A brand-new catalytic system: a Pd-based catalyst directly attached on the inner walls of the reactor which independently catalyzed the Heck reaction

Article abstract of DOI:10.1039/c5ra10340a

A Pd-based catalyst directly attached on the inner surface of the reactor as a new catalytic system has been achieved and developed via an electrospinning technique followed by an impregnation-reduction and a simple carbonization process. The application of the reactor was demonstrated using the Heck reaction. This work showed many potential advantages in numerous fields.

Full text of DOI:10.1039/c5ra10340a

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A brand-new catalytic system: a Pd-based catalyst
directly attached on the inner walls of the reactor
which independently catalyzed the Heck reaction†
Cite this: RSC Adv., 2015, 5, 56157
Received 1st June 2015
Accepted 16th June 2015
Qingrun Meng, Jie Bai* and Chunping Li
DOI: 10.1039/c5ra10340a
www.rsc.org/advances
A Pd-based catalyst directly attached on the inner surface of the polyacrylonitrile (PAN) and palladium chloride (PdCl ) to form
2
reactor as a new catalytic system has been achieved and developed via PAN–PdCl bers. The bers was followed by an impregnation–
2
an electrospinning technique followed by an impregnation–reduction reduction and subsequent calcination methods, so the Pd salt
and a simple carbonization process. The application of the reactor was attached to the PAN was reduced to Pd nanoparticles and our
demonstrated using the Heck reaction. This work showed many group has previously demonstrated that this kind of catalyst
potential advantages in numerous fields.
exhibited excellent catalytic activity for the Heck reaction.
Therefore a stable catalytic system making use of supported
Pd NPs would undoubtedly allow the Heck reaction to be carried
out. In this way, herein, we designed a unique catalytic reactor
to develop a brand-new catalytic system. To the best of our
knowledge, this is the rst report that directly electrospun PAN–
Carbon–carbon cross-coupling reactions are among the most
useful and most widely studied synthetic transformations. One
of the most important processes, the Heck coupling reaction,
has been abundantly used in syntheses and widely studied in
2
PdCl bers to the inner surface of the reactor and the catalyst
1–5
recent decades. Palladium is the metal of choice for these
reactions and several catalysts rely on Pd(II) and Pd(0) species,
can attach to the reactor rmly even suffering reduction, pre-
oxidation and carbonization processes. The effectiveness of
this catalytic system is demonstrated for the Heck coupling of
iodobenzene with acrylates. This work opens the door to the
development of a novel catalytic system and the inspired Pd
materials are active with a variety of substrates, suggesting that
the system is versatile and may translate to various other
chemical reactions.
The digital photos of the reactor are shown in Fig. S1.† The
entire reactor that was designed consists of two parts: the
bottom of the reactor was used for the collection of the catalyst
and the top part was equipped with a thermometer and
condenser tube. The two parts were xed and rotated by a screw
opening. Scheme 1 shows all of the steps for the preparation of
three types of catalyst. Firstly, we directly electrospun PAN–
6
mostly in the form of Pd complexes. Transition metal nano-
particles (NPs) are particularly attractive, nding signicant
7
8
applications in optics, electronics, and especially in catal-
ysis.
9–11
It is well believed that a high efficiency in catalysis oen
requires nanometric and uniform size as well as a homoge-
neous distribution of the catalyst throughout a suitable
substrate. Moreover, preventing nanoparticles from aggregation
is one of the most important issues for preserving their prop-
erties. In this respect, Pd NPs are widely utilized as a catalyst for
C–C cross-coupling reactions, as witnessed by the numerous
12
reviews present in the literature.
However conventional homogeneous palladium catalysts
have some disadvantages in separation and recovery that limit
the industrial and synthetic applications of the Heck reaction.
In recent years, immobilizing metal catalysts onto solid
supports has received great attention. A variety of organic and
inorganic supports have been reported
nanobers prepared though the electrospinning technique turn
PdCl
process of NH
2
bers to the inner surface of the reactor, followed by a
2+
2
2
NH reduction, in which most of the Pd ions
13
can be reduced to Pd(0). Aer that, the reactor was placed in a
14–16
and among them,
ꢀ
tube furnace at 250 C in air for two hours to prepare the Pd/
PANOF catalyst. In the same way, we obtained the Pd/LTCF-
17–19
out to be the simplest and ideal supports.
