Encapsulating palladium nanoparticles inside ethylenediamine functionalized and crosslinked chlorinated poly(vinyl chloride) nanofibers as an efficient and stable heterogeneous catalyst for coupling reactions

Article abstract of DOI:10.1016/j.jpcs.2020.109674

Palladium chloride and chlorinated poly (vinyl chloride) (CPVC) mixture were prepared into homogeneous solution, followed by electrospinning to make uniform nanofibers with average diameter of ~460 nm. Then, these composite nanofibers were treated in ethy

Full text of DOI:10.1016/j.jpcs.2020.109674

Journal Pre-proof
Encapsulating palladium nanoparticles inside ethylenediamine functionalized and
crosslinked chlorinated poly(vinyl chloride) nanofibers as an efficient and stable
heterogeneous catalyst for coupling reactions
Min Qin, Qingqing Wang, Yijun Du, Linjun Shao, Chenze Qi, Hongyu Tao
PII:
S0022-3697(20)30844-1
https://doi.org/10.1016/j.jpcs.2020.109674
PCS 109674
DOI:
Reference:
To appear in: Journal of Physics and Chemistry of Solids
Received Date: 2 April 2020
Revised Date: 27 June 2020
Accepted Date: 21 July 2020
Please cite this article as: M. Qin, Q. Wang, Y. Du, L. Shao, C. Qi, H. Tao, Encapsulating palladium
nanoparticles inside ethylenediamine functionalized and crosslinked chlorinated poly(vinyl chloride)
nanofibers as an efficient and stable heterogeneous catalyst for coupling reactions, Journal of Physics
and Chemistry of Solids (2020), doi: https://doi.org/10.1016/j.jpcs.2020.109674.
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Author statement
Min Qin: Preparation and characterization of Pd@ACPVC catalyst
Qingqing Wang: Evaluation of catalytic performance of Pd@ACPVC catalyst
Yijun Du: Supervision
Linjun Shao: data analysis, writing-original draft preparation.
Chenze Qi: Characterization
Hongyu Tao: data analysis

Encapsulating palladium nanoparticles inside ethylenediamine functionalized
and crosslinked chlorinated poly(vinyl chloride) nanofibers as an efficient
and stable heterogeneous catalyst for coupling reactions
Min Qin, Qingqing Wang, Yijun Du*, Linjun Shao*, Chenze Qi, Hongyu Tao
Zhejiang Key Laboratory of Alternative Technologies for Fine Chemicals Process, Shaoxing
University, Zhejiang 312000, China
*Address correspondence to Linjun Shao (shaolinjun@usx.edu.cn)

Abstract
Palladium chloride and chlorinated poly(vinyl chloride) (CPVC) mixture were prepared into
homogeneous solution, followed by electrospinning to make uniform nanofibers with average
diameter of ~460 nm. Then, these composite nanofibers were treated in ethylenediamine solution
to functionalize and crosslink the CPVC molecules inside the nanofibers to improve their
chelating ability and solvent resistance. The functionalization and crosslinking of CPVC
molecules inside composite nanofibers were confirmed by SEM, FT-IR, EA and PALS
characterizations. The catalytic performance of these palladium encapsulated CPVC nanofibers
(Pd@ACPVC) have been evaluated by the Heck and Sonogashira reactions. The catalysis results
demonstrate that this Pd@ACPVC catalyst was very effective and stable to catalyze the coupling
reaction of aromatic iodides with alkenes (Heck reaction) or phenyl acetylene (Sonogashira
reaction) to afford the coupling products in moderate to excellent yields. Due to the regular
fibrous structure, the Pd@ACPVC could be readily separated and recovered from reaction
mixture. In addition, the Pd@ACPVC could be separately reused for 10 times for Heck reaction
and 8 times for Sonogashira reaction without significant decrement of coupling yields. After
careful investigation, the palladium leaching from Pd@ACPVC in the reuse procedure could be
ascribed to the loss of chelating groups (amino groups) and expansion of free volume holes in
the composite nanofibers. The excellent catalytic activity and stability of Pd@ACPVC could be
attributed to the strong chelating ability of amino groups, encapsulation of palladium
nanoparticles and ultrafine fiber.
Keywords: Palladium nanoparticles; chlorinated poly(vinyl chloride) nanofibers; electrospinning;
coupling reaction; free volume.

