Coupling reactions of aromatic halides with palladium catalyst immobilized on poly(vinyl alcohol) nanofiber mats

Article abstract of DOI:10.1016/j.apcata.2011.11.018

Nanoporous poly(vinyl alcohol) (PVA) nanofiber mats prepared by means of electrospinning have been used for the immobilization of palladium catalyst. Thermal treatment of the palladium-loaded PVA nanofiber mats results in the cross-linking of the matrix PVA molecules as well as the reduction of the divalent palladium (Pd²⁺) into zerovalent palladium (Pd⁰) species. The palladium oxidation states were examined by X-ray photoelectron spectroscopic (XPS) analysis. The PVA nanofiber morphology was characterized by scanning electron microscopy (SEM). The catalytic activity and recyclability of the prepared heterogeneous palladium catalysts have been evaluated for the Ullmann, Heck-Mizoroki and Sonogashira coupling reactions of aromatic halides. The large structure of the Pd/PVA nanofiber mats can greatly facilitate its separation and recycling, and the high catalytic activity and stability of the prepared Pd/PVA nanofiber mats have been attributed to the chelation of palladium species with the abundant hydroxyl functional groups on the PVA matrix surface area.

Full text of DOI:10.1016/j.apcata.2011.11.018

Applied Catalysis A: General 413–414 (2012) 267–272
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Coupling reactions of aromatic halides with palladium catalyst immobilized on
poly(vinyl alcohol) nanofiber mats
Linjun Shao, Weixin Ji, Pengdu Dong, Minfeng Zeng, Chenze Qi∗, Xian-Man Zhang∗
Institute of Applied Chemistry and Chemistry Department, Shaoxing University, Shaoxing, Zhejiang Province 312000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Nanoporous poly(vinyl alcohol) (PVA) nanofiber mats prepared by means of electrospinning have been
used for the immobilization of palladium catalyst. Thermal treatment of the palladium-loaded PVA
nanofiber mats results in the cross-linking of the matrix PVA molecules as well as the reduction of
the divalent palladium (Pd2+) into zerovalent palladium (Pd0) species. The palladium oxidation states
were examined by X-ray photoelectron spectroscopic (XPS) analysis. The PVA nanofiber morphology
was characterized by scanning electron microscopy (SEM). The catalytic activity and recyclability of the
prepared heterogeneous palladium catalysts have been evaluated for the Ullmann, Heck–Mizoroki and
Sonogashira coupling reactions of aromatic halides. The large structure of the Pd/PVA nanofiber mats can
greatly facilitate its separation and recycling, and the high catalytic activity and stability of the prepared
Pd/PVA nanofiber mats have been attributed to the chelation of palladium species with the abundant
hydroxyl functional groups on the PVA matrix surface area.
Received 1 October 2011
Received in revised form
12 November 2011
Accepted 15 November 2011
Available online 23 November 2011
Keywords:
Poly(vinyl alcohol) (PVA) fiber mats
Heterogeneous palladium catalyst
Ullmann homocoupling
Heck–Mizoroki and Sonogashira
cross-couplings
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
reactions can take place only on the heterogeneous surface, rather
than all catalytic centers are accessible in the homogeneous system.
The importance of palladium catalyst in synthetic organic chem-
istry can hardly be overestimated because palladium catalysis has
become a routine tool for the preparation of fine chemicals and
pharmaceutically active compounds in both academic studies and
industrial productions [1]. However, the majority of the palla-
dium catalysis is performed in the homogeneous system, in which
the catalyst suffers from the difficult separation from the reaction
media, nonreusability and deactivation due to aggregation of pal-
ladium particles [2]. Moreover for the homogeneous catalysis, the
final products are frequently contaminated by residual transition
metal at an unacceptable level, which could be difficult to remove,
especially for large scale productions of pharmaceuticals where
metal contamination is closely monitored [3]. Thus, it is not sur-
prising why relatively few homogeneous catalysis systems have
been commercialized [4].
