Room temperature catalytic reduction of nitrobenzene to azoxybenzene over one pot synthesised reduced graphene oxide decorated with Ag/ZnO nanocomposite

Article abstract of DOI:10.1016/j.catcom.2019.02.026

We report herein, a one-pot synthetic route for the synthesis of reduced graphene oxide decorated Ag/ZnO nanocomposite and studied its catalytic activity as simple, recyclable and efficient catalyst for one-pot conversion of nitrobenzene to azoxybenzene. It was observed that 5–10 nm Ag-nanoparticles supported on 40–60 nm ZnO nanorod decorated on reduced graphene oxide was formed with a silver loading of 1.6 wt%. The effect of different reaction parameters were investigated and studied in detail. A nitrobenzene conversion of 96% with 98% selectivity of azoxybenzene was achieved without the use of any external additives.

Full text of DOI:10.1016/j.catcom.2019.02.026

Accepted Manuscript
Room temperature catalytic reduction of nitrobenzene to
azoxybenzene over one pot synthesised reduced graphene oxide
decorated with Ag/ZnO nanocomposite
Bappi Paul, S. Vadivel, Nishant Yadav, Siddhartha Sankar Dhar
PII:
S1566-7367(19)30060-3
DOI:
https://doi.org/10.1016/j.catcom.2019.02.026
CATCOM 5637
Reference:
To appear in:
Catalysis Communications
Received date:
Accepted date:
8 February 2019
26 February 2019
Please cite this article as: B. Paul, S. Vadivel, N. Yadav, et al., Room temperature catalytic
reduction of nitrobenzene to azoxybenzene over one pot synthesised reduced graphene
oxide decorated with Ag/ZnO nanocomposite, Catalysis Communications, https://doi.org/
1
0.1016/j.catcom.2019.02.026
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ACCEPTED MANUSCRIPT
Room temperature catalytic reduction of nitrobenzene to azoxybenzene over one pot
synthesised reduced graphene oxide decorated with Ag/ZnO nanocomposite
a
b
c
a
Bappi Paul *, S. Vadivel , Nishant yadav , Siddhartha Sankar Dhar *
aDepartment of Chemistry, National Institute of Technology Silchar, Silchar-788010, Assam,
India
bDepartment of Chemistry, PSG College of Technology, Coimbatore 641004, India
c
Department of Chemistry, Pandit Prithi Nath College, kanpur- 208001, India
*
Corresponding authors. Tel: +91-03842-242915; fax: +91-03842-224797
Email: bappipaulnits@gmail.com (B. Paul); ssd_iitg@hotmail.com (S.S. Dhar)
Abstract
We report herein, a one-pot synthetic route for the synthesis of reduced graphene oxide
decorated Ag/ZnO nanocomposite and studied its catalytic activity as simple, recyclable and
efficient catalyst for one‐pot conversion of nitrobenzene to azoxybenzene. It was observed that
5
–10 nm Ag-nanoparticles supported on 40–60 nm ZnO nanorod decorated on reduced
graphene oxide was formed with a silver loading of 1.6 wt %. The effect of different reaction
parameters were investigated and studied in detail. A nitrobenzene conversion of 96% with
9
8% selectivity of azoxybenzene was achieved without the use of any external additives.
Keywords: Nitrobenzene; Selective reduction; Azoxybenzene; High conversion; High
selectivity.
Introduction
Selective catalytic reduction of nitrobenzene into valuable intermediates such as aniline,
azobenzene, nitroso benzene and azoxybenzene are one of the challenging reactions in the field
of pharmaceutical industries ranging from dyes, polymer stabiliser and food additives [1-3].
Among those, the preparations of azoxybenzene have attracted special importance due to its
1

