Synthesis and characterization of Ni nanoparticles-polyvinylamine/SBA-15 catalyst for simple reduction of aromatic nitro compounds

Article abstract of DOI:10.1016/j.catcom.2011.02.019

This work concerns the preparation and characterization of Nickel nanoparticle-PVAm/SBA-15, a polymer-inorganic hybrid composite, which was effectively employed as a novel heterogeneous catalyst for reduction of aromatic nitro compounds in the presence of

Full text of DOI:10.1016/j.catcom.2011.02.019

Catalysis Communications 12 (2011) 955–960
Contents lists available at ScienceDirect
Catalysis Communications
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Short Communication
Synthesis and characterization of Ni nanoparticles-polyvinylamine/SBA-15 catalyst
for simple reduction of aromatic nitro compounds
a,
b
a
Roozbeh Javad Kalbasi ⁎, Amir Abbas Nourbakhsh , Farzaneh Babaknezhad
a
Department of Chemistry, Islamic Azad University, Shahreza Branch, 311-86145, Shahreza, Isfahan, Iran
Department of Ceramic, Islamic Azad University, Shahreza Branch, 311-86145, Shahreza, Isfahan, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
This work concerns the preparation and characterization of Nickel nanoparticle-PVAm/SBA-15, a polymer-
inorganic hybrid composite, which was effectively employed as a novel heterogeneous catalyst for reduction
of aromatic nitro compounds in the presence of NaBH as a reducing agent. The reaction was carried out in
4
room temperature and an aqueous medium. The catalyst showed good activity for the reduction of a number
of aromatic nitro compounds yielding the respective amine compounds in excellent yield (~98%) at room
temperature along with good reusability.
Received 25 December 2010
Received in revised form 20 February 2011
Accepted 22 February 2011
Available online 1 April 2011
Keywords:
Polymer-inorganic hybrid material
SBA-15
©
2011 Elsevier B.V. All rights reserved.
Nickel nanoparticles
Aromatic nitro compounds
Reduction
1
. Introduction
macromolecular organic ligands [8,9]. However, these kinds of
composites (nanoparticles-polymer) usually suffers from such dis-
advantages as absence of complete heterogeneity and high temper-
ature annealing which generally causes thermal degradation of
organic polymers. In addition, to avoid the problems associated with
metal nanoparticles such as homogeneity, recyclability and the sepa-
ration of the catalyst from reaction system, some other works focused
on immobilizing metal nanoparticles on suitable support materials
such as immobilization in pores of heterogeneous supports [10,11]
like ordered mesoporous silica.
The reduction of aromatic nitro compounds to corresponding
anilines [1,2] is one of the most important transformations in synthetic
organic chemistry as the anilines obtained are commercially important
starting materials and intermediates for a number of valuable com-
pounds such as pharmaceuticals, agrochemicals and dyes. Among the
methods used for reducing the aromatic nitro compounds, catalytic
transfer hydrogenation is the best one being simple, eco-friendly with
safe operation [3,4].
The nanoparticles dispersed on the supports often exhibited
specific physical and chemical properties [5], and gave rise to high
catalytic activity for the reduction of a wide variety of nitrobenzenes
Inorganic–organic hybridization for surface functionalization of
mesoporous silica with organic groups is important for practical
applications in catalysis owing to the inert nature of the amorphous
silica surface [12]. Polymer/mesoporous composite materials have
long been extensively studied for their combined advantages of
mesoporous inorganic materials (e.g. thermal stability) and organic
polymers (e.g. pH stability and chemical functionality). Recently, in
our previous studies [13–15], polymer/SBA-15 hybrids were used as
catalysts.
In this study, polyvinylamine/SBA-15 composite (PVAm/SBA-15),
which combines advantages of both polymer and mesoporous silica,
was synthesized as a heterogeneous support for nickel nanoparticles
to avoid polymer–nanoparticle homogeneity (polymer–nanoparticle
hybrids are soluble in most of solvents so they act as homogeneous
catalysts). So Ni-PVAm/SBA-15 was synthesized for the first time;
and it is used as a new and efficient heterogeneous catalyst for swift
reduction of aromatic nitro compounds to corresponding aromatic
amino compounds in room temperature and an aqueous medium in
the presence of sodium borohydride as a reducing agent.
[6]. Ni nanoparticles in particular being cheap need mild reaction
conditions for high yield of products in short reaction times. The Ni-
nanoparticles are greener as compared to traditional Raney nickel
catalyst in respect of producing undesired toxic wastes [7].
In catalytic applications, a uniform dispersion of nanoparticles and
an effective control of particle size are usually expected. It should be
mentioned that nanoparticles frequently aggregate to yield bulk like
materials, which greatly reduce the catalytic activity and selectivity.
Since metal nanoparticles tend to lose reactivity as they precipitate or
aggregate, they must be embedded in a matrix such as polymer or
⁎
Corresponding author. Tel.: +98 321 3213211; fax: +98 321 3213103.
E-mail address: rkalbasi@iaush.ac.ir (R.J. Kalbasi).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.02.019

