Preoxidated polyacrylonitrile fiber mats supported copper catalyst for Mizoroki-Heck cross-coupling reactions

Article abstract of DOI:10.1016/j.apcata.2013.08.010

Porous preoxidated polyacrylonitrile (PAN) fiber mats, which was prepared by electrospinning and then followed by thermal treatment, has been used for the immobilization of copper catalyst. FT-IR/ATR spectra and XRD pattern show that the linear polymer structure of the PAN fiber mat can be converted into the ladder molecular structure at the high temperature. The morphology of the copper catalyst supported on the preoxidated PAN fiber mats was examined by scanning electron microscopy (SEM). The catalytic activity and stability of the fiber mat supported copper catalyst have been evaluated using the Mizoroki-Heck cross-coupling reaction of aryl iodides with different acrylates. It was found that the catalytic activities of fiber mat supported copper catalyst could be significantly promoted by addition of small amount of additives, such as ethylene glycol.

Full text of DOI:10.1016/j.apcata.2013.08.010

Applied Catalysis A: General 468 (2013) 26–31
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Preoxidated polyacrylonitrile fiber mats supported copper catalyst for
Mizoroki–Heck cross-coupling reactions
∗
Linjun Shao, Chenze Qi
Zhejiang Key Laboratory of Alternative Technologies for Fine Chemicals Process, Shaoxing University, Zhejiang Province 312000, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Porous preoxidated polyacrylonitrile (PAN) fiber mats, which was prepared by electrospinning and then
followed by thermal treatment, has been used for the immobilization of copper catalyst. FT-IR/ATR spec-
tra and XRD pattern show that the linear polymer structure of the PAN fiber mat can be converted into
the ladder molecular structure at the high temperature. The morphology of the copper catalyst sup-
ported on the preoxidated PAN fiber mats was examined by scanning electron microscopy (SEM). The
catalytic activity and stability of the fiber mat supported copper catalyst have been evaluated using the
Mizoroki–Heck cross-coupling reaction of aryl iodides with different acrylates. It was found that the cat-
alytic activities of fiber mat supported copper catalyst could be significantly promoted by addition of
small amount of additives, such as ethylene glycol.
Received 21 March 2013
Received in revised form 15 June 2013
Accepted 8 August 2013
Available online xxx
Keywords:
Copper catalyst
Mizoroki–Heck cross-coupling reaction
Porous polyacrylonitrile fiber
Electrospinning
© 2013 Elsevier B.V. All rights reserved.
1
. Introduction
The vinylation of aryl halides (Mizoroki–Heck reaction) is one
more challenge to develop robust heterogeneous copper catalyst
for the potential industrial applications because the catalytic activ-
ity of copper catalyst is generally much lower than that of palladium
catalyst.
of the most powerful methods for the carbon–carbon (C C) chem-
ical bond formation in synthetic organic chemistry [1,2], but the
related cross-coupling reactions generally require palladium cat-
alyst [3]. In view of the economy and environmental protection,
it is necessary, but yet very challenged, to recover and recycle
the toxic and expensive palladium catalyst for these transforma-
tions, especially for the homogeneous catalytic systems. Certainly,
it is desirable to develop easily recyclable heterogeneous catalysts
using relative cheaper and less toxic transition metals such as cop-
per [4–8], cobalt [9–11] and nickel [12,13]. Clearly, copper is the
transition metal of choice and it has indeed been used for numer-
ous coupling reactions such as Ullmann, Henry and Mizoroki–Heck
reaction [4,14–18].
Elevation of the reaction temperature will increase the catalytic
activity, but the high reaction temperature may be detrimental
for relatively unstable chemical compounds besides higher energy
consumption. Thus, addition of reaction promoter has been con-
sidered as an economic and practical approach to enhance the
activities of copper catalysts. The copper catalytic activities for var-
ious coupling reactions were found to be significantly improved by
addition of chelating ligands such as ␤-diketones [22], tetraalky-
lammonium and phosphonium cations [23], zinc chloride [24] and
others [25,26]. We have shown that addition of small amount of
ethanol could significantly increase the Pd/C catalyzed Ullmann
homocoupling of various aryl iodides and bromides [27].