Our previous
ꢀ
3
50 and Pd/LTCF-450 catalysts as follows: 250 C annealing
20
research reported our ndings in electrospinning mixtures of
ꢀ
ꢀ
for 2 hours in air; heating up to 350 C and 450 C at a rate of 2
C min and annealing for 2 hours in nitrogen. Finally, we
ꢀ
ꢁ1
Chemical Engineering College, Inner Mongolia University of Technology, Hohhot, added the reactants of the Heck reaction, reacted at the specic
0
†
1
10051, People’s Republic of China. E-mail: baijie@imut.edu.cn
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra10340a
temperature and time.
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Fig. 1 SEM images of (A) PAN/PdCl
2
nanofibers and (B) Pd/PAN
2 2
nanofibers obtained after they were reduced by NH NH and taken
from the inner surface of the reactor; the inset shows corresponding
digital photos.
Scheme 1 Schematic illustration of the preparation of three types of
catalyst.
Pd/PANOF (Fig. 2A2), Pd/LTCF-350 (Fig. 2B2) and Pd/LTCF-
2
Fig. S2† shows the FTIR spectra for the PAN/PdCl bers, Pd/
4
50 (Fig. 2C2) there were no obvious changes in the sizes of
PAN bers aer the hydrazine hydrate solution treatment, and
Pd/PANOF, together with Pd/LTCF-350 and Pd/LTCF-450.
Fig. S2A and B† show the electrospun PAN/PdCl2 composite
nanobers before and aer the hydrazine hydrate solution
treatment with the characteristic bands of the nitrile (2237
Pd NPs. They were roughly spherical in shape and were
randomly distributed on the surface of the nanobers with a
moderate density and their average sizes were 3.8 nm,
5
.3 nm, and 3.71 nm, respectively. The Pd NPs were stabilized
in place even aer the Heck reaction mechanism thus
allowing for efficient reuse in subsequent steps and this
ꢁ
1
cm ) groups coming from the polyacrylonitrile comonomer.
ꢁ1
2
The characteristic peak at around 2927 cm for CH conrms

nding was in accord with the next discussion in Fig. S4.†
ꢁ1
methylene, and the triplet at 1099 cm represents the C–CN
group. At the same time, in Fig. S2C–E,† the thermal treatment
resulted in the essential disappearance of the nitrile (2237
As shown in Fig. 3, their X-ray photoelectron spectroscopy
(
5
(
XPS) spectra possess three peaks centered at 284.9, 400.02, and
32.1 eV, corresponding to C 1s, N 1s, and O 1s, respectively
Fig. 3A). PAN is a nitrogen-rich precursor, and the presence of
ꢁ
1
cm ) groups. That means, the structure of PAN had changed,
the linear PAN molecular chain structure could have been
converted into the aromatic ladder structure aer the thermal
nitrogen species facilitates the loading of Pd nanoparticles onto
the support and enhances the capacity for adsorbing metal ions
in aqueous solution, due to the binding between nitrogen
species and metals usually making them hard to remove during
2
1
ꢁ1
treatment. The absorption peaks at 1620 cm for the C]N
ꢁ1
bonds, and at 1377 and 758 cm
for the C]C–H group
appeared, presumably due to the dehydrogenation.
A comparison of the UV-vis spectra of the samples is dis-
played in Fig. S3.† The red spectrum (Fig. S3A†) presented for
25
thermal treatment. The high-resolution N 1s peak in the XPS
spectra is displayed in Fig. 3B. The signals at 400.4 eV are
assigned to the pyridine type nitrogen. However, another new
the PdCl /PAN bers displays a feature absorbance band at 244
2
nm. This absorbance is likely due to the interaction between the
2
+
22,23
Pd and the polymer.
Upon reduction, represented by the
black spectrum (Fig. S3B†), this band disappeared with the
formation of a broad absorbance characteristic of nanoparticle
formation, indicating that bivalent palladium was reduced to
24
zerovalent palladium.
