Catalytic reduction of nitroarenes with polymeric palladium nanoparticles

Article abstract of DOI:10.1080/15533174.2010.522669

An efficient synthesis of Pd nanoparticles in water has been developed using PEG10k as the capping agent. The PEG10k-Pd NPs (2 mol%) efficiently catalyzes reduction of nitro compounds performed in air. The stability of the catalyst allows an extensive recycle in the reduction of nitro compounds. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15533174.2010.522669

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Synthesis and Reactivity in Inorganic, Metal-Organic,
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Catalytic Reduction of Nitroarenes with Polymeric
Palladium Nanoparticles
Rupesh Kumar a , Veena Yadav b , Shweta Gupta a , Rekha Lagarkha b & Harish Chandra a
a Department of Chemistry, University of Delhi, Delhi, India
b Department of Chemistry, Bundelkhand University, Jhansi, India
Published online: 02 Feb 2011.
To cite this article: Rupesh Kumar , Veena Yadav , Shweta Gupta , Rekha Lagarkha & Harish Chandra (2011) Catalytic
Reduction of Nitroarenes with Polymeric Palladium Nanoparticles, Synthesis and Reactivity in Inorganic, Metal-Organic,
and Nano-Metal Chemistry, 41:1, 114-119
To link to this article: http://dx.doi.org/10.1080/15533174.2010.522669
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:114–119, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2010.522669
Catalytic Reduction of Nitroarenes with Polymeric
Palladium Nanoparticles
Rupesh Kumar,1 Veena Yadav,2 Shweta Gupta,1 Rekha Lagarkha,2
and Harish Chandra1
1Department of Chemistry, University of Delhi, Delhi, India
2Department of Chemistry, Bundelkhand University, Jhansi, India
use of different catalysts under a wider range of reaction con-
dition yields the corresponding amines quantitatively without
the production of waste products. Because of the exothermic
nature of the reaction and the need for a closed high-pressure
system, numerous safety precautions have to be taken. Palla-
dium charcoal system is flammable at room temperature so
these systems have handling problems. We report for the first
time a new palladium catalyst that is highly active in reduc-
tion of nitro compounds in one pot. Furthermore, the catalyst
is recyclable, inexpensive, thermally stable, and active in water.
Palladium nanoparticles have proven to be an attractive cat-
alyst in many useful transformations,[7] including the hydro-
genation of alkenes and alkynes,[8] the oxidation of alcohols,[9]
and carbon-carbon coupling reactions.[10] Nanoparticles have
been immobilized on inorganic solid supports[11] or embedded in
organic polymers,[12] dendrimers,[13] multilayer polyelectrolyte
films,[14] and ionic liquids for separation and reuse,[15] How-
ever, the immobilization often suffers from problems such as
low reactivity, degradation, palladium leaching, and difficult
synthetic procedures. Very recently, a simple one-pot method
of making recyclable palladium catalyst through generation of
palladium nanoparticles from Pd(OAc)2 and Polyethylene gly-
col was reported.[16] The reduction of aromatic nitro compounds
with hydrazine or hydrazine hydrate represents a special vari-
ation of the catalytic reduction, where hydrazine is the source
of the hydrogen. The hydrogen can be generated by a variety
of H-transfer catalysts.[17−19] Especially with the use of noble
metal catalysts, such as Pd, Pt, or Ru, the catalytic hydrazine
reduction gives high yields comparable to or better than the
catalytic hydrogenation. Therefore, the search continues for a
better catalyst for the synthesis of aniline derivatives in terms
of mild reaction conditions, operational simplicity, economic
viability, and selectivity.[20] The use of simple and widely avail-
able polymers, like PEGs, as a non-toxic, inexpensive, non-
ionic, thermally stable, recoverable, non-volatile polymers have
been used for various transformations.[21−26] Transition metal
nanoparticles have wide ranging applications in catalysis. How-
ever, due to their large surface area and surface energy, they
tend to agglomerate during the reactions and therefore need to
An efficient synthesis of Pd nanoparticles in water has been
developed using PEG10k as the capping agent. The PEG10k-Pd
NPs (2 mol%) efficiently catalyzes reduction of nitro compounds
performed in air. The stability of the catalyst allows an extensive
recycle in the reduction of nitro compounds.
Keywords catalysts, nanoparticles, palladium, PEG10k, reduction
1. INTRODUCTION
Metal nanoparticles have become attractive key materials
in both academic and industrial areas because of their unique
chemical and physical properties, such as catalytic, optical,
and magnetic properties distinct from those of bulk metals or
atoms.[1−4] Catalytic hydrogenation of nitroarenes over hetero-
geneous catalysts is a simple method for the production of aro-
matic amines. However, the application of conventional cat-
alytic systems to the reduction of nitroarenes containing other
reducible functional groups, e.g., halogen, carbonyl, cyano, ben-
zyloxy, and alkenic groups, is problematic because the reac-
tion is often accompanied by the reduction of these functional
groups.[5] It is well known that the catalytic properties of het-
erogeneous catalysts are dependent on the particle size of the
metal and the surface structure of the supports.[6] Transition
metal nanoparticles are effective catalysts for chemical trans-
formations due to their large surface area and a unique combi-
nation of reactivity, stability, and selectivity. In most cases, the
Received 8 June 2010; accepted 26 July 2010.
The authors are pleased to acknowledge financial support from DST,
New Delhi [Grant No. SR/FTP/CS-62/2006] for this investigation. The
authors are also thankful to the Department of Chemistry and USIC of
D.U for providing technical support.
Address correspondence to Rupesh Kumar, Department of Chem-
istry, University of Delhi, Delhi-110007, India. E-mail: chembio005
@yahoo.co.in
114

