Sulfonic groups functionalized preoxidated polyacrylonitrile nanofibers and its catalytic applications

Article abstract of DOI:10.1016/j.apcata.2012.07.034

A SO₃H-bearing nanofiber mat was synthesized and investigated as a novel heterogeneous acid catalyst. Preoxidated polyacrylonitrile nanofiber mat was prepared via electrospinning and heat treatment, and then reacted with chlorosulfuric acid to introduce the sulfonic groups. The nanofiber mat owned high acidity of 2.99 mmol/g. The preoxidation and sulfonation were examined by FT-IR spectroscopy, elemental analysis and X-ray diffraction spectroscopy (XRD). The fiber morphologies were characterized by scanning electron microscopy (SEM). The catalytic activities and reuse of the prepared nanofiber mat solid acid catalyst have been evaluated for the acetalization and esterification. The regular fiber mat structure could significantly facilitate the recovery and reuse of the catalyst. The excellent catalytic performance and easy recycling made the novel fiber mat solid acid hold great potential for the green chemical processes.

Full text of DOI:10.1016/j.apcata.2012.07.034

Applied Catalysis A: General 443–444 (2012) 133–137
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Sulfonic groups functionalized preoxidated polyacrylonitrile nanofibers and
its catalytic applications
∗
Linjun Shao, Guiying Xing, Luyao He, Ji Chen, Hangqing Xie, Xuezheng Liang, Chenze Qi
Institute of Applied Chemistry, Shaoxing University, Shaoxing, 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:
Received 8 April 2012
Received in revised form 4 July 2012
Accepted 27 July 2012
Available online 3 August 2012
A SO H-bearing nanofiber mat was synthesized and investigated as a novel heterogeneous acid cata-
lyst. Preoxidated polyacrylonitrile nanofiber mat was prepared via electrospinning and heat treatment,
3
and then reacted with chlorosulfuric acid to introduce the sulfonic groups. The nanofiber mat owned
high acidity of 2.99 mmol/g. The preoxidation and sulfonation were examined by FT-IR spectroscopy,
elemental analysis and X-ray diffraction spectroscopy (XRD). The fiber morphologies were character-
ized by scanning electron microscopy (SEM). The catalytic activities and reuse of the prepared nanofiber
mat solid acid catalyst have been evaluated for the acetalization and esterification. The regular fiber
mat structure could significantly facilitate the recovery and reuse of the catalyst. The excellent catalytic
performance and easy recycling made the novel fiber mat solid acid hold great potential for the green
chemical processes.
Keywords:
Solid acid
Preoxidation
Electrospun
Nanofiber
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
Electrospinning is a facile method to produce non-woven mats
with large surface area to volume ratio and excellent mechan-
ical strength. These electrospun mats can be used in various
areas such as filtration, composite materials, tissue engineering
and catalysis [8–10]. Till now, a number of synthetic and bio-
related polymers have been successfully fabricated into sub-micro
or nanofibers [11–13]. Acrylonitrile and acrylonitrile based poly-
mers were the most widely used electrospinning materials due
to their excellent spinning ability and chemical stability, and
their electrospinning mats were excellent supporting materials
for catalyst active species [14–16]. The nanofiber mats sup-
ported catalyst has the advantage of much easier separation
and recycling over the fine particles supported catalyst from the
reaction media. Moreover, the fiber mats hold the potential for
the preparation of membrane reactor for continuous production
[17–19].
In our previous study, we have prepared the polyvinyl alco-
hol (PVA) nanofiber mats to immobilize palladium catalysts for
the homocoupling and crosscoupling reactions of various aro-
matic halides [20]. In this article, preoxidated polyacrylonitrile
nanofiber mat was prepared by electrospinning and heat treat-
ment. This prepared fiber mat was then sulfonated by chlorosulfuric
acid to synthesize sulfonic group functionalized nanofiber mat
solid acid catalyst. The resulting nanofiber mat acid catalyst was
applied to the acetalization and esterification. To the best of our
knowledge, polyacrylonitrile preoxidated nanofibers have not yet
been used for the preparation of the heterogeneous acid catalysts
till now.
