Facile method for the synthesis of core/shell Fe3O4@SiO2@SiO2-SH-Au: a super magnetic nanocatalyst for water-medium and solvent-free alkyne hydration

Article abstract of DOI:10.1007/s13738-016-0851-9

Abstract: In a simple approach, gold was successfully anchored to the surface of thiol-functionalized super paramagnetic Fe₃O₄ nanoparticles as heterogeneous catalyst. The core/shell Fe₃O₄@SiO₂@SiO₂-SH-Au was prepared in a facile procedure, without using any reducing reagents by in situ reduction of Au³⁺. The catalyst was characterized by TEM, SEM, IR, XRD and EDX. The catalyst with loaded iron oxide nanoparticles can be recovered from the reaction mixture by an external magnet and reused without considerable decrease in activity even after four runs. As demonstrated experimentally, the catalyst shows remarkably high catalytic efficiencies in promoting water-medium and solvent-free alkyne hydration reaction. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s13738-016-0851-9

J IRAN CHEM SOC
DOI 10.1007/s13738-016-0851-9
ORIGINAL PAPER
Facile method for the synthesis of core/shell Fe3O4@SiO2@
SiO2‑SH‑Au: a super magnetic nanocatalyst for water‑medium
and solvent‑free alkyne hydration
Shahnaz Rostamizadeh1 · Hamid Estiri1 · Mohammad Azad1
Received: 5 March 2015 / Accepted: 30 March 2016
© Iranian Chemical Society 2016
Abstract In a simple approach, gold was successfully
anchored to the surface of thiol-functionalized super para-
magnetic Fe3O4 nanoparticles as heterogeneous catalyst.
The core/shell Fe3O4@SiO2@SiO2-SH-Au was prepared
in a facile procedure, without using any reducing reagents
by in situ reduction of Au3+. The catalyst was character-
ized by TEM, SEM, IR, XRD and EDX. The catalyst with
loaded iron oxide nanoparticles can be recovered from the
reaction mixture by an external magnet and reused without
considerable decrease in activity even after four runs. As
demonstrated experimentally, the catalyst shows remark-
ably high catalytic efficiencies in promoting water-medium
and solvent-free alkyne hydration reaction.
Keywords Magnetic nanocatalyst · Au nanoparticles ·
Recyclable catalyst · Alkyne hydration · Heterogeneous
catalysis
Introduction
The alkyne hydration reaction, leading to the creation of
carbonyl derivatives, is of main interest considering the
wide availability of alkynyl substrates and the fundamen-
tal importance of the carbonyl stile compounds in modern
organic synthesis [1]. The alkyne hydration reaction pos-
sesses a long history. In 1881, Kucherov’s observation has
led to the development of a wide variety of metal-based
catalytic systems [2]. This type of reaction is generally
directed in organic solvents, and in most cases, presence of
a Brønsted acid is essential.
Graphical abstract
The increasing environmental consciousness of the
chemical community has led to the search for more effi-
cient and environmentally friendly methods for chemical
syntheses [3]. Because of the environmental acceptabil-
ity, abundance, and low cost of water, organic reactions in
water have received increased attention [4, 5]. Thus, the
development of efficient procedures for alkyne hydration in
water is highly appreciated.
Although, in recent years the heterogeneous and\or
homogeneous gold catalysts were used for this reaction,
but the utilization of a simple, efficient and reusable cata-
lyst remains an unresolved challenge [6–12]. Therefore,
the development of a magnetic catalytic system involving
Au nanoparticles with superior recoverability would be of
great advantage.
More recently, Au nanoparticles are widely used in a
series of reactions due to their unique physical and chemi-
cal properties [13–15]. Au nanoparticle catalysts are
* Shahnaz Rostamizadeh
shrostamizadeh@yahoo.com; rostamizadeh@kntu.ac.ir
1
Department of Chemistry, K. N. Toosi University
of Technology, P.O. Box 15875-4416, Tehran, Iran
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J IRAN CHEM SOC
Scheme 1 Preparation of
Fe3O4@SiO2@SiO2-SH-Au
Fig. 1 The TEM (a) and SEM (b) images of Fe3O4@SiO2@SiO2-SH-Au
mainly employed for reactions including oxidation [16–
18], cross-coupling [19–21] reactions, etc. Utilization of
more environmentally friendly gold catalysts accompanied
by practical and efficient processes for catalyst separa-
tion and recovery provide both economic and ecological
benefits.
