Multifunctional Fe3O4@: N SiO2@ m SiO2/Pr-Imi-NH2·Ag core-shell microspheres as highly efficient catalysts in the aqueous reduction of nitroarenes: Improved catalytic activity and facile catalyst recovery

Article abstract of DOI:10.1039/c6ra00120c

Core@shell nanoparticles with a superparamagnetic iron oxide core, a middle nonporous silica shell, and an outer organo functionalized mesoporous silica shell were synthesized and evaluated for immobilizing Ag nanoparticles. The iron oxide nanoparticles cores were synthesized via improved chemical coprecipitation method and they were silica-coated by surface silylation via a Stober sol-gel process. Afterward, magnetic core-shell structured mesoporous silica microspheres, Fe₃O₄@nSiO₂@mSiO₂, were synthesized by silylation of Fe₃O₄@nSiO₂ surface using tetraethyl orthosilicate through surfactant directed self-assembly on the surface, followed by etching of the surfactant molecules. Further modification of the surface by grafting of 3-chloropropyl triethoxysilane, nucleophilic substitution reaction of the chloride with imidazole and then quaternization with 2-bromo ethylamine hydrobromide produced the 2-amino ethyl-3-propyl imidazolium bromide functionalized magnetic core-shell structured mesoporous silica microspheres, Fe₃O₄@nSiO₂@mSiO₂/Pr-Im-NH₂. Finally, the target nanocomposite, Fe₃O₄@nSiO₂@mSiO₂/Pr-Im-NH₂·Ag was formed by embedding the silver nanoparticles into the mesoporous nanocomposite. The organic-inorganic nanocomposite was characterized by FT-IR spectroscopy, transmission electron microscopy (TEM), elemental analysis (CHN), vibrating sample magnetometer (VAM), thermogravimetric analysis (TGA) and differential thermal analysis (DTA), X-ray diffraction (XRD) and Brunauer-Emmett-Teller (BET). The catalytic activity test with the nitroarene reduction in an aqueous medium by using NaBH₄ reveals that this Ag-supported nanocomposite is highly active for a wide range of substrates, suggesting highly promising application potentials of the magnetic core-shell-structured mesoporous organosilica. This method has the advantages of high yields, a cleaner reaction, simple methodology, short reaction times, easy workup, and greener conditions. In addition to the facility of this methodology, it also enhances product purity and promises economic as well as environmental benefits.

Full text of DOI:10.1039/c6ra00120c

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PAPER
Multifunctional Fe3O4@nSiO2@mSiO2/Pr-Imi-
NH2$Ag core–shell microspheres as highly efficient
catalysts in the aqueous reduction of nitroarenes:
improved catalytic activity and facile catalyst
recovery
Cite this: RSC Adv., 2016, 6, 41871
*
Mina Jafari Nasab and Ali Reza Kiasat
Core@shell nanoparticles with a superparamagnetic iron oxide core, a middle nonporous silica shell, and an
outer organo functionalized mesoporous silica shell were synthesized and evaluated for immobilizing Ag
nanoparticles. The iron oxide nanoparticles cores were synthesized via improved chemical
coprecipitation method and they were silica-coated by surface silylation via a Stober sol–gel process.
