Thiol Functionalized Cross-Linked Chitosan Polymer Supporting Palladium for Oxidative Heck Reaction and Reduction of p-Nitrophenol

Article abstract of DOI:10.1007/s10562-017-2174-6

Abstract: A novel porous cross-linked chitosan polymer was prepared by Schiff reaction of chitosan (CS) with terephthalic aldehyde (TPA), followed by modification with trimethoxysilylpropanethiol. Such functional CS polymer (CS-TPA) could serve as both support and stabilizer for homogeneous palladium (Pd²⁺) catalyst. The obtained heterogeneous catalyst (Pd/CS-TPA-SH) exhibited high catalytic activity and selectivity for the oxidative Heck reaction of benzene and ethyl acrylate. The excellent performance is mostly attributed to the –SH groups stabilized Pd²⁺ species. Furthermore, Pd/CS-TPA-SH can also be used as the catalyst for the reduction of p-nitrophenol at room temperature, showing high efficiency and steady reusability. Graphical Abstract: A novel thiol functionalized cross-linked chitosan polymer supporting palladium is synthesized and proved to be a highly efficient catalyst for oxidative Heck reaction and nitrophenol reduction. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-017-2174-6

Catal Lett
DOI 10.1007/s10562-017-2174-6
Thiol Functionalized Cross-Linked Chitosan Polymer Supporting
Palladium for Oxidative Heck Reaction and Reduction
of p-Nitrophenol
1
1
1
1
1
Chenjun Zhang · Dan Lu · Pingping Jiang · Jingjing Li · Yan Leng
Received: 9 June 2017 / Accepted: 21 August 2017
©
Springer Science+Business Media, LLC 2017
Abstract A novel porous cross-linked chitosan polymer
was prepared by Schiff reaction of chitosan (CS) with tere-
phthalic aldehyde (TPA), followed by modification with
trimethoxysilylpropanethiol. Such functional CS polymer
(
CS-TPA) could serve as both support and stabilizer for
2
+
homogeneous palladium (Pd ) catalyst. The obtained het-
erogeneous catalyst (Pd/CS-TPA-SH) exhibited high cata-
lytic activity and selectivity for the oxidative Heck reaction
of benzene and ethyl acrylate. The excellent performance is
2
+
mostly attributed to the –SH groups stabilized Pd species.
Furthermore, Pd/CS-TPA-SH can also be used as the cata-
lyst for the reduction of p-nitrophenol at room temperature,
showing high efficiency and steady reusability.
Graphical Abstract A novel thiol functionalized cross-
linked chitosan polymer supporting palladium is synthesized
and proved to be a highly efficient catalyst for oxidative
Heck reaction and nitrophenol reduction.
Keywords Chitosan (CS) · Palladium ·
Functionalization · Oxidative Heck reaction · Nitrophenol
reduction
1
Introduction
Palladium (Pd)-catalyzed carbon–carbon (C–C) bond-form-
ing reactions based on C–H functionalization have attracted
significant attention owing to high atom economy and low
cost of reactants. In particular, oxidative Heck reactions are
of strategic importance in forming C–C bonds in synthetic
chemistry and industrial applications. In traditional [1–5],
the oxidative Heck reaction was usually performed under
homogeneous conditions. However, these reaction systems
often require expensive organic ligands to stabilize the
Pd catalyst and broaden the range of their reactivity, and
the recovery of unstable and potentially toxic Pd catalysts
*
Yan Leng
yanleng@jiangnan.edu.cn
1
The Key Laboratory of Synthetic and Biological Colloids,
Ministry of Education, School of Chemical and Material
Engineering, Jiangnan University, Wuxi 214122, China
Vol.:(0
1
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3

C. Zhang et al.
remains as a pressing issue [6, 7]. Considering the intrinsic
advantages of heterogeneous catalysts in catalyst recovery,
operation and reuse, tremendous efforts have been made
towards the development of heterogeneous Pd catalysts [8,
cross-linking technique for the production of porous cross-
linked CS with rigid structures.