956
R.J. Kalbasi et al. / Catalysis Communications 12 (2011) 955–960
2
. Experimental Method
the mixture drop by drop in 20–30 min. After that, the solution was
stirred for 3 h. Then, adding the same amount of NaBH was repeated
4
2
.1. Catalyst Preparation
and again the mixture was stirred for 3 h. Afterwards, the solution was
filtered and washed sequentially with deionized water and methanol
Mesoporous silica SBA-15 was prepared through the method
4 2
to remove excess NaBH and NiCl and was dried in room temperature
described in the literature recently [16].
to yield nickel nanoparticle-polyvinylamine/SBA-15 composite (Ni-
PVAm/SBA-15) (Scheme 1).
Ni nanoparticles-polyvinylamine/SBA-15 (Ni-PVAm/SBA-15)
composite was synthesized in three steps. At first, polyacryl amide/
SBA-15 was prepared by in-situ method. SBA-15 (0.5 g) and acryl
amide (0.25 g) in THF (7 mL) were placed in a round bottom flask.
Benzoyl peroxide (0.025 g) was added and the mixture was heated to
The Ni content of the catalyst was estimated by decomposing the
known amount of the catalyst by perchloric acid, nitric acid, fluoric
acid, hydrochloric acid and Ni content were estimated by atomic
absorption spectrometer. The Ni content of the catalyst estimated by
−1
7
0–75 °C for 5 h while stirring. The solvent was removed and the
atomic absorption spectrometer was 1.66 mmol g
.
mixture dried at 60 °C overnight under vacuum to gain 0.7 g of white
powder of polyacryl amide/SBA-15 (PAA/SBA-15) composite. In the
2.2. General Procedure for the Reduction of Aromatic Nitro Compounds
2 2
second step, PAA/SBA-15 (0.1 g), H O (15 mL), Ca(OCl) (0.4 g) and
stirring bar were placed into a round bottom flask. The stirrer was
started and the mixture was refluxed for 6 h. It was then filtered,
washed with water, n-hexane and finally dried at 60 °C overnight
under vacuum to yield the yellow polyvinyl amine/SBA-15 (PVAm/
SBA-15) composite.
In a typical procedure, a mixture of nitro benzene (2 mmol),
NaBH (8 mmol) and Ni-PVAm/SBA-15 (0.12 g) in water (3 mL) was
4
placed in a 25 mL round bottom flask. The suspension was stirred at
room temperature. Completion of the reaction was monitored by Thin
Layer Chromatography (TLC), using n-hexane/ethylacetate (15:5) as
an eluent. After the completion of the reaction, for the reaction work-
up, the catalyst was removed from the reaction mixture by filtration,
and aniline was extracted from the aqueous medium by ether as an
organic phase. Then, ether was removed from aniline by evaporation;
In the third step, 2 mL aqueous solution of NiCl
added to the obtained PVAm/SBA-15 (0.1 g) together with 3 mL of
O. The mixture was heated for 5 h at 80 °C. Then, the solution of
NaBH [0.23 g (6 mmol)] dissolved in 5 mL methanol was added to
2 2
·6H O (1 M) was
H
2
4
Scheme 1. Preparation of Ni-PVAm/SBA-15.

