Palladium supported on metal–organic framework as a catalyst for the hydrogenation of nitroarenes under mild conditions

Article abstract of DOI:10.1002/aoc.5607

Sustainable development demands an environmentally friendly and efficient method for the hydrogenation of organic molecules, including the hydrogenation of functionalized nitroarenes. In this study, a highly active and selective metal–organic framework-supported palladium catalyst was prepared for the catalytic hydrogenation of nitroarenes. High selectivity (>99%) and excellent yield (98%) of aniline were realized after 2 hours in ethanol under hydrogen (1 atm) at room temperature. The reductions were successfully carried out in the presence of a wide range of other reducible functional groups. More importantly, the catalyst was very stable without the loss of its catalytic activity after five cycles.

Full text of DOI:10.1002/aoc.5607

Received: 20 September 2019
DOI: 10.1002/aoc.5607
Revised: 7 January 2020
Accepted: 30 January 2020
F U L L P A P E R
Palladium supported on metal–organic framework as a
catalyst for the hydrogenation of nitroarenes under mild
conditions
1
1
1
1
1
Lingxiang Bao
| Zongbao Yu | Teng Fei | Zhiyuan Yan | Jiazhe Li
|
1
,2
1,2
Chenghui Sun
| Siping Pang
1
School of Materials Science and
Engineering, Beijing Institute of
Technology, Beijing, 100081, China
Sustainable development demands an environmentally friendly and efficient
method for the hydrogenation of organic molecules, including the hydrogena-
tion of functionalized nitroarenes. In this study, a highly active and selective
metal–organic framework-supported palladium catalyst was prepared for the
catalytic hydrogenation of nitroarenes. High selectivity (>99%) and excellent
yield (98%) of aniline were realized after 2 hours in ethanol under hydrogen
2
Key Laboratory for Ministry of Education
of High Energy Density Materials, Beijing
Institute of Technology, Beijing, 100081,
China
Correspondence
(1 atm) at room temperature. The reductions were successfully carried out in
Chenghui Sun and Siping Pang, School of
Materials Science and Engineering,
Beijing Institute of Technology, Beijing
the presence of a wide range of other reducible functional groups. More impor-
tantly, the catalyst was very stable without the loss of its catalytic activity after
five cycles.
1
00081, China.
Email: sunch@bit.edu.cn; pangsp@bit.
edu.cn
K E Y W O R D S
Funding information
heterogeneous catalysts, MOFs, nitroarenes, palladium, reduction
National Natural Science Foundation of
China, Grant/Award Numbers: 21576026,
21975023
1
| INTRODUCTION
environmentally friendly and efficient method for the
production of functionalized anilines.
Functionalized anilines are versatile intermediates for
agrochemicals, pharmaceuticals, pigments, dyes and fine
In recent years, metal–organic frameworks (MOFs)
have emerged as the most promising stabilizing agents
for supported metal nanoparticles owing to their well-
defined structure, large surface areas and chemical
[1]
chemicals. Because of their importance, many methods
[
2]
for the reduction of nitroarenes have been developed.
In general, the methods can be categorized into two
[
12]
tunability.
Their special structures limit the inter-
[3]
types.
One is a stoichiometric reduction of the
action/contact between incorporated metal nanoparticles,
corresponding nitroarenes using sodium hydrosulfite,
avoiding the agglomeration and leaching of the metal
[4]
[13]
iron, tin or zinc. However, this method often causes
environmental issues such as large amounts of waste
acids and residues generated during the reaction. The
second is catalytic hydrogenation of nitro compounds
nanoparticles.
Among their advantages, the most sig-
nificant one is the designability of the organic ligands in
the MOF material, thus controlling the size and proper-
ties of the MOFs, which is unmatched by traditional
[
5]
[6]
[7]
[14]
over supported metal catalysts, e.g. Au, Ni, Fe,
porous materials.
For example, the MOF [Zn(pip)
[8]
[9]
Ru and Co catalysts. Nevertheless, the reported cata-
lysts usually require harsh conditions, such as a high
(bpy)]n (MOF-1, H pip
=
5-(prop-2-yn-1-yloxy)
2
0
isophthalic acid, bpy = 4,4 -bipyridine) is constructed by
an alkyne-functionalized ligand and Zn ions which shows
[10]
reaction temperature
and a high hydrogen pres-
Therefore, it is highly desirable to develop an
[11]
[15]
sure.
a high noble metal loading.
Appl Organometal Chem. 2020;e5607.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5607

