Selective hydrogenation of nitroarenes using gum acacia supported Pt colloid an effective reusable catalyst in aqueous medium

Article abstract of DOI:10.1016/j.catcom.2011.02.027

An efficient green synthesis of Pt nanoparticles in water has been developed using gum acacia, both as reducing and stabilizing agent. The colloidal platinum efficiently catalyzes hydrogenation of nitroarenes to arylamines employing molecular hydrogen as the reductant. Excellent yields of products were obtained with a wide range of substrates and the catalyst was reused for several cycles with consistent activity.

Full text of DOI:10.1016/j.catcom.2011.02.027

Catalysis Communications 12 (2011) 1009–1014
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Selective hydrogenation of nitroarenes using gum acacia supported Pt colloid an
effective reusable catalyst in aqueous medium
⁎
B. Sreedhar , D. Keerthi Devi, Deepthi Yada
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology (Council of Scientific and Industrial Research), Hyderabad 500607, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient green synthesis of Pt nanoparticles in water has been developed using gum acacia, both as
reducing and stabilizing agent. The colloidal platinum efficiently catalyzes hydrogenation of nitroarenes to
arylamines employing molecular hydrogen as the reductant. Excellent yields of products were obtained with a
wide range of substrates and the catalyst was reused for several cycles with consistent activity.
© 2011 Elsevier B.V. All rights reserved.
Received 25 November 2010
Received in revised form 21 February 2011
Accepted 28 February 2011
Available online 9 March 2011
Keywords:
Hydrogenation
Nitroarenes
Platinum nanoparticles
Amines
Gum acacia
Colloidal solution
1. Introduction
for modern organic chemists. Consequently, it is highly desirable to
develop environmentally benign processes that can be conducted in
Organic amines are important starting materials for the manufac-
ture of a wide variety of chemicals such as dyestuffs, pharmaceuticals,
agrochemicals, surfactants, pesticides and polymers etc [1]. It is well
known that amino groups can easily be replaced by other groups (H, F,
Cl, Br, I, OH etc.) via the corresponding diazonium salts [2]. In this
regard, the selective reduction of nitro compounds to amines is a
synthetically important transformation for the synthesis of valuable
starting materials and intermediates [3]. A variety of reagents [4–11]
and heterogeneous catalysts [12,13] have been reported for the
reduction of nitro compounds to the corresponding amines.
Colloidal metal particles are of continuing interest owing to their
extremely large surface area, monometallic and bimetallic noble metal
colloids have been used as active catalysts for various organic
transformations. On the other hand, ultrafine particles often agglom-
erate to form either lumps or secondary particles resulting in the
reduction of the total surface or the interfacial energy of the system.
Therefore, it is very important to stabilize the particles against adverse
agglomeration at both synthesis and catalytic usage stages.
aqueous media [15,16]. Another window of green chemistry is to take
into consideration the development of heterogeneous catalysts for the
organic transformations.
There is considerable interest in exploiting natural polymer
macrostructures, and in particular those of polysaccharides, to create
high-performance and environmentally friendly catalysts that are
chemically stable and biodegradable. Gum acacia (GA) is a highly
branched, neutral or slightly acidic arabinogalactan polysaccharide
nontoxic and biocompatible. It is widely used in food and pharmaceu-
tical industry as an additive or emulsifying agent and as stabilizer for
various novel nanomaterials [17].
Recently, we have reported the synthesis of GA stabilized silver
nanoparticles at room temperature [18] and GA stabilized palladium
nanoparticles for the direct one-pot reductive amination of aldehydes
with nitroarenes [19]. In continuation, we have developed an efficient
straightforward approach for the synthesis of GA stabilized Pt
nanoparticles and used for the selective hydrogenation of nitroarenes
to amines (Scheme 1).
Green or sustainable chemistry has now attained the status of a
major scientific discipline and the studies in this area have led to the
development of cleaner and relatively benign chemical processes with
many new technologies being developed each year [14]. The use of
water as a medium for organic reactions is one of the latest challenges
2. Experimental
2.1. General information
H2PtCl6.6H2O was purchased from Sigma-Aldrich and gum acacia
was purchased from S.D Fine Chemicals, Pvt. Ltd. India and used as
received. All other chemicals and solvents were obtained from
commercial sources.
⁎
Corresponding author.
E-mail address: sreedharb@iict.res.in (B. Sreedhar).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.02.027

