Chemoselective hydrogenation of nitrobenzenes activated with tuned Au/h-BN

Article abstract of DOI:10.1016/j.jcat.2018.12.008

The azo- and hydrazo compounds are of great importance in pharmaceuticals, agrochemicals, and chemistry. The controlled reduction of nitroarenes to their coupled products such as aromatic azo and hydrazo compounds has been an interesting area of research synthetically and mechanistically. Herein, we report that the chemoselective catalytic hydrogenation of nitrobenzenes to hydrazobenzenes via azobenzenes can be achieved over gold nanoparticles supported by hexagonal boron nitride nanoplates. It is found that the catalytic process can be successfully conducted not only in N₂ but also in air with isopropanol alcohol/KOH. Complete conversion of nitrobenzenes and high selectivity of azobenzenes and hydrazobenzenes have been achieved in one pot under N₂ or air atmosphere. Furthermore, as usual unstable intermediates in the reduction process of nitrobenzenes, azobenzenes and hydrazobenzenes can be alternatively harvested as the main product by controlling reaction time or atmosphere. This work shows promise for direct and chemoselective synthesis of azo- and hydrazo compounds under mild conditions in a controllable manner.

Full text of DOI:10.1016/j.jcat.2018.12.008

Journal of Catalysis 370 (2019) 55–60
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Chemoselective hydrogenation of nitrobenzenes activated with tuned
Au/h-BN
Qiuwen Liu a,b, Yan Xu , Xiaoqing Qiu , Caijin Huang b, , Min Liu c,
a
a,
⇑
⇑
⇑
a
College of Chemistry and Chemical Engineering, Central South University, Changsha 41083, PR China
State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350116, PR China
College of Physical Science and Electronics, Central South University, Changsha 41083, PR China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The azo- and hydrazo compounds are of great importance in pharmaceuticals, agrochemicals, and chem-
istry. The controlled reduction of nitroarenes to their coupled products such as aromatic azo and hydrazo
compounds has been an interesting area of research synthetically and mechanistically. Herein, we report
that the chemoselective catalytic hydrogenation of nitrobenzenes to hydrazobenzenes via azobenzenes
can be achieved over gold nanoparticles supported by hexagonal boron nitride nanoplates. It is found that
Received 19 November 2018
Revised 10 December 2018
Accepted 11 December 2018
the catalytic process can be successfully conducted not only in N
KOH. Complete conversion of nitrobenzenes and high selectivity of azobenzenes and hydrazobenzenes
have been achieved in one pot under N or air atmosphere. Furthermore, as usual unstable intermediates
2
but also in air with isopropanol alcohol/
Keywords:
Chemoselective hydrogenation
Nitrobenzenes
2
in the reduction process of nitrobenzenes, azobenzenes and hydrazobenzenes can be alternatively har-
vested as the main product by controlling reaction time or atmosphere. This work shows promise for
direct and chemoselective synthesis of azo- and hydrazo compounds under mild conditions in a control-
lable manner.
Gold
Hexagonal boron nitride
Ó 2018 Elsevier Inc. All rights reserved.
1
. Introduction
The yield of the target product is always a key topic for organic
tion reactions of nitroaromatics to azo-derivatives, which brings
the tedious product separation. Therefore, it will be extremely
desirable to realize the conversion of nitrobenzenes to azoben-
zenes or hydrazobenzenes controllably in one pot with good selec-
tivity and yield.