R.J. Kalbasi et al. / Catalysis Communications 12 (2011) 955–960
957
1
so, a pure product was obtained. The product was identified with H
1
3
NMR, C NMR and FT-IR spectroscopy techniques.
3
. Results and Discussion
3
.1. Catalyst Characterization
The powder X-ray diffraction patterns of mesoporous silica SBA-
5, PVAm/SBA-15 and Ni-PVAm/SBA-15 samples are shown in Fig. 1.
1
Typically, the low angle diffraction pattern shows three peaks at 2θ
values of 0.5°–3°, including one strong peak (100) and two weak
peaks (110) and (200) (Fig. 1a)‚ which corresponded to a highly
ordered hexagonal mesoporous silica framework [17]. The PVAm/
SBA-15 and Ni-PVAm/SBA-15 samples showed the same patterns at
2θ values of 0.5°-3°, indicating that the long-range order of the SBA-15
framework was well retained after the immobilization (Fig. 1b, c).
In the amorphous Ni-PVAm/SBA-15, Ni peak could not be seen in
XRD due to the homogeneity of the composite and Ni-PVAm/SBA-15
showed an amorphous pattern at 2θ values of 20°–40° (Fig. 1d). In order
to prove the existence of the Ni nanoparticles in the composite, the
composite catalyst was exposed to temperature (200 °C and 400 °C). On
heating, amorphous Ni- changed to crystalline, thus appearing as a peak,
the intensity of which increased at higher temperature. The XRD
patterns of Ni-PVAm/SBA-15 sample at two temperatures (200 °C and
Fig. 2. FT-IR spectra of (a) PVAm/SBA-15 and (b) Ni-PVAm/SBA-15.
1436 cm− is the bending vibration absorption of the N–H bond.
1
−1
Moreover, the presence of peaks at around 2800–3000 cm
corre-
sponds to the aliphatic C–H stretching in PVAm/SBA-15. According to
our previous study [14], the characteristic band at 1089 cm− is due to
the Si–O stretching in Si–O–Si structure of SBA-15. The appearance of
the above bands shows that PVAm was attached to the surface of SBA-
15, and the PVAm/SBA-15 composite was obtained.
1
4
2
00 °C) can be seen in Fig. 1e and Fig. 1f, respectively. After calcining at
00 °C (Fig. 1e), XRD patterns of residue showed that the broadening
and low intensity of peaks could be attributed to nano size of Ni particles
described in later section). Nevertheless, the peaks at 2θ=44.29,
(
−1
corresponding to the plane (111) of fcc nickel, could be observed. The
same procedure can be seen in Fig. 1f, nickel nanoparticles peak is
shown at 2θ=44.29.
As shown in Ni-PVAm/SBA-15 spectrum, the band around 1090 cm
−1
was shifted to lower wavenumbers (999 cm ) (red shift). This might be
due to the introduction of the Ni groups. The peak intensity of the
−1
Fig. 2 shows the FT-IR spectra of PVAm/SBA-15 and Ni-PVAm/SBA-
spectrum of Ni-PVAm/SBA-15 at 1346 cm is lower than that of PVAm/
SBA-15 and also the peak is shifted to lower wavenumbers. These might
result from interaction between the Ni particles and N–H groups.
1
5. In the FT-IR spectrum of PVAm/SBA-15 (Fig. 2a), the band at
2+
Fig. 3a shows the DRS-UV of PVAm/SBA-15, Ni -PVAm/SBA-15
and Fig. 3b shows the DRS-UV of Ni-PVAm/SBA-15. DRS UV of Ni2+
-
PVAm/SBA-15 is shown because we want to show the difference
between DRS UV of Ni2 -PVAm/SBA-15 and Ni-PVAm/SBA-15. There
is no characteristic peak in PVAm/SBA-15. In the case of Ni(II) amin
complex, there are no LMCT transitions at wavelengths longer than
+
2
50 nm, so the peaks involve in this case are d–d transitions only
(
Fig. 3a). Assuming octahedral coordination of Ni(II), the observed
3
absorption bands can be attributed to the electronic transitions of T1g
(
(
3
3
3
1
3
P)← A2g (F) (365 nm), T1g (F)← A2g (F) (592 nm), E
g
(D)← A2g
F) (745 nm) (Fig. 3a) [18]. As shown in Fig. 3b, Ni-PVAm/SBA-15
shows the feature around 250 nm [19] due to the Ni nanoparticles. So
2+
comparing the two spectra, it can be found that all peaks of Ni
disappeared as a result of the reduction of Ni2 to Ni .
+
0
The TEM micrographs of SBA-15 and Ni-PVAm/SBA-15 were
depicted in Fig. 4. The TEM micrographs of SBA-15 materials
(
Fig. 4a) showed the well-ordered hexagonal arrays of pores with
one-dimensional channel indicating 2-D hexagonal mesopores.
In order to reveal the morphology and structure for Ni nanopar-
ticles, TEM experiment is carried out for the Ni-PVAm/SBA-15 (Fig. 4b,
c). As shown in Fig. 4b, spherical nickel nanoparticles (average
diameter of ~3 nm) were seen to be dispersed mainly inside the pores
of SBA-15. TEM images of Ni-PVAm/SBA-15 confirm that the
hexagonally arranged mesopores of SBA-15 are retained and no
damage of the periodic structure of the silicate framework was
observed. Fig. 4b clearly shows the uniform dispersion of nickel
nanoparticles inside the SBA-15 channels. Fig. 4c shows Ni particles
with a spherical shape and relatively narrow size distribution in the
range of 2–5 nm.
EDX analysis from area shown in Fig. 4c, confirmed the presence
Fig. 1. XRD patterns of (a) mesoporous silica SBA-15 (b) PVAm/SBA-15 (c) Ni-PVAm/
SBA-15 (2θ=0.5–5) (d) amorphous Ni-PVAm/SBA-15 (2θ=15–70) (e) Ni-PVAm/SBA-
of nickel nanoparticles and Na which attributed to NaBH
reducing agent.
4
used as a
15 burned in 200 °C for 2 h and (f) Ni-PVAm/SBA-15 burned in 400 °C for 2 h.

