Pd nanoparticles stabilized with phosphine-functionalized porous ionic polymer for efficient catalytic hydrogenation of nitroarenes in water

Article abstract of DOI:10.1039/c9nj05734j

Small palladium nanoparticles stabilized with phosphine-functionalized porous ionic polymer (Pd@P(QP-TVP)) were successfully prepared through a free-radical copolymerization, successive anion-exchange and chemical reduction method. Physicochemical characterization studies suggested that the prepared catalyst featured large surface area, a hierarchically porous structure, amphiphilic surface wettability, and strong electron interaction between Pd nanoparticles and the polymer scaffold. We demonstrated the use of the solid catalyst for water-mediated reduction of nitrobenzene with H₂ as a hydrogen source. Notably, a low Pd dosage was sufficient for a high yield (99.7%) of aniline with a remarkable turnover frequency (TOF) of 5982 h^-1. Furthermore, the Pd@P(QP-TVP) catalyst can be easily recovered and reused at least 5 times without significant loss of activity. Additionally, a number of functional nitroarenes can be efficiently transformed to arylamines in high yields under optimal conditions. Thus, this work provided a highly active, stable and heterogeneous Pd catalyst for the environmentally benign and cost-effective hydrogenation of nitroarenes.

Full text of DOI:10.1039/c9nj05734j

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Chechia Hu, Tzu-Jen Lin et al.
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Pd nanoparticles stabilized by phosphine-functionalized porous
ionic polymer for efficient catalytic hydrogenation of nitroarenes
in water
Received 00th January 20xx,
Accepted 00th January 20xx
Yizhu Lei *, Zaifei Chen, Guosong Lan, Renshu Wang, Xiao-Yu Zhou
DOI: 10.1039/x0xx00000x
www.rsc.org/
Small palladium nanoparticles stabilized by phosphine-functionalized porous ionic polymer (Pd@P(QP-TVP))
was successfully prepared through a free-radical copolymerization, successive anion-exchange and chemical
reduction method. Physicochemical characterizations suggested that the prepared catalyst featured big
surface area, hierarchically porous structure, amphiphilic surface wettability, and strong electron interaction
between Pd nanoparticles and the polymer scaffold. We demonstrated the use of the solid catalyst for
water-mediated reduction of nitrobenzene with H2 as a hydrogen source. Notably, a low Pd dosage was
sufficient for high yield (99.7%) of aniline with a remarkable turnover frequency (TOF) of 5982 h-1.
Furthermore, the Pd@P(QP-TVP) catalyst can be easily recovered and reused at least 5 times without
significant loss of activity. Additionally, a number of functional nitroarenes can be efficiently transformed to
arylamines in high yields under the optimal conditions. Thus, this work provided a highly active, stable and
heterogeneous Pd catalyst for the environmentally benign and cost-effective hydrogenation of nitroarenes.
coupling reactions, as well as the hydrogenation of nitroarenes.
11-13 Among the noble metals, palladium is especially important
due to its outstanding properties and extensive applications in
Introduction
Aromatic amines, an important class of NH2-containing organic
chemicals, have been widely used as organic intermediates for
a wide range of fine chemicals, such as agrochemicals,
pharmaceuticals, and dyestuffs.1-3 However, traditional non-
catalytic synthesis method of aromatic amines using Fe/HCl or
sulfide as reducing agent suffers from large amounts of waste
and poor atom economy.4,5 Therefore, a number of synthetic
approaches, such as catalytic amination, catalytic
hydrogenation and electrochemical methods, have been
developed.6 Among various synthetic approaches, catalytic
hydrogenation of nitroarenes with H2 as a hydrogen source
represents one of the most favorable processes due to its
advantages of high selectivity, cost-effectiveness and low
waste production.6-8
Noble metal nanoparticles (NPs) with small diameters (1-10
nm) have been increasingly the focus of catalysis research in
recent decades.9-11 Due to the high surface-to-volume ratios
and quantum size effects, these noble metal nanoparticles
exhibited excellent catalytic activities in a variety of organic
reactions, including many hydrogenation, oxidation and cross-
various catalytic reactions.14-18 However, like other metal NPs,
Pd NPs are thermodynamically unstable and easy to aggregate,
which usually result in a rapid decrease in the catalytic
activity.19,20 In addition to the catalytic activity, easy recovery
and stable reuse of palladium catalysts are also crucial for their
practical applications due to the high price of Pd metal.21-23
Therefore, immobilization of Pd NPs onto the solid supports
has been attempted to resolve these issues. To date, a variety
of supports were developed to stabilize Pd NPs for the
catalytic hydrogenation of nitroarenes.24-29 For most the
established methods of nitroarenes reduction, flammable
sodium borohydride or toxic hydrazine was used as the
hydrogen source. From the viewpoint of green synthesis and
practical application, H2 should be most suitable hydrogen
source for this reaction. However, with H2 as a hydrogen
source, the efficiency and/or stability of the currently available
supported Pd NPs are still unsatisfactory.