Super magnetic nanoparticles work as an excellent frame-
work for supporting various nanocatalysts as they allow
easy separation of the catalyst from the reaction mixture
using an external magnetic field [22–25]. Lately, many
excellent studies about magnetic composite nanocatalysts
have been reported [26–29].
Recently, incorporation of a magnetic element (e.g.,
Fe3O4) to provide an additional functionality to the origi-
nal material has been widely used in the field of catalysis.
The new prepared catalyst Fe3O4@SiO2@SiO2-SH-
Au with loaded iron oxide nanoparticles can be recovered
from the reaction mixture by an external magnet and reused
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J IRAN CHEM SOC
Fig. 2 The XRD patterns of
Fe3O4@SiO2@SiO2-SH-Au in
the region of 10.0° (2θ) to 80.0°
(2θ)
Experimental
Chemicals and instruments
The reagents and solvents used in this work were obtained
from Fluka or Merck and used without further purification.
Melting points were recorded on a Buchi B-540 appara-
tus. IR spectra were recorded on an ABB Bomem Model
1
FTLA200-100 instrument. H and 13CNMR spectra were
measured on a Bruker DRX-300 spectrometer, at 300
and 75 MHz, using TMS as an internal standard. Chemi-
cal shifts (δ) were reported relative to TMS, and coupling
constants (J) were reported in hertz (Hz). X-ray powder
diffraction (XRD) was carried out on a Philips X’Pert dif-
fractometer with Co Kα radiation. The structure and mor-
phology of the nanocatalysts were characterized by trans-
mission electron microscope (TEM) and were recorded
on a Philips CM-10 instrument on an accelerating volt-
age of 100 kV and scanning electron microscopy (SEM)
analyses were performed by a VEGAIITESCAN electron
microscope (Czech Republic) operating at an accelerating
voltage of 15 kV. The chemical compositions of the sam-
ples are evaluated by scanning electron microscopy (SEM,
SAMX) equipped with an energy-dispersive X-ray (EDX).
Fig. 3 The EDX spectrum of Fe3O4@SiO2@SiO2-SH-Au
without considerable decrease in activity even after four
runs.
However, so far only a few methods for the synthesis
of Au nanoparticle functionalized magnetic silica spheres
were reported in the literature, all of which used reduc-
ing reagents such as H2 and NaBH4 in the final stage for
the conversion of Au3+ to Au0 [30, 31]. In the core/shell
Fe3O4@SiO2@SiO2-SH-Au, in situ reduction of Au3+ with
HS functionalized magnetic silica sphere resulted in the
formation of some –SO3H functional groups. We have syn-
thesized a catalyst (α-Fe2O3)-MCM-41-SH-Au with this
method previously [29].
Catalyst preparation
In the present work, the core/shell Fe3O4@SiO2 was
functionalized with (3-mercaptopropyl) trimethoxysilane
(MPMS) and subsequently Au was anchored to Fe3O4@
SiO2@SiO2-SH.
Synthesis of thiol‑modified silica‑coated magnetic
microspheres
Due to soft–soft interaction between Au and S and the
resulting semi-covalent bonds with approximately 45 kcal/
mol strength [32], we joined −SH on the surface of the
magnetic nanoparticles for deposition of Au.
Then in a green approach, Fe3O4@SiO2@SiO2-SH-
Au has been used as a green and novel gold magnetic
nanocatalyst for water-medium and solvent-free alkyne
hydration.
The Fe3O4@SiO2 microspheres were prepared using a
published method [33]. Fe3O4@SiO2 (0.1 g) microspheres
were dispersed in 30 mL of toluene and ultrasonicated
for 20 min. Then MPMS (0.35 mL) was added drop wise
into the dispersion and ultrasonicated for another 10 min.
Afterward, the mixture was refluxed for 8 h. The mixture
was then separated by an external magnet and washed
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J IRAN CHEM SOC
Fig. 4 The IR spectra of the
(a) Fe3O4@SiO2@SiO2-SH; (b)
Fe3O4@SiO2@SiO2-SH-Au
Loading of Au on thiol‑modified silica‑coated magnetic
microspheres
Table 1 Optimization of reaction condition of phenylacetylene for 2 h
In a typical synthesis, 0.1 g of above synthesized Fe3O4@
SiO2@SiO2-SH was dispersed in 10 mL of ethanol by soni-
cation for 20 min and then 10 mL of 0.025 M ethanol solu-
tion of HAuCl4 was added dropwise with vigorous stirring
to this mixture at room temperature. After stirring for 24 h,
the Fe3O4@SiO2@SiO2-SH-Au was collected by an exter-
nal magnet and washed two times with deionized water and
two times with ethanol (Scheme 1). The amount of gold in
Fe3O4@SiO2@SiO2-SH which was determined by EDX
was found to be (1.6 Au/g).