Heterogenization of a homogeneous catalyst can significantly
improve the isolation and recycling of the transition metal, but it
will inevitably reduce the catalytic activities because the catalytic
reaction occurs only on the heterogeneous surface, rather than all
accessible catalytic centers in the homogeneous system. In our pre-
vious study, palladium catalyst has been successfully supported
on crosslinked poly(vinyl alcohol) fiber mat, polyacrylonitrile fiber
mat and chitosan microsphere, which all showed excellent catalytic
performance for C C coupling reactions [19–21]. However, it poses
On the other hand, the solid supports are typically fabricated
into fine particles to maximize the surface area in order to enhance
the activities of the heterogeneous catalysts, but it is a tedious and
time-consuming process to separate and recycle the particulate
catalysts due to the high pressure drop required for filtration. The
large size of the porous PAN fiber mat has the advantage for easy
recovery and recycling as the solid support to immobilize the tran-
sition metals [28]. In the present work, we would like to report the
Mizoroki–Heck cross-coupling reactions as catalyzed by the preox-
idated polyacrylonitrile (PAN) fiber mat supported copper catalyst.
The PAN fiber mat supported copper catalyst can be conveniently
recovered and reused, and the catalytic activity could be signifi-
cantly improved by addition of small amount of chelating ligand.
∗ Corresponding author. Tel.: +86 575 8834 5682; fax: +86 575 8834 5682.
E-mail address: qichenze@usx.edu.cn (C. Qi).
0
926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.08.010

L. Shao, C. Qi / Applied Catalysis A: General 468 (2013) 26–31
27
2
. Experimental
Cyclization
CN CN CN
PAN fiber
Dehydrogenation
2.1. Materials
All organic reagents are commercial products of the highest
CN CN CN
N
N
N
purity available (98%) and used without further purification. Poly-
acrylonitrile (PAN) was synthesized according to our previous
procedure [29]. The quantitative analysis was performed on a Shi-
madzu (GC-14B) gas chromatograph. The morphology images of
the electrospun fiber mats were recorded with a scanning elec-
tron microscope (SEM) (Jeol, jsm-6360lv, Japan). Samples were
coated with a 2–3 nm layer of Au to make them conductive for
the SEM analysis. Elemental analysis was performed on a EuroEA
Dehydrogenation
Cyclization
N
N
N
Preoxidated PAN fiber
Fig. 1. Approach for the synthesis of preoxidated PAN fiber.
2
.4. General procedure for Cu/PrePAN fiber mat separation and
reuse
3
000 from Leeman, USA. FT-IR/ATR spectra were recorded on a
FT-IR spectrometer (Nicolet, Nexus-470, USA) with the accessories
of attenuated total reflection. Phase composition of the prepared
samples was determined by means of X-ray powder diffraction
After completion, the heterogeneous Cu/PrePAN fiber mat was
filtered, and then washed with deionized water and ethanol. The
recovered catalyst was then dried at 30 C for 12 h under reduced
(
XRD) (Rigaku D, max-3BX, Japan). Inductively coupled plasma-
◦
atomic emission spectroscopy (ICP-AES) analysis was performed
on a Leemann ICP-AES Prodigy XP (Leeman Labs, USA). X-ray pho-
toelectron spectra (XPS) measurements were carried out on a RBD
upgraded PHI-5000C ESCA system with an Al/Mg anode (Perkin
Elmer, USA).
pressure.
3. Results and discussion
3.1. Characterization of the Cu/PrePAN fiber mat
2.2. Preparation of preoxidated PAN fiber mat supported copper
Electrospinning is a versatile polymer processing technique
catalyst
for the preparation of the ultrafine nanofibers with diameters
in the nanometer to submicrometer range. Polyacrylonitrile and
acrylonitrile-based polymers are among the most common sub-
strates for electrospinning due to their excellent spinning ability
and chemical stability [30]. Because of the high surface area to
volume ratios, the ultrafine fiber mats have many different appli-
cations such as in water treatment, sensor, and tissue engineering
[31]. The linear PAN molecular chain structure could be converted
into the aromatic ladder structure after the preoxidation step
[32–35] (Fig. 1). Additional higher temperature treatment (above
PAN was firstly dissolved in N,N-dimethylformide (DMF) to form
a homogeneous solution at a concentration of 8 wt.% of PAN. The
resultant solution (20 g) was then slowly added with a K CuI solu-
2
3
tion prepared from 0.24 g CuI (1.33 mmol) and 0.44 g KI (2.66 mol)
in 6.0 g DMF, followed by electrospinning with a syringe (20 mL)
and a blunt-end capillary (0.8 mm ID). An electrically ground alu-
minum foil was placed under the syringe at a distance of 12 cm
as a collector. The feed rate of the PAN solution was maintained
at 1.0 mL/h by a micro-infusion pump (WZ-50C6, Zhejiang Smiths
Medical Instrument, China) under a voltage of 12 kV (GDW-a, Tian-
jin Dongwen High-Voltage Power Supply Company, China). After
the copper catalyst coated PAN fiber mats were dried under vac-
uum at room temperature, they were placed between two stainless
◦
500 C) under nitrogen or argon atmosphere led to the conversion
of the preoxidated PAN fiber into carbon nanofiber, which can be
used as the supporting materials for the palladium catalyst [36].