1. Introduction
Palladium catalysts have attracted much attention for their high catalytic performance in organic
reactions, which have been widely applied in chemical industry and academic researches for
many carbon-carbon bond transformations [1,2]. Homogeneous palladium catalysts are highly
active and selective to catalyze the reactions, but suffer the problems of high cost, toxicity and
difficulties in separation and recovery [3]. Hence, much effort has been devoted to developing
heterogeneous palladium catalysts for easy recovery and reuse [3-9]. However, it is still a great
challenging to mitigate the leaching of palladium species from the heterogeneous catalysts in
catalysis procedure. Therefore, exploiting facile methods to prepare highly active and stable
heterogeneous palladium catalysts is still in great demand.
In order to retain palladium activity and simplify preparation procedure, palladium species
are generally deposited on the surface of solid matrices with larger specific surface area [10-12].
However, destruction of solid matrices, severe reaction conditions (e.g. high reaction temperature)
and polar solvent would weaken the bound strength between solid matrices with palladium
species [13,14], resulting in the aggregation and leaching of palladium nanoparticles in catalysis.
Thus, although the synthesis of ultrafine palladium nanoparticles on solid matrices has been
achieved by using ligands and capping agents [15,16], the inhibition of palladium aggregation
and leaching in catalysis procedure was still a great challenging. As the capping agents can
promote the formation of ultrafine palladium nanoparticles, we have speculated that that the
stability of palladium nanoparticles could be improved by encapsulating them inside the solid
polymer matrices due to the capping effect from the polymer chains. Moreover, the catalytic
activity of these encapsulated palladium nanoparticles could be retained provided the reactants
molecules could freely access the catalytic sites [17-19].

Electrospinning is a versatile method to fabricate polymer ultrafine fibers directly from
polymer solutions or melts. The larger specific surface area of these fibers makes them hold great
potential in catalysis. The polymer nanofibers are ideal solid matrices to encapsulate the
palladium nanoparticles for their ultrafine fiber, which facilitates the diffusion of reactants and
products inside the nanofibers [20-22]. Moreover, compared with the particulate solid matrices,
the nanofiber structure not only can make the fibers achieve a relatively larger specific area, but
also can be readily separated and recovered from the reaction mixture by simple filtration
[23,24].
Till now, various inorganic and organic materials have been used as solid matrices to
support palladium catalyst [25]. Chlorinated poly(vinyl chloride) (CPVC) and poly(vinyl
chloride) (PVC) have been used as the supporting matrices for its low cost, excellent mechanical
property and easy modification [26-29]. In our previous studies [30,31], CPVC has been
electrospun into nanofibers for the deposition of palladium nanoparticles. The CPVC was readily
electrospun into well-defined nanofibers, but these CPVC nanofibers were easily dissolved in
many organic solvents (e.g. chloroform, tetrahydrofuran and N,N- dimethylfomamide). Therefore,
crosslinking of CPVC molecules were essential to improve the solvent resistance of CPVC
nanofibers. Herein, ethylenediamine was used to crosslink and functionalize the CPVC
molecules inside the palladium encapsulated CPVC composite nanofibers. The stability of
palladium nanoparticles would be improved by the chelating ability of amino groups and capping
effect of crosslinked CPVC molecules (Scheme 1). The catalytic performance of these palladium
encapsulated CPVC composite nanofibers (Pd@ACPVC) was evaluated by Heck and
Sonogashira reactions.
---Scheme 1---

2. Experimental section
2.1. Materials
Chlorinated polyvinyl chloride (CPVC, Average molecular weight: 85000) was bought from
Hangzhou Qianjing Chemical Co., Ltd (Zhejiang, China). Ethylenediamine and the reagents for
Heck and Sonogashira reactions (Analytical grade) were purchased form Aladdin Chemical Co.
Ltd (Shanghai, China). Palladium chloride (PdCl2) (Chemical grade) was bought from Hangzhou
Changqing Chemical Industrial Co., Ltd (Zhejiang, China). All the reagents were directly used
as received without further purification.
2.2. Preparation of palladium doped CPVC nanofiber via electrospinning
CPVC powder (2.0 g), PdCl2 (50 mg) and NaCl (50 mg) were put into a flask containing 6.3 mL
tetrahydrofuran (THF) and 12.0 mL N,N-dimethylformamide (DMF). The mixture was stirred
magnetically for 12 hours to afford red homogeneous solution. Then, the solution was transferred
into an electrospinning equipment to make palladium encapsulated CPVC nanofibers (FM1206,
Beijing Future Material Sci-tech Co., Ltd, China). The electrospinning parameters were as
follows: needle gauge: 21 G; feeding rate: 0.08 mL/h; applied voltage: 25 kV, work distance: 25
o
cm. The resultant nanofiber mat (Pd@CPVC) was dried under vacuum at 80 C to remove the
residual solvent.
2.3. Crosslinking of CPVC molecules inside the nanofibers
The Pd@CPVC fiber mat (0.2 g) was added into a flask containing 4.5 mL ethylenediamine and
o
12.7 mL ethanol. After reacting at 60 C for 12 hours, the fiber mat was filtrated and washed
three times by water. The fiber mat was dried under vacuum at 80 oC for 12 hours and signed as
Pd@ACPVC.