Entry
Solvent
Temperature °C
Yield %
1
1,4-Dioxane/H2O (2:1)
MeOH/H2O (2:1)
1,4-Dioxane/H2O (1:1)
MeOH/H2O (1:1)
H2O
R.t
R.t
R.t
R.t
R.t
R.t
80
80
80
80
80
70
70
60
75
50
65
70
85
85
88
85
92
95
2
3
4
5
6
Solvent-free
7
1,4-Dioxane/H2O (2:1)
MeOH/H2O (2:1)
1,4-Dioxane/H2O (1:1)
MeOH/H2O (1:1)
H2O
Solvent‑free and water‑medium alkyne hydration
reactions
8
9
10
11
12
The solvent-free and water-medium alkyne hydrations were
carried out in a 10 mL reaction vial for a specified tempera-
ture and time. For solvent-free alkyne hydration, alkyne
(0.3 mmol), was added to 50 mg Fe3O4@SiO2@SiO2-
SH-Au (containing 0.004 mmol Au); it was then stirred at
70 °C for 2 h. For water-medium alkyne hydration, alkyne
(0.3 mmol) and 50 mg Fe3O4@SiO2@SiO2-SH-Au (con-
taining 0.004 mmol Au) were taken in water (4 ml) and
the mixture was stirred at 80 °C for 2 h for the synthe-
sis of different ketones. After completion of the reaction,
Solvent-free
Reaction condition: phenylacetylene (0.3 mmol), solvent (4 ml) and
50 mg Fe3O4@SiO2@SiO2-SH-Au
with toluene and distilled deionized water to give Fe3O4@
SiO2@SiO2-SH as brown powder.
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J IRAN CHEM SOC
Table 2 Hydration reaction catalyzed by various catalysts
catalyst Fe3O4@SiO2@SiO2-SH-Au could be observed
directly through TEM images (Fig. 1a). The SEM images
show that the diameter of resulting microspheres is ~50 nm
(Fig. 1b).
Entry
Catalyst
Time (h)
Yield %
1
2
3
4
5
No catalyst
24
24
24
24
2
Trace
<10
<10
<10
92
Fe3O4
The crystallinity and phase composition of the mate-
rials were characterized using wide-angle X-ray diffrac-
tion (WAXRD). Figure 2 displays the WAXRD pattern
of the sample. The sample of Fe3O4@SiO2@SiO2-SH-
Au showed a relatively well-defined XRD pattern, with
peaks corresponding to Au and Fe3O4, and a broad dif-
fraction band around 23° of 2θ associated with amor-
phous silica phase. In contrast the XRD pattern of the
composite material shows peaks at 2θ = ~ 38.1º, ~44.2º,
~64.8º, and ~78º which correspond to Au (111), (200),
(220), and (311) reflections indicating the formation
of Au(0) nanoparticles with a face centered cubic (fcc)
structure [34]. For Fe3O4, all the diffraction peaks of
(220), (311), (400), (422), (333), and (440) were in good
agreement with the face centered cubic (fcc) structure of
magnetite [35].
Fe3O4@SiO2
Fe3O4@SiO2@SiO2-SH
Fe3O4@SiO2@SiO2-SH-Au
Reaction condition: phenylacetylene (0.3 mmol), H2O (4 ml), 50 mg
catalyst and 80 °C
the reaction mixture was filtered and purified by passing
through a column of silica gel, eluting with 10 % EtOAc in
hexane to afford the pure products.
Results and discussion
Characterization of Fe3O4@SiO2@SiO2‑SH‑Au
The elemental composition of the Fe3O4@SiO2@SiO2-
SH-Au samples was determined by EDX analysis. The
result shown in Fig. 3 reveals that the as-prepared products
contain Fe, Si, Au, S, C, and O.
The newly prepared catalyst Fe3O4@SiO2@SiO2-SH-Au
was characterized with TEM, SEM, XRD, EDX and IR.
