Rapid, efficient and selective reduction of aromatic nitro compounds with hydrazine hydrate in the presence of the plain and supported platinum nanoparticles as catalysts

Article abstract of DOI:10.1007/s13738-014-0430-x

The current study aimed at application of the plain and supported platinum nanoparticles as a heterogenous catalyst for the reduction of aromatic nitro compounds. Monodispersed platinum nanoparticles were synthesized by reduction of H₂PtCl₆ by ethanol in the presence of polyvinyl pyrrolidone as a stabilizer, and then were immobilized on four types of zeolites. The obtained catalyst granules were characterized by X-ray diffractometry and transmission electron microscopy. The study then focused on elaboration of the catalytic activity of the nano catalysts under different operational conditions. It was found that reaction is adequately rapid at ambient temperature, and by utilizing a sufficient amount of catalyst, can be completed in nearly 30 min. Among the utilized zeolitic supports, zeolite 4A had the highest performance, but the mechanism of its synergetic effect on the activity of platinum nano catalyst was not found and requires more investigation.

Full text of DOI:10.1007/s13738-014-0430-x

J IRAN CHEM SOC
DOI 10.1007/s13738-014-0430-x
ORIGINAL PAPER
Rapid, efficient and selective reduction of aromatic nitro
compounds with hydrazine hydrate in the presence of the plain
and supported platinum nanoparticles as catalysts
•
Soofia Mehdizadeh Seyed Javad Ahmadi
•
•
Sodeh Sadjadi Mohammad Outokesh
Received: 3 August 2013 / Accepted: 7 February 2014
Ó Iranian Chemical Society 2014
Abstract The current study aimed at application of the
plain and supported platinum nanoparticles as a heteroge-
nous catalyst for the reduction of aromatic nitro com-
pounds. Monodispersed platinum nanoparticles were
synthesized by reduction of H2PtCl6 by ethanol in the
presence of polyvinyl pyrrolidone as a stabilizer, and then
were immobilized on four types of zeolites. The obtained
catalyst granules were characterized by X-ray diffractom-
etry and transmission electron microscopy. The study then
focused on elaboration of the catalytic activity of the nano
catalysts under different operational conditions. It was
found that reaction is adequately rapid at ambient tem-
perature, and by utilizing a sufficient amount of catalyst,
can be completed in nearly 30 min. Among the utilized
zeolitic supports, zeolite 4A had the highest performance,
but the mechanism of its synergetic effect on the activity of
platinum nano catalyst was not found and requires more
investigation.
molecules used as pharmaceuticals, agronomics, colorants
and antioxidants, and can be readily synthesized from aryl
nitro compounds via reduction methods [1]. Numerous new
reagents have been developed for the reduction of aromatic
nitro compounds. Nowadays, most large-scale aromatic
amines are being produced by catalytic hydrogenation of
the corresponding nitroarenes with a large variety of cata-
lysts (e.g. Ni, Cu, Co, Cr, Fe, Sn, Ag, Pt, Pd, Zn, Ti, Mo,
metal oxides and sulfides) [2]. Catalytic hydrogenation
employs highly diffusible, low molecular weight, flamma-
ble hydrogen gas and requires pressure equipment [3]. To
overcome these difficulties several new methods have been
developed. Hydrazine reduction is an attractive alternative
to the reduction of aromatic nitro compounds because it
produces harmless by-products such as nitrogen gas and
water. This reduction can be done using hydrazine and a
variety of metal catalysts [4] such as Zn, Mg, FeCl3ꢀ6H2O-
activated carbon, iron(III) oxide and iron oxide hydroxide
[5]. With the use of noble metal catalysts, such as Pd, Pt or
Ru, but also with the application of Ni or Cu, the catalytic
hydrazine reduction gives high yields comparable to or
better than the catalytic hydrogenation [2].
Keywords Platinum nanoparticle ꢀ Zeolite supports ꢀ
Aromatic nitro compounds reduction
Over recent years, nanometer materials have attracted
interest throughout the scientific community. Their elec-
tronic, magnetic, optical, biological, mechanical and cata-
lytic properties make nanometer materials attractive
alternatives to their bulky materials [6, 7]. Of them, the
special catalytic activities of nanomaterials intrigued our
interest. In the fabrication of the nano catalyst, one way to
achieve large surface areas is to use a mesoporous support
and fill its cages with the aimed metals. Using this method,
it is possible to produces a homogeneously dispersed cat-
alyst [8].