Afterward, magnetic core–shell structured mesoporous silica microspheres, Fe3O4@nSiO2@mSiO2, were
synthesized by silylation of Fe3O4@nSiO2 surface using tetraethyl orthosilicate through surfactant
directed self-assembly on the surface, followed by etching of the surfactant molecules. Further
modification of the surface by grafting of 3-chloropropyl triethoxysilane, nucleophilic substitution
reaction of the chloride with imidazole and then quaternization with 2-bromo ethylamine hydrobromide
produced the 2-amino ethyl-3-propyl imidazolium bromide functionalized magnetic core–shell
structured mesoporous silica microspheres, Fe3O4@nSiO2@mSiO2/Pr-Im-NH2. Finally, the target
nanocomposite, Fe3O4@nSiO2@mSiO2/Pr-Im-NH2$Ag was formed by embedding the silver nanoparticles
into the mesoporous nanocomposite. The organic–inorganic nanocomposite was characterized by FT-
IR spectroscopy, transmission electron microscopy (TEM), elemental analysis (CHN), vibrating sample
magnetometer (VAM), thermogravimetric analysis (TGA) and differential thermal analysis (DTA), X-ray
diffraction (XRD) and Brunauer–Emmett–Teller (BET). The catalytic activity test with the nitroarene
reduction in an aqueous medium by using NaBH4 reveals that this Ag-supported nanocomposite is
highly active for a wide range of substrates, suggesting highly promising application potentials of the
magnetic core–shell-structured mesoporous organosilica. This method has the advantages of high
yields, a cleaner reaction, simple methodology, short reaction times, easy workup, and greener
conditions. In addition to the facility of this methodology, it also enhances product purity and promises
economic as well as environmental benefits.
Received 3rd January 2016
Accepted 2nd March 2016
DOI: 10.1039/c6ra00120c
www.rsc.org/advances
candidates for catalytic applications and evoke new windows for
the exploration in scientic research.
1. Introduction
On the other hand, the direct reduction of aromatic nitro
compounds to the corresponding anilines is one of the toughest
challenges today for researchers and has extensive application
in organic synthesis.3 Although a variety of methods has been
well documented4–9 for this purpose, however, some of these
protocols bear drawbacks such as long reaction time, use of
a toxic and expensive catalyst, carcinogenic solvent, unavail-
ability and reusability of the catalyst. Therefore, there is still
need for a green catalyst which can dominate one or more
drawbacks and also an environmentally benign procedure to
synthesize aromatic amines.
Magnetic core–shell-structured mesoporous silica micro-
spheres are of the great interest to researchers from a wide
range of disciplines, including magnetic uids, catalysis,
biotechnology/biomedicine, magnetic resonance imaging, data
storage, and environmental remediation.1,2 In addition, this
class of material possesses many excellent characteristics, such
as good biocompatibility, rigidity, chemical stability, high
surface areas, large pore volumes, uniform and tuneable pore
sizes, controllable surface functionalization, and resistance to
microbial attack. All these properties make them promising
By considering all the above-mentioned points and in
continuation of our research to develop green chemistry by
using water as reaction medium and molecular host–guest
Chemistry Department, College of Science, Shahid Chamran University, Ahvaz,
61357-4-3169, Iran. E-mail: akiasat@scu.ac.ir; Fax: +98 61 333728044; Tel: +98 61
333728044
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systems,10–12 herein, we have focused our interest on the for several times. The nal product was collected and dried at 60
synthesis of core–shell structured microsphere with a super- ꢀC. The mesoporous SiO2 shells were achieved by treating the
paramagnetic core, a middle nonporous silica shell and an obtained Fe3O4@nSiO2 microspheres with a hydrothermal
outer organo functionalized mesoporous silica shell, in order to method. Typically, 0.1 g of the as-synthesized Fe3O4@nSiO2
render its suitable as a host to support silver nanoparticles. The microspheres was dispersed in a mixed solution of ethanol
catalytic activity of the synthesized nanocomposite, Fe3O4@ (60 mL), deionized water (80 mL), cetyltrimethyl ammonium
nSiO2@mSiO2/Pr-Im-NH2$Ag was investigated for the reduction bromide (CTAB, 0.3 g), and 1.1 mL of aqueous ammonia
of nitroarenes to the corresponding aromatic amines by using (25 wt%) by ultrasonication for 15 min. Then 0.5 mL of TEOS
NaBH4 as reducing agent in aqueous medium.
was added into the above suspension under stirring. Aer
reacting for 16 h at room temperature, the product was collected
by magnetic separation, washed rst with an ethanol/HCl (95/5,
v/v) solution to remove the surfactant template, and then with
ethanol and deionized water. The nal product was dried at 60
ꢀC, and denoted as Fe3O4@nSiO2@mSiO2.
