Tungstate-supported silica-coated magnetite nanoparticles: a novel magnetically recoverable nanocatalyst for green synthesis of nitroso arenes

Article abstract of DOI:10.1007/s11696-019-00708-x

Tungstate ion was heterogenized on the silica-coated magnetite nanoparticles and applied for the selective oxidation of anilines to nitroso arenes—with hydrogen peroxide/urea as oxidant in dimethyl carbonate as solvent—in moderate–good yields (40–96%). The catalyst was characterized using different techniques including Fourier-transform infrared spectroscopy, X-ray powder diffraction, vibrating sample magnetometry, scanning electron microscopy, energy dispersive X-ray and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The catalyst was easily recovered using an external magnet and reused for six times.

Full text of DOI:10.1007/s11696-019-00708-x

Chemical Papers
https://doi.org/10.1007/s11696-019-00708-x
ORIGINAL PAPER
Tungstate‑supported silica‑coated magnetite nanoparticles: a novel
magnetically recoverable nanocatalyst for green synthesis of nitroso
arenes
Masoume Jadidi Nejad1 · Elahe Yazdani1 · Maryam Kazemi Miraki1 · Akbar Heydari1
Received: 1 August 2018 / Accepted: 5 February 2019
© Institute of Chemistry, Slovak Academy of Sciences 2019
Abstract
Tungstate ion was heterogenized on the silica-coated magnetite nanoparticles and applied for the selective oxidation of
anilines to nitroso arenes—with hydrogen peroxide/urea as oxidant in dimethyl carbonate as solvent—in moderate–good
yields (40–96%). The catalyst was characterized using diferent techniques including Fourier-transform infrared spectros-
copy, X-ray powder difraction, vibrating sample magnetometry, scanning electron microscopy, energy dispersive X-ray and
inductively coupled plasma atomic emission spectroscopy (ICP-AES). The catalyst was easily recovered using an external
magnet and reused for six times.
Keywords Nitrosobenzene · Tungstate · Magnetite · Nano catalyst · Urea hydrogen peroxide
Introduction
and Halligudi 2007; Gowenlock and Richter-Addo 2004).
However, reactions of these reagents are so violent which
produce complex mixtures of over-oxidized, non-regio
and chemoselective side products (Tayebee and Alizadeh
2007). To overcome these problems, catalytic strategies
using milder oxidants such as aqueous hydrogen peroxide,
tert-butyl peroxide and urea hydrogen peroxide (UHP) have
been devised (Defoin 2004; Priewisch and Rück-Braun
2005; Yost and Gutmann 1970; Di Nunno et al. 1970).
However, the composition of the obtained product mixture
delicately depends on the nature and stoichiometry of the
oxidant, reaction conditions like time and temperature and
most importantly the nature of the catalyst used (Alizadeh
and Tayebee 2005). The catalysts for amine oxidation can
be classifed into three classes, i.e., metallic ions such as
ferrous, oxides (Croston et al. 2002) and complexes of met-
als like Mo, W, V, Zr and Re (Defoin 2004; Priewisch and
Rück-Braun 2005; Yost and Gutmann 1970; Di Nunno et al.
1970), metal nanoparticles like Au (Fountoulaki et al. 2016)
and surface-mediated catalysts like silicate-based molecu-
lar sieves (Reddy and Sayari 1994; Fields and Kropp 2000;
Hosseini et al. 2018). Amongst these classes of catalysts,
only molybdenum and tungstate are selective for nitroso
compounds (Bordoloi and Halligudi 2007; Gowenlock and
Richter-Addo 2004). Recently, tungstate-activated catalytic
oxidations have received considerable interest due to their
high activity and selectivity (Hosseini et al. 2018). However,
Nitroso compounds are versatile building blocks in polymer,
dyes, agrochemicals and pharmaceutical industries (Biradar
et al. 2008). Their easy preparation from inexpensive start-
ing materials renders them cost-efective intermediates in
synthetic organic chemistry (Hunger 2007). Furthermore,
regioselective characteristics of nitroso compounds reac-
tions, brings up a useful method for synthesis of natural-
product simulated molecules (Huang et al. 2016).
Nitroso compounds are classically prepared by oxida-
tion of anilines with various organic or inorganic oxidants
(Defoin 2004; Priewisch and Rück-Braun 2005; Yost and
Gutmann 1970; Di Nunno et al. 1970).