As shown in Fig. 1A and B, the electrospun PAN/PdCl
2
composite nanobers before and aer the hydrazine hydrate
solution treatment align in random orientations and interweave
to form a brous lm. The surface of the PAN/PdCl nanobers
2
was smooth and their average ber diameters were uniform
within 150 nm, aer the hydrazine hydrate solution reduction,
the morphology of the nanobers almost did not change, the
average diameters of nanobers shrunk to 130 nm. Also, the

brous character of these ber mats was still retained. The inset
section gave us visual evidence that the nanobers could rmly
attach onto the inner surface of the reactor, or even suffer a
liquid reduction process.
Upon further treatment with different temperatures of
ꢀ ꢀ ꢀ
2
50 C (Fig. 2A1), 350 C (Fig. 2B1) and 450 C (Fig. 2C1),
these bers appeared to be slightly smaller with average
diameters of 110–150 nm, which is likely due to the thermal
treatment. Nevertheless, the brous character of the bers
was still retained at all stages. TEM images demonstrate
that with the increase of the carbonization temperature,
Fig. 2 SEM images of Pd/PANOF (A1), Pd/LTCF-350 (B1) and Pd/LTCF-
450 (C1); the insets provide the corresponding digital photos. TEM
images of Pd/PANOF (A2), Pd/LTCF-350 (B2) and Pd/LTCF-450 (C2);
the insets show the size distribution histograms, respectively.
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a
peak which appears at 398.4 eV in Fig. 3B3 is assigned to the Table 1 Solvent effect on the Heck reaction
pyridine nitrogen. This is mainly because the samples obtained
at 450 C are not fully conductive, indicating that aromatization
b
b
Entry
Solvent
Conversion (%)
Selectivity (%)
ꢀ
is not completed at that temperature but had already occurred
1
2
3
4
EG
DMF
DMSO
Toluene
8.67
95.55
94.21
27.09
2.25
79.94
74.39
26.66
26
partially. The XPS spectrum in the Pd 3d region of Pd/PANOF
Fig. 3C1), Pd/LTCF-350 (Fig. 3C2) and Pd/LTCF-450 (Fig. 3C3)
indicated the presence of a mixture of Pd(0) and Pd(II) as is
(
5
/2
a
evident from the measured binding energy of the Pd 3d and
Reaction conditions: iodobenzene (1 mmol); n-butyl acrylate (1.5
3
/2
mmol); triethylamine (3 mmol); catalyst: Pd/PANOF in 10 mL solvent;
temperature: 100 C; reaction time: 24 h. Determined from the GC
analysis.
3d
electrons at 335.1 and 340.1 eV for Pd(0) and 338.1 and
ꢀ
b
343.3 eV for Pd(II), respectively.
Upon conrmation of the fabrication of Pd metallic nano-
particles, the catalyst and system were studied for catalytic
reactivity for the formation of C–C bonds. Specically, we
focused on the Mizoroki–Heck cross-coupling reaction and this
reaction was selected for two specic reasons. First, the nal
organic product of the catalytic reaction possesses a newly
formed C–C bond, which is important for a vast variety of elds.
Second, many of the reagents, both aryl halides and alkenes, are
commercially available to be used for facile characterization of
the reactivity of the newly produced Pd based reactor system.
Additionally, the solubility of the reagents and the stability of
the system can be tailored to t a variety of solvent polarities
and reaction conditions, thus facilitating a broad set of analyses
of the Pd nanomaterial catalytic activity.
A series of bases were examined for the cross-coupling
reaction and the related results are summarized in Table S1.†
Examination of entries 1–3 of Table S1† shows that inorganic
bases (either strong alkalis or weak inorganic salts) scarcely
promote the Heck reaction in this catalytic system whereas the
organic base (triethylamine) is able obviously to enhance. It is
not difficult to explain, as the system of the solvent is DMF and
this kind of organic solvent is almost insoluble for all three
inorganic bases. As a result, the function of the bases is much
weaker than the organic base. So triethylamine is an optimal
choice of bases.