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BAO ET AL.
MOF-supported metal nanoparticles have been found
to show excellent catalytic performance towards a wide
[
16]
range of organic reactions, including oxidation,
[17]
photocatalytic oxidation and reduction,
hydrogena-
SCHEME 1 Reduction of nitroarenes [Colour figure can be
viewed at wileyonlinelibrary.com]
[
18]
tion
and so on. However, there remains an absence of
reports of the use of MOF-supported nanoparticles for
the production of functionalized anilines under mild
conditions.
Herein, we report an efficient, convenient and high-
yield protocol using MOF-supported palladium
nanoparticles (Pd@MOF-1; Figure 1) as a catalyst for the
reduction of nitroarenes under ambient conditions
solid was then dried under vacuum at 313 K. The
amount of Pd adsorbed into MOF-1 was estimated with
inductively coupled plasma atomic emission spectrome-
try (ICP-AES).
(Scheme 1).
2.2 | General procedure for
hydrogenations
2
2
| EXPERIMENTAL
In a typical protocol, the nitroarene compound (1 mmol)
and the Pd@MOF-1 catalyst (1 mol%) were added into
oven-dried glassware that was sealed with a rubber sep-
tum. Then, the reaction mixture was stirred under a
hydrogen atmosphere with IKA RCT basic magnetic stir-
.1 | Synthesis of Pd@MOF-1
The synthesis procedure and crystal structure of MOF-1
are detailed in the supporting information according to
the literature.
ꢀ
rer bars at 25 C after degasification (flushed with high-
[15]
Pd@MOF-1 was prepared using a wet
purity argon three times to remove O ). Completion of
2
chemical method. To 50 mg of activated MOF-1 in 30 ml
the reaction was monitored by TLC. After the comple-
tion of the reaction, the catalyst was removed from the
solution by filtration, and the obtained filtrate was ana-
lyzed using LC-MS and purified by silica gel chromatog-
raphy using an appropriate eluent for NMR analysis.
Every experiment was conducted twice to ensure accu-
−
2
of ethanol, 4.2 mg (2.4 × 10 mmol) of PdCl in 10 ml
2
of ethanol was added drop by drop. The mixture was
kept stirring at room temperature for 2 hours to adsorb
2+
the Pd
ions into MOF-1. Subsequently, 4.5 mg
(
12 mmol, 5 equiv. with respect to PdCl ) of NaBH was
2
4
2+
1
13
added for the reduction of Pd ions with constant stir-
ring for another 1 hour. Finally, the mixture was filtered
and washed three times with fresh ethanol. The grey
racy. The characterization data of H NMR and
C
NMR spectra of products are provided in the supporting
information.
FIGURE 1 Structure of Pd@MOF-1
[Colour figure can be viewed at
wileyonlinelibrary.com]

PD@MOF-1 AS A CATALYST FOR THE HYDROGENATION OF NITROARENES
3 of 8
2
+
2.3 | Recycling of Pd@MOF-1 for
congregated Pd particles, which indicated that the Pd
hydrogenations
ions were adsorbed homogeneously (Figure 3). Such an
even distribution may be because the ethynyl moieties
responsible for stabilizing Pd nanoparticles were homo-
geneously present in every channel of MOF-1. A TEM
image of Pd@MOF-1 is shown in Figure 4a. It shows that
the distribution of Pd particles did not change after the
reduction. The size of the Pd nanoparticles in the as-
prepared Pd@MOF-1 catalyst is in the range 1–2 nm
(Figure 4b).
After the completion of the reaction, the catalyst was
removed from the solution by filtration. The catalyst was
washed three times with ethanol. The grey solid was
dried under vacuum at 313 K for 3 hours to remove the
residual solvent, and then reused as the catalyst in a sub-
sequent run. The recyclability of the Pd@MOF-1 catalyst
was investigated for the hydrogenation of nitrobenzene
under the same reaction conditions mentioned before.
X-ray photoelectron spectroscopy (XPS) is an excel-
lent tool for confirming the change in the valence of
metal ions. In this case, the Pd 3d spectra of
3
3
| RESULTS AND DISCUSSION
.1 | Characterization of materials
PdCl @MOF-1 and Pd@MOF-1 were obtained to confirm
2
A solvent infiltration method was adopted to load Pd
nanoparticles into MOF-1 in a stepwise manner starting
from PdCl precursor. To obtain further insight into the
2
adsorption process, transmission electron microscopy
(TEM), scanning electron microscopy (SEM) and the
corresponding energy-dispersive X-ray spectroscopic
2+
(
EDS) mapping were carried out using Pd -adsorbed
MOF-1 (PdCl @MOF-1). SEM-EDS mapping analyses
2
(
Figure 2) of PdCl @MOF-1 showed a uniform distribu-
2
tion of Zn, Pd and Cl throughout the entire MOF-1
2+
crystal and demonstrated that the Pd was uniformly
dispersed with no accumulation in specific areas. More-
over, TEM analyses of PdCl @MOF-1 showed no notably
2
FIGURE 3 TEM image of PdCl @MOF-1
2
FIGURE 2 SEM-EDS mapping of
PdCl
2
@MOF-1 [Colour figure can be
viewed at wileyonlinelibrary.com]

