Palladium immobilized on: In situ cross-linked chitosan superfine fibers for catalytic application in an aqueous medium

Article abstract of DOI:10.1039/c8nj02183j

Chitosan composite superfine fibers with a diameter of 321 ± 99 nm were prepared by electrospinning with PEO as the co-spinning polymer and itaconic acid as the in situ cross-linking agent. Itaconic acid was homogeneously dispersed in chitosan fibers and the in situ cross-linking endowed the prepared chitosan composite fibers with improved solvent resistance, thermostability and mechanical strength. Palladium species immobilized on these annealed chitosan fibers can efficiently catalyze the reduction reaction of 4-nitrophenol as well as the Mizoroki-Heck reaction of aromatic iodides with n-butyl acrylate in an aqueous medium. Furthermore, the larger fiber structure can facilitate the recovery and reuse of the palladium catalyst. The remarkable catalytic performance and easy recovery make this novel fiber-supported palladium catalyst hold great potential applications in green chemistry.

Full text of DOI:10.1039/c8nj02183j

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Palladium immobilized on in situ crossꢀlinked chitosan superfine
fibers for catalytic application in the aqueous medium
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Lulu Liang,a Li Nie,a Minjuan Jiang,a Fusheng Bie,b Linjun Shao,*, a Chenze Qi,a
X. Man Zhang,b Xuejing Liu*, b
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aZhejiang Key Laboratory of Alternative Technologies for Fine Chemicals
Process, Shaoxing University, Zhejiang Province 312000, People’s Republic of China,
and bCollege of Chemistry, Chemical Engineering and Material Science, Zaozhuang
University, Zaozhuang, Shandong, 277160, China
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*Address correspondence to Linjun Shao (shaolinjun@usx.edu.cn)
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Abstract
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Chitosan composite superfine fibers with diameter of 321 ± 99 nm were prepared by
electrospinning with PEO as the coꢀspinning polymer and itaconic acid as the in situ
crossꢀlinking agent. Itaconic acid was homogeneously dispersed in the chitosan fiber
and the in situ crossꢀlinking enabled the prepared chitosan composite fiber with
improved solvent resistance, thermostability and mechanical strength. Palladium
species immobilized on these annealed chitosan fibers can efficiently catalyze the
reduction reaction of 4ꢀnitrophenol as well as the MizorokiꢀHeck reaction of aromatic
iodides with nꢀbutyl acrylates in aqueous medium. Furthermore, the larger fiber
structure can facilitate the recovery and reuse of the palladium catalyst. The
remarkable catalytic performance and easy recovery make this novel fiber supported
palladium catalyst hold great potential applications in the green chemistry.
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Keywords: Chitosan; Electrospinning; Crossꢀlinking; Palladium; Heterogeneous
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Introduction
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Chitosan, derived from deacetylation of biopolymer chitin, has excellent
biocompatibility, biodegradability and adsorption properties, thus it holds great
potential applications in medicine, food, chemistry and waste water treatment [1ꢀ3].
As chitosan contains abundant amine and hydroxyl groups, which could chelate with
the active transition metal species, it has been fabricated into various shapes as
excellent stabilizers and solid matrices to support the transition metal catalysts [4ꢀ7].
Recently, chitosan superfine fibers have attracted great interest as the solid matrices
for immobilization of transition metal catalysts because of its easier recovery and
larger specific surface area [8ꢀ10].
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Electrospinning is a unique technique to fabricate the polymer solution or melt
into fibers with diameter ranging from hundreds nanometer to several microns [11].
Trifluoroacetic acid (TFA) is often required as the solvent for chitosan to be processed
by the electrospinning though it is expensive and toxic [12]. Blending chitosan with a
second water soluble polymer (e.g. polyethylene oxide and polyvinyl alcohol) as the
coꢀspinning agent is another effective way to prepare uniform chitosan fibers by
electrospinning [13ꢀ15]. Moreover, a carefully selected polymer could even improve
the physical and chemical properties of the composite fibers. Since chitosan fibers are
unstable and easily soluble in the acidic solution, crossꢀlinking is usually required by
treatment with the crossꢀlinking agent (e.g. glutaraldehyde and epichlorohydrin)
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[16ꢀ18]. However, the mechanical strength and chelating ability of chitosan fibers
could be reduced after crossꢀlinking with glutaraldehyde because glutaraldehyde was
primarily reacted with the surface amine groups of chitosan fiber, whereas the fiber
chitosan chains could not be homogeneously crossꢀlinked [18].
