A highly effective Ag-RANEY nickel hybrid catalyst for reduction of nitrofurazone and aromatic nitro compounds in aqueous solution

Article abstract of DOI:10.1039/c7ra04343k

RANEY nickel reduced Ag⁺ ions to form ultrafine spherical silver nanoparticles over itself, and the obtained hybrid material was used as catalyst for efficient and selective reduction of aromatic nitro compounds, and nitrofurazone as a non-aromatic example, in aqueous solution by using NaBH₄ as reducing agent. Other silver nanostructures with different morphologies such as silver nano-flowers were also prepared and their efficiency in the reduction process was evaluated. Ag-RANEY nickel catalyst however, had superior advantages including mild reaction conditions, higher conversion yield, and reduction in aqueous solution at near ambient temperature. The catalyst was also recoverable and showed 5% decrease in efficiency after six successive runs.

Full text of DOI:10.1039/c7ra04343k

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A highly effective Ag–RANEY® nickel hybrid
catalyst for reduction of nitrofurazone and
aromatic nitro compounds in aqueous solution
Cite this: RSC Adv., 2017, 7, 29938
*
Alireza Khorshidi
and Bahareh Ghorbannezhad
RANEY® nickel reduced Ag+ ions to form ultrafine spherical silver nanoparticles over itself, and the obtained
hybrid material was used as catalyst for efficient and selective reduction of aromatic nitro compounds, and
nitrofurazone as a non-aromatic example, in aqueous solution by using NaBH4 as reducing agent. Other silver
nanostructures with different morphologies such as silver nano-flowers were also prepared and their
efficiency in the reduction process was evaluated. Ag–RANEY® nickel catalyst however, had superior
advantages including mild reaction conditions, higher conversion yield, and reduction in aqueous solution at
near ambient temperature. The catalyst was also recoverable and showed 5% decrease in efficiency after six
successive runs.
Received 18th April 2017
Accepted 4th June 2017
DOI: 10.1039/c7ra04343k
rsc.li/rsc-advances
Introduction
Nitroaromatic compounds are generally prepared via nitration of
simple aromatic compounds1 and are extensively being used in
various industries such as plastics, dyeing, agriculture, etc. As
a result, nitroaromatic compounds are usually present in indus-
trial effluents and agricultural waste water.2 Presence of these
compounds in this waste, even in small quantities, is highly
undesirable and has drastic adverse effects on the environment
and human beings. Hence, there is an ongoing demand for new
Scheme 1 Reduction of various nitro compounds in water at near
ambient temperature catalyzed by Ag–RANEY® nickel.
methods of removal or conversion of these compounds to less temperature or pressure, use of harmful organic solvents, need of
harmful products. Anilines are an important class of precursors for special apparatus and expensive reagents or catalysts, prevent their
the synthesis of pharmaceuticals, dyes, and other biologically practical application. RANEY® nickel, on the other hand, is an
active compounds such as Glu-P-1 and Trp-P-2,3–5 and can be outstanding catalyst in the eld of transfer hydrogenation.20–23 Its
produced by selective reduction of nitroaromatic compounds. performance in the catalytic hydrogenation of nitroaromatic
While traditional methods such as reduction with iron powder in compounds however, still needs to be improved. Hereby, we report
presence of hydrochloric or acetic acid6 are gradually being elim- a combination of silver nanoparticles and RANEY® nickel as an
inated due to environmental issues, catalytic hydrogenation effective catalyst in the reduction of a variety of nitroaromatic
processes have emerged as more effective alternatives. Some of the compounds as well as nitrofurazone, as a non-aromatic example,
recent reports in this context include application of silver nano- by NaBH4 (Scheme 1).
