fe ions in the basic solution and
ture. Textural characteristics of the outgassed samples were silica-coated magnetite nanoparticles were prepared by the
21
obtained from nitrogen physisorption at 77 K using a Bru- St ¨o ber method. The Fe O @SiO CH (CH ) NH nanoparticle
3
4
2
2
2 2
2
nauer, Emmett and Teller surface area and porosity analyzer was prepared by covalent graing of silanol groups of Fe
instrument. @SiO surface and 3-aminopropyltriethoxysilane (APTS).
3
-
O
4
2
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Magnetic nanoparticles of 1a was prepared by simple
direct addition of heteropolytungstostannate of [P W O -
2
18 68
IV
12ꢀ
(
HOSn OH) ]
aqueous solution to Fe O @SiO CH (CH ) NH
3 4 2 2 2 2 2
3
nanoparticle. The magnetic nanoparticle of Fe O @SiO @CH (-
3
4
2
2
2 2 2
CH ) NH was protonated by the acidic hydrogen and the
hydrogen of hydroxide groups in the structure of K11H[P
2
W
18
12ꢀ
-
IV
IV
3 2 2 18 3
O68(OHSn OH) ]$20H O. The anionic [P W O68(OHSn OH) ]
was attached to positively charged magnetic nanoparticles, Fe
3
-
+
O
4
@SiO
2
@CH
2
(CH
2
)
2
NH
3
nanoparticle through electrostatic
interactions. The nanocatalyst synthesis pathway is shown in
Scheme 1.
FT-IR spectra of 1a, Fe O @SiO CH (CH ) NH and 1 are
3
4
2
2
ꢀ
2 2
2
1
shown in Fig. 2. The peak at 573 cm in Fe
3
O
4
2 2
@SiO CH -
(
CH
2 2 2
) NH and 1a spectra could be attributed to Fe–O stretching
24
ꢀ1
vibration. Two bands at 1070 and 800 cm are ascribed to the
symmetrical and asymmetrical vibrations of the Si–O–Si bonds.
The very strong bands appearing in the 700–1090 cm in spec-
trum of 1a are clearly assigned to W–O–W, W]O and P–O
stretching modes of [P W O (HOSn OH) ] . The results show
Fig. 2 IR spectra of Fe
b) and 1 (c).
O
3 4
@SiO
2
CH
2
2
(CH )
2
NH
2
(a), 1a nanocomposite
25
(
ꢀ1
IV
12ꢀ
2
18 68
3
force magnetometer (AGFM) at room temperature (Fig. 5). A
decrease in saturation magnetization was observed for 1a,
which could be attributed to the increased mass of [P W -
that small shis to the higher wavenumbers were observed
compared to 1. The C–H stretching vibration bands of the
2
18
ꢀ
1
26
anchored propyl group appeared at 2925 and 2850 cm
.
These
IV
12ꢀ
O
68(HOSn OH)
3
]
. Even with this reduction in the saturation
results prove the formation of nanocomposite 1a.
magnetization, complete magnetic separation of 1a was ach-
ieved in <20 s by placing a magnet near the vessels containing
the aqueous dispersion of the nanoparticles. These results
indicate the polyoxometalate coating on the magnetic support.
The SEM images show the morphological features of 1a,
which conrm that the nanoparticles were almost spherical
3 4
Fig. 3 shows the XRD results of Fe O (a), 1a (b), and 1 (c).
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
Diffraction peaks with 2q ¼ 18.2 , 30.2 , 35.6 , 43.2 ,53.7 , 57.2 ,
ꢁ ꢁ
2.7 and 74.4 are attributed to Fe O , which indicates a cubic
3 4
6
27
structure of the magnetite. The peaks corresponding to 1 are also
observed in 1a, which is a conrmation of the successful synthesis
of nanocomposite. The particle diameters that were calculated
from the Scherrer formula are in the range of 18–30 nm.
(Fig. 6). The nanocomposite shows uniform shape with a mean
size less than 30 nm, which is consistent with the XRD results.
Magnetic interactions occur between the nanoparticles, which
leads to some particle aggregations as shown in the SEM
images.
Nitrogen adsorption–desorption isotherms of 1a nano-
composite show a type IV isotherm with a type H1 hysteresis loop
in the high range of relative pressure (Fig. 7A(a)). For the values of
high relative pressure, condensation takes place giving a sharp
The EDX spectrum obtained for 1a (Fig. 4) revealed that Fe, O,
Si, C, N, Sn,
Fe @SiO @CH
P
(CH
and
W
[P
peaks are associated with
+
IV
11ꢀ
3
O
4
2
2
2 2
) NH
3
2
W
18
O
68(HOSn OH)
3
]
. These
results demonstrate that the mentioned polyoxometalate was
immobilized onto the surface of the functionalized support.
