Chitosan microspheres-supported palladium species as an efficient and recyclable catalyst for Mizoroki-Heck reaction

Article abstract of DOI:10.1039/c7nj01918a

Chitosan microspheres-supported palladium catalysts (Pd@CM) for Mizoroki-Heck reaction have been successfully prepared by electrospraying PdCl₂/chitosan mixture in a trifluoroacetic acid (TFA) aqueous solution. Swelling capacity measurement and thermal analysis demonstrated that the palladium species in the chitosan microspheres could cross-link the chitosan efficiently. FT-IR spectra demonstrated that the chitosan in the microspheres could be cross-linked by glutaraldehyde, and the formed Schiff bond could chelate the palladium species efficiently. High resolution transmission electron microscopy (HR-TEM) indicated that the entrapment of palladium species in chitosan microspheres could enhance its dispersion. Mizoroki-Heck reaction catalysis results demonstrated that the catalytic activity of Pd@CM was associated with the size of chitosan microspheres. The Pd@CM with a suitable size was even more active than the catalyst with palladium species adsorbed on the surface of chitosan microspheres (Pd-CM). Moreover, the entrapment of palladium species in chitosan microspheres could reduce its aggregation and leaching in the catalysis procedure, which in turn increased the reusability of Pd@CM catalyst. The high catalytic activity and stability of Pd@CM have been attributed to the entrapment of palladium species within the cross-linked chitosan microspheres and the chelation of palladium species with the abundant amine and hydroxyl groups on chitosan chain.

Full text of DOI:10.1039/c7nj01918a

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Page 1 of 27
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Chitosan Microspheres Supported Palladium Species as an Efficient and
Recyclable Catalyst for Mizoroki-Heck Reaction
Linjun Shao, Chenze Qi*
Zhejiang Key Laboratory of Alternative Technologies for Fine Chemicals Process, Shaoxing University,
Zhejiang Province 312000, People’s Republic of China
* Corresponding authors. Fax: +86 575 8834 5682. E-mail address: 312087578@qq.com.

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Abstract
Chitosan microspheres supported palladium catalysts (Pd@CM) for Mizoroki-Heck reaction
has been successfully prepared by electrospraying PdCl2/chitosan mixture in trifluroacetic
acid (TFA) aqueous solution. Swelling capacity measurement and thermal analysis
demonstrated that the palladium species in the chitosan microspheres could cross-link the
chitosan efficiently. FT-IR spectra demonstrated that the chitosan in the microspheres could
be cross-linked by glutaraldehyde and the formed Schiff bond could chelate the palladium
species efficiently. High resolution transmission electron microscopy (HR-TEM) indicated
that the entrapment of palladium species in chitosan microspheres could enhance its
dispersion. Mizoroki-Heck reaction catalysis results demonstrated that the catalytic activity
of Pd@CM was associated with the size of chitosan microspheres. The Pd@CM with
suitable size was even more active than the catalyst with palladium species adsorbed on the
surface of chitosan microspheres (Pd-CM). Moreover, the entrapment of palladium species
in chitosan microspheres could reduce its aggregation and leaching in the catalysis
procedure, which in turn increased the reusability of Pd@CM catalyst. The high catalytic
activity and stability of Pd@CM have been attributed to the entrapment of palladium
species within the cross-linked chitosan microspheres and the chelation of palladium species
with the abundant amine and hydroxyl groups on chitosan chain.
Keywords: Palladium; Chitosan microsphere; Electrospraying; Mizoroki-Heck reaction;
Heterogeneous.

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Introduction
Palladium catalyzed reaction have played an important role in chemical industrials and
academic researches for the preparation of fine chemicals and pharmaceutically active
compound.1,2 The majority of palladium catalysis is performed in homogeneous system for
its high activity and selectivity. However, the homogeneous catalysts usually suffer the
problems of the recovery and reuse of palladium and ligands from reaction media and
purification of final products.3 Moreover, the content of residual palladium in final products
is frequently reached to an unacceptable level, which would seriously limit the application
of products in pharmaceuticals, food, cosmetics and so on.4 The complete removal of
palladium from the products is usually tedious and costly, especially for large scale
production. Immobilization of homogeneous palladium on/in solid matrices is a facile
method to solve these problems.5-9 Moreover, the immobilized palladium catalyst could be
readily recovered from the reaction mixture and recycled, which could increase the overall
production and minimize the waste generation, resulting in a green and sustainable chemical
transformation process.
Recently, natural polymers have attracted more and more attention for their
environment-friendly and biodegradable properties.10-12 Chitosan is the N-deacetylation
product of chitin, which is the second abundant natural biopolymer. As the abundant amine
and hydroxyl groups on chitosan chain could chelate with transition-metal ions efficiently,
chitosan hold great potential as a versatile platform for supporting active species. Till now,
chitosan has been fabricated into various forms for the immobilization of transition metal
catalyst, such as powders, gels, spheres and fibers.13-16 In our previous study,17,18 chitosan