particles stabilized by polyaminocyclodextrin,7 Fe(II) complexes
and nano zero-valent iron,8 gold nanoparticles,9 NaBH4/RANEY®
Experimental
Materials and methods
nickel,10 Ag nanoparticles supported on cellulosic bers,11 palla-
dium nanoparticles,12 and platinum single-atom catalysts.13 Other
reagents such as hydrazine/RANEY® nickel,14 Na2S/NEt4Br,15
Sm(0),16 H2/RANEY® nickel,17 metal carbonyls such as Mo(CO)6,18
and Pd/Fe bimetal nanoparticles embedded in P-doped meso-
porous carbons,19 have also been reported. However, limitations
associated with these methods, such as long reaction time, high
RANEY®-nickel was purchased from W. R. Grace and Co. All of
the other chemicals were purchased from Sigma-Aldrich and
used without further purication. Flower-like silver nano-
particles were prepared according to the literature.24 Silver
nanoparticles deposited over the surface of RANEY® nickel
were prepared via two different methods. For the reduction of
Ag+ ions with NaBH4, the method of Mirkin,25 without using
hydrogen peroxide and PVP was used. For the reduction of Ag+
ions with RANEY® nickel itself as a reducing agent, the
Department of Chemistry, Faculty of Sciences, University of Guilan, P. O. Box:
41335-1914, Iran. E-mail: khorshidi@guilan.ac.ir; Fax: +98 13 33 33 32 62; Tel:
+98 911 339 7159
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following method was used. RANEY® nickel (100 mg) was using a Philips X'pert diffractometer with mono chromatized
dispersed in 500 mL of double distilled water for 15 min in Cu Ka radiation at 40 kV and 20 mA (Ni lter, 2q 10 to 70ꢀ with
a round-bottom ask located in an ultrasonic bath. Then the a step size of 0.05ꢀ and a count time of 1 s). Scanning electron
mixture was simultaneously stirred by means of a mechanical micrographs were obtained on a Jeol JSM 6400. TEM images
stirrer rotating at 1500 rpm and 100 mL of 0.5 mM AgNO3 were obtained on a transmission electron microscope (TEM-
solution was injected at once. The trend of reduction was PHILIPS MC 10) with an acceleration voltage of 80 kV. ICP-
monitored by sodium chloride test. Aer 30 min, the product OES analysis was performed on a PerkinElmer Optima 8300
(Ag–RANEY® nickel) was separated by centrifugation at ICP-OES spectrometer.
2000 rpm, washed with acetone and dried under reduced
pressure.
General procedure for reduction of nitroaromatic
compounds. To 100 mL of a 0.2 mM solution of the corre-
sponding nitroaromatic compound in water, 5 mg of Ag–
Results and discussion
Characterization of the catalysts
Silver nanostructures with a drastically different morphology
were prepared and their efficiency in reduction of nitroaromatic
compounds was evaluated. Low-magnication SEM micro-
graphs of the ower-like silver nanoparticles, Ag nanoparticles
prepared by reduction with NaBH4, and those prepared by
reduction with RANEY® nickel are shown in Fig. 1a–c. From
Fig. 1a, it is clear that the obtained nanoowers have rod-like
tips of about 60–90 nm in diameter. Reduction of Ag+ with
NaBH4 on the other hand, resulted in aggregates of disordered
silver nanoparticles (Fig. 1b) over the surface of RANEY® nickel.
RANEY® nickel catalyst was added and the suspension was
stirred at 35 C by a mechanical stirrer rotating at 1200 rpm.
ꢀ
Then 1.0 mL of freshly prepared 0.04 M NaBH4 in water was
added and the reaction proceeded until complete reduction of
nitro groups as was monitored by measuring of the absorbance
of the solution under investigation at lmax of the nitro
compound, aer ltration of the catalyst.
Instrumentation
UV-Vis spectra were recorded on a Perkin Elmer LAMBDA 25 Reduction with RANEY® nickel however, resulted in aggregate
spectrophotometer. XRD measurements were performed by free surfaces (Fig. 1c), and a closer look by a transmission
Fig. 1 SEM image of flower-like silver nanoparticles (a), aggregated Ag nanoparticles prepared by reduction with NaBH4 (b), aggregate free
surface of Ag–RANEY® nickel catalyst prepared by reduction of Ag+ with RANEY® nickel (c), and its TEM image (d) with corresponding particle
size distribution chart (e) and EDX analysis (f).