Therefore, the solid matrices are generally fabricated into small
fine particles to maximize the surface area to enhance the catalytic
activities. However, the separation and recycling of the particu-
late catalysts from the reaction media are usually a tedious and
time-consuming process due to the high pressure drop by filtration.
On the other hand, the palladium leaching is a common problem
for the heterogeneous palladium catalysts [6,7]. Surface derivati-
zation of the solid matrices with special ligand functional groups
including phosphine and N-heterocyclic carbine could improve the
chelation with the transition metal species to minimize the leach-
ing, but the ligand synthesis as well as the surface modification
are usually very challenging [2,8,9]. Nevertheless, only few low-
leaching and ligand-free heterogeneous palladium catalysts have
been reported in the literature [10]. Recently, we have demon-
strated that the ground powder of the natural pearl shells is an
excellent solid support for the immobilization of the palladium
catalyst [11]. The high stability of the heterogeneous palladium cat-
alyst has been attributed to the chelation of palladium species with
the abundant hydroxyl and amino functional groups of the shell
powder surface area. These results promoted us to explore the inex-
pensive, non-toxic and biodegradable poly(vinyl alcohol) (PVA) as
the supported matrix to immobilize palladium catalyst because
PVA has plentiful surface hydroxyl functional groups. To the best of
our knowledge, electrospun PVA nanofiber mats have not yet been
used for the preparation of the heterogeneous palladium catalysts.
Catalyst reuse can greatly increase the overall productivity and
cost effectiveness while minimizing both waste generation and
contamination of the metal traces in the final products, resulting
in a greener and more sustainable chemical transformation process
[5]. But the heterogenization of a homogeneous catalysis will signif-
icantly reduce the accessible catalytic centers because the catalytic
∗
Corresponding authors. Fax: +86 575 8834 5682.
E-mail addresses: qichenze@usx.edu.cn (C. Qi), xian-man.zhang@usx.edu.cn
(X.-M. Zhang).
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.11.018

268
L. Shao et al. / Applied Catalysis A: General 413–414 (2012) 267–272
Fig. 1. Scanning electron microscopic (SEM) images of the electrospun nanofiber mats prepared using different PVA weight concentrations: (a) 3.0%, (b) 5.0%, (c) 6.0% and
(d) 7.0%.
2. Results and discussion
because the nanofiber diameter (148 71 nm) of the mats obtained
using 6.0% PVA weight concentration is significantly smaller than
that (504 290 nm) obtained using 7.0% PVA weight concentration
(Fig. 1).
2.1. Preparation and characterization of heterogeneous Pd/PVA
nanofiber mats
Palladium deposition was achieved by dissolution of PVA poly-
mer substrate in disodium tetrachloropalladate (Na2PdCl4) acetic
acid solution, followed by electrospinning. Many experimental
methods have been developed for reduction of the divalent Pd2+
precatalyst into zerovalent palladium (Pd0) active species in liter-
ature [18]. We have shown that the divalent Pd2+ species can be
readily reduced into the active Pd0 species in alcoholic media [19].
Thus, we believed that the precatalyst divalent Pd2+ species should
be reduced by the hydroxyl functional groups of the solid matrix
PVA molecules. Indeed, characteristic color change was observed
from brown to black during a thermal treatment at the temper-
ature over 150◦ C. The color change is clearly associated with the
formation of the zerovalent palladium (Pd0) active species (black),
i.e. Pd/PVA nanofiber mats. The reduction of the divalent Pd2+ into
Pd0 species has further been confirmed by the XPS analysis as
shown in Fig. 2, which evidently indicates that the thermal treat-
ment decreases the palladium electron binding energy (3d5/2 and
3d3/2) by ∼1.1 eV. These observations are consistent with the char-
acteristic electron binding energy shift reported in the literature for
the reduction of the divalent palladium into the metallic palladium
species [20,21].