The conversion efficiency and selectivity of organic reactions
can be controlled with the assistance of the suitable catalysts
[2,4,16,17]. The Pt and Pd catalysts modified with harmful addi-
tives (e.g. Pb and V) have been found to improve the selectivity
of the hydrogenation of nitrobenzenes under harsh reaction condi-
tions of high temperature and high pressure [18]. Comparatively,
gold (Au) has long been considered to be less catalytically active
until Bond et al. reported the hydrogenation of olefins over sup-
ported Au catalysts [19]. Henceforth, the supported Au catalysts
are actively pursued in the reduction of nitrogen oxides and the
hydrogenation of organic compounds [4]. For instance, Corma
and co-workers reported that the Au nanoparticles supported on
reactions and their industrial applications [1–3]. In particular, how
to improve the selectivity of the aromatic azo- and hydrazo-
compounds from the hydrogenation of nitroarenes has long been
a challenging issue, since the azo- and hydrazo-compounds play
essential roles in the production of food additives, polymers, phar-
maceuticals, herbicides and dyes [4,5]. Generally, azo- compounds
are prepared by reducing nitroaromatics or oxidizing anilines
through the azo coupling reaction using nitrite salts [6,7] or toxic
oxidants [8]. On the other hand, as compared with azo-
compounds, the hydrazo-substrates are frequently obtained by
reducing azo- or azoxybenzene with Zn or Al powder [9–11] or
by electrochemical reduction of nitroarenes using Pb-containing
electrodes [12,13]. Few synthetic methods threw the spotlight on
the hydrazo-compounds in the process of hydrogenation of
nitroarenes likely due to their instability and extremely low yield
2
TiO exhibited up to 98% yield for the selective hydrogenation of
[
14,15]. The aromatic hydrazo- as well as azoxy-compounds are
nitroarenes into the corresponding aromatic azo- compounds
[20]. Zhu et al. further proposed a photocatalytic pathway for the
reduction of nitroaromatic compounds into aromatic azo- com-
commonly regarded as unwanted by-products in the most reduc-
pounds over Au supported on ZrO
IPA)/KOH as hydrogen source [21]. Moreover, the Au nanoparti-
cles supported on meso-CeO , Al , and other oxides, have also
2
utilizing isopropyl alcohol
⇑
Corresponding authors.
E-mail addresses: xq-qiu@csu.edu.cn (X. Qiu), cjhuang@fzu.edu.cn (C. Huang),
(
2
2 3
O
minliu@csu.edu.cn (M. Liu).
https://doi.org/10.1016/j.jcat.2018.12.008
0
021-9517/Ó 2018 Elsevier Inc. All rights reserved.

5
6
Q. Liu et al. / Journal of Catalysis 370 (2019) 55–60
been demonstrated to be active in the reduction of nitro group of
organic compounds [22,23]. As a matter of fact, the reaction path-
ways and yield of the nitrobenzenes hydrogenation are highly
depend on the both Au particle size and the nature of the support
tometer with Cu ka1 radiation. Fourier transform infrared spectra
(FTIR) were collected on a nicolet 670FTIR spectrometer using KBr
as a transmission standard and a diluting reagent. UV–Vis spectra
were recorded on a Varian Cary 500 Scan UV–Vis system. Raman
spectra were acquired on a Renishawnin ViaReflex Raman Micro-
phrobe with 633 nm excitation. The morphologies and microstruc-
tures of the samples were investigated by an Agilent SPM5500
Atomic Force Microscope (AFM), filed emission scanning electron
microscopy (FESEM) on a Hitachi New Generation SU8100 appara-
tus and transition electron microscopy (TEM) on a TECNAIF30
[
22,24]. The oxides supporters can generally activate the oxygen in
the reaction system. Consequently, selectively catalytic reduction
of nitrobenzene over the oxides supported Au catalysts is fre-
quently performed under poor oxygen conditions, i.e. N , H or
2 2
CO atmosphere, thereby increasing the cost and potential danger
in industrial practice [14,25,26]. Thus, it is quite urgent to find
novel materials to support Au nanoparticles and endow the cata-
lysts with high selectivity for hydrogenation of nitrobenzene even
under the air atmosphere. Unlike the conventional oxides, hexago-
nal boron nitride (h-BN) is a non-oxide compound constructed
from lightweight and abundant elements. h-BN possesses out-
standing physicochemical properties such as atomically smooth
surfaces, nontoxicity, chemical and thermal stability, and thermal
conductivity [27,28]. What’s more, as a layered two-dimensional
material structurally analogue to graphite/graphene, h-BN can pro-
vide a platform to support the metallic nanoparticles and offer the
1
3
instrument under an acceleration voltage of 200 kV. C NMR spec-
tra of final products were acquired using a Bruker Avance III 500
spectrometer equipped with an 11.7 T wide bore superconducting
1
3
magnet, at Larmor frequencies of 500 MHz for C.