Based on the properties of cross-linked CS and Pd, we
hypothesized that functionalized cross-linked CS could
not only support but also stabilize Pd well and catalyze
oxidative Heck reaction and 4-nitrophenol reduction effec-
tively. To the best of our knowledge, there have been no
reports about using thiol functionalized cross-linked CS as
a porous support to stabilize metal ions. The present study
is directed toward the development of porous cross-linked
CS polymer through Schiff base reaction of CS and tereph-
thalic aldehyde. Functional thiols (–SH) groups were then
covalently bonded to the CS by reacting with –OH sites.
The obtained functional cross-linked CS polymer was sub-
sequently employed as a support for Pd catalyst, and the
resulting solid catalyst were tested to catalyze oxidative
Heck reaction and 4-nitrophenol reduction.
9
]. The immobilization or entrapping of Pd on porous sup-
ports is a promising and commonly used method to prepare
heterogeneous Pd catalysts, and various porous materials,
such as silica [10], zeolites [11, 12], activated carbons [13],
metal–organic frameworks (MOFs) [14], and organic poly-
mer [15, 16], have been developed and used for supporting
the Pd catalysts. Nevertheless, these porous materials are
high-cost and their existing synthesis methods are usually
energy-consuming, time-demanding, and procedure tedious.
Therefore, developing an environmentally-friendly, easy-to-
make, and cost-effective porous materials for the preparation
of Pd-supported catalyst is highly desirable.
Recent developments for cleaner, sustainable chemistry
are being driven by a shift from petrochemical-based feed-
stocks to biological materials due to the limited availabil-
ity of fossil fuel and growing environmental concerns [17].
There is considerable interest in exploiting well-designed
natural macromolecular materials, and in particular those
of polysaccharides, to create high-performance and environ-
mentally friendly catalysts [18]. Chitosan (CS) is a particu-
larly attractive polysaccharide for application as a catalyst
support owing to its low cost, non-toxicity, biocompatibility,
and a great quantity of hydroxyl (–OH) and amine groups
2 Experimental Section
2.1 Materials and Characterization
Chitosan were purchased from the Chemical Reagent
Co., Ltd and used without any further purification. The
degree of deacetylation of chitosan (C804726-100g)
is higher than 95%. All the other chemicals (analytical
grade) were purchased from Aladdin Chemical Reagent
Co., Ltd. Fourier transform infrared (FT-IR) spectra were
recorder on a Nicolet 360 FT-IR instrument (KBr discs)
(
–NH ), which can be used for the further functionalization
2
as well as immobilization of metal complexes [19–24]. For
example, CS derivatives obtained by treatment with various
aldehydes have been prepared to enhance copper adsorp-
tion or to promote copper catalyzed Huisgen cycloadditions
−
1
in the 4000–400 cm region. Thermogravimetric (TG)
analysis was performed with a STA409 instrument a heat-
[
25]. Chitosan microspheres can be used as an efficient pal-
−
1
ladium carrier, and the surface functional groups enable the
modulation of Pd coordination and activity [26]. Apart from
catalysis, CS derivatives have been employed for extensive
applications in food [27], agriculture [28, 29], biomedi-
cal [30], biosensing [31], adsorption of heavy metals [32,
ing rate of 10 °C/min in nitrogen flow from 50 to 800 °C.
The nitrogen adsorption/desorption isotherms and pore
size distribution curves were measured at the temperature
of liquid nitrogen (77 K) using a BELSORP-MINI ana-
lyzer. Scanning electron microscopy (SEM) images were
performed on a SUPERSCAN SSX-550 electron micro-
scope. Transmission electron microscopy (TEM) images
were obtained with JEOL JEM-2100 electron microscope
operated at 200 kV. The amount of Pd in the catalyst was
measured by a Jarrell-Ash 1100 ICP-AES spectrometer.
X-ray photoelectron spectroscopy (XPS) was conducted
on a PHI 5000 Versa Probe X-ray photoelectron spectrom-
eter equipped with Al Karadiation (1486.6 eV). The C 1 s
peak at 285.2 eV was used as the reference for the bind-
ing energies. Field emission scanning electron microscope
(FESEM; Hitachi S-4800, accelerated voltage: 5 kV)
accompanied by Energy dispersive X-ray spectrometry
(EDS; accelerated voltage: 20 kV) was used to study the
morphology and the elements distribution.