Although PVA can strongly chelate with the transition metal
species because of the numerous surface hydroxyl functional
groups, it is very difficult to fabricate it into suitable solid matrix
with a large surface area due to the semi-crystalline structure and
high melting point (230 ◦C) [12]. Electrospinning is a versatile poly-
mer processing technique using a high electrical charge to draw
ultrafine nanofibers with diameters in the nanometer to submi-
crometer range. The resultant porous nanofiber mats have very
high surface area to volume ratios, resulting in a wide range of
applications such as in water purifications, medical tissue repair,
drug delivery, etc. [13,14]. The large structure of the nanofiber mats
supported catalyst also has the advantage of much easier separa-
tion and recycling over the fine particles supported catalyst from
the reaction media.
A series of PVA samples with different concentrations have been
prepared to examine the nanofiber morphology by the optimiza-
tion of the electrospinning conditions including electrical voltage,
tip-target distance, flow rate and polymer substrate concentra-
tion [12,15]. The scanning electron microscopy (SEM) images of
the yielded PVA nanofiber mats are shown in Fig. 1. Spindle-like
beads (Fig. 1a and b) were observed for the nanofiber mats prepared
using 3.0 and 5.0% of the PVA weight concentrations. The bead for-
mation could be attributed to the inefficient chain entanglement
at relatively low PVA substrate concentrations [16]. Droplets were
obtained instead of nanofiber mats regardless of the electrospin-
ning voltage when over 8.0% PVA weight concentration was used.
Interestingly, a uniform and bead-free structure of the nanofiber
mats could be achieved when the PVA weight concentration was
at 6.0 or 7.0% [17]. The PVA weight (6.0%) concentration was cho-
sen for the subsequent preparation of the PVA nanofiber mats
The reduction of the divalent Pd2+ species must be concomi-
tantly coupled with the oxidation of the PVA matrix molecules.
The FT-IR spectra (Fig. 3) indicate that the carbonyl absorp-
tion peak at ∼1733 cm−1 is significantly increased during the
reduction of the divalent Pd2+ species into the zerovalent palla-
dium active species, suggesting that some of the PVA hydroxyl
groups have been oxidized into the carbonyl functional groups
as shown in Scheme 1. X-ray diffraction (XRD) analysis (not
shown) indicates that there is no characteristic diffraction peak
at 40◦ for the metallic palladium, suggesting that the zerovalent

L. Shao et al. / Applied Catalysis A: General 413–414 (2012) 267–272
269
Pd3d5/2
337.47
28
21
14
7
a
Average fiber diameter = 247.9
114.4
Pd3d3/2
342.79
0
0.00
338.6
0.15
0.30
0.45
0.60
0.75
0.90
343.9
28
21
14
7
b
b
a
Average fiber diameter = 198.4 94.3 nm
0
0.00
0.15
0.30
0.45
0.60
0.75
0.90
330
332
334
336
338
340
342
344
346
348
350
Binding energy (eV)
Fig. 4. Nanofiber diameter distributions for (a) precatalyst Na2PdCl4/PVA nanofiber
mats and (b) Pd/PVA nanofiber mats.
Fig. 2. X-ray photoelectron spectra (XPS): (a) precatalyst Na2PdCl4/PVA nanofiber
mats and (b) Pd/PVA nanofiber mats.
cross-linking of the PVA nanofiber molecules and the reduction of
the divalent Pd2+ into zerovalent Pd0 species.
The SEM images (Fig. S1 of the supplementary material) show
that the fiber diameters of the Pd/PVA are considerably smaller than
those of the precatalyst Na2PdCl4/PVA nanofiber mats. The fiber
diameter distributions for the Na2PdCl4/PVA and Pd/PVA nanofiber
mats can be obtained from their SEM images and the results are
summarized in Fig. 4. The shrinking of the mats fiber diameter by
the thermal treatment can be attributed to the partial dehydration
of the hydroxyl functional groups among the PVA polymer chains,
resulting in the cross-linking of the PVA nanofiber mats. Ther-
mogravity analysis (TGA) (Fig. S2 of the supplementary material)
shows that the weight loss from the dehydration is not signifi-
cant up to 250 ◦C for the palladium-loaded PVA nanofiber mats,
indicating that the oxidative degradation of the PVA polymer bone
structures should be negligible for all of the applications performed
in this study.