Ionic liquids, which represent an intriguing class of solvent
with variable physicochemical properties, have recently been
widely used as both solvent and stabilizers for transition metal
NPs.10-11,30-31 Although the strategy of stabilizing metal NPs by
ionic liquids has been successfully applied in various organic
reactions, it still suffer from some drawbacks, such as the
requirement of large volumes of ionic liquids, and the
inevitable leaching of ionic liquids during work-up.32 To this
end, metal NPs immobilized on supported ionic liquids,33-35
School of Chemistry and Materials Engineering, Liupanshui Normal University,
Liupanshui, Guizhou 553004, PR China. Email: yzleiabc@126.com
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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poly(ionic liquids),35,36 and porous ionic polymer (PiPs),37-39 and successive chemical reduction. The results ofVaiecwtiAvrtiitcyle tOenslintes
have been recently developed. PiPs, which represent a new indicated that the prepared Pd@P(QP-TVDPO) Ie: 1x0h.i1b0i3t9e/dC9eNxJc0e5l7le34nJt
kind of porous organic polymers (POPs), could be easily activity for nitroarenes hydrogenation in water, outperforming
prepared from free radical polymerization,23,40-43 and commercial Pd/C and Ru/C, as well as Pd NPs supported on
nucleophilic substitution reactions.44-47 When used as supports porous poly(vinyl-functionalized quaternary phosphonium salt)
for metal NPs, PiPs can exhibit the features of POPs, such as and poly(tris (4-vinylphenyl) phosphine. Nano size, well
large surface areas, tunable pore structures, and nice stabilized Pd particles and electron-donation effect of
confinement of NPs growth, which are very favorable for phosphine ligand, as well as the amphiphilic surface wettability
heterogeneous catalysis.43 Besides, PiPs could show some account for the excellent activity. Furthermore, Pd@P(QP-TVP)
additional advantages: (1) enable the easy loading of metallic catalyst was quite stable and can be reused at least five times.
NPs via ion-exchange and successive chemical reduction; (2)
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stabilize the incorporated NPs via electrostatic interaction
between ionic sites and metallic NPs; (3) surface wettability of
Results and discussion
PiPs could be easily adjusted by altering the ratio of
hydrophilic and hydrophobic units, thus allowing fabrication of
Characterization of the catalysts
FT-IR spectroscopy was used to study the structural feature of P(QP-
amphiphilic PiPs that enable easy accessibility of both
TVP). As shown in Fig. 1 (a), FT-IR spectrum of P(QP-TVP) showed
hydrophobic and hydrophilic substrates in water.23,35-39
the characteristic peaks of C-H stretching vibrations of the saturated
However, like the stabilization effect of metallic NPs in ionic
methane (-CH2-) and methine (-CH-) groups at 2983, 2930 and 2830
liquids, electrostatic interactions between metallic NPs and
cm−1, and C-C stretching vibrations of the benzene ring at 1603,
ionic sites in the PiPs are rather weak, and thus, they are often
1500 and 1447 cm−1. Compared to FT-IR spectra of QP and TVP
not sufficient to prevent NPs from aggregation under the
monomers, the characteristic peaks of C-C (1627 and 987 cm−1) and
reaction conditions of catalysis.48-50 Recently, metallic NPs
C-H (3089 and 3059 cm−1) vibrations of the terminal -C=CH2 groups
stabilized by heteroatom donor-modified ionic liquid have
were disappeared in FT-IR spectrum of P(QP-TVP). These results
been developed on the basis that the heteroatom could bind
inferred that the copolymerization reaction of QP and TVP
to the metallic NPs and supplement the weak electrostatic
monomers was finished.
interactions, thus providing long term reusability in
catalysis.32,48-49,51 With this consideration, many heteroatom
donor-modified ionic liquids and supported ionic liquids have
also been developed, previous results revealed that the
strategy of heteroatom donor modification not only can
benefit the stabilization of metallic NPs, but also could
enhance the reaction rates and/or selectivity via the strong
electron-donation effect of heteroatom.48-49,51-53 Although
metallic NPs stabilized by heteroatom donor modified ionic
liquids have advanced significantly, metallic NPs stabilized by
heteroatom donor-modified porous ionic polymer has rarely
been reported.
Fig. 1 FT-IR spectra of (a) P(QP-TVP), (b) QP and (c) TVP.
The structural feature of P(QP-TVP) and Pd@P(QP-TVP) was also
investigated by solid-state 13C and 31P NMR spectra. As depicted in
Fig. 2a, P(QP-TVP) and Pd@P(QP-TVP) showed the same
characteristic peaks in the solid-state 13C NMR spectra, suggesting
no obvious structure change of P(QP-TVP) after Pd immobilization.
Scheme 1 Synthesis of Pd@P(QP-TVP).
