Tuning the Catalytic Activity of Tertiary-Amine Functionalized Polyacrylonitrile Fibers by Adjusting the Surface Microenvironment

Article abstract of DOI:10.1002/cctc.201700515

Polyacrylonitrile fibers were functionalized with different amines to construct special tridimensional catalytic microenvironments at the surface. The polarity of the microenvironments at the surface of the fiber catalyst was tuned by changing the ratio o

Full text of DOI:10.1002/cctc.201700515

Accepted Article
Title: Tuning the catalytic activity of tertiary-amine functionalized
polyacrylonitrile fiber by adjusting the surface micro-environment
Authors: Pengyu Li, Yuanyuan Liu, Jian Cao, Minli Tao, and Wenqin
Zhang
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ChemCatChem
10.1002/cctc.201700515
Tuning the catalytic activity of tertiary-amine functionalized
polyacrylonitrile fiber by adjusting the surface micro-environment
[a]
Pengyu Li, Yuanyuan Liu, Jian Cao, Minli Tao* and Wenqin Zhang*
[
12]
Polyacrylonitrile fiber was deeply functionalized with different amines
to construct special tridimensional catalytic micro-environments in
the surface. The polarity of the micro-environment in the surface of
fiber catalyst was tuned by changing the ratio of hydrophobic and
hydrophilic amines. The catalytic performances of these micro-
environments were examined via cyanation reaction in water.
Among these fiber catalysts with different micro-environments, the
one modified by more hydrophobic benzylamine (PANBTF) shows
much higher catalytic activity than that modified by hydrophilic
ethanolamine (PANETF). The PANBTF can efficiently and rapidly
catalyze the cyanation reaction of benzaldehyde and ethyl
selectivity for intramolecular reactions and so on.
Polyacrylonitrile fiber (PANF), which is known as “artificial wool”,
is a common and widely used material in industry and our daily
life. Furthermore, it is also appropriate to be the catalyst support
as the advantages of low cost, simple production technology,
high strength, lightly density and mildew resistant, etc. What’s
more, it has abundant cyano groups which can be easily
transformed into carboxyl, aminocarbonyl and other functional
groups. Besides, PANF can be knitted into a variety of shapes,
which is very suitable for the fixed-bed reactors in industrial
production. Without doubt, PANF is a fine raw material to
[
13]
cyanoformate with a yield of 94% at RT in water in 40 min. Moreover, prepare heterogeneous catalysts.
a hydrophobic micro-environment promoted mechanism has been
In our previous works, PANF has been used as support to
[
14]
[15]
[16]
proposed to explain the high and specific catalytic activity of PANBTF. immobilize amines,
L-proline,
ionic liquids (ILs)
DMAP,
and so on.
quaternary
[
17]
[18]
[13a, 19]
In addition, this fiber catalyst exhibited excellent recyclability and
reusability (8 times) without further treatment, as well as it performed
well in flow chemistry test and scaled-up experiment which indicating
a potential application of PANBTF for industry production.
ammonium salt,
When
we used DMAP functionalized polyacrylonitrile fiber (PANDMAPF)
to catalyze the three-component condensation reaction of p-
chlorobenzaldehyde, malononitrile and α-naphthol, we found
that only high polar solvents like water or methanol were used,
could we obtained the corresponding substituted 2-amino-2-
[
16a]
chromene.
The reason may be that, in high polar solvents,
Introduction
naphthol is sparingly soluble, which could be forced to
accumulate in the surface of PANDMAPF and contacted with
active catalytic point, thus accelerate the reaction to produce the
final product. The research illustrated that the relative polarity
constructed by the polymer segment in the fiber, the introduced
functional group and the solvent, was a critical factor in the
organic reaction catalyzed by polyacrylonitrile fiber catalyst.
Possibly, reduce the polarity of the surface of the fiber catalyst
or enhance the solvent’s polarity will make the organic reactant
more easily to access the micro-environment in the fiber catalyst
and contact the catalytic sites to expedite the organic reaction.
Nowadays, green chemistry has been world widely valued in
academic and industrial fields. Two main directions in green
process are to minimize the waste production (use
environmentally benign solvents) and to maximize catalyst
[1]
efficiency.