The morphologies and structural features of the synthesized
Table 3 Solvent-free and water-medium alkyne hydrations over the Fe3O4@SiO2@SiO2-SH-Au catalyst
R2
O
R2
Catalyst
R1
R1
Entry
R1
Ph
R2
H2O (ml)
Temperature (°C)
Time (h)
Yield %
1
H
H
H
H
H
H
H
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
0
80
80
80
80
80
80
80
80
80
70
70
70
70
70
70
70
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
92
90
92
93
90
86
87
84
91
95
94
95
88
95
93
91
2
4-MePh
4-ClPh
4-MeOPh
4-BrPh
2-BrPh
2-ClPh
Ph
3
4
5
6
7
8
CH3
H
9
n-C6H13
Ph
10
11
12
13
14
15
16
a
H
4-MePh
4-MeOPh
Ph
H
0
H
0
CH3
H
0
n-C6H13
2-BrPh
2-ClPh
0
H
0
H
0
See reaction conditions in “Experimental” Section
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J IRAN CHEM SOC
Ph
HO
Au
H
Ph
2
O
Ph
H
Au
O
Au
Ph
H
H
Ph
OH
H
H
H
Au
O+
Ph
H
Scheme 2 Plausible reaction mechanism of alkyne hydration reac-
tion
the coordination of the HS-ligand with gold nanoparticles
(Fig. 4b).
Reaction condition optimization
Various reaction conditions were investigated including
routine solvents for this reaction [6–8] and temperature
variation. To optimize the reaction conditions, the same
reaction was carried out in each solvent at room tem-
perature and at 80 °C. As can be seen the reactions using
H2O at 80 °C and solvent-free at 70° C gave the best
results (Table 1, Entries 11 and 12). Water has been cho-
sen because of being green and cost effective. Solvent-free
conditions would be suitable for liquid alkynes, but would
have dramatically reduced yields with solid alkynes. Use of
water as solvent is suitable for all alkynes.
Fig. 5 Catalyst recovery at the end of the reaction
Figure 4 shows the FT-IR spectra of the catalyst. The
characteristic bands of Fe–O at 570.0 and 465.5 cm−1 were
attributed to the Fe–O stretch bands. The data were consist-
ent with the values reported for Fe3O4 in the literature [36].
Two strong bands at about 1095.5 and 948.7 cm−1 were
assigned to νas (Si–O–Si) and νs (Si–O–Si), respectively
[37]. Two strong bands at 3428.8 and 1626.9 cm−1 were
assigned to water stretches and bends [38]. MPMS dis-
played characteristic –CH2 stretching bands at 2983 cm−1
(Fig. 4a, b). The weak peak appearing at about 2502 cm−1
is due to the vibration of the HS group (Fig. 4a). In com-
parison with the HS-, the Au-HS displayed no peak due to
In continuation, to show the unique catalytic behavior of
Fe3O4@SiO2@SiO2-SH-Au and to show the role of gold in
this catalyst, this reaction was performed in the presence of
Table 4 Comparison of phenylacetylene formation by Fe3O4@SiO2@SiO2-SH-Au Catalyst with literature reported methods
Entry
Catalyst
Reaction conditions
Time (h)/yield (%) [lit.]
1
2
3
4
5
6
7
8
9
AuCl3
MeOH, tetrachloroauric acid, reflux
BF3·Et2O, THF, 45 °C
H2O, MeOH, CF3SO3H, 60 °C
KPF6; H2O-MeOH, 80 ºC
1,4-Dioxane/H2O (2:1)
H2SO4, MeOH/H2O, reflux
H2O, 80 ºC
24/60 [3]
4/98.8 [4]
1/98 [5]
(triphenylphosphine)gold(I)-p-tolylsulfonate
(Ph3P)AuCH3
gold(I) − NHC complexes H2O
[(IPr)AuCl]AgSbF6
12/100 [8]
18/85 [7]
1.5/100 [2]
1.5/99 [6]
2/98 [29]
2/92a
t-NBu4[Au(C6F5)2Cl2]
Au-HS/SO3H-PMO(Et)
(α-Fe2O3)-MCM-41-SH-Au
Fe3O4@SiO2@SiO2-SH-Au
H2O, 80 ºC
H2O, 80 ºC
a
Present study
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J IRAN CHEM SOC
catalytic amounts of Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2@
SiO2-SH and in the absence of any catalyst (Table 2). It was
observed that only Fe3O4@SiO2@SiO2-SH-Au was able to
catalyze this reaction (Table 2, entry 5). In addition, this
table shows that the catalytic activity of Fe3O4 is very little
(Table 2, entry 2–4), and only it’s magnetic property facili-
tates the efficient recovery of the catalyst from the reaction
mixture. So we can say Fe3O4 only helps in separation of
the catalyst nanoparticles.
the solution transparent and redispersed quickly again
via shaking or ultrasonication when the magnetic field
was removed, demonstrating desired magnetic control of
Fe3O4@SiO2@SiO2-SH-Au, which was particularly appro-
priate for its practical applications in catalysis.
