Palladium nanoclusters entrapped in polyurea: A recyclable and efficient catalyst for reduction of nitro-benzenes and hydrodechlorination of halogeno-benzenes

Article abstract of DOI:10.1007/s11426-010-4027-7

Polyurea-entrapped palladium nanoclusters have been prepared by interfacial polymerization in W/O emulsion and showed high thermal stability and chemical stability with the content of 0.12 mmol g^-1 Pd. This catalyst exhibited dual catalytic activity for reduction of nitro compounds and hydrodehalogenation of aromatic chlorides in atmospheric hydrogen with 100% yield for reduction of nitro compounds and >99% yield for hydrodehalogenation of aromatic chlorides. This immobilizing method was particularly effective and eliminated the need of special chelating groups.

Full text of DOI:10.1007/s11426-010-4027-7

SCIENCE CHINA
Chemistry
July 2010 Vol.53 No.7: 1520–1524
doi: 10.1007/s11426-010-4027-7
• ARTICLES •
Palladium nanoclusters entrapped in polyurea: A recyclable and
efficient catalyst for reduction of nitro-benzenes and
hydrodechlorination of halogeno-benzenes
JI HongBing1*, LONG QingPing1, HE YunBing1 & YAO XingDong2
1The Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong Province;
School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China
2College of Chemistry & Ecological Engineering, Guangxi University for Nationalities, Nanning 530006, China
Received March 10, 2010; accepted June 9, 2010
Polyurea-entrapped palladium nanoclusters have been prepared by interfacial polymerization in W/O emulsion and showed
high thermal stability and chemical stability with the content of 0.12 mmol g1 Pd. This catalyst exhibited dual catalytic activity
for reduction of nitro compounds and hydrodehalogenation of aromatic chlorides in atmospheric hydrogen with 100% yield for
reduction of nitro compounds and >99% yield for hydrodehalogenation of aromatic chlorides. This immobilizing method was
particularly effective and eliminated the need of special chelating groups.
polyurea-entrapped palladium nanocluster, interfacial polymerization, reduction, hydrodehalogenation, immobilization
methods often suffer from such problems as low reactivity,
1 Introduction
palladium leaching, and requirement of special chelating
groups, leading to the requirement of alternative methods
for immobilization of palladium species.
The palladium species has been used as an attractive cata-
lyst for numerous useful organic reactions [1], e.g., hydro-
genation of nitro compounds [2], hydrodechlorination of
aromatic chlorides [3], cyclization reactions [4] and car-
bon-carbon coupling reactions [5]. The need for practical
and economic translation of these laboratory methods to
large-scale operations and the trend of clean production
have prompted the development of various strategies for
immobilizing palladium species which may allow for re-
covery and reuse of catalysts [6]. Palladium on carbon was
a typical heterogenous catalyst for organic synthesis but
quite sensitive to trace impurities and problematic for reuse
[7]. The palladium species has also been immobilized on
inorganic solid supports via absorption [8] or embedded in
organic polymers via complexation [9]. However, these
Polyurea, which is insoluble in aqueous and many or-
ganic solvents, has been demonstrated as a promising sup-
port due to its good thermal and chemical stability. Polyurea
as a support could eliminate the need for special ligands due
to its strong chelation effect on metal species [10]. Micro-
encapsulated Pd(OAc)2 in polyurea by interfacial polymeri-
zation approach in O/W emulsion system has been demon-
strated as a recoverable and reusable catalyst for hydro-
genation of alkenes and ketones, hydrogenolysis of epox-
ides, and cross-coupling reactions [11–15]. To avoid the use
of toxic solvent dichloromethane and expensive polymers,
such as polymethylene diisocyanate and polyphenylene