Moreover, the preoxidated PAN fibers have much higher strength
and higher resistance toward heat and corrosion. Generally, high
and adequate stretching is usually imposed in the thermal treat-
ment process to avoid the formation of amorphous carbon, which
had no physical strength. Recently, Liu et al. [37] have reported
that well-defined aligned preoxidated PAN fiber bundles could be
prepared by washing, drying desification, damp-heat drafting, and
preoxidation process. Herein, it was found that keeping the PAN
fiber mat between two steel sheets had the same effect as that of
using tension block. By this way, it was simple and convenient to
prepared preoxidated PAN fiber mat. Herein, a convenient method
was developed by fixing the fiber mat between two steel sheets.
Fig. 2a presents the well-defined Cu/PAN fiber mats as obtained
when the PAN concentration was chosen at 6.0 wt.%. Slight shrink-
age of the Cu/PrePAN fiber mats is shown in Fig. 2b after the heat
treatment. The elemental analysis indicates that the preoxidated
Cu/PAN fiber mats contain C: 50.99%; N: 18.59%; H: 3.48%. The nitro-
gen content of the preoxidated Cu/PAN fiber mats is much higher
than that (2.64%) of the carbon fiber [36]. Thus, we have examined
the preoxidated PAN fiber mats as the solid supporting materials
for the immobilization of the catalytic cuprous ion because nitrogen
atom is the main chelating center for the transition metal ions.
Fig. 3 shows the FT-IR/ATR spectra for the PAN fibers, the PAN
fibers after treated at high temperatures for 1.5 and 3.5 h, together
with the Cu/PrePAN fiber mats. Examination of Fig. 3 shows that
the thermal treatment resulted in the essential vanishment of the
◦
steel sheets, followed by baking in a muffle furnace at 200 C
for one and a half hours to prepare the preoxidated PAN fiber
mat supported copper catalyst (Cu/PrePAN fiber mat). The pre-
oxidated PAN fiber mat supported copper catalyst could also be
prepared by the same procedure except for using CuCl2 instead of
CuI2.
2.3. General procedure for the Cu/PrePAN fiber mats catalyzed
Mizoroki–Heck cross-coupling reactions
To a flask containing Cu/PrePAN fiber mat (56 mg, 0.043 mmol)
in 5.0 mL DMF, added with aryl iodide (0.5 mmol), CH CO K
3
2
(
(
343 mg, 3.5 mmol), acrylate (1.0 mmol) and ethylene glycol
0.387 mmol). The reaction mixture was allowed to stir at 130 C,
◦
and the reaction progress was monitored by TLC and/or GC/MS
analysis. After completion, the reaction mixture was cooled down
to room temperature and then quenched by addition of 10 mL of
water, followed by extraction (3× 20 mL) with ethyl acetate for
three times. The combined organic layer was washed with water
(
3× 20 mL) and then the solvent was removed under a reduced
pressure. The residual mixture was purified by silica gel column
chromatography with a mixture of petroleum ether and ethyl
acetate as eluent. All of the cross-coupling products were charac-
1
terized by H NMR and GC/MS analysis.
−1
IR absorption peaks at 2243 cm for the C N group, at 2939 and

2
8
L. Shao, C. Qi / Applied Catalysis A: General 468 (2013) 26–31
Fig. 2. SEM images of Cu/PAN fiber mat (a) and Cu/PrePAN fiber mat (b).