3
. Results and discussion
3.1. Nanoparticle composition and morphology
The h-BN powder samples were prepared by thermal decompo-
sition of the mixture of boron oxide and urea in the presence of
copper nitrate according to the methods in our previous literatures
‘
‘naked” nanoparticle surfaces with highly active sites [29–31]. Our
previous work demonstrated that h-BN serves as an excellent
plane-like support for functional nanoparticles used as catalysts,
rendering an improved catalytic performance [29,32].
[
33,34]. The FESEM, TEM, and AFM images (Fig. S1 and Fig. S2)
show that the h-BN samples exhibit a plate-like shape with the
average diameter of 50–70 nm and ca. 2 nm in thickness. Au
Herein, we report that chemoselective hydrogenation of
nitrobenzene to azobenzene and hydrazobenzene can be achieved
catalytically in one pot under nitrogen or air atmospheres. The
reaction was catalyzed over Au nanoparticles supported on h-BN
nanoplates (noted as Au/BN) in the presence of IPA/KOH. We
attempt to perform the reduction reaction in an industrial-
favourable and controllable manner in order to directly harvest
azobenzene or hydrazobenzene as the main final product with high
selectivity.
nanoparticles are loaded on h-BN nanoplates by reducing HAuCl
with NaBH in the presence of lysine. The intensities of XRD peaks
corresponding to Au increase with the increased amount of Au
Fig. 1a). The X-ray photoelectron spectroscopy (XPS) signals at
7.8 and 84.2 eV are attributed to Au 4f5/2 and Au 4f7/2 [21], respec-
4
4
(
8
tively, suggesting Au exists in metallic state (Fig. 1b and Fig. S3).
The diffusion reflectance spectra and Raman spectra of the samples
are presented in Fig. S4. The optical absorption in the range 500–
6
00 nm is owing to the localized surface plasmon resonance of
2
. Experimental
.1. Preparation of Au/BN composites
A certain amount of HAuCl (10 g/L) was added to the as-
Au nanoparticles [35] (Fig. S4a). A slight redshift of the B–N trans-
verse stretching vibration (about 1380 cm ) demonstrates the
interaction of Au nanoparticles and the surface of h-BN nanoplates
ꢀ
1
2
(Fig. S4b) [36,37]. As shown in Fig. 2 and Fig. S5, Au nanoparticles
4
(average size of 5–6 nm) are well distributed on the h-BN surfaces.
Clearly, no obvious difference in the size of Au nanoparticles was
observed for 3.7% Au/BN and 20.9% Au/BN, but there is a higher
dis-tribution density of nanoparticles for 20.9% Au/BN. As illus-
trated in Fig. 2b and d, the selected area electron diffraction (SAED)
indicates Au nanoparticles have a face-centered cubic phase (FCC)
which is consistent with the XRD results (Fig. 1a).
obtained h-BN colloid (20.1 mg/mL, 5 mL). 2 mL of lysine (31 mg/
mL) was then added, and the mixture was stirred vigorously for
3
0 min. 10 mL of aqueous solution of NaBH
dropped into the mixture in 20 min. After keeping stirring for
h, the sample was left for 16 h in the dark without stirring. The
4
(3.8 mg/mL) was
1
precipitate was filtrated and washed with deionized water and
ethanol, and then dried at 60 °C. The resulting sample was defined
as x % Au/BN, where x was the Au content in the composites. The
details for the sample characterization and catalytic activity test
experiments are given as the Supporting Information.