ACCEPTED MANUSCRIPT
use as precursor for Wallach rearrangement that has been applicable in coloration of resin, dye,
lacquer and also provide a simple route for synthesizing hydroxyazobenzene[4].
Azoxybenzene is produced as an intermediate in the reduction of nitroarenes or the oxidation
of aniline. These reaction steps are quite complex and lead to different reaction intermediates.
Therefore, the challenge lies in controlling the selectivity to the target product via manipulation
of reaction conditions at a reasonable substrate conversion has becomes an important research
interest [5,6]. Various methods have been reported for oxidative coupling of aniline and
reducing nitroarenes using homogenous and heterogeneous catalyst.[6, 7-9] However, in most
of the cases product separation from the reaction mixtures and the recyclability of the catalysts
become two major issues in liquid-phase reaction. Therefore, development of more benign
recyclable catalyst that works under mild conditions in reducing nitroarenes is in high demand.
Nanostructured ZnO continues to draw significant attention because of its potential
application in energy generators, sensors, lasers as well as catalysts in chemical reactions [10].
Recent studies have shown that ZnO NPs modified with noble metals such as Ag or Au enable
larger area of active sites for improved catalytic activity [11]. Noble metal supported ZnO
nanostructured materials although have shown better activity due to synergistic effect between
metal and metal oxides, however due to poor diffusion and steric factors, their activity and
selectivity might be affected. Therefore, such heterodimeric nanocatalysts if supported on
porous carbon, surface area may substantially be increased due to their highly even distribution
in the pores. Graphene oxide (GO) has proved to very efficient porous carbon support for
anchoring metal nanoparticles due to its large specific surface area and both sides accessible of
its nanosheets [12]. Recently, some studies have demonstrated that transition metal
nanoparticles supported on porous carbon nanosheets work as efficient catalysts for organic
transformation [13-16]. We report herein, a one-pot hydrothermal synthesis of Ag/ZnO
embedded rGO and studies of its catalytic activity for selective reduction of nitrobenzene to
2

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azoxybenzene using hydrazine hydrate (N
hydrate (N .H O) is considered as a very suitable reagent for the reduction of nitrobenzene,
since only water and N are generated as by-products [17].
2
H
4
.H O) as reducing agent. In this study, hydrazine
2
2
H
4
2
2
2
. Experimental
.1 Synthesis of Ag/ZnO-rGO
2
Graphene oxide (GO) nanosheets were prepared by our previously reported method [18]. 0.68
g of ZnCl (5 mmol) and 0.084 g of AgNO (0.5 mmol) were slowly added to 20 mL of 1.8 mg
2
3
-1
mL GO aqueous solution while stirring. Subsequently, 1.20 g of urea (20 mmol) and 4 g of
PEG 4000 (1 mmol) were slowly added to the aqueous mixture and was further sonicated for
3
0 min. Subsequently 60 ml of deionized water was added followed by hydrothermal heating
at 180C for about 24 h. The resulting solid was cooled to room temperature, washed with
deionized water and ethanol several times. Finally the product was dried at 70°C in a vacuum
oven for 6 h.
2
.2 Catalytic activity measurements
The catalytic activity of Ag/ZnO-rGO catalyst was tested in the selective reduction of different
aromatic nitro where nitrobenzene was taken as model substrate for reduction reaction as
summarised in Table 2. Reduction of nitrobenzene was carried out in a double neck round
bottom flask at room temperature (25 °C) using 20 mg catalyst, 15 mmol nitrobenzene and 5
mL ethanol as solvent, where 22.5 mmol hydrazine was added dropwise as reducing agent.
Small aliquots were collected from the reaction mixture at regular intervals for analysis. At the
end of the reaction, the solid catalyst was separated by filtering and products were analysed
using a gas chromatograph (GC, Agilent 7890) connected with a HP-5 capillary column (30 m
length, 0.28 mm id and 0.25 μm film thickness) and a flame ionization detector (FID).
3
. Results and Discussion
3