2.4. Reduction of Pd2+ cations into Pd0 nanoparticles
The Pd@ACPVC was immersed in 12.7 mL ethanol solution containing 1.0 g NaBH4. After
stirring for 5.0 hours, the Pd@ACPVC was recovered by filtration and dried under reduced
pressure at 25 oC for 12 hours. The palladium content in the Pd@ACPVC was 0.94 wt.%, which
was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES).
For comparison, CPVC nanofibers were also prepared by the same method as that of
Pd@CPVC but without the addition of PdCl2 and NaCl. Then, the CPVC nanofibers were
reacted with ethylenediamine by the identical procedure as that of Pd@ACPVC to synthesize
aminated and crosslinked CPVC nanofibers (ACPVC).
2.5. Typical procedure for Heck reaction
Aromatic iodide (1.0 mmol), alkene (2.0 mmol), triethylamine (2.5 mmol) and glycol (0.2 g)
were added into a tubular reactor containing 25 mg Pd@ACPVC fiber mat (Pd content: 2.21
μmol) and 3.2 mL N,N-dimethylacetamide (DMAc). The reaction mixture was carried out with
magnetically stirring at 110 oC. The reaction progress was monitored by thin layer
chromatography (TLC) and gas chromatograph/mass spectrometer (GC/MS). After reaction, 10
mL water was added into the reaction mixture to quench the reaction and then the reaction
mixture was extracted with ethyl acetate (3 × 10 mL). Solvent of extracted solution was removed
under reduced pressure to afford the crude product, which was further purified by silica gel
chromatography. The coupling products were confirmed by 1H NMR spectra and mass spectra.
The Pd@ACPVC catalyst was recovered by filtration and then washed by DMAc and water.
o
Then, the Pd@ACPVC catalyst was dried at 80 C under reduced pressure for reuse. The reuse
procedure is the same as that of Pd@ACPVC catalyzed Heck reaction.

Characterization data of Heck reaction products was shown as follows:
Butyl cinnamate: 1H NMR (400 MHz, CDCl3) δ 7.62 (d, J = 16.0 Hz, 1H), 7.50 – 7.42 (m, 2H),
7.36 – 7.29 (m, 3H), 6.38 (d, J = 16.0 Hz, 1H), 4.20 – 4.11 (m, 2H), 1.63 (dt, J = 14.6, 6.8 Hz,
2H), 1.45 – 1.29 (m, 2H), 0.91 (t, J = 7.4 Hz, 3H). MS m/z (%): 204 (23.5), 148 (71.5), 147
(59.2), 131 (99.9), 103 (13.3), 77(22.5).
1
(E)-butyl 3-(2-fluorophenyl)acrylate: H NMR (400 MHz, CDCl3) δ 7.81 (d, J = 16.2 Hz, 1H),
7.54 (td, J = 7.6, 1.5 Hz, 1H), 7.35 (ddd, J = 15.3, 5.3, 1.7 Hz, 1H), 7.20 – 7.06 (m, 2H), 6.54 (d,
J = 16.2 Hz, 1H), 4.22 (t, J = 6.7 Hz, 2H), 1.70 (dt, J = 14.6, 6.8 Hz, 2H), 1.53 – 1.35 (m, 2H),
0.97 (t, J = 7.4 Hz, 3H). MS m/z (%): 222 (14.2), 167 (14.1), 166 (41.0), 149 (99.9), 146 (23.2),
121 (34.3), 101 (48.9), 75 (14.3).
1
(E)-butyl 3-(4-fluorophenyl)acrylate: H NMR (400 MHz, CDCl3) δ 7.64 (d, J = 16.0 Hz, 1H),
7.56 – 7.47 (m, 2H), 7.12 – 7.03 (m, 2H), 6.36 (d, J = 16.0 Hz, 1H), 4.21 (t, J = 6.7 Hz, 2H),
1.69 (dt, J = 14.6, 6.8 Hz, 2H), 1.51 – 1.36 (m, 2H), 0.97 (t, J = 7.4 Hz, 3H). MS m/z (%): 222
(19.7), 165 (27.8), 166 (93.0), 149 (99.9), 121 (35.4), 101 (35.17).
1
(E)-butyl 3-(2-bromophenyl)acrylate: H NMR (400 MHz, CDCl3) 8.05 (d, J = 15.9 Hz, 1H),
7.61 (d, J = 8.0 Hz, 2H), 7.32 (s, 1H), 7.29 – 7.19 (m, 1H), 6.39 (d, J = 15.9 Hz, 1H), 4.23 (t, J =
6.7 Hz, 2H), 1.75 – 1.66 (m, 2H), 1.45 (dd, J = 15.0, 7.5 Hz, 2H), 0.97 (t, J = 7.4 Hz, 3H). MS
m/z (%): 282 (5.2), 209 (169), 211 (162), 147 (99.9), 102 (28), 1101 (10.2)
1
(E)-butyl 3-(4-chlorophenyl)acrylate: H NMR (400 MHz, CDCl3) δ 7.63 (d, J = 16.0 Hz, 1H),
7.46 (d, J = 8.5 Hz, 2H), 7.36 (d, J = 8.5 Hz, 2H), 6.42 (d, J = 16.0 Hz, 1H), 4.21 (t, J = 6.7 Hz,
2H), 1.69 (dt, J = 14.6, 6.8 Hz, 2H), 1.44 (dd, J = 15.1, 7.5 Hz, 2H), 0.96 (dd, J = 13.8, 6.4 Hz,
3H). MS m/z (%) 238 (21.6), 182 (99.9), 165 (88.6), 137 (27.9), 102 (37.6), 75 (25.3).

