Simple synthesis of sub-nanometer Pd clusters: High catalytic activity of Pd/PEG-PNIPAM in Suzuki reaction

Article abstract of DOI:10.1016/S1872-2067(17)62797-9

Ultra-small metal nanoclusters have high surface energy and abundant active sites, and therefore their catalytic activities are usually significantly higher than those of larger nanoparticles. A temperature-responsive copolymer, namely poly(ethylene glycol)-co-poly(N-isopropylacrylamide) (PEG-PNIPAM) was synthesized as the first step, and then ultra-small Pd clusters stabilized within PEG-PNIPAM copolymer micelles were formed by direct reduction. Pd nanoclusters of size less than 2 nm showed outstanding catalytic activity in the Suzuki coupling reaction. The reaction between iodobenzene and phenylboronic acid was completed in as little as 10 s (turnover frequency = 4.3 × 10⁴ h^?1). A yield of 64% was achieved in 5 min in the reaction between chlorobenzene and phenylboronic acid. The catalyst showed significant deactivation during three consecutive runs. However, this composite catalyst consisting of Pd/PEG-PNIPAM can be easily recycled based on the reversible phase transition of temperature-responsive PEG-PNIPAM. This catalyst therefore has good potential for practical applications.

Full text of DOI:10.1016/S1872-2067(17)62797-9

Chinese Journal of Catalysis 38 (2017) 651–657
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Simple synthesis of sub‐nanometer Pd clusters: High catalytic
activity of Pd/PEG‐PNIPAM in Suzuki reaction
Zhe Chen a, Yu Liang a, Da‐Shuang Jiaa, Zhi‐Min Cui b,*, Wei‐Guo Songc,#
a College of Environmental Science and Engineering, North China Electric Power University, Beijing 102206, China
b School of Chemistry and Environment, Beihang University, Beijing 100191, China
c Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Ultra‐small metal nanoclusters have high surface energy and abundant active sites, and therefore
their catalytic activities are usually significantly higher than those of larger nanoparticles. A tem‐
perature‐responsive copolymer, namely poly(ethylene glycol)‐co‐poly(N‐isopropylacrylamide)
(PEG‐PNIPAM) was synthesized as the first step, and then ultra‐small Pd clusters stabilized within
PEG‐PNIPAM copolymer micelles were formed by direct reduction. Pd nanoclusters of size less than
2 nm showed outstanding catalytic activity in the Suzuki coupling reaction. The reaction between
iodobenzene and phenylboronic acid was completed in as little as 10 s (turnover frequency = 4.3 ×
104 h−1). A yield of 64% was achieved in 5 min in the reaction between chlorobenzene and phenyl‐
boronic acid. The catalyst showed significant deactivation during three consecutive runs. However,
this composite catalyst consisting of Pd/PEG‐PNIPAM can be easily recycled based on the reversible
phase transition of temperature‐responsive PEG‐PNIPAM. This catalyst therefore has good potential
for practical applications.
Received 12 January 2017
Accepted 26 February 2017
Published 5 April 2017
Keywords:
Temperature responsive
Hydrophilic polymer
Pd nanoclusters
Suzuki reaction
© 2017, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
These clusters often suffer from significant corrosion and have
poor stabilities [11]. Several synthetic methods such as im‐
Ultra‐small metal clusters are among the best heterogene‐
ous catalysts. Their high surface energies and sur‐
face‐to‐volume ratios give them high atom efficiencies [1–4]. In
some cases, the catalytic activities of small nanoclusters of cer‐
tain critical sizes are significantly higher than those of larger
nanoparticles. For noble‐metal clusters, the critical diameter is
around 2 nm [5–8]. These sub‐2‐nm nanoparticles have out‐
standing catalytic activities in many organic reactions such as
the Suzuki reaction [8,9] and propene epoxidation [10]. How‐
ever, these small metal clusters are more difficult to synthesize
and use in practical applications than are larger nanoparticles.
pregnation, hydrothermal reduction, and radiolysis have been
used to produce sub‐nanometer Pd clusters [12–15]. In these
methods, stabilizers such as functionalized polymers [16–18],
dendrimers [19], and metal–organic frameworks [20] are gen‐
erally used to stabilize the catalyst because their functional
groups have high affinities with PdCl4 −, and this prevents ag‐
2
glomeration of the newly formed Pd clusters [21].
