Chemoselective Transfer Hydrogenation of Nitroarenes Catalyzed by Highly Dispersed, Supported Nickel Nanoparticles

Article abstract of DOI:10.1021/acscatal.5b00969

A recyclable highly dispersed Ni/SiO₂ catalyst was prepared by atomic layer deposition. Chemoselective reduction of nitroarenes was studied using the prepared Ni/SiO₂ as the catalyst and hydrazine hydrate as a hydrogen donor. Different kinds of nitroarenes were converted to the corresponding anilines with high yields. The high activity of the catalysts could be a result of the highly dispersed Ni nanoparticles. (Chemical Presented).

Full text of DOI:10.1021/acscatal.5b00969

Letter
pubs.acs.org/acscatalysis
Chemoselective Transfer Hydrogenation of Nitroarenes Catalyzed by
Highly Dispersed, Supported Nickel Nanoparticles
Chengjun Jiang,†,§ Zeyu Shang,‡,§ and Xinhua Liang*,‡
†Department of Chemical and Biological Engineering, Zhejiang University of Science & Technology, Hangzhou, Zhejiang 310023,
People’s Republic of China
‡Department of Chemical and Biochemical Engineering, Missouri University of Science and Technology, Rolla, Missouri 65409,
United States
S
* Supporting Information
ABSTRACT: A recyclable highly dispersed Ni/SiO2 catalyst was prepared by
atomic layer deposition. Chemoselective reduction of nitroarenes was studied
using the prepared Ni/SiO2 as the catalyst and hydrazine hydrate as a
hydrogen donor. Different kinds of nitroarenes were converted to the
corresponding anilines with high yields. The high activity of the catalysts could
be a result of the highly dispersed Ni nanoparticles.
KEYWORDS: atomic layer deposition (ALD), nickel nanoparticles, heterogeneous catalysis, chemoselective, transfer hydrogenation,
nitroarenes
catalysts.22 These advantages have expanded the ALD
1. INTRODUCTION
applications in the catalysis area. As a part of our effort in
Aromatic amines are useful intermediates in the preparation of
this area, we demonstrated the excellent performance of
dyes, pharmaceuticals, and agricultural chemicals, which can be
platinum nanoparticles deposited by ALD for different liquid
obtained by the reduction of aromatic nitro compounds. A
phase catalytic reactions.24,25 Recently, Gould et al.26
variety of procedures involving metal catalysts and other
demonstrated that Ni nanoparticles deposited by ALD had
reducing agents are available for this purpose;1−4 however, the
excellent performance in propylene hydrogenolysis reactions. It
selective reduction of a nitro group in the presence of other
was demonstrated that smaller metal nanoparticles showed
reducible functional groups in a molecule is a challenging task.
higher activity for hydrogenation of nitroarenes.11,27 It is
In addition, reduction of aromatic nitro compounds often stops
expected that nickel nanoparticles deposited by ALD would
at an intermediate stage, yielding hydroxylamines, hydrazines,
have excellent properties for the reduction of NO2 groups.
azoarenes, or azoxyarenes as byproducts.5−7 Recently, many
Here, we report the chemoselective reduction of aromatic
catalysts involving metal nanoparticles (e.g., Au,8−10 Ag,11 Fe,12
nitro compounds to the corresponding anilines by highly
Fe−Ni,13 Ni−B,14,15 Ru,16,17 and Pd18,19) have been employed
dispersed ALD nickel nanoparticles, supported on SiO2 in
for the reduction of nitro groups. Among the catalysts, gold
combination with a convenient hydrogen donor. The reaction
showed the highest activity and selectivity; however, gold is a
conditions were optimized using different amounts of hydrazine
noble metal and very expensive. Nickel and iron are good
hydrate. Reduction reactions of different nitroarenes were
reducing agents and much less expensive than gold. Various
studied to evaluate the performance of the catalysts.
liquid-phase-based methods were applied to prepare Ni and Fe
nanoparticle catalysts for the reduction of nitroarenes. These
2. EXPERIMENTAL SECTION
2.1. Materials. All solvents, hydrazine hydrate, and
reactants were provided by J&K Chemical. Silica nanoparticles
catalysts showed relatively high selectivity toward aromatic
amines, but the activity was much lower than that of gold.12−14
It is very important that an efficient, cost-effective, and
(20−30 nm in diameter) and silica gel particles were purchased
chemoselective catalyst be developed for the hydrogenation
from US Research Nanomaterials and Sigma-Aldrich, respec-
of nitroarenes.