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had been prepared into porous spheres and nanofibers as the supporting matrices of
palladium and these chitosan supported palladium catalysts were very active and efficient to
catalyze the carbon-carbon coupling reaction. Moreover, the recovery and recycling of these
chitosan supported palladium catalysts were also very convenient.
Fore most heterogeneous palladium catalyst, palladium species were usually anchored
on the surface of solid matrices with larger specific surface in order to increase the collision
probability with substrates.6-9 Palladium leaching in catalysis process is a common problem
for these surface adsorbed palladium catalysts due to the weaker coordination of zero-valent
palladium with solid matrices in harsh reaction, such as high temperature. Surface chemical
modification of solid matrices with special chelating groups has been widely used to
minimize the palladium leaching in catalysis. However, the synthesis of special ligands and
surface modification of solid matrices are usually challenging.19-21 Chitosan supported
palladium heterogeneous catalysts also suffer the problem of palladium leaching in catalytic
process.14-18 Recently, we discovered that the palladium leaching could be obviously
decreased by entrapping the palladium species in interpenetrating polymer networks of
poly(vinyl alcohol) nanofiber.22 However, due to the low interaction of palladium with
poly(vinyl alcohol), the stability and palladium leaching of this catalyst is still not satisfied.
In the present paper, palladium species was entrapped in chitosan microspheres by
electrospraying the PdCl2/chitosan mixture in trifluoroacetic acid (TFA) aqueous solution
(Scheme 1). We believed that the high chelating ability of amine and hydroxyl groups on
chitosan chain and cages formed by cross-linked chitosan chain could decrease the
palladium leaching obviously in the catalysis process. Mizoroki-Heck reaction was

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employed to evaluate the catalytic activities and stabilities of these entrapped palladium
catalysts. The catalysis results proved that the entrapped palladium catalyst with suitable
size had even better catalytic activity and stability than the surface adsorbed palladium
catalyst and the palladium leaching also decreased.
Pd2+
Crosslinking
Electrospraying
Reduction
CH2OH
O
CH2OH
O
CH2OH
O
O
O
HO
HO
H2N
H2N
H2N
Cage
Chitosan
-OH or -NH2
Pd0
Chitosan
Crosslinking
Scheme 1. Schematic representation for the preparation of chitosan supported palladium
microspheres.
Experimental
Materials
Chitosan (Mn = 2.0 × 105, Pharmaceutical grade, Deacetylated degree: 95%) was bought
from Zhejiang Aoxing Biotechnology Co. Ltd (Zhejian, China). Aromatic iodides and
olefins (Analytic grade) were bought from Aladdin Industrial Co. Ltd (Shanghai, China).
PdCl2 (Chemical grade) was bought from Hangzhou Changqing Chemical Industrial Co. Ltd
(Zhejiang, China). Deionized water was used in all experiments.
Preparation of chitosan supported palladium catalyst
50 mg PdCl2, 34 mg NaCl and 1.0 g chitosan were dissolved in 40 g trifluoroacetic acid
(TFA) aqueous solution (TFA content: 20 wt.%) to get PdCl2/chitosan solution (chitosan