The broad peaks at around 130 ppm were assignable to the
Considering the excellent electron-donating property and
extensive applications of phosphine ligands in palladium
catalysis,54-57 herein, we initiate the synthesis of Pd NPs
stabilized by phosphine-modified porous ionic polymer
(Pd@P(QP-TVP)), and study its catalytic performance in
hydrogenation of nitroarenes with H2 as a hydrogen source
and water as the solvent. The hyper-cross-linked porous ionic
polymer, P(QP-TVP), was facilely synthesized through the
radical copolymerization of vinyl-functionalized quaternary
phosphonium salt (QP) and tris(4-vinylphenyl) phosphine (TVP).
Then, Pd@P(QP-TVP) was obtained via the anion-exchange
aromatic carbons42,55 of the polymers. While the bands at around 40
ppm could be attributed to the polymerized vinyl groups.42,55 The
solid-state 31P NMR spectra of P(QP-TVP) and Pd@P(QP-TVP) were
shown in Fig. 2b. 31P NMR spectrum of P(QP-TVP) displayed two
characteristic peaks at 20.7 ppm and -7.3 ppm, associating with
phosphonium salt42 and PPh3 units,55,56 respectively. The solid-state
31P NMR spectrum of Pd@P(QP-TVP) mainly displayed shoulder
peaks at 28.5 and 20.7 ppm. The signal at 20.7 ppm was assignable
to phosphonium salt in the framework; while the new signal at 28.5
ppm could be assigned to P atom coordinated with Pd NPs, showing
the strong interaction of Pd NPs with PPh3 unit.56,57
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Pd@P(QP-TVP) decreased slightly. These decreaseVsiewofArtipclheyOsnilcinael
properties could be assigned to the partiaDlOIp:o10re.10f3il9li/nCg9NwJ0it5h73P4dJ
nanoparticles. Physical properties of Pd@PQP and PdNPs@POP-
PPh3 were also listed, they showed BET surface areas of 638 and
1056 m2 g-1, respectively.
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Fig. 2 The solid-state NMR spectra of P(QP-TVP) and Pd@P(QP-TVP):
(a) 13C NMR spectra; (b) 31P NMR spectra.
To gain insight into the structural information and electronic
property of Pd species, P(QP-TVP) and Pd@P(QP-TVP) samples were
evaluated with X-ray photoelectron spectroscopy (XPS). Pd 3d5/2-3/2
and P 2p XPS spectra fitted, after Tougaard’s background subtraction,
to their chemical components using Gaussian-Lorentzian
asymmetric functions by the XPSPeak 4.1 software were shown in
Fig. 3. As depicted in Fig. 3 (a), Pd 3d spectrum of Pd@P(QP-TVP)
showed the characteristic peaks of 3d3/2,5/2 doublets. The peaks at
around 340.1 (Pd 3d3/2) and 334.9 eV (Pd 3d5/2) were attributed to
Pd0 species, and the peaks around 342.0 (Pd 3d3/2) and 336.2 eV (Pd
3d5/2) were assigned to PdII species, which probably resulted from
the reoxidation of Pd0.58 Compared with Pd 3d5/2-3/2 spectrum of
Pd@PQP in Fig. 3 (b), Pd 3d binding energies of Pd@P(QP-TVP) had
a red shift. This electron-rich character of Pd species should be
attributed to the electron donating effect of the phosphine ligand in
the Pd@P(QP-TVP). The ratios of Pd0/PdII in Pd@P(QP-TVP) and
Pd@PQP, which were estimated by the proportion of relative peak
areas, were 1.15 and 1.02, respectively. XPS spectra of the P 2p
region were shown in Fig. 3b. The peaks at around 132.8 and 131 eV
were assigned to the phosphonium salt and PPh3 units, respectively.
Compared to P(QP-TVP), P 2p XPS spectrum of Pd@P(QP-TVP)
showed a blue shift of binding energy. This result further verified
the electron donating behavior of the phosphine ligand in
Pd@P(QP-TVP).
Fig. 3 (a) Pd 3d XPS spectra of Pd@P(QP-TVP) and Pd@PQP; (b) P 2p
XPS spectra of Pd@P(QP-TVP), P(QP-TVP) and Pd@PQP.
Table 1 Physical properties of the prepared samples.
Pore volume Average pore
Catalysts
SBET (m2·g −1)
(cm3/g) a
0.80
radius (nm) b
4.7
P(QP-TVP)
830
726
638
Pd@P(QP-TVP)
0.63
0.56
3.8
3.5
Pd@PQP
PdNPs@POP-
PPh3
1056
2.05
7.8
a Single point adsorption total pore volume of pores at P/Po = 0.95.
b Adsorption average pore diameter (4V/A by BET).