The Suzuki cross‐coupling reaction between an aryl halide
and an arylboronic acid is an important process in modern
organic synthesis and offers a reliable pathway for the con‐
struction of aryl–aryl bonds. Suzuki reactions involving inactive
* Corresponding author. Tel: +86‐10‐82316160; E‐mail: cuizhm@iccas.ac.cn
# Corresponding author. Tel: +86‐10‐62557908; E‐mail: wsong@iccas.ac.cn
This work was supported by the National Natural Science Foundation of China (51502089, 51302008) and the Fundamental Research Funds for the
Central Universities (2016MS03).
DOI: 10.1016/S1872‐2067(17)62797‐9| http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 38, No. 4, April 2017

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Zhe Chen et al. / Chinese Journal of Catalysis 38 (2017) 651–657
but inexpensive aryl halides such as aryl chlorides are an at‐
tractive goal for the chemical industry. Generally, it is difficult
to catalyze the reactions of aryl chlorides using heterogeneous
Pd catalysts. The development of efficient heterogeneous cata‐
lysts for the reactions of aryl chlorides will be an important
breakthrough [22–23].
cooled to 0 °C in an ice‐water bath. A solution of
2‐chloropropionyl chloride (1.98 g, 15.80 mmol) in benzene
(10 mL) was added dropwise under stirring. The mixture was
stirred at room temperature for 24 h. The sediment was re‐
moved by suction filtration and most of the benzene was volati‐
lized by rotary evaporation. The solid was recrystallized from
diethyl ether, and PEG with 2‐chloropropionyl groups at both
chain ends (PEG‐2) was obtained by vacuum drying.
Smart materials that respond to changes in their environ‐
ment (e.g., pH and temperature) play important roles in cataly‐
sis [24–27]. Poly(N‐isopropylacrylamide) (PNIPAM) is a tem‐
perature‐responsive smart material; it shows a sharp phase
transition in water at the lower critical solution temperature
(LCST) [28–29]. Such smart polymers are excellent supports
for noble metals, e.g., in Pd@PNIPAM [18], Pd‐doped
C/Co@PNIPAM‐PPh2 [27], hollow PNIPAM/Ag spheres [30–31],
and Au@poly(glycidylmethacrylate‐co‐NIPAM) [32]. Noble‐
metal nanoparticles of size around 5 nm in such composites
have shown good temperature‐responsive catalytic behaviors,
but their high catalytic activities have not been investigated.
In this study, we used single‐electron‐transfer living‐radical
polymerization (SET‐LRP) [33] to synthesize a hydrophilic
twin‐tail polymer containing a poly(ethylene glycol) (PEG) ring
and two linear PNIPAM chains. The PEG‐PNIPAM chains can act
both as reducing agents and catalyst supports in grafting ul‐
tra‐small Pd nanoclusters to form a Pd/PEG‐PNIPAM compo‐
site. The Pd/PEG‐PNIPAM environment can be enriched with
both hydrophilic and hydrophobic reactants, and therefore it
shows extremely high catalytic activity in Suzuki cross‐coupling
reactions. The reaction between iodobenzene and phenyl‐
boronic acid in a mixture of ethanol and water (volume ratio of
1:1) was completed in 10 s (turnover frequency (TOF) = 4.3 ×
104 h−1) at 80 °C. The reaction with chlorobenzene gave 65%
conversion in 5 min. The Pd/PEG‐PNIPAM catalyst can be easi‐
ly recycled because of the reversible phase transition of
PNIPAM.