4
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BAO ET AL.
FIGURE 4 (a) TEM image of
Pd@MOF-1. (b) Particle size
distribution of Pd@MOF-1 [Colour
figure can be viewed at
wileyonlinelibrary.com]
the variation in valence in the Pd species and the role of
NaBH . As shown in Figure 5, two obvious peaks were
4
observed in the Pd 3d spectrum of PdCl @MOF-1, with
2
binding energies of 338.3 and 343.6 eV that corresponded
to the characteristics of Pd(II). This indicated that all Pd
2+
species in PdCl @MOF-1 existed in the form of Pd
2
before the reduction treatment of NaBH . After reduc-
4
tion, the peaks of Pd 3d5/2 and Pd 3d3/2 appeared at bind-
ing energies of 335.3 and 340.7 eV, which demonstrated
[15]
the formation of Pd nanoparticles.
The formation of Pd nanoparticles was also confirmed
from powder X-ray diffraction (PXRD) analysis
ꢀ ꢀ
(
Figure 6). The high-angle broad peaks at 40.1 and 46.3
in the PXRD pattern of Pd@MOF-1 were ascribed to the
face-centered cubic Pd (111) and (200) planes, indicating
the formation of Pd nanoparticles. However, no charac-
teristic diffraction peaks of face-centered cubic Pd
FIGURE 6 Comparison of PXRD patterns of MOF-1,
PdCl @MOF-1 and Pd@MOF-1 [Colour figure can be viewed at
2
wileyonlinelibrary.com]
ꢀ
ꢀ
ꢀ
nanoparticles at 67.6 , 81.2 and 85 were found, which
indicated that Pd nanoparticles may be well dispersed.
The PXRD pattern of Pd@MOF-1 also confirmed that the
crystallinity of MOF-1 remained intact after Pd
3.2 | Catalytic hydrogenation of
nitroarenes
[15]
insertion.
After the successful synthesis of the catalyst, we endeav-
ored to identify the best catalytic system for the reduction
of nitroarenes using nitrobenzene as a model substrate.
As evident from Table 1, the solvent had a strong effect
on the catalytic efficiency in this catalytic system; metha-
nol and ethanol gave 88% and 98% yields within 2 hours,
respectively
(Table
1).
Among
the
others,
dimethylformamide and tetrahydrofuran gave reaction
products in moderate yields (60% and 44%, respectively).
The reaction occurred slowly in toluene. Considering the
requirements of green development, ethanol was herein
selected as the optimal solvent.
Based on the optimized reaction conditions, the cata-
lytic activity of Pd@MOF-1 for the hydrogenation of
nitrobenzene was evaluated. Pd@MOF-1 exhibited
higher activity compared with commercial Pd/C catalyst
(
Table 2) and other reported Pd heterogeneous catalysts
2
FIGURE 5 Pd 3d core-level XPS spectra of PdCl @MOF-1 and
Pd@MOF-1 [Colour figure can be viewed at wileyonlinelibrary.
com]
(supporting information, Table S2). To investigate the ori-
gin of the superior catalytic activity of Pd@MOF-1, the