2. Experimental
2.1. General
Iron(II) chloride tetrahydrate (99%), iron(III) chloride hexahy-
drate (98%), cetyl trimethyl ammonium bromide (CTAB),
ammonia, concentrated HCl, tetraethyl orthosilicate (TEOS), 3-
chloropropyltriethoxysilane (CPTES) and nitrobenzene deriva-
tives were purchased from Fluka and Merck companies and
used without further purication. The products were charac-
terized by comparison of their physical data, FT-IR, H & 13C
NMR, spectra with known samples. The purity determination of
the products and reaction monitoring were accomplished by
TLC on silica gel PolyGram SILG/UV 254 plates. FT-IR spectra of
the powders were recorded using BOMEM MB-Series 1998 FT-IR
spectrometer. The TGA curve of the nanocomposite wꢁa1s recor-
2.4. Synthesis of Fe3O4@nSiO2@mSiO2-propyl-3-aminoethyl
imidazolium bromide, Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2
2.4.1. Graing of the propyl chloride unites on the surface
of the Fe3O4@nSiO2@mSiO2 microspheres, synthesis of Fe3-
O4@nSiO2@mSiO2@Pr–Cl.
To
prepare
Fe3O4@nSiO2@
mSiO2@Pr–Cl, 0.4 mL (1.66 mmol) of CPTES was added slowly
into a mixture of 0.2 g of Fe3O4@nSiO2@mSiO2 and 20 mL of dry
toluene. The reaction mixture was reuxed in an oil bath. Aer
24 h of reuxing, the ask was cooled to room temperature. The
solid phase was ltered and washed twice with dry toluene, once
with ethanol and distilled water to remove the un-reacted
ꢀ
ded on a BAHR SPA 503 at heating rates of 10 C min under
air atmosphere, over the temperature range of 25–600 ꢀC. TEM
images were taken using a Zeiss-EM10C at 80 kV. X-ray
diffraction (XRD) patterns of samples were taken on Philips X-
ray diffraction Model PW 1840. The magnetic properties were
investigated with a vibrating magnetometer (Meghnatis Dag-
high Kavir Co., Kashan, Iran).
ꢀ
CPTES. The nanocomposite was then dried at 100 C for 24 h.
2.4.2. Graing of the propyl-3-aminoethyl imidazolium
bromide unites on the surface of the Fe3O4@nSiO2@mSiO2
microspheres, synthesis of Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2.
To a solution of imidazole (0.136 g, 2 mmol) in 25 mL of dry
toluene, sodium hydride (0.048 g, 2 mmol) was added and
stirred under a nitrogen atmosphere at room temperature for 2
h to give sodium imidazole.14 Then Fe3O4@nSiO2@mSiO2@Pr–
Cl (0.3 g) was added and the mixture was reuxed under
a nitrogen atmosphere for 24 h. The resulting product was
ltered and washed with ethanol (3 ꢂ 20 mL) and dried under
vacuum at 100 ꢀC for 8 h to give Fe3O4@nSiO2@mSiO2@Pr-Imi.
Then, to the suspension of Fe3O4@nSiO2@mSiO2@Pr-Imi (0.3
g, 2 mmol) in 25 mL of acetonitrile, 2-bromo ethyl amine
hydrobromide (0.408 g, 2 mmol) was slowly added and the
mixture was reuxed at 80 ꢀC for 12 h. The excess 2-bromo ethyl
amine hydrobromide was removed by ltration, followed by
repeated washing with ethanol. The resulting solid was washed
with NaOH (0.05 g, 1.25 mmol) for neutralization and dried in
2.2. Preparation of Fe3O4 superparamagnetic nanoparticles
Superparamagnetic nanoparticles (MNPs) were prepared via
improved chemical coprecipitation method.13 According to this
method, FeCl2$4H2O (6.346 g, 31.905 mmol) and FeCl3$6H2O
(15.136 g, 55.987 mmol) were dissolved in 640 mL of deionized
water. The mixed solution was stirred under N2 at 90 ꢀC for 1 h.