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concentration: 2.5 wt. %). The electrospraying procedure was carried out on electrospinning
equipment (FM1206, Beijing Future Material Sci-tech Co., Ltd, China). 500 mL 10 wt.%
NaOH aqueous solution, connected to the ground, was put on a magnetic stirrer as the
collector. 10 mL PdCl2/chitosan solution was put in to syringe, and then electrosprayed into
the NaOH aqueous solution. The electrospraying parameters were as follows: needle gauge:
21 G; feed rate: 5.0 mL/h; applied voltage: 18 kV; work distance: 10 cm; and rotation speed
of magnetic stirrer: 400 rpm.
After electrospraying, 25 mL glutaraldehyde aqueous solution (concentration: 50%)
was added to the NaOH aqueous solution and then stirred overnight to cross-link the
chitosan microspheres. Then, the solution was filtrated and the microspheres were put in 50
mL H2O. 0.5 g NaBH4 was added to reduce the Pd2+ into Pd0. After reduction, the
microspheres became completely black. The solution was then filtrated and the
microspheres were washed by H2O and ethanol (3 × 20 mL), respectively. The resultant
microspheres were dried under vacuum at room temperature to remove the residual solvent
and named as Pd@25CM18. We also prepared Pd@50CM18 and Pd@50CM00 catalysts
with chitosan concentration of 5.0 wt.% by the same method with and without the appliance
of high voltage.
For comparison, chitosan microspheres without loading of palladium were also
prepared by electrospraying 5.0 wt.% chitosan solution. The immobilization of palladium on
these chitosan microspheres was the same with our previous report:17 50 mg PdCl2 and 34
mg NaCl were dissolved in 50 mL H2O and stirred for 2 hrs to get homogeneous yellow
Na2[PdCl4] solution. Then, the chitosan microspheres (1.0 g) were added to the Na2[PdCl4]

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solution and stirred overnight, and the solution became colorless, indicating the completely
adsorption of palladium ion. Then, 0.5 g NaBH4 was added to reduce the Pd2+ into Pd0, and
the chitosan microspheres became black. This surface adsorbed palladium catalyst
(Pd-50CM18) was separated by filtration and dried under a reduced pressure for 12 h.
Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) characterization
showed that the palladium contents in the Pd@25CM18, Pd@50CM18, Pd-50CM18 and
Pd@50CM00 were 2.93%, 2.95%, 2.84% and 2.86%, respectively.
Swell capacities of chitosan microsphere supported palladium catalysts
50 mg chitosan microsphere supported palladium catalysts were immersed in 20 g
N,N-dimethylacetamide (DMAc) for 2 and 5 hrs, respectively. Then, the microspheres were
recovered by filtration through a sintered filter funnel (pore size: 3~4 µm). The swelling
capacities of the samples were calculated by the following equation (Equation 1):
m2 - m1
Q =
m1
Equation 1
Where m1 and m2 are the mass of the dried and swollen samples, respectively. The Q value
was calculated as grams of DMAc solution soaked per gram of microsphere sample.
General procedure for the Mizoroki-Heck cross-coupling reaction
The reaction conditions were adapted from our previous studies:17 chitosan supported
palladium catalyst (10 mg, palladium content: 2.8 µmol) was added into the 3.0 g DMAc
solution containing aromatic iodides (0.7 mmol), olefins (1.4 mmol) and triethylamine (5.25
o
mmol) in 25 mL tubular reactor. The mixture was stirred magnetically at 110 C and the
reaction mixture was analyzed by GC/MS.

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Hot filtration test
The reaction was firstly carried out with the same condition as that in “General procedure
for the Mizoroki-Heck cross-coupling reaction”. After reacting for 1 h, the reaction mixture
was filtrated off at the reaction temperature. The filtrate was then transferred into another 25
mL tubular reactor and the reaction was restarted under the same condition.
Reuse of the chitosan microspheres supported palladium catalysts
The chitosan microspheres supported palladium catalyst was recovered from the reaction
mixture by centrifugation and then was reused for subsequent catalysis directly.
Mizoroki-Heck reaction accruing within the Pd@25CM18 catalyst
50 mg Pd@25CM18 was immersed in the p-iodoanisole solution (p-iodoanisole: 0.2 g,
DMAc: 5.0 g). Then the solution was put in an open bottle and stirred magnetically at 120
oC. After the evaporation of DMAc, the Pd@25CM18 catalyst was pulverized in a mortar
and then characterized by scanning electron microscope-energy dispersive X-ray
spectroscopy (SEM-EDX).
Characterization
The morphologies of the microspheres were characterized by a scanning electron
microscope (SEM) (Jeol, Jsm-6360lv, Japan) and the element analysis was examined with
an energy dispersive X-ray spectroscopy (EDX) (Oxford, X-act, Britain). The diameters of
chitosan microspheres and Pd/chitosan microspheres were acquired from the SEM images.
The chemical structures of chitosan microspheres and Pd/chitosan microspheres were
analyzed by Fourier transform infrared spectroscopy (FT-IR) with the accessories of
attenuated total reflection (Nicolet, Nexus-470, USA). The rotary viscosity was tested using