1733
1073
1091
b
a
750
1000
1250
1500
1750
3000
3500
4000
2.2. Reductive homocoupling of aromatic iodides as catalyzed by
Pd/PVA nanofiber mats
Wavelength (nm)
Fig. 3. FT-IR/ATR spectra: (a) PVA nanofiber mats and (b) Pd/PVA nanofiber mats.
Symmetrical biaryls are important intermediates in organic
chemistry, but they are traditionally synthesized by the copper-
mediated Ullmann reductive homocoupling [25]. The Ullmann
reactions generally require drastic reaction conditions such as the
high temperatures (over 200 ◦C) together with the consumption
of a stoichiometric amount of copper metal into toxic copper
halide waste [26]. The palladium catalyzed reductive homocou-
pling has been demonstrated as a much more convenient and
efficient method for the biaryl synthesis [27].
Scheme 1.
The Pd/PVA heterogeneous catalyst was first tested and opti-
mized for the model reductive homocoupling of iodobenzene in
DMSO solution. The initial reaction conditions were adapted from
the parameters optimized for the homogeneous and heterogeneous
catalytic systems [19,28]. While cesium fluoride (CsF) is the most
efficient base [19,28], potassium acetate (KOAc) is the most efficient
base among the screened bases including CsF, acetates, carbon-
ates, bicarbonates and hydroxides for the Pd/PVA nanofiber mats
catalysis in DMSO solution.
Several representative solvent systems have been examined for
the Pd/PVA nanofiber mats catalyzed homocoupling of iodoben-
zene and the results are summarized in Table 1. Surprisingly, no
biphenyl product was observed when carried out in the aqueous
solution (entry 1). Entry 6 of Table 1 shows that the reductive
homocoupling of iodobenzene gives the complete conversion of
palladium metal dispersed on the PVA nanofiber mats has amor-
phous structure [6].
PVA polymers are generally cross-linked through chemical reac-
tions with bifunctional reagents such as glutaraldehyde [22] or
glyoxal [23] to minimize dissolution for the applications in polar
solvents because the cross-linking can significantly reduce the
intrinsic solubility and dissolution of polymeric materials [22–24].
Interestingly, the PVA molecules were found to be cross-linked
during the thermal treatment for the reduction of the precatalyst
Na2PdCl4/PVA nanofiber mats. The PVA nanofiber mats were still
noticeably swollen in dimethyl sulfoxide (DMSO) solution after
being treated at 150 ◦C for 1 h, whereas they were over cross-
linked after being treated at 210 ◦C for 1 h. Thus, the precatalyst
Na2PdCl4/PVA nanofiber mats were heated at 170 ◦C for 1 h for the

270
L. Shao et al. / Applied Catalysis A: General 413–414 (2012) 267–272
Table 1
Table 3
Solvent effects on the Pd/PVA nanofiber mats catalyzed reductive homocoupling of
Heck–Mizoroki cross-coupling reactions of various aromatic halides with different
iodobenzenea.
acrylatesa.
R1
Cat.(2 mol%)/3eq KOAc
DMSO, 110 oC
Entry
Solvent
Conversionb (%)
Yield (%)b
Biphenyl
X
R1
+
Benzene
1
2
3
4
5
6
7
Waterc
Xylene
Ethanol
3-Pentanol
DMF
∼0
0.82
19.7
54.6
29.5
100
100
∼0
∼0
Entry
Aryl halides
Vinylic substrates
Time (h)
Yield (%) (cis:trans)b
0.8
0
1
2
3
4
5
6
7
C6H5I
n-Butyl acrylate
n-Butyl acrylate
n-Butyl acrylate
n-Butyl acrylate
n-Butyl acrylate
Methyl acrylate
n-Butyl acrylate
2.0
2.0
2.0
2.0
4.0
4.0
2.0
36
97.5 (0.4:99.6)
94.6 (2.5:97.5)
97.0 (1.2:98.8)
97.5 (0.2:99.8)
94.5 (0.5:99.5)
97.1 (0.3:99.7)
1.6 (0:100)
5.8
2.8
27.6
87.0
91.0
13.9
51.8
1.7
10.9
9.0
3-FC6H4I
4-FC6H4I
4-BrC6H4I
4-CH3C6H4I
C6H5I
DMSO
Xylene + DMSOd
a
Reaction conditions: 1.0 mmol iodobenzene, 0.05 mmol Pd/PVA nanofiber mats
catalyst, 7.5 mmol potassium acetate in 5.0 ml of solvent at 110 ◦C for 7.0 h unless
otherwise indicated.