80 mL of NH3$H2O (25%) was injected into the reaction mixture
rapidly, stirred under N2 for another 1 h and then cooled to
room temperature. The precipitated particles were washed with
hot water and separated by magnetic decantation. Finally,
ꢀ
magnetic NPs were dried under vacuum at 70 C.
ꢀ
an oven at 80 C for 6 h.
2.3. Preparation of Fe3O4@nSiO2@mSiO2 microspheres
Fe3O4@nSiO2 core–shell microspheres were prepared via
a hydrolysis reaction. In a typical synthesis, 0.1 g of as-prepared
Fe3O4 microspheres was rstly washed with 50 mL of HCl
2.5. Immobilization of silver nanoparticles onto the
Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2
solution (0.1 M), and then dispersed in a solution containing 80 0.3 g of Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2 was dispersed in
mL of ethanol and 20 mL of H2O mixed with 1 mL of aqueous 100 mL freshly prepared an aqueous solution of NaBH4 (0.003
ammonia (25 wt%). Aer 15 min of ultrasonication, 0.5 mL of M) and the mixture was stirred for 1 h in an ice bath. To the
tetraethyl orthosilicate (TEOS) was added dropwise into the suspension, an aqueous solution of AgNO3 (100 mL of 0.001 M)
mixture and under violent stirring at room temperature the was added drop wise with constant stirring. Aer 2 h, the ice
reaction continued for 24 h. The product was separated by an bath was removed and the suspension was stirred for 3 h.
external magnet and washed with ethanol and deionized water Finally, the nanocomposite, Fe3O4@nSiO2@mSiO2@Pr-Imi-
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NH2$Ag, was ltered and washed with water for several times load the Ag nanoparticles on the SiO2 surface. In this process,
and dried at 45 ꢀC. The nanocomposite was stored in a dark NaBH4 was selected as the reductant to load Ag nanoparticles
colour bottle.
on the Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2 particles, because
NaBH4 could reduce the Ag+ to Ag0. As shown in Scheme 1,
Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2$Ag was contained a super-
paramagnetic core, a middle nonporous silica shell, and an
outer Ag-functionalized mesoporous silica shell.
This organic–inorganic hybrid nanocomposite contains
Brønsted basic ionic liquid unites. It is thought that this unites
can enhance the hydrophobicity as well as the hydrophilicity of
silica porous and therefore, it can be used as a solid–liquid
phase transfer catalyst in the nitroarenes reduction by NaBH4 in
water. In addition, this arms can probably separate the sup-
ported silver nanoparticles and prevents their agglomeration.
The structure of the resulting hybrid nanocomposite, Fe3-
O4@nSiO2@mSiO2@Pr-Imi-NH2$Ag, was comprehensively
characterized by FT-IR spectroscopy, transmission electron
microscopy (TEM), CHN analysis, thermogravimetric analysis
2.6. Procedure for reduction of nitro compounds, catalyzed
by Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2$Ag
In a 50 mL round bottom ask, nitroaromatic compound
(1 mmol), NaBH4 (5 mmol) and nanocomposite (0.02 g) were
added and completely mixed at room temperature. Then water
(5 mL) was added to the mixture and the resulting suspension
was stirred at 95 ꢀC for the time shown in Table 3. Aer
completion of the reaction as indicated by TLC [using Et2O/n-
hexane as eluent: 1/5], the insoluble supported nanocatalyst was
isolated with the aid of an external magnetic eld and the
aniline product was extracted from liquid with diethyl ether.
3. Results and discussions
The route for the preparation of the magnetic core–shell-
structured mesoporous organosilica having Brønsted basic
ionic liquid unites and its successful application as support for
Ag nanoparticles is schematically illustrated in Scheme 1. For
this gold, Fe3O4 nanoparticles were prepared by co-precipitation
method and coated with silica through the hydrolysis of TEOS.