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B. Sreedhar et al. / Catalysis Communications 12 (2011) 1009–1014
GA-Pt colloidal
nanoparticles
NO2
NH2
2.2. Synthesis of GA–Pt colloidal solution
H2O, rt, 1Atm H2 (Baloon)
An aliquot of 5 ml of the aqueous H2PtCl6.6H2O (2.5 mg) solution
was mixed with 5 ml of 1% aqueous solution of gum acacia. The
reaction mixture was heated at 100 °C for 6 h. Progress of the
platinum reduction was monitored spectrophotometrically at regular
intervals of time 60 min up to 24 h. The solid sample of the catalyst
was prepared from the colloidal solution by using acetone as an anti-
solvent. After addition of acetone the precipitated catalyst was
centrifuged and dried in air and thoroughly characterized by TEM,
XRD, FTIR and XPS.
Scheme 1. Hydrogenation of nitrobenzene to aniline using GA-Pt colloidal nanoparticles.
2.3. Typical experimental procedure for the hydrogenation of nitroarenes
In a typical procedure, to 5 ml of GA–Pt colloidal solution (0.24 mol%
of platinum) 1 mmol of nitroarene was added and hydrogen balloon
was fitted to the flask. The reaction was stirred at room temperature
until completion of the reaction as indicated by TLC. After completion of
the reaction the product was extracted with diethyl ether (5 ml×4) and
purified by column chromatography using hexane and ethyl acetate as
an eluent. The products were characterized by comparison of their NMR,
IR and mass spectra with those of authentic samples.
Fig. 1. UV–Vis absorption spectra of GA–Pt nanoparticles dispersed in water heated at
100 °C recorded at different time intervals. Inset shows as prepared GA–H2PtCl6
solution (a) before and (b) after heating at 100 °C for 24 h.
TEM images were taken on PHILIPS TECNAI FE12 operating at
120 kV and HRTEM images on TECNAI FE30 at 300 kV. UV–Vis
spectrophotometric characterization was carried out on Perkin Elmer
Lamda 750. X-ray diffraction measurements were recorded using a
Rigaku diffractometer (Cu radiation, λ=0.1546 nm) running at 40 kV
and 40 mA. X-ray photoelectron spectroscopy (XPS) measurements
were carried on Kratos AXIS 165 using Mg Kα radiation
(hν=1253.6 eV). IR spectra were recorded on Thermo Nicolet
Nexus 670 spectrophotometer.
2.4. Reuse of the catalyst
After completion of the reaction the productwas extracted into ether
and to remove traces of entrapped ether in GA–Pt colloidal solution
vacuum was applied before using the catalyst for further reactions. The
catalyst showed consistent activity for five cycles. The above mentioned
reaction was repeated and the filtrates were collected for determining
the catalyst leaching after each run. The amount of catalyst leached was
determined by ICP-AES and no leaching was found.
Fig. 2. TEM imagesof GA–Pt nanoparticles (a) fresh catalyst (b) used catalyst (c) HRTEM of fresh catalyst (d) histogram of fresh and used GA–Pt catalyst and (e) SAED pattern of GA–Pt catalyst.

B. Sreedhar et al. / Catalysis Communications 12 (2011) 1009–1014
1011
Table 1
Comparative study of different supported platinum catalysts for hydrogenation of
nitroarenes.a
Entry
Catalyst
Time
Yield (%)
1
2
3
4
GA–Pt
Pt/C
Pt–Al2O3
Pt–ZrO2
6
12
12
12
91
47
0
0
a
Reaction conditions: nitrobenzene (1 mmol), Pt catalyst in water (5 ml, 0.24 mol%
platinum).
of Pt nanoparticles. With the elapsed time, it is clearly seen that the
peak at 261 nm gradually disappears, which can be attributed to the
steady reduction of PtCl26− ions [20].
Fig. 3. XRD pattern of GA–Pt nanoparticles.
3.1.2. TEM analysis
Fig. 2a and b shows the TEM studies of both GA–Pt fresh and used
catalyst. The formation of monodispersed nanoparticles can be
attributed due to the effective passivation of the surfaces and
suppression of the growth of the nanoparticles through strong
interactions with the particles via their functional molecular groups
of gum acacia namely, hydroxyl groups of arabinose, rhamnose and
galactose and carboxylic groups of glucoronic acid moieties. Gum
acacia has been shown to be an effective capping agent due to the well
established chemical bonding between –OH and –COOH functional
groups and the surface of the metallic nanoparticles, thereby forming
a transparent colloidal suspension (Fig. 1b).
The histogram, Fig. 2d clearly shows that the average diameter of
the particles both in fresh and used catalyst is in the range 2–3 nm. As
shown in the HRTEM image Fig. 2c, the majority of the Pt
nanoparticles could be discernible as single crystals of a face-centered
cubic (fcc) lattice, because clear (111) lattice planes are observed to
cover all of the particles if the particles are viewed in a proper
direction (e.g., the (110) direction). The lattice spacing, 0.23 nm, is
consistent with that of bulk Pt [21] and the XRD results. The selected-
area ED, Fig. 2e shows four Debye–Scherrer concentric rings assigned
to (111), (200), (220), and (311) planes [22], consistent with the fcc
phase of the platinum nanoparticles and have been confirmed by the
XRD results.
Fig. 4. FTIR spectra of (a) pure gum acacia and (b) gum acacia–platinum nanoparticles.
3. Results and discussion
3.1. Characterization of GA–Pt catalyst
3.1.1. UV–Vis absorption analysis
UV–Vis spectroscopy was employed to characterize the reduction
procedure and progress of formation of GA–Pt nanoparticles. Fig. 1
shows the time-dependent UV–Vis absorption spectra of a solution
containing GA and H2PtCl6 heated at 100 °C over a period of 24 h.
Fig. 1a and b shows the photographs of the as prepared GA–H2PtCl6
solution before and after heating at 100 °C for 24 h, respectively. As
can be seen from the figure, the color of the solution gradually turned
from pale yellow to dark brown, providing evidence for the formation
3.1.3. XRD analysis
Fig. 3 shows the XRD pattern of gum acacia stabilized Pt
nanoparticles synthesized at 100 °C. Peaks consistent with the face-
centered cubic (fcc) expected for Pt are clearly observed. The broad peak
at 39.5° is characteristic of (111) peak of zerovalent Pt with face-
centered cubic (fcc) structure. The other four diffraction peaks with 2θ
values of 46.3, 68.1, 80.9 and 85.7° corresponding to the major
Fig. 5. XPS high resolution narrow scans of (a) C 1s and (b) Pt 4f for GA–Pt nanoparticles.

