Aminated chlorinated polyvinylchloride nanofiber mat-supported palladium heterogeneous catalysts: Preparation, characterization and applications

Article abstract of DOI:10.1039/c4ra08469a

Chlorinated polyvinylchloride (CPVC) nanofiber mats were prepared by an electrospinning technique, and then treated with amines of different chemical structures, followed by immobilization of palladium catalysts (CPVC-NH₂-Pd), which have been demonstrated as efficient, stable and easily recyclable heterogeneous catalysts. Their catalytic activities could be correlated with the binding energies of the palladium species with the amine chelating ligands.

Full text of DOI:10.1039/c4ra08469a

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Aminated chlorinated polyvinylchloride nanofiber
mat-supported palladium heterogeneous catalysts:
preparation, characterization and applications†
Cite this: RSC Adv., 2014, 4, 53105
Received 11th August 2014
Accepted 8th October 2014
Linjun Shao, Chenze Qi* and Xian-Man Zhang*
DOI: 10.1039/c4ra08469a
www.rsc.org/advances
Chlorinated polyvinylchloride (CPVC) nanofiber mats were prepared
Nanober mats have been used as the supporting matrixes
by an electrospinning technique, and then treated with amines of for immobilizing palladium and other transition metal catalysts
different chemical structures, followed by immobilization of palladium because of their high specic surface area, allowing high
6
catalysts (CPVC–NH
2
–Pd), which have been demonstrated as effi- dispersion of the catalytic active species. In addition, the
cient, stable and easily recyclable heterogeneous catalysts. Their characteristic intact structure of the nanober mat allows much
catalytic activities could be correlated with the binding energies of the easier removal of the heterogeneous catalysts from the reaction
palladium species with the amine chelating ligands.
mixture by simple ltration. However, the reported nanober
mats-supported palladium heterogeneous catalysts usually
suffer relatively low stability, mainly due to the relatively weak
Catalysis plays a signicant role in synthetic organic chemistry
as it can greatly facilitate chemical transformations. Hetero-
7–9
absorption of the palladium species onto the nanober mats.
1
We hypothesized that introduction of strong chelating ligands
onto the surface of the highly porous nanober mats should
increase chelation of the palladium species with the surface
molecules of the supporting matrixes, which might improve the
catalytic activities and stabilities while retaining the large intact
nanober mat structure. Unfortunately, most of the strong
chelating ligands cannot survive the oxidative degradation at
the high temperature and electric voltage required by the elec-
trospinning protocol. Our strategy was inspired by the easy
geneous catalysis has received tremendous interest because
catalyst reuse and recycling can further improve the overall
productivity and cost-effectiveness, thereby minimizing the
waste generation and catalyst contamination, leading to a
greener and more sustainable chemical transformation
2–4
process.
However, there are still many challenges for
preparing efficient, stable and easily recyclable palladium
heterogeneous catalysts. First, the zero-valent palladium
species generated from the catalytic cycle is prone to aggregate
and then lose the catalytic activity by formation of palladium
black. Second, heterogenization reduces the accessibility and in
turn decreases the catalytic activity because the reactants can
only access the surface-bound catalytic centers, rather than all
amination of polyvinylchloride with amines under mild reac-
10,11
tion conditions.
Thus we have prepared the nanober mat
using chlorinated polyvinylchloride (CPVC), then modied the
surface with different amines, followed by immobilization of
the palladium catalyst (CPVC–NH –Pd) as depicted in Fig. 1.
2
2–4
accessible in a homogeneous system. Finally, the solid sup-
ported matrices are generally fabricated in small ne particles
to maximize the surface area to mass ratio, evolving into a
tedious and time-consuming process by separation and recy-
cling of the particulate catalysts from the reaction mixture due
to the high pressure drop by ltration, especially in the large-
Herein, we describe the use of this approach to construct a
series of CPVC–NH2 nanober mat-supported palladium
heterogeneous catalysts, which have been demonstrated as
highly efficient, stable and recyclable heterogeneous catalysts
for the Heck cross-coupling reactions.
CPVC was shown as an excellent polymeric material for
preparing the nanober mats by means of electrospinning
technique. The CPVC concentration and feeding rate might be
5
scale industrial applications.
ꢀ
1
used up to 20 wt% and 2.5 ml h , respectively. However, the
resultant CPVC nanober mats were soluble in the polar
organic solvents such as DMSO, DMF, chloroform, etc., limiting
Zhejiang Key Laboratory of Alternative Technologies for Fine Chemicals Process,
Shaoxing University, Zhejiang Province 312000, People's Republic of China. E-mail:
chenzeqi@usx.edu.cn; xian-man.zhang@usx.edu.cn; Fax: +86 575 88345681; Tel: their applications in the organic solutions. No improvement for
+
†
1
86 575 88345681
the solvent resistance was achieved when the CPVC nanober
mats were treated with mono amines such as n-propyl amine
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra08469a
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2
Fig. 3 SEM-EDS image for the CPVC–NH –Pd catalyst.
on the untreated CPVC nanober mats, the palladium content
of the CPVC–NH –Pd heterogeneous catalyst was determined to
2
be about 3.0 wt% based on the ICP-AES analysis.