Catalyst-free methods include peracid oxidants, espe-
cially Caro acid, perbenzoic and peracetic acid (Bordoloi
This article is dedicated to memory of Mahdi Zeinoddin.
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11696-019-00708-x) contains
supplementary material, which is available to authorized users.
* Akbar Heydari
heydar_a@modares.ac.ir
1
Chemistry Department, Tarbiat Modares University, P.O.
Box 14155-4838, Tehran, Iran
Vol.:(0123456789)
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Chemical Papers
Scheme 1 Pathway to synthesis
1. NH3(aq)
2. TEOS
1.Na2WO4
2. Calcine 6000C
Fe(II)
Fe3O4@ SiO2
Fe(III)
FSW
of FSW nanoparticles
+
FS
Fig. 1 FT-IR spectra of FS and FSW nanoparticles
Fig. 3 Vibrating sample magnetometry of FSW nanoparticles
Experimental
All chemicals and solvents were of reagent grade and pur-
chased from Merck and used without further purifcation.
Amine derivatives were purchased from Sigma-Aldrich with
reagent grade purity. FT-IR spectra were obtained over the
region 400–4000 cm−1 with a Nicolet IR100 FT-IR with
spectroscopic grade KBr. X-ray powder difraction (XRD)
patterns were obtained at room temperature with a Philips
X-pert 1710 difractometer with Co Kα (λ = 1.78897 Å),
40 kV voltage, 40 mA current and in the range 10–90°
(2θ) with a scan speed of 0.020° s−1. Scanning electron
microscopy (SEM) (Philips XL 30 and S-4160) was used
to study the catalyst morphology and size. Magnetic satura-
tion of the catalyst was investigated using a vibrating mag-
netometer/alternating gradient force magnetometer (VSM/
AGFM, MDK Co., Iran). ICP analysis was performed using
a ICP-OES, Varian 730-ES instrument. 1H NMR spectra
were recorded with a Bruker Avance (DRX 500 MHz, DRX
250 MHz) in pure deuterated chloroform solvent with tetra-
methylsilane as internal standard.
Fig. 2 XRD patterns of FS and FSW nanoparticles
these systems lack enough efciency for large-scale pro-
ductions, due to cumbersome recycling of the catalyst and
contamination of the fnal product either in homogeneous
systems or solid supported ones (Hosseini et al. 2018; Hos-
seini Eshbala et al. 2017). In the present study, on the con-
tinuation of our on-going research on green organic synthe-
sis, we introduce the application of magnetic nanoparticles
supported tungstate as an efcient and easily recyclable
nanocatalyst for selective oxidation of anilines to the cor-
responding nitroso compounds in dimethyl carbonate as the
reaction media (Ma’mani et al. 2011). The importance of
application of dimethyl carbonate as the reaction media is
the enhanced solubility of organic substrates in this solvent
as compared to water solubilities, as well as being a green
and environmentally benign solvent.
Preparation of Fe3O4@SiO2–NaWO4 (FSW)
Preparation of silica‑coated magnetite nanoparticles
(Fe3O4@SiO2)
Magnetite nanoparticles were prepared according to the
previously reported literature (Liu et al. 2017). Separate
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Fig. 4 SEM image of a FS and
b FSW nanoparticles
solutions of 10 mmol FeCl3·6H2O and 5 mmol FeCl2·4H2O
in 40 mL deionized water were mixed and vigorously stirred
at 80 °C. Subsequently, 10 mL of aqueous ammonia solu-
tion (25%) was added in small portions to pH 11. After 2 h,
40 mL of EtOH and 5 mL of tetraethyl orthosilicate (TEOS)
were added to the mixture in a stepwise manner. The result-
ing suspension was stirred mechanically for about 18 h at
40 °C. The resulting black solid was magnetically fltered
and washed several times with water and EtOH and dried
overnight at 60 °C.
Immobilization of tungstate on the surface of silica‑coated
magnetite nanoparticles
1 g of Fe3O4@SiO2 was dispersed in 50 mL of water with
the aid of an ultrasonic bath. Then a solution of 1 mmol
Na2WO4·2H2O in 50 mL water was added gradually to this
dispersion. The mixture was vacuum dried using a rotary
evaporator and calcined for 6 h at 60 °C.