958
R.J. Kalbasi et al. / Catalysis Communications 12 (2011) 955–960
Table 1
Surface area analyses of samples.
Sample
BET surface area
Pore volume
Pore diameter
(nm)
2
−1
3
−1
(m g
)
(cm g
)
Mesoporous silica SBA-15
PVAm/SBA-15
Ni-PVAm/SBA-15
1430
856
59
1.9
1.1
0.9
9.9
8.8
2.8
PVAm/SBA-15 and Ni-PVAm/SBA-15 exhibit a smaller specific surface
area, pore size and volume in comparison to those of pure SBA-15,
which might be due to the presence of polymer and metal
nanoparticles on the surface of the SBA-15 [14]. Total pore volume
3
−1
3 −1
of SBA-15 (1.90 cm g ) was decreased to 0.85 cm g
after the
deposition of Ni particles. Multiple point BET surface area of SBA-15,
PVAm/SBA-15 and Ni-PVAm/SBA-15 were determined 1430, 856 and
2
−1
5
8.5 m g , respectively. Although there are significant decreases in
surface area and total pore volume, SBA-15 pores were not blocked by
deposition of polymers and Ni nanoparticles.
3.2. Effect of Reaction Conditions on Nitrobenzene Reduction
The reduction of nitro benzene was chosen as a model reaction to
test the catalytic activity of the Ni-PVAm/SBA-15 for the reduction of
aromatic nitro compounds. This reduction reaction was carried out in
water as a solvent and in room temperature.
To identify the effect of nickel nanoparticle concentration on the
reduction reaction, different amounts of NiCl
The amount of NiCl ·6H O was changed from 0 mmol to 2.0 mmol
while the other values were constant (the molar ratio of NiCl to NaBH
was 1:6). As shown in Table 2, the catalytic activity was improved by
increasing the amount of NiCl ·6H O and 2.0 mmol showed the best
2 2
·6H O were examined.
2
2
2
4
Fig. 3. DRS-UV spectra of (a) PVAm/SBA-15 and Ni2+-PVAm/SBA-15; Ni-PVAm/SBA-15.
2
2
The BET specific surface areas and the pore size of mesoporous
silica SBA-15, PVAm/SBA-15 and Ni-PVAm/SBA-15 had been calcu-
lated using Brunauer–Emmett–Teller (BET) and Barrett–Joyner–
Halenda (BJH) methods, respectively (Table 1). It was clear that
catalytic activity. As the catalytic reaction mechanism involves Ni-
−
nanoparticle mediated electron transfer from BH
4
ion to the nitro
−
compounds, the amount of H sites on the catalyst surface increased
by increasing nickel nanoparticles, and a larger amount of hydride can
Fig. 4. TEM images of (a) mesoporous silica SBA-15 (b,c) Ni-PVAm/SBA-15.

