Transition metal nanoparticles supported on mesoporous polyaniline catalyzed reduction of nitroaromatics

Article abstract of DOI:10.1016/j.catcom.2013.03.034

Transition metal nanoparticles supported on mesoporous polyaniline (Meso-PANI) were synthesized by the self assembly of dual surfactants followed by the in situ reduction of metal precursors in aqueous solution. Catalysts were investigated in the reductio

Full text of DOI:10.1016/j.catcom.2013.03.034

Catalysis Communications 37 (2013) 64–68
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Transition metal nanoparticles supported on mesoporous polyaniline
catalyzed reduction of nitroaromatics
Mahesh Tumma, Rajendra Srivastava ⁎
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar 140001, Punjab, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Transition metal nanoparticles supported on mesoporous polyaniline (Meso-PANI) were synthesized by the self
assembly of dual surfactants followed by the in situ reduction of metal precursors in aqueous solution. Catalysts
were investigated in the reduction of nitroaromatics in the presence of NaBH as a reducing agent. Among the
4
catalysts investigated, Cu(10%)-Meso-PANI exhibited the highest activity. Cu supported on Meso-PANI exhibited
significantly high activity compared to Cu supported on conventional PANI. Recycling experiments suggest that
catalysts can be reused without significant loss in catalytic activity.
Received 26 January 2013
Received in revised form 7 March 2013
Accepted 26 March 2013
Available online 3 April 2013
Keywords:
Mesoporous polyaniline
Reduction
©
2013 Elsevier B.V. All rights reserved.
Nitroaromatics
Metal nanoparticles
Sodium borohydride
1
. Introduction
and distribution of metal nanoparticles [25–27]. Among the conducting
polymers, polyaniline (PANI) has been proved to be an excellent candi-
date due to its unique redox property and high electrical conductivity
[25–29]. Recently, we reported the morphological controlled synthesis
of micro-/nano PANI [28]. Very recently, we developed the synthesis
protocol for the preparation of Meso-PANI and its application is
Amines are important synthetic intermediates for the production
of several organic compounds used in dye, pigment, agrochemical,
polymer, and pharmaceutical industries [1–4]. Several routes are
known for the synthesis of aliphatic amines, which includes reductive
amination [5], reduction of nitriles [6], and direct amination of alco-
hols [7,8]. The most common approach for the synthesis of aromatic
amine is based on the reduction of nitroaromatics. In conventional
approach, catalytic hydrogenation is adopted to prepare aromatic
2 2
shown in H O and glucose sensing [29]. We are exploring the pos-
sibility of supporting various transition metal nanoparticles on
Meso-PANI and find their applications in catalysis. In this study, we re-
port a novel method to synthesize transition metal nanoparticles support-
ed on Meso-PANI and investigate their application in the reduction of
nitroaromatics (Scheme 1) and compared the activity with metal nano-
particles supported on PANI.
2
amines [9–13]. High temperature and high H pressure are some of
the limitations associated with the catalytic hydrogenation process. To
overcome these limitations, the precious noble metal catalysts have
been used in combination with various reducing agents [14–17]. Howev-
er, the high cost and limited availability of these metals led researchers to
find more economical, easily available, and environmentally friendly
alternatives. Recently, several studies based on transition metal mediat-
ed catalytic routes (including metal complexes) have been developed
2. Experimental
PANI and Meso-PANI were prepared according to the reported
procedures [29]. PANI and Meso-PANI were decorated with metal
nanoparticles by the in situ reduction of metal halide in aqueous solu-
tion. In this study, a variety of transition metal nanoparticles support-
ed Meso-PANI were prepared (represented as M(x%)-Meso-PANI;
where M = Cu, Ni, Co, Fe, and Mn; and x = input metal loading).
In a typical synthesis of Cu(10%)-Meso-PANI, Meso-PANI (500 mg)
was dispersed in 500 ml of deionized water with magnetic stirring
at room temperature for 0.5 h and then with ultrasonication for an-
[18–24]. Although some advancement has been made in the catalytic re-
duction of nitroaromatics using heterogeneous catalysts [9–24], still the
development of fast, cost effective, and mild catalytic route for the reduc-
tion of nitroaromatics with negligible metal contamination is of high
demand.
Conducting polymer–metal nanocomposite materials have been
widely used in catalysis [25–27]. Metal particles govern the catalytic
properties, whereas the polymer matrix provides flexible functionali-
ties to control host–guest interaction, thereby, ensuring the growth
other 0.5 h to obtain a uniform solution. CuCl
added to the resultant solution and stirred for another 5 min to en-
sure complete mixing. Further, 100 ml of 0.2% NaBH was added
2 2
.2H O (150 mg) was
4
and the reaction mixture was stirred at ambient condition for 24 h.
The resulting hybrid material was collected by centrifuging with an
⁎
Corresponding author. Tel.: +91 1881 242175; fax: +91 1881 223395.
E-mail address: rajendra@iitrpr.ac.in (R. Srivastava).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.03.034