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o
NDJ-8S type rotary viscometer at room temperature (20 C) (Shanghai Pingxuan Scientific
Instrument Co. Ltd, China). 1H NMR spectra were recorded in CDCl3 (Bruker, AVANCE III
400 MHz, Switzerland) using TMS as the internal reference. Inductively coupled
plasma-atomic emission spectroscopy (ICP-AES) analysis was performed on a Leemann
ICP-AES Prodigy XP (Leeman Labs, USA). The thermal stability of the fibers was analyzed
by a thermo gravimetric analyzer (TGA) (Beijing Scientific Instrument Factory, China)
under air atmosphere. The heating ramp was from 100 to 500 oC at a rate of 10 oC/min. The
quantitative analysis of Mizoroki-Heck reaction product was determined by Agilent GC/MS
(Agilent, GC6890/5975 MSD, USA). Palladium dispersion was analyzed by high-resolution
transmission electron microscopy (HR-TEM) (Jeol, JEM-2100F, Japan) and X-ray
diffraction (XRD) (Empyrean, PANalytical, Netherlands), respectively. The diameter
distribution of palladium species in chitosan microspheres were adopted from the HR-TEM
images. X-ray photoelectron spectroscopy (XPS) measurements were carried out at room
temperature and relative elemental ratios on the sample surface were determined on an
Al/Mg anode with a power of 250 W (Thermo Scientific, ESCALAB_250Xi, USA).
Results and Discussion
Characterization of chitosan supported palladium microspheres
Electrospraying is a one-step method to generate polymer microspheres, and is a
modification of electrospinning which is widely used to prepare micro/nanofibers.23,24
Acetic acid aqueous solution was the widely used solvent for chitosan. However, the
chitosan in acetic acid aqueous solution was usually gelatinized after addition of even small

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amount of palladium ion due to the strong chelating ability of chitosan with palladium ion.25
Herein, PdCl2/chitosan homogeneous solution was obtained with TFA aqueous solution (20
wt.%) as the solvent and could be readily prepared into microspheres by electrospraying
(Figure 1). As the amine and hydroxyl groups in the chitosan chain could chelate with
palladium ion strongly, the rotational viscosity of chitosan solution (5 wt.%) increased to
860 mPa.S from 650 mPa.S after addition of PdCl2 and the diameter of electrosprayed
microspheres increased to 419.87 85.43 µm (Pd@50CM18) from 313.15 45.12 µm
(Pd-50CM18) (Figure 1A-D). When the concentration of PdCl2/chitosan mixture solution
decreased to 2.5 wt%, the rotational viscosity of PdCl2/chitosan mixture solution dropped
down to 140 mPa.S and the corresponding microspheres become flat with diameter of
313.15 45.12 µm (Figure 1E and 1F). Examination of Figure 1C and 1E demonstrated that
the chitosan concentration could influence the size and morphology of chitosan
microspheres significantly. For comparison, Pd@50CM00 with lager size (Average diameter
= 788.10 ±69.90 µm) was prepared by dropping the PdCl2/chitosan solution directly into
the NaOH solution without the appliance of high voltage (Figure 1G and 1H).

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9
(A)
Average diameter = 313.15
±
45.12 μm
(B)
8
7
6
5
4
3
2
1
0
200
220
240
260
280
300
320
m)
340
360
380
400
Diameter (
μ
5.0
(C)
(D)
Average diameter = 419.87
±
85.43 μm
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
200
250
300
350
400
m)
450
500
550
Diameter (
μ
10
8
Average diameter = 298.42
± 110.38 μm
(F)
(E)
6
4
2
0
100
150
200
250
300
350
400
450
500
Diameter (
μ
m)
6
5
4
3
2
1
0
Average diameter = 788.10
±
69.90
μ
m
(G)
(H)
600
650
700
750
800
m)
850
900
950
Diameter (
μ
Figure 1. SEM images and the corresponding microsphere diameter distributions of (A, B)
Pd-50CM18, (C, D) Pd50@CM18, (E, F) Pd@25CM18 and (G, H)
Pd@50CM00. (The scale bar is 1 mm.)