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<
Fig.1>
The amount of Ag present in Ag/ZnO-rGO catalysts were estimated by ICP-AES. The
crystalline nature and exposed phase of prepared catalyst were determined by powder X-ray
diffraction (XRD) analysis (Fig.1). The diffraction peaks are found to be consistent with those
of the reported data of ZnO (JCPDS File no. 89-1397) (Fig.1 (c)). The Ag/ZnO-rGO
nanocomposite of catalyst showed two additional peaks at 2θ values 38.13° and 25.4°
corresponds to (111) planes of face-centered cubic Ag (JCPDS File no. 89-3722), and (002)
plane of rGO (Fig. 1(d)). It is also observed from the XRD pattern of the spent catalyst that the
metallic Ag does not change its phase even after the catalytic reaction (Fig.1(e)). The
physicochemical properties of the fresh and spent catalyst is summarised in Table S1 in ESI.
<
Fig. 2.>
The TEM image of Ag/ZnO-rGO nanocomposite synthesized is shown in Figure 2. From the
TEM image it can be seen that Ag particles are distributed homogeneously on the ZnO nanorod
and anchored uniformly over rGO. TEM showed a rod-like structure of ZnO with diameter 40-
6
0 nm and Ag NPs of sizes approximately 5-10 nm randomly attached to it. The high
magnification TEM image of Ag/ZnO-rGO nanocomposite (Fig.S1 in ESI) and the EDS
spectrum showed the existence of Zn, O, Ag and C in the material, which clearly indicates the
formation of Ag/ZnO-rGO composite (Fig.S2 in ESI).
To characterize the ordered and disordered crystal structure of GO, Raman spectra were
recorded (Fig.S3 in ESI). Both GO and Ag/ZnO-rGO shows two prominent peaks at 1298 and
-1
1
595 cm , which correspond to D-band (defect (D) peak) and G-band (graphite (G) peak)
respectively[19]. The D-band/ G-band (I /I ) intensity ratio of these peaks provide the disorder
levels of the GO. The I /I of GO was determined to be 1.12 whereas upon hydrothermal
D
G
D
G
reduction, the I /I
D G
of Ag/ZnO-rGO is increased to 1.46.
4

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Ag/ZnO-rGO nanohybrid featured characteristics of a type IV isotherm with H hysteresis loop
3
associated with aggregate of plate-like particles forming slit-like pores with a surface area and
2
3
pore volume of 522.15 (m /g) and 1.07(cm /g), respectively (Fig.S3 in ESI). The pore size
distribution as indicated by Barrett Joyner Halenda (BJH) shows that most of the pores range
from 5 to 60 nm with an average pore size of 27.12 nm (Fig.S4 inset in ESI).
<
Fig. 3.>
The valence state of Ag species in Ag/ZnO-rGO was estimated by X-ray photoelectron
spectroscopy (XPS), where we can clearly see the presence of metallic silver in the form of
corresponding Ag 3d5/2 and Ag 3d3/2 with binding energy values of 367.4 eV and 373.4 eV,
respectively (Fig. 3). The XPS spectra of Zn 2p with binding energy at 1021.4 eV (2p3/2) and
2
+
1
044.5 eV (2p1/2) confirms the existence of Zn element is in the state of Zn . The high
resolution C 1s spectra were deconvoluted into three distinct peaks, that correspond to the C–
C bond at 285.6 eV and the carbon atoms bonded to different functional moieties of arginine
C–O at 286.2 eV and C=O at 290.1 eV. The corresponding XPS analysis of the spent catalyst
confirms that the oxidation state of Ag 3d binding energy remains same during the catalysis
(Figure S5 in ESI).
2
.3 Catalytic activity
After successful synthesis and characterization of nanostructured Ag/ZnO-rGO, we examined
its catalytic activity for one-pot reduction of different aromatic nitro compound where
nitrobenzene was taken as model substrate and various parameter such as time, temperature,
nitrobenzene: N
2
H
4
.H O molar ratio and catalyst amount were studied in order to get best yield
2
and selectivity of azoxybenzene. When the catalytic experiments were carried out in presence
of N
2
H
4
.H
2
O as the reducing agent and ethanol as a solvent keeping the nitrobenzene:
N
2
H
4
.H
2
O mole ratio constant (1:1.5) at room temperature for 120 min the catalyst showed
maximum nitrobenzene conversion of 96% with 98% selectivity towards azoxybenzene at
5