The most advantageous aspect of this system is that the
catalyst attached on the surface of the reactor can be readily
recycled from the reaction media by simple washing even with
no need for ltration and then reused for the subsequent
studies. The Pd-based catalyst together with the reactor builds
up an overall catalytic system and independently catalyzes the
Heck reaction. The recyclability of all three catalysts was
examined (Table 2). All three types of catalysts presented high
catalytic activities in the rst three runs. However, a consid-
erable decrease in the catalytic activity was observed aer it
was reused four times. We examined the samples by TEM aer
the 4th run (Fig. S4†) and found an amount of the Pd nano-
particles fell off the bers. A comparison of the activities of
three catalysts indicated that the activities of Pd/PANOF cata-
lyst had dropped signicantly and this may be mainly because
the polyacrylonitrile preoxidated ber (PANOF) did not
undergo a process of carbonization and therefore some active
groups still existed on the bers that led to the loss of the Pd
NPs during the reaction. This nding agrees with the FTIR
analysis in Fig. S2.† As the carbonization temperature rises,
the properties of the catalyst from preoxidated ber gradually
converts to carbon bers and therefore the Pd/LTCF-450 type
catalyst presents more stability aer four cycles as expected.
Additionally, it is also noteworthy that a small part of the
catalyst fell off the reactor during the reaction and washing
process and this was another reason for the dropping of the
catalytic activities.
For the catalytic analysis of the system, relatively mild
conditions were initially employed for the coupling of iodo-
benzene with n-butyl acrylate. For this reaction and system, 1
mmol iodobenzene was co-dissolved in a solvent with 1.5 mmol
of acrylates. This solution was added directly to the reactor and
the reaction was le for 24.0 h with no stirring. The temperature
ꢀ
of the system was 100 C instead of boiling point.
The solvent effects for the Heck coupling of iodobenzene
with n-butyl acrylate in four different solvents are summarized
in Table 1. Examination of Table 1 shows that DMF is the effi-
cient solvent for the Pd/PANOF catalyzed cross-coupling
reaction.
As shown from the recycling experiments, the Pd/LTCF-450
catalyst exhibited a better catalytic activity and recyclability
than the other two catalysts. So we chose the Pd/LTCF-450
catalyst to further investigate the Heck reaction using iodo-
benzene with different olen substrates. However, a lower
selectivity was found for the cross-coupling reaction of
Fig. 3 (A) The X-ray photoelectron spectra for Pd/PANOF, Pd/LTCF-
3
50 and Pd/LTCF-450; (B) N 1s spectra; (C) Pd 3d spectra.
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a
Table 2 Repeated uses of three types of catalyst in the Heck reaction
Pd/PANOF
Pd/LTCF-350
Pd/LTCF-450
b
b
b
b
b
b
Run
Conversion (%)
Selectivity (%)
Conversion (%)
Selectivity (%)
Conversion (%)
Selectivity (%)
1
2
3
4
95.55
90.04
89.89
64.56
79.94
84.88
75.18
67.28
94.03
87.70
90.89
71.13
88.88
92.50
60.74
61.29
99.34
100
100
69.86
71.95
72.70
73.26
81.26
a
ꢀ
Reaction conditions: iodobenzene (1 mmol); n-butyl acrylate (1.5 mmol); triethylamine (3 mmol); solvent 10 mL DMF; temperature: 100 C;
b
reaction time: 24 h. Determined from the GC analysis.
a
Table 3 Heck reaction of iodobenzene with different substrates
Experimental
b
b
Entry
1
R
Product
Conversion (%) Selectivity (%)
Polyacrylonitrile (PAN, Mw ¼ 80 000) was purchased from
Kunshan Hongyu Plastics Co. Ltd. N,N-Dimethylformamide
C
2
3
5
H
H
H
3
5
9
O
O
O
2
2
2
96.53
96.06
72.64
69.86
37.33
(DMF, C
iodobenzene (C
4 6 2
9%), methyl acrylate (C H O
3
H
7
NO, AR, 99.5%), palladium chloride (PdCl
I, CP, 97%), triethylamine (C 15N, AR,
, CP, 98%), ethyl acrylate
, CP, 98%), acrylonitrile (C N, CP, 98%), acrylic acid
, CP, 98%) and n-butyl acrylate (C , CP, 98%) were
2
, AR),
6
H
5
6
H
9
2
3
4
C
C
100
(C
5
H
H
8
4
O
O
2
3 3
H
(C
3
2
7 12 2
H O
purchased from Sinopharm.