CATALYTIC REDUCTION OF NITROARENES
115
FIG. 1. (a) QELS data of polymeric PEG10k-Pd: plot of population distribution in percentile size distribution in nanometre. Mean: 106.6, standard deviation:
58.43, polydispersity: 0.300, (b) TEM micrographs of polymeric PEG10k-Pd nanoparticles, average size 112 nm, and (c) TEM micrographs of polymeric
PEG10k-Pd nanoparticles after seventh cycle. (Figure is provided in color online.)
be stabilized for effective utilization.
aggregate formation before the reduction of palladium ions.
Herein, we report a novel and facile route for prepara- With increasing the reaction time, the solution turned gradually
tion of Pd nanoparticles by exploiting PEG molecular weight brownish, indicating the formation of palladium nanoparticles
10000(MW10k), which was found to act as both reducing agent (Scheme 2). The reaction mixtures turned black, which indicated
and stabilizer.[16] It was noted that prepared nano-Pd provides the formation of palladium. The mixing ratio of the PEG10k and
the active and recyclable heterogeneous catalysts for the nitro palladium ions (PEG10k/ [Pd2+]) affected the formation of the
group reduction in the absence of phosphine ligands. The re- spherical aggregates of the palladium nanoparticles.
action was insensitive to the air and moisture with high yields,
and polymeric PEG10k-Pd nanoparticles can be readily recycle
for more than 8 times (Figure 3) without deactivation. Poly-
meric PEG10k-Pd nanoparticles were found to act as efficient
reduction catalyst for the conversion of nitroarenes to anilines
2.2. Typical Experiment Procedure for the Reduction of
Aromatic Reduction Compounds
2.2.1. Reduction of nitroarenes (entries 1–14)
Reduction of nitroarenes was performed in a 10 mL round
(Scheme 1) derivatives in high TONs without affecting the re-
bottom flask in the presence of nitroarenes (entries 1–14)
duction of halogen or other functional groups.
(1.0 mmol), polymeric PEG10k-Pd NP (2 mol% PEG10k-Pd
NPs), and hydrazine hydrate (8 equivalents) in open atmosphere
at 100◦C. After the reaction was completed, polymeric PEG10k-
Pd NPs were removed by centrifugation. The solution was fil-
2. EXPERIMENTAL
2.1. Preparation of Polymeric PEG10k-Pd Nanoparticles
Palladium acetate Pd (OAc)2 and polyethylene glycol
mol.wt.10k were added to water as a solvent and heated at 90◦C
for 3 h. Immediately after the two solutions were mixed, the solu-
tion became slightly yellow and slightly turbid, which indicated
tered through a pad of celite, saturated aqueous NaHCO3(10
mL) was added to the filtrate and extracted with ethyl ac-
etate. The combined organic layers were dried over anhydrous
Na2SO4, concentrated under reduced pressure, and purified by
the column chromatography to give the aniline derivatives (en-
tries 1–14). The analysis of the products was determined by
1HNMR and 13CNMR and mass spectra. Removed polymeric
PEG10k-Pd NPs were washed with water and ether and then
dried under vacuum for further use.
NH2
NO2
PEG10k-Pd NP ,
N2H4.H2O(8 equiv)
100 C,80-130 min.
X = -CH , -Cl -OH , NH
X
X
3
,
2
3. RESULTS AND DISCUSSION
The average diameters were measured by TEM analysis
(Figure 1b). Polymer PEG10k was used for the immobi-
lization of palladium, where the metal particles could be
highly dispersed on the surface in all cases with average
particle size 112 nm (polymeric PEG10k-Pd nanoparticles
characterized by QELS, TEM data (Figure 1a and b). Metallic
SCH. 1. Reduction of nitroarenes.
90 oC, 3h
PEG10K( aq).
Pd(OAc)2(aq)
PEG10k-Pd NPs
SCH. 2. Synthesis of polymeric PEG10k-Pd nanoparticles.