C6H5Br
14.3 (0:100)
a
Reaction conditions: 1.0 mmol aromatic halide, 2.0 mmol acrylate, 0.02 mmol
Pd/PVA nanofiber mats, 3.0 mmol potassium acetate in 5.0 ml DMSO at 110 ◦C.
b
Iodobenzene conversion, biphenyl and benzene yields were determined from
the GC/MS measurements based on the amount of iodobenzene.
b
Cross-coupling yields were determined from the GC/MS measurements based
c
At 100 ◦C for 10 h.
Volume ratio of xylene and DMSO is 1:1.
on the amount of aromatic halide substrate.
d
Table 2
2.3. Heck–Mizoroki cross-coupling as catalyzed by Pd/PVA
nanofiber mats
Pd/PVA nanofiber mats catalyzed reductive homocoupling of various substituted
aromatic halidesa.
Pd/PVA5 mol%
X
Heck–Mizoroki reaction is the palladium catalyzed cross-
coupling of aromatic halides with alkenes, which allows an
E-selective carbon-carbon bond formation between sp2 hybridized
carbons [29]. The reaction has been considered as one of the most
powerful and straightforward methods for the construction of the
sp2–sp2 carbon–carbon bonds [30]. The exceptional attractiveness
of the Heck–Mizoroki reaction for the organic chemical synthe-
sis of substituted alkenes has stimulated tremendous interest in
searching for more environmentally benign and ligand-free het-
erogeneous catalysts [31].
KOAc, DMSO/xylene
R
R
R
Entry
Substrate
C6H5I
Yield b (%)
1
2
3
4
5
6
7
8
94.0
80.8
93.3
91.4
90.1
95.3
97.6
17.4d
2-CH3C6H4I
4-CH3C6H4I
2-ClC6H4I
3-ClC6H4I
4-FC6H4I
2-IC5H4Nc
C6H5Br
The prepared Pd/PVA nanofiber mats have been examined to
catalyze the Heck–Mizoroki cross-coupling reactions of several
representative aromatic halides with n-butyl acrylate or methyl
acrylate in DMSO solution at 110 ◦C. Examination of Table 3
shows that the Pd/PVA nanofiber mats catalysis affords excellent
cross-coupling yields (>95%) and selectivity (>97%, trans-products)
in 2–4 h, indicating that the prepared Pd/PVA nanofiber mats
are an efficient, selective and specific heterogeneous catalyst for
the Heck–Mizoroki cross-coupling reactions of aromatic iodides
with acrylates. However, the related catalytic activity is relatively
low for the cross-coupling reaction of bromobenzene (entry 7 of
Table 3).
a
Reaction conditions: 1.0 mmol aromatic halide, 0.05 mmol Pd/PVA nanofiber
mats, 7.5 mmol potassium acetate in 5.0 ml of DMSO/xylene (v:v = 1:1) at 110 ◦
C
in 7 h, unless otherwise indicated.
b
Biaryl yields were determined from the GC/MS measurements based on the
amount of aromatic halide substrate.
c
2-Iodopyridine.
In 40 h.
d
iodobenzene and 87% biphenyl yield in DMSO in about 7 h, whereas
the biphenyl yield is only 27% when carried out in DMF under the
same conditions (entry 5). Dehalogenation is the primary pathway
for the Pd/PVA nanofiber mats catalyzed reduction of iodobenzene
in ethanol (entry 3) and 3-pentanol (entry 4), which are similar
to the results observed for the homogeneous palladium catalysis
[19]. Although the biphenyl product is negligible in xylene (entry
2), addition of xylene into DMSO solution does not suppress the
homocoupling pathway, but instead slightly improves it (entry 7).