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The chemical structures of chitosan microspheres and Pd/chitosan microspheres were
characterized by the FT-IR spectra (Figure 2). After treated by glutaraldehyde, a new
absorption peak at 1664 cm-1 attributed to C=N (Schiff band) was observed and the intensity
of absorption bands at 2927 and 2868 cm-1 signed to –CH2- was obviously increased, which
demonstrated that the chitosan has been reacted with glutaraldehyde. After immobilization
of palladium species, the absorption peat at 1664 cm-1 was red shifted, indicating the
coordination between the C=N bond with the palladium species.
100
99
98
1664
1626
97
96
95
94
93
92
2868
2927
Chitosan microspheres
1637
(before glutaraldehyde cross-linking)
Chitosan microspheres
(after glutaraldehyde cross-linking)
Pd-50CM18
Pd@50CM18
4000 3600 3200 2800 1800 1600 1400 1200 1000
Wavenumber (cm-1)
800
Figure 2. FT-IR/ATR spectra of chitosan microspheres before and after glutaraldehyde
cross-linking, Pd-50CM18 and Pd@50CM18.
The swelling capacities of the microspheres were measured (Table 1). Due to the
cross-linking of chitosan chain by palladium species chelation with the surrounding amino
and hydroxyl groups, the swelling capacity of Pd@50CM18 was lower than that of
Pd-50CM18. Compared with the Pd@50CM18, the Pd@25CM18 was small and flat, so the
Pd@25CM18 should have larger surface area than Pd@50CM18. The larger surface area

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would facilitate the permeation of DMAc into the Pd@25CM18 more quickly and readily
while the Pd@50CM18 would took longer to be soaked. As a result, the swelling rate of
Pd@25CM18 was faster than that of Pd@50CM18. After immersed in DMAc for 5 hrs, the
swelling capacities of Pd@25CM18 and Pd@50CM18 were similar.
Table 1. Swelling capacities of Pd-50CM18, Pd@50CM18 and Pd@25CM18 in DMAc.
Time
(hrs)
2
Pd-50CM18
(g/g)
Pd@50CM18
(g/g)
Pd@25CM18
(g/g)
Entry
1
2
0.4784
0.2650
0.3196
5
0.5368
0.4209
0.4303
As the catalysts are used in relatively high temperature, the thermal stabilities are very
critical for their practical applications. Figure 3 showed that the cross-linking of chitosan
chain by palladium and glutaraldehyde could increase the thermal stabilities of chitosan
microspheres. As shown in Figure 2, DTG analysis showed that the chitosan microspheres
had one peak at 265 oC while the Pd@50CM18 before glutaraldehyde cross-linking had two
peaks at 265 and 287 oC, due to the partial cross-linking of chitosan by palladium. The peak
o
increased to 312 C after cross-linked by glutaraldehyde, indicating that the Pd@50CM18
would be stable in reaction temperature (110 oC).

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110
100
90
80
70
60
50
40
30
20
10
0
Chitosan microspheres
Pd@50CM18 (before glutaraldehyde cross-linking) 1.8
Pd@50CM18 (After glutaraldehyde cross-linking)
1.5
1.2
0.9
265
312
287
0.6
0.3
0.0
100
200
300
400
500
Temperature (oC)
Figure 3. Thermal analysis results of chitosan microspheres, Pd@50CM18 before and after
glutaraldehyde cross-linking.
The dispersion of palladium species on/in chitosan microspheres were analyzed by
XRD and HR-TEM (Figure 4 and 5). As shown in Figure 4, the absorption bands at 40o, 46o
and 68o were attributed to the face centered cubic (FCC) palladium, which were the (111),
(200) and (220) planes of palladium crystal (Figure 3).17 The palladium absorption peaks of
Pd@50CM18 was obviously weaker than those of Pd-50CM18, indicating that the
dispersion of palladium species in Pd@50CM18 was more homogeneous. Figure 5 clearly
showed that the palladium species aggregated on the surface of Pd-50CM18 while the
palladium species dispersed homogeneously in the Pd@50CM18 (Figure 5A and 5C).
Moreover, the size of palladium species in the Pd@50CM18 was much smaller than these in
the Pd-50CM18 (Figure 5B and 5D). As we know, high dispersion of palladium species
could ensure the catalysts with high activity. Figure 6 showed that the palladium 3d5/2
electron binding energy of Pd@50CM18 after reduction was about 1.49 eV lower than that