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In this study, the crossꢀlinker itaconic acid was used to incorporate into the
chitosan composite fibers. After electrospinning, in situ thermal crossꢀlinking was
induced to improve the solvent resistance, thermostability and mechanical strength for
the composite fiber. Moreover, the crossꢀlinked chitosan composite fibers could
effectively chelate with the palladium species (Figure 1). This novel chitosan fiber
supported palladium catalyst exhibites remarkable activity and stability to catalyze the
reduction reaction of 4ꢀnitrophenol as well as the MizorokiꢀHeck reaction of aromatic
iodides with olefins in aqueous medium.
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H2
N
OH
NH2
OH
OH
O
OH
O
OH
O
NH2
NH2
COOH
O
OH
HO
HO
O
HO
HO
HOOC COOH
NH2
NH
2 n
NH2
NH2
COOH
OH
NH2
Electrospinning
H2N
CS
HO
O
NH2
OH
O
HO
n
O
IA
PEO
Co-spinning agnet
: PEO
: CS
In situ crosslinker
HOOC COOH: IA
In situ crosslinker
O
H2
N
H2
OH
N
O
OH
NH
OH
NH2
H2N
NH2
O
HN
HN
OH
O
NH
Pd
C
C
C
Thermal
C
C
O
O
O
O
C
O
NH
NH
C
O
C O
Immobilizaiton
treatment
NH2
NH2
H2
N
H2N
HO
OH
NH2
NH2
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Figure 1. Schematic diagram for fabrication of the PdꢀCS/PEO/IA fibers.
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Experimental
Materials
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Chitosan (CS) (pharmaceutical grade, Mn = 2.0 × 104, deacetylated degree: 95 %) was
bought from Zhejiang Aoxing Biotechnology Co., Ltd (Zhejiang, China). PdCl2
(chemical purity) was purchased from Hangzhou Changqing Chemical Industrial Co.
Ltd (Zhejiang, China). Poly(ethylene oxide) (PEO) (average molecular weight = 1.0 ×
106), itaconic acid (IA) (analytical grade), cetryltrimethyl ammonium bromide (CTAB)
(analytical grade), tetrabutyl ammonium bromide (TBAB) (analytical grade) and
PEGꢀ2000 were bought from Aladdin Industrial Co. Ltd (Shanghai, China). Deionized
water was used in all experiments.
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Preparation of the cross-linked CS/PEO/IA composite fibers
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Chitosan (0.95 g), PEO (0.05 g) and IA (0.20 g) were dissolved in 19 g 80% acetic
acid aqueous solution and stirred overnight to achieve homogeneous solution. The
solution was then applied into the electrospinning apparatus (FM1206, Beijing Future
Material Sciꢀtech Co., Ltd, China) to prepare chitosan composite fibers. The
optimized electrospinning parameters were: inner diameter of needle: 0.51 mm; feed
rate: 1.2 mL/h; applied voltage: 18 kV; work distance: 15 cm. The resultant fiber mat
o
was dried under vacuum to remove the residual solvent at 100 C, followed by
o
annealing at 150 C for 12 hrs. The CS/PEO fibers were also prepared using the
similar preparation protocol for the CS/PEO/IA composite fibers without addition of
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IA.
Immobilization of palladium on cross-linked CS/PEO/IA composite fibers
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The CS/PEO/IA fiber mat (1.0 g) was added to a roundꢀbottom flask containing 25 mg
PdCl2, 17 mg NaCl and 5 g H2O (Na2PdCl4 aqueous solution), which was allowed to
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stir at 25 C for 12 hrs, followed by addition of 0.5 g hydrazine (80%) to reduce the
Pd2+ into Pd0 species. After reduction, the fiber mat turned back, indicative of
palladium immobilization on the fibers. The fibers supported palladium catalyst
(PdꢀCS/PEO/IA) was filtered, and then dried under a reduced pressure at 50 oC for 12
hrs.