M. Tumma, R. Srivastava / Catalysis Communications 37 (2013) 64–68
65
0
0
0
0
.25
.20
.15
.10
210
180
150
120
90
Scheme 1. Reduction of nitroaromatics over transition metal nanoparticles supported
on Meso-PANI.
0.05
.00
0
0
10 20 30 40 50
Pore diameter (nm)
Table 1
Textural properties of materials synthesized in this study.
Sample
S
BET
Total pore
volume (cm /g)
Metal content
(%)
2
3
(m /g)
6
0
0
0
PANI
35.4
83.6
76.3
71.6
80.4
78.8
33.2
0.034
0.085
0.075
0.072
0.081
0.076
0.030
-
-
Meso-PANI
Cu(10%)-Meso-PANI
7.80
7.55
0.85
0.83
0.82
3
a
Cu(10%)-Meso-PANI
Cu(1%)-Meso-PANI
a
Cu(1%)-Meso-PANI
Meso-PANI
Cu(1%)-PANI
a
Cu(10%)-Meso-PANI
PANI
Recovered catalyst after 5th recycle.
-30
ultracentrifuge. Atomic absorption spectroscopy was used to determine
the metal loading in the materials (Table 1). For the comparative study,
Cu(1%)-PANI was also prepared by following the procedure mentioned
above.
Details about the materials, catalysts characterization, and procedure
adopted for the catalytic investigation have been provided in the
supporting information.
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
Fig. 2. N
2
adsorption–desorption isotherms of PANI, Meso-PANI, and Cu(10%)-Meso-PANI
samples (Inset shows their pore size distributions).
(
Fig. 1(a)). A broad low-angle peak for Meso-PANI confirmed the ex-
3
. Results and discussion
istence of the ordered mesostructured PANI (Inset, Fig. 1a). Besides
reflections corresponding to Meso-PANI [29], several more reflections
were also observed in the XRD pattern of Cu(10%)-Meso-PANI. The
diffraction peaks located at 2θ values of 43.3°, 50.4°, 74.0°, and 90.0°
correspond to (111), (200), (220), and (311) respectively, for metallic
Cu in the fcc lattice (JCPDS, File No. 85-1326) (Fig. 1(b)). Cu(1%)-
Meso-PANI sample shows XRD reflections only corresponding to Meso-
PANI. No reflection for Cu-nanoparticles was observed in the XRD pattern
of Cu(1%)-Meso-PANI confirming that Cu nanoparticles are incorporated
3
.1. Synthesis and characterizations of catalysts
XRD patterns for PANI and Meso-PANI matched well with the
reported literature (Fig. 1a) [29]. In the XRD pattern of Meso-PANI,
two peaks at 2θ = 20.5° and 25.3° were observed. First peak is related
to the periodicity parallel to the polymer chain, while the latter peak
represent the periodicity perpendicular to the polymer chain [29]
(b)
(
a)
1
2
3
4
5
6
2θ
Cu(10%)-Meso-PANI
Cu(1%)-Meso-PANI
PANI
Meso-PANI
1
0
20
30
40
50
20 30 40 50 60 70 80 90
2
θ
2θ
Fig. 1. XRD patterns of (a) PANI and Meso-PANI (Inset shows the low-angle XRD pattern of Meso-PANI) (b) Cu(10%)-Meso-PANI and Cu(1%)-Meso-PANI samples.