Water is the most readily available and
environmentally friendly solvent, more and more chemists have
[2]
explored aqueous phase organic reactions. Reuse of catalysts
for organic synthesis is another strategy for sustainable
chemistry. In recent years, the immobilization of homogeneous
catalysts onto solid supports has attracted much attention and
In this work, the tertiary-amine functionalized polyacrylonitrile
[
3]
been well investigated. Currently, a variety of materials have
[13b]
fiber catalyst (PAN
T
F)
was used as a template fiber catalyst
[
4]
[5]
[6]
been used as supports, such as silicas, zeolites, polymers,
and to explore the effect of surface polarity on the catalytic
activity of the fiber catalyst based on its easy preparation. The
polarity of the fiber catalyst was tuned by adjust the ratio of N,N-
dimethyl-1,3-propanediamine (DMPDA) with benzylamine or
ethanolamine. The influences of the polarities of surface micro-
environments on their catalytic activities were examined by a
cyanation reaction of aldehyde with ethyl cyanoformate in water.
[
7]
[8]
[9]
activated charcoal,
mesoporous materials,
metal oxides,
metal particles
and
[10]
etc. Among these materials, polymer
supported catalysts often display superior catalytic activity, high
regioselectivity and many other special performances due to the
unique micro-environment created for the reactants within the
[
11]
polymer support. For example, polymer catalyst can improve
catalyst stability, enhance regioselectivity and increase
Results and Discussion
[a]
Dr. P. Li, Ms. Y. Liu, Dr. J. Cao, Dr. M. Tao, Prof. Dr. W Zhang
Department of Chemistry, School of Science
Tianjin University
Synthesis of the fiber catalysts
Collaborative Innovation Center of Chemical Science and
Engineering.
Tianjin 300072, P. R. China.
The DMPDA functionalized polyacrylonitrile catalyst PAN
synthesized according to our previous work,
T
F was
and the PANBTF,
PANETF were prepared by a two-step method as follows
Scheme 1). The extents of functionalization determined by
[13b]
E-mail: zhangwenqin@tju.edu.cn
Supporting information for this article is given via a link at the end of
the document.
(
weight gain and acid exchange capacity were summarized in
Table 1.
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ChemCatChem
10.1002/cctc.201700515
O
O
The water retention is also summarized in Table 1, it represents
the capability of the modified fiber to absorb water. Because of
the benzyl group is hydrophobic and the hydroxyethyl is
NC
NC
CN
CN
H
2
N
N
2
H O,
N
H
N
N
NC
NC
NC
NC
refluxed
N
H
hydrophilic, the PAN
Therefore, the water retention capability of PAN
PAN F (Table 1, entries 2 and 3). The PAN F has a much higher
water retention rate than that of PAN F(Table 1, entries 1 and 3),
which suggestes that the dimethylamino group is basic and can
be act as a good electron donor to form hydrogen bond with
B
F is more hydrophobic than that of PAN
E
F.
T
PAN F
B
F is lower than
O
O
E
T
NC
CN
NC
NC
NC
NC
NC
NC
R
N
R
N
H
2
H O,
H
2
N
R
H
2
H O,
H
2
N
N
E
NC
NC
O
refluxed
refluxed
CN
CN
N
H
N
PAN
B
F: R=
Ph
OH
PANBTF: R=
PANETF: R=
Ph
B
water. Once introducing tertiary-amine group into the PAN F, the
PAN
E
F: R=
OH
water retention rate of the PANBTF is increased (Table 1, entry 4).
And the hydrophilicity of the PANBTF is gained with the
increasing of the tertiary-amine functionality which proved by the
enhancement of water retention from 16.5% to 71.5% (Table 1,
entries 4-8).
Scheme 1. Preparation of the fiber catalysts..
Table 1. Synthesis of the fiber catalysts.