Examination of the IR spectra (Fig. 1S of the ESI†) shows that
there are two broad characteristic IR absorptions at 1630 and
Fig. 1 Schematic illustration for the preparation of the aminated CPVC
nanofiber mat-supported palladium catalysts.
ꢀ
1
3418 cm , which can be attributed to the >NH functional group
of the CPVC–NH nanober mats. The slightly blue shis of the
2
amine IR absorption bands aer immobilization of the palla-
dium catalyst can be attributed to the chelation of the amine
groups with the palladium species. Nevertheless, the elemental
using the literature procedure reported for the amination of
11
polyvinylchloride. Most intriguingly, a signicant improve-
ment of resistance was obtained to the polar organic solvents
when the CPVC nanober mats were treated with bidentate
amines such as 1,3-propylenediamine, termed as CPVC–NH2
nanober mats. For example, no much morphological variation
was observed for the CPVC–NH2 nanober mats even aer
soaked in DMF solution for 36 hours (also see Fig. 2c). More-
over, the mechanical tensile strength was found to increase
signicantly from 6.50 ꢁ 1.34 MPa (CPVC nanober mat) to
analysis showed that the resultant CPVC–NH nanober mat
2
contained 1.16 ꢁ 0.06% weight of nitrogen, i.e. 0.08 mol per 100
g of the amine functional group content, providing the
conclusive evidence for the amination.
The morphological features of the prepared nanober mats
have been examined by means of the scanning electron
microscopic (SEM) technique. The essentially identical SEM
images of the CPVC nanober mats (Fig. 2a), CPVC–NH2
nanober mats (Fig. 2b) and CPVC–NH –Pd nanober mats
1
1.08 ꢁ 1.56 MPa (CPVC–NH
2
nanober mat) when treated with
2
(Fig. 2d) suggested that the amination and palladium immo-
1,3-propylenediamine. The signicant tensile strength increase
bilization had negligible effects on the morphological structure
of the nanober mats.
from the treatment of bidentate amines signied a cross-linking
structure of the CPVC–NH2 nanober mats. The CPVC and
2
The palladium deposition of the CPVC–NH –Pd heteroge-
CPVC–NH nanober mats were deposited with the palladium
2
neous catalysts was further examined by the X-ray diffraction
(XRD) and SEM-EDX analysis. The weak Pd(111) characteristic
pattern (Fig. 2S†) was consistent with the amorphous structure
of the immobilized palladium species on the CPVC–NH2
nanober mats. The SEM-EDX image (Fig. 3) of the
catalyst according to the similar procedure as reported previ-
ously. Although negligible palladium species was immobilized
5
Table 1 The catalytic activities and chelating energies of the aminated
a
CPVC nanofiber mat supported palladium heterogeneous catalysts
b
c
ꢀ1
Amine ligands
,3-Propanediamine
Ethylenediamine
Diethylenetriamine
Conversion (%) Yield (%) DH (kJ mol )
1
97 ꢁ 3
92 ꢁ 3
89 ꢁ 1
93 ꢁ 2
88 ꢁ 1
84 ꢁ 1
84 ꢁ 2
83 ꢁ 3
ꢀ34
ꢀ93
ꢀ123
ꢀ206
—
Tetraethylenepentamine 85 ꢁ 1
Pd/C
90 ꢁ 4
a
Reaction conditions: palladium catalyst: 50 mg; iodobenzene: 0.70
CO K: 5.25
mmol; n-butyl acrylate: 1.4 mmol; DMSO: 3.0 ml; CH
3
2
ꢂ
b
mmol; reaction temp.: 110 C; time: 1.0 hour. Cross-coupling yields
were determined from the GC/MS measurements based on the
amount of aromatic halide substrate. Results were the average from
c
Fig. 2 SEM images: (a) CPVC nanofiber mat, (b) CPVC–NH
2
nanofiber three separate experiments. Gaussian 03 calculation: B3LYP function
mat aminated by 1,3-propanediamine, (c) aminated CPVC nanofiber with SDD//6-31G(d) basis set.
mat after soaked in DMF for 36 hours and (d) CPVC–NH –Pd catalyst.
2
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activity, stability and recyclability of the prepared
CPVC–NH –Pd heterogeneous catalysts in both DMSO and
2
aqueous solutions. The initial reaction conditions were adapted
7,8
from our previous studies, which were optimized for the
chitosan microsphere supported palladium catalyst. The
conversion and cross-coupling yields were determined to assess
2
the catalytic activities of the resultant CPVC–NH –Pd hetero-
geneous catalysts.
Examination of Table 1 shows that the catalytic activities of
the CPVC–NH –Pd heterogeneous catalysts are considerably
2
higher than that of the carbon supported palladium catalyst
under the same conditions. Most interestingly, the activities of
the prepared CPVC–NH –Pd catalysts were dependent upon the
2
Fig. 4 Linear correlation of the catalytic activities with the chelation
energies of the palladium species with the amine ligands (Table 1).
chemical structure of the multiamines although their morpho-
logical and mechanical features are essentially identical.