Acid catalyzed reactions have played an important role in chem-
ical industries for the production of various important chemicals
1,2]. Over 15 million tonnes of sulfuric acid is annually consumed
[
as an unrecyclable catalyst, which caused serious environmental
problems. Therefore, preparation of high efficient and recyclable
acid catalyst is highly demanded. Recently, a number of hetero-
geneous solid acids have been synthesized as the replacement of
the homogenous sulfuric acid [3–5]. Besides avoiding the envi-
ronmental pollution, the reusability of the heterogeneous catalyst
can greatly increase the overall productivity and cost effective-
ness while minimizing both waste generation and contamination
of the final products, resulting in a greener and more sustainable
chemical transformation process [6,7]. Unfortunately, compared to
homogenous catalysts, the catalytic activities of heterogeneous cat-
alysts would significantly reduced due to the steric hindrance of the
supporting materials. In order to reduce the steric effect, the solid
supporting materials were generally fabricated into small parti-
cles to maximize the contacting area between the substrate and
catalytic active species. However, the separation and recycling of
these particulate catalysts from the reaction mixture was usually a
tedious and time-consuming process due to the high pressure drop
by filtration.
∗ Corresponding author. Fax: +86 575 8834 5682.
E-mail address: qichenze@usx.edu.cn (C. Qi).
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.07.034

1
34
L. Shao et al. / Applied Catalysis A: General 443–444 (2012) 133–137
2
. Experimental part
continuously from the reaction mixture. The reaction was refluxed
for 2 h to complete the reaction.
2.1. General remarks
2
.6. General procedure for the SF-NF mat catalyzed esterification
All organic reagents were commercial products of the highest
and acrylation
purity available (98%) and used for the reaction without further
purification. Polyacrylonitrile (PAN) was synthesized according to
the literature [21]. The quantitative analysis was performed on
a Shimadzu (GC-14B) gas chromatograph. The morphologies of
electrospun fiber mats were recorded with a scanning electron
microscope (SEM) (Jeol, jsm-6360lv, Japan). Samples for SEM were
coated with a 2–3 nm layer of Au to make them conductive. The ele-
mental analysis was taken on the EuroEA 3000 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 samples was determined by means of X-ray
powder diffraction (XRD) (Rigaku D, max-3BX, Japan). Surface area
was measured by TriStar II 3020 from Micromeritics, USA.
The mixture of acetic acid or acetic anhydride (24 mmol), alcohol
or aniline (20 mmol), and SF-NF mat (50 mg) was put in a round
bottomed flask and stirred at 60 C for 3 h.
◦
3. Results and discussion
3.1. Characterization of the SF-NF mat
Electrospinning is a well-established technique to produce
ultrafine fiber mats with diameters in the nanometer to submi-
crometer range and with large surface area to volume ratio. The
morphologies of the electrospun mats can be tuned by variation of
the polymer concentrations [20–22]. Well-defined fiber mat can be
achieved when the PAN concentration was at 8.0 wt.% (Fig. 1a). If
the PAN fiber was treated under air without the fixation of stain-
less steel sheet, the PAN would be oxidated into amorphous carbon
without any physical strength. Compared with the PAN fibers,
shrinkage and fracture of some preoxidated PAN fibers happened
due to the heat treatment (Fig. 1b). It is interesting to find that the
diameter of fibers was significantly decreased after the sulfona-
tion step (Fig. 1c and d). As the solution of the chlorosulfuric acid
solution had become slight yellow after the sulfonation, it can be
concluded that the strong acidity of chlorosulfuric acid could dis-
solve the small molecules in the fiber, which produced during the
preoxidation step. As a result, the fiber diameter of SF-NF mat was
further reduced to 180.1 ± 4.1 nm and the corresponding surface
2.2. Preparation of PAN nanofiber mat
PAN was dissolved in N,N-dimethylformide (DMF) and stirred
overnight to get a homogeneous solution at a concentration of
wt.%. The resulting solution was directly subjected into a syringe
20 mL) with a blunt-end capillary (0.8 mm ID). A sheet of alu-
8
(
minum foil, connected to the ground, was put under the syringe
at a distance of 12 cm as the collector. A voltage of 10 kV (GDW-
a, Tianjin Dongwen High-Voltage Power Supply Company, China)
was applied to the PAN solution. The feed rate of the PAN solution
was kept at 1.0 mL/h by a micro-infusion pump (WZ-50C6, Zhejiang
Smiths Medical Instrument, China). The resultant mats were dried
under vacuum at room temperature to remove residual solvent.