Introduction
Aryl amines are synthetically important compounds which
act as precursors to the synthesis of many interesting
S. Mehdizadeh ꢀ M. Outokesh
Department of Energy Engineering, Sharif University
of Technology, Azadi Ave., P.O. Box 113658639, Tehran, Iran
S. J. Ahmadi ꢀ S. Sadjadi (&)
Nuclear Science and Technology Research Institute, End
of North Karegar Ave., P.O. Box 1439951113, Tehran, Iran
e-mail: sadjadi.s.s@gmail.com
In this paper, we presented the synthesis and charac-
terization of supported platinum on zeolites. Zeolites were
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J IRAN CHEM SOC
mixture was quickly added to 1.5 g of the selected zeolite,
and refluxed for 3 h. The solvent then was evaporated and
the obtained catalysts granules were calcined at 450 °C for
12 h in a stream of muffle furnace. The aforementioned
procedure was repeated for four types of zeolite (NY, 4A,
FAU, ANA) to yield the corresponding supported catalyst.
NH2
NO2
R1
R2
nanoPt/Zeolite
NH2NH2
R1
R2
rt.
R4
R4
R3
R3
1
2
General procedure for aromatic nitro compounds
reduction
Scheme 1 Reduction of aromatic nitro compounds by hydrazine
hydrate with the plain and supported platinum nanoparticles
selected as the support because of their nano sized cavities
and their high surface areas, shape/size selectivity, and ease
of separation of their granules from reaction mixtures. This
material has high catalytic activity for the reduction of
aromatic nitro compounds to the corresponding aromatic
amine with hydrazine hydrate under mild reaction condi-
tions (Scheme 1).
About 0.03 g of the plain platinum or 0.1 g supported
platinum catalyst was added to a solution of 1 mmol aro-
matic nitro compounds and 3 mmol hydrazine hydrate.
After 30 min stirring, the reaction mixture was filtered to
isolate the solid product. Then solid product was diluted
with tetrahydrofuran (THF) and the catalyst was removed
by simple centrifugation (the catalyst is not soluble in THF)
and then the solvent was evaporated to afford products.
Materials and methods
Results and discussion
Materials and instruments
Synthesis and characterization of catalyst
Hexachloroplatinic acid (H2PtCl6ꢀ6H2O (99.9 %, metals
basis)), hydrazine hydrate, aromatic nitro compounds, and
poly vinylpyrrolidone (PVP, Mw = 29,000) were pur-
chased from Merck, AG, Germany. Four types of zeolites
(NY, 4A, FAU, ANA) were used in the current study and
were obtained from Pars Zeolite Chemical Co.
Monodisperse platinum particles of 20–30 nm were syn-
thesized by modified alcohol reduction methods according
to the literature [9]. Methanol served as solvent for dis-
solving Pt salts and PVP, and as a reducing agent of Pt
according to the following reaction:
The particle size of the nanocrystalline particle was
examined using transmission electron microscopy (TEM,
Philips EM208S at 100 kV). The X-ray diffraction (XRD)
pattern of the catalyst powders was provided by a (Philips
PW 1800) diffractometer. The active functional groups were
identified by a (FT-IR Brucker Tensor 27) spectrometer.
Melting points of the product samples were measured using
the capillary tube method with an (Electro thermal 9200)
apparatus. Gas chromatography experiments were carried
out on an (Network GC System-Agilent 5973) instrument.
H2PtCl6 þ 2CH3OH ! Pt0 þ 2CH2O þ 6HCl
Platinum particle sizes were measured by XRD (Fig. 1)
and TEM (Fig. 2). All diffraction peaks of X-ray were
indexed to the monoclinic crystal system of Pt in Fig. 1.
Figure 2 shows that the particles are uniform and have a
˚
narrow size distribution. 4A with a pore diameter of 4 A
was used as a catalyst support due to its high surface area
and ordered mesoporous structure.