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The palladium content of the PdꢀCS/PEO/IA catalyst was determined according
to the following protocol: 20 mg PdꢀCS/PEO/IA was completely decomposed in 5 mL
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mixture of fuming HNO3 and H2SO4 (v/v: 1/1) at 80 C, then diluted to 50 mL with
deionized water. The palladium concentration was determined by means of
inductively coupled plasmaꢀatomic emission spectroscopy, which was allowed to
calculate the palladium content (1.35%) for the PdꢀCS/PEO/IA catalyst.
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Reduction of 4-nitrophenol catalyzed by the fiber catalyst
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Fresh prepared NaBH4 aqueous solution (2.0 mL, NaBH4 content: 0.65 mmol),
4ꢀnitrophenol aqueous solution (2.0 mL, pꢀnitrophenol content: 6.5 ꢁmol) and 46 mL
water were added into a 100 mL flask, followed by addition of the PdꢀCS/PEO/IA (5.0
mg, Pd: 0.63 ꢁmol), then stirred vigorously to reduce the diffusion influence at room
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temperature. The reductive reaction was monitored by UVꢀVis analysis. After
completion, the PdꢀCS/PEO/IA catalyst was recovered by filtration and washed with
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water and ethanol. The recovered catalyst was dried under reduced pressure at 50 C
for 12 hrs.
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General procedure for the Mizoroki-Heck reaction catalyzed by the fiber catalyst
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Aromatic iodides (0.7 mmol), olefins (1.4 mmol), PdꢀCS/PEO/IA (50 mg, Pd: 6.3
ꢁmol), CTAB (63 ꢁmol) and CH3COOK (2.1 mmol) and deionized water (5.0 g) was
added to a 20 mL tubular reactor equipped with a magnetic stir bar. The mixture was
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allowed to stir at 100 C for 24 hrs. The reaction was extracted with ethyl acetate for
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three times (3 × 20 mL). The combined organic layer was washed with water, and
then dried over anhydrous Na2SO4, and the solvent was removed under reduce
pressure. The coupling products were obtained by purification using silica gel
chromatography with a mixture of petroleum ether and ethyl acetate (v/v: 8/1), and
the chemical structures were confirmed by 1H NMR spectroscopy. The PdꢀCS/PEO/IA
catalyst was filtered from the mixture, then washed with N,N –dimethylformamide,
water and ethanol. The recovered PdꢀCS/PEO/IA catalyst was dried under reduced
pressure at 50 oC for 12 hrs.
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Characterization
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The morphologies of the fiber mats were observed with a scanning electron
microscope (SEM) (Jeol, Jsmꢀ6360lv, Japan). The SEM samples were coated with a
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2ꢀ3 nm layer of Pt to make them conductive. The fiber diameters were acquired from
the SEM images. The elemental analysis was determined by means of an energy
dispersive Xꢀray spectroscopy (Oxford EDX System). The chemical structures of
fiber mats were analyzed by Fourier transform infrared spectroscopy (FTꢀIR) with the
accessories of attenuated total reflection (Nicolet, Nexusꢀ470, USA). The
thermostability of the fiber mat was analyzed using a thermogravimetric analyzer
(TGA) (Beijing Scientific Instrument Factory, China) in air. The heating ramp was set
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from 40 to 550 C at a rate of 20 C/min. UVꢀVis absorption spectrum was recorded
on a UVꢀ1800PC spectrometer (Mapada, China). The quantitative analysis of
MizorokiꢀHeck reaction products was carried out via Agilent GC/MS (Agilent,
GC6890/5975 MSD, USA). The dispersion of palladium nanoparticles in the
PdꢀCS/PEO/IA was analyzed by Xꢀray diffraction (XRD) (Empyrean, PANalytical,
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Netherlands). H 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). Inductively coupled plasmaꢀatomic emission spectroscopy
(ICPꢀAES) analysis was performed on a Leemann ICPꢀAES Prodigy XP (Leeman
Labs, USA). The mechanical analysis was carried out on the electronic fabric strength
tester (YG065HC/PC, Shandong Lanzhou Electron Instrument Co., Ltd, China). The
specific surface area of the CS/PEO/IA fiber mat was determined by the standard
BrunaverꢀEmmettꢀTeller (BET) method on the specific surface area and porosity
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analyzer (Empyrean, Micromeritics, USA). Xꢀray photoelectron spectroscopic (XPS)
was performed by angleꢀresolved photoemission spectroscopy (ADESꢀ400, VG Co.