R.J. Kalbasi et al. / Catalysis Communications 12 (2011) 955–960
959
Table 2
Effect of NiCl
Table 5
The catalyst reusability.
O molar ratio on the catalytic activity.a
a
2
·6H
2
NiCl
0
2
·6H
2
O (mmol)
Reaction time (min)
Yield (%) b
Cycle
Reaction time (min)
Yield (%) b
45
45
45
45
45
45
0
70
90
90
N90
98
fresh
20
20
20
20
20
20
98
98
98
98
98
98
0.6
1.0
1.2
1.5
2.0
1
2
3
4
5
a
a
Reaction conditions: nitrobenzene (2 mmol), Ni-PVAm/SBA-15 (0.1 g), H
(8 mmol), room temperature.
Isolated yield.
2
O (3 mL),
Reaction conditions: nitrobenzene (2 mmol), Ni-PVAm/SBA-15 (0.12 g),
(8 mmol), room temperature.
Isolated yield.
2
H O
NaBH
4
(3 mL), NaBH
4
b
b
be transferred to the nitro compounds through the catalyst. Therefore,
yield by raising catalyst amounts, 0.12 g was the best value and the
excess amount didn't have any effect on the catalytic activity.
The reusability of the catalyst was studied by using Ni-PVAm/SBA-
15 in recycling experiments. In order to regenerate the catalyst, after
completion of the reaction, it was separated by filtration, washed
several times with deionized water and ether, then dried and used in
the subsequent run. By using the regenerated sample after five cycles,
the yield was constant (Table 5).
2 2
higher concentrations of NiCl ·6H O precursor showed better cata-
lytic activity.
In order to investigate the effect of SBA-15 on catalytic activity,
different kinds of Ni-PVAm/SBA-15 catalysts in which the amount of
the SBA-15 were 0.0, 0.25, 0.5 and 1 g, for polymerization of 0.25 g
acryl amide respectively, prepared and used in the reduction of
aromatic nitro compounds. Based on our previous study [14], the
amine contents of catalysts were found to be 14.00, 13.80 and
Nitro benzene was chosen as a model for the reduction reaction
over Ni-PVAm/SBA-15. In addition, this catalyst shows an excellent
activity on the reduction of other aromatic nitro compounds (Table 6).
−1
1
2.60 meq g
for the amounts of SBA-15 (0.25, 0.50 and 1.00 g),
respectively. It can be observed that by increasing the ratio of SBA-15
to PVAm , the amount of amine sites decreased. Since nickel
nanoparticles were placed on the amine sites, less amine site caused
less nickel concentration. The best result (98% yield in 45 min) was
obtained when ratio of SBA-15 (g) to Acryl amide (g) was 0.5:0.25.
Obviously, the greater amount of the SBA-15 did not lead to the higher
catalytic activity of the Ni-PVAm/SBA-15 as a catalyst. It should be
noticed that when SBA-15 or PVAm/SBA-15 used as catalysts, the
reaction did not proceed significantly.
4. Conclusion
In conclusion, we demonstrated a facile nickel nanoparticle
preparation inside modified mesoporous silica and the utilization of
this composite as a new heterogeneous catalyst system. Reduction of
Table 6
a
The effect of NaBH
investigated using 0.1 g of the catalyst and 2.0 mmol of nitro benzene.
The results showed that by increasing the amount of NaBH , the
percentage of yield increased (Table 3). With further increase in the
NaBH amount, the percentage of yield remained the same. 8 mmol
4
amount on the reduction reaction was
Reduction of various aromatic nitro compounds over Ni-PVAm/SBA-15.
Yield (%) b
Entry
1
Substrate
Product
Time (min)
20
4
98
4
2
3
4
2
3
4
98
98
98
was the best value because fewer values were not enough for the
reduction of the mentioned amount of aromatic nitro compounds and
the excess values didn't have any effect on the reaction.
The variation of catalytic activity with the amount of catalyst was
studied (Table 4). In general, there was an upward trend of aniline
Table 3
a
Effect of reducing agent on the nitrobenzene reduction.
NaBH
4
(mmol)
Reaction time (min)
Yield (%) b
5
6
2
98
98
4
8
1
.0
.0
0.0
45
45
45
70
98
98
a
2
Reaction conditions: nitrobenzene (2 mmol), Ni-PVAm/SBA-15 (0.1 g), H O (3 mL),
room temperature.
85
b
Isolated yield.
7
8
4
98
98
Table 4
a
Effect of catalyst amount on the nitrobenzene reduction.
Amount of catalyst (g)
0
Reaction time (min)
Yield (%) b
42
120
120
45
20
20
0
98
98
98
98
0
0
0
0
.06
.1
.12
.14
a
a
Reaction conditions: nitrobenzene (2 mmol), catalyst (Ni-PVAm/SBA-15), NaBH
O (3 mL), room temperature.
Isolated yield.
4
Reaction conditions: nitro aromatic compound (2 mmol), Ni-PVAm/SBA-15
(8 mmol), H O (3 mL), room temperature.
Isolated yield.
(
8 mmol), H
2
(0.12 g), NaBH
4
2
b
b

960
R.J. Kalbasi et al. / Catalysis Communications 12 (2011) 955–960
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