Elemental analyses (EA)
Entry
Catalyst
Time (h)
Weight
gain (%)
Water
retention
%)
Functional
degree
(mmol/g)
The elemental analysis data of PANF and modified fibers are
shown in Table S1. As benzylamine has higher C and H
contents and lower N content than that of PANF, the C and H
contents (74.52% and 5.83%, respectively) increased and the N
content (20.75%) decreased as expected (Table S1, entries 1
and 2). Similarly, compared to PANF, the C, H and N contents of
[
a]
[
b]
(
1
2
3
PAN
PAN
PAN
T
F
3.5
12
4
14.2
20.1
10.7
3.1
53.7
14.8
21.8
16.5
19.5
30.2
49.4
71.5
60.2
1.13
B
F
F
1.56
E B
PAN F, PAN F, PANETF and PANBTF changed according to the
E
1.58
modified small molecule benzylamine, ethanolamine or N,N-
dimethyl-1,3-propanediamine (Table S1, entries 3-6), which
demonstrated the successful grafting of these small molecules.
After being reused one time, the EA data of PANBTF-1were
almost the same with PANBTF (Table S1, entries 6-7).Even
though the PANBTF was reused eight times, the EA data of
PANBTF-8 only has slightly chang which may be caused by the
absorption of reactants and product in the reaction process. All
in all, the PANBTF is chemical stable after being reused in the
cyanation reaction for many times.
[
[
[
[
[
c]
c]
c]
c]
c]
4
5
6
7
8
9
PANBT
PANBT
PANBT
PANBT
PANBT
PANET
F
0.5
F
1.0
F
1.5
F
2.0
F
2.5
F
0.5
1
0.28
6.8
0.58
1.5
2
13.4
18.4
23.5
13.4
1.10
1.46
2.5
1
1.79
1.12
[
a] Weight gain = [(W
PANF and functionalized PANFs. [b] Determined by titration. [c] The footnote
number means the reaction time of PAN F modified further by DMPDA.
2 1 1 1 2
−W )/W ]×100%, where W and W are the weight of
Fourier-transfer infrared spectroscopy (FTIR)
B
The samples of the fiber catalysts were pulverized and mixed
into KBr pellets and their FTIR spectra were shown in Figure 1.
-1
Before modification, the 2243 cm absorption band in the
spectrum of PANF (Figure 1a) is attributed to the -C≡N
stretching vibration. Because of the existence of methyl acrylate
units in PANF, there has a corresponding adsorption peak of -
Compared with the introduction of DMPDA and ethanolamine
Table 1, entries 1 and 3), the introduction of benzylamine needs
(
-1
C=O at 1734 cm . After the modification, the stretching
longer reaction time of 12 h (Table 1, entry 2). It suggested that
the steric hindrance and the less hydrophilicity interfering the
benzylamine to access the inner surface of the PANF which
contains abundance of hydrophilic cyano groups. To obtain the
same functional degree, 3.5 h was required to synthesis the
-1
-1
vibrations peaks of C≡N at 2243 cm and -C=O at 1734 cm
reduce, which suggests that the -C≡N and -C=O have been
consumed (Figure 1b-h). Besides, a broad peak appears at
-1
3
700-3150 cm is assigned to the N-H in the -CONH- moiety
formed from the aminolysis of cyano and eater groups in PANF
T
PAN F, while only 1.5 h and 1 h were needed to obtain the
-1
(
Figure 1b, d, and f-h). However, the broad band at 3400 cm in
the spectra of PAN F (Figure 1c) is assigned to the stretching
vibration of O-H, which signifies the successful introduction of
PANBTF and PANETF, respectively (Table 1, entries 1, 6 and 9).
It was speculated that the surface of PANF was broken to a
certain extent which caused the relevant inner chains were
exposed in the step of preparing the PAN F and PAN F,
B E
therefore expanded the reaction area and accelerated the
functionalization speed in following step.
E
-1
the hydroxyethyl moeity. For PANETF, the 3400 cm band is a
superposition peak of N-H in -CONH- and O-H in hydroxyethyl
-1
(
Figure 1e). Moreover, the broad peak at 1647 cm is attributed
to the amide I (mainly C=O stretching vibration in -CONH-) and
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ChemCatChem
10.1002/cctc.201700515
-1
1
560 cm is assigned to the amide II (mainly C-N stretching
T
immobilization with DMPDA, the WCA of PAN F is decreased to
vibration and N-H bending vibrations in -CONH- ) (Figure 1b-h),
which are another evidences for the reaction of -C≡N in PANF
with -NH in the amines and the formation of -CONH-, namely,
2
the successful immobilization of the amines into the surface of
the fiber. Furthermore, the spectrum of the PANBTF (Figure 1f) is
almost the same with those of the PANBTF-1 (Figure 1g) and
PANBTF-8 (Figure 1h), which suggest the excellent recoverability
of PANBTF.