Nitrogen adsorption-desorption isotherms of Pd@P(QP-TVP) and
P(QP-TVP) in Fig. 4 displayed obvious steep steps of nitrogen gas
uptake at low relative pressure (P/P0 < 0.01), indicating the
abundance of micropore; while the obvious hysteresis loops at
medium to high pressure regions reflected the abundance of
mesopore. Consistently, pore-size distribution analysis of Pd@P(QP-
TVP) (insert) also confirmed the presence of micropore and
mesopore. For solid catalyst, such a hierarchical pore structure with
abundant micropores and mesopores would be beneficial for
heterogeneous catalysts by accelerating the mass transport of
The BET (Brunauer-Emmett-Teller) surface areas and average pore
width of the prepared samples were shown in Table 1. P(QP-TVP)
had BET surface area of 830 m2 g-1. After immobilization of Pd NPs,
BET surface area, pore volume and average pore radius of
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reactants and products.43
hydrophobic. Therefore, the differences in surface VhieywdArrotipclhe iOlincliintye
could be ascribed to the different compoDsiOtiIo:n10.o10f 3t9h/Ces9eNJp0o57ro34uJs
solids. To investigate the lipophilicity of the prepared Pd@P(QP-TVP),
contact angle measurement of Pd@P(QP-TVP) towards toluene
droplet was also tested. As depicted in Fig. 6 (d), toluene droplets
were totally absorbed by the sample without any residue,
suggesting the high lipophilicity of Pd@P(QP-TVP). The results of
contact angle measurements revealed that the prepared Pd@P(QP-
TVP), which was composed of hydrophilic QP and hydrophobic TVP
monomers, exhibited good surface amphiphilicity. For
heterogeneous catalyst in water, good surface hydrophilicity could
enable the catalyst well dispersed in water, while the nice
lipophilicity could enrich organic substrates into the porous
framework of the catalyst, and thus, accelerating the reaction
rate.63,64
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Fig. 4 Nitrogen adsorption (solid)-desorption (open) isotherms of
Pd@P(QP-TVP) and P(QP-TVP), and pore size distribution curve
(inset) of Pd@P(QP-TVP) calculated by NLDFT method.
SEM and TEM characterizations were used to analyze the
morphology and Pd size of the prepared samples. As shown in Fig.
5(a), representative SEM image of Pd@P(QP-TVP) revealed that the
sample was comprised of loosely packed and irregular-shape small
particles. TEM images of Pd@P(QP-TVP) in Fig. 5(b-c) suggested that
Pd nanoparticles were well distributed in the supporting material
with very narrow size distribution, and average diameter of the
ultrafine Pd clusters was about 2.7 nm. In Fig. 5 (d), TEM image of
Pd@PQP displayed that Pd@PQP had bigger particles size of Pd
clusters with an average diameter of 3.6 nm. Considering the same
immobilization and reduction method, the differences in particle
size of Pd clusters between Pd@P(QP-TVP) and Pd@PQP might be
attributed to the coordination effect of P ligand towards Pd species
during the reduction reaction. Indeed, similar phenomena have
been reported in a number of literatures.59-61
Fig. 6. Images of contact angle measurements: (a) PdNPs@POP-PPh3
for water, (b) Pd@P(QP-TVP) for water, (c) Pd@PQP for water, (d)
PdNPs@POP-PPh3 for toluene.
Thermostability of P(QP-TVP) and Pd@P(QP-TVP) were examined
using thermogravimetric analysis (TGA) in the atmosphere of
nitrogen. As depicted in Fig. 7, P(QP-TVP) and Pd@P(QP-TVP)
displayed similar TGA curves. The initial weight losses below 300 °C
could be attributed to desorption of the physisorbed guest
molecules, which is common for porous materials. Significant
weight losses occurred in the range of 400-600 °C, resulting from
the decomposition of polymer support. TGA analyses suggested that
the prepared polymer and catalyst had high thermostability and
seem could be stable at least up to a temperature less than 400 °C.
Fig. 5 (a) SEM and (b-c) TEM images of Pd@P(QP-TVP), (d) TEM
image of Pd@PQP.
For organic reactions in water, surface wettability of the solid
catalyst has great influence on its catalytic performances.62,63
Therefore, contact angle measurement was conducted to test
surface wettability of the prepared samples. As shown in Fig. 6 (a-c),
contact angles of PdNPs@POP-PPh3, Pd@P(QP-TVP) and Pd@PQP
towards water droplets were 115°, 32° and 0°, respectively. This
indicated that Pd@P(QP-TVP) and Pd@PQP had good hydrophilic
property, while PdNPs@POP-PPh3 showed water-repellent property.
The QP monomer is relatively hydrophilic, while TVP is relatively
Fig. 7 TGA curves of P(QP-TVP) and Pd@P(QP-TVP).
Catalytic hydrogenation of nitroarenes
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With the expected material in hand, we selected the hydrogenation H2 pressure of 2.0 MPa, 99.7% conversion of nitrobenzene with a
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of nitrobenzene as a model reaction to explore the catalytic high TOF of 5982 h-1 was achieved. Thus, under relatively mild
DOI: 10.1039/C9NJ05734J
performance of the prepared catalyst. Firstly, catalytic performances reaction conditions, a highly active catalyst was obtained for
of several solid catalysts were investigated and shown in Table 2. nitrobenzene hydrogenation.