3000
2000
1000
0
Synthesis of the plain platinum nanoparticle
The synthesis procedure was started by adding 133 mg PVP
into a mixture of 20 cm3 of 6.0 mM H2PtCl6ꢀ6H2O aqueous
solution and 180 cm3 pure ethanol. The mixture was then
refluxed for 3 h. Afterwards the solvent was evaporated,
and the obtained black precipitate was thoroughly washed
with water and ethanol, and dried at ambient condition.
Preparation of Pt/zeolite
10
30
50
70
90
110
130
2θ (°)
About 20 cm3 of H2PtCl6ꢀ6H2O aqueous solution (6 mM)
was mixed with 133 mg PVP and 180 cm3 of ethanol. The
Fig. 1 XRD patterns of Pt particles. Pt nanoparticles (filled square)
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J IRAN CHEM SOC
Fig. 2 Transmission electron microscopy of Platinum nanoparticles
Fig. 4 Transmission electron microscopy of Pt/zeolite 4A
Fig. 5 Comparison of different support for p-nitrotoluene reduction
reduction of p-nitrotoluene to p-toluidine was examined on
various zeolite solid supports (NY, 4A, FAU, ANA). This
reaction was carried out with 0.1 g of catalysts at room
temperature under solvent-free conditions for 30 min. The
data presented in Fig. 5 clearly show the efficiency of 4A-
supported platinum in the reduction of p-nitrotoluene in
good yield (98 %).
Fig. 3 XRD patterns of Pt particles on zeolite 4A
Platinum/zeolite catalyst was prepared as discussed in
experimental section. The loading of the platinum on the
zeolite sample was measured by dissolving the sample in
aqua regia, and analyzing the Pt content of the supernatant
by ICP method. The loading was found to be around
3 wt%. These materials were characterized by XRD
(Fig. 3) and TEM (Fig. 4) measurements. The Pt reflec-
tions are seen in the XRD patterns of 4A catalysts (Fig. 3).
TEM images of Pt/4A samples (Fig. 4) show that the
particles are well-dispersed in the entire channel structures.
To show the importance of support in catalytic activity,
same reaction was carried out with platinum nanoparticles;
for an exact comparison the net weight of platinum in
zeolite-supported catalyst was calculated (0.003 g) and
used for comparative test. The desired product was
obtained in a very low yield in this condition but with
0.06 g of catalyst the product was obtained in a good yield
(93 %) after 30 min. Comparison of supported and non-
supported platinum nanoparticles catalyst showed that the
platinum nanoparticles is less active than the supported one
even with an effective amount of platinum twenty times
less than what was used in the reaction with the platinum
nanoparticles. This effect cannot be simply attributed to the
greater surface-to-volume ratio (A/V) of the supported
Catalytic reaction
Two series of experiments were performed to identify and
optimize the ideal solid support material and the appro-
priate catalyst to effect the reduction of p-nitrotoluene as
model reaction. First, the effect of supports on the
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indicate that there is an increase in the amount of product
with increase in reaction time. The optimum reaction time
has been found to be 30 min.
Table 1 Optimization of the operational conditions
No.
Time (min)
Amount of Pt/zeolite catalysta
Yield (%)b
1
2
3
4
5
6
7
8
9
180
120
60
30
10
5
0.1
0.1
0.1
0.1
0.1
0.1
0.03
0.05
0.2
98
98
97
97
74
58
69
81
99
A brief summary of the optimization experiments for
reduction of p-nitrotoluene is presented in Table 1. Inter-
estingly, it was found that platinum nanoparticles sup-
ported on 4A zeolite with low loading (3 wt%) are efficient
catalysts and gave exclusively p-toluidine in 98 % yield in
30 min under solvent-free conditions at room temperature.
To further extend the scope of the reaction, several
other substituted aryl nitro compounds were utilized as
substrates in the reaction with hydrazine hydrate in the
presence of Pt/zeolite catalyst (Table 2). Both electron-
withdrawing and electron-donating substituents were tol-
erated at various positions on the benzene ring. The cor-
responding aryl amines derivatives were obtained in good
yields (Table 2).
30
30
30
Reaction conditions: p-nitrotoluene 1 mmol, hydrazine hydrate:
3 mmol at room temperature
a
The loading of metals was adjusted to 3 wt%
b
Yield of purified products (based on the p-methylaniline)
The optimal reduction conditions were applied to further
nitro aromatic derivatives, examining the selectivity of the
reduction for nitro substituents in the presence of other
sensitive functionalities (Table 2). Nitroarenes containing
different reducible functional groups such as carboxylic
and amides were chemoselectively reduced to their corre-
sponding aryl amines in good yields (Table 2, entries 5–7).