56°, which means PAN F is more hydrophilic than PANF. When
T
the fiber was modified by benzylamine and DMPDA, the WCA of
PANBTF increases to 69°due to the hydrophobic property of
benzyl moiety (Figure 2c), indicating the formation of more
hydrophobic micro-environment beneath the fiber surface. On
the contrary, the hydrophilic micro-environment was tuned by
the introduction of ethanolamine into the fiber surface, and the
WCA of PANETF decreased further to 37° as expected (Figure
2d). These results are consistent with the water retention
experiments (Table 1), which shows that different micro-
environments beneath the fiber surface were constructed.
Figure 2. WCA images of fibers.
T E B
Figure 1. FTIR spectra of (a) PANF, (b) PAN F, (c) PAN F, (d) PAN F, (e)
PANETF, (f) PANBTF, (g) PANBTF-1, (h) PANBTF-8.
Mechanical properties
Scanning electron microscopy (SEM)
The breaking strengths of the fibers were measured and shown
in Table S2. PANF has the highest breaking strength of 11.0 cN
Figure S1 presents the SEM images of PANF, PAN
PAN F, PANETT, PANBTF, PANBTF-1 and PANBTF-8. The
diameter of PANF is around 25 μm (Figure S1a). After being
modified once, the diameters and the surface of PAN F, PAN
and PAN F become wider and rougher than those of PANF
Figure S1b-d), indicating the successful modifications.
Compared with PAN F and PAN F, the surfaces of PANETT and
T E
F, PAN F,
(
Table S2, entry 1). After being functionalized, the breaking
strength of PAN F, PANETF and PANBTF were declined to 9.4 cN,
.7 cN and 8.2 cN, respectively (Table S2, entries 2-4). After the
cyanation, the breaking strength of PANBTF-1 keeps unchanged
Table S2, entries 5) and the fiber catalyst retain 74% of its
original physical strength even after being used eight times
Table S2, entries 6). These results show that the fiber catalyst
B
T
8
T
E
F
B
(
(
E
B
(
PANBTF are increasingly coarser and the bodies of the fiber
catalysts swelled further because of the second step of
modification (Figure S1e and f). After PANBTF used to catalyze
the cyanation reaction over one and eight times, very few
changes occur on the diameters and surfaces of PANBTF-1 and
PANBTF-8 (Figure S1g and h), which means PANBTF is an
has excellent mechanical strength to meet further application
Catalytic activities
Cyanation reaction is an important method to obtain
cyanohydrins, which is a crucial building blocks for numerous
excellent recoverable catalyst.
.
[
21]
biologically active compounds,
thus tremendous efforts have
[
22]
been paid to synthesize cyanohydrins.
In this paper, the
The measurement of water contact angles of the fiber
catalysts
catalytic activity of different amines functionalized PANFs were
examined via a cyanation reaction of aldehyde and ethyl
cyanoformate, and the results are listed in Table 2.
In this work, water contact angle (WCA) was used to
characterize the hydrophobicity performance of material surface
by measuring the thin films made by the fiber materials (Figure
[
a]
2). As shown in Figure 2, the WCA of unmodified PANF is 78°
Table 2. Catalytic activities of different amine functionalized PANF.
[
20]
which conforms well with the literature datum.
After
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ChemCatChem
10.1002/cctc.201700515
during the cyanation reaction can also indicate the high catalytic
activity of the PANBTF (Figure S15).