For the preparation of mesoporous silica-encapsulated Fe3-
O4@nSiO2@mSiO2 nanoparticles, Fe3O4@nSiO2 was coated
with silica through the hydrolysis of TEOS in the presence of
CTAB as a pore-forming agent. Further modication of the
surface by graing of 3-chloropropyl triethoxysilane, nucleo-
philic substitution reaction of the chloride with imidazole and
then quaternization with 2-bromo ethylamine hydrobromide
was produced 2-amino ethyl-3-propyl imidazolium bromide
functionalized magnetic core–shell-structured mesoporous
silica
microspheres,
Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2.
Finally, the nanocomposite was used as a host to support silver
Fig. 1 The FT-IR spectra of (a) Fe3O4, (b) Fe3O4@nSiO2@mSiO2, (c)
NPs. In the preparation process, one very important step was to Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2.
Scheme 1 Schematic illustration of the steps in the synthesis of Fe3O4@nSiO2@mSiO2/Pr-Im-NH2$Ag.
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(TGA), vibrating sample magnetometer (VAM), X-ray diffraction C]C, C]N bands of the imidazole ring, respectively. The bands
(XRD) and Brunauer–Emmett–Teller (BET).
in the range of 2800–3000 cmꢁ1 corresponded to the stretching
Fig. 1 shows the FT-IR spectra of the Fe3O4 (a), Fe3O4@ vibration of C–H bonds of the methylene groups that indicates
nSiO2@mSiO2 (b), Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2 (c) in the successful graing of organic groups to silica in Fig. 1c.
range of 400–4000 cmꢁ1. The characteristic band of Fe–O
The X-ray diffraction (XRD) pattern of Fe3O4@nSiO2@
appeared at about 582 cmꢁ1 in all spectrums. Fig. 1b indicates mSiO2@Pr-Imi-NH2$Ag is shown in Fig. 2. It exists a broad peak
absorption bands at 1090, 989, 801 cmꢁ1 that corresponded to at 2q ¼ 22 that indicates the coated SiO2 shell is amorphous.
the stretching vibration of Si–O–Si, Si–OH, and Si–O–Fe bonds, The obvious diffraction peaks at 2q values of 38.1, 44.3, 64.4,
respectively, that indicating the SiO2 shell was indeed coated and 77.3, were corresponding to the (111), (200), (220), and (311)
onto the surfaces of Fe3O4 nanoparticles. In addition, the FT-IR planes of silver (marked B), respectively. In addition, the peaks
spectra of the Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2 exhibited two located at 2q ¼ 30.5, 35.7, 43.29, 53.68, 57.18, and 62.79 were
new peaks display at 1511 and 1615 cmꢁ1 which were assigned to
Fig. 2 The X-ray diffraction pattern for Fe3O4@nSiO2@mSiO2@Pr-
Imi-NH2$Ag.
Fig. 4 TEM images of Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2$Ag core–
shell nanostructures.
Fig. 3 VSM magnetization curves of the Fe3O4 (a), Fe3O4@nSiO2@
mSiO2/Pr-Imi-NH2 (b) and Fe3O4@nSiO2@mSiO2/Pr-Imi-NH2$Ag (c)
nanoparticles.
Fig. 5 TGA-DTA analysis of Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2.
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Table 2 Reduction of nitroarenes by NaBH4 in the presence of Fe3-
O4@nSiO2@mSiO2/Pr-Imi-NH2$Ag in water (NaBH4/nitro compound:
5 : 1, temperature: 95 ꢀC)
Entry
Substrate
Product
Time (min)
Yield (%)
1
40
98
2
20
98
3
4
5
55
45
35
95
90
80
Fig. 6 Nitrogen adsorption–desorption (a) and pore size distributions
(b) isotherms of Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2.
