Polyacrylonitrile fiber mat supported solid acid catalyst for acetalization

Article abstract of DOI:10.1007/s00706-011-0706-0

A novel polyacrylonitrile hybrid fiber mat supported solid acid catalyst was prepared by electrospin-ning, and its catalytic activities were carefully investigated through acetalization reactions. The results showed that this hybrid fiber mat exhibits high activity for the reactions, with average yields over 95%. Besides having catalytic activities similar to the solid acid, the heterogeneous solid acid/polyacrylonitrile mat can be reused in six runs without significant loss of catalytic activities. The large size of the hybrid fiber mat greatly facilitates recovery of the catalyst from the reaction mixture. The high and stable catalytic activities of the hybrid fiber mat hold great potential for green chemical processes and preparation of membrane reactors in the future.

Full text of DOI:10.1007/s00706-011-0706-0

Monatsh Chem (2012) 143:1199–1203
DOI 10.1007/s00706-011-0706-0
ORIGINAL PAPER
Polyacrylonitrile fiber mat supported solid acid catalyst
for acetalization
Linjun Shao
Xuezheng Liang
•
Yijun Du
•
•
Guiying Xing
•
Weixi Lv
•
Chenze Qi
Received: 26 September 2011 / Accepted: 13 December 2011 / Published online: 12 January 2012
Ó Springer-Verlag 2012
Abstract A novel polyacrylonitrile hybrid fiber mat
supported solid acid catalyst was prepared by electrospin-
ning, and its catalytic activities were carefully investigated
through acetalization reactions. The results showed that
this hybrid fiber mat exhibits high activity for the reactions,
with average yields over 95%. Besides having catalytic
activities similar to the solid acid, the heterogeneous solid
acid/polyacrylonitrile mat can be reused in six runs without
significant loss of catalytic activities. The large size of the
hybrid fiber mat greatly facilitates recovery of the catalyst
from the reaction mixture. The high and stable catalytic
activities of the hybrid fiber mat hold great potential for
green chemical processes and preparation of membrane
reactors in the future.
unrecyclable sulfonic acids [3–5]. Sulfonated carbonaceous
materials are used in some of the most widely used
methods to synthesize solid acids via carbonization and
sulfonation [6–8]. However, almost all solid acids prepared
in this way are powders. Separation and recycling of par-
ticulate catalysts from the reaction mixture is a tedious and
time-consuming process due to the large pressure drop in
filtration.
Electrospinning is a facile method to fabricate organic,
inorganic, and hybrid fibers with diameters ranging from
tens of nanometers to several micrometers [9–11]. Such
thin fibers are widely used in fiber textiles, sensors, bio-
materials, and catalyst carriers due to their unique surface
area-to-volume ratios. Recently, both natural (silk fibroin,
casein, cellulose, chitosan, etc.) and synthetic polymers
[polystyrene, polyvinyl alcohol, polycaprolactone, poly-
sulfone, polyacrylonitrile (PAN), etc.] have been suc-
cessfully prepared into fiber mats by electrospinning [12,
Keywords Membrane Á Acidity Á Catalysis Á
Green chemistry
13]. When a catalyst is coelectrospun into a polymer mat
Introduction
matrix, the active molecular species are anchored both on
the surface and in the interior of the fiber. Due to the low
diffusion resistance of the mat fiber network, the sub-
strate can diffuse on the surface and into the fiber easily,
and all the active species of the catalyst can be fully
used.
Acid catalysts have played an important role in chemical
industries for production of various important chemicals
[
1, 2]. Over 15 million tons of sulfuric acid is consumed
annually as an unrecyclable catalyst, resulting in great
environmental pollution. Preparation of environmentally
friendly acid catalysts is highly demanded, and various
solid acids have been developed for replacement of
Due to their excellent spinnability and chemical stabil-
ity, acrylonitrile-based homo- and copolymers have been
the most widely used polymers to prepare fiber mats as
supporting materials for catalyst [14, 15]. However, to the
best of our knowledge, they have not yet been applied to
immobilize solid acid catalysts. The much larger structure
of the fiber mats results in much easier separation and
recycling of solid acid from the mixture. Furthermore,
these fiber mats could be prepared into a membrane reactor
for continuous catalytic reaction in the future.
L. Shao Á Y. Du Á G. Xing Á W. Lv Á X. Liang Á C. Qi (&)
Institute of Applied Chemistry, Shaoxing University,
Shaoxing, China
e-mail: qichenze@usx.edu.cn
123