These observations are further supported by the time-courses of
the biphenyl and benzene yields as shown in Fig. S3 of the sup-
plementary material. Addition of the less polar xylene could also
reduce the swelling of the PVA nanofiber mats especially for the
relatively slow homocoupling reactions in DMSO.
Table 2 summarizes that the optimized Pd/PVA nanofiber mat
catalysis is also applicable to other aromatic iodides although
the homocoupling biphenyl yield is very low for bromobenzene.
The new palladium catalytic system works well for the reductive
homocoupling of the aromatic iodides bearing either an electron
donating or an electron accepting substituent. The relatively lower
homocoupling yield of 2-methyliodobeneze could be attributed to
the steric hindrance of the o-methyl group (entry 2), but the steric
effects seem to be less severe for the relatively small chlorine (entry
4). Excellent biaryl yield was also obtained for the Pd/PVA nanofiber
mats catalyzed reductive homocoupling of 2-iodopyridine (entry
7).
2.4. Sonogashira cross-coupling as catalyzed by Pd/PVA nanofiber
mats
In view of the high catalytic activity and selectivity for the
Heck–Mizoroki cross-coupling with aromatic iodides (Table 3),
we extended our interests to explore the Pd/PVA nanofiber mats
to catalyze the Sonogashira cross-coupling because both the
Heck–Mizoroki and Sonogashira reactions involve the palladium
catalyzed cross-coupling of aromatic halides with unsaturated
hydrocarbon. The Sonogashira reaction is known as the most pow-
erful and straightforward method for the construction of the sp2–sp
carbon–carbon bonds [32]. The prepared Pd/PVA nanofiber mats
have been examined for the Sonogashira reaction of phenylacety-
lene with several representative aromatic halides. Although the
cross-coupling yield is relatively low for bromobenzene (entry 6
of Table 4), the Pd/PVA nanofiber mats catalyzed cross-coupling
reactions afford excellent cross-coupling yields (88–99%) for the
aromatic iodides in DMSO solution, indicating that the prepared
Pd/PVA nanofiber mats are also an efficient heterogeneous catalyst
for the Sonogashira cross-coupling reaction of aromatic iodides.

L. Shao et al. / Applied Catalysis A: General 413–414 (2012) 267–272
271
Table 4
Table 5
Pd/PVA nanofiber mats catalyzed reductive Sonogashira cross-coupling reactions
of various aromatic halides with phenylacetylenea. .
Pd/PVA 2 mol%
Heck–Mizoroki cross-coupling reactions of iodobenzene with n-butyl acrylate as
catalyzed by different palladium catalystsa.
Entry
Palladium catalyst
Yieldb (%)
X
+
Base, solvent
R
R
1
2
3
4
PVA/PdCl2 nanofiber mats
PdCl2
Pd(PPh3)2Cl2
Pd(dppf)Cl2
93.4
73.4
93.2
99.0
Entry
Substrate
Yieldb (%)
1
2
3
4
5
6
C6H5I
99.5
95.6
99.0
88.1
88.3
35.0c
a
Reaction conditions: 1.0 mmol iodobenzene, 2.0 mmol n-butyl acrylate,
3.0 mmol potassium acetate and 0.02 mmol palladium catalyst in 5.0 ml DMSO at
110 ◦C for 0.5 h.
2-CH3C6H4I
4-CH3C6H4I
4-FC6H4I
2-ClC6H4I
C6H5Br
b
Cross-coupling yields as determined from the GC/MS measurements based on
the amount of iodobenzene.
a
Reaction conditions: 1.0 mmol aromatic halide, 1.2 mmol phenylacetylene,
chelation of palladium species with the abundant hydroxyl groups
on the PVA matrix surface.