UK). C1s of 284.6 eV was used as the internal reference for calibration.
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Results and Discussion
Characterization of the Pd-CS/PEO/IA catalyst
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Blending chitosan with PEO for electrospinning is a unique efficient technique to
prepare continuous chitosan superfine fibers [12,13,17,21]. The CS/PEO fibers with
diameter of 321 ± 99 nm were successfully prepared with merely 5% of PEOꢀloading
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(Figure 1A). Although the CS/PEO fiber mat was annealed at 150 C for 12 hrs, the
fiber structure was found to be completely damaged after submersing in 50 wt.%
acetic acid solution for 24 hrs (Figure 2B). The CS/PEO/IA fiber mats with 10%, 20%
and 30% loadings of IA were also prepared to examine their solvent resistance (Figure
2C and S1). Figure 2D and S1 shows that the CS/PEO/IA fibers with 10% loading of
IA has been seriously swollen, whereas the fibers with 20% and 30% loadings of IA
are well retained after submersing in 50 wt.% acetic acid solution for 24 hrs. Thus,
20% loading of IA was chose to crosslink the CS/PEO/IA fibers for this study.
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(A)
(B)
(C)
(D)
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Figure 2. SEM images of the CS/PEO fiber mat (A, B) and the CS/PEO/IA fiber mat
with 20 wt.% of IA loading (C, D) before and after submersing in 50 wt.% acetic acid
solution for 24 hrs.
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The chemical structures of chitosan composite fiber mats were analyzed by
FTꢀIR spectra (Figure 3). Compared with the FTꢀIR spectrum of the CS/PEO fiber mat,
incorporation of IA into the fiber resulted in observation of a new absorption peak at
1706 cmꢀ1 (>C=O of carboxyl group) for the CS/PEO/IA fiber mat. After annealing
treatment, the absorption at 1706 cmꢀ1 disappeared and a new band appeared at 1306
cmꢀ1 (amide III), accompanied by intensifying absorption peak at 1653 cmꢀ1 (amide I)
[22]. These results indicate that the CS/PEO/IA fibers have been crossꢀlinked by
reactions of the carboxyl groups on IA with the amino groups of the chitosan to form
the amide functional groups. After immobilization of the palladium species, the
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absorption peaks at 1563 cmꢀ1 (ꢀNH2) and 1653 cmꢀ1 (>C=O) are shifted, suggesting
coordination of the fiber surface chitosan with the palladium species, thus the
crossꢀlinked CS/PEO/IA can effectively chelate with the palladium species.
CS/PEO fiber mat
100
CS/PEO/IA fiber mat before annealing
95
CS/PEO/IA fiber mat after annealing
1706
90
Pd-CS/PEO/IA fiber mat
1306
1563
1653
1650
85
80
1571
4000
3500
3000
2500
2000
1500
1000
Wavenumber (cm-1)
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Figure 3. FTꢀIR spectra for the CS/PEO fiber mat and the CS/PEO/IA fiber mat
before and after annealing treatment, together with the PdꢀCS/PEO/IA fiber mat.