3
3] and drug delivery [34]. However, chemical instability,
lower surface area, and lower dispersion ability are the major
issues associated with CS derivatives.
Cross-linked CS has become a new frontier of chitosan
derivatives. Many interesting properties are expected, such
as excellent mechanical property, good thermal stability,
increasing anticoagulant and low hydrophilicity [35]. Fur-
ther chemical modification may also improve the special
properties of CS. Surprisingly, although there has been a
worldwide realization that cross-linked CS can provide
promising raw materials for the development of hetero-
geneous catalysts [36], their use as catalyst supports is
still in its infancy. This is probably because of that most
of the existing cross-linked CSs are still nonporous with a
very low surface area. Thus, it is necessary to find efficient
1
3

Thiol Functionalized Cross-Linked Chitosan Polymer Supporting Palladium for Oxidative Heck…
2
2
1
.2 Preparation of Catalysts
(GC, SP-6890A) equipped with a FID detector and a capil-
lary column (SE-54 60 m×0.32 mm×0.25 μm). After the
reaction, the catalyst Pd/CS-TPA-SH was recovered by cen-
trifugation and washed with water and ethanol, and then was
oven dried and subsequently used for the next run.
.2.1 Synthesis of CS‑TPA
.0 g of CS was dissolved in 19 mL of 2.0% acetic acid
solution. Terephthalic aldehyde (TPA, 0.42 g) and sodium
bisulfite (0.65 g, as solubilizer) were dissolved in 8 mL
water, and the obtained solution was added into the above
CS solution. The mixture was vigorously stirred for 10 min
at room temperature, and the homogeneous hydrogel were
obtained. Then, the hydrogel was dried under vacuum
freeze-drying for 24 h and turned to white powder. The
white powder was washed with a large amount of deionized
water to neutral and then dried at 50 °C under vacuum for
2.3.2 4‑Nitrophenol Reduction
A 25 mL bottom flask was charged with 0.25 mL newly
prepared 0.5 M NaBH solution and 3 mL 0.17 mM 4-nitro-
4
phenol solution, followed by the addition of catalyst Pd/
CS-TPA-SH (0.02 g). The resulting solution was stirred at
30 °C for 30 s. The solid catalyst was filtered off, and the
liquid was determined by UV–Visible spectrometer (Hitachi
U-2900). The recovered catalyst was washed with deionized
water and dried at 50 °C under vacuum, and then used for
the next run.
1
2 h to give the resulting product CS-TPA (1.3 g, 91.5%
yield).
2
.2.2 Synthesis of CS‑TPA‑SH
For the surface modification of CS-TPA, CS-TPA (0.5 g) was
firstly dispersed in 15 mL toluene, and trimethoxysilylpro-
panethiol (5 mL) was added into it. The mixture was heated
to 110 °C and stirred for 24 h. On completion, the solid
product CS-TPA-SH was separated by filtration, washed with
toluene and ethanol, respectively, and dried at 100 °C under
3 Results and Discussion
3.1 Preparation and Characterization of Catalysts
The synthetic strategy adopted for the preparation of func-
tional cross-linked CS material is shown in Scheme 1. Ini-
tially, terephthalic aldehyde was chosen as the cross-linking
agent to react with the CS via Schiff base reaction to yield
insoluble cross-linked network CS-TPA. CS-TPA was sub-
sequently treated with trimethoxysilylpropanethiol to give
the functional cross-linked network CS-TPA-SH. The syn-
thesized CS-TPA-SH was then characterized by FT-IR and
TGA, and its surface area and pore size distribution were
determined by nitrogen adsorption–desorption analysis at
77 K.
vacuum. CS-TPA-NH was prepared in the same way by
2
modification of CS-TPA with aminopropyltrimethoxylsilane.