diisocyanate, a new immobilization system for encapsulat-
ing inorganic metal salts in polyurea via interfacial polym-
erization in W/O emulsion was developed for the first time,
and the microencapsulated NiCl2 has been demonstrated to
*Corresponding author (email: jihb@mail.sysu.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2010
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JI HongBing, et al. Sci China Chem July (2010) Vol.53 No.7
1521
be a recoverable and reusable catalyst for benzaldehyde
reduction [16, 17]. However, when we extended this
method to encapsulate PdCl2 in polyurea, the encapsulated
PdCl2 microcapsules could not be formed, which may be
attributed to the strong chelation between PdCl2 and poly-
urea. Surprisingly, PdCl2 could be well-immobilized in the
form of nanoclusters after its reduction, and the immobi-
lized Pd catalyst exhibited excellent catalytic performance
for reduction of nitro compounds and hydrodehalogenation
of aromatic chlorides in atmospheric hydrogen. This process
provides a novel and cost-effective method to immobilize
inorganic Pd salt via interfacial polymerization in the form
of polyurea with Pd(0) nanoclusters instead of microcapsules.
2 Experimental
Figure 1 TEM micrograph of the Pd-PU (magnification × 100000, scale:
20 nm).
2.1 Preparation of polyurea-entrapped palladium nano-
clusters
The Pd(0) nanocluster entrapped in polyurea was prepared
as follows: 15 mL aqueous solution containing 0.1 g PdCl2
was dispersed into 40 mL cyclohexane containing 3.5 g
2,4-toluene diisocyanate and 1.8 g Span 80 to form W/O
emulsion under stirring. Then, 1 mL triethylamine was
added to initiate the polymerization. After 10 h, the result-
ing mixture was reduced with H2, filtrated, washed and
dried to give Pd(0) nanocluster entrapped in polyurea (Pd-PU).
2.2 Typical reaction of reduction
0.10 g of the described Pd-PU was added into 5 mL of
n-hexane under stirring with a magnetic stirrer, and the hetero-
geneous solution was kept at 30 C for 30 min by 15 mL/min
of atmospheric hydrogen flow. Then 1.5 mmol nitrobenzene
was added to the above solution to start the reduction. The
solid catalyst was recycled by filtration. The filtrate was
analyzed by GC (Shimadzu GC-14CPF, CBP1-M25-025
column, 0.2 mm × 25 m) and naphthalene as the internal
standard. The yields of the reaction were calculated on the
basis of the GC results.
Figure 2 DSC and TG curves of the Pd-PU.
Because the immobilization of Pd was attained by the
interaction between palladium and other atoms in Pd-PU,
the possible interactive elements including C, N and O have
been investigated using XPS characterization, as shown in
Figure 3. The binding energy and intensity of C1s peaks for
both blank polyurea and Pd-PU remain unchanged, indicat-
ing the chemical environment of C was hardly influenced
by the entrapment of Pd. However, the situation is different
for N1s and O1s. The N1s peak in the Pd-PU shifted posi-
tively by 0.3 eV, and the O1s peak shifted negatively by
1.0 eV, compared with those in the blank polyurea. With the
entrapment of Pd, the electron density for N was a little
electron-deficient, while the electron density for O was
electron-enriched. Accordingly, the formation of Pd-PU is
described in Figure 4.
3 Results and discussion
3.1 Material characterization
TEM micrograph shows that Pd(0) species was dispersed in
nanometers in polyurea, as shown in Figure 1. The thermal
stability of modified polyurea measured by differential
scanning calorimetry (DSC) and thermogravimetry (TG) is
shown in Figure 2, and the improved thermal stability of
Pd-PU was observed in comparison with the blank polyurea.
The corresponding DSC and TG peaks of Pd-PU shifted
from 298 to 328 C. The content of Pd in Pd-PU was deter-
mined using AAS, and the result was 0.12 mmol g1.
3.2
Catalytic performance of polyurea-entrapped
palladium nanoclusters
The catalytic activity of Pd-PU was firstly demonstrated