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electron microscope revealed the formation of ultrane spher- RANEY® nickel and it has been shown that their crystalline
ical silver nanoparticles ranging from 10 to 40 nm throughout structure does not dehydrate considerably during evacuation at
the entire surface (Fig. 1d) based on the corresponding size elevated temperature.26 It should also be noted that the Ag
distribution chart (Fig. 1e). Energy dispersive X-ray analysis content of the Ag–RANEY® nickel catalyst was determined by
also, revealed the simultaneous presence of Ni, Al and Ag ICP-OES method and the result was similar to that obtained by
(Fig. 1f).
In the XRD diffractogram of the ower-like silver nano-
EDX analysis (0.83 w% vs. 0.71 w%).
Taking into account the requirements of sustainable and
particles (Fig. 2a), three peaks at 38ꢀ, 44ꢀ and 74ꢀ correspond to green chemistry, including mild reaction conditions, using
111, 200 and 220 planes of face-centered-cubic (FCC) silver on non-toxic solvents and in situ preparation of reactive compo-
the basis of standard values in the card (JCPDS le no. 04-783). nents such as hydrogen, reduction of nitrobenzene in water was
Moreover, formation of some hcp phase, based on the reec- selected as a model reaction to determine the optimum condi-
tions at 36ꢀ and 40ꢀ indexed to (0004) and (1102) planes of hcp tions such as catalyst type, catalyst loading, pH, and tempera-
silver is evident. In the XRD pattern of the Ag nanoparticles ture. NaBH4 was used as the source of hydrogen because it is
prepared by reduction with RANEY® nickel (Ag–RANEY® fairly an inexpensive reagent. Control experiments showed that
nickel, Fig. 2b), in addition to the (111) and (200) reections of the reduction efficiency depends on the catalyst type. From
Ni at 44.5ꢀ and 52ꢀ, corresponding reections of fcc silver were Table 1 it is clear that under the same conditions, Ag–RANEY®
also present. Other peaks at 28.5ꢀ, 46.5ꢀ and 47.5ꢀ correspond to nickel resulted in the best reduction efficiency in terms of the
alumina trihydrates. Generally, RANEY® nickel consists of apparent rate constant being calculated as ln C/C0 ¼ ꢁkt (where
metallic nickel, and aluminum as either metal or Al2O3$3H2O. C is the residual and C0 is the initial concentration of the
The alumina trihydrates were identied as components of the nitrobenzene, measured at 269 nm at room temperature in pH
¼ 7.0). Higher activity of Ag–RANEY® nickel may be attributed
to the uniform distribution of ultrane silver nanoparticles
which provides higher surface area for these nanoparticles, and
more active sites would be available for the catalytic reduction.
Fig. 3 Reduction efficiency in terms of conversion (%) vs. catalyst
loading, after 5 min under general experimental conditions.
Fig. 2 XRD pattern of flower-like silver nanoparticles (a), and Ag–
RANEY® nickel catalyst (b).
Table
1 Reduction of nitrobenzene with NaBH4 using different
catalysts
Catalyst
loading (mg)
Apparent rate
Entrya
Catalyst type
constant, k (minꢁ1
)
1
2
None
—
5
0.001
0.159
Flower-like Ag
nanoparticles
RANEY® nickel
Ag aggregates on
RANEY® nickel
Ag–RANEY® nickel
3
4
5
5
0.254
0.342
5
5
0.435
a
All of the reactions were carried out according to the general
experimental procedure at room temperature.
Fig. 4 Trend of nitrobenzene reduction at different temperatures.
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the reduction efficiency. Based on these data, 5.0 mg of the Ag–
RANEY® nickel catalyst was selected as the optimum amount.
In order to investigate the effect of temperature on the
reduction of nitrobenzene, the reaction was carried out at
different temperatures of 25, 35, and 45 ꢀC under an optimized
catalyst loading of 5.0 mg, and C/C0 vs. time was plotted. From
Fig. 4, it is clear that an increase in the temperature results in
slightly better efficiency.
It was also found that the reduction of nitrobenzene follows
a pseudo rst order kinetic with the apparent rate constant
being calculated as ln C/C0 ¼ ꢁkt. The value of k for 5.0 mg of
the catalyst was found to be 0.457, 0.568, and 0.605 minꢁ1 at 25,
35, and 45 C, respectively (Fig. 5). Based on these data, 35 C
was selected as the optimum temperature for reduction of
nitroaromatic compounds, which in turn, has the advantage of
proximity to the ambient temperature.