2
.2.3 Synthesis of Pd/CS‑TPA‑SH
The CS-TPA-SH (0.4 g), dichloromethane (20 mL), and pal-
ladium acetate (0.15 g) were added into a 25 mL round bot-
tom flask. After stirring at 50 °C for 24 h, the mixture was
cooled to room temperature, the solid product Pd/CS-TPA-
SH was collected by centrifugation and washed thoroughly
with deionized water and ethanol, respectively, then dried
The FT-IR spectra of CS, CS-TPA and CS-TPA-SH
are showed in Fig. 1. The characteristic bands for the sac-
charide structure of CS are appeared at the wavenumbers
overnight under vacuum. Pd/CS-TPA-NH and Pd/CS-TPA
2
−
1
−1
were prepared accordingly.
of 1594 cm (–NH groups) and 1150–1010 cm (anti-
2
symmetric stretching of the C–O–C bridge). For the cross-
−
1
2
.3 Catalytic Activity Measurements
linked CS-TPA, the –NH groups at 1594 cm of CS dis-
2
−
1
appeared, and the new bands at 1640 cm assigned to the
−
1
2
.3.1 Oxidative Heck Reaction
C=N group and 1409–1527 cm assigned to the stretching
vibrations of aromatic ring were observed for CS-TPA. After
the further treatment with trimethoxysilylpropanethiol, the
FT-IR spectrum of CS-TPA-SH shows characteristic bands
Typically, the catalyst Pd/CS-TPA-SH (0.04 g, 0.013 mmol%
based on Pd), AcOH (3.0 mL), ethyl acrylate (53 μL,
−
1
0
.5 mmol), benzene (1.8 mL, 20 mmol) and TBHP (76 μL)
of Si–O–Si (1049–1110 cm ). The above phenomenon
indicates the successful formation of –SH functionalized
cross-linked network. The thermal stability of CS-TPA and
CS-TPA-SH was analysed by TG-DTG, as also shown in
Fig. 1, the weight loss before 150 °C is plausibly attributed
to the loss of adsorbed and bound water, and the weight
loss between 150 and 700 °C mostly due to the decomposi-
tion of organic polymer groups. Notably, for CS-TPA-SH,
were added in to a Schlenk tube via a syringe. The resulting
mixture was stirred at 100 °C for 18 h, and then centrifuged.
The product was separated by column chromatography
1
method and characterized by H NMR (400 MHz, CDCl ),
3
δ = 7.69 (d, 1H, –CH), 7.50 (m, 2H, –CH), 7.35 (m, 3H,
–
3
CH), 6.43 (d, 1H, –CH), 4.27 (m, 2H, –CH ) and 1.33 (m,
2
H, –CH ). The filtrate was analyzed by gas chromatography
3
1
3

C. Zhang et al.
Scheme 1 Schematic illustration outlining the preparation and structure of Pd/CS-TPA-SH
B
0.01
.00
A
100
0
(a)
-0.01
-0.02
-0.03
-0.04
1
594
1
0801010
1
150
80
CS-TPA
CS-TPA-SH
o
1
66 C
(b)
1409
1
527
-0.05
1
640
100 200 300 400 500 600 700 800
1
110
60
o
1
049
Temperature ( C)
CS-TPA
CS-TPA-SH
(c)
40
4000
3500
3000
2500
2000
1500
1000
500
100
200
300
400
500
600
)
700
800
cm-1
)
Temperature
oC
(
Wavenumber
(
Fig. 1 A FT-IR spectra of a CS, b CS-TPA, and c CS-TPA-SH, B TG and DTG curves of CS-TPA and CS-TPA-SH
an obvious weight loss step at 166 °C, which is absent in the
1
1
1
80
60
40
DTG curve for CS-TPA is mainly attribute to the decomposi-
tion of –SH groups.