2.3. Preparation of PEG‐PNIPAM by SET‐LRP
A mixture of PEG‐2 (1.00 g, 0.25 mmol), NIPAM (1.29 g,
11.50 mmol), and dimethylformamide (10.5 mL) was deoxy‐
genated for 60 min by bubbling with nitrogen. After all the in‐
gredients had dissolved, the tube containing the solution was
immersed in liquid nitrogen. CuCl (45.50 mg, 0.46 mmol) and
Me6TREN (132 mg, 0.46 mmol) in water (7 mL) were added.
The mixture was degassed by performing three
freeze–pump–thaw cycles, and the tube was sealed under vac‐
uum. The sealed tube was kept at 25 °C for 24 h; polymeriza‐
tion was achieved under stirring with a magnetic stirrer at 800
rpm. The mixture was purified by dialysis against distilled wa‐
ter to give a molecular‐weight cut‐off of 3500. Lyophilization
gave PEG‐PNIPAM as a white powder.
2.4. Pd nanoparticle loading on PEG‐PNIPAM
PEG‐PNIPAM (50 mg) was dispersed in distilled water (30
mL) and the dispersion was stirred for 10 min. PdCl2 (0.0564
mol/L, 368 μL) was added and the mixture was stirred for 5 h.
The mixture was purified by dialysis against distilled water to
remove H+, Cl−, and Pd2+ ions. Lyophilization gave the
Pd/PEG‐PNIPAM composite as a light‐brown powder.
2.5. Suzuki cross‐coupling reaction
2. Experimental
The Pd/PEG‐PNIPAM catalyst (3 mg) was dissolved in water
(3 mL). Iodobenzene (0.15 mmol), phenylboronic acid (0.3
mmol), K2CO3 (0.3 mmol), and pentamethylbenzene (0.15
mmol) were added to ethanol (3 mL) under stirring. The two
solutions were mixed under magnetic stirring at a speed of 800
rpm and the reaction was performed at 80 °C in air. A sample of
the reaction mixture was collected and analyzed using
high‐performance liquid chromatography (HPLC) and gas
chromatography (GC).
2.1. Materials
PEG with two hydroxyl end groups (number‐average mo‐
lecular mass (Mn) = 1900 g/mol, Acros) was dried over anhy‐
drous toluene by azeotropic distillation. N‐Isopropylacrylamide
(NIPAM, 97%, Kohjin Co., Japan) was recrystallized from a
benzene/n‐hexane mixture (65/35 v/v). Tris[2‐(dimethyla‐
mino) ethyl]amine (Me6TREN) was synthesized using a previ‐
ously reported method [34]. 2‐Chloropropionyl chloride and
CuCl (99.99%) (Aldrich) were used as received, without further
purification. Triethylamine was stirred for 12 h with KOH and
then refluxed with toluene‐4‐sulfonyl chloride before use. Io‐
dobenzene, phenylboronic acid, pentamethylbenzene (Alfa
Aesar), and K2CO3 (Beijing Chemicals Co.) were analytical grade
and used as received.
2.6. Characterization
Gel‐permeation chromatography (GPC; Waters 515 HPLC
pump, Waters) was used to determine the molecular mass and
polydispersity index of Pd/PEG‐PNIPAM. The eluent was tet‐
rahydrofuran (THF) at a flow rate of 1.0 mL/min at 40 °C. Poly‐
styrene samples with narrow molecular mass distributions
were used as calibration standards. Transmission electron mi‐
croscopy (TEM; JEOL 1011F) was performed at 100 kV. Fouri‐
er‐transform infrared (FTIR) spectroscopy (Thermo NICOLET
iZ1) was performed using the KBr pellet method in the range
2.2. Reaction of PEG with 2‐chloropropionyl chloride
In a typical synthesis, PEG (6.30 g, 1.58 mmol) was mixed
with triethylamine (1.59 g, 15.80 mmol) and the mixture was

Zhe Chen et al. / Chinese Journal of Catalysis 38 (2017) 651–657
653
400−4000 cm−1. The spectra were recorded in single‐beam
mode at a resolution of 4 cm−1. Proton nuclear magnetic reso‐
nance (1H NMR, 300 MHz) spectroscopy was performed using a
Varian 400 MHz spectrometer at room temperature with CDCl3
as the solvent. The Pd content of the catalyst was determined
using inductively coupled plasma atomic emission spectrosco‐
py (ICP‐AES; Shimadzu ICPE‐9000). Elemental analysis was
performed using X‐ray photoelectron spectroscopy (XPS; ES‐
CALab220i‐XL electron spectrometer, VG Scientific) with 300
W Al Kα radiation. The temperature dependence of the optical
transmittance of Pd/PEG‐PNIPAM was investigated using ul‐
traviolet‐visible (UV‐vis) spectroscopy (Shimadzu UV1601).