Further-more, the catalytic activities could be linearly correlated
with the chelating energies of these amine ligands with the
palladium species as determined by a density functional theory
calculation at the B3LYP/6-31G(d)-SDD level as summarized in
Fig. 4. The linear correlation clearly suggests that the coordi-
nation with the stronger chelating ligand will result in more
2
+
difficult reduction of the divalent palladium (Pd ) into the zero-
0
valent palladium (Pd ) species, leading to a lower catalytic
activity.
Cross-coupling products were obtained in excellent yields
from both organic and aqueous solutions (Fig. 5). There were
negligible effects on the catalytic activity of the recovered
CPVC–NH –pd catalyst from the aqueous solution reaction even
2
Fig. 5 Reuse of the CPVC–NH
aqueous solutions.
2
–Pd catalyst in both DMSO and aer seven recycling times. But the catalytic activity was found
to gradually decrease aer ve recycling times for the recovered
CPVC–NH
shown in Fig. 5. The relatively lower stability of the
–Pd heterogeneous catalyst clearly indicated that the CPVC–NH –Pd catalyst in the DMSO solution is presumably
2
–Pd catalyst from the DMSO solution reaction as
CPVC–NH
2
2
immobilized palladium species was uniformly dispersed onto associated with the stronger chelation of the palladium species
the surface of the nanober mats. The invisible palladium with the solvent DMSO molecules relative to the water mole-
species from the SEM images (Fig. 2d) could be attributed to its cules. This conclusion is consistent with the DFT calculations,
relatively small size, which was estimated to be about 3.2 nm which clearly showed that the chelation energy of DMSO with
1
2
ꢀ1
ꢀ1
from the Scherrer equation.
Pd (DH: ꢀ224 kJ mol ; DG: ꢀ150 kJ mol ) was much larger
The palladium catalyzed Heck cross-coupling reactions have than that of Pd with H O (DH: ꢀ137 kJ molꢀ ; DG:
1
2
ꢀ
1
widely been used to construct the carbon–carbon bond trans- ꢀ72 kJ mol ). Furthermore, the ICP-AES analysis showed that
2
13–15
formations from two sp hybridized carbons.
In this study, the palladium contents of the recovered CPVC–NH –Pd cata-
2
we employed the Heck cross-coupling reaction of aromatic lysts were 2.32 ꢁ 0.11% and 2.43 ꢁ 0.13% aer ve cycles from
iodides and acrylates as a model reaction to assess the catalytic the DMSO and H O solutions, respectively.
2
a
Table 2 Heck cross-coupling reactions of various aromatic halides with different acrylates
b
Entry
Aromatic iodides
Acrylates
Time (hours)
Yield (%)
1
2
3
4
5
6
7
4-FC
4-BrC
4-ClC
6
H
4
I
CH
CH
CH
CH
CH
CH
CH
2
2
2
2
2
2
2
¼ CHCO
¼ CHCO
¼ CHCO
¼ CHCO
¼ CHCO
¼ CHCO
¼ CHCO
2
2
2
2
2
2
2
(n-Bu)
(n-Bu)
(n-Bu)
(n-Bu)
(n-Bu)
(n-Bu)
6
6
6
8
8
3
3
94/91
92/90
97/94
94/88
99/95
90/84
98/92
6
H
4
I
I
6
H
4
4-CH
4-NO
3
C
C
6
H
H
4
4
I
I
2
6
C
C
6
H
H
5
I
I
6
5
CH
3
a
ꢂ
b
Reaction condition: aromatic iodides: 0.70 mmol; acrylate: 1.4 mmol; DMSO: 3.0 ml; CH
3
CO
2
K: 5.25 mmol; reaction temperature: 110 C. Cross-
coupling yields were determined from the GC/MS measurements/isolated yield based on the amount of aromatic halide substrate.
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The prepared CPVC–NH
2
–Pd catalysts were also applicable to
2 D. J. Cole-Hamilton, Science, 2003, 299, 1702–1706.
the Heck reactions of other aromatic iodides and acrylates.
Table 2 presents few representative results as catalyzed by the
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1,3-propylenediamine modied CPVC nanober mat supported
palladium catalyst. Clearly, the new palladium heterogeneous
catalysts worked very well for the Heck cross-coupling of the
aromatic iodides bearing either an electron donating or an
electron accepting substituent.
In summary, CPVC nanober mat has been prepared by
electrospinning. Amination of the CPVC nanober mats with
multiamines could improve the chelating ability, solvent resis-
tance and mechanical properties. The catalytic activities could
be tuned by the chemical structures of the multiamines. The
CPVC–NH
2
-supported palladium has been demonstrated as an
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Acknowledgements
1
This work was supported by the Zhejiang Province Natural
Science Foundation (LQ14E030003).
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1
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Notes and references
1
P. Barbaro and F. Liguori, Chem. Rev., 2009, 109, 515–529.
53108 | RSC Adv., 2014, 4, 53105–53108
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