3.2. Optimization of reaction conditions
To investigate the catalytic performance of the as-obtained cat-
alysts in the reduction reaction, nitrobenzene was selected as the
substrate to optimize reaction conditions including Au loading,
reaction temperature, and atmosphere. As shown in Fig. S6 and
Fig. 3, three main products have been observed in our experiments
and appeared sequentially from azoxybenzene, azobenzene to
hydrazobenzene, which obeys the condensation route of Haber
model [38]. No other intermediates, such as nitrosobenzene and
phenylhydroxylamine, were observed by GC-MS analyses, imply-
ing that conversion of these compounds to azoxybenzene is very
fast. It can be clearly seen from Fig. 3 that the conversion of
nitrobenzene is up to about ca. 100% within 30 min for the Au/
BN catalysts with Au contents >3.7 wt% in the composites. It should
be mentioned that the BN and Au alone exhibited very limited
activity for the conversion of nitrobenzene. Moreover, the selectiv-
ity for each product (azoxybenzene, azobenzene and hydrazoben-
zene) is different and varies with the Au content of the catalysts.
2
.2. Activity test
Nitrobenzene (0. 31 mmol) and 0.06 mmol KOH were dissolved
in 3 mL isopropyl alcohol. 10 mg catalyst (Au/BN composites) was
added to the mixture. The reaction was carried out in nitrogen or in
air at 90 °C under stirring unless otherwise specified. The products
were analyzed using an Agilent 7820A GC equipped with HP-FFAP
column and a Varian Cary 500 Scan UV–Vis spectrometer.
2
.3. Characterization
The actual contents of Au for the as-obtained Au/BN composites
analyzed by inductively coupled plasma-atomic emission spec-
troscopy (ICP-AES). Powder X-ray diffraction (XRD) patterns were
obtained at room temperature on a Bruker D8 Advance diffrac-

Q. Liu et al. / Journal of Catalysis 370 (2019) 55–60
57
∗
b
♦
∗
h-BN
Au
84.2
a
8
7.8
♦
∗
Au 4f7/2
Au 4f5/2
∗
∗
♦
♦
30.1% ♦
20.9%
1
4.7%
8
.2%
3
.7%
h-BN
1
0
20
30
40
50
60
70
80
90
88
86
84
82
2-theta (degree)
Banding Energy (eV)
Fig. 1. (a) XRD patterns of Au/BN samples (the percentage is the mass ratio of Au with h-BN). From left to right, the (0 0 2), (1 0 0), (1 0 1) and (1 1 0) planes of h-BN (JCPDS
card No. 34-0421) are marked by the sign ‘r’ and the ‘*’ represents the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of Au (JCPDS card No. 04-0784), respectively. (b) XPS Au core-
level spectra of the sample 20.9% Au/BN.
Fig. 2. High-resolution TEM images and selected area electron diffraction of 20.9% Au/BN (a, b) and 3.7% Au/BN (c, d).
The catalyst with 20.9% Au content (noted as 20.9% Au/BN) is the
most active for the catalytic complete reduction of nitrobenzene
to the final product hydrazobenzene, and thus was selected for fur-
ther study. The catalysts with Au content over 20.9% exhibit an
aggregation of Au nanoparticles, lowering their effective exposed
surface and active sites. In addition, we also investigated the effect
of reaction temperature on hydrogenation of nitrobenzene using
the catalyst 20.9% Au/BN. The selectivity of the final product hydra-
zobenzene increases with the temperature and reaches 100% at
90 °C (Fig. S6). Considering the boiling point (82.45 °C) of iso-
propanol, other temperatures higher than 90 °C have not been
attempted.
Fig. 3b shows the evolution for the reduction of nitrobenzene on
20.9% Au/BN at 90 °C. As the reaction proceeds, the selectivity of

5
8
Q. Liu et al. / Journal of Catalysis 370 (2019) 55–60
orange-red to colourless (Fig. 3b), showing that hydrazobenzene
1
00
100
80
60
40
20
0
a
becomes the main final product at this stage. Therefore, from the
product profile the main product azobenzene or hydrazobenzene
varies with reaction time, providing a reliable way to harvest
azobenzene and hydrazobenzene as expected with high selectivity
by monitoring reaction time.
8
6
4
2
0
0
0
0
0
NB
AB
It is worth mentioning that hydrazobenzene cannot be directly
detected by gas chromatograph (GC) equipped with FID detector.