116
R. KUMAR ET AL.
TABLE 1
Reduction of substituted nitroarenes with hydrazine hydrate in presence of polymeric PEG10k-Pd NPs to corresponding anilines
Entry
1
Substrate
Product
Time (min.)
120
Yield (%)
90
Reference
28
2
3
110
90
90a
92a
–
–
4
5
6
80
110
120
95a
80
–
–
–
85a
7
130
80a
–
8
9
100
100
90a
99
–
32
10
11
12
110
100
120
97
98
98
32
32
32
(Continued on next page)

CATALYTIC REDUCTION OF NITROARENES
117
TABLE 1
Reduction of substituted nitroarenes with hydrazine hydrate in presence of polymeric PEG10k-Pd NPs to corresponding anilines
(Continued)
Entry
13
Substrate
Product
Time (min.)
110
Yield (%)
98
Reference
32
14
120
99
32
Reaction conditions – All reactions were carried out with nitroarenes (1 mmol) hydrazine hydrate (8eq) and polymeric PEG10k-PdNPs
(2.0 mol %) at 100◦C for 80–130 minutes in open atmosphere. a. Yields refer to those of isolated one. Products characterized by 1H, 13C NMR
and mass spectroscopic data.
natures of particles were confirmed by UV spectroscopy data exhibited good to high catalytic efficiency in the reduction of the
(Figure 2). The prominent peaks of PEG (at 2θ = 19.062, nitro group. Entry[28] 1(Table 1) was consumed within 120 min,
23.205, 26.103, and 39.673◦) were presented in this pattern, and higher turnover frequencies were reached. The selectivity
indicating the presence of pure polymer, also the characteristic of the reduction of p-chloroaniline Entry 2 (Table 1) was highly
peak of Pd (about 2θ = 39.673◦, hkl values at 111) was pre- dependent on the polymeric PEG10k-Pd nanoparticles. The op-
sented in XRD data. Polymer PEG10000 is highly crystalline timization of reaction conditions were carried out and it was
due to which peaks of palladium metal in XRD pattern are found that the catalysts, polymeric PEG10k-Pd nanoparticles
not sharp, which is confirmed by individual data (XRD, see loading 2 mol% at 100◦C and 8 equivalent hydrazine hydrate
Electronic Supplementary Information).
have highest activity for nitrobenzene directly reduction to ani-
It was observed that the size of the nanoparticles increases line under mild conditions. A blank experiment (without poly-
with increasing amount of the stabilizer PEG10k. When ratio meric PEG10k-Pd catalysts) was conducted with nitrobenzene,
PEG10k/Pd2+ is 4, the size of polymeric PEG10k-Pd nanoparti- and it was found that no reduction of nitro group was observed.
cles was observed 112 nm. By increasing the ratio PEG10k/Pd2+ The PEG10k might cover the surface of smaller particles more
the particle size increases more than 112 nm. The polymeric effectively and block the approach of reactants. El Sayed and
PEG10k-Pd nanoparticles synthesized was used in the catalytic co-workers found[29] that catalytic activity and nanoparticle sta-
reduction of nitro group of nitroarene derivatives (Scheme 1). bility were anticorrelated, with capping agents that gave good
The reaction of nitroarene derivatives was carried out in the nanoparticle stability yielding slow reaction rates. In our exper-
presence of the catalyst in N2H4.H2O at 100◦C temperature iments, an increased amount of PEG10k not only increased the
(Table 1). All of the polymeric PEG10k-Pd nanoparticles used size of the nanoparticle, but also increased its stability, which
UV DATA
14
12
10
8
PEG 10k-Pd Nps
PEG 10k polymer
Pd(OAc2)
6
4
2
0
190
210
230
250
270
290
310
330
350
Wavelength (nm)
FIG. 2. Displayed the experimental UV–vis spectra of the PEG10k-Pd nanoparticles, Pd(OAc)2, and PEG10k, respectively. (Figure is provided in color online.)

118
R. KUMAR ET AL.
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