(E)-butyl 3-o-tolylacrylate: 1H NMR (400 MHz, CDCl3) δ 7.97 (d, J = 15.9 Hz, 1H), 7.59 – 7.51
(m, 1H), 7.31 – 7.16 (m, 3H), 6.36 (d, J = 15.9 Hz, 1H), 4.21 (t, J = 6.7 Hz, 2H), 2.44 (s, 3H),
1.76 – 1.63 (m, 2H), 1.51 – 1.38 (m, 2H), 0.97 (t, J = 7.4 Hz, 3H). MS m/z (%): 218 (24.2), 147
(30.5), 146 (12.2), 145 (99.9), 116(73.6), 115(64.8), 117 (47.3), 91 (21.8).
(E)-butyl 3-m-tolylacrylate: 1H NMR (400 MHz, CDCl3) δ 7.65 (d, J = 16.0 Hz, 1H), 7.36 – 7.15
(m, 4H), 6.43 (d, J = 16.0 Hz, 1H), 4.20 (t, J = 6.7 Hz, 2H), 2.36 (d, J = 5.2 Hz, 3H), 1.75 – 1.62
(m, 2H), 1.44 (dd, J = 15.0, 7.5 Hz, 2H), 0.96 (t, J = 7.4 Hz, 3H). MS m/z (%): 218 (30.0), 162
(69.2), 147 (44.7), 145 (99.9), 117 (27.7), 116 (15.1), 115 (53.8), 91 (25.5).
(E)-butyl 3-p-tolylacrylate: 1H NMR (400 MHz, CDCl3) δ 7.63 (d, J = 16.0 Hz, 1H), 7.54 – 7.39
(m, 2H), 6.96 – 6.81 (m, 2H), 6.31 (d, J = 16.0 Hz, 1H), 4.19 (t, J = 6.7 Hz, 2H), 3.82 (d, J = 8.0
Hz, 3H), 1.74 – 1.61 (m, 2H), 1.51 – 1.35 (m, 2H), 1.00 – 0.65 (m, 3H). MS m/z (%):218 (34.9),
162 (95.6), 161 (26.17), 147 (22.3), 145 (99.9), 115 (46.5), 117 (28.8).
1
Methyl cinnamate: H NMR (400 MHz, CDCl3) δ 7.70 (d, J = 16.0 Hz, 1H), 7.53 (dd, J = 6.5,
2.9 Hz, 2H), 7.46 – 7.29 (m, 3H), 6.45 (d, J = 16.0 Hz, 1H), 3.81 (s, 3H). MS m/z (%): 162
(55.0), 161 (27.0), 132 (10.1), 131 (99.9), 103 (59.8), 102 (15.4), 77 (32.2).
(E)-1,2-diphenylethene: 1H NMR (400 MHz, CDCl3) δ 7.53 (d, J = 7.5 Hz, 4H), 7.37 (t, J = 7.6
Hz, 4H), 7.26 (d, J = 3.7 Hz, 2H), 7.12 (s, 2H). MS (EI) m/z (%): 180 (99.9), 179 (57.8), 178
(336), 165 (23.6), 89 (15.6)
1
(E)-butyl 3-(4-nitrophenyl)acrylate: H NMR (400 MHz, CDCl3) δ 7.62 (d, J = 16.0 Hz, 1H),
7.49 – 7.41 (m, 2H), 7.35 (dd, J = 8.7, 2.0 Hz, 2H), 6.41 (d, J = 16.0 Hz, 1H), 4.21 (t, J = 6.7 Hz,
2H), 1.69 (dt, J = 14.6, 6.8 Hz, 2H), 1.51 – 1.36 (m, 2H), 0.96 (t, J = 7.4 Hz, 3H). MS m/z (%):
239 (11.2), 194 (54.5), 193 (43.7), 176 (99.9), 177 (19.2), 102 (38.1), 56 (33.4), 130 (31.6).