With a molar ratio of Pd/substrate at 1/1000, Pd@P(QP-TVP)
afforded 98.6% conversion of nitrobenzene at 40 °C in 60 min (Table
2, entry 1), showing much higher catalytic activities than the
analogous Pd@PQP and PdNPs@POP-Ph3P, as well as commercial
Pd/C and Ru/C catalysts (Table 2, entries 2-4). In view of the
physicochemical properties of Pd@P(QP-TVP), the superior catalytic
activity of Pd@P(QP-TVP) might be attributed to its good surface
amphiphilicity and nice electron donating effect of P ligand. The
good surface hydrophilicity enabled the catalyst well disperse in
water; while the excellent lipophilicity could enrich organic
substrates into the porous framework of the catalyst, and thus,
providing a microenvironment with the high concentrations of
organic substrates.63,64 Meanwhile, the nice electron-donating effect
of P ligand could increase the surface electronic density of Pd
nanoparticles, and thus, enhancing their catalytic activity for the
hydrogenation of electrophilic nitro compounds.56 This conjecture
was also in agreement with the experimental results. Pd@PQP had
good surface amphipathicity and large BET surface area, but the
weak electron-donating ability of PQP towards Pd nanoparticles
resulted in its lower activity. Conversely, POP-Ph3P had nice
electron-donating ability towards Pd nanoparticles,22 however, the
hydrophobic surface property of PdNPs@POP-Ph3P led to its inferior
catalytic activity.
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Table 2 Catalytic performance of different catalysts for nitrobenzene
hydrogenation.a
Conv.
(mol%)
98.6
64.5
33.7
Sel.
(mol%)
100
100
99.1
100
100
Entry Catalysts
TOF (h-1)
1
2
3
4
Pd@P(QP-TVP)
Pd@PQP
PdNPs@POP-Ph3P
Pd/C (5wt%)
Ru/C (5wt%)
986
645
334
392
175
39.2
17.5
5
a
Reaction conditions: nitrobenzene (0.62 g,
5
mmol),
Pd(Ru)/substrate (1:1000, mole ratio), 1.0 MPa H2, H2O (10 ml),
40 °C, 60 min.
Next, the influences of reaction parameters such as catalyst usage,
reaction temperature and H2 pressure on activity were investigated.
As shown in Fig. 8a, the conversion of nitrobenzene decreased with
rising the molar ratio of substrate/Pd. Accordingly, the TOF
increased first, and then decreased slightly. With a molar ratio of
Pd/substrate at 1/4000, 50.7% conversion of nitrobenzene with
100% selectivity of aniline was preserved, achieving a high TOF of
2028 h-1. Therefore, the molar ratio of 1/4000 was selected for
investigating the effect of reaction temperature. Fig. 8b displayed
the effect of reaction temperature, and the results suggested that
nitrobenzene conversion significantly increased with rising reaction
temperature from 25 to 60 °C. At 60 °C, a high TOF of 3984 h-1 was
obtained with 99.6% conversion of nitrobenzene. Further increasing
the reaction temperature, the selectivity of decreased slightly. With
a lower Pd/substrate molar ratio at 1/6000, we then studied the
effect of H2 pressure. As depicted in Fig. 8c, the conversion of
nitrobenzene increased gradually with rising H2 pressure from 0.1 to
2.5 MPa. The higher hydrogen pressure resulted in larger hydrogen
solubility, which is beneficial for the hydrogenation reaction. Under
Fig.
8 The influence of reaction conditions in nitrobenzene
hydrogenation. (a) the molar ratio of substrate/Pd (1.0 MPa H2, 0.62
g nitrobenzene, 40 °C, 60 min), (b) reaction temperature (molar
ratio of substrate/Pd is 4000, 7.8 mg Pd@P(QP-TVP), 0.62 g
nitrobenzene, 1.0 MPa H2, 60 min), (c) hydrogen pressure (molar
ratio of substrate/Pd is 6000, 5.2 mg Pd@P(QP-TVP), 0.62 g
nitrobenzene, 60 °C, 60 min).
Besides the catalytic activity, the reusability of heterogeneous
catalysts is also crucial for their practical applications. To this end,
hydrogenation of nitrobenzene was selected to evaluate the
reusability of Pd@P(QP-TVP). After the catalytic reaction, the
mixture was separated by centrifugation. The obtained solid catalyst
was extracted with diethyl ether, washed with methanol and water,
and then, directly used for the next catalytic run. As shown in Fig. 9a,
after reusing for five times, Pd@P(QP-TVP) was still capable of
achieving 92.8% conversion of nitrobenzene with a high TOF of 5568
h-1. This indicated that Pd@P(QP-TVP) was quite robust under the
reaction conditions. Moreover, Pd content of the recycling
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Pd@P(QP-TVP) was analyzed by ICP. After reusing five times, 1.7 aggregation of Pd nanoparticles (average diameVteierw, Ar3ti.c0le Onnlmine)
wt% of Pd loading was still preserved in the recycling Pd@P(QP-TVP), occurred up after reusing five times.DOTIh: u10s,.103th9/eCs9eNJr0e5s7u3l4tJs
thus suggesting its excellent leaching-resistance ability. TEM image demonstrated that Pd@P(QP-TVP) was an active, robust and
(Fig. 9b) of the recycled Pd@P(QP-TVP) suggested that a slight heterogeneous catalyst for nitrobenzene hydrogenation in water.