Moreover, with the increasing of the hydrophobic benzyl group
from PANBT
entries 5-7) and then gradually decline to 78% (Table 2, entries
-9). For the reason that there are three kinds of micro-
environments with different polarities during the increasing of the
tertiary-amine group (Figure S2). In the beginning, the fiber
F0.5 to PANBTF2.5, the yields rise to 94% (Table 2,
[
b]
Entry
Catalyst
Solvent
Time (min)
Yield (%)
NR
NR
32
8
1
2
3
4
5
6
7
8
9
PAN
PAN
PAN
B
F
F
H
H
H
H
H
H
H
H
H
H
H
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
30
30
30
30
30
30
30
30
30
40
50
40
40
40
40
E
B
catalyst (namely PAN F) has a hydrophobic micro-environment
T
F
constructed by benzyl group and polymeric segment (Figure S2,
section 1). The fiber has no catalytic site hence no activity. After
being modified by DMPDA, a new micro-environment which
contains dimethylamino group, i.e. the catalytic center, appears.
PANET
PANBT
PANBT
PANBT
PANBT
PANBT
PANBT
PANBT
PANBT
PANBT
PANBT
PANBT
F
F
F
F
F
F
F
F
F
F
F
F
18
0.5
1.0
1.5
2.0
2.5
1.5
1.5
1.5
1.5
1.5
1.5
65
(
Figure S2, section 2). That’s an effective micro-environment
86
where the reactants can get in and access to the active center
and then react with each other. The catalytic activity of fiber
catalyst is determined by two factors, the one is the effect of the
quantity of the tertiary-amine group and the other is the effect of
hydrophobic performance of the fiber. At first, the quantity effect
of tertiary-amine is predominant, both catalytic activity and
hydrophobicity are high. However, the high hydrophobicity of the
fiber catalyst causes the difficulty leaving of the product from the
micro-environment in fiber. And the product will encase the
catalytic site which will prevent further contacting of reactants
with the catalytic site. As a result, the cyanation reaction
94
88
78
10
11
12
13
14
15
99
98
CH
3
OH
43
EtOH
25
catalyzed by PANBT
, entry 5). With the increase of the hydrophilic tertiary-amine
group, the hydrophobic of the fiber catalyst is dropped. Until the
two effects reach an equilibrium state at PANBT 1.5 , the fiber
F0.5 obtains a moderate yield of 65% (Table
2
1,4-Dioxane
Cyclohexane
0
0
F
catalyst exhibited the highest catalytic activity (Table 2, entry 7).
Whereafter, with the introduction of more tertiary-amine group
the effect of hydrophobicity occupies the leading position, along
with the decrease of hydrophobicity, the reactants become
harder to access to the active catalytic point and the fiber’s
catalytic activity declined. At last, the tertiary-amine group is
excess, a greater polarity micro-environment (Figure S2, section
[
a] Reaction conditions: benzaldehyde (1.0 mmol), ethyl cyanoformate (1.5
mmol), solvent (5 mL) and fiber catalyst (0.1 mmol, 10 mol% tertiary-amide
group compared to aldehyde) was stirred at room temperature. [b] Yield by
HPLC with ethyl benzoate as internal standard, based on A1. [c] The footnote
B
number means the reaction time of PAN F being modified further by DMPDA.
3) formed by tertiary-amine groups. While, for the high polarity of
The PAN
entries 1 and 2), while the tertiary-amine functionalized fiber
catalyst PAN F could catalyze the cyanation reaction with a low
yield of 32% (Table 2, entry 3). When PAN F was modified by
hydrophilic ethanol amine, the yield declined to 18% (Table 2,
entry 4). However, when the reaction was carried out with
PANBTFs which were modified by both of DMPDA and
B
F and PAN
E
F failed to catalyze the cyanation (Table 2,
the micro-environment, only few hydrophobic reactants could
close to this section.
T
What’s more, with PANBTF1.5 (in short as PANBTF) as catalyst,
the yield increased to 99% when prolonged the reaction time to
40 min (Table 2, entry 10), while, the yield kept nearly unchang
when the time was extended to 50 min (Table 2, entry 11).
Besides, this catalyst system has been tested in different
solvents, as expected, the yields decreased with the lower of the
polarities of solvents (Table 2, entries 10, 12-15). The reason
may be that, the reactants dissolved well in a nonpolar solvent
but hard to accumulate in the micro-environment of the fiber
catalyst to generate the product (Table 2, entries 14-15). Above
all, with PANBTF as the optimum catalyst, reacting for 40 min in
water was selected for further research of this reaction.