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JI HongBing, et al. Sci China Chem July (2010) Vol.53 No.7
Figure 3 The XPS curves of C, N and O elements in Pd-PU and blank polyurea.
Figure 4 The formation profile of Pd-PU.
with reduction of various nitro compounds. The reactions
were carried out in n-hexane at ambient temperature in at-
mospheric hydrogen. Various nitro compounds were re-
duced with high yields, as shown in Table 1.
As shown in Table 1, nitrobenzene and its derivatives
could be smoothly converted to the corresponding amines.
The substituted group at different positions of the benzene
ring influenced the reaction speed and increased in the order
of o-, m-, and p-substituted nitro-benzene due to steric hin-
drance (entries 2–4). Two nitro groups at the same benzene
ring, e.g., o-dinitrobenzene, could also be completely re-
duced (entry 6).
The activity of recycled Pd-PU was tested using nitro-
benzene reduction as the typical reaction. Pd-PU was re-
covered by simple filtration and reused for three times
without any loss in activity or selectivity, showing the en-
trapment of Pd in polyurea is an effective immobilization
method (Table 2). Analysis of the product mixtures gave no
detection of palladium ions by the AAS method whose de-
tection limit was 1 ppb. When the solvents, such as ethanol,
toluene and water, were adopted to mix the Pd-PU, and
heated even at 110 C for 5 h, no palladium ions could be
detected, which indicated the chemical stability of Pd-PU.
Polyurea itself was capable of chelating with the metal spe-
cies, eliminating the need to synthesize special chelating
groups and incorporate them within the polymer matrix.
Inspired by the catalytic performance of nitro com-
pounds’ reduction, hydrodechlorination of aromatic chlo-
rides was also investigated with the same catalytic system.
With a similar procedure, hydrodechlorination of various
aromatic chlorides in methanol at 60 °C under atmospheric
hydrogen gave the corresponding dechlorinated product
with a high yield, as shown in Table 3.
As shown in Table 3, the aromatic chlorides were effi-
ciently converted to the corresponding dechlorinated prod-
ucts. It is noteworthy that the coexisting nitro and chloride
groups could be simultaneously reduced and dechloried
with longer reaction time.

JI HongBing, et al. Sci China Chem July (2010) Vol.53 No.7
Table 1 Reduction of nitro compounds in the presence of Pd-PU nanoclusters a)
1523
Entry
Substrate
Product
Time (h)
Conv. (%)
100
Yield (%)
100
1
2
2
3
4
5.5
4.5
3
100
100
100
100
100
100
5 b
10.5
100
100
100
6
4.5
100
a) Reaction conditions: substrate (1.5 mmol), Pd-PU (0.1 g), n-hexane (5 mL), 30 °C, H2 (1 atm); b) THF as the solvent.
Table 2 Reduction of nitrobenzene catalyzed by recycled Pd-PU a)
Run
1
2
3
Yield (%)
100
100
100
a) Reaction conditions: substrate (1.5 mmol), Pd-PU (0.1 g), n-hexane (5 mL), 30 C, H2 (1 atm), 2 h.
Table 3 Hydrodechlorination of aromatic chlorides in the presence of Pd-PUa)
Entry
1
Substrate
Product
Time (h)
1.5
Conv. (%)
>99
Yield (%)
>99
2
3
4.5
4.5
>99
>99
>99
>99
4
5
3.5
10
>99
>99
>99
>99
6
7
10.5
9
>99
>99
>99
>99
a) Reaction conditions: substrate (2 mmol), Pd-PU (0.1 g), methanol (5 mL), 60 C, H2 (1 atm).

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JI HongBing, et al. Sci China Chem July (2010) Vol.53 No.7
Table 4 Hydrodechlorination of benzyl chloride catalyzed by recycled Pd-PU a)
Run
1
2
3
Yield (%)
>99
>99
>99
a) Reaction conditions: substrate (2 mmol), Pd-PU (0.1 g), methanol (5 mL), 60 C, H2 (1 atm), 1.5 h.
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