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Table 3 Comparison of Pd@P(QP-TVP) with the typical Pd-loaded catalysts for nitrobenzene hydrogenation.
Entry Catalyst
Catal. usagea Conditions
T(min) Yield(%)
Run
Ref.
Present
work
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1
Pd@P(QP-TVP)
1/6000
H2O, 60 °C, 20 bar H2
60
99.7
5
2
3
4
5
6
7
8
PdNPs-PAAS
Pd/PVA
1/100
1/500
H2O, RT, 1 bar
180
180
90
100
99.3
100
>99
>99
98
6
[8]
H2O, 45 °C, 20 bar H2
H2O, 25 °C, 70 psi H2
EtOH, 40 °C, 10 bar
EtOH, RT, 1 bar
7
[24]
[32]
[56]
[62]
[63]
[64]
PdNP@PPh2-PEGPIILP 0.47/100
6
Pd/PPh3@FDU-12
Pd@NAC-800
Pd/OCNTs
1/6000
60
5
0.5/100
0.75/100
120
240
60
10
10
10
H2O, 60 °C, 1 bar
H2O, 50 °C, 1 bar
Pd@CQDs@Fe/Hybrid 1.25/100
100
a Catalyst usage (molar ratio of Pd /substrate).
catalytic activity and green solvent (water). Therefore,
Pd@P(QP-TVP) is promising catalytic system for
a
nitrobenzene hydrogenation.
Table 4 Scope of the catalyst.
Entry
R
T (min)
60
Con.(%)
99.4
Sel.(%)
100%
100%
100%
99.5%
100%
100%
1
2
3
4
5
6
4-CH3
2-CH3
4-OMe
2-OMe
4-F
120
60
98.2
96.5
120
60
96.9
98.8
4-NH2
80
99.4
7
90
97.6
100%
Fig. 9 (a) Reuse of Pd@P(QP-TVP) for nitrobenzene hydrogenation
(molar ratio of substrate/Pd is 6000, 5.2 mg Pd@P(QP-TVP), 0.62 g
nitrobenzene, 10 ml H2O, 60 °C, 60 min), (b) TEM image of the
recovered catalyst after reusing five times.
The comparison results of Pd@P(QP-TVP) with some recent
Pd NPs catalysts for the hydrogenation of nitrobenzene with H2
were listed in Table 3. The results indicated that the obtained
Pd@P(QP-TVP) herein featured low catalytic loading, high
Reaction conditions: nitroaromatic (5 mmol), Pd@P(QP-TVP),
Pd/substrate (1:6000, mole ratio), 2.0 MPa H2, 60 °C, H2O (10 ml), 1
h.
Next, the scope of the Pd@P(QP-TVP) catalyst in the
hydrogenation of nitroarenes was also tested with several
typical nitroarenes. As summarized in Table 4, nitroarenes
substituted with functional groups, such as -CH3, -OCH3, -F, -
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NH2, were well tolerated, affording excellent yields of
corresponding aromatic amines within 120 min (Table 4, chloride (PQP, Scheme S1 in the Supporting Information) was
entries 1-6). Of these group, ortho-CH3 and ortho-OCH3 synthesized by the same procedures described in literature.42
substituted nitrobenzenes also gave the target products in Synthesis of supported Pd NPs catalysts
high yields (Table 4, entries 2 and 4). As a representative example
of heterocyclic nitroarenes, 3-nitropyridine reacted smoothly under approach of anion exchange with Na2PdCl4 and successive reduction
the reaction conditions (Table 4, entry 7).
ARTICLE
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Polymerized
(4-vinylbenzyl)-tris-(4-vinylphenyl)-phosphonium
View Article Online
DOI: 10.1039/C9NJ05734J
Pd@P(QP-TVP) and Pd@PQP were prepared via a two-step
by NaBH4. In a typical run, Na2PdCl4 (29 mg, 0.099 mmol) was
dissolved in a 30 mL methanol-water (V/V=1:1) mixed solvent. Then,
the polymer support (0.5 g) was added to the solution. After stirring
at room temperature for 12 h, the solid was separated by filtration,
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Conclusions
washed thoroughly with deionized water. Successively,
5 mL
In conclusion, we have designed Pd nanoparticles stabilized by
phosphine-functionalized porous ionic polymer for efficiently
catalytic hydrogenation of nitroarenes in water with H2 as a
hydrogen source. Comparison results of the catalytic activity of
the prepared catalyst with that of Pd@PQP and PdNPs@POP-
Ph3P confirmed the contribution of phosphine ligand-modified
porous ionic polymer to the catalysis. The results established
that the incorporation of phosphine ligand in the polymer
network could improve anchoring and activity of Pd species
through electrostatic interactions, meanwhile suppressing the
Pd NPs from aggregation and leaching. The recyclability test
indicated the good recyclability of the catalyst, up to five
reaction runs, with a slight loss of the catalytic activity. Due to
the simple synthetic method and excellent catalytic
performance, this work provides not only a highly active
catalyst for nitroarenes reduction, but also a strategy to
prepare noble metal NPs loaded on porous ionic polymer with
high catalytic performance.