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B. Sreedhar et al. / Catalysis Communications 12 (2011) 1009–1014
reflections of crystal planes (200), (220), (311) and (222), respectively.
Nanoparticle size was also calculated from the line broadening of (111)
reflection using Debye–Scherrer equation and found to be 1.418 nm in
consonance with the size of the nanoparticles observed with TEM.
FTIR spectrum of the gum acacia capped Pt nanoparticles (Fig. 4). When
acacia molecule anchor to the nanoparticle surfaces the stretching
frequencies originating from the functional groups of acacia are expected
to shift accordingly. The stretching vibration of –OH, 3417.5 cm−1 shifts
to 3415.6 cm−1, asymmetric and symmetric stretching of vibration of
–CH2, 2927.1 cm−1 and 2149.1 cm−1 are shifted to 2925.2 cm−1 and
2149.0 cm−1, respectively. The peaks at 1611.2 cm−1 (C=O stretching of
carbonyl group, typical saccharide absorption) and 1426.8 cm−1 (C=O
stretching of – COOH group and – OH bending of acid group, respectively)
3.1.4. FTIR analysis
To investigate the protecting mechanism of gum acacia via the
functional groups of gum acacia (namely, –COOH and –OH) was
examined by comparing the FTIR spectrum of neat gum acacia and the
Table 2
Hydrogenation of nitroarenes using colloidal GA–Pt nanoparticles.a
Entry
1
Nitroarene
Product
Time (h)
6
Yield (%)b
91
NO2
NH2
NH2
NH2
NO2
NO2
2
3
4
6
6
92
90
NO2
NO2
NH2
NH2
6
6
95
90
H3CO
H3CO
5
6
OCH3
NO2
OCH3
NH2
8
82
HO
F
HO
F
NO2
NH2
8
8
90
88
7
8
NO2
NO2
NH2
NH2
Cl
Cl
9
8
8
72
85
Cl
Cl
NO2
NH2
10
Br
I
Br
I
NO2
NH2
NH2
NH2
11
12
8
8
8
8
68
NO2
NO2
75
I
I
H
O
H
O
13
14
87c
84c
NH2
NO2
O
O
a
Reaction conditions: aromatic nitro compound (1 mmol), GA–Pt colloidal nanoparticles in water (5 mL, 0.24 mol% of platinum) at room temperature under H2 atmosphere (1 atm).
b
c
Isolated yields.
Reactants initially dissolved in methanol and then aqueous catalyst added.