7.5 mmol potassium acetate, 0.02 mmol Pd/PVA nanofiber mats in 5.0 ml DMSO at
110 ◦C in 4 h, unless otherwise indicates.
b
Cross-coupling yields were determined from the GC/MS measurements based
on the amount of aromatic halide substrate.
2.6. Comparison of heterogeneous and homogeneous palladium
catalytic activities
c
In 9 h.
Heterogeneous catalysis is generally less active than the
corresponding homogeneous one primarily due to the limited
surface-bound catalytic centers [30]. Several commonly used and
commercially available homogeneous palladium catalysts together
with the prepared Pd/PVA nanofiber mats have been tested to cat-
alyze the Heck–Mizoroki cross-coupling reaction of iodobenzene
with n-butyl acrylate under the same reaction conditions. Exam-
ination of Table 5 shows that the PVA nanofiber mats supported
palladium catalyst has comparable catalytic activity with these
commonly used homogeneous palladium catalysts, suggesting that
the chelation of palladium species with the abundant hydroxyl
functional groups on the PVA matrix surface may enhance the
catalytic activities to compensate the less surface-bound catalytic
centers.
2.5. Separation and recyclability of the Pd/PVA nanofiber mats
catalyst
The heterogeneous catalysts have the advantage for their rel-
atively easy separation and recycle of the expensive transitional
metals. The prepared Pd/PVA nanofiber mats not only have large
structure for easier separation, but also have excellent stabil-
ity as shown for the Ullmann homocoupling and Heck–Mizoroki
cross-coupling reactions. The catalytic activities of the recovered
Pd/PVA nanofiber mats retained over five cycles for the Ullmann
homocoupling of iodobenzene. Even more recycling times with-
out loss of the catalytic activities have been obtained for the
Pd/PVA nanofiber mats catalyzed Heck–Mizoroki cross-coupling of
iodobenzene with n-butyl acrylate as shown in Fig. 5. The palladium
contents have been determined to be 0.276 0.003, 0.223 0.001
and 0.164 0.002 g for each gram of the fresh catalyst, the recov-
ered catalysts after three and six recycles respectively, indicating
that the average palladium leaching is about 6.5% for each cycle of
application. These results clearly demonstrate that the palladium
leaching from the Pd/PVA nanofiber mats is not significant dur-
ing the reaction and washing process. The remarkable stability of
the prepared Pd/PVA nanofiber mats must be associated with the
3. Conclusions
In summary, a highly active and recyclable heterogeneous pal-
ladium catalyst (Pd/PVA nanofiber mats) has been prepared by
the immobilization of palladium species onto the cross-linked
poly(vinyl alcohol) nanofiber mats, which are not only insoluble in
most organic solvents and water, but can also function as a reduc-
ing agent and stabilizer. The prepared Pd/PVA nanofiber mats have
been demonstrated as an efficient and selective heterogeneous cat-
alyst for the reductive Ullmann homocoupling, the Heck–Mizoroki
cross-coupling and Sonogashira cross-coupling of aromatic halides.
The large structure of the nanofiber mats enables easier separation
and recycling of the heterogeneous catalyst than the fine pow-
ders supported catalysts from the reaction media. The remarkable
stability and recyclability of the prepared Pd/PVA nanofiber mats
have been attributed to the chelation of palladium species with the
abundant hydroxyl functional groups on the PVA matrix surface.
The high catalytic activities combined with the easier separation
and recycle make this unique environmentally benign heteroge-
neous palladium catalyst to be attractive for large industrial-scale
applications.
100
80
60
40
20
0
4. Experimental
4.1. General remarks
1
2
3
4
5
6
7
8
9
Recylce Runs
Poly(vinyl alcohol) (Mn = 77,000) and xylene were purchased
from Sinopharm Chemical Reagent Company (Shanghai, China).
Palladium catalysts were supplied by Shanghai D&R Fine Chemical
Company (Shanghai, China). Acetic acid and DMSO were obtained
from Guangdong Xilong Chemical Company (Guangdong, China).