Fig. 5 EDS analysis of FSW nanoparticles
Fig. 6 TEM micrographs of a
FS and b FSW nanoparticles
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Chemical Papers
NO
General procedure for amine oxidation to nitroso
arene
NH2
Oxidant(3 eq.)
Catalyst(mg)
Solvent, temp
To a solution of 1 mmol amine in 2 mL DMC, was added
3 mmol UHP and 0.03 g catalyst. The mixture was stirred
at room temperature for 2 h. Then the catalyst was removed
using magnetic decantation. The reaction vessel was washed
with water and extracted with dichloromethane (3×10 mL).
The organic layer was dried with Na2SO4 and the solvent
was evaporated under vacuum. The crude product was puri-
fed using column chromatography if needed.
Scheme 2 Oxidation of aniline to nitrosobenzene
Table 1 Optimization of reaction conditions of aniline oxidation
Entry Catalyst
(mg)
Oxidant Solvent
Temp. (°C) Yielda (%)
1
2
3
4
5
6
7
8
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.02
0.05
–
UHP
UHP
UHP
UHP
UHP
UHP
UHP
UHP
UHP
UHP
UHP
H2O2
DMF
DMSO
THF
n-Hexane
CHCl3
CH2Cl2
CH3OH
CH3CH2OH r.t.
CH3CN
H2O
DMC
DMC
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
–
–
Results and discussion
20
40
55
50
60
55
50
75b, 20c
90
70
50
–
50
40
82
93
–
Scheme 1 shows the process of catalyst synthesis. The
core–shell nanoparticles of Fe3O4@SiO2 were prepared in
the frst step. Next, the tungstate moiety was linked to the
shell during the calcination process. The resultant nano-
particles were characterized using diferent spectroscopic
methods including Fourier-transform infrared spectroscopy
(FT-IR), XRD, vibrating sample magnetometry (VSM),
SEM–EDS and ICP analysis.
9
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
60
90
r.t.
r.t.
r.t.
r.t.
r.t.
10
11
12
13
14
15
16
17
18
19
20
21
Variations in FT-IR spectrum of prepared fnal nanopar-
ticles as compared to those of magnetite and silica-coated
magnetite nanoparticles, proves the successful immobiliza-
tion of tungstate moiety on the surface of the nanoparticles
(Fig. 1). Peaks at 3400 cm−1 can be ascribed to stretching
vibrations of surface hydroxyl groups. Stretching vibrations
of Si–O–Si groups appeared at around 1090 cm−1. Char-
acteristic peaks of W–O and Fe–O bonds can be seen at
590 cm−1 and 835 cm−1, respectively (Khoshnavazi et al.
2017; Habibi-Yangjeh and Shekofteh-Gohari 2017).
Oxone DMC
TBHP DMC
UHP
UHP
UHP
UHP
UHP
UHP
UHP
DMC
DMC
DMC
DMC
DMC
DMC
DMC
0.03d
0.03e
–
–
Reaction conditions: amine (1 mmol), oxidant (3 mmol), solvent
(2 mL), 2 h
X-ray difraction pattern of the prepared nanoparticles is
illustrated in Fig. 2. The reported reference peaks of mag-
netite, i.e., 21.38, 35.27, 41.62, 50.71, 63.28, 67.64 and 74.6
corresponding to crystalline faces of (111), (220), (311),
(400), (422), (511) and (440) are observed in the pattern
demonstrating the crystalline purity of the particles core
(Kazemi Miraki et al. 2017). A broad band at about 30 is
demonstrative of silica shell. The new assigned bands are
due to immobilized tungstate groups (Yildiz et al. 2014; Yan
et al. 1992) which is in accordance with FT-IR fndings. The
bands observed at difraction angles (2θ) of 32.1, 37.9, 50.5,
57.3 61.1 and 76.5 are corresponding to sodium tungstate
crystalline faces of (220), (311), (331), (422), (511) and
(620) respectively (JCPD car no. 12-722).
aIsolated yield
bNitroso benzene product
cNitrobenzene product
dFe3O4 nanoparticles
eFe3O4@SiO2 nanoparticles
permanent magnet. The relatively low amount of saturation
magnetization as compared to bare magnetite nanoparticles
(73 emu g−1) is due to silica coating and tungstate moieties
as well (Ghonchepour et al. 2017).