tively. The silica gel particles were 30−75 μm in diameter with
Atomic layer deposition (ALD) is a layer-by-layer gas phase
process, which has been employed to uniformly deposit
an average pore size of 15 nm and a Brunauer−Emmett−Teller
surface area of ∼270 m2/g. Bis(cyclopentadienyl)nickel was
purchased from Alfa Aesar.
conformal films on porous nanostructures.20,21 The growth of
these films can be controlled at the atomic level through self-
limiting, sequential binary reactions on the substrate surface.
ALD also allows the uniform deposition of highly dispersed
metal nanoparticles (2 nm or smaller). Metal nanoparticles,
such as Pt and Pd, have been successfully deposited on a porous
substrate by ALD.22,23 This is very important for the metal
nanoparticles in catalysis application, especially for supported
2.2. Ni Nanoparticles Deposited on SiO2 by ALD. Ni
ALD was carried out using bis(cyclopentadienyl)nickel
Received: May 8, 2015
Revised: July 14, 2015
© XXXX American Chemical Society
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Table 1. Evaluation of the Effects of Hydrazine Amounts and Solvent on the Reduction of 4-Fluoronitrobenzene to p-
a
Fluoroaniline Catalyzed by One Cycle of Ni ALD-Deposited on Silica Gel Particles
Entry
Catalyst
Temp (°C)
Solvent
NH2NH2, equiv
Selectivity (%)
Yield (%)
1
2
no catalyst
Ni/SiO2
Ni/SiO2
Ni/SiO2
Ni/SiO2
Ni/SiO2
Ni/SiO2
Ni/SiO2
Ni/SiO2
Ni/SiO2
recycle1
recycle2
recycle3
recycle4
100
60
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
THF
4
4
4
4
4
6
8
8
8
8
8
8
8
8
0
0
98.1
97.8
98.2
97.9
98.1
98.2
97.5
97.9
98.5
98.5
98.0
97.8
97.2
59.2
75.8
95.0
71.7
96.5
97.8
92.7
94.2
95.8
95.7
95.2
94.3
94.0
3
80
4
100
120
100
100
100
100
100
100
100
100
100
5
6
7
8
9
CH3OH
t-BuOH
EtOH
EtOH
EtOH
EtOH
10
11
12
13
14
a
Reaction conditions: 6 mmol of nitroarenes, 4−8 equiv of NH2NH2, 10 mL of solvent, and 10 mg of Ni/SiO2 catalysts (except entry 1) were added
into the reactor. The reactions were carried out at different temperatures for 8 h.
(NiCp2) and hydrogen as precursors at 300 °C in a fluidized
bed reactor. The ALD reactor system was described in detail
previously.28 Both silica nanoparticles (20−30 nm) and porous
silica gel particles were used as substrates. In this study, one
cycle of Ni ALD was applied on the silica gel particles, and
different cycles of Ni ALD were applied on the silica
nanoparticles. The details of Ni ALD are introduced in the
Supporting Information.
was employed to verify the complete removal of the organic
ligand from the ALD precursor.
3. RESULTS AND DISCUSSION
3.1. Characterization of Ni/SiO2. The ICP−AES result
showed that the Ni loading of one cycle of Ni ALD on the silica
gel particles was 0.73 wt %. Figure 1 shows the HRTEM images
of ALD-deposited Ni nanoparticles (2 cycles and 15 cycles) on
20−30 nm silica nanoparticles and the size distribution of the
Ni nanoparticles. The average size of the Ni nanoparticles was
2.4 and 2.6 nm for the 2 cycle- and 15 cycle-samples,
respectively. This particle size was similar to the values reported
by Gould et al.26 We believe that the average size of Ni
nanoparticles deposited on silica gel particles, with only 1 cycle
of Ni ALD, should be ∼2.4 nm or even smaller. The TEM
images also indicated that the Ni nanoparticles were highly
dispersed on the silica support. Figure S1 shows the Ni (2p)
high-resolution XPS spectrum for Ni supported on a silica gel
sample. Carbon peaks were also observed on the survey scan of
the XPS spectrum (Figure S2). To verify whether there were
substantial organic ligands from the Ni precursor left on Ni
particles, FTIR was applied to analyze the organic groups.