2
2
area was increased to 3.15 m /g from 2.07 m /g.
2.3. Preparation of sulfonic groups functionalized PAN
The preoxidatin step could change the linear PAN molecular
chains into aromatic ladder structure, which could enhance the
chemical stability and solvent resistance of the fiber mat and also
stabilize the attached sulfonic groups [23,24]. Therefore, it can be
concluded that the preoxidation step would be helpful for the SF-NF
mat catalyst with high activity and stability. Fig. 2 shows the FT-IR
spectra of fiber mats at each step. After preoxidation, the intensi-
preoxidated nanofiber mat (SF-NF mat)
PAN fiber mat was located at the centre of two stainless steel
sheets. The stainless steel sheets were then heated in a muffle fur-
◦
nace at 200 C for 2.5 h to prepare PAN preoxidated fiber mat. PAN
preoxidated nanofiber mat (1.0 g) was added to the chlorosulfuric
acid/dichloromethane solution (20 mL, chlorosulfuric acid concen-
−
1
−1
ties of absorption peak at 2243 cm assigned to C≡N, 2939 cm
◦
tration: 10 wt.%) and stirred gently for 12 h at 10 C. On completion,
−
1
−1
and 1454 cm attributed to C H, and band at 1070 cm assigned
to C–CN were significantly weaken or disappeared; while the
the nanofiber mat was taken out and washed with dichloromethane
repeatedly to remove the residue chlorosulfuric acid. The resulting
SF-NF mat was dried under a reduced pressure at room temperature
for overnight, and then was stored in a desiccator until application.
−
1
−1
bands at 1640 cm and 1590 cm (due to C N, C C mixed), and
−
1
−1
the bands in 1345 cm and 810 cm (due to C
intensify due to dehydrogenation. Meanwhile, a shoulder at
730 cm attributed to C O appeared because of oxidation reac-
C H) began to
−
1
1
2.4. Swelling ability of SF-NF mat
tion [24]. As shown in Fig. 3, the XRD pattern of preoxidated PAN
◦
◦
fiber mat shows two diffraction peaks at 26 and 43 , which is sim-
ilar to the characteristic peaks of the (0 0 2) and (1 0 1) planes in
graphite [25,26]. These spectroscopic results clearly indicated that
the linear PAN structure had been changed into aromatic ladder
structure. In the sulfonation step, the sulfuric acid aqueous solu-
tion (50 wt.%) and chlorosulfuric acid in dichloromethane solution
Swelling ability of the SF-NF mat was characterized in solvent
at room temperature according to the following equation (Eq. (1)):
M − M
d
Degree of swelling (%) =
× 100
(1)
M
d
where M is the weight of the SF-NF mat after submersion in solvent
for 24 h, Md is the weight of original mat in its dry state.
(10 wt.%) had been employed as the sulfonating agent. With sulfu-
ric acid as the sulfonating agent, the preoxidated PAN fibers were
partly dissolved and seriously broken. The different intensities of
−
1
2.5. General procedure for the SF-NF mat catalyzed acetalization
bands at 1045 cm
(S = 0) for preoxidated PAN fiber mats sul-
reaction
fonated by chlorosulfuric acid and sulfuric acid also indicated that
the chlorosulfuric acid was better than sulfuric acid as sulfonat-
ing agent. Thus, chlorosulfuric acid was chosen as the sulfonating
agent. The acidity of the SF-NF mat was 2.99 mmol/g, which was
determined through the neutralization titration [27]. The elemen-
tal analysis of SF-NF mat gave the results: C: 34.54%; N: 13.42; H:
2.66%; S: 4.17%. These results indicated that there existed other
Aldehyde or ketone (20 mmol), 5 mL cyclohexane,
a diol
(
24 mmol) and the catalyst (20 mg) were mixed together in a three
necked round bottomed flask equipped with a magnetic stirrer and
a thermometer, and a Dean–Stark apparatus which was consti-
tuted with manifold and condenser was used to remove the water

L. Shao et al. / Applied Catalysis A: General 443–444 (2012) 133–137
135
Fig. 1. SEM images of PAN fiber mat (a), preoxidated PAN fiber mat (b) and SF-NF mat (c, d).