2.6. Typical procedure for Sonogashira reaction
Aromatic iodide (1.0 mmol), alkyne (2.0 mmol), K2CO3 (3.0 mmol), glycol (0.2 g) and CuI (0.02
mmol) were added into a tubular reactor containing 15 mg Pd@CPVC fiber mat and 3.2 mL
o
DMAc. The reaction was carried out at 110 C with magnetic stirring. The post treatment was
the same as that of Pd@CPVC catalyzed Heck reaction. The coupling products were confirmed
by the 1H NMR spectra and mass spectra.
Characterization data of Sonogashira reaction products was shown as follows:
1
1,2-Diphenylethyne: H NMR (400 MHz, CDCl3) δ7.55-7.59 (m, 3H), 7.42-7.44 (m, 1H),
7.35-7.37 (m, 3H), 7.27-7.29 (m, 1H), 7.22-7.25 (m, 1H); MS m/z (%) 178 (99.9), 176 (20.5),
179 (15.2), 177 (11.3), 152 (11.2), 151 (8.6), 76 (7.6), 89(6.4), 150(5.7), 126(5.1).
1
1-Fluoro-4-(2-phenylethynyl)benzene: H NMR (400 MHz, CDCl3) δ 7.26-7.30 (m, 2H),
7.43-7.45 (m, 3H), 7.48-7.51(m, 1H), 7.54-7.57(m, 1H), 7.60-7.64(m, 2H). MS m/z (%) 196
(99.9), 197 (15.5), 194 (13.3), 170 (9.0), 98 (8.0), 175 (7.5), 195 (6.8), 85 (5.2), 169 (4.4), 144
(4.0).
1
1-Bromo-2-(2-phenylethynyl)benzene: H NMR (400 MHz, CDCl3) δ 7.57 – 7.49 (m, 4H),
7.41 – 7.29 (m, 6H). MS m/z (%) 258 (96.0), 256 (99.9), 176 (69.5), 151 (17.6), 88 (18.6).
1
1-Chloro-2-(2-phenylethynyl)benzene: H NMR (400 MHz, CDCl3) δ 7.58-7.61(m, 4H),
7.46-7.78(m, 6H). MS m/z (%) 212 (99.9), 176 (37.0), 151 (13.0).
1-Methyl-2-(2-phenylethynyl)benzene: 1H NMR (400 MHz, CDCl3) δ 7.56-7.58(m, 2H), 7.51(d,
1H, J=7.6 Hz), 7.43-7.46 (m, 3H), 7.33(d, 2H, J=3.0 Hz), 7.22-7.26(m, 1H), 2.47(s, 3H). MS m/z
(%) 192 (99.9), 191 (96.1), 189 (36.1), 165 (26.2), 190 (18.7), 193 (15.1), 115 (11.5), 95 (7.0),
163 (5.5), 164 (4.8).

1
1-Methyl-4-(2-phenylethynyl)benzene: H NMR (400 MHz, CDCl3) δ 6.96-7.00(m, 4H),
7.12-7.15 (m, 2H), 7.15-7.17(m, 2H), 7.25 (m, 1H), 2.31(s, 3H). MS m/z (%) 192 (99.9), 191
(49.0), 189 (25.0), 193 (15.9), 165 (15.5), 190 (12.3), 115 (5.3), 95 (4.7), 139 (4.0), 163 (3.9).
1
2-(2-Phenylethynyl)thiophene: H NMR (400 MHz, CDCl3) δ 7.50 (dd, J = 4.9, 1.6 Hz, 2H),
7.32 (dd, J = 5.0, 1.7 Hz, 3H), 7.29 – 7.24 (m, 2H), 6.99 (dd, J = 5.1, 3.7 Hz, 1H). MS m/z (%)
184 (99.9), 152 (17.5), 139 (22.3).
2.7. Instrumentation
The fiber surface morphologies were recorded by scanning electron microscopy (SEM) (Jeol,
Jsm-6360lv, Tokyo, Japan). Nanofiber diameters were determined from their corresponding
SEM images. For each sample, at least fifty nanofibers were counted for the determination of
mean diameter. Surface mapping was done on an energy dispersive X-ray spectroscopy (Oxford
EDX system) (SEM-EDS) (X-act, Oxford, Britain). The chemical structures of the fiber mats
were analyzed by Fourier transform infrared spectroscopy (FT-IR) with the accessory of
attenuated total reflection (Nicolet, Nexus-470, Wisconsin, USA). Thermal stability of the fiber
mats in atmospheric air was performed on a thermogravimetric analyzer (TGA) (Beijing
o
Scientific Instrument Factory, Beijing, China) with heating rate of 10 C/min. The palladium
dispersion in Pd@ACPVC was analyzed by X-ray diffraction (XRD) (Empyrean, PANalytical,
Almelo, Netherlands). The electronic states of palladium were determined by X-ray
photoelectron spectroscopy (Escalab250Xi, Thermo Fisher Scientific, USA). C1s of 284.6 eV
was used as the internal reference for calibration.
Positron annihilation lifetime spectroscopic (PALS) measurements were done using a
fast-fast lifetime spectrometer (EG&G ORTEC Co., Tennessee, USA) at room temperature.