−1
−
1
1
454 cm for the C H group, and at 1070 cm for the C CN group.
(111)
3
2
2
000
500
000
−
1
At the same time, the IR absorption peaks at 1640 and 1590 cm for
−
1
the C N and C C bonds, and at 1345 and 810 cm for the C
group appeared, presumably due to the dehydrogenation.
C
H
(002)
The essentially identical IR spectra for the PAN nanofiber mats
after heated for 1.5 and 3.5 h, suggesting that the thermal treat-
ment for 1.5 h is enough for the preoxidation of the PAN fibers.
XRD pattern (Fig. 4) shows two characteristic diffraction peaks at
Cu/PrePAN fiber mat
(101)
1500
1000
500
0
◦ ◦
2
6 and 43 for the preoxidated PAN fiber mats, which is simi-
lar to the characteristic peaks for the (0 0 2) and (1 0 1) planes of
graphite [35,38,39], indicating that the linear PAN chain structure
had been converted into the aromatic ladder structure. The diffrac-
PAN fiber mat-1.5 h
CuI
◦
tion peak at 25 was the (1 1 1) plane of CuI, indicating some CuI
was aggregated.
3
.2. The catalytic activities for the Mizoroki–Heck cross-coupling
20
30
40
50
60
70
80
90
o
2theta ( )
The solvent effects for the Mizoroki–Heck cross-coupling of
iodobenzene with n-butyl acrylate in four different solvents are
summarized in Table 1. Examination of Table 1 shows that eth-
ylene glycol is the efficient solvent for the Cu/PrePAN fiber mats
catalyzed cross-coupling reaction. The catalytic activity of Cu cat-
alyst was significantly lower than that of Pd catalyst (entry 2 and
entry 5). Therefore, it is very important to develop an efficient cat-
alytic system for Cu catalyst. These results promoted us to examine
the effects of solvent on the cross-coupling reactions.
Fig. 4. X-ray diffraction (XRD) spectra for CuI, PAN fiber mat after thermal treatment
for 1.5 h, and Cu/PrePAN fiber mat.
by addition of small amount of ethylene glycol (0.66 mmol) as
shown in Fig. 5. Similarly, we have shown that the Pd/C catalyzed
Ullmann-type homocoupling reactions could be significantly
improved by addition of a small amount of ethanol (0.86 mmol)
[27]. The significant enhancement of the catalytic activity has been
attributed to the reduction of the oxidated Pd(II) species into the
active Pd(0) species by ethanol.
Interestingly, the cross-coupling yields of the iodobenzene
with n-butyl acrylate in DMF could be significantly improved
A series of additional additives have been examined for the
cross-coupling reaction and the related results were summarized
in Table 2. Examination of entries 1–3 of Table 2 shows that consid-
erable amount (60–86%) of the iodobenzene substrate was reduced
by the addition of zinc, glucose or triphenylphosphine, but resulted
in negligible formation of the corresponding cross-coupling n-
butyl cinnamate. These results clearly indicate that zinc, glucose
or triphenylphosphine were only involved with the reduction of
1
00
PAN
9
9
9
9
9
8
8
8
6
4
2
0
8
6
PAN-1.5h
PAN-3.5h
Table 1
Solvent effects on the cross-coupling reaction of iodobenzene with n-butyl acrylate.a
Cu/PrePAN
Conversion (%)c
Yield (%)c
Entry
Catalyst
Solvent
C N
C-H
1
2
3
4
Cu/PrePAN
Cu/PrePAN
Cu/PrePAN
Cu/PrePAN
Pd/PrePAN
DMF
DMSO
Ethylene glycol
Xylene
DMSO
7.70
21.85
94.00
2.98
7.70
21.85
81.00
2.98
C=C-H
800
C=N =C-H
b
5
100
98.28
4
000
3500
3000
2400
2000
1600
1200
a
Reaction conditions: iodobenzene (0.5 mmol); n-butyl acrylate (1.0 mmol);
-
1
potassium acetate (3.5 mmol); catalyst (0.043 mmol) in 5.0 mL solvent; tempera-
Wavenumber (cm )
◦
ture: 130 C; reaction time: 10 h.
b
Preoxidated PAN fiber supported PdCl2 catalyst; reaction time: 3 h.
Cross-coupling yields as determined by GC–MS analysis.