Table 1
3.2. The catalytic activities for the acetalization
Degree of swelling of SF-NF mat in different solvent.
Solvent
Degree of swelling (%)
Various carbonyl compounds and diols have been employed to
evaluate the catalytic activities of the SF-NF mat. The results in
Table 2 clearly demonstrated that the SF-NF mat was very effi-
cient to catalyze the acetalization reaction with high yields for
most reactions. The aldehydes transformed to the correspond-
ing acetals smoothly under the reaction conditions (Entry 1–2 in
Table 2). Cyclohexanone was also worked very well with almost
complete conversion for the low steric hindrance of carbonyl group
(Entry 4 in Table 2). However, due to the steric effect, the butanone
owned relatively low activity with moderate yields, especially with
1,4-butanediol (Entry 3 in Table 2). Through increasing the reac-
tion time, butanone could also be acetalized by 1,4-butanediol to
synthesize corresponding 1,4-ketal with excellent yield. As the five-
membered ring was more stable than the seven-membered ring,
DMF
Cyclohexane
105.55
94.11
123.53
1
,2-Ethanediol
acidic groups in the SF-NF mat besides sulfonic groups, such as
carboxyl group, which was consistent with the FT-IR spectra results.
The solvent resistance was determined by measuring the
swelling ability of SF-NF mat in polar and non polar solvent. As
shown in Table 1, the degrees of swell of SF-NF mat were in the
range from 94% to 124% after immersed in solvents for 24 h. This
result demonstrated that the SF-NF mat was stable and could be
used in different reaction condition.
1
1
1
04
02
00
30000
(002)
2
5000
9
9
9
9
9
8
8
8
8
6
4
2
0
8
6
4
C-CN
20000
15000
10000
C-H
PAN fiber mat
Preoxidated PAN fiber mat
Preoxidated PAN fiber mat/H SO
C
N
C=O
S=O
C=C-H
(101)
50
2
4
SF-NF mat
C=C
C=N
5
000
0
=C-H
4
000
3500
3000
2500
2000
1500
1000
500
20
30
40
60
70
80
90
-
1
Wavelength (cm )
2θ (degree)
Fig. 2. FT-IR spectra of the electrospun mats.
Fig. 3. XRD pattern of preoxidated PAN fiber mat.

1
36
L. Shao et al. / Applied Catalysis A: General 443–444 (2012) 133–137
Table 2
a
The SF-NF mat catalyzed acetalization.
Entry
Ketone/aldehyde
Chloroacetaldehyde
Benzaldehyde
Diol
Conversion (%)
Yield (%)
1,2-Ethanediol
1,4-Butanediol
1,2-Ethanediol
1,4-Butanediol
1,2-Ethanediol
99.99
99.99
99.99
98.47
94.38
99.60
99.50
97.33
92.55
94.00
47.18
88.76b
99.80
99.70
1
2
3
Butanone
1,4-Butanediol
47.18
b
88.95
1
1
,2-Ethanediol
,4-Butanediol
99.99
99.99
4
Cyclohexanone
a
◦
Catalytic condition: 20 mmol ketone or aldehyde; 24 mmol diol; 20 mg catalyst in 5 mL cyclohexane at 150 C for 2 h; with Dean–Stark apparatus.
Reaction time: 4 h.
b
Table 3
Acetalization
Esterification
a
The SF-NF mat catalyzed esterification and acylation.