99.34
98.39
For the fabrication of the Pd/PANOF, Pd/LTCF-350 and Pd/
LTCF-450 catalysts, the homogeneous solution used for elec-
trospinning was prepared by dissolving PAN in DMF at a
concentration of 5 wt% by stirring for 12 h. Palladium chloride
CHO
2
(
PdCl
in which the monomer of PAN and PdCl
0, and then intensively stirred for 24 hours. Fabrication of the
PdCl –PAN nanobers was typically achieved by electrospinning
2
) powder was mixed into a PAN–DMF blending solution
was at a molar ratio of
5
CN
92.30
73.10
2
5
2
a
Reaction conditions: iodobenzene (1 mmol); substrates (1.5 mmol);
a blending solution containing PdCl –PAN–DMF, the blending
ꢀ
2
triethylamine (3 mmol); solvent 10 mL DMF; temperature: 100 C;
reaction time: 24 h. Determined from the GC and GC-MS analysis.
b
solution was passed through a dropper with winding copper
wire. The positive voltage applied to the winding copper wire
was 16 kV and the distance between the winding copper wire
and the collector was 12 cm. It is worth mentioning that the
collector in this experiment was the reactor. In other words, the
iodobenzene with acrylic acid, presumably due to the stereo-
hindrance effect that produced by-products (Table 3).
TEM images of the three catalysts aer the 4th run (Fig. S4†)
show that the morphology and size of the palladium NPs aer
four cycles had better uniform dispersion and no signicant
agglomeration. Their average sizes are 4.06 nm, 5.34 nm, and
as-spun PAN–PdCl
the reactor. Aer that, 2 mol L N H aqueous solutions were
2
bers were collected on the inner surface of
ꢁ1
2
4
added into the reactor for 2 hours. Aer washing and drying in
ambient air, the reactor was placed in a tube furnace and
ꢀ
stabilized in air for 2 h at 250 C to prepare the polyacrylonitrile
3.88 nm, respectively, which are only slightly larger than that in
preoxided ber supported palladium catalyst (Pd/PANOF).
Similarly, we successfully obtained the Pd/LTCF-350 and Pd/
LTCF-450 catalysts on the basis of stabilizing in air for 2 h at
fresh Pd/PANOF, Pd/LTCF-350 and Pd/LTCF-450 (Fig. 2). It
should also be noted that an amount of Pd nanoparticles fell off
the bers indicating a possible mechanism for the deactivation
of the catalyst due to the organic activating group.
ꢀ
ꢀ
ꢀ
2
50 C and then heating up to 350 C and 450 C for 2 h in
nitrogen.
In a typical procedure, a mixture of iodobenzene (1 mmol),
In summary, we successfully designed a brand new catalytic
system in which a palladium based catalyst can be directly
attached on the inner wall of the reactor. It has indeed been
demonstrated that the new system containing Pd/PANOF, Pd/
LTCF-350 and Pd/LTCF-450 catalysts, together with the reactor
was shown to be very highly active and selective for the Mizor-
oki–Heck cross-coupling with iodobenzene and acrylates. The
high catalytic activities, easier separation and recycling make
this unique environmentally benign heterogeneous palladium
catalyst and the novel system attractive for large industrial-scale
applications.
acrylate (1.5 mmol), triethylamine (3 mmol) and DMF as a
solvent (10 mL) were added into the reactor. Aer the reactor
was screwed down, the reaction mixture was allowed to heat to
ꢀ
1
00 C for 24 hours in a nitrogen atmosphere. Aer completion,
the reaction mixture was cooled down to room temperature, the
reactor was opened and the mixture was ltered. The products
were examined by a 7890A gas chromatograph (GC) equipped
with a FID detector, the chromatographic conditions were as
follows according to different reaction substrates, the
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temperature of the injection port and detector ranged from 220
to 280 C. The temperature of the oven adopted temperature
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ꢀ
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The authors gratefully acknowledge the support of the National
Natural Science Foundation of China (21266016).
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