Fig. 5. Dependence of Heck–Mizoroki cross-coupling yields with the recycled num-
bers for the Pd/PVA nanofiber mats catalyzed cross-coupling of iodobenzene with
n-butyl acrylate. Reaction conditions: 1.0 mmol iodobenzene, 2.0 mmol acrylate,
0.02 mmol Pd/PVA nanofiber mats, 3.0 mmol potassium acetate in 5.0 ml DMSO at
110 ◦C for 2.0 h.

272
L. Shao et al. / Applied Catalysis A: General 413–414 (2012) 267–272
All solvents and chemicals are analytical grade or the best grade
available and used without further purification. The quantitative
analysis was performed on an Agilent GC/MS instrument with a
programmable split/splitless injector, which was set at 270 ◦C. The
oven-temperature was initially started at 140 ◦C and then ramped
to 270 ◦C at a rate of 10 ◦C/min, and maintained for 2.0 min at each
step. NMR spectra were recorded in CDCl3 (Bruker, AVANCE III
400 MHz, Switzerland) and the proton chemical shifts are reported
in ppm relative to TMS as the internal reference. Multiplicities are
reported as: singlet (s), doublet (d), and multiplet (m). The mor-
phologies of the electrospun PVA nanofiber mats were recorded on
a scanning electron microscope (Jeol, jsm-6360lv, Japan). Samples
for the SEM analysis were first coated with a 2–3 nm layer of gold
to make them conductive. The nanofiber diameter was determined
from the SEM micrographs. FT-IR/ATR spectra were recorded on a
FT-IR spectrometer (Nicolet, Nexus-470, USA) with the accessories
of attenuated total reflection. 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
(VG, ESCALAB, UK). TGA was performed using a thermogravimet-
ric analyzer (Mettler, SDTA851, Switzerland) from 20 to 600 ◦C at a
rate of 10 ◦C/min under a nitrogen atmosphere.
(1.0 mmol), acrylate (2.0 mmol) or phenylacetylene (1.2 mmol),
Pd/PVA nanofiber mats (0.02 mmol) and an appropriate amount of
potassium acetate base. The resulting solution was allowed to stir
at 110 ◦C and the reaction progress was monitored by TLC and/or
GC/MS analysis. The reaction mixture was cooled down to room
temperature after completion, and then quenched with 10 ml of
water and extracted three times with ethyl acetate (3 × 20 ml).
The combined organic layer was washed with water, saturated
brine, and then dried over anhydrous Na2SO4. Solvent was removed
under a reduced pressure. The residue was purified to afford the
cross-coupling product by silica gel chromatography with a mix-
ture of petroleum ether and ethyl acetate. All of the cross-coupling
products were characterized by 1H NMR spectroscopy and mass
spectroscopy.
Acknowledgements
This work was generously supported by the Natural Science
Foundation of Zhejiang Province, People’s Republic of China and
Shaoxing University.
Appendix A. Supplementary data
4.2. Preparation of Pd-Loaded PVA nanofiber mats
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2011.11.018.
Appropriate amount of PVA polymer substrate was suspended
in a 13.0 M acetic acid aqueous solution, which was allowed to stir
at room temperature until complete dissolution. The resultant solu-
tion was then slowly mixed with the Na2PdCl4 solution, which
was prepared with 4.2 g of PdCl2 (23.5 mmol) and 4.0 g of NaCl
(68.4 mmol) in 13.0 M acetic acid. The final solution was subjected
to electrospinning for the PVA nanofiber mats through a syringe
(20 ml) with a blunt-end capillary (1.1 mm ID) as spinneret at a
flow rate of 1.0 ml/min. A voltage of 21 kV (Tianjin Dongwen High-
Voltage Power Supply Company, Tianjin, China) was applied with
a distance of 12 cm between the syringe tip and the collector. The
resulting nanofiber mats were dried at room temperature under
vacuum for 12 h. The cross-linking of the electrospun PVA nanofiber
mats was optimized at the temperature of 150, 170 and 210 ◦C for
1 h under an argon atmosphere.
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