Figure S3 shows that there were no characteristic peaks for the
cyclopentadienyl group. This means that all organic groups of
the precursor were removed during the ALD hydrogen
reduction process.
3.2. Catalytic Reduction of 4-Fluoronitrobenzene. To
optimize the reaction conditions, the reduction of 4-
fluoronitrobenzene was initially investigated with different
amounts of hydrazine and different solvents at different
temperatures (Table 1). Ni/silica gel particles were used as
catalysts in all catalytic reactions. The recyclability of the
catalyst was also investigated by recycling the catalyst for
reduction of 4-fluoronitrobenzene. As shown in Table 1, the
control experiment showed that no reaction was processed
without a catalyst. The temperature was a key factor in the
reduction of 4-fluoronitrobenzene. At 60 °C, when 4 equiv of
hydrazine hydrate was added, the selectivity to p-fluoroaniline
was 98.1%, and the yield was only 59.2%. The yield increased
2.3. General Procedure of Nitroarenes Reduction with
Hydrazine Hydrate Catalyzed by Ni/SiO2. Hydrazine
hydrate was chosen as the hydrogen donor for the reduction
of aromatic nitro compounds. Ni supported on silica gel
particles was applied to catalyze the reaction. In a typical
reaction, nitroarenes (6 mmol), NH2NH2 (4−8 equiv), solvent
(10 mL), and Ni/SiO2 catalysts (10 mg) were added into a
sealed copper tube with an open end and 12 mL in volume.
The reaction was carried out at atmospheric pressure and an
appropriate temperature (see Table 1). The products of the
reaction were monitored by a LC-20AT high-performance
liquid chromatography equipped with a Hypersil ODS (C18)
column and SPD-20A/20AV UV−vis detector. Methanol and
water (80% methanol and 20% water) were employed as the
mobile phase at a flow rate of 1 mL/min. The mixture was
separated at 25 °C with 254 nm as the detection wavelength.
The identities of all products were compared with authentic
samples. For the recycling studies of 4-fluoronitrobenzene
reduction with hydrazine hydrate, the reaction conditions were
as follows: 6 mmol of 4-fluoronitrobenzene, 10 mg of Ni/SiO2,
48 mmol of hydrazine hydrate (in each cycle), and 10 mL of
ethanol, for 8 h at 100 °C. After each run, the catalyst (Ni/
SiO2) was isolated by a magnet and rinsed thoroughly with
ethanol, since the Ni/SiO2 particles were highly magnetic.
2.4. Catalyst Characterization. Inductively coupled
plasma-atomic emission spectroscopy (ICP−AES) was em-
ployed to analyze the Ni content of the catalysts. The Ni
particles were directly observed by high-resolution transmission
electron microscopy (HRTEM). X-ray photoelectron spectros-
copy (XPS) was applied to verify the composition of Ni on the
silica particles. Fourier transform infrared spectroscopy (FTIR)
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The ALD-prepared Ni/SiO2 catalysts were highly magnetic;
therefore, for separation, the catalyst particles could be easily
collected using a magnet. The catalyst particles were recycled
four times. Both the selectivity and yield only slightly decreased
after cycling, as shown in Table 1, which indicated that the
ALD-prepared Ni/SiO2 catalysts could maintain good activity
and stability in the liquid phase reactions.