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As the heterogeneous palladium catalysts are often used at relatively high
reaction temperature, the thermostabilities of supporting materials are critical for their
practical applications. Figure 4 shows that weigh loss of the CS/PEO/IA fiber mats
before and after annealing treatment took place in three major stages. The first stage at
50ꢀ130 oC was assigned to loss of the adsorbed water. The predominant weight losses
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of the second (175ꢀ375 C) and third stages (450ꢀ550 C) may be attributed to the
thermal oxidation, and carbonization of chitosan and PEO. The thermostability of the
CS/PEO/IA fiber mat has been improved after annealing treatment. Moreover, tensile
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strength test shows that the tensile strength of the CS/PEO/IA fiber mat also increases
from 4.30 MPa to 5.21 MPa after annealing treatment. The high tensile strength of the
fiber mat could improve the resistance from damage in the following harsh reaction
conditions. The BET surface was determined to be 9.53 m2/g for the CS/PEO/IA fiber
mat. The strong chelating ability, larger specific surface area, excellent mechanical
strength and solvent resistance make this novel CS/PEO/IA fiber mat an ideal polymer
matrix to support transition metal catalyst.
120
Before heat treatment
After heat treatment
1.6
1.2
0.8
0.4
0.0
80
40
0
Stage II
Stage III
Stage I
100
-40
200
300
400
500
Temperature (oC)
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Figure 4. Thermal analysis results of CS/PEO/IA fiber mat before and after annealing
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treatment.
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SEMꢀEDX characterization shows that the palladium species has been deposited
homogeneously on the PdꢀCS/PEO/IA fiber mat (Figure 5). Besides SEMꢀEDX
characterization, the PdꢀCS/PEO/IA was further analyzed by XRD (Figure 6). As
chitosan is a partially crystalline polysaccharide due to its regular chain, it has a
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strong reflection peak at 18.4o [20,22]. The reflection peaks at 39.6°, 45.9° and 67.5°
are associated with the (111), (200) and (220) planes of the face centered cubic (FCC)
palladium particles [23ꢀ25]. Based on the Scherrer equation [24,25], the reflection
peaks suggest an average size of palladium particles (d) closed to 9.3 nm. XPS
analysis shows that the palladium 3d5/2 and 3d3/2 electron binding energies of the
PdꢀCS/PEO/IA catalyst are about 1.3 eV lower after than before reduction, indicating
the reduction of the divalent Pd2+ into the Pd0 species (Figure S2).
Element
wt.%
29.46
17.41
43.55
1.55
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C
N
O
Cl
Pd
8.03
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Total:
100.00
Figure 5. SEMꢀEDX analysis of the PdꢀCS/PEO/IA.
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CS
2500
2000
1500
1000
500
39.4
Pd(111)
45.6
Pd(200)
67.8
Pd(220)
0
0
10
20
30
40
50
60
70
80
90
2
θ
(degree)
1
2
Figure 6. XRD pattern of the PdꢀCS/PEO/IA.
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Reduction of 4-nitrophenol catalyzed by the Pd-CS/PEO/IA catalyst
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Since nitrophenols and their derivatives are highly toxic pollutants derived from
pesticides, herbicides, insecticides and synthetic dyes, it is important to reduce them
into much less toxic aminophenols [5,26,27]. The catalytic activity of the
PdꢀCS/PEO/IA catalyst has been investigated for the reductive reaction of
4ꢀnitrophenol with excess NaBH4 in aqueous medium. The reductive reaction is very
slow without addition of the palladium catalyst (Figure 7), but the 4ꢀnitrophenol
conversion can reach nearly 100 % within 30 min when catalyzed with the
PdꢀCS/PEO/IA.
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3.0
3.0
(b)
1
2
3
4
(a)
4-nitrophenol
4-nitrophenol
2.5
2.5
0 min
3 min
6 min
10 min
15 min
30 min
0 min
3 min
6 min
10 min
15 min
30 min
2.0
2.0
1.5
1.0
0.5
0.0
1.5
1.0
4-aminophenol
4-aminophenol
0.5
0.0
250
300
350
400
450
500
250
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
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Figure 7. UVꢀVis spectra for the reductive reactions of 4ꢀnitrophenol: (a) no catalyst
and (b) with the PdꢀCS/PEO/IA. Reaction condition: 0.65 mmol NaBH4, 6.5 ꢁmol
4ꢀnitrophenol and 0.63 ꢁmol PdꢀCS/PEO/IA in 50 mL water at 25 oC for 30 min.
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The reductive reaction of 4ꢀnitrophenol into 4ꢀaminophenol was believed to
proceed via the pseudoꢀfirstꢀorder kinetics in excess of NaBH4 when catalyzed by
metal catalysts [27]. The reaction was examined using the pseudoꢀfirstꢀorder kinetic
model according to the following kinetic equation (Eq 1).