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before reduction, indicating the successful reduction of divalent Pd2+ into Pd0 species, which
also demonstrated that the palladium species in the chitosan microsphere were accessible.
6000
Pd@50CM18
Pd-50CM18
5000
4000
40
3000
2000
1000
0
46
68
0
10 20 30 40 50 60 70 80 90
(o)
2
θ
Figure 4. XRD patterns of Pd-50CM18 and Pd@50CM18.
(A)
Average diameter = 7.74
± 1.72 nm
12
10
8
(B)
6
4
2
0
4
5
6
7
8
9
10
11
Diameter (nm)
Average diameter = 2.99
30
(C)
±
0.81 nm
(D)
25
20
15
10
5
0
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Diameter (nm)
Figure 5. HR-TEM images and corresponding palladium particle diameter distributions of

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(A, B) Pd-50CM18 and (C, D) Pd@50CM18. (The scale bar is 100 nm).
400
395
336.45
390
385
380
375
370
365
360
355
350
345
340
335
After reduction
337.94
Before reduction
332
334
336
338
340
342
Binding energy (eV)
Figure 6. X-ray photoelectron spectra of the Pd@50CM18 before and after reduction.
Catalytic comparison of chitosan supported palladium microspheres
Mizoroki-Heck reaction is an efficient method to construct carbon(sp2)-carbon(sp2)
bond.26-28 In order to increase the collision probability of palladium species with substrates,
palladium species were usually anchored on the surface of solid matrices with larger
specific surface.6-9 With palladium species entrapped in solid matrices, it is widely believed
that the collision probability of palladium species with substrates would be greatly reduced
and the catalytic activity would be low. As shown in Figure 7, the yield of Mizoroki-Heck
reaction of iodobenzene with n-butyl acrylate was only ~6 % after 5 hrs with Pd@50CM00
as the catalyst. However, the Pd@50CM18 and Pd@25CM18 were very active to catalyze
the Mizoroki-Heck reaction and Pd@25CM18 was even more active than the surface
adsorbed palladium catalyst (Pd-50CM18). It can be found that the size of chitosan
microspheres of entrapped palladium catalysts could affect the catalytic activity obviously

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through the comparison of the catalytic activities of entrapped palladium catalysts. Thus, it
can be concluded that small size of chitosan microspheres could enhance the permeation of
reaction substrates into the chitosan microspheres and subsequently increase the collision
probability of palladium species in the chitosan microspheres with substrates. The hot
filtration experiment was carried out to verify the heterogeneity of Pd-50CM18 and
Pd@25CM18. The catalysts were removed from reaction mixtures after reacting for 60 min.
After this hot filtration, the filtrate did not exhibit any catalytic activity, indicating the
reactions were carried out in a heterogeneous way.29,30
Pd-50CM18
Pd@25CM18
100
Pd@50CM18
Pd@50CM00
Pd@25CM18
80
(hot filtration)
Pd-50CM18
(hot filtration)
60
40
20
0
0
50
100
150
200
250
300
Time (min)
.
Figure 7. Mizoroki-Heck reaction of iodobenzene with n-butyl acrylate as catalyzed by
Pd-50CM18, Pd@25CM18, Pd@50CM18 and Pd@50CM00.
Effect of catalyst loading on the Mizoroki-Heck cross-coupling reactions
The effect of catalyst loading on Mizoroki-Heck reaction of iodobenzene with n-butyl
acrylate was evaluated (Table 2). The yield increased gradually with the increment of
catalyst loading. Excellent yield was obtained by using 10 mg Pd@25CM18 with PhI/Pd