ꢀ
ꢀ
Fig. 5 Plot of ln C/C0 vs. time at 25, 35, and 45 ꢀC for reduction of
nitrobenzene in presence of 5.0 mg of the catalyst.
It was also found that reduction efficiency depends on the
catalyst loading. Fig. 3 shows that increase in the catalyst
loading from 3.0 to 10.0 mg results in a non-linear increase in
With the optimized conditions in hand, a variety of nitro-
aromatic compounds were used to evaluate generality and
Table 2 Reduction of various nitroaromatic compounds employing Ag–RANEY® nickel/NaBH4 in aqueous solution
Apparent rate
Entrya
1
Nitroaromatic compound
Productb
constant, k (minꢁ1
)
0.568
0.386
2
3
4
0.229
0.528
5
0.124
6
7
0.139
0.061
8
0.035
a
b
All of the reactions were carried out according to the general experimental procedure. Products were identied from their MS spectra in
comparison with authentic samples.
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Fig. 6 Nitrofurazone.
scope of this reductive protocol, and typical results are shown in
Table 2.
In order to extend applicability of the Ag–RANEY® nickel/
NaBH4 protocol to non-aromatic nitro compounds, nitro-
furazone (Fig. 6) was selected due to its contribution in
carcinogenesis.
Fig. 8 Reusability of the recycled catalyst.
reducing agents.16,30 When nitrosobenzene or phenylhydroxyl-
Nitrofurazone is a broad-spectrum antibiotic used in live-
stock, and has been detected in tissues and milk of animals.27,28
Signicant correlation between nitrofurazone reduction and
cellular DNA damage has been reported.29 Thus reduction of
nitrofurazone residues may prevent release of this drug into the
animal food and subsequent risk to human health. When 100
mL of a 0.2 mM solution of nitrofurazone in water was treated
with 5 mg of Ag–RANEY® nickel catalyst and 1.0 mL of freshly
amine was treated with Ag–RANEY® nickel and NaBH4 in
ꢀ
aqueous solution at 35 C, the nal product was aniline and it
can be concluded that the following reaction path rationalized
the overall reduction.
Ph-NO2 / Ph-NO / Ph-NHOH / Ph-NH2
ꢀ
prepared 0.04 M NaBH4 at 45 C, a continuous change in the
UV-Vis spectrum of the solution was observed. Fig. 7 shows the
recorded spectra aer 15, 30, 45 and 60 min of stirring of the
reaction mixture. Clearly, the band at 324 nm originating from
the extended conjugated structure of nitrofurazone tends to
disappear, while a distinct evolution in the band at 220 nm,
corresponding to n / p* transitions of NH2 groups, was
observed. These changes, coincides with the color change of the
solution from yellow to colorless. Under these circumstances,
89% of the starting nitrofurazone was reduced based on the
measurement of the absorbance of the solution at 324 nm.
In order to evaluate reusability of the catalyst, reduction of
nitrobenzene was carried out in presence of the recycled cata-
lyst. Fig. 8 shows the reduction efficiency against successive
reaction runs. As it is clear, aer six runs, only 5% decrease in
efficiency in terms of conversion percent was observed.
Conclusions
In conclusion, we have devised a new catalyst based on silver
nanoparticles distributed over the surface of RANEY® nickel for
efficient reduction of nitro compounds in aqueous solution
under mild conditions. Highlights of the present work include
near ambient reaction temperature, use of water as a green
solvent, in situ production of the reducing agent from readily
available NaBH4, and above all, reusability which promises
minimization of the catalyst waste.
Acknowledgements
The authors are grateful to the Research Council of University of
Guilan for the partial support of this study.
To get an insight into the reaction mechanism of reduction
of nitrobenzene, we assumed that if the reaction proceeds via
polar intermediates such as nitrosobenzene, phenylhydroxyl-
amine and hydrazine, then each of these intermediates must
follow the same path, leading to the same product. Formation
of such intermediates is oen observed with many other
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