100
80
Entry
Alcohol/amine
Acid/anhydride
Conversion (%)
Yield (%)
99.40
1
2
3
Aniline
1,3-Diaminopropane
n-Butanol
99.99
94.50b
32.69
b
84.98
Acetic acid
32.23
9
8.98c
98.52c
6
0
0
4
5
6
Metanol
Isopropanol
n-Butanol
99.99
94.34
99.99
99.60
93.17
99.58
Acetic anhydride
4
a
Catalytic condition: 20 mmol aniline or alcohol; 24 mmol acetic acid or acetic
◦
anhydride; 50 mg catalyst; 60 C; 3 h.
b
Catalytic condition: 20 mmol 1,3-diaminopropane; 48 mmol acetic acid; 50 mg
20
0
◦
catalyst; 60 C; 3 h.
c
Catalytic condition: 20 mmol n-butanol; 24 mmol acetic acid or acetic anhy-
dride; 50 mg catalyst; reflux, 2.0 h; with Dean–Stark apparatus.
1
2
3
4
5
6
Run times
1
,2-ethanediol worked better with carbonyl compound than 1,4-
butanediol.
Fig. 4. The reuse of the SF-NF mat.
3.4. Comparison of SF-NF mat with other acid catalyst
3.3. The catalytic activities for the esterification and acylation
Compared with homogenous catalyst, the reduction of catalytic
Table 3 shows that the SF-NF fiber mat was also very effi-
activity of heterogeneous catalyst is associated with the disfavored
kinetics of the biphasic catalytic system [28]. A comparative study
on the catalytic activities of the SF-NF mat with homogeneous cat-
alysts was carried out (Table 4). The conversions and yields clearly
showed the SF-NF fiber mat had comparable catalytic activities
cient for the acylation of monoamine by acetic acid (Entry 1 in
Table 3). Due to the low reactivity of the second amine group,
the 1,3-diaminopropane was acylated to afford the correspond-
ing biacrylated product in moderate yield (Entry 2 in Table 3).
Because the byproduct of water would significantly deceler-
ate the esterification, it is not surprising to find that the yield
was very low for the esterification of acetic acid and n-butanol
Entry 3 in Table 3). Same as the acetalization, the esterifica-
tion could also be significantly enhanced through removing the
water from the reaction continuously by Dean–Stark apparatus
Entry 3 in Table 3). Replacement of acetic acid by acetic anhy-
dride was also an effective way to promote the esterification
Entry 4–6 in Table 3). The isopropanol owned comparably low
reactivity for the esterification due to the high steric hindrance
Entry 5 in Table 3).
with the homogeneous H SO4 and p-toluene sulphonic acid cat-
2
alysts. We can also found that SF-NF mat was even more efficient
than the particle solid acid [29]. The values of TOF also showed that
the SF-NF mat was an efficient acid-catalyst for the acetalization.
The comparable catalytic activity of the SF-NF mat to traditional
homogeneous acid catalyst indicated by the present results demon-
strated that it had great potential to replace homogeneous acid
catalyst for green processes. Furthermore, compared with the par-
ticle solid acid [29], the fiber structure not only facilitated the
recovery of the catalyst, but also could be prepared into membrane
reactor for large scale industrial production.
(
(
(
(
Table 4
a
The catalytic activity comparison of different catalysts.
TOF (s 1
−
)
Entry
Catalyst
Amount (mg)
Aldehyde/Ketone
Conversion (%)
Yield (%)
Benzaldehyde
Cyclohexanone
Benzaldehyde
Cyclohexanone
Benzaldehyde
Cyclohexanone
Benzaldehyde
Cyclohexanone
39.13
99.91
52.21
99.99
30.56
99.99
53.32
99.99
39.13
87.40
52.21
91.35
30.56
92.60
53.32
96.32
3.20
(1.36) 8.16
14.97
(4.78) 28.68
10.61
(5.79) 34.74
14.86
1
2
3
H2SO4
10
10
20
20
p-Toluene sulphonic acid
Solid acid [26]
SF-NF mat
4
(4.64) 27.84
a
◦
Reaction condition: 20 mmol benzaldhyde/cyclohexanone; 24 mmol 1,2-ethenediol; 10 or 20 mg catalyst in 5 mL cyclohexane at 150 C for 20 min; with Dean–Stark
apparatus.

L. Shao et al. / Applied Catalysis A: General 443–444 (2012) 133–137
137
4
. Reuse of SF-NF mat
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