Figure 2 shows the N adsorption–desorption isotherm
2
and the corresponding pore size distribution of the CS-
120
00
TPA. It displays a typical IV type isotherm, giving a steep
1
increase at a relative pressure of 0.6<P/P <1, indicating
0
80
1
10
100
the presence of mesopores structure. The BET surface area
Pore diameter dp (nm)
2
60
40
of CS-TPA was measured to be 70.8 m /g, and the pore size
distribution shows a relative narrow pore size distribution
with the most probable pore size of 12 nm. However, after
modification with –SH, the BET surface area of CS-TPA-
20
0
2
SH significantly decreased to <2 m /g. This is presumably
0.0
0.2
0.4
0.6
0.8
1.0
because of that –SH groups blocked the porous channel of
the CS-TPA-SH. The morphological features of CS-TPA and
CS-TPA-SH were determined by SEM in Fig. 3a, b. CS-TPA
exhibits a rich coral-like morphology whereas CS-TPA-SH
Relative pressure
(
p/p
o)
Fig. 2 N sorption isotherm and BJH pore size distribution of CS-
2
TPA
1
3

Thiol Functionalized Cross-Linked Chitosan Polymer Supporting Palladium for Oxidative Heck…
species were observed. Previous reports have demonstrated
that –SH groups is capable of stabilizing the Pd(II) species
37]. Therefore, Pd ligated to polymer support may form
[
2
+
a strong coordinated Pd of SH-to-Pd in the catalyst Pd/
CS-TPA-SH. The TEM image and EDS elemental mapping
image in Fig. 4b reveal that Pd elements were uniformly
dispersed in the polymer support without any aggregation.
The above results indicate that the present catalyst Pd/CS-
TPA-SH may exhibit a great potential in catalyzing the C–H
olefination reactions due to its high content and dispersion
2
+
of Pd species.
3
.2 Catalytic Activity for Oxidative Heck Reaction
Fig. 3 SEM images of a CS-TPA, b CS-TPA-SH, and c EDS elemen-
tal mapping of S element for CS-TPA-SH
In recent year, oxidative coupling reactions are attracting
more and more interests owing to it provides various meth-
ods to carbon–carbon and carbon-heteroatom. The oxida-
tive Heck reaction is one of these reaction and converts
C–H bond to C–C bond under Pd(II) species. To assess the
catalytic performance of the prepared catalysts, catalytic
tests were first performed for the oxidative Heck reaction
of benzene with ethyl acrylate, and the results are shown in
Table 1. Pd/CS-TPA-SH led to a solid–liquid heterogeneous
catalysis, and exhibited a high yield of 85% with 97% selec-
tivity (entry 2). It was comparable to the activity of homoge-
reveals irregular fluffy structure. The EDS mapping image
for CS-TPA-SH validates the homogeneous distribution
of S element on the polymer framework (Fig. 3c). These
observations further suggest the successful grafting of –SH
groups onto the polymer framework of CS-TPA via covalent
attachment.
The above results indicate that CS-TPA-SH is a solid
porous material with highly cross-linked framework and
abundant functional –SH groups, which should enable effi-
cient metal complexation. Subsequently, the synthesized CS-
TPA-SH was used as a support for the immobilization of Pd
to offer the heterogeneous Pd catalyst (Pd/CS-TPA-SH) by
neous Pd(OAc) , which also showed 98% yield (entry 1). For
2
comparison, two type of control catalysts Pd/CS-TPA and
Pd/CS-TPA-NH were prepared and applied in this oxidative
2
immersion CS-TPA-SH in Pd(OAc) dichloromethane solu-
Heck reaction. However, the –SH free catalyst Pd/CS-TPA
showed only 65% yield and 90% selectivity (entry 3), and
the –NH functionalized catalyst Pd/CS-TPA-NH offered a
2
tion, followed by washing and drying. About 0.37 wt% Pd
content in the obtained catalyst was determined by ICP-AES.
According to the previous literature reports [37], Pd nano-
particles cannot catalyze the C–H olefination reactions, and
the activation of a C–H bond can only be initiated by Pd(II).