3.3. Reduction of Nitro Compounds into Amines with
Hydrazine Hydrate Catalyzed by Ni/SiO2. To investigate
and determine the scope and limitations of this reductive
methodology, a series of nitroarenes were examined. As shown
in Table 2, for a reaction time of 8 h, the selectivity and yield of
most nitroarene reduction reactions were higher than 90%. For
example, the selectivity and yield of aniline were 95.2% and
93.8%, respectively. Turnover frequency (TOF), which is
defined as the reactant molecules reacting per active site in unit
time, was calculated to investigate and determine the activity of
the catalyst (Table 2). The conversion of nitroarenes was close
to 100% for 8 h of reaction; thus, it would not be accurate to
calculate TOF at that high conversion. To calculate an accurate
TOF, reactions were also carried out for 2 h. The active metal
surface area (ASA) and dispersion of the catalysts were
calculated by using the measured size of the Ni nanoparticles in
eqs 1 and 2:26
L × f
d × z
ASA (m2/g) =
(1)
ASA × M
N × A × L
dispersion (%) =
(2)
where L is the weight fraction of metal loading, f is the particle
shape correction factor (f = 6 for a sphere), d is the average
diameter of the supported particles measured by TEM (assume
the size of Ni particles on silica gel is 2.4 nm), z is the metal
density, M is the formula weight of the supported metal, N is
Avogadro’s number, and A is the metal atomic surface area (for
Ni, A = 6.494 Å2/atom). On the basis of eq 1, the ASA for the
Ni/SiO2 catalyst is 2.05 m2/g. The metal dispersion was then
calculated from eq 2 to be 42%. On the basis of the conversion
and reaction time (conversion at a reaction time of 2 h), the
TOF was calculated. As shown in Table 2, almost all of the
TOF of entries 1−8 were higher than 0.7 s−1, which is pretty
high in the reduction of nitroarene systems.
Liu et al.9 applied Au/meso-CeO2 as the catalyst for the
reduction of nitrobenzene and got a TOF of 20 h−1 (5.56 ×
10−3 s−1). Petkar et al.13 employed a Fe−Ni bimetallic system
to catalyze the reduction of aromatic nitro compounds and got
a TOF of 4.76 × 10−3 s−1 for nitrobenzene. As shown in Table
2, the TOF of nitrobenzene in our system was 0.78 s−1, which
was more than 2 orders of magnitude higher than the reported
values. The high catalytic activity of the catalysts could be due
to the highly dispersed Ni nanoparticles prepared by ALD. In
the above-mentioned previous studies, the metal nanoparticle
catalysts were normally prepared by the liquid impregnation
method, and the metal nanoparticles were very large. In our
current study, the Ni nanoparticles were prepared by ALD, and
the particle sizes were smaller than 3 nm. The effect of particle
size on the catalytic activity was reported previously. Mahata et
al.27 reported that smaller Ni nanoparticles showed higher
activity for hydrogenation of nitroarenes. Shimizu et al.11 also
demonstrated that catalyst activity increased with the decrease
in silver nanoparticle size for hydrogenation of nitroarenes. In
addition, Gould et al. reported that the surface structures of the
Figure 1. HRTEM images of (a) 2 cycles and (b) 15 cycles of ALD-
deposited Ni nanoparticles on 20−30 nm silica particles. The inlet
images show the size distribution of Ni nanoparticles.
with increases in the temperature up to 100 °C (75.8% at 80 °C
and 95.0% at 100 °C); however, both selectivity and yield
decreased at 120 °C, which was due to some side reactions at
higher temperatures. On the basis of this initial study, 100 °C
was selected as the reaction temperature in the following
studies. At 100 °C, the yield increased to 96.5% and 97.8%
when the hydrazine amount was increased to 6 and 8 equiv,
respectively. Solvents normally have a significant influence on
the performance of the catalyst.29,30 In this study, the different
solvents (tetrahydrofuran (THF), methanol, and tertiary
butanol) were used for this reaction. The selectivity and yield
showed no significant differences, which indicated that the
solvents would not affect this reaction in this catalytic system.
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Table 2. Chemoselective Reduction of Nitro Compounds to Amines with Hydrazine Hydrate Catalyzed by 1 Cycle of Ni ALD-
Deposited on Silica Gel Particles
Reaction conditions: 6 mmol of nitroarenes, 48 mmol of NH2NH2, 10 mL of ethanol, and 10 mg of Ni/SiO2 catalysts were added into the reactor.
a
b
The reactions were carried out at 100 °C for 2 and 8 h, respectively. Values in parentheses are the selectivity of the reactions after 2 h. Values in
c
parentheses are the yields of the reactions after 2 h. TOF values were calculated using the results with a reaction time of 2 h.
ALD Ni nanoparticles were different from those of the larger Ni
particles prepared by an incipient wetness method.26 This could
also be a reason for the high catalytic activity of the catalysts.