Scheme 1. Synthesis of linear PEG‐PNIPAM copolymer.
trace peak was sharp and symmetric, confirming
well‐controlled polymerization. No tailing or shoulders at the
lower‐ or higher‐molecular‐mass side were observed, indicat‐
ing that only one type of product was obtained. The Mn,NMR was
7763, which is very close to Mn,GPC. The presence of PNIPAM in
the PEG‐PNIPAM polymer was further confirmed by FTIR (Fig.
1(c)). The peaks at 1547 (N–H) and 1649 (C=O) cm−1 confirmed
that PNIPAM was successfully grafted on PEG [21‐22].
3. Results and discussion
3.1. Identification of PEG‐PNIPAM structure
Linear PEG‐PNIPAM played a key role in formation of the
Pd/PEG‐PNIPAM composite. It was synthesized using a previ‐
ously reported method (Scheme 1) [33]. 2‐Chloropropionyl
chloride reacted with the hydroxyl groups of PEG to generate
initiation sites for SET‐LRP. Then NIPAM monomers were
grafted on both sides of PEG‐1 and polymerized in situ with
initiation by the 2‐chloro group. If all the NIPAM molecules are
polymerized, the resulting polymerization degree of NIPAM at
each side is 25.
The structure of the PEG‐PNIPAM block copolymer was de‐
termined using 1H NMR spectroscopy (Fig. 1(a)). All the proton
chemical shifts matched those in the PNIPAM block well. The
characteristic chemical shifts of the PNIPAM block were clearly
observed at 6.4 (4), 4.01 (2), and 1.18 (1) ppm, from N–H,
–CH–, and –CH3, respectively [33,35]. The chemical shift of the
methylene proton of PEG (3) was observed at 3.63 ppm. A
comparison of the areas of the methylene proton peak of PEG
(3.63 ppm) and the methine proton peak of PNIPAM (4.01
ppm) shows that the polymerization degree of PNIPAM was 50,
i.e., 25 for each initiation site. The polymerization degree is in
good agreement with the reagent ratio. GPC analysis of the
PEG‐PNIPAM composite showed that the polymerization pro‐
cess was well controlled (Fig. 1(b)). GPC analysis of
PEG‐PNIPAM in THF gave an Mn,GPC of 7841 and a polydispersi‐
ty index [weight‐average molecular mass/Mn) of 1.23. The GPC
3.2. Loading of sub‐2‐nm Pd nanoclusters on thermoresponsive
PEG‐PNIPAM polymer
A simple method was used to load Pd nanoclusters on the
PEG‐PNIPAM copolymer to form a Pd/PEG‐PNIPAM composite.
When the block copolymer was added to PdCl2 aqueous solu‐
tion, Pd2+ bonded to the NH groups of the PEG‐PNIPAM poly‐
mer via electrostatic interactions. The NH2 groups then acted as
reducing sites, and Pd2+ was reduced to Pd0 by the surrounding
NH2 groups [36,37]. The color of the reaction mixture turned
from bright yellow to light brown after a few minutes, indicat‐
ing the formation of Pd nanoclusters. ICP‐AES showed that the
Pd loading on the composite was 4.4 wt%.