SEM image of nanoparticles (FS and FSW) is depicted in
Fig. 4. The morphology was evaluated to be almost spherical
in shape. No considerable change in the morphology was
observed after functionalization.
Hysteresis loop of the prepared nanoparticles is revers-
ible, with no remanence and saturation magnetization of
28 emu g−1 in the applied magnetic feld of − 10,000 to
+ 10,000 Oersted (Fig. 3). These results are indicative of
superparamagnetic character of nanoparticles resulting in
easy separation from reaction mixture using an external
Energy dispersive X-ray (EDS) spectroscopy was per-
formed to speculate the atomic composition of prepared
nanoparticles (Fig. 5). Fe, Si and O are attributed to iron
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O
H
H
N
H
NH2
O
O
Hydrogen peroxide-Urea
O
MeO
O
MeO
OMe
+ MeOH
(1)
NH2
Urea,MeOH, CO2
1
HO
2-
NH
O
HO
O
O
O
Na2WO4 .2H2O
W
OH
O
O
N
Scheme 3 Reaction pathway of oxidation of aniline to nitrosobenzene
oxide core and silica shell of nanoparticles and W and Na
peaks are totally related to tungstate-immobilized groups.
The percentage of tungsten element observed in the EDS
table, is equivalent to 0.563 mmol of this element in 1 g of
the fnal nanoparticles; while EDS results are not quantita-
tively reliable, as it is a surface probe, the precise amount of
tungsten element was determined using inductively couple
plasma (ICP) analysis. The results showed the presence of
0.557 mmol of tungsten per gram of nanoparticles.
yields do not largely depend to solvent polarity. However,
DMC plays a diferent role in the reaction which is shown in
the reaction mechanism (Scheme 3). With the optimized sol-
vent in hand, various oxidants were tested in the model reac-
tion (entries 12–14). Using hydrogen peroxide and oxone as
oxidant, lower product yields were obtained; surprisingly
when tert-butyl hydrogen peroxide was applied, no product
was detected (entry 14). The efciency of the oxidation reac-
tion using DMC as solvent and UHP as oxidant in elevated
temperatures were tested (entries 15–16). It was observed
that at higher temperatures formation of nitro compounds
precedes over nitroso formation. In addition, efect of lower
and higher amount of catalyst was studied (entries 17–18).
As it was expected, no product was detected in the absence
of the catalyst (entry 19). Furthermore, reaction did not pro-
ceed when magnetite and silica-coated magnetite nanoparti-
cles were used as catalyst (entries 20–21). Overall, DMC and
UHP at ambient temperature were much more selective for
production of nitroso compound in excellent yields.
Transmission electron micrographs of FS and FSW nan-
oparticles are shown in Fig. 6. Core–shell nature of both
nanoparticles is observable.
After characterization of nanoparticles, their catalytic
activity was examined in selective oxidation of aniline to the
nitroso benzene as a model reaction (Scheme 2). The opti-
mized reaction condition was studied and the results are out-
lined in Table 1. Initially, the efect of solvent was surveyed
in the formation of nitroso benzene while the oxidant (UHP)
applied, was kept constant (entries 1–11). In DMF and
DMSO no product formation was observed (entries 1–2).
Considerably, low product yield was obtained in THF (entry
3). The yield of product was moderate in alkane, chlorinated
solvents, alcohols and acetonitrile (entries 4–9). The reaction
in water was not so selective and 20% of nitro product was
observed (entry 10). The oxidation reaction was efcient and
highly selective for nitroso product in dimethyl carbonate as
solvent (entry 11). It can be suggested that, polar protic sol-
vents, i.e., water and alcohols promote the reaction by easy
dissolution of oxidant (UHP), while non-polar solvents dis-
solve the amine substrate which result in comparable product
yield in spite of low solubility of oxidant. Hence the product
After achieving the best reaction condition, substrate
scope of the reaction was surveyed. For electron donating
substituents like methoxy, hydroxy and amino group mod-
erately good yields of corresponding nitroso compound
was obtained (2e–f, 2i–j); while for electron withdrawing
groups like Cl, Br and nitro product yield was diminished
(2g–h and 2k). Even though for double nitro substituted
amine, no product formation was detected (2l). Where two
amino groups were present in the starting material, the
protocol was selective to oxidation of only one and no
over or di-oxidated product was observed (2e–f) (Table 2).