The hydrogenation of nitroarenes often stops at an
intermediate stage, yielding various byproducts. For hydro-
genation of halogenated nitroarenes, the hydrogenolysis of the
C−X, in which X represents halogen, always happens.31
Because of the high activity of ALD Ni nanoparticles, the
hydrogenation of nitroarenes did not stop at the intermediate
stage. The Ni nanoparticles also showed no activity in
hydrogenolysis of the C−X bond in our current study, since
no dehalogenation products were observed. Gawande et al.32
also reported that no dehalogenation occurred in the
hydrogenation of halogenated nitroarenes using Fe3O4−Ni
nanoparticles. This was our hypothesis for the high selectivity
of our ALD Ni catalysts. More studies are being conducted to
reveal the effects of the Ni particle size on the activity and
selectivity of the reactions.
The ALD catalysts showed excellent selectivity and yield of
the reduction of nitroarenes with fluorine and chlorine. In
particular, no defluorination and dechlorination byproducts
were observed (entries 2−5), and the selectivities of all
reactions were higher than 94%. This result was very good
compared with other reported results with different kinds of
catalyst systems in which the selectivities were always lower
than 90% or even lower.9,30,33 The high selectivity of the
reaction is highly desirable in industry because these
byproducts do not need to be separated, which is a costly
process. For example, for entry 5, the product is solely 3-chloro-
4-fluoro-benzenamine, which is a raw material for norfloxacin.
The effective control of defluorination and dechlorination could
help provide high-purity raw material for norfloxacin; however,
the selectivity and yield of the transfer hydrogenations of p-
nitrobenzoic acid and methyl p-nitrobenzoate were lower than
90% and 80%, respectively. This was possible largely because of
the existence of other electron-withdrawing groups on the
reactant molecules. Compared with the reported catalytic
systems, our catalytic system is much simpler in terms of both
catalyst preparation and experimental procedure. It is an
efficient, cost-effective, and chemoselective catalyst for the
reduction of nitroarenes.
4. CONCLUSIONS
Highly dispersed nickel nanoparticles were deposited on both
porous silica gel particles and 20−30 nm dense silica
nanoparticles. The prepared Ni/silica gel catalysts can activate
hydrazine hydrate as a reducing agent in the transfer
hydrogenation of aryl nitro compounds into the corresponding
amines with high selectivity and high yield. To the best of our
knowledge, this is the first report of ALD-prepared nickel
nanoparticles for the transfer hydrogenation of aryl nitro
compounds. The current work has extended the utility of a
readily available catalyst (Ni/SiO2) toward new applications in
catalysis and organic chemistry.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b00969.
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Ni ALD experimental details, characterization details of
Letter
(25) Shang, Z.; Patel, R. L.; Evanko, B. W.; Liang, X. H. Chem.
Commun. 2013, 49, 10067−10069.
(26) Gould, T. D.; Lubers, A. M.; Neltner, B. T.; Carrier, J. V.;
Weimer, A. W.; Falconer, J. L.; Medlin, J. W. J. Catal. 2013, 303, 9−15.
(27) Mahata, N.; Cunha, A.; Orfao, J. J. M.; Figueiredo, J. L. Appl.
Catal., A 2008, 351, 204−209.
XPS and FTIR, XPS spectra, and FTIR spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: liangxin@mst.edu.
■
(28) Liang, X. H.; Hakim, L. F.; Zhan, G.-D.; McCormick, J. A.;
George, S. M.; Weimer, A. W.; Spencer, J. A., II; Buechler, K. J.;
Blackson, J.; Wood, C. J.; Dorgan, J. R. J. Am. Ceram. Soc. 2007, 90,
57−63.
Author Contributions
§C.J. and Z.S. contributed equally.
Notes
(29) Verho, O.; Nagendiran, A.; Tai, C.-W.; Johnston, E. V.; Backvall,
̈
The authors declare no competing financial interest.
J.-E. ChemCatChem 2014, 6, 205−211.
(30) Wienhofer, G.; Sorribes, I.; Boddien, A.; Westerhaus, F.; Junge,
K.; Junge, H.; Llusar, R.; Beller, M. J. Am. Chem. Soc. 2011, 133,
̈
ACKNOWLEDGMENTS
■
12875−12879.
This work was supported by the University of Missouri
Research Board. C.J. thanks financial support from the Chinese
Postdoctoral Science Foundation (2014M551745) and the
Zhejiang Province Natural Science Foundation
(LY15B060008). The authors thank Clarissa A. Wisner and
Brian Porter of the Materials Research Center at the Missouri
University of Science and Technology for the TEM analysis
and XPS analysis, respectively.
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