66
M. Tumma, R. Srivastava / Catalysis Communications 37 (2013) 64–68
Fig. 3. (a) TEM images (i and ii) of Cu(10%)-Meso-PANI and corresponding diffraction pattern (iii) of Cu nanoparticles. (b) Energy-dispersive X-ray spectra of Cu(10%)-Meso-PANI.
inside the mesoporous wall of Meso-PANI. XRD patterns of M(1%)-
Meso-PANI samples were found to be similar to that of Cu(1%)-
Meso-PANI. Among the materials investigated, catalytic activity for
Cu(10%)-Meso-PANI was found to be the best; therefore, the detailed
characterization for only Cu(10%)-Meso-PANI is provided.
Surface area and pore volume of catalysts were calculated from
2
the N adsorption–desorption studies. PANI samples synthesized in
this study exhibited type IV isotherm but with different hysteresis
loops (Fig. 2). Meso-PANI and Cu(10%)-Meso-PANI exhibited H3 hys-
teresis loops, which show that materials are having non-rigid aggre-
gates like particles and signify the presence of slit shape pores,
whereas, PANI exhibited H4 hysteresis. Materials exhibited broad pore
size (4–10 nm) distribution (Fig. 2, inset). BET surface area for Cu
metal decorated Meso-PANI was found to be less than Meso-PANI but
Table 2
Comparative catalytic activity in the reduction of p-nitroaniline over various catalysts
investigated in this study.
S. No.
Catalyst
Time
cProduct yield (%)
1
2
3
4
5
6
6
7
8
9
Cu(10%)-Meso-PANI
Cu(10%)-Meso-PANI
Cu(1%)-Meso-PANI
Cu(1%)-Meso-PANI
Cu(1%)-Meso-PANI
Cu(1%)-PANI
Ni(1%)-Meso-PANI
Co(1%)-Meso-PANI
Fe(1%)-Meso-PANI
45 min
45 min
45 min
1.5 h
1.5 h
1.5 h
1.5 h
1.5 h
1.5 h
1.5 h
2.5 h
2.5 h
2.5 h
3.5 h
3 h
100
98
55
100
96
32
52
50
18
12
100
100
100
100
100
a
a
Table 3
Influence of solvent in the reduction of p-nitroaniline over Cu(10%)-Meso-PANI.
aProduct yield (%)
S. No.
Solvent
1
2
3
4
5
6
7
8
9
Ethanol
Methanol
Water
Ethanol: Water (1:1)
THF
Isopropanol
Acetonitrile
Dichloromethane
Toluene
100
24
88
100
10
12
0
Mn(1%)-Meso-PANI
b
10
11
12
13
14
CuSO
CuCl
4
b
2
b
b
3 2
Cu(NO )
Cu(OAC)
2
b
CuI
0
0
Reaction condition: p-nitroaniline (1 mmol), solvent (10 ml, EtOH: water = 1:1), catalyst
(
50 mg), NaBH
Catalytic activity results after 5th recycle.
4
(5 mmol), temperature (313 K).
a
b
c
Reaction condition: p-nitroaniline (1 mmol), solvent (10 ml), catalyst (50 mg), NaBH
4
0.01 mmol of the catalyst was used.
(5 mmol), temperature (313 K), reaction time (45 minutes).
a
GC yield.
GC yield.