Magnetic properties of Fe O @SiO CH (CH ) NH and 1a
3
4
2
2
2 2
2
nanocomposite were analyzed using an alternating gradient
Scheme 1 The synthesis pathway for preparation of 1a nanocomposite.
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3 4
Fig. 3 The XRD diffraction patterns of Fe O (a) 1a nanocomposite (b) and 1 (c).
IV
of the bulk K H[P W O (HOSn OH) ]$20H O (1) are shown in
11
2
18 68
3
2
Fig. 7A(b) and B(b), respectively.
The catalytic applications
The catalytic applications of (1) in the Knoevenagel
condensation. First, in this study, the catalytic activity of tin(IV)-
containing POM of 1 was examined in Knoevenagel condensa-
tions. We found that 1 catalyzes Knoevenagel condensation as
a heterogeneous and homogeneous catalyst under mild condi-
tions. The Knoevenagel reaction is one of the most important
reactions to form C–C bonds for preparing key derivatives in
Fig. 4 EDX analysis of 1a nanocomposite.
adsorption volume increase, which conrms a mesoporous
2
ꢀ1
character for 1a (specic BET surface area, 56.99 m g ).
The pore size distributions were calculated using the Barret–
Joyner–Halenda (BJH) method applied to the adsorption branches
of the isotherms. Fig. 7B(a) shows the pore size distribution for 1a
nanocomposite. As shown in the gure a hierarchical set of pores
˚
˚
˚
centered near 50 A and 80 A (the average pore diameter 96.99 A)
could be observed in the nanocomposite. As expected, the surface
IV
area of the crystallite K11H[P
2
W
18
O
68(HOSn OH)
3 2
]$20H O (1) is
very small (BJH adsorption cumulative surface area of pores,
2
ꢀ1
5
.318 m g ). The BET analysis illustrates that surface area of 1
markedly increased by immobilization on silica-coated magnetite
nanoparticles of Fe O @SiO CH (CH ) NH . The nitrogen
3
4
2
2
2 2
2
adsorption–desorption isotherms and the pore size distribution
Fig. 5 Alternating gradient force magnetometer (AGFM) of Fe
3 4
O @-
SiO CH (CH NH and 1a nanocomposite at room temperature.
2
2
)
2 2
2
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Fig. 6 SEM images of 1a nanoparticles.
Fig. 7 Nitrogen adsorption–desorption isotherms (A) and pore size distribution (B) of 1a and 1.
28–30
perfume, polymers and pharmaceuticals.
This reaction is
carried out using active methylene compounds and an aldehyde
or a ketone in the presence of a base or Lewis acid as cata- conditions
Table
1
Knoevenagel condensation under various reaction
29
a
3
1–35
lysts.
The bases used as both heterogeneous and homoge-
36
37
neous catalysts. For example, zeolites and hydrotalcites are
used under heterogeneous conditions, while ethylenediamine,
piperidine or amino acids such as glycine and b-alanine are
38,39
used under homogeneous conditions.
The Knoevenagel
condensation of benzaldehyde with malononitrile as a model
ꢁ
was examined using 1 at 25 C in water and ethanol. The results
b
Entry Cat (X g)
Solvent
Time Conversion Yield (%)
are summarized in Table 1 (entries 2 and 5).
1
2
3
4
5
6
7
1 (0.020)
H
H
H
EtOH
EtOH
EtOH
2
2
2
O
O
O
1 h
1 h
2 h
1 h
1 h
2 h
100
100
100
100
100
100
40
85
90
85
85
87
80
—
The yields of reaction show that 1 is much more effective
3
ꢀ
1 (0.010)
1 (0.0050)
1 (0.020)
1 (0.010)
1 (0.0050)
Catalyst free
than the Keggin POMs of [PW12
O
40
]
O
(acid or sodium salt) and
(potassium salt) under
7
ꢀ
the mono-lacunary POM of [PW11
39
]
40
similar conditions. The high activity of 1 compared with its
similar compounds is attributed to the stannous atoms in 1.
The Knoevenagel condensation catalyzed by 1 in water was
carried out with good yields of 85–95% (Table 2). Reaction in
2
H O/EtOH 24 h
a
Reaction conditions: benzaldehyde (1 mmol), malononitrile (1.2
b
absolute ethanol by POM of 1 as a heterogeneous catalyst was mmol), solvent (1 ml), rt, air. Isolated yield.
also carried out and we found that the products were formed
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Table
malononitrile
2
Knoevenagel condensation of aromatic aldehydes with with good yields of 85–95% (Table 3). The catalyst was reused at
a
least ve times without any signicant decrease in catalytic
activities, and the structure of the catalyst remained unchanged
aer recycling.