Time resolution was 190 ps at a 60Co prompt peak of 1.18 MeV and 1.33 MeV. A 22Na positron
source (6.0 × 105 Bq) on 3 µm thick Kapton films was sandwiched within two identical fiber
mats. Analysis of positron lifetime spectrum was performed using a three-component fitting LT
9.0 program.
The fiber mat (~10 mg) was first dissolved in a mixture of fuming nitric acid and 98%
sulfuric acid (v/v: 1/3) and then diluted to 50 mL with deionized water. The palladium
concentration was measured by inductively coupled plasma-atomic emission spectroscopy (ICP)
(Leeman ICP-AES Prodigy XP, New Hampshire, USA).
The quantitative analysis of the coupling products was performed on an Agilent GC/MS
instrument (Agilent, GC6890/5975 MSD, California, USA). The coupling products were
1
dissolved in CDCl3 solution and characterized by H NMR spectra (AVANCE III 400 MHz,
Bruker, Fällanden, Switzerland).
3. Results and Discussion
3.1. Characterization of the palladium encapsulated CPVC nanofibers
PdCl2/CPVC mixture solution was readily electrospun into uniform composite nanofibers due to
the excellent spinnability of CPVC (Fig. 1a),. The abundant C-Cl groups in CPVC molecules
provided the sites for chemical modification and crosslinking by nucleophilic regents, so the
CPVC can react with ethylenediamine to form crosslinked spatial net configuration to improve
the solvent resistance of CPVC nanofibers. In addition, the amino groups inside the nanofibers
after crosslinking can provide the chelating sites for palladium nanoparticles. As shown in Fig.
1b, the composite nanofibers become thicker after reacting with ethylenediamine. Fig. 1c shows

that the Pd@ACPVC was insoluble in THF, indicating that the CPVC molecules in Pd@ACPVC
nanofibers had been sufficiently crosslinked by ethylenediamine.
---Fig. 1---
The reaction of CPVC with ethylenediamine was first analyzed by FT-IR spectra (Fig. 2).
The new absorption peaks at 3275 cm-1 and 1540 cm-1 were attributed to the stretching and
deformation vibration of amino groups, indicating that the CPVC molecules had been reacted
with ethylenediamine. Due to the elimination of HCl from CPVC, a new peak at 1662 cm-1
corresponding to –C=C- groups was found [26]. In addition, the new absorption peak at 1086
cm-1 (due to C-O group) demonstrates the partially hydrolysis of C-Cl bond in CPVC molecules.
After encapsulation of palladium nanoparticles, the absorption peak assigned to amino groups
was blue shifted to 1563 cm-1 from 1540 cm-1, indicating the coordination of amino groups with
palladium nanoparticles. Besides FT-IR spectra, XPS characterization shows that the amination
process raised the palladium electron binding energies (3d5/2 and 3d3/2) separately from 337.68
eV to 337.98 eV and 342.88 eV to 343.28 eV, which evidently indicated the coordination of
palladium with amino groups (Fig. 3). The reduction of Pd2+ cations into Pd0 species was
confirmed by decrement of palladium electron binding energies. In addition, the elemental
analysis result shows that the nitrogen content in the Pd@ACPVC were 6.843 ± 0.502 %,
indicating that the CPVC molecules had been successfully reacted with ethylenediamine.
---Fig. 2---, --Fig. 3---
Because temperature is an important factor for the palladium catalyzed reactions, the
thermal stability of supporting matrices is critical for application of supported palladium
o
catalysts. As shown in Fig. 4, the weight loss at temperature below 150 C was ascribed to the

loss of physically absorbed water. The crosslinking of CPVC molecules made the decomposition
o
o
temperature of nanofibers increased to 236 C from 176 C. Thus, the thermal stability of
Pd@ACPVC could satisfy the high reaction temperature in the following catalysis reactions.
---Fig. 4---
SEM-EDS analysis shows that the palladium species were homogeneously dispersed in the
Pd@ACPVC nanofibers (Fig. 5). In addition, nitrogen and oxygen elements were also found in
the fiber, which confirmed the amination and hydrolysis of CPVC molecules. Moreover, the
XRD pattern of Pd@ACPVC before reduction had only one intense peak at 2θ = 23 o, which was
ascribed to crystal phase of CPVC polymer (Fig. 6a) [32]. After reduction, a new weak peak at
o
2θ = 40 assigned to (111) plane of face centered cubic (FCC) Pd0 crystal appeared [33]. This
result also demonstrates that the reducing agent had diffused into the nanofiber to reduce Pd2+
cations into Pd0 nanoparticles. In addition, TEM image shows that the palladium nanoparticles
with mean diameter of ~3.13 nm dispersed homogeneously inside the nanofibers (Fig. 6b). Thus,
we believed that the reactants can also migrate inside the nanofiber to the palladium species to
complete the catalysis reaction.
----Fig. 5---, ---Fig. 6---
3.2. Heck reaction
Heck reaction is a powerful method for the construction of sp2-sp2 carbon-carbon bonds in the
synthesis of various fine chemicals and pharmaceutically active compounds [34,35]. Herein,
Heck reaction was employed as a model reaction to evaluate the catalytic performance of
Pd@ACPVC. As shown in Fig. 7, increments of reaction temperature and catalyst loading can
promote Pd@ACPVC catalyzed Heck reaction of iodobenzene with n-butyl acrylate. The Heck