Table 1 Optimization of the amount of the Fe3O4@nSiO2@mSiO2/Pr-
Imi-NH2$Ag as catalyst for the reduction of nitrobenzene in water at
6
7
50
45
90
97
95 ꢀ
C
Entry
Catalyst (g)
NaBH4 (mmol)
Time (min)
Yield (%)
1
2
3
4
5
6
—
6
5
5
5
6
6
240
60
45
45
45
40
Trace
85
98
98
99
0.01
0.02
0.05
0.05
0.1
98
8
30
92
Scheme 2 Reduction of the nitroarenes.
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corresponding to the (220), (311), (400), (422), (511), and (440)
planes of Fe3O4, (marked P), respectively.
The textural property of Fe3O4@nSiO2@mSiO2/Pr-Im-NH2
was characterized via nitrogen gas porosity measurements.
The magnetic properties of the synthesized Fe3O4 and Fig. 6 shows the N2 adsorption–desorption isotherms and pore
silica coated Fe3O4 nanoparticles were analyzed by VSM, as size distributions. The nitrogen sorption isotherms of magnetic
shown in Fig. 3. The saturated magnetization value (Ms) of core–shell nanocomposite exhibited typical type IV isotherm
the Fe3O4 (a), Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2 (b) and with a hysteresis loop of type H2 at P/P0 between 0.4 and 1.0
Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2$Ag (c) were 69, 10 and 7 which is characteristic of mesoporous materials (according to
emu gꢁ1, respectively. The reason of observed reductions the IUPAC classication), characteristics of pores with inde-
could be explained because of SiO2 coating and supporting nite form and size, according to the TEM images. In addition,
Ag nanoparticles on the surface of Fe3O4 nanoparticles.
the measured BET surface area, BJH pore volume and pore size
In order to further characterize the catalyst, and to conrm is 45.42 m2 gꢁ1, 0.31 cm3 gꢁ1 and 8 nm, respectively. The
the immobilization of the organic components on the pore appropriate surface area and pore volume can provide a favor-
surface, CHN analysis was utilized. The results of elemental able space for anchoring Ag NPs.
analysis showed the presence of carbon (7.6%), nitrogen (2.5%)
and hydrogen (9.8%).
Aer characterization, the catalytic performance of this
heterogeneous organocatalyst has been systematically studied
In addition, the Ag content of the nanocomposite was in the aqueous reduction of nitroarenes by NaBH4. The nitro-
determined by atomic absorption spectroscopy using standard benzene reduction was examined as a model reaction to deter-
addition method and conrmed by UV-vis spectroscopy mine whether the use of Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2$Ag
(loading ca. 0.0556 ꢃ 0.001 mmol gꢁ1).
was efficient and to investigate the optimized conditions
The morphology and particle size distribution of Fe3O4@- (Table 1).
nSiO2@mSiO2@Pr-Imi-NH2$Ag nanostructure was examined by
Aer some experiments, it was found that for facile reduc-
TEM. Fig. 4 reveals that Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2$Ag tion of 1 mmol of nitrobenzene, the use of 5 mmol of NaBH4 in
has spherical shape and core–shell structured magnetic silica the presence of Fe3O4@nSiO2@mSiO2@Pr-Imi-NH2$Ag (0.02 g)
microspheres with nano dimension ranging under 30 nm.
at 95 ꢀC was the best condition. In order to elucidate the role of
TGA analysis of the nanoparticle was also used to determine catalyst, a control reaction was set up in the absence of a cata-
the content of organic functional groups of the sample and their lyst. It was found that in the absence of catalyst only trace
thermal stability. Fig. 5 showed the thermogravimetric analysis- amount of the desired product was observed on the TLC plate
derivative thermogravimetric analysis (TGA-DTG) for Fe3O4@ even aer 6 h of heating.