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ratio of 248/1. These results clearly demonstrated the Pd@25CM18 was a highly active and
efficient heterogeneous palladium catalyst for the Mizoroki-Heck reaction.
Table 2. The effect of added amount of catalyst on the yield of the reaction.a
Entry
Catalyst amount (mg)
PhI/Pd
2482
496
248
99
Yield (%)b
1
2
3
4
5
1
32
64
97
98
98
5
10
25
50
50
a
Reaction conditions: 0.7 mmol iodobenzene, 1.4 mmol n-butyl acrylate, and 5.25 mmol
triethylamine in 3.0 g DMAc at 110 oC for 5 hrs.
b
Cross-coupling yields, based on the amount of aromatic iodide, were determined from
GC/MS measurements.
Pd@25CM18 catalyzed Mizoroki-Heck reactions
Several representative aromatic iodides and olefins were employed to evaluate the catalytic
performance of Pd@25CM18. Reaction of aromatic iodides with n-butyl acrylate (Entry 1-9
in Table 3) shows that the Pd@25CM18 catalyzed system was tolerant for both electron-rich
and electron-deficient aromatic iodides to obtain the corresponding reaction products in
good yields. The aromatic iodides with electron-rich groups (Entry 1-5 in Table 3) acted

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more efficiently than these with electron-deficient groups (Entry 6-9 in Table 3). Due to the
steric hindrance, the position and size of groups attached on the aromatic iodides would
decrease its activity in Mizoroki-Heck cross-coupling (Entry 1, 2, 5-9 in Table 3). The
Pd@25CM18 was also efficient to catalyze the Mizoroki-Heck cross-coupling of
iodobenzene with mono-substituted olefins (Entry 10 and 11 in Table 3). However, the
related catalytic activity was relatively low for the cross-coupling reaction of bromobenzene
(Entry 12 in Table 3).
Table 3．Pd@25M18 catalyzed Mizoroki-Heck reaction of aromatic iodides with olefins.a
4 ‰ Pd@25M18
+
R2
I
R2
110 oC, 5 hrs
R1
R1
Entry
Aryl halides
2-FPhI
Vinylic substrates
n-butyl acrylate
n-butyl acrylate
n-butyl acrylate
n-butyl acrylate
n-butyl acrylate
n-butyl acrylate
n-butyl acrylate
n-butyl acrylate
n-butyl acrylate
n-methyl acrylate
Yield (%, trans)c
1
2
85
86
85
85
83
89
91
93
89
98
4-FPhI
3
4-ClPhI
4
4-BrPhI
4-NO2PhI
2-CH3PhI
3-CH3PhI
4-CH3PhI
4-CH3OPhI
PhI
5
6
7
8
9
10

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11
PhI
PhBr
CH2=CH-Ph
82
16
12b
n-butyl acrylate
a Reaction conditions: 0.7 mmol aromatic iodide, 1.4 mmol olefin, 10 mg Pd@25CM18 (Pd
content: 2.8 µmol), and 5.25 mmol triethylamine in 3.0 g DMAc at 110 oC for 5 hrs.
b The reaction time was 24 hrs.
c
Cross-coupling yields, based on the amount of aromatic iodide, were determined from
GC/MS measurements.
The reuse of chitosan supported palladium microspheres
Although chitosan could chelate with Pd2+ ion strongly, the chelating strength would be
greatly wreaked when Pd2+ ion was reduce into Pd0. Moreover, the harsh reaction condition,
such as high catalysis temperature, would also decrease the chelating strength of chitosan
chain with palladium species. Thus, it is very difficult to avoid the aggregates and leaching
of palladium in catalysis process for the palladium species adsorbed on the surface of solid
matrices. With palladium species entrapped in the chitosan microspheres, the palladium
species could be anchored by both the chelating groups of amine and hydroxyl groups and
cages formed by the cross-linked chitosan chain. Figure 8 showed that the catalytic activity
of Pd-50CM18 decreased at the 13th run while the Pd@25CM18 was still active at the 14th
run. The palladium content analysis in the recovered catalyst indicated that the palladium
leaching of Pd@25CM18 was obviously lower than that of Pd-25CM18. Thus, it is an
efficient way to decrease the palladium leaching and increase the reuse times by entrapping
palladium species in chitosan microspheres.