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room temperature where aniline, and azobenzene were detected as the side products. The effect
of reaction time was also studied and the results also showed an increasing trend in both the
nitrobenzene conversion and azoxybenzene selectivity with increasing time (Fig. 4). The
results indicated that after 120 min of reaction, the maximum selectivity of azoxybenzene
(∼98%) was achieved, but after 150 min the selectivity of azoxybenzene dropped to ∼92% due
to formation of azobenzene. The conversion of nitrobenzene was found to be 100% with 54%
azoxybenzene selectivity, whereas with increase in the temperature up to 70 °C, although the
conversion increases steadily the selectivity drops down due to the formation of azobenzene as
the major by-product which may be due to the over reduction of nitrobenzene which takes place
at high temperature (Fig. S6 in ESI). When the molar ratio of nitrobenzene and N
: 0.5, we observed very low conversion of nitrobenzene ( 74%) with 80% azoxybenzene
selectivity, which may be due to the lesser interaction between the reactant and active reducing
species for catalysis. On increasing the mole ratio of nitrobenzene and N .H O up to 1:2.5),
2
H
4
.H
2
O was
1
2
H
4
2
the conversion reached up to 100% and the selectivity dropped to up to 80% with the increase
in azobenzene formation, because of over reduction of nitrobenzene (Fig. S7 in ESI).
<
Fig. 4.>
In order to explore the individual role of Ag, ZnO and reduced graphene oxide in its catalytic
activity, a series of experiments were conducted. A blank run in the absence of the catalyst
does not show any catalytic activity under the optimized reaction conditions (Table 1, entry
1
3). When the Ag/ZnO catalyst was synthesized by the conventional impregnation method
(imp), it showed very poor catalytic activity due to larger particle sizes and excessive leaching
of the metals (Table 2, entry 5). Furthermore, the Ag/ZnO-rGO nanostructured catalyst showed
a greater conversion for azoxybenzene production, compared to bare Ag or ZnO catalyst (Table
2
, entries 3 and 4). The reasons for excellent catalytic activity of Ag/ZnO-rGO prepared by the
present method is believed to be due to higher specific surface area and higher dispersion of
6

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very small sized Ag NPs supported on ZnO leading to easy accessibility of reactants on active
catalytic sites. Thus, taking into account of all the reaction parameters as a standard Ag/ZnO-
rGO as a catalyst the scope of the reaction was explored next. The results are summarized in
Table 2. Various substituted nitroarenes bearing electron withdrawing and electron donating
groups were examined. It was observed that a variety of structurally different aromatic
nitroarenes were selectively transformed to their corresponding products in high to excellent
yield.
<
<
Table 1.>
Table 2.>
The activity of the recovered catalyst after 5 consecutive cycle did not show any significant
decline (Fig.5). Negligible amount of leaching of Ag and ZnO was detected (concentration of
both metals were <2 ppb). To confirm that there is no leaching of Ag particles during the
reaction, we have performed the conventional hot filtration test, where we have removed the
catalyst after 90 min of the reaction and the yield of the product obtained at 90 min was noted
[
20]. The reaction was further continued without the catalyst for further 150 min more hours
but we did not observe any increase in the yield. This confirms that there occurs no leaching
of Ag NPs from the ZnO during the reaction.
<
Fig. 5.>
4
. Conclusion
In conclusion, one-pot synthetic route for the synthesis of reduced graphene oxide (rGO)
decorated Ag/ZnO nanocomposite via homogeneous precipitation of Zn(II) and simultaneous
reduction of Ag(I). The catalyst was found to be efficient catalyst for the selective reduction
of nitrobenzene to azoxybenzene with 96% nitrobenzene conversion and 98% azoxybenzene
7

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selectivity at room temperature with hydrazine as reducing agent. The catalyst also shows true
heterogeneity as its effectiveness remains almost same after five cycles of reuse.
Acknowledgements
The authors are thankful to SAIF IIT Bombay and SAIF North Eastern Hill University for
providing analytical results. Director, NIT Silchar, is acknowledged for his support and
encouragement.
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Figure, and table captions
Figure1. Powder XRD patterns of (a) standard Ag(0), (b) standard ZnO, (c) synthesized ZnO
nanoparticle (d) Fresh Ag/ZnO-rGO nanocatalyst and (e) Spent Ag/ZnO-rGO nanocatalyst.
Figure2. TEM images of Ag/ZnO-rGO nanoparticles.
Figure 3. XPS spectrum of the Ag/ZnO-rGO nanoparticles.
Figure 4. Effect of time on nitrobenzene reduction. [ ] Conversion of nitrobenzene, [ ]
selectivity to azoxybenzene, [ ] selectivity of aniline, [ ] selectivity to
azobenzene. Reaction Condition solvent = ethanol; nitrobenzene = 15 mmol;
the weight of the catalyst = 20 mg; substrate : hydrazine molar ratio = 1 : 1.5.
Figure 5. Recyclability test of Ag/ZnO-rGO nanoparticals
Table 1. Reductive coupling of nitrobenzene to azoxybenzenea
Table 2. Reductive coupling of various nitrobenzene catalyzed by Ag/ZnO-rGO
nanocomposite
9