Catalytic applications of 1a in the oxidation of suldes to
sulfoxides. As sulfoxides and sulfones are useful synthetic
intermediates, the selective oxidation of suldes to sulfoxides or
sulfone is a very important class of reaction in synthetic organic
b
Time (min)
Yield (%) chemistry. Different synthetic methods for the controlled
Entry
R
cat. 1
Conversion
cat. 1
oxidation of conventional suldes have been previously re-
ported for fundamental transformation. Vanadium Schiff base
complexes immobilized on mesoporous silica were used for the
1
2
3
4
5
6
4-CH
4-CH
4-NO
3-NO
4-ClC
2-ClC
3
3
2
2
C
OC
6
H
4
80
10
20
10
60
60
100
100
100
100
100
100
95
95
95
95
92
87
6
H
4
oxidation of aromatic suldes using tert-butyl hydroperoxide as
C
C
6
H
H
4
41
6
4
an oxidant in acetonitrile by Jain et al. Styrene-based
peroxotungstate-modied polymer-immobilized ionic liquid-
6
H
H
4
6
4
phase catalysts [PO
4
{WO(O
2
)
2
}
4
]@ImPIILP (Im ¼ imidazo-
a
Reaction conditions: aromatic aldehyde (1 mmol), malononitrile (1.2 lium) were synthesized by Doherty et al. and used remarkably
b
2
mmol), H O (1 ml), rt, air. Isolated yield.
efficient systems for the selective oxidation of suldes under
42
mild conditions in batch and continuous ow processes. An
alkali exchanged form of mesoporous polytriallylamine and
a Cr-graed material Cr-MPTA-1 were prepared via facile in situ
radical polymerization of a triallylamine template by Bhaumik
et al. and used successfully as an efficient catalyst for the liquid-
Table
malononitrile
3 Knoevenagel condensation of aromatic aldehydes with
a
2 2
phase partial oxidation of suldes to sulfoxides using H O as
43
an oxidant in acetonitrile.
The catalytic activity of 1a was investigated in the selective
oxidation of suldes to sulfoxides using 30% H O as a green
2
2
oxidant at room temperature. To optimize the reaction condi-
tion, methyl phenyl sulde was used as a model compound. The
amount of catalyst and the effect of solvent type were studied on
the rate of the reaction (Table 4). The appropriate amount of
catalysts needed to carry out the reaction was 0.030 g. As shown
b
Time (min)
cat. 1
Yield (%)
Entry
R
Conversion
cat. 1
1
2
3
4
5
6
4-CH
4-CH
4-NO
3-NO
4-ClC
2-ClC
3
3
2
2
C
OC
6
H
4
80
60
20
20
60
30
100
100
100
100
100
100
95
95
95
95
90
85
6
H
4
2 3
in Table 4, solvents H O and CH CN were examined. The results
C
C
6
H
H
4
show that by changing of water to acetonitrile, sulfone was the
reaction product indeed of sulfoxide. Owing to the toxicity of
acetonitrile, water was selected as a suitable solvent for this
purpose. Investigations showed that the oxidation reaction
remained incomplete without catalysts under optimized
conditions (Table 4, entry 5).
6
4
6
H
H
4
6
4
a
Reaction conditions: aromatic aldehyde (1 mmol), malononitrile (1.2
b
mmol), EtOH (1 ml), rt, air. Isolated yield.
a
Table 4 Optimization of the reaction conditions with respect to the effect of catalyst and solvent on the oxidation of methyl phenyl sulfide
b
b
Entry
Cat (mg)
Solvent
Time (min)
Y 1 (%)
Y 2 (%)
Conversion
1
2
3
4
5
1a (10)
1a (30)
1a (30)
Catalyst free
1 (30)
MeCN
MeCN
3 h
60
5
24 h
10 h
—
—
90
30
30
90
92
—
—
40
100
100
100
—
H
2
H
2
H
2
O
O
O
—
a
b
2 2
Reaction conditions: methyl phenyl sulde (1 mmol), solvent (1.25 ml), H O (6 mmol, 30%), rt, air. Isolated yield.