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100
90
80
70
60
50
40
30
3
2
1
0
Pd@25CM18
Pd-50CM18
0
2
4
6
8
10
12
14
16
Reuse times
Figure 8. The reuse and palladium leaching of Pd-50CM18 and Pd@25CM18.
Characterization of the recovered chitosan supported palladium microspheres
The recovered Pd-50CM18 and Pd@25CM18 after reused for 14 times were characterized
by SEM (Figure 9). It can be found that the Pd-50CM18 and Pd@25CM18 were broken into
small particles after 14 reuses. As the fresh Pd@25CM18 was flat, the break of
Pd@25CM18 was more seriously than that of Pd-50CM18, which might be an important
reason responsible for the palladium leaching of Pd@25CM18.

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10
Average diameter = 214.08
±
94.66 μm
(A)
(B)
8
6
4
2
0
50
100
150
200
250
300
350
400
450
Diameter (μm)
28
(C)
Average diameter = 86.83
±
46.85 μm
(D)
24
20
16
12
8
4
0
0
50
100
150
200
250
Diameter (μm)
Figure 9. SEM images and corresponding microsphere diameter distributions of (A, B)
Pd-50CM18 and (C, D) Pd@25CM18 after reused for 14 times. (The scale bar is 500 µm).
The dispersions of palladium species in the recovered Pd-50CM18 and Pd@25CM18
after reused for 14 times were characterized by HR-TEM (Figure 10). Compared with the
fresh Pd-50CM18 and Pd@25CM18 (Figure 5), the diameters of palladium particles were
obviously increased. However, the palladium particles in Pd@25CM18 were still much
smaller and dispersed much more homogeneously than those on Pd-50CM18.

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5
Average diameter = 27.17
± 5.32 nm
(A)
(B)
4
3
2
1
0
15
20
25
30
35
40
Diameter (nm)
6
Average diameter = 10.37
± 2.12 nm
(C)
(D)
5
4
3
2
1
0
6
7
8
9
10
11
12
13
14
15
Diameter (nm)
Figure 10. HR-TEM images and corresponding palladium particle diameter distributions of
(A, B) Pd-50CM18 and (C, D) Pd@25CM18 after reused for 14 times. (The
scale bar is 100 nm).
Catalysis occurring in the Pd@25CM18 catalyst
As shown above, the Pd@25CM18 catalyst is even more efficient and stable to catalyze the
Mizoroki-Heck reaction than the Pd-50CM18 catalyst. The high stability of Pd@25CM18
could be ascribed to the anchor of chelating amine groups and cross-linked chitosan chain,
but the high catalytic activity of Pd@25CM18 cannot only be attributed to the palladium
species on the surface of Pd@25CM18 because the majority of the palladium species were
located within the chitosan microspheres. So the majority of the Pd@25CM18 catalyzed
Mizoroki-Heck reaction should take place within the Pd@25CM18 like that inside the
cross-linking meshes of swollen IPNs.31 In order to verify this specification, p-iodoanisole,

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which was the largest substrate among the examined aromatic iodides, was used to examine
whether the substrate could permeate into the Pd@25CM18. As shown in Figure 11, iodine
element was detected within the Pd@25CM18, which demonstrated that the p-iodoanisole
has successfully permeated into the Pd@25CM18. As small diameter of chitosan
microspheres would make the permeation of substrates and departure of products
conveniently, the Pd@25CM18 is the most active catalyst to catalyze the Mizoroki-Heck
reaction.
Scanning area
Figure 11. SEM-EDX observation of Pd@25CM18.
Conclusions
In summary, a highly active and recyclable heterogeneous palladium catalyst (Pd@25CM18)
has been prepared by the entrapment of palladium species into the cross-linked chitosan
microspheres. This novel heterogeneous palladium catalyst has been demonstrated as an
efficient catalyst for the Mizoroki-Heck reaction of aromatic iodides with olefins. The high
catalytic activity of Pd@25CM18 has been attributed to the high dispersion of palladium

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species and small size of chitosan microspheres while the remarkable stability and
recyclability could be ascribed to the chetation of palladium with the abundant amine and
hydroxyl groups on the chitosan chain and the entrapment of palladium species in the
cross-linked chitosan chain. The high catalytic activity and stability make this unique
environmentally benign heterogeneous palladium catalyst to be attractive for large
industrial-scale applications.
Acknowledgment
The authors acknowledge the financial support from Zhejiang Province Natural Science
Foundation (LQ14E030003).
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