B. Sreedhar et al. / Catalysis Communications 12 (2011) 1009–1014
1013
GA-Pt colloidal
nanoparticles
NH2
NH2
NO2
+
H2O, rt,
1Atm H2 (Baloon)
O2N
O2N
H2N
45% yield in the ratio 2:1
Scheme 2. Hydrogenation of dinitrobenzene by using GA-Pt colloidal nanoparticles.
show a shift to 1645.7 cm−1 and 1428.9 cm−1, respectively. The extra
shoulder peak observed at 1760.4 cm−1 indicates the attachment of
COO− to the surface of Pt nanoparticles. The interaction between acacia
and Pt nanoparticles via both the –COO and –OH functional groups
facilitates the encapsulation of Pt nanoparticles with a protective layer of
acacia molecules thereby providing steric stabilization of the Pt
nanoparticles.
catalysts screened, results from the high surface area and strong
hydrogen trapping property of platinum nanoparticles compared to
supported platinum catalysts.
A variety of nitroaromatic compounds are investigated in this work.
The results indicate that nitroarenes can be readily converted to the
corresponding amines in excellent yields. For example, the reaction of
nitrobenzene (Table 2, entry 1) gave 91% of aniline after 6 h. Similarly,
4-nitrotoluene (Table 2, entry 2) was reduced to p-toludine in only 3 h
by this method in excellent yield (92%). Furthermore, 2-nitrotoluene,
4-nitroanisole, 2-nitroanisole and 4- nitrophenol were effectively
reduced to 2-aminotoluene, 4-aminoanisole, 2-aminoanisole and
4- aminophenol, respectively in good yield (Table 2, entries 3–6).
We next examined the reduction of halogen substituted nitroaro-
matic compounds (Table 2, entries 7–12). The results show that these
compounds can be readily converted to the corresponding anilines in
excellent yields. However, concomitant hydrogenolysis of halogens
was not observed under these mild reaction conditions. When we
carried out the reaction with reducible group, such as carbonyl
compounds, corresponding anilines were obtained chemoselectively
in excellent yields (Table 2, entries 13 and 14). Dinitrobenzene on the
other hand gave a mixture of both 4-aminonitrobenzene and 1,4-
diaminobenzene in the ratio 2:1 though the yield was moderate
(Scheme 2). Unfortunately we observed that the present method is
not amenable for aliphatic nitro compounds. Furthermore, the GA–Pt
colloidal solution was reused for several times with moderate loss in
catalytic activity (yield 91% in the first cycle and 84% in the fifth cycle)
as shown in Table 3.
3.1.5. XPS results
XPS survey scan of GA–Pt nanoparticles showed the presence of
oxygen, O 1s (at 535 eV), carbon, C 1s (at 285 eV), and platinum, Pt 4f
(at 72 eV). The observed XPS high resolution narrow scan spectrum of
C 1s for GA–Pt nanoparticles is shown in Fig. 5a. The observed C 1s
peaks could be deconvoluted into five peaks, and the binding energies
are attributed due to the energies distinct for substituted carbon
moieties. The observed C 1s peaks at 283.9, 284.6, 285.4, 286.6 and
287.5 eV are characteristic of Pt–C, –C–C–/–CH–, a carbon singly
bonded to one oxygen atom (–C–O–), –O–C–O–, and carboxylic acid
group (–O–C=O–), respectively [23–26].
The Pt 4f spectra shows the presence of a main contribution
together with a minor component at higher BE values. Curve fitting
analysis showed that the Pt 4f spectra for GA–Pt nanoparticles
resulted from two pairs of spin-orbit components as shown in Fig. 5b.
On the basis of previous measurements, the Pt 4f7/2 and 4f5/2 peaks
found at BE=71.2 eV and 74.6 eV, respectively have been attributed
to metallic platinum Pt(0) in agreement with literature [23]. The
second set of peaks for Pt 4f7/2 and Pt 4f5/2 observed at BE=73.2 eV
and 76.4 eV respectively, occurring at higher BE values can be
attributed due to Pt atoms with lower charge density. Though the
presence of Pt in two oxidation states is very clear from Fig. 5b the
higher intensity in Pt nanoparticles clearly indicates the metallic form
of the sample. The contribution due to the BE peak at~73.2 eV can be
attributed to the presence of Pt atoms on the surface capping on Pt
nanoparticles. However, under the reaction conditions most of the
unreduced catalyst also gets reduced and contribute to high yields of
the desired product.
4. Conclusion
In conclusion, the reduction of a variety of organic nitro
compounds to the corresponding amines in good to excellent yields
has been achieved using colloidal GA–Pt nanoparticles. This method is
mild, exceedingly efficient and highly selective as wide range of
functional groups is well tolerated under the reaction conditions. The
simple procedure for catalyst preparation and reusability of the
catalyst is expected to contribute to its utilization for the development
of benign chemical processes and products.
3.2. Catalytic activity of GA–Pt nanoparticles
Acknowledgements
The aim of the present study is to evaluate the efficiency of the GA–Pt
colloidal nanoparticles in water for the hydrogenation of nitroaromatic
compounds under mild reaction conditions (Scheme 1). No product was
formed when the reaction was run in the absence of the catalyst. Then
the reaction was carried out with different platinum catalysts such as
GA–Pt colloidal nanoparticles, Pt/C, Pt–Al2O3, and Pt–ZrO2 and the
results are shown in Table 1. The reaction with Pt/C gave the product in
moderate yield and no product was formed with Pt–Al2O3, and Pt–ZrO2.
The enhanced activity of GA–Pt catalyst over other supported platinum
The authors thank Indo French Centre for the Promotion of
Advanced Research (IFCPAR), New Delhi for financial assistance.
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