aqueous NaBH4 (56.74 mg, 1.5 mmol) was added dropwise into the
suspension of the resultant polymer in water (30 mL) at 0 °C and
then treated by ultrasonic wave for 60 min. The resultant powders
were collected by filtration, washed with water and methanol, and
then dried under vacuum at 50 °C for 12 h. ICP analysis suggested
that the palladium contents in Pd@P(QP-TVP) and Pd@PQP were
1.7 wt% and 1.8 wt%, respectively.
Pd NPs supported on polymerized tris(4-vinylphenyl) phosphine
(PdNPs@POP-Ph3P) with a 1.7 wt% palladium loading was prepared
as our previous report.22 The structure and composition of
PdNPs@POP-Ph3P had been carefully characterized previously.
Characterization of catalysts
Fourier Transform Infrared Spectroscopy (FT-IR) was collected on
a Bruker Equinox 55 FT-IR spectrophotometer (KBr disks) over a
range of 4000-400 cm-1. X-ray photoelectron spectroscopy (XPS) was
carried out on a VG multilab 2000 spectrometer equipped with Mg-
AlKa X-ray source. The spectra of Pd 3d5/2-3/2 and P 2p XPS were fitted
with a Gaussian–Lorentzian function (G:L = 80:20) and a tougaard
background function using the XPSPEAK 4.1 software. Solid-state 13
C
NMR and 31P NMR spectra were collected on a Bruker AVANCE III
600 Bruker spectrometer. Nitrogen sorption isotherms and pore size
distribution curves were acquired on a Micrometrics ASAP 2020
system at 77 K, and the samples were degassed at 90 °C for 12 h
before analysis. Scanning electron microscopic (SEM) images were
recorded on a Hitachi S‐4800 instrument. Transmission electron
microscopy (TEM) analysis was viewed on a FEI Tecnai G2 F30
instrument. The CHNO elemental analysis was performed on an
Elementar Vario MICRO Elemental analyzer. The amount of P species
in the polymer was determined by ICP technique, before
measurement, the sample was combusted in an oxygen bomb
calorimeter and then dissolved in water for analysis. The amount of
Cl was analyzed using ion chromatograph analysis. Quantitative
analysis of Pd species was collected on an inductively coupled
plasma atomic emission spectroscopy (ICP, PerkinElmer Optima
8000). Contact angle analysis was carried out using a Dataphysics
Experimental
Materials
N,N-Dimethylacetamide, 4-vinylbenzyl chloride, nitroarenes,
sodium tetrachloropalladate (II) (Na2PdCl4, 99.95%), Pd/C, Ru/C and
sodium borohydride (NaBH4) were purchased from Energy Chemical
and used as received. 2,2’-Azobis(2-methylpropionitrile) (AIBN) was
supplied by Aladdin Chemistry Co., Ltd. (Shanghai, China). Tris(4-
vinylphenyl) phosphine was synthesized as the procedures
described in our previous work.57 (4-Vinylbenzyl)-tris-(4-
vinylphenyl)- phosphonium chloride (QP) was prepared by the
reaction of tris(4-vinylphenyl) phosphine and 4-vinylbenzyl chloride
under N2 atmosphere in acetone solvent.42 H2 (99.99%) was
supplied by a local manufacturer.
Synthesis of catalyst supports
P(QP-TVP) was synthesized by a solvothermal, free-radical and
cross-linked copolymerization method. In a typical procedure, QP
(2.4 mmol, 1.18 g), TVP (2.4 mmol, 0.82 g) and AIBN (50 mg) were
dissolved in 20 mL of N,N-dimethylacetamide in a 100 mL Teflon
autoclave. After replacing air with nitrogen and stirring at room
temperature for 2 h, the autoclave was heated statically in an oven
at 100 °C for 24 h. After the solvothermal reaction, the autoclave
was cooled to room temperature, and a monolith solid was
obtained. The resulting monolith solid was extracted with ethanol
and dried under vacuum. The obtained white solid was designated
as P(QP-TVP) (1.98 g). Elemental composition of P(QP-TVP): C, 79.51;
H, 6.25; P, 7.09; Cl, 4.12%.
1
OCA20 system. H NMR spectra were acquired on a Bruker AVANCE
III 400 NMR spectrometer. Quantitative analysis of the reaction
liquid was performed on a gas chromatography (Scientific™ TRACE™
1310) equipped with a FID detector and a capillary column (TRACE
TR-WAX).