PD@MOF-1 AS A CATALYST FOR THE HYDROGENATION OF NITROARENES
5 of 8
TABLE 1 Effect of various solvents for the reduction of
nitrobenzene using Pd@MOF-1
support. The mean size of Pd nanoparticles is 12.72 nm,
revealing that MOF-1 is crucial for stabilizing the metal
nanoparticles.
a
b
Entry
Solvent
Time (h)
Yield (%)
In addition, the results presented in Table 2 show that
the size of Pd (Figure 4 and Figure S5) in those samples
increased in an order opposite to that of the catalytic
activity (Pd@MOF-1 > Pd/C > Pd nanoparticles). This
could be rationalized by the fact that smaller particles
possess a larger number of surface atoms that can
improve the conversion rate of the catalytic reaction.
Considering all of these results, the superior catalytic
activity of Pd@MOF-1 was attributed to the MOF-1 as
support. MOF-1 can not only act as a stabilizer for metal
1
2
3
4
5
Ethanol
2
2
2
2
2
98
88
60
44
15
Methanol
Dimethylformamide
Tetrahydrofuran
Toluene
a
Reaction conditions: nitrobenzene (1 mmol), catalyst (1 mol%), solvent
8 ml), room temperature, H balloon.
(
b
2
Isolated yield.
nanoparticles, but also enable the superior H enrich-
a
2
TABLE 2 Catalytic hydrogenation of nitrobenzene
ment property of the catalyst which would effectively
enhance the catalytic efficiency, as also demonstrated in
b
Entry
Catalyst
Time (h)
Yield (%)
[
20]
previous reports.
1
2
3
4
Pd@MOF-1
Pd/C
2
2
2
2
98
15
10
—
To establish the scope of the catalyst, several
nitroarenes were adopted under the same reaction condi-
tions, with the results summarized in Table 3. It was
obvious that the compounds were reduced with high
yields regardless of the diversity of structure (Table 3,
entries 2–19). Alkyl-substituted nitroarenes showed high
conversion, with up to 96% yield (Table 3, entries 2–4).
However, the activity was significantly influenced by the
position of the substituents on the aromatic ring. For
example, the decreased reaction rate of ortho-
methylnitrobenze relative to the meta and para ana-
logues demonstrates a steric effect (Table 3, entries 2–4).
Moreover, naphthalene-substituted nitroarenes took
more time (5 hours) than methyl-substituted nitroarenes
(2–3 hours) but were still capable of producing good
yields (89%; Table 3, entry 5).
In general, electron withdrawing/donating groups,
such as nitro and methoxy groups, did not have a signifi-
cant influence on the reaction. For instance,
4-nitroanisole, 4-nitrothioanisole, 3,4-dimethoxynitro-
benzene and 4-methoxy-3-methylnitrobenzene were
equally well transformed into the corresponding anilines
(Table 3, entries 6–9). Notably, the presence of sulfur
atoms resulted in a slower reaction rate; it took three
times longer (6 hours) than nitrobenzene to reduce
thioether (Table 3, entry 7). Moreover, diaminobenzene
was easily accessible from the corresponding dinitroben-
zene (Table 3, entries 10 and 11).
Pd nanoparticles
MOF-1
a
Reaction conditions: nitrobenzene (1 mmol), catalyst (1 mol%), ethanol
8 ml), room temperature, H balloon.
(
b
2
Isolated yield.
specific surface areas of Pd@MOF-1 and Pd/C were mea-
sured via nitrogen adsorption. As depicted in Figure S3,
the surface area of the Pd/C catalyst was estimated to be
71.9 m g , that of the Pd@MOF-1 catalyst being only
3.9 m g . In consideration of good porosity observed
2
−1
8
2
2
−1
[15]
from the single-crystal X-ray structure,
a fraction of
the pores accessible to H molecules may not be accessi-
ble to N2 molecules especially at cryogenic
hydrogen sorption measurements being
conducted at 298 K. The H2 adsorption ability of
Pd@MOF-1 is much better than that of Pd/C as expected
Figure S4), revealing that MOF-1 plays very important
roles in H adsorption. To examine this assumption, free-
standing Pd nanoparticles were prepared using the same
method as for Pd@MOF-1. As shown in Figure S4, the
H adsorption ability of Pd@MOF-1 is dozens of times
2
[19]
temperatures,
(
2
2
that of the Pd nanoparticles.
Control reactions over MOF-1 or Pd nanoparticles
were also carried out. The results show that the activity
of MOF-1 is negligible (Table 2). The use of Pd@MOF-1
leads to almost complete conversion of nitrobenzene in
20 minutes, whereas only 10% conversion occurred in
the presence of Pd nanoparticles under otherwise identi-
cal reaction conditions. The significant difference could
be partially due to the H enrichment capability of MOF-
(Figure S4). Meanwhile, the TEM images of the Pd
nanoparticles (Figure S5) show that the Pd nanoparticles
are mostly agglomerated without the stabilization of the
The catalyst also showed promise for selective reduc-
tion of substrates bearing other reducible groups (Table 3,
entries 10–17). Esters and ketones remained untouched
under the reaction conditions (Table 3, entries 13–15).
The activities of 3-nitroacetophenone and 4-nitro-
acetophenone were slightly influenced by the nature/
position of the substituents on the aromatic ring (Table 3,
entries 14 and 15). In addition, nitroarenes bearing
1
2
1