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ln (Ct/C0) = ln (At/A0) =ꢀkapp
t
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( 1 )
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where C0 and Ct are the 4ꢀnitrophenol concentrations at initial time and time t,
respectively. The concentration ratio Ct/C0 can be determined from the absorbance
ratio of 4ꢀnitrophenol At (at time t) to A0 (at initial time). As shown in Figure 8, the
apparent rate constant (kapp) were obtained to be 1.76 × 10ꢀ4 sꢀ1 and 1.91 × 10ꢀ3 sꢀ1 for
no catalyst and with the PdꢀCS/PEO/IA, respectively. Interestingly, the rate constant
for the PdꢀCS/PEO/IA is comparable with the literature results [27,28].
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0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
No catalyst
Y=-0.03-1.76
R=0.9530
Pd-CS/PEO/IA
× X
10-4
Y=0.15-1.91
R=0.9970
× X
10-3
-200
0
200 400 600
800 1000 1200 1400 1600 1800 2000
Time (s)
1
2
3
Figure 8. Plot of ln(At/A0) verses reaction time for the reductive reaction of
4ꢀnitrophenol by no catalyst and the PdꢀCS/PEO/IA catalyst.
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The heterogeneous catalysts have advantage for their relatively easy separation
and reuse of the expensive transition metals. Generally, supporting materials are
prepared into small fine particles to maximize the surface area to enhance the catalytic
activities. Compared to the tiny particles, the larger fiber structure ensures the easy
separation of the PdꢀCS/PEO/IA catalyst from the reaction mixture. As shown in
Figure 9, the PdꢀCS/PEO/IA catalyst can be reused for 5 times without obvious loss of
activity.
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90
80
70
60
50
40
30
20
10
0
1
2
3
4
5
Recycling Times
1
2
3
4
Figure 9. Dependence of the 4ꢀnitrophenol conversion with the recycling times for
the PdꢀCS/PEO/IA. Reaction condition: 0.65 mmol NaBH4, 6.5 ꢁmol 4ꢀnitrophenol
and 0.63 ꢁmol PdꢀCS/PEO/IA in 50 mL water at 25 oC for 30 min.
5
Mizoroki-Heck reaction catalyzed by the Pd-CS/PEO/IA
6
7
MizorokiꢀHeck reaction was one of the most powerful and straightforward methods
for construction of the sp2ꢀsp2 carbonꢀcarbon bonds [29,30], which attracts the
interests to search for more environmental benign reaction conditions [31,32].
Heterogeneous palladium catalyst has also attracted much attention for their easy
recovery and reuse of expensive and toxic palladium catalyst [33ꢀ35]. Furthermore,
different green solvents have been evaluated for this reactions [36ꢀ38]. Among the
various solvent, water is clearly the ideal green solvent. In order to improve the
solubility of organic substrates in water, a phase transfer agent (PTA) is generally
needed to promote the reaction. The quaternary ammonium salt can stabilize the
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zeroꢀvalent palladium species, and the long alkyl chain can also increase the
dispersion of reaction substrates [39,40]. Examination of entries 1ꢀ4 in Table 2 shows
that CTAB is the most effective PTA to promote the reaction, and the MizorokiꢀHeck
reaction yield increases with the CTAB loading (Entry 4ꢀ6 in Table 1). The optimal
nꢀbutyl cinnamate yield (89%) can be obtained when the CTAB/Pd ratio reaches 10,
and no further improvement of the reaction yields has been observed with higher
CTAB/Pd ratios.
8
9
Table 1. Effect of phase transfer reagent on the PdꢀCS/PEO/IA catalyzed
MizorokiꢀHeck reactions of iodobenzene with methyl acrylate a
Entry
PTA
PTA/Pd ratio
Conversionb
Yieldb
(%)
21
25
30
70
89
96
98
(%, trans)
1
2
3
4
5
6
7
No
0
17
22
27
61
76
89
90
PEGꢀ2000
TBAB
CTAB
CTAB
CTAB
CTAB
1
1
1
5
10
15
10
11
a Reaction conditions: 0.7 mmol iodobenzene, 1.4 mmol methyl acrylate, 2.1 mmol
potassium acetate, 6.3 ꢁmol PdꢀCS/PEO/IA, and PTA in 5.0 g water at 100 oC for 24
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2
hrs.
b GCꢀMS conversion and yield based on the amount of iodobenzene.