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a
2 2
Table 5 Selective oxidation of sulfides to sulfoxides using H O catalyzed by 1a
b
Entry
1
Catalyst
Substrate
Product
Time (min)
Yield (%)
1a
5
90
95
2
1a
5
3
4
1a
1a
5
5
92
90
5
1a
5
95
a
b
2 2 2
Reaction conditions: catalyst (30 mg), H O (6 mmol, 30%), sulde (1 mmol), H O (1.25 ml) rt, air. Isolated yield.
In order to generalize the scope of the reaction, a series of
The catalytic applications of 1a in the oxidative amination of
various suldes such as benzylic, linear and cyclic suldes were aldehydes. The importance of amides in organic chemistry is
subjected to the experiment under optimized conditions well recognized as they are an integral part of polymers, natural
44,45
(Table 5). According to the data listed in Table 5, the reaction products and pharmaceuticals.
Hence, a variety of catalytic
was performed in a very short time with high yields. To show the methods have been developed for the synthesis of amide
46–48
chemoselectivity of this method, the suldes that contain functionality from aldehydes.
functional groups such as OH and COOCH were subjected to methods of preparing amides is the oxidative amination of
the reaction. The functional groups remained unchanged in the aldehydes. Recently, the palladium nanoparticles immobi-
One of the most common
3
49
oxidation reaction (Table 5, entries 4 and 5).
lized on amino functionalized metal–organic framework MIL-
a
Table 6 Oxidative amidation of aldehydes
b
Entry
X
Time (h) cat. 1a
Conversion
Yield (%)
1
2
3
4
5
6
H
3.5
3
4
3
3.5
4
100
100
100
100
100
100
94
91
90
96
89
86
4-Cl
4-Me
4-NO
2-NO
2
2
2,4-Cl
2
a
2
Reaction conditions: aldehyde (1 mmol), hydroxyl amine hydrochloride (1.2 mmol), sodium bicarbonate (1.2 mmol), catalyst (20 mg), and H O (1
ꢁ
b
ml) at 90 C. Isolated yield.
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Catalysts recovery and reuse
The recycling and stability of the catalyst were investigated. The
catalyst could be easily recovered using an external magnet and
further recycled. The recovered catalyst aer the rst use was
washed thoroughly with ether and reused in a subsequent
reaction. The strong coordination of POM with functionalized
Fe
free from leaching, and reusable for a greater number of cycles
ve times, (Fig. 8)). The FT-IR spectra showed no signicant
structural changes for the catalyst aer ve consecutive runs
Fig. 9).
3 4
O magnetite nanoparticles makes the catalyst stable, nearly
(
(
Fig. 8 The recycling experiment of 1a for the oxidation of methyl
phenyl sulfide (blue) and the recycling experiment of 1a for the
oxidation oxidative amination of aldehyde (red).
Conclusion
In summary, heteropolytungstostannate of K11H[P
2
W
18
O
68
-
1
01(Cr) was used as an active heterogeneous catalyst for the
IV
(HOSn OH)
3
]$H
2
O (1) was used as a heterogeneous and
oxidative amination of aldehydes with hydrogen peroxide as an
homogeneous catalyst in the high selectivity and efficient
Knoevenagel condensation and for fabrication of magnetically
recoverable catalyst via electrostatic interaction. The new type of
magnetically recoverable nanocomposite was characterized
using IR, SEM, XRD, EDX and AGFM. The magnetically recov-
erable nanocatalyst is utilized as a high-selectivity and efficient
catalyst in the oxidation of suldes to sulfoxides and the
oxidative amination of aldehydes. The catalyst could be easily
separated from the reaction mixture and used for the next run
over ve times without any obvious decrease in the catalytic
activity.
50
oxidant. In these methods, there are several disadvantages
such as the problem of catalyst separation from the reaction
mixture and thus less recyclability, harsh conditions and long
reaction times.
IV
12ꢀ
Thus, [P
2
W
18
O
68(HOSn OH)32
]
was supported on amine-
modied silica-coated magnetite nanoparticles and their cata-
lytic performance was evaluated in the preparation of amides
from aldehydes and hydroxylamine hydrochloride in an
aqueous medium (Table 6). The products were conrmed by
comparison with the literature. As the results show, the catalytic
system displayed high performance in the synthesis of active
and inactive amides. The reaction time of aldehydes was lower
in the presence of electron-withdrawing groups (Table 6, entries
Conflicts of interest
2
, 4, 5, 6) than in the presence of electron-donating groups
(Table 6, entry 3). Furthermore, the reaction time was affected The authors declare that they have no competing interests.
by the substituted position. For example, the reaction of the
ortho-derivatives (Table 6, entries 5 and 6) despite having an
electron-withdrawing group has a longer reaction time. High References
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