To show the merit of this study, we compared the
obtained results with the results reported. For this purpose,
the reaction of phenylacetylene hydration was chosen as
the model reaction and comparison was carried out on the
basis of reaction conditions, reaction time and percentage
yields obtained (Table 4). It is worth mentioning that this
method is a greener than catalysts entry 1–6 and the recov-
ery of catalyst is easier than entry 7. In addition, the prepa-
ration of this core/shell catalyst is simpler than entry 8 as a
mesoporous catalyst.
The stereochemistry of water addition (syn vs. anti) with
metal catalysts is not yet known; some researchers have
discussed the necessity of an additional coordination site
for water in the context of a proposed inner sphere (syn)
addition mechanism [39, 40]. Therefore, we suggest for the
gold (0) catalyzed addition of water to alkynes, a syn-addi-
tion mode involving activation of both water and the alkyne
by Au which has been proposed (Scheme 2).
Evaluation of the catalytic activity of Fe3O4@SiO2@
SiO2‑SH‑Au trough the alkyne hydration reaction
After optimization of the reaction condition and with the
above initial satisfactory catalytic results, the core–shell
structured nanoparticles Fe3O4@SiO2@SiO2-SH-Au were
investigated for the alkyne hydration reaction in H2O and
under solvent-free conditions. To show the versatility of
this reaction, a variety of alkynes, possessing aliphatic,
internal and external alkynes were employed for the forma-
tion of corresponding ketones (Table 3).
This catalyst showed excellent catalytic activity in the
hydration of different alkyne substrates with excellent
yields (84–95 %). It can be noted that the presence of sub-
stituents on the p-position of benzene had very little influ-
ence on the catalytic efficiencies, showing 90–93 % yields
for the corresponding products (Table 3, entries 2–5). Sub-
stituents on the o-position of benzene with more steric hin-
drance, showed 86 and 87 % yields for the corresponding
products (Table 3, entries 6 and 7). The process was also
applicable to aliphatic alkyne in excellent yield (91 %,
entry 9).This table shows that steric hindrance played a
certain role in the reaction efficiency. Excellent yields were
also obtained in solvent-free alkyne hydration even at rela-
tively lower temperature (70 °C). On one hand, this may
be attributed to high polar character of nanostructures with
high surface areas which could efficiently adsorb moisture
(as the water source). On the other hand, the water mole-
cules that coordinated with the acidic sites of the silica sur-
face were sufficiently nucleophilic to react with the electro-
philic species involved in the reaction [6].
Conclusion
In summary, due to the high price of gold and the impor-
tance of the environment, we have developed a novel Core/
shell Fe3O4@SiO2@SiO2-SH-Au nanocatalyst in a facile
procedure, without using reducing reagents by in situ reduc-
tion of Au3+. The magnetically recyclable catalyst could be
recovered in a facile manner from the reaction mixture with
an external magnet, and used in subsequent runs without
observation of significant decrease in activity even after 4
runs. These ligand-modified Au magnetic nanoparticles are
highly efficient and stable heterogeneous catalysts for alkyne
hydration reaction. In a green approach these reactions pro-
ceeded efficiently in aqueous media and solvent-free condi-
tions without using any organic solvents and acids.
Acknowledgments We gratefully acknowledge the partial support
The reusability and stability of the heterogeneous sys-
tem is a critical requirement for its applications. The recy-
clability of the Fe3O4@SiO2@SiO2-SH-Au catalyst was
investigated in the alkyne hydration reaction using pheny-
lacetylene as substrate. After each cycle of the reaction,
the catalyst was recovered by simple magnetic separa-
tion, washed and then reused in the next reaction run. As
shown in Fig. 5a, the results demonstrate that the catalyst
can be reused for four runs without obvious loss of cata-
lytic activity and selectivity. As shown in Fig. 5b, the com-
posite Fe3O4@SiO2@SiO2-SH-Au readily aggregated in
a few seconds upon an external magnetic field, leaving
of this work by K. N. Toosi University of Technology.
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