Fig. 3. FT-IR/ATR spectra for PAN fiber mat, PAN fiber mats after thermal treatment
for 1.5 and 3.5 h, and Cu/PrePAN fiber mat.
c

L. Shao, C. Qi / Applied Catalysis A: General 468 (2013) 26–31
29
1
1
05
00
Table 4
Mizoroki–Heck cross-coupling reaction of iodobenzene with n-butyl acrylate as
catalyzed by different copper catalysts.
a
Catalyst
Conversion (%)b
Yield (%)b
9
9
8
8
7
7
6
5
0
5
0
5
0
5
Cu/PrePAN
CuI
CuI/KI
100
99.40
80.36
95.01
∼0
91.90
98.26
∼0
CuCl2/PrePAN
Conversion
Yield
a
Reaction conditions: iodobenzene (0.50 mmol); n-butyl acrylate (1.0 mmol);
potassium acetate (3.5 mmol); ethylene glycol (0.66 mmol); catalyst (0.043 mmol)
◦
in 5.0 mL DMF; temperature: 130 C; reaction time: 10 h.
b
Determined from the GC–MS analysis.
cross-coupling protocol is also applicable to a wide variety of aryl
iodides and olefins as shown in Table 3. Examination of entries 1–8
of Table 3 shows that the ethylene glycol promoted Cu/PrePAN cat-
alyzed Mizoroki–Heck cross-coupling reaction works well for aryl
iodides bearing with either an electron-donating groups such as
o-Me (entry 1), p-Me (entry 2), p-OMe (entry 3), or an electron-
withdrawing groups such as p-NO2 (entry 7).
0
.5
1.0
1.5
2.0
2.5
3.0
3.5
Amount of Ethylene Glycol (mmol)
Fig. 5. Effect of ethylene glycol amount on the Cu/PrePAN fiber mat catalyzed cross-
coupling of iodobenzene with n-butyl acrylate in DMF.
Examination of entries 8–11 of Table 3 shows that the ethylene
glycol promoted Cu/PrePAN catalytic system is also efficient for
the Mizoroki–Heck cross-coupling reaction of iodobenzene with
different olefin substrates. However, much lower catalytic reac-
tivity and selectivity was found for the cross-coupling reaction of
iodobenzene with methyl methacrylate (entry 11 of Table 3), pre-
sumably due to the larger steric hindrance of methyl methacrylate.
Consequently, biphenyl was obtained with a yield of 35% from the
homocoupling of iodobenzene.
Table 2
Effects of additives on the cross-coupling reaction of iodobenzene with n-butyl
acrylate.a
Entry
Additive
Conversion (%)b
Yield (%)b
1
2
3
4
5
6
Triphenyl phosphine
Glucose
Zinc
Ethanol
Propane diamine
Ethylene glycol
67.16
59.65
86.16
100
100
100
2.05
3.03
∼0
96.12
95.35
99.40
a
Reaction conditions: iodobenzene (0.5 mmol); n-butyl acrylate (1.0 mmol);
3.3. Comparison of the heterogeneous Cu/PrePAN fiber mat
catalyst with homogeneous copper catalyst
potassium acetate (3.5 mmol); catalyst (0.043 mmol); additive (0.66 mmol) in 5.0 mL
◦
DMF; temperature: 130 C; reaction time: 10 h.
b
Determined by GC–MS analysis.
Heterogeneous catalysis is generally less active than corre-
sponding homogeneous catalysis due to the less accessible catalytic
centers in the heterogeneous catalysis. Examination of Table 4
shows that the heterogeneous Cu/PrePAN fiber mat catalyst (entry
1) is much more efficient than the homogeneous copper (I)
iodide (CuI) (entry 2), even slightly better than the homoge-
neous CuI/KI catalyst (entry 3). As expected, no catalytic activity
iodobenzene substrate. On the other hand, the addition of ethanol,
propanediamine and ethylene glycol not only completely con-
sumes the iodobenzene substrates (entries 4–6 of Table 2), but also
leads to over 95% cross-coupling yields, suggesting that the addi-
tives with small size were efficient to promote the Mizoroki–Heck
cross-coupling reactions while the additives with larger size could
promote the reduction of aryl iodides.