To further study the catalytic activity of the PANBTF, the kinetitoc
of the cyanation reaction of benzaldehyde and ethyl
cyanoformate catalyzed by PANBTF was performed and the
results were shown in Figure S3. With PANBTF as the catalyst,
the reaction could be finished in 40 min (initial 20 min turnover
frequency (TOF), 22.8/h). After used one time, the kinetic curve
E
benzylamine, the yields were much higher than PAN
entries 5-9), which suggested that the catalytic activities of
PAN F, PANETF and PANBTF were closely related to the
T
F (Table 2,
T
polarities of the micro-environments in the fiber catalysts. For
the PANBTF, a relatively hydrophobic micro-environment was
constructed by the dimethylamino group (catalytic activity
center), the benzyl group and polymeric segments. Where the
hydrophobic reactants easily go into the fiber surface and
contact with the catalytic sites to process the reaction efficiently
and exclusively. While for catalyst PANETF, the micro-
environment is hydrophilic for its high polarity, which repel the
reactants from the catalytic centers, not to mention conduct
chemical reaction. The content change of the benzaldehyde
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ChemCatChem
10.1002/cctc.201700515
of the PANBTF-1 (initial 0.5 h TOF, 22.5/h) was analogous to that
of the original catalyst system. Moreover, when the fiber catalyst
was used eight times, the TOF of the PANBTF-8 could reach
1H) belong to the intermediate IV’ which suggest that cyanation
reaction of benzaldehyde and ethyl cyanoformate is proceed by
pathway b. Moreover, if the cyanation reaction proceed by
-
20.4/h during the first 20 min.
pathway b, the CN (namely the intermediate III’) mast exist here
can be detected by FeSO
cyanoformate and PANBTF into water and stirred 2 min at RT.
Then added some FeSO powder into the mixture and we found
the mixture and the fiber catalyst turning blue immediately
Figure S11b and c). On the other hand, the mixture of ethyl
cyanoformate, FeSO and water did not change its colour even
4
. Therefore, we putted the ethyl
O
O
O
O
O
O
N
H
NC
NC
NC
NC
NC
NC
4
N
H
N
N
H
N
N
H
N
O
O
CN
CN
O
Ph
N
H
Ph
O
O
(
II
II '
4
O
O
stirred 30 min at the same conditions (Figure S11a). All in all,
the NMR-monitoring experiment and the CN detection test can
both demonstrate the existence of pathway b.
NC
NC
N
N
Ph
NC
NC
O
NC
NC
N
N
O
H
a
b
H
-
O
CN
O
N
N
O
CN
O
H
O
I
O
Ph
III
III '
O
Ph
NC
OEt
O
O
NC
NC
NC
NC
N
N
Ph
N
H
N
O
H
O
O
O
O
O
CN
O
O
O
NC
IV '
Ph
IV
Ph
CN
Scheme 2. Possible “micro-environment promote” catalytic process.
From the results discussed above, two kinds of “micro-
environment promoted” mechanisms of the cyanation reaction of
benzaldehyde and ethyl cyanoformate are proposed (Scheme 2).
In the beginning, benzaldehyde and ethyl cyanoformate enter
into the relative hydrophobic micro-environment I to form II or II’.
For pathway a, the nitrogen atom of the tertiary-amine group
reacts with benzaldehyde to from an inner-salt-type intermediate
III. Then, the atomic oxygen of the intermediate III as a
nucleophile attacks the carbonyl carbon to form the intermediate
IV and a cyanide anion is generated at the same time. The last
procedure is a rapid in cage nucleophilic reaction, namely, the
formed cyanide anion attacks the benzyl carbon to afford the
target ethylcyano carbonate. For pathway b, the tertiary-amine
group initially reacts with ethyl cyanoformate to generate a
cyanide anion and a quaternary ammonium cationic III’. Next,
the cyanide anion as a strong nucleophile attacks the carbonyl
carbon of benzaldehyde to give the intermediate IV’. Finally, a
fast in cage nucleophilic attack of the cyanoalkoxide on the
carbonyl carbon in intermediate III’ to obtain the corresponding
product. After the pathway a or b, the PANBTF is recovered and
can be reused for the next cycle.