Fig. S7a gives the GC results of commercial hydrazobenzene (Ark
Pharm Co., Ltd.), exhibiting two signals corresponding to aniline
and azobenzene, respectively. This is likely due to the instability
of hydrazobenzene under redox atmosphere and high temperature
within FID detector [20]. Our colourless final product also has two
similar signals corresponding to azobenzene and aniline on GC
(Fig. S7b), leading to a misunderstanding for the production of ani-
HAB
AOB
0
5
10
15
20
25
30
35
Au mass ratio (%)
1
3
line. Moreover, the evidence of C NMR spectra proves that nei-
ther aniline nor azoxybenzene is formed in the final product but
it is hydrazobenzene (Fig. S8), which is different from that reported
2
by Zhu et al. over ZrO supported Au under light illumination [21].
Therefore, the yield of hydrazobenzene presented in this work was
evaluated by the Lambert-Beer law with a detection wavelength of
4
40 nm corresponding to its characteristic peak (Fig. S9). Most
importantly, the Au/BN composite also shows high conversion
activities in the highly selective reduction of other nitroaromatic
compounds to produce the corresponding aromatic azo com-
pounds and aromatic hydrazo compounds (Table 1). Meanwhile,
the Au/BN catalyst shows excellent stability (Fig S10).
1
00
100
80
60
40
20
0
b
8
6
4
2
0
0
0
0
0
NB
3.3. Mechanism of hydrogenation reaction on Au/BN
AB
HAB
AOB
Furthermore, to explore the effect of air on the hydrazobenzene,
we removed the catalyst and left the solution to be cooled down to
room temperature and exposed to air. Interestingly, as seen in
Fig. 4, the solution gradually turned from colourless to orange-
red and the product hydrazobenzene was oxidized to azobenzene
in the presence of O
post-oxidation treatment is also an efficient way to convert hydra-
zobenzene to azobenzene just by introducing O . Inspired by this
case, an interesting question was raised that how the nitrobenzene
hydrogenation reaction proceeds in O which is different from that
carried out under H or N atmosphere reported by the literature
20,21]. Fig. 5 shows the evolution of the products, demonstrating
2
, but further oxidation does not proceed. This
0
5
10
15
20
25
30
Time (min)
2
Fig. 3. (a) Reduction of nitrobenzene over the as-prepared Au/BN samples with
different Au contents within 30 min. (b) Reduction of nitrobenzene over the 20.9%
Au/BN samples with different time. Top panel: photos of the transformation of
nitrobenzene to hydrazobenzene. The reaction was conducted under nitrogen
atmosphere at 90 °C for 30 min. Nitrobenzene: NB, azobenzene: AB, hydrazoben-
zene: HAB, azoxybenzene: AOB.
2
2
2
[
that the reduction of nitrobenzene to hydrazobenzene can also
proceed in air. By comparing with the results depicted in Fig. 3,
the existence of O
delay on the formation of hydrazobenzene, and lowers the final
yield of hydrazobenzene. This is due to the presence of O resulting
in the consumption of activated Hydrogen (H-Au) and the oxida-
tion of hydrazobenzene (Fig. 6). The main product azobenzene or
2
obviously slows the reaction rate, such as a
azoxybenzene decreases from 100% down to almost zero in 15 min.
Meanwhile, the conversion of nitrobenzene and the selectivity of
azobenzene increase up to 100%. The turnover frequency of 20.9%
2
ꢀ1
Au/BN catalyst is calculated to be 212 h , much higher than that
hydrazobenzene can also be controlled by reaction time under O
2
ꢀ1
of copper/graphene composites (24 h ) in the similar experimen-
tal conditions [39]. Interestingly, although at the reaction time of
atmosphere (Fig. 5). In addition, in our work the selectivity of the
nitro reduction tuned in favor of azo- and hydrazo products might
be due to the synergic effect between the metal and the support in
the Au catalyst system [41–45].