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<Fig.1>
<Fig.2>
1
0

ACCEPTED MANUSCRIPT
<Fig.3>
<Fig.4>
1
1

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<Fig.5>
Table 1.
NO2
O
Ag/ZnO-rGO, N H .H O
2
4
2
2
N N
Ethanol, RT
Entry
Catalyst
Selectivity S c
(%)
P
b
Yield Y d
A
C
(
T
Aniline
Azo
benzene
Azoxy
benzene
%)
(%)
1
2
3
4
5
6
7
8
9
Agcom
6.8
18
24
27
22
6
96
96
100
74
100
1
43
51
38
64
16
100
-
-
2
14
-
-
47
31
56
14
58
-
2
3
44
6
20
-
-
18
6
-
3
1
6
6
com
ZnO
us
Ag
us
ZnO
22
26
-
98
97
54
80
80
-
imp
Ag/ZnO
rGO
-
e
f
Ag/ZnO-rGO
Ag/ZnO-rGO
Ag/ZnO-rGO
Ag/ZnO-rGO
Ag/ZnO-rGO
No Catalyst
94
93
54
59
80
-
g
h
1
1
1
0
1
3
i
a
Reaction conditions: solvent (ethanol) = 5 ml, substrate (nitrobenzene) = 15 mmol, the weight of the catalyst =
2
0 mg, silver loading = 1.6 wt%, reaction temperature = room temperature; time = 120 min; nitrobenzene :
b
hydrazine molar ratio = 1 :1.5;
nitrobenzene reacted/initial moles of nitrobenzene used] × 100. SP: selectivity of the product calculated by total
moles of the product formed/total moles of nitrobenzene converted; YA (Yield of azoxybenzene) = conversion ×
selectivity/100; ∼Ag nanoparticles supported on ZnO-rGO. Catalyst after 5 reuse; Ag/ZnO-rGO nanocatalyst.
When the reaction temperature was 80 °C. When nitrobenzene: hydrazine (molar ratio) = 1:0.5; when
nitrobenzene: hydrazine (molar ratio) = 1:2.5. com = commercial; us = bare Ag and ZnO prepared by our method;
imp = impregnation method.
T
C : conversion of nitrobenzene based upon the FID-GC results = [moles of
c
d
e
f
g
h
i
1
2

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Table 2.
Entries
Substrate
C (%)
Product
Selectivity (%)
Yield (%)
T
O
9
8
9
6
1
2
N N
NO2
NO2
O
H C
3
N
N
84
62
85
7
4
H C
3
CH3
Cl
Cl
O
Cl
90
9
4
3
4
NO2
N N
O
83
Cl
NO2
NO2
90
90
Cl
N N
Cl
92
86
O
5
77
H CO
H3CO
N N
OCH3
3
a
Reaction conditions: solvent (ethanol) = 5 ml, substrate = 15 mmol, the weight of the Ag/ZnO-rGO catalyst = 20 mg,
silver loading = 1.6 wt%, time = 120 min; nitrobenzene: hydrazine molar ratio = 1: 1.5.
1
3

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Highlights




One-pot hydrothermal synthesis of reduced graphene oxide decorated Ag/ZnO
5–10 nm Ag-nanoparticles supported on 40–60 nm ZnO nanorod
Room temperature catalytic reduction of nitrobenzene to azoxybenzene.
Nitrobenzene conversion of 96% with 98% selectivity of azoxybenzene.
1
4