Figure 3. NMR-monitoring Cyanation reaction of benzaldehyde and ethyl
cyanoformate
Under the optimal conditions, a series of aldehydes were
evaluated for the reaction and the results were summarized in
Table 3. It was found that benzaldehydes with electron
withdrawing groups could get yields up to 96% (Table 3, entries
5-7), which exhibited higher activities than those with electron
donating groups (Table 3, entries 2-4). On account of the
electron donating group can increase the electron cloud density
of the carbonyl carbon so as to decrease the activity of the
aldehydes. Similarly, heteroaromatic aldehydes with high density
of electron cloud also presented moderate yields (Table 3,
entries 9 and 10). For the aldehydes with conjugative effect
which could decline the electropositive of carbonyl carbon such
as cinnamic aldehyde and 1-naphthaldehyde afforded the
slightly low yields of 80% and 82%, respectively (Table 3, entries
In order to determine which pathway does the reaction undergo,
N,N-dimethylbutylamine act as a succedaneum of the PANBTF to
catalyze the cyanation reaction of benzaldehyde and ethyl
cyanoformate in DMSO-d6 and the reaction was monitored by
NMR. As shown in Figure 3, peaks at δ = 10.02 (s, 1H), 7.92 (d,
2
H), 7.71 (t, 1H), 7.61 (t, 2H), δ = 4.32 (q, 2H), 1.29 (t, 3H), and
δ = 2.35 (t, 2H), 2.24 (s, 6H), 1.40 (m, 2H), 1.18 (m, 2H), 0.86 (t,
H) belong to the unconsumed benzaldehyde, ethyl
11 and 12). Satisfactorily, aliphatic aldehydes all gave high
yields from 95% to 98% and the reason might be that the
carbonyl carbon of aliphatic aldehydes were more active than
aromatic aldehydes for nucleophilic reaction (Table 3, entries 13
and 12). In all, we could say that PANBTF is an excellent
heterogeneous catalyst for cyanation reaction.
3
cyanoformate and the catalyst N,N-dimethylbutylamine,
respectively (Figure S4-7). In addition, peaks at δ = 7.51 (s, 5H),
6
.67 (s, 1H), 4.23 (q, 2H), 1.24 (t, 3H) are attributed to the
1
product C1 (Figure S6). Besides, compared to the H NMR of
mandelonitrile (Figure S8) and its deprotonated compound
(
Figure S9), new peaks appeared at δ = 7.42 (m, 5H), 5.77 (s,
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ChemCatChem
10.1002/cctc.201700515
[
a]
Table 3. Cyanation reaction of aldehyde and ethyl cyanoformate.
pump
PANT-TA
F
[
b]
-1
Entry
R
Prouduct
C1
Yield (%)
TOF (h )
14.1
12.3
13.2
12.6
14.4
14.1
14.2
12.8
12.7
12.1
12.0
12.3
14.7
14.2
14.4
14.2
1
2
3
4
5
6
7
8
9
C6H5
94
2-MeOC
3-MeOC
4-MeOC
6
6
6
H
H
H
4
4
4
C2
82
C3
88
C4
84
Figure 4. Schematic diagram of the continue-flow reaction system
4-FC
4-ClC
4-CF
4-C
6
H
4
C5
96
6
H
4
C6
94
One of the advantages of PANF supported fiber catalyst is its
flexibility and tenacity and can be woven into a variety of shapes,
which is suitable for the fixed-bed reactors. Here we applied the
PANBTF to a simple continue-flow reactor by filling PANBTF into a
glass tube and used the cyanation reaction as a template
reaction to test its continue-flow property. The detailed process
of the continue-flow reaction system was drawn in Figure 4. A
solution of benzaldehyde and ethyl cyanoformate was pumped
into the glass reactor at RT. A time-on-stream format and the
results are shown in Figure S13. From which we can see that
after 72 h, the yield of the cyanation reaction has been basically
unchanged, which indicate the high potential application value in
industry.