The chemo-selective reduction of chloro-substituted ni-
troarenes (Table 2, entries 4, 6) was also achieved in a
good yield without undergoing any dehalogenation.
Finally, the efficacy of the present method for the syn-
thesis of p-toluidine was compared with other reported
procedures (Table 3) [2, 11–16]. It revealed that 4A-sup-
ported platinum catalyst is an efficient catalyst in the
synthesis of p-toluidine.
sample, because this ratio for various zeolites is nearly
equal, though their catalytic activities are quite different. In
addition the plain platinum nanoparticles, whose sizes are
in the range of few nanometers, do not have much smaller
A/V ratio. A possible interpretation of these phenomena is
that cavities of zeolites could stabilize platinum nano
clusters and enhance the durability of the catalysts [10],
and as these cavity sizes become larger from 4A to ANA
the yield of product decreases.
Control experiments show that neither hydrazine nor
zeolite alone causes the reduction of the nitro compounds
which indicated that the use of platinum is absolutely
necessary for this transformation. In the second series of
experiments, a series of model reactions were performed to
determine the optimal reaction conditions for the reduction
of p-nitrotoluene with hydrazine in the presence of 4A-
supported platinum catalyst (Table 1). First, to study the
effect of temperature, this reaction was examined under
solvent-free conditions at 70 °C for 30 min with 0.1 g 4A-
supported platinum catalyst and p-toluidine was obtained
in 98 % yield. Performing the reaction at room temperature
had no effect on the yield, therefore the room temperature
was chosen for all further experiments.
From Tables 2 and 3, the obvious advantages of the
current method over previously known methods are: (a) the
reduction is carried out in solvent-free conditions;
(b) selectivity of the reduction of nitro compound in the
presence of other reducible groups; (c) easy to operate
under simple experimental conditions; (d) rapid reduction;
(e) high yields of substituted amines.
Conclusion
The effect of dose of catalyst on p-nitrotoluene con-
version and product selectivity was studied with
0.03–0.3 g of catalyst. Generally, the p–nitrotoluene
conversion was increased with an increase in the amount
of catalyst. With 0.03 g of catalyst, p-nitrotoluene con-
version was found to be 69 % and it increased to 98 %
with 0.1 g catalyst. A further increase of catalyst had no
effect on yield.
In conclusion, we have developed solid supported nano
platinum as heterogeneous catalysts and their successful
application for reduction of nitroaryl compounds to corre-
sponding aryl amines. Easy hydrogen transfer capability of
this catalyst has been well documented at low reaction
temperature (room temperature) with a larger substrate
scope. Conventional reductions of halogenated nitroarenes
carrying potentially reducible substituents with hydrazine
hydrate employing noble metal catalysts are accompanied
During the catalytic reduction of p-nitrotoluene in the
presence of this catalyst, TLC analysis showed that the
formation of product is starting after 5 min. The results
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J IRAN CHEM SOC
Table 2 Reduction of aryl nitro compounds with hydrazine hydrate in the presence of Pt/zeolite catalyst
No.
1
Reagent
Product
Yield (%)a
98
2
95
3
4
97
93
5
96
6
93
7
95
Reaction conditions: aryl nitro compounds 1 mmol, hydrazine hydrate: 3 mmol at room temperature
The loading of metals was adjusted to 3 wt%
a
Yield of purified products
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Table 3 Comparison with other catalysts
Entry Catalyst
Temperature (°C)
Solvent
Time
(min)
Yield
(%)
References
1
2
3
Pt/zeolite
FeY
r.t
–
30
60
98
84
97
–
Refluxed in hexane
78
Hexane
[11]
[2]
FeO(OH)
Ethanol or
water
150
4
5
6
7
Iron(III)oxide-MgO
60
60
70
Methanol
360
420
85
99
90
89
89
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Graphene-Fe3O4 nanocomposite
Ethanol
–
–
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FeCl3ꢀ6H2O
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900 resin)
Microwave (30 to
[45 W) 108
7
8
MeOH:H2O
(3:7)
90
82
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Acknowledgments The author is thankful from Nuclear Science
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