o
reaction could be carried out with satisfied yields at temperature of 110 C and Pd@ACPVC
loading of 0.221 mol%, which were used in the catalysis reaction afterward. Interestingly, this
Pd@ACPVC catalyst had similar catalytic activity with the palladium nanoparticles deposited on
functionalized CPVC nanofibers [30,31]. Thus, it can be concluded that encapsulation of
palladium nanoparticles inside the ACPVC nanofibers have no obviously negative effect on the
catalytic activity, which can be ascribed to the ultrafine fiber.
---Fig. 7---
The Pd@ACPVC catalysis protocol was used to catalyze the Heck reaction with additional
aromatic halides with alkenes, and the related results were summarized in Table 1. Examination
1-7 in Table 1 shows that the Pd@ACPVC was very active to catalyze the Heck reaction of
aromatic iodides with n-butyl acrylate to synthesize the products in moderate to excellent yields.
Besides the n-butyl acrylate, the Pd@ACPVC can also effectively catalyze the Heck reaction of
iodobenzene with other two alkenes (Entry 8 and 9 in Table 1). Unfortunately, the catalytic
activity of Pd@ACPVC was comparably low for the Heck reactions of aromatic bromides with
n-butyl acrylate, which could be attributed to the greater bond strength of carbon-bromine bond
(Entry 10-12 in Table 1). As many palladium nanoparticles deposited on polymer solid matrices
were also not very active to catalyze the Heck reactions of aromatic bromides, the encapsulation
of palladium nanoparticles inside nanofibers was not the reason responsibility for the low
catalytic activity of Pd@ACPVC for Heck reactions of aromatic bromides [36,37].
---Table 1---
3.3. Sonogashira reaction
Sonogashira reaction is the cross-coupling reaction of aromatic halides with terminal alkynes,

which provides a powerful and versatile method for the construction of sp2-sp carbon-carbon
bonds [38,39]. Herein, the catalytic activity of Pd@ACPVC was also examined for Sonogashira
reactions. Fig. 8 clearly shows that excellent coupling yield of iodobenzene with phenyl
acetylene was achieved with Pd@ACPVC loading of 0.133 mol%.
---Fig. 8---
Then, the Pd@ACPVC has been examined for Sonogashira reactions of phenyl acetylene
with several additional representative aromatic halides. Examination of entry 1-7 in Table 2
shows that the Pd@ACPVC can effectively catalyze the Sonogashira reactions of aromatic
iodides with phenyl acetylene to afford the products with satisfied yields (84-98%). In addition,
the aromatic iodides with electron-deficient groups were more active than these with
electron-rich groups. Moreover, examination of entry 8-10 in Table 2 shows that the
Pd@ACPVC was also efficient to catalyze the Sonogashira reactions of iodobenzene with
different aromatic alkynes with satisfied yields (81-92%). Unfortunately, the catalytic activity of
Pd@ACPVC was also relatively low for the aromatic bromides (Entry 7 and 8 in Table 2).
---Table 2---
3.4. Reuse of the Pd@ACPVC catalyst
The easy separation, recovery and reuse were the fascinating characteristics of heterogeneous
palladium catalysts. Due to the regular structure, the fiber catalyst can be readily separated and
recovered by simple filtration for reuse. The Heck reaction of iodobenzene with n-butyl acrylate
and Sonogashira reaction of iodobenzene with phenyl acetylene were again employed to assess
the reusability of Pd@ACPVC. Fig. 9 shows that the Pd@ACPVC could be separately reused 10
times for Heck reaction and 6 times for Sonogashira reaction without significant decrement of

coupling yields. Compared with palladium nanoparticles deposited on ACPVC nanofibers
[30,31], the reusability of Pd@ACPVC catalyst was significantly improved. Thus, encapsulation
of palladium nanoparticles inside the nanofibers can indeed improve its stability and reusability.
---Fig. 9---
Because the palladium aggregation and leaching were the two main reasons responsible for
the loss of catalytic activity, the palladium content and dispersion of the recovered Pd@ACPVC
catalyst were analyzed by ICP-AES and SEM-EDS (Fig. 10). The palladium contents of the
recovered Pd@ACPVC catalyst determined by ICP-AES and SEM-EDS were 0.48% and 0.35%
(For Heck reaction), indicating that the palladium leaching was not significant. In addition,
SEM-EDS analysis shows that the palladium species were still dispersed evenly in the
Pd@ACPVC nanofibers. Moreover, compared with the fresh Pd@ACPVC catalyst, the nitrogen
content was decreased while the oxygen content was increased, which could be ascribed to the
degradation of ACPVC matrix. In addition, FT-IR analysis of the recovered Pd@ACPVC shows
that a new peak at 1726 cm-1 attributed to C=O groups appeared, which also confirmed the
degradation of polymer matrix (Fig. 2). Interestingly, the fibrous structure of Pd@ACPVC was
still well retained even after 10 cycles (Fig. 11), which could be ascribed to the excellent
mechanical property of CPVC polymer. Thus, it can be concluded that the chelating groups
(amino groups) were partially lost in the catalysis procedure due to the degradation of polymer
matrix, which would weaken the interaction between palladium nanoparticles with ACPVC
polymer matrix.
---Fig. 10---, ---Fig. 11---