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Chemical Papers
Table 2 FSW catalyzed
oxidation of anilines to
corresponding nitroso arenes
NO
NH2
UHP(3 eq.)
Catalyst(30 mg)
R
R
DMC, r.t., 2 h
1a-l
2a-l
NO
NO
NO
NO
2b, 95%
NO
2c,90%
NO
2a, 90%
2d, 96%
NH2
NH2
2f, 65%
2e, 70%
NO
NO
NO
NO
OH
O
Br
Cl
2h, 85%
2i, 70%
2j, 70%
2g, 90%
NO
NO
NO2
NO2
2l, N.D.
NO2
2k, 40%
Reaction conditions: amine (1 mmol), UHP (3 mmol), catalyst (30 mg), DMC (2 mL), room tem-
perature, 2 h
Isolated yield
A reaction pathway of amine oxidation to nitroso arene
in the presence of supported tungstate catalyst is depicted
in Scheme 3. Initially, tunstate is converted to correspond-
ing peroxo ion through reaction with UHP (Strukul 2013)
and hydroxyl amine is produced as an intermediate. When
hydroxyl amine undergoes the same reaction, the desired
product of nitroso arene is formed. The role of dimethyl
carbonate as solvent is the ultimate activation of UHP; frst
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Chemical Papers
consecutively washed three times with dichloromethane,
methanol and acetone, dried at ambient temperature and
the vessel was charged with reagents to react for the next
run. This process was repeated for 6 cycles with negligible
decrease in the catalyst activity (Fig. 7). Furthermore, ICP
analysis of the reaction mixture indicated no leaching of
tungstate from the catalyst support.
The recycled catalyst was characterized after sixth run
and it showed no signifcant changes in its structure (Fig. 8).
Because most of the products have been characterized,
analytical identifcation of the synthesized adducts is limited
to the infrared, MS and melting point for some products.
Spectral data for selected derivatives:
Fig. 7 Recycling of FSW in the oxidation of aniline to nitrosobenzene
3‑Methylnitrosobenzene (2c) Dark yellow solid; 90% yield;
mp: 65–67 °C; IR (KBr) ν (cm−1)=3110, 2923, 1614, 1456,
1261, 1107,753, 624. 1H NMR (500 MHz, CDCl3): δ 3.34
(s,3H), 6.27 (dd, J=1.02–8.15 Hz, 1H), 7.15 (t, J=7.6 Hz,
1H), 7.53 (td, J=7.65–0.65, 1H), 7.58 (dt, J=1.4–7.65, 1H).
it is converted to its corresponding peroxide with liberation
of methanol. Then urea activates the resulting peroxide via
hydrogen bonding (Kraïem et al. 2016).
Reusability of the prepared catalyst was evaluated in the
model reaction. After the reaction was considered com-
pleted, the catalyst was separated from the reaction mixture
with the aid of a permanent external magnet. Then it was
2‑Aminonitrosobenzene (2e) Green solid; 70% yield; mp:
80–82 °C; IR (KBr) ν (cm−1) = 3386, 2922, 2360, 1624,
Fig. 8 a FT-IR, b XRD, c VSM, d TEM, e SEM and f EDS of recycled FSW (after 6th run)
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amines with aqueous hydrogen peroxide. Adv Synth Catal
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1504, 1347, 1254, 1105, 1024, 745. MS (EI, 70 eV): m/z
(%)=122 (M+, 100), 92 (78), 90 (13), 76 (10), 75 (9), 65
(82), 63 (15), 52 (14).
1H NMR (500 MHz, CDCl3): δ 6.95 (t, J=7.8 Hz, 2H),
7.35 (dd, J=7.65–1.3, 2H), 8.68 (s, 2H).
Defoin A (2004) Simple preparation of nitroso benzenes and nitro ben-
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lines to nitroso-compounds. J Chem Soc C 10:1433–1434. https
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4‑Aminonitrosobenzene (2f) Dark green solid; 65% yield;
mp 146–148 °C; IR (KBr) ν (cm−1) = 3354, 2924, 1657,
1614, 1600, 1510, 1327, 1286, 1236, 1103, 834, 632. MS
(EI, 70 eV): m/z (%)=122 (M+, 100), 92 (62), 85 (11), 71
(19), 65 (61), 57 (21).