Furthermore, XPS analysis was carried out to demonstrate
the electron valence state of the Pd species. As illustrated
in Fig. 4a, the XPS spectrum of Pd/CS-TPA-SH presents
the Pd 3d peaks at 342.7 and 337.6 eV corresponding to
2
2
very low yield of 36% with 81% selectivity (entry 4). This
2+
catalytic activity order seems reasonably related to the Pd /
0
Pd species in the catalysts [37]. Subsequently, the catalyst
2
+
Pd/CS-TPA-SH was treated with NaBH to reduce Pd to
4
0
0
Pd . However, the obtained deep gray catalyst Pd /CS-TPA-
SH only gave a very low yield of 19%, which further con-
firms that the active sites for the oxidative Heck reaction
2
+
2+
0
2+
Pd 3d and Pd 3d , respectively, and there is no Pd
should be Pd species. We noticed that during the catalyst
5
/2
3/2
Fig. 4 a XPS spectra of Pd and
b TEM images of CS-TPA-SH.
Inset EDS elemental mapping
of Pd element
A
B
2+
337.6 eV
Pd
342.7 eV
350 348 346 344 342 340 338 336 334 332 330
Binding energy (eV)
1
3

C. Zhang et al.
Table 1 Catalytic effect of catalysts for the oxidative Heck reaction
of benzene and ethyl acrylate
Entry
Cat
Yield (%)
Selecivity (%)
1
2
3
4
5
Pd(OAc)2
Pd/CS-TPA-SH
Pd/CS-TPA
98
98
65
36
19
98
97
90
81
92
Pd/CS-TPA-NH
2
0
Pd /CS-TPA-SH
Fig. 5 Optimization of the substrates ratio and the reaction time. The
ratio showed in picture is molar ratio of benzene to ethyl acrylate,
AcOH as the solvent (3 mL), TBHP (76 μL), temperature (100 °C),
catalysts amount (0.04 g). The conversion and selectivity were deter-
mined by GC with dodecane as the internal standard
Reaction conditions: benzene (1.8 mL, 20 mmol), ethyl acrylate (53
μL 0.5 mmol), AcOH as the solvent (3 mL), TBHP (76 μL), tem-
perature (100 °C), time (18 h), catalysts amount (0.04 g, 0.002 g
Pd(OAc) was used for entry 1). The conversion and selectivity were
2
determined by GC with dodecane as the internal standard. Byproduct
was biphenyl
with 10–40 nm diameters could be evidently observed in
the TEM image of the recovered Pd/CS-TPA-SH (Fig. 6c).
From the above spectral results, one can conclude that the
preparation, upon immersion in Pd(OAc) dichloromethane
2
2
+
0
0
solution, the resulted catalyst Pd/CS-TPA-SH turned yellow,
transformation of Pd to Pd and the agglomeration of Pd
but the catalysts Pd/CS-TPA and Pd/CS-TPA-NH became
particles are the main reasons for the deactivation of the
recycled catalyst, which is in accordance with that of the
previous works [38, 39].
2
grey and dark grey, respectively. The above results indicate
that within the catalysts Pd/CS-TPA and Pd/CS-TPA-NH ,
2
0
2+
a small part of Pd nanoparticles are formed form Pd in
the presence of abundant –OH groups in CS-TPA and –NH
3.3 Catalytic Activity for Reduction of p-Nitrophenol
2
groups in CS-TPA-NH acting as gentle reducing agents,
2
whereas the strong coordination of thiol-to-Pd in CS-TPA-
Aromatic amines are important feedstocks for the prepa-
ration of pharmaceuticals, pigments, agrochemicals, and
other organic chemicals. It has been mostly obtained from
the nitroarenes by using reducing agents, such as sulfides
[40, 41], Fe [42, 43], and Zn [44, 45]. Recently, supported
metal catalysts have been demonstrated to be very economic
and effective for the reduction of nitroarenes because of high
catalytic activity and ease of recovery and reuse [46, 47]. We
found that the present catalyst Pd/CS-TPA-SH could also
be applied to the reduction of p-nitrophenol and exhibit
excellent performance. As can be seen in Fig. 7a, p-nitro-
2
+
SH enables the stabilization of Pd [29], which is respon-
sible for the high activity of Pd/CS-TPA-SH in the oxidative
Heck reaction. The influence of the substrates ratio and the
reaction time were investigated, and the results are compiled
in Fig. 5.