M. Tumma, R. Srivastava / Catalysis Communications 37 (2013) 64–68
67
Table 4
It may further be noted that the formation of CuH by the reaction of Cu
and H needed high pressure [32]. CuH is thermally unstable and
undergoes decomposition under the reaction conditions to generate
Cu and hydrogen to accomplish the reduction of nitro groups via a
transfer-hydrogenation process [33].
Catalysts were found to be recyclable. After completion of the reac-
tion, catalyst was separated by the centrifugation from the reaction mix-
ture, which was washed first with ethyl acetate (thrice) followed by
washing with water (twice). Recovered catalyst was dried in vacuum
at 353 K for 8 h and used in the fresh reaction. No significant loss in cat-
alytic activity was observed even after 5 recycle (Table 2). Hot-filtration
method further confirmed that no catalytic species was leached into the
reaction mixture. Cu(1%)-Meso-PANI and Cu(10%)-Meso-PANI were
found to be stable even after their reuse, which was confirmed by XRD
Catalytic reduction of nitroaromatics over Cu(10%)-Meso-PANI and Cu(1%)-Meso-PANI.
2
S.
Substrate
Product
Cu(10%)-
Meso-PANI (%)
Yield Cu(1%)-
Meso-PANI (%)
bYield
No.
1
.
.
3 hr
100 a24 h
52
2
45 min
100 1.5 h
100 3.0 h
100
3.
2.0 h
95
4
5
.
.
1.5 h
2.0 h
100 2.5 h
100 4.0 h
93
96
(
Fig. S2, supporting information) and N
Scope of this reaction was extended to other nitroaromatics such as
2
-adsorption studies (Table 2).
6
7
.
.
2.5 h
2.0 h
100 4.0 h
100 3.0 h
90
80
nitrobenzene and substituted nitrobenzene. A wide range of substituted
nitroaromatic compounds were reduced by this procedure to produce
the corresponding aromatic amines. Reduction of nitrobenzene re-
quired much longer time than the reduction of substituted nitroben-
zene (Table 4). It may be noted that high Cu loading is required to
obtain the quantitative yield of aniline upon the reduction of nitroben-
zene. In general, ortho-substituted nitrobenzene took longer time than
para-substituted nitrobenzene during the reduction process (Table 4).
The reduction of p-chloro nitrobenzene proceeded selectively, without
any dehalogenation (Table 4).
In general, the reactions were very clean, giving the amines in high
yields. No intermediate product was observed during or after the reac-
tion for various nitroaromatics. However, an intermediate product was
observed during the reduction of nitrobenzene using Cu(1%)-Meso-
PANI. Intermediate product was separated by column chromatography
and identified by using IR and NMR. Isolated intermediate product was
identified as azobenzene. To understand the mechanistic pathway, two
parallel reduction reactions of nitrosobenzene and azobenzene were
conducted. Under this condition, nitrosobenzene gave aniline whereas,
azobenzene gave 1,2-diphenylhydrazine as major product and aniline
as a minor product. Since we did not observe any intermediate during
the reaction (except in the case of the reduction of nitrobenzene over
Cu(1%)-Meso-PANI), hence, we can conclude that this reaction pro-
ceeds via nitrosobenzene derivative, which consumed instantly as
soon as it formed during the reaction and therefore, we did not identify
nitrosobenzene as intermediate. Whereas, the reduction of azobenzene
is very slow process, and hence, it was detected in the case of Cu(1%)-
Meso-PANI.
Reaction condition: reactant (1 mmol), solvent (10 ml, EtOH: water = 1:1), catalyst
50 mg), NaBH (5 mmol), temperature (313 K).
(
4
a
Reaction was conducted at 353 K.
Isolated yield.
b
much higher than Cu metal decorated on PANI (Table 1). Uniform dis-
persion of Cu nanoparticles in the Meso-PANI matrix was confirmed
using TEM (Fig. 3a), however, in the TEM image, it was noticed that
the particles were not clearly monodispersed. The selected area elec-
tron diffraction patterns show diffuse ring pattern for the Cu nano-
particles present in Cu(10%)-Meso-PANI (Fig. 3a). EDS analysis further
confirms the incorporation of Cu in Cu(10%)-Meso-PANI (Fig. 3b).
UV–visible study also confirms the formation of Cu nanoparticle deco-
rated Meso-PANI material (See Fig. S1 and other details in supporting
information).
3
.2. Catalytic activity
4
-Nitroaniline was chosen as a model substrate to find the opti-
mum reaction condition. The reduction of 4-nitroaniline by NaBH
in the absence of metal catalyst was quite slow and the reaction
was not initiated even after 8 h at 313 K. Thus a combination of catalyst
and NaBH
and EtOH:H
of 4-nitroaniline using NaBH
yield for p-phenylenediamine in 45 min (Table 2). Among the solvents
investigated, EtOH:H O (1:1) was found to be the best for the reduction
4
4
was investigated. The combination of Cu(10%)-Meso-PANI
O (1:1) was found to be very effective for the reduction
as a reducing agents, which gave 100 %
2
4
2
4
. Conclusion
of nitroaromatics, when dissolution of reactants and catalytic activities
are taken into account (Table 3).
In summary, we reported the reduction of nitroaromatics to aromat-
Support plays an important role for the uniform dispersion with
suitable dimension of metal nanoparticles, which is very important
for high catalytic activity [30]. It is interesting to note that the activity
of Cu(1%)-PANI is lower than that of Cu(1%)-Meso-PANI (Table 2).
High catalytic activity of Cu(1%)-meso-PANI can be correlated with
the high surface area of Meso-PANI compared to PANI, which pro-
vides favourable high dispersion of Cu nanoparticles and enhance
the accessibility of reactant molecules to catalytic active sites. For
comparative study, several M(1%)-Meso-PANI were prepared and in-
vestigated in the reduction of 4-nitroaniline. Among the M(1%)-Meso-
PANI, Cu(1%)-Meso-PANI exhibited the highest activity, whereas
Mn(1%)-Meso-PANI exhibited the lowest activity (Table 2). It was
very interesting to note that commercially available Cu(II) and Cu(I)
salts were also found to be active for the reduction of 4-nitroaniline in
ic amines by using transition metal incorporated Meso-PANI. Among
the catalysts investigated, Cu(10%)-Meso-PANI exhibited the highest
activity. The simple operation, use of economical catalyst & reducing
agent, mild reaction conditions, short reaction time, high yields of
amines, reduction of wide range of substituted nitroaromatics, and re-
cyclability of the catalyst make this protocol an attractive route for the
reduction of aromatic nitro compounds.
Acknowledgments
We thank CSIR, New Delhi, for financial support under CSIR
(
01(2423)/10/EMR-II).
4
the presence of NaBH , however, their activity was found to be lower
Appendix A. Supplementary data
than that of Cu(1%)-Meso-PANI (Table 2).
It is known that Cu nanoparticles form CuH by transfer hydrogena-
tion in the presence of reducing agent under the reaction conditions [31].
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2013.03.034.

68
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