3
4
MizorokiꢀHeck reactions require suitable bases to neutralize the acidic
byproducts, otherwise they would be suppressed. Examination of entry 1 in Table 2
shows that the coupling yield of methyl cinnamate is only 27% without base. Entries
3ꢀ6 in Table 2 shows that the potassium compounds are more effective than the
sodium compounds, whereas carbonate is more effective than hydroxide at promoting
the PdꢀCS/PEO/IA catalyzed MizorokiꢀHeck reaction. Triethylamine has comparable
activity with K2CO3 as base (Entry 7 in Table 2). Among the tested bases, potassium
acetate is the most effective base to promote the PdꢀCS/PEO/IA catalyzed
MizorokiꢀHeck reaction with yield of 89% (Entry 2 in Table 2). Thus, potassium
acetate has been chosen for the following experiments.
5
6
7
8
9
10
11
12
13
14
Table 2. Base effects on the PdꢀCS/PEO/IA catalyzed MizorokiꢀHeck reactions of
iodobenzene with methyl acrylate.a
Entry
Base
Conversionb (%)
Yieldb (%)
1
2
3
4
5
No
29
96
70
75
32
27
89
54
71
32
KOAc
KOH
K2CO3
NaOH
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7
Na2CO3
Et3N
67
81
66
67
1
2
3
aReaction conditions: 0.7 mmol iodobenzene, 1.4 mmol methyl acrylate, 2.1 mmol
base, 6.3 ꢁmol PdꢀCS/PEO/IA, and 63 ꢁmol PTA in 5.0 g water at 100 oC for 24 hrs.
bGCꢀMS conversion and yield based on the amount of iodobenzene.
4
5
6
7
8
9
Table 3 shows that high temperature is beneficial for both iodobenzene
conversion and methyl cinnamate yield. No coupling product was detected when
o
the reaction was carried out below 80 C, whereas iodobenzene was almost
o
consumed for the reaction with methyl acrylate at 100 C. Thus, the optimal
o
reaction temperature is 100 C for the PdꢀCS/PEO/IA catalyzed MizorokiꢀHeck
reaction.
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11
Table 3. Temperature effects on the PdꢀCS/PEO/IA catalyzed MizorokiꢀHeck
reactions of iodobenzene with methyl acrylate.a
Entry
Temperature (oC) Conversionb (%)
Yieldb (%)
1
2
3
4
80
~0
55
86
96
~0
46
83
89
90
95
100
12
13
aReaction conditions: 0.7 mmol iodobenzene, 1.4 mmol methyl acrylate, 2.1 mmol
base, 6.3 ꢁmol PdꢀCS/PEO/IA, and 63 ꢁmol PTA in 5.0 g water for 24 hrs.
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bGCꢀMS conversion and yield based on the amount of iodobenzene.
2
3
4
5
6
7
8
9
The PdꢀCS/PEO/IA catalyst is also effective for the MizorokiꢀHeck reactions of
other aryl iodides with olefins, as evident in Table 4. Entries 2ꢀ5 in Table 4 shows that
the PdꢀCS/PEO/IA catalytic system is tolerant for both electronꢀrich and
electronꢀdeficient aromatic iodides to produce the corresponding reaction products in
excellent yields. The relatively low coupling yield of 1ꢀiodoꢀ2ꢀmethylbenzene with
nꢀbutyl acrylate could be attributed to the steric hindrance of 2ꢀmethyl group (Entry 6
in Table 4). The PdꢀCS/PEO/IA catalyst also exhibits high activity for the
monoꢀsubstituted acrylates (Entries 7 and 8 in Table 4).