Fig. 2 shows a TEM image of the Pd/PEG‐PNIPAM composite
and a high‐resolution TEM (HRTEM) image of the Pd clusters.
The TEM image shows a thin sheet of PEG‐PNIPAM. Ultra‐small
Pd clusters are densely loaded on the polymer. The Pd diame‐
ters calculated from Fig. 2(b) and 2(c) are around 2 nm. The
HRTEM image in Fig. 2(c) shows a lattice spacing of 0.224 nm,
corresponding to the (110) plane of Pd. The oxidation state of
Pd in the Pd/PEG‐PNIPAM composite was determined using
Fig. 1. (a) 1H NMR spectrum of PEG‐PNIPAM in CDCl3, (b) GPC trace for PEG‐PNIPAM, and (c) FTIR spectra of PEG and PEG‐PNIPAM polymers.

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Zhe Chen et al. / Chinese Journal of Catalysis 38 (2017) 651–657
transparent again. The phase transition provides strong evi‐
dence of PEG‐PNIPAM formation. This transition enables the
catalyst to be recycled easily.
Although this phase transformation was observed in aque‐
ous solution, we believed that the thermo‐responsive ability of
PNIPAM would also be useful in catalysis in organic solutions.
In this work, the activity of the catalyst in Suzuki reactions per‐
formed in a mixed water/ethanol solution was studied. We
hypothesized that above the LCST, a hydrophobic nanoenvi‐
ronment would be formed in the solution, and this nanoenvi‐
ronment would be enriched with hydrophobic guest molecules
(such as iodobenzene or phenylboronic acid in our catalytic
reaction) in water. Nanoreactor enrichment would greatly ac‐
celerate the reaction, as shown in other studies [18,41].
3.3. Catalytic performance of Pd/PEG‐PNIPAM composite
We performed Suzuki cross‐coupling reactions as model
reactions to evaluate the catalytic ability of the composite
nanoreactor. The iodobenzene substituents were varied to test
the effects of substitution. The results for Suzuki reactions per‐
formed using various reagents are listed in Table 1. The results
show that the composite showed excellent activity in all these
reactions. When iodobenzene was used, only the cross‐coupling
product biphenyl was detected by HPLC and GC; no other by‐
products were produced. The yield of biphenyl reached 100%
in 10 s (Table 1, run 1), with a TOF of 4.3 × 104 h−1. When the
reagent/catalyst ratio was increased from 120 to 1600, 100%
yields were reached in 3 min, with a TOF of 3.2 × 104 h−1 (Table
1, run 3). This high activity is remarkable for heterogeneous
catalysis, and shows the advantage of using PEG‐PNIPAM as a
catalyst support. A commercial Pd/C was used as the catalyst
for comparison, and 30 min was needed to reach 100% con‐
version under the same reaction conditions as in Table 1, run 1.
When bromobenzenes with different substituents were used,
all the bromobenzenes showed high reactivity. The product
yields were 80–100% in 3 min (Table 1, runs 4–7). Even chlo‐
Fig. 2. (a) and (b) TEM images of Pd/PEG‐PNIPAM composite at differ‐
ent magnifications, (c) HRTEM image of Pd nanoclusters, and (d) XPS
profile of Pd in Pd/PEG‐PNIPAM composite.
XPS; the binding energy of 340.5 eV shows that Pd2+ was suc‐
cessfully reduced to metallic Pd0 by PEG‐PNIPAM (Fig. 2(d)).