3
C
6
H
4
C7
95
6
H
5
C
6
H
4
C8
86
2-Furanyl
2-Thienyl
C9
85
10
11
12
13
14
15
16
C10
C11
C12
C13
C14
C15
C16
81
2-C
6
H
5
C
2
H
2
80
1-Naphthyl
n-Propyl
82
98
Isopropyl
95
Scale-up of the cyanation reaction catalyzed by PANBTF.
Cyclopentyl
96
2-Cyclohexenyl
95
The cyanation reaction was scaled up with benzaldehyde (5.31
g) and ethyl cyanoformate (7.43 g) under the optimized
conditions and the corresponding yield was 96% (9.8 g). The
[
a] Reaction conditions: aldehyde (1.0 mmol), ethyl cyanoformate (1.5
mmol), water (5 mL) and PANBTF (0.1 mmol, 10 mol% tertiary-amine group
compared to aldehyde) was stirred at room temperature for 40 min. [b]
Isolated yield.
result further indicates that the PANBTF has a potential
application in industry.
Comparison of the cyanation reaction in different catalytic
systems on cyanation reaction
Reusability of the catalyst PANBT
F
Compared with different catalyst systems under their optimized
conditions (Table 6), the PANBTF still has advantages for it’s
easy preparation, high catalytic activity (which can finish the
cyanation reaction at RT in 40 min with a high yield of 94%),
environmentally friendly solvent (water) and excellent reusability
The recyclability of the catalyst PANBTF was investigated by
cyanation reaction in water and the results were listed in Figure
S12. When the reaction completed, the fiber catalyst was taken
out and washed with ethyl acetate to remove the adsorbed
reactants or product. After that, the recycled fiber catalyst was
reused directly in the next cycle without any further treatments.
After 8 cycles, the yield slightly declined from 94% to 89%,
which indicated that the PANBTF has an excellent recoverability.
(
can be reused at least 8 times).
Conclusions
Flow chemistry of cyanation reaction catalyzed by PANBTF.
In this work, different amines functionalized polyacrylonitrile fiber
catalysts were prepared and their catalytic activities were tuned
by adjusting the polarities of the surface micro-environment. For
the cyanation reaction, a relatively hydrophobic fiber catalyst
(
PANBTF) shows the highest catalytic activity, which indicates
that the surface micro-environment plays an important role in the
catalytic activities of fiber catalysts. Furthermore, among all the
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ChemCatChem
10.1002/cctc.201700515
tested solvents, water presents the best result (with 99% yield in
standard (ethyl benzoate) was added into the effluent reaction solution
and the yield was determined by HPLC.
40 min at RT). In addition, a hydrophobic micro-environment
promoted mechanism have been proposed and verified to
explain the outstanding catalytic activity of the PANBTF. Besides,
the PANBTF catalyst can be reused at least eight times without Acknowledgements
further treatments and performed well in flow chemistry and
scaled-up experiments, which suggesting its potential
application in industry. All in all, PANBTF is a green, recoverable,
We gratefully acknowledge the support from the National Natural
Science Foundation of China (No. 21572156 and No. 21306133).
efficient, and easily prepared heterogeneous catalyst,
a
hydrophobic micro-environment in the surface of the fiber
catalyst can efficiently enhance its catalytic activity.
Keywords: Cyanation reaction • Fiber catalyst • Green
chemistry • Micro-environment • Polyacrylonitrile fiber
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FULL PAPER
Controllable micro-environment:
It’s a guideline for the design of the
polyacrylonitrile fiber catalyst which
catalytic activity has been controlled
by tuning the polarity of the micro-
environment and applied to the
cyanation reaction in water. For
PANBTF, it has the advantages of
easy preparation, excellent activity
Pengyu Li, Yuanyuan Liu, Jian Cao,
Minli Tao* and Wenqin Zhang*
Page No. – Page No.
Tuning the catalytic activity of
tertiary-amine functionalized
polyacrylonitrile fiber by adjusting
the surface micro-environment
(
80-98%), catalytic recyclability (at
least 8 cycles) and so on. What’s
more, its well performation in flow
chemistry and acaled-up experiment
indicate a potential application for
industry.
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