6
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BAO ET AL.
a
TABLE 3 Reduction of various nitroarenes using Pd@MOF-1
b
b
Entry
Substrate
Time (h)
Yield (%)
Entry
13
Substrate
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
Nitrobenzene
2
2
2
3
5
3
6
4
4
6
6
5
98
96
96
94
89
92
96
90
90
89
86
91
Methyl 4-nitrobenzoate
4-Nitroacetophenone
3-Nitroacetophenone
4-Nitrobenzamide
4-Nitrobenzaldehyde
4-Nitrochlorobenzene
4-Iodonitrobenzene
5-Nitroquinoline
3
3
3
3
5
6
6
4
3
4
3
3
96
96
90
91
82
76
84
87
90
90
96
—
4-Nitrotoluene
14
3-Nitrotoluene
15
2-Nitrotoluene
16
1-Nitronaphthalene
4-Nitroanisole
17
d
18
e
4-Nitrothioanisole
3,4-Dimethoxynitrobenzene
4-Methoxy-3-methylnitrobenzene
1,4-Dinitrobenzene
1,3-Dinitrobenzene
2-(4-Nitrophenyl)ethanol
19
20
21
22
23
9
5-Nitroisoquinoline
6-Nitroquinoline
c
10
11
12
c
7-Nitroquinoline
[c]
24
8-Nitroquinoline
a
Reaction conditions: substrate (1 mmol), catalyst (1 mol%), ethanol (8 ml), room temperature, H
2
balloon.
b
Isolated yield.
c
ꢀ
5
0 C.
d
20% of aniline product was obtained.
e
1
3% of aniline product was obtained.
was smoothly reduced with high selectivity affording the
corresponding aniline in 82% yield (Table 3, entry 17).
For halide-containing substrates, little hydrogenolysis of
the C&bond;X bond was observed (Table 3, entries
18 and 19), but still retaining good yields (>76%).
Heterocyclic compounds, such as quinolines, were
successfully employed. Mononitro-substituted quinolines
were chemoselectively hydrogenated in good to excellent
yields (87–96%), regardless of the position of the nitro
substituent on the benzene ring (Table 3, entries 20–23).
Interestingly, compared with other quinolines mentioned
before (Table 3, entries 20–23), the substrate bearing a
nitro group at the 8-position exhibited extremely low
reactivity, TLC showing no new dot after 3 hours with
slight heating.
FIGURE 7 Recycling of Pd@MOF-1 for reduction of
nitrobenzene [Colour figure can be viewed at wileyonlinelibrary.
com]
Stability is an essential characteristic of an excellent
catalyst in a solid–liquid reaction, particularly when the
reactant or the product has a strong interaction with
the supported metal particles, which may accelerate the
amides showed excellent conversion, producing the
corresponding anilines (Table 3, entry 16). 4-Nitro-
benzaldehyde bearing a sensitive aldehyde functionality
[21]
leaching of metal nanoparticles from the supports.
FIGURE 8 (a) TEM image of
recovered Pd@MOF-1 after the fifth
catalytic cycle. (b) Particle size
distribution of recovered Pd@MOF-1
[Colour figure can be viewed at
wileyonlinelibrary.com]

PD@MOF-1 AS A CATALYST FOR THE HYDROGENATION OF NITROARENES
7 of 8
The stability of Pd@MOF-1 was further investigated
using recycling experiments, and the results are depicted
in Figure 7. Pd@MOF-1 demonstrated stable recovery by
simple filtration from the reaction mixture. Only a negli-
gible decline (1–3%) in the catalytic activity of
Pd@MOF-1 was observed when the catalyst was used
for five successive cycles in the hydrogenation of
nitrobenzene.
Moreover, the average particle diameter and size dis-
tribution of freshly prepared Pd@MOF-1 catalyst and of
catalyst after the fifth cycle were compared. TEM ana-
lyses of the Pd@MOF-1 catalyst after reuse showed parti-
cle size and size distribution (mean size = 1.41 nm)
similar to those of the freshly prepared catalyst, and no
agglomeration of the used catalyst was observed
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4
| CONCLUSIONS
[
In summary, the general reduction of nitroarenes to the
corresponding amines in good to excellent yields
(76–98%) was achieved using Pd nanoparticles supported
on MOF. This method was easy and exceedingly efficient.
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ated under these reaction conditions. Other notable
advantages of this catalytic system included high isolated
yields, the use of hydrogen as a sustainable source, an
easy handling procedure and the reusability of the
catalyst.
[
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