The phase transition of PNIPAM is affected by its molecular
mass and concentrations of other polymers [38]. Studies have
shown that the PEG content greatly affects the phase‐transition
behavior of PNIPAM‐g‐PEG [39]. The LCST of PNIPAM in aque‐
ous solution is typically 32 °C [40]. Fig. 3 shows the tempera‐
ture dependence of the optical transmittance at 500 nm for 1
g/L aqueous suspension of Pd/PEG‐PNIPAM. The solution
temperature was kept at 30 °C, and then increased to 50 °C at a
rate of 1 °C/min. The light‐brown clear solution at 30 °C, which
had a transmittance of nearly 70%, became cloudy and the
transmittance decreased to about 3% at 45 °C. The LCST of this
composite was determined to be about 41 °C. The inset in Fig. 3
shows digital images of the sample at 30 and 50 °C. When the
cloudy suspension was cooled to room temperature, it became
robenzene
showed
excellent
reactivity
over
this
Pd/PEG‐PNIPAM catalyst, giving a 65% product yield in 5 min
at 80 °C (Table 1, run 8). These results show that the composite
is extremely active in Pd‐catalyzed C–C cross‐coupling reac‐
tions.
80
60
40
When the Pd/PEG‐PNIPAM was used in three consecutive
runs, the catalyst showed significant deactivation. The iodo‐
benzene conversion decreased to 5% in 10 s after three con‐
secutive runs. A TEM image of the used composite showed that
the Pd nanoclusters aggregated into large particles of size sev‐
eral hundred nanometers. Because of their high surface energy,
the sub‐2 nm Pd nanoparticles have a high tendency to aggre‐
gate, and the interactions between PNIPAM and Pd are not
sufficient to anchor the Pd nanoparticles. Prevention of aggre‐
gation of the sub‐2‐nm Pd nanoparticles in Pd/PEG‐PNIPAM
could enable their activity to be maintained. In future work,
stabilizing methods such as combination with silica or other
porous polymers or silica that have stronger affinities with Pd
will be studied. However, these large Pd particles can be easily
recovered by centrifugation. ICP‐AES analysis detected only a
30oC 50oC
20
0
30
35
40
45
50
Temperature (oC)
Fig. 3. Temperature dependence of optical transmittance at 500 nm for
1 g/L aqueous suspension of Pd/PEG‐PNIPAM.

Zhe Chen et al. / Chinese Journal of Catalysis 38 (2017) 651–657
655
Table 1
Suzuki cross‐coupling reactions catalyzed by Pd/PEG‐PNIPAMa.
Run
1
Reagent 1
Reagent 2
T/°C
80
Time
10 s
Yield
100%
TOF (h−1)
4.3×104
2
25
80
80
80
1 min
3 min
3 min
3 min
100%
100%
100%
90%
7.2×103
3.2×104
2.4×103
2.16×103
3 b
4
5
6
7
8
80
80
80
3 min
3 min
5 min
80%
96%
65%
1.92×103
2.3×103
936
a Reaction condition: 80 °C, 0.15 mmol iodobenzene, 0.3 mmol phenylboronic acid, 0.3 mmol K2CO3, 0.15 mmol pentamethylbenzene and 3 mg cata‐
lyst were added to 3 mL ethanol and 3 mL H2O mixture. Reagent/catalyst ratio was 120.
b reagent/catalyst ratio was 1600.
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catalyst,
the
environment
around
Pd/PEG‐PNIPAM was enriched with both hydrophilic and hy‐
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Zhe Chen et al. / Chinese Journal of Catalysis 38 (2017) 651–657
Graphical Abstract
Chin. J. Catal., 2017, 38: 651–657 doi: 10.1016/S1872‐2067(17)62797‐9
Simple synthesis of sub‐nanometer Pd clusters: High catalytic
activity of Pd/PEG‐PNIPAM in Suzuki reaction
Zhe Chen, Yu Liang, Da‐Shuang Jia, Zhi‐Min Cui *, Wei‐Guo Song*
North China Electric Power University; Beihang University; Institute
of Chemistry, Chinese Academy of Sciences
A temperature‐responsive copolymer, i.e., PEG‐PNIPAM, was syn‐
thesized and stabilized ultra‐small Pd clusters were formed on it
by direct reduction. The sub‐2‐nm Pd nanoclusters showed out‐
standing catalytic activities in Suzuki reactions.
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