nSiO2@mSiO2/Pr-Im-NH2. The TGA curve showed that the emis-
Using the optimized reaction conditions, the activity and the
sion mass fraction of wꢀater was about 2.65% when the tempera- scope of the catalyst was explored in the reduction of a variety of
ture was less than 200 C and the other decrease in weight was nitroarenes (Scheme 2, Table 2). As shown in Table 2, electron
10.70% in the temperature range from ca. 200 to 730 ꢀC should be withdrawing/donating groups have a very small inuence on the
ascribed to the decomposition of groups graed to the silica reaction times and yields. In all cases, aniline derivatives were
surface during hydrothermal treatment. It could easily be seen found to be the only product of the reactions and the azoxy, azo
that the total content of the magnetic nucleus and silica shell in and hydrazo compounds as the usual side products in the
the Fe3O4@nSiO2@mSiO2/Pr-Im-NH2 is about 86.6%. In addition, reduction of nitroarenes were not observed.
the results of TGA prove that the attachment of organic groups
Noteworthy is that highly chemoselective reactions were
onto the surface of Fe3O4@nSiO2@mSiO2 nanoparticle and it can achieved in the presence of other functional groups such as
be seen from the organic groups content which is similar to the halogen, carboxylic acid, and ester groups. The silver particles
results of elemental analysis (EA). On the basis of the result of EA immobilized on silica gel are stable in the presence of oxygen
about 0.107 g gꢁ1 of organic ligand is graed on the carriers.
for several months.
Table 3 Comparison of the present system with the some recently reported procedures in the nitrobenzene reduction
No.
Catalytic system
Reaction conditions
Yield
Ref.
1
2
3
4
Polymer supported palladium nanoparticles
Silica gel supported PEG and zinc powder
NAP–Mg–Pd(0)/PSa
Silver nanoparticles immobilized to rice
husk–SiO2–aminopropylsilane composite
Ag/SiO2
NaBH4, H2O, 6 h, R.T, catalyst (0.04 g)
Water, reux, 1 h, catalyst (0.1 g)
96%
86%
97%
95%
15
16
17
18
Et3N, PMHS, H2O, 80 ꢀC, 2 h, catalyst (0.02 g)
NaBH4, H2O, reux, 50 min, catalyst (0.5 g)
5
6
7
EtOH, pressure to 2 MPa, 140 ꢀC, 3 h, catalyst (0.5 g)
NaBH4, EtOH/water (1/1), 5 ꢀC, 4 h, catalyst (0.3 g)
EtOH/water, H2 (4 bar, total pressure), 30 ꢀC, 50 min,
catalyst (0.07 g)
100%
95%
99%
19
20
21
Scrap automobile catalyst
(c-Pt + Mo)/Cb
8
Present catalyst
NaBH4, H2O, reux, 45 min, catalyst (0.02 g)
98%
This work
a
b
Polysiloxane-stabilised “Pd” nanoparticles on NAP–magnesium oxide supports. Pt NPs supported on activated carbon and promoted by
a molybdenum salt.
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novel methodology was also enhanced product purity and
promises economic as well as environmental benets.
Acknowledgements
We gratefully acknowledge the support of this work by Shahid
Chamran University Research Council.
Notes and references
Fig. 7 Recyclability of Fe3O4@nSiO2@mSiO2/Pr-Imi-NH2$Ag in the
nitrobenzene reduction in water after 40 min.
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NH2$Ag nanocatalyst was compared with that of a number of
previously reported catalysts in the reduction of nitroarenes
reaction (Table 3). The results demonstrate that, although, the
former catalytic systems have a reasonably high activity for
conversion of a set of various nitroarenes to their corresponding
aniline derivatives, but the reactions were performed with
a large amount of catalyst and in longer times than with the
present catalyst.
The reusability of the catalyst was also tested. For this gold,
aer completion of the reduction of nitrobenzene, the Fe3-
O4@nSiO2@mSiO2@Pr-Imi-NH2$Ag was easily separated from
the reaction mixture by means of an external magnetic eld and
washed with water and diethyl ether and then dried and reused
for four times under one constant set of operating conditions
ꢀ
(NaBH4/nitrobenzene: 5 : 1, temperature: 95 C, time: 40 min).
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ChemCatChem, 2013, 5, 431.
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