Examination of Table 2 shows that ethylene glycol is the most
efficient additive of the screened compounds. Most interestingly,
the ethylene glycol promoted Cu/PrePAN catalyzed Mizoroki–Heck
of the Cu(II)Cl /PrePAN catalyst fiber mats was observed for the
2
Mizoroki–Heck cross-coupling reaction of iodobenzene with n-
butyl acrylate under the same reaction conditions, providing the
conclusive evidence that the effective catalytic species is the Cu(I)
rather than Cu(II) ion. Also, no Cu(I) ion can be formed from the
Table 3
Mizoroki–Heck cross-coupling reactions of aryl iodides with olefins.a
Entry
Aromatic iodides
Olefins
Time (h)
Conversion (%)b
Yield (%)b
trans:cis)
(
1
2
3
4
5
6
7
8
9
o-MeC6H4I
p-MeC6H4I
p-MeOC6H4I
p-BrC6H4I
o-ClC6H5I
p-ClC6H4I
p-NO2C6H4I
C6H5I
n-Butyl acrylate
n-Butyl acrylate
n-Butyl acrylate
n-Butyl acrylate
n-Butyl acrylate
n-Butyl acrylate
n-Butyl acrylate
n-Butyl acrylate
Methyl acrylate
Acrylonitrile
10
10
24
10
10
10
24
10
10
10
24
99.76
84.97
100
100
99.98
100
99.92
100
99.95
99.93
99.95
98.71 (98.39:1.61)
84.97 (98.98:1.02)
98.61 (99.98:0.02)
98.24 (99.50:0.50)
95.74 (98.79:1.21)
87.98 (97.50:2.50)
86.30 (99.99:0.01)
99.40 (99.99:0.01)
89.10 (99.99:0.01)
97.92 (57.15:42.85)
44.90 (59.17:40.83)
C6H5I
C6H5I
C6H5I
1
1
0
1
Methyl methacrylate
(
Biphenyl: 35.15%)
a
Reaction conditions: aryl iodide (0.50 mmol); olefin (1.0 mmol); potassium acetate (3.5 mmol); ethylene glycol (0.66 mmol); catalyst (0.043 mmol) in 5.0 mL DMF;
◦
temperature: 130 C; reaction time: 10 h.
b
Determined from the GC–MS analysis.

3
0
L. Shao, C. Qi / Applied Catalysis A: General 468 (2013) 26–31
Table 5
significantly improved by addition of small amounts of additives
a
Catalytic activities of the recycled Cu/PrePAN fiber mat.
such as ethylene glycol. The heterogeneous Cu/PrePAN fiber mat
catalytic system was shown to be very highly active and selec-
tive for the Mizoroki–Heck cross-coupling of aromatic iodides with
acrylates. The high catalytic activities together with the easier sep-
aration and recycling make this unique environmentally benign
heterogeneous copper catalyst to be attractive for large industrial-
scale applications.
Run
Conversion (%)
Yield (%)b
Copper content (wt.%)
1
2
3
4
100.00
99.78
65.30
42.72
99.40
96.66
65.30
42.72
4.90 ± 0.01
3.78 ± 0.01
a
Reaction conditions: iodobenzene (0.50 mmol); n-butyl acrylate (1.0 mmol);
potassium acetate (3.5 mmol); catalyst (0.043 mmol); additive (0.66 mmol) in 5.0 mL
◦
DMF; temperature: 130 C; reaction time: 10 h.
b
Acknowledgment
Determined from the GC–MS analysis.
This work was supported by the Zhejiang Science and Technol-
ogy Innovation Team of Zhejiang Province Science and Technology
Hall (2010R50014-16).
Cu2p3/2 (932.6)
Cu2p1/2 (952.5)
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
Recovered Cu/PrePAN fiber mat
afte one recycle
2
013.08.010.
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Heterogenization can greatly facilitate the separation and
recycling of the homogeneous catalyst to reduce the reaction
cost and minimize waste generation, ultimately leading to a
more economic and environmentally benign process. Moreover,
the fiber mat supported heterogeneous copper catalyst can be
readily recycled from the reaction media by simple filtration and
then reused for the subsequent studies. However, a considerable
decrease of the catalytic activity was observed for the recycled het-
erogeneous copper catalyst after it was reused for three times. The
copper content of the third recycled catalyst was lost by about
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3% (entry 3 of Table 5). As the Cu(II) has no catalytic activity
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