Catalytic hydrogenation of nitroarenes
Catalytic hydrogenation of nitroarenes was carried out in a 50 mL
Teflon-lined stainless-steel autoclave. In a typical procedure, Pd
catalyst and water (10 ml) was added into the reactor. The mixture
was stirred at room temperature for five minutes. Then, the reactor
was charged with nitroarenes, and flushed four times with H2. After
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18 X. F. Wu, P. Anbarasan, H. Neumann and M. Beller, VAienwgAerwtic.leCOhnelmine.
pressurizing with H2 to the desired pressure, the autoclave was
heated to preset temperature and stirred for the given time. After
the reaction, the reactor was cooled to room temperature and
carefully depressurized. The reaction mixture was extracted with
diethyl ether for five times. The combined liquid was quantitatively
analyzed by GC using N,N-dimethylformamide as an internal
Inter. Ed., 2010, 49, 9047-9050.
DOI: 10.1039/C9NJ05734J
19 M. N. Shaikh, RSC Adv., 2019, 9, 28199-28206.
2. M. T. Islam, R. Saenz-Arana, H. Wang, R. Bernal and J. C. Noveron,
New J. Chem., 2018, 42, 6472-6478.
21 L. Qiu, R. McCaffrey, Y. Jin, Y. Gong, Y. Hu, H. Sun, W. Park and W.
Zhang, Chem. Sci., 2018, 9, 676-680.
22 Y. Wan, Z. Chen, D. Liu and Y. Lei, Catal., 2018, 8, 586.
23 Q. Wang, X. Cai, Y. Liu, J. Xie, Y. Zhou and J. Wang, Appl. Catal. B:
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24 X. Wang, C. Huang, X. Li, C. Xie and S. Yu, Chem. Asian J., 2019, 14,
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26 G. Wang, S. Yuan, Z. Wu, W. Liu, H. Zhan, Y. Liang, X. Chen, B. Ma
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27 M. Shokouhimehr, K. Hong, T. H. Lee, C. W. Moon, S. P. Hong, K.
Zhang, J. M. Suh, K. S. Choi, R. S. Varma and H. W. Jang, Green
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28 S. Sadjadi, M. Akbari, E. Monflier, M. M. Heravi and B. Leger, New
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29 J. Li, L. Zhang, X. Liu, N. Shang, S. Gao, C. Feng, C. Wang and Z.
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30 Z. He and P. Alexandridis, Phys. Chem. Chem. Phys., 2015, 17,
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31 J. Dupont and J. D. Scholten, Chem. Soc. Rev., 2010, 39, 1780-
1804.
32 S. Doherty, J. G. Knight, T. Backhouse, A. Bradford, F. Saunders, R.
A. Bourne, T. W. Chamberlain, R. Stones, A. Clayton and K.
Lovelock, Catal. Sci. Technol., 2018, 8, 1454-1467.
33 F. Valentini, H. Mahmoudi, L. A. Bivona, O. Piermatti, M.
Bagherzadeh, L. Fusaro, C. Aprile, A. Marrocchi, L.Vaccaro and L.
Vaccaro, ACS Sustainable Chem. Eng., 2019, 7, 6939-6946.
34 B. Xin and J. Hao, Chem. Soc. Rev., 2014, 43, 7171-7187.
35 K. Manojkumar, A. Sivaramakrishna and K. Vijayakrishna, J.
Nanoparticle Res., 2016, 18, 103.
36 W. Qian, J. Texter and F. Yan, Chem. Soc. Rev., 2017, 46, 1124-
1159.
37 D. Xu, J. Guo and F. Yan, Prog. Poly. Sci., 2018, 79, 121-143.
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39 J. K. Sun, M. Antonietti and J. Yuan, Chem. Soc. Rev., 2016, 45,
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1
standard. The structure of the product was confirmed by H NMR.
For recycling test of the catalyst, the recovered catalyst was washed
with methanol and water for three times, and then, directly used for
the next catalytic run.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We acknowledge the financial support by the National Natural
Science Foundation of China (21763017), the Science and
Technology Fund Project of Guizhou Province (qian ke he ji chu
[2018]1414]), the Youth Talent Growth Project of Educational
Department of Guizhou Province (qian jiao he KY [2016]264),
the Scientific and Technological Innovation Platform of
Liupanshui (52020-2018-03-02 and 52020-2017-02-02), the
Academician Workstation of Liupanshui Normal University
(qiankehepingtairencai [2019]5604 hao), and the Fund of
Liupanshui Normal University (LPSSYZDXK201602).
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Table of contents
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Small palladium nanoparticles stabilized by phosphine-functionalized porous ionic
polymer was successfully prepared. The catalyst displayed high catalytic activity for
water-mediated hydrogenation of nitroarenes. Nano size, electron-donation effect of
phosphine ligand, and amphiphilic surface wettability account for the excellent catalytic
performance.