After the reaction, as the catalyst Pd/CS-TPA-SH can be
easily recovered by filtration, the reusability of the catalyst
was examined for subsequent five runs under the described
conditions (Fig. 6a). The Pd/CS-TPA-SH can be reused with
five times without significant decrease of catalytic activity.
In order to check the stability of Pd/CS-TPA-SH, ICP-AES,
TEM, and XPS were performed for the recycled catalyst.
ICP-AES elemental analysis result shows that there is almost
no detectable Pd species in the reaction medium, indicat-
ing the heterogeneous nature for the present catalyst. The
XPS investigation on the recovered Pd/CS-TPA-SH reveals
phenol (p-NP) aqueous solution with NaBH shows yellow
4
color, and its absorption peak at 400 nm assigned to p-NP
is observed in UV–Visible spectrum. When the catalyst Pd/
CS-TPA-SH was added into it, the absorption peak intensity
at 400 nm decreased as the reaction proceeded, and a new
absorption peak at 300 nm corresponding to p-aminophenol
(p-AP) appeared. The reaction was completed in only 30 s as
the absorption peak intensity at 400 nm and the yellow color
of the reaction mixture disappeared. In this reaction system,
2
+
four binding sites assignable to Pd (342.6 and 337.5 eV)
0
and Pd (340.9 and 335.7 eV) (Fig. 6b). This observation
2
+
indicates that 36% Pd species escape the strong coordi-
nation of SH-to-Pd during catalysis and are subsequently
due to the presence of NaBH , the catalytically active palla-
4
0
0
converted to naked Pd , which has no activity for the C–H
dium for the reduction of p-nitrophenol is most probably Pd
olefination reactions. Furthermore, aggregated Pd particles
species in Pd/CS-TPA-SH. A five recycling test in Fig. 7b
1
3

Thiol Functionalized Cross-Linked Chitosan Polymer Supporting Palladium for Oxidative Heck…
Fig. 6 Reusability of Pd/CS-
TPA-SH in the oxidative Heck
reaction. Reaction conditions:
benzene (1.8 mL, 20 mmol),
ethyl acrylate (53 μL 0.5 mmol),
AcOH as the solvent (3 mL),
TBHP (76 μL), temperature
(
100 °C), time (18 h), catalysts
amount (0.04 g). b TEM image
and c Pd 3d XPS spectra of the
reused Pd/CS-TPA-SH
NO2
NH2
Cat
Room temperature
HO
HO
7
6
5
4
3
2
1
0
A
B
0
0
0
0
0
0
0
0
5
1
3
6
s
0 s
0 s
0 s
300
400
500
600
1
2
3
4
5
λ(nm)
Run
Fig. 7 a Time dependent UV–Visible spectra of 4-nitrophenol reduc-
tion of Pd/CS-TPA-SH, [inset shows an optical image of color change
from 4-nitrophenol (yellow) to 4-aminophenol (colorless)]. The pre-
sented reaction was studied using the Pd/CS-TPA-SH (0.02 g) at a
temperature of 30 °C. b Recycling runs for the reduction of 4-nitro-
phenol by Pd/CS-TPA-SH
1
3

C. Zhang et al.
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In summary, we have developed a novel porous cross-linked
CS bearing –SH groups as a support of Pd(II) catalyst. The
resulting catalyst Pd/CS-TPA-SH led to the heterogeneous
oxidative Heck reaction of benzene and ethyl acrylate with
outstanding catalytic activity and selectivity. The results
of characterizations have demonstrated that the Pd(II) spe-
cies stabilized by –SH groups should be responsible for
the excellent catalytic performance of Pd/CS-TPA-SH. In
addition, the developed catalyst could also be used for the
reduction of p-nitrophenol at room temperature, presenting
high efficiency and ready reusability. This work presents a
novel strategy of the utilization of CS for the developing of
functional porous material which can further be used for
immobilization of metal catalysts, offering a new generation
of heterogeneous catalysts for the organic synthesis industry.
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Funds for the Central Universities (JUSRP51623A) and MOE and
SAFEA for the 111 Project (No. B13025).
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