10
11
Table 4. PdꢀCS/PEO/IA catalyzed MizorokiꢀHeck reactions of various aromatic
halides with different olefins in the aqueous medium.a
O
I
O
9 ‰ PdꢀCS/PEO/IA
CTAB, KAc, H2O
R2
+
R2
R1
O
O
R1
12
Entry
Aromatic
iodides
PhI
Olefin
Conversion Yield
(%)
95
(%, trans)
1
nꢀbutyl acrylate
89
2
3
4
4ꢀFPhI
4ꢀBrPhI
4ꢀClPhI
nꢀbutyl acrylate
nꢀbutyl acrylate
nꢀbutyl acrylate
95
99
98
87
91
90
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7
8
4ꢀCH3PhI
2ꢀCH3PhI
PhI
nꢀbutyl acrylate
nꢀbutyl acrylate
methyl acrylate
CH2=CHꢀPh
99
96
96
88
97
87
89
81
PhI
1
2
3
a Reaction conditions: 0.7 mmol aromatic iodides, 1.4 mmol olefin, 2.1 mmol
potassium acetate, 6.3 ꢁmol PdꢀCS/PEO/IA, and 63 ꢁmol CTAB in 5.0 g water at 100
oC for 24 hrs.
4
b GCꢀMS conversion and yield based on the amount of aromatic halides.
5
6
The reusability of PdꢀCS/PEO/IA for the MizorokiꢀHeck reaction has been
examined, and the results have been summarized in Figure 10. The catalytic activities
of the recovered PdꢀCS/PEO/IA catalyst retain over five cycles for the MizorokiꢀHeck
reaction of iodobenzene with nꢀbutyl acrylate. Moreover, the fiber structure of the
recovered PdꢀCS/PEO/IA catalyst is still well retained (Figure S3). The palladium
content (0.83%) in the recovered PdꢀCS/PEO/IA catalyst indicates that only about
26% of the immobilized palladium species had been leached after 5 cycles of
applications. The remarkable stability of the PdꢀCS/PEO/IA catalyst can be associated
with the strong chelation of the palladium species with the amine groups on the fiber
surface.
7
8
9
10
11
12
13
14
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100
80
60
40
20
0
Conversion
Yield
1
2
3
4
5
Recycling Times
1
2
3
4
5
Figure 10. Dependence of the MizorokiꢀHeck reaction conversions and yields with
the recycling times for the PdꢀCS/PEO/IA catalyst. Reaction condition: 0.7 mmol
iodobenzene, 1.4 mmol nꢀbutyl acrylate, 2.1 mmol potassium acetate, 6.3 ꢁmol
PdꢀCS/PEO/IA, and 63 ꢁmol CTAB in 5.0 g water at 100 oC for 24 hrs.
6
7
Conclusions
8
9
In summary, we have demonstrated a facile approach to fabricate the crossꢀlinked
chitosan superfine fibers. In situ crossꢀlinking was induced in the fiber to enhance its
solvent resistance, thermostability and mechanical strength. This crossꢀlinked chitosan
fiber holds great potential as the supporting material to chelate the palladium catalyst,
larger surface area of superfine fiber structure, and stability in aqueous medium. As a
demonstration, palladium catalyst has been immobilized on these crosslinked
CS/PEO/IA superfine fibers, which was shown as an efficient and stable
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heterogeneous palladium catalyst for the reductive reaction of 4ꢀnitrophenol as well as
the MizorokiꢀHeck reactions of aromatic iodides with olefins in the aqueous medium.
3
4
Acknowledgement
5
6
The authors thanks very much for the financial support from the National Natural
Science Foundation of China (Grant No. 11705117).
7
8
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Graphical Abstract
CH2OH
O
CH2 CH2
PEO
O
Reduction
Mizoroki-Heck
100
80
60
40
20
0
O
n
O
HO
n
H2
N
O
+
OH
HO
O
Chitosan (CS)
Itaconic acid (Crosslinker)
Electrospinning
9.7 % catalyst
HO
NO2
HO
NH2
NaBH4
O
O
In situ crosslinking
Pd immobilization
I
9 ¡ë catalyst
n-Bu
O
+
n-Bu
KAc, H2O
O
1
2
3
4
5
Recycling Times