Fields JD, Kropp PJ (2000) Surface-mediated reactions. 9. Selective
oxidation of primary and secondary amines to hydroxylamines.
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Fountoulaki S, Gkizis PL, Symeonidis TS, Kaminioti E, Karina A,
Tamiolakis I, Armatas S, Lykakis IN (2016) Titania-supported
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nitroso compounds in the presence of hydrogen peroxide. Adv
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0957
1H NMR (500 MHz, CDCl3): δ 4.38 (s, 2H), 6.62 (d,
J=9.05 Hz, 2H), 8.07 (d, J=9.05 Hz, 2H).
4‑Nitronitrosobenzene (2k) Pale yellow solid, 40% yield;
mp: 185–187 °C; IR (KBr) ν (cm−1) = 3416, 2923, 1624,
1549, 1343, 1107, 756. 1H NMR (500 MHz, CDCl3): δ 7.26
(s,2H), 8.42 (s, 2H).
Ghonchepour E, Yazdani E, Saberi D, Aref M, Heydari A (2017)
Preparation and characterization of copper chloride supported
on citric acid-modifed magnetite nanoparticles (Cu2+ -CA@
Fe3O4) and evaluation of its catalytic activity in the reduction of
nitroarene compounds. Appl Organomet Chem 31:e3822. https://
doi.org/10.1002/aoc.3822
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compounds. Chem Rev 104:3315–3340. https://doi.org/10.1021/
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Conclusions
Habibi-Yangjeh A, Shekofteh-Gohari M (2017) Novel magnetic Fe3O4/
ZnO/NiWO4 nanocomposites: enhanced visible-light photocata-
lytic performance through p-n heterojunctions. Sep Purif Technol
184:334–346. https://doi.org/10.1016/j.seppur.2017.05.007
Hosseini Eshbala F, Mohanazadeh F, Sedrpoushan A (2017) Tung-
state ions (WO=4 ) supported on imidazolium framework as novel
and recyclable catalyst for rapid and selective oxidation of benzyl
alcohols in the presence of hydrogen peroxide. Appl Organomet
Chem 32:3597. https://doi.org/10.1002/aoc.3597
Sodium tungstate-supported magnetite nanoparticles were
prepared and characterized. The catalytic activity of the
nanoparticles was screened in the selective oxidation
of anilines to the corresponding nitroso arenes in good
yields using urea/hydrogen peroxide oxidant, in dimethyl
carbonate as a green reaction media. The method showed
acceptable tolerance to electron donating and withdrawing
substituents. The catalyst was easily recovered with the
magnetic power and reused for 6 cycles; although a rela-
tively signifcant drop in product yield was observed after
6 cycles, it can be considered acceptable to a large extent.
Hence this catalytic system brings up a new vision in the
selective catalytic oxidation of anilines to corresponding
nitroso compounds.
Hosseini SH, Tavakolizadeh M, Zohreh N, Soleyman R (2018) Green
route for selective gram-scale oxidation of sulfdes using tung-
state/triazine-based ionic liquid immobilized on magnetic nano-
particles as a phase-transfer heterogeneous catalyst. Appl Orga-
nomet Chem 32:e3953. https://doi.org/10.1002/aoc.3953
Huang J, Chen Z, Yuan J, Peng Y (2016) Recent advances in highly
selective applications of nitroso compounds. Asian J Org Chem
5:951–960. https://doi.org/10.1002/ajoc.201600242
Hunger K (2007) Industrial dyes: chemistry, properties, applications.
Wiley, New York, pp 110–112
Kazemi Miraki M, Yazdani E, Ghandi L, Azizi K, Heydari A (2017)
Mild and eco-friendly chemoselective acylation of amines in
aqueous medium using a green, superparamagnetic, recover-
able nanocatalyst. Appl Organomet Chem 31:e3744. https://doi.
org/10.1002/aoc.3744
Acknowledgements We acknowledge Tarbiat Modares University for
partial support of this.
Khoshnavazi R, Bahrami L, Rezaei M (2017) Heteropolytungstostan-
nate as a homo- and heterogeneous catalyst for Knoevenagel con-
densations, selective oxidation of sulfdes and oxidative amination
of aldehydes. RSC Adv 7:45495–45503. https://doi.org/10.1039/
C7RA06112A
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