1
5 min azoxybenzene is completely converted to azobenzene, no
formation of hydrazobenzene was observed. The transformation
of azobenzene to hydrazobenzene appears to be blocked in the
presence of nitrobenzene until all the nitrobenzenes are depleted
For full understanding the reaction mechanism, we further col-
lected and analysed the trace amount of by-products in the reac-
tion system, and proposed a feasible mechanism (Fig. 6). As
shown in Table S1, small amount of acetone and hydrogen was
generated during the reaction, indicating that KOH abstracts
hydrogen atoms from isopropyl alcohol, thereby producing the
acetone and the activated H-donor [46,47], which could been con-
firmed by acetone generated in reaction (Fig. S11). This process
could be facilitated over the surface of Au and generates H-Au spe-
cies by binding hydrogen atoms on the Au surfaces [26,48] i.e. the
Step I depicted in Fig. 6. Then, in II and III step, these H-Au species
capture oxygen atoms from N-O bonds to yield azoxybenzene,
[
40]. For this reason, the conversion of azobenzene to hydrazoben-
zene could be a rate-determining step during the reduction reac-
tion. Additionally, from the reaction profile, full consumption of
nitrobenzene and azoxybenzene are occurred at 15 min without
formation of hydrazobenzene, revealing that azoxybenzene is not
directly hydrogenated to hydrazobenzene, which is different from
the case of direct hydrogenation of azoxybenzene to hydrazoben-
zene [40]. Furthermore, after 15 min, the accumulated azobenzene
tends to be reduced and almost completely converted to hydra-
zobenzene in 30 min while the solution colour turns from

Q. Liu et al. / Journal of Catalysis 370 (2019) 55–60
59
Table 1
Reduction of substituted nitro compounds to hydrazobenzene compounds. All reactions were carried out with 10 mg of 20.9% Au/BN catalyst, 0.31 mmol of reactant, 0.06 mmol of
KOH, and 3 mL of IPA at 90 °C under N
2
atmosphere.
Reactant
Final product
Raction time
Yield (%)
100a
3
3
3
0 min
0 min
0 min
>90b
>90b
3
6
0 min
0 min
>90b
>80b
a
Detection by UV–Vis spectrometer.
NMR yield.
b
1
00
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
NB
AOB
AB
HAB
1
00
AB
HAB
8
6
4
2
0
0
0
0
0
0
10 20 30 40 50 60 70 80 90
Time (min)
Fig. 5. Reduction of NB in air over the 20.9% Au/BN catalyst at 90 °C.
0
20 40 60 80 600
Time (min)
800
1000
Fig. 4. Time-conversion plots for the transformation of hydrazobenzene to
azobenzene in air after a complete reduction reaction of nitrobenzene (see text
for details). Top panel: photos of the transformation of hydrazobenzene to
azobenzene in air.
azobenzene, and HO-Au species that were consumed by iso-
propanol by evolving H O [49], as demonstrated by evaporating
2
reaction solution which made the white anhydrous cupric sulfate
turn into blue (Fig. S12). At the IV step, H-Au keeps away from
nitrobenzene at last to reduce azobenzene to be hydrazobenzene
due to itself reducing capacity [49].
With H-Au and HO-Au produced and consumed consequently,
the catalytic reduction of nitrobenzene proceeds rapidly on the
Fig. 6. Proposed mechanism for hydrogenation of nitrobenzene to hydrazobenzene
over Au nanoparticles supported by h-BN nanoplates.
surface of Au under N
introduced O , which could remove the hydrogen from the Au sur-
faces [49], just lowering the catalytic activity and the reaction rate
Fig. 5). Meanwhile, the H atoms from the H-Au species could be
collided with each other and thus produce H along with reaction
2
atmosphere. It even still conducting after
proceeding [50,51], this conjecture is based on a small quantity of
H generated rather than that equivalent of H produced. By sum-
2 2
ming together, the reaction for the overall equation as following
equation:
2
(
2

6
0
Q. Liu et al. / Journal of Catalysis 370 (2019) 55–60
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Acknowledgement
This project was sponsored by the National Natural Science
Foundation of China (NSFC, Grant No. 11305091), Hunan Provincial
Natural Science Foundation of China (Grant No. 2017JJ2326), and
the Open Project Program of the State Key Laboratory of Photo-
catalysis on Energy and Environment (Grant No. SKLPEE-
KF201703) Fuzhou University.
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