3.5. Comparison of Catalysts
In order to evaluate the catalytic performance of the Pd@ACPVC catalyst, some reported
supported palladium catalysts for the Heck reaction of iodobenzene with alkene were
summarized in Table 3. The Pd@ACPVC has comparable or better catalytic activity and stability
for the Heck reaction. In addition, the recovery and reuse of this fibrous Pd@ACPVC catalyst
are quicker and easier than those of the other particulate catalysts.
---Table 3---
3.6. Characterization of internal structure of composite nanofibers
As the palladium nanoparticles were encapsulated inside the nanofiber, the internal structure of
composite nanofibers not only restricted the leaching and aggregation of palladium nanoparticles,
but also allowed the diffusion of reactants and products. Herein, PALS was employed to analyze
the inner structure of nanofibers. It is well known that PALS was a sensitive method to assess the
size and concentration of the free volume holes of condensed materials [45-47]. The PALS
spectra can be resolved into three components (τ1, τ2 and τ3) and the long lived component τ3 is
signed to the o-Ps pick-off annihilation in free volume holes in amorphous regions of polymeric
materials. Thus, the radius of free volume holes (R) can be calculated based on the following
equation (Equation 1) [45-47]:
1
R
1
2πR
Sin
2
1
+
τ3
R + 0.1656
2π
R + 0.1656
Equation 1
As the crosslinking restricts the movement of CPVC molecules in composite nanofibers, the
free volume size decreased after crosslinking reaction (Table 4). After reuse, the size of free
volume holes was increased while the corresponding concentration was reduced, which could be

ascribed to degradation of ACPVC polymeric matrix. The CPVC polymer could be degraded
under high temperature due to the dehydrochlorination and oxidation reactions [48,49]. As a
result, the capping effect of palladium nanoparticles by the crosslinked CPVC molecules was
also weaken, resulting in the aggregation and leaching of palladium nanoparticles.
---Table 4---
4. Conclusions
In summary, we have prepared uniform Pd2+/CPVC composite nanofibers by electrospinning.
Then, the CPVC molecules inside the nanofibers were readily functionalized and crosslinked by
ethylenediamine. These composite nanofibers have exhibited high catalytic activity and stability
for Heck and Sonogashira reactions of aromatic iodides, which can be attributed to the chelating
ability of amino groups on polymeric chain, encapsulation of palladium nanoparticles inside the
nanofibers and ultrafine fiber. The reuse of these composite nanofibers would lead to the loss of
functional groups (amino groups) and expansion of free volume holes, resulting in the leaching
of palladium nanoparticles. The high catalytic activity and easy operation make this unique
environmentally benign heterogeneous palladium catalyst to be attractive for large
industrial-scale applications.
Acknowledgement
The authors acknowledge the financial support from the National Natural Science Foundation of
China (No. 11905132 and 11975157).

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Table 1. Pd@ACPVC catalyzed Heck reaction of aromatic halides with alkenes.a
2.21 mol.‰ Pd@ACPVC
R2
X +
R2
R1
R1
Yield ( )b
Entry
R1
X
R2
Time (h)
％
5
10
5
77
90
85
97
57
87
44
94
31
90
43
92
19
81
98
74
98
43
84
10
52
15
77
1
2
3
4
5
6
2-F-
I-
I-
I-
I-
I-
I-
-COO(CH2)3CH3
4-F-
2-Br-
-COO(CH2)3CH3
-COO(CH2)3CH3
-COO(CH2)3CH3
-COO(CH2)3CH3
-COO(CH2)3CH3
10
5
10
5
4-Cl-
10
5
2-CH3-
3-CH3-
10
5
10
5
7
8
8
4-CH3-
H-
I-
I-
I-
-COO(CH2)3CH3
-COO(CH2)3CH3
-COOCH3
10
5
5
H-
10
5
9
H-
H-
I-
-Phc
24
5
10
11
Br-
Br-
-COO(CH2)3CH3
-COO(CH2)3CH3
24
5
4-F-
24

5
19
61
12
4-NO2-
Br-
-COO(CH2)3CH3
24
a: Reaction condition: aromatic halide: 1.0 mmol; alkene: 2.0 mmol; triethylamine: 2.5 mmol;
Pd@ACPVC: 25 mg (0.221 mol%); glycol: 0.2 g; DMAc: 3.2 mL; Reaction temperature: 110 oC
(for aromatic iodides) or 130 oC (for aromatic bromides).
b:
Product yields, based on the amount of aromatic iodides, were determined from GC/MS
measurements.
c: Phenyl group.