1
200
L. Shao et al.
Results and discussion
weight loss when the temperature was higher than the
initial decomposition temperature. The solid acid/PAN was
stable at temperature below 230 °C.
Polymer concentration is known to be critical for elec-
trospinning [16]. A mixture of fiber and droplet appears if
the concentration is too low; in contrast, it is difficult to
force the solution through a syringe needle if the concen-
tration is too high. Well-defined fibers with diameter of
Comparative study of the catalytic activities of the
hybrid fiber mat with H SO and solid acid was undertaken
2
4
firstly using the acetalization reaction between cyclohexa-
none and ethylene glycol (Table 1; Fig. 3). Table 1 shows
that the conversions and yields were similar for the ace-
talization reaction catalyzed by the three catalysts when the
reaction time was 2 h. However, it was very important to
remove the water continuously from the mixture by a
Dean–Stark apparatus. Otherwise, the reaction could be
greatly hindered. In Fig. 1, it can be seen that H SO was
4
87.2 ± 68.4 nm were fabricated from PAN at concen-
tration of 8 wt%. (Fig. 1a). Therefore, 8 wt% PAN
concentration was chosen for subsequent solid acid/PAN
mat preparation. Compared with that of the PAN mat, the
fiber diameter of solid acid/PAN mat increased to
7
15.4 ± 182.2 nm due to the increased solid content of the
2
4
solid acid/PAN solution (Fig. 1b).
more effective than solid acid, which in turn was better
than solid acid/PAN mat at promoting the acetalization
reaction. The high effectiveness of H SO is associated
The acidity of the hybrid fiber mat was 0.71 mmol/g
while that of solid acid was 2.4 mmol/g, determined by
neutralization titration. As the content of solid acid in the
hybrid fiber mat was only 33.33%, the acidity loss of the
solid acid when added to the hybrid fiber mat was very low.
Elemental analysis of solid acid/PAN mat gave the results:
C, 69.12%; H, 6.51%; N, 18.46%; S, 2.36%; O, 3.55%.
This result is consistent with our previous report [17] that
the S content in solid acid is 7.6%. Results of thermal
gravimetric analysis (TGA) of PAN and solid acid/PAN are
shown in Fig. 2. The results indicate that PAN was stable
at temperature below 330 °C but decomposed rapidly at
temperature between 330 and 400 °C. Addition of solid
acid to the PAN mat made the initial decomposition tem-
perature decrease to about 230 °C. However, compared
with PAN, the solid acid/PAN mat showed much slower
2
4
with its high acidity. Compared with solid acid, the loss of
activity of the hybrid fiber mat could be attributed to steric
hindrance of the polymer chain around the active species of
solid acid and the loss of acidity.
The effect of the reactant molar ratio was then consid-
ered. As shown in Table 2, conversion increased as the
amount of ethylene glycol increased. It is believed that
addition of ethylene glycol could advance the reaction as
the intermolecular collisions between ethylene glycol and
cyclohexanone increased. The molar ratio of 1:1.5 was the
best and was chosen for subsequent study.
To examine the suitable scope of the solid acid/PAN
fiber mat catalytic system, the catalytic activities for a
variety of various carbonyl compounds and diols were
Fig. 1 SEM images of PAN
mat (a), solid acid/PAN mat (b),
and solid acid/PAN mat
recycled for six times (c)
1
23

Polyacrylonitrile fiber mat supported solid acid catalyst
1201
tested. As shown in Table 3, all anticipated carbonyl
compounds were successfully transformed to the corre-
sponding products with conversions above 96% and yields
over 95%, clearly demonstrating that the solid acid/PAN
fiber mat was very efficient for the acetalization reactions.
Aliphatic aldehydes acted more efficient than aliphatic
ketones (entries 1–4). However, cyclic ketones could also
be acetalized to afford the corresponding 1,3-acetals with
high yields (entries 5–8). The conversions and yields were
excellent for aromatic aldehydes (entries 9, 10). These
results clearly demonstrate that the reactivity of a carbonyl
compound was determined by the steric hindrance of the
substrate. Due to the higher stability of six-membered ring,
neopentyl glycol worked better with carbonyl compound
than ethylene glycol.
1
00
PAN
PAN/Solid acid
9
9
8
8
7
7
6
6
5
5
0
5
0
5
0
5
0
5
0
100
200
300
400
500
600
700
Temperature/°C
Fig. 2 TGA curves of PAN and solid acid/PAN mat
The performance of the recycled catalyst for reaction in
up to six successive runs is shown in Fig. 4. The prepared
solid acid/PAN fiber mat catalyst is much easier to separate
from the reaction mixture by filtration than powder catalyst
because of the large size of the fiber mat. The catalytic
activities of recovered solid acid/PAN fiber mat catalyst
decreased slightly for acetalization of cyclohexanone and
ethylene glycol. Furthermore, scanning electron micros-
copy (SEM) images of the recycled catalyst showed that
the fiber diameter of solid acid/PAN mat (987.0 ?
Table 1 Acetalization reactions of cyclohexanone and ethylene
glycol catalyzed by different acid catalysts
Catalyst
H
2
SO
4
Solid acid
Solid acid/PAN mat
20
Amount/mg
20
7
?
-2
a
H /10 mmol
Conversion/%
Yield/%
40.82
98.53
98.35
1.68
99.93
99.93
1.68
b
98.48
98.45
77.10
75.91
b
Catalytic conditions: 20 mmol cyclohexanone, 30 mmol ethylene
glycol; reflux; 2.0 h; with Dean–Stark apparatus
3
08.2 nm) was greatly increased due to the swelling but the
fiber morphology was not damaged (Fig. 1c). These results
demonstrate that this solid acid/PAN fiber mat was stable
in the reaction, and could be recovered and reused
conveniently.
a
Content of hydrogen ion in the catalyst
b
Without Dean–Stark apparatus
100
Experimental
9
9
8
8
6
2
8
4
The quantitative analysis was performed on a Shimadzu
(GC-14B) gas chromatograph. The injector port tempera-
H SO4
Solid acid
Solid acid/PAN mat
2
ture was set at 250 °C. The oven temperature program was
initially set at 100 °C and maintained for 2 min, then
ramped to 250 °C at 20 °C/min. The morphologies of
electrospun fiber mats were recorded with a scanning
electron microscope (JSM-6360LV; JEOL, Japan). Sam-
ples for SEM were coated with a 2–3-nm layer of Au to
make them conductive. The diameter of fibers was deter-
mined from SEM micrographs. Elemental analysis was
undertaken on a EuroEA 3000 (Leeman, USA). The ther-
mal stabilities of PAN and solid acid/PAN mat were
analyzed with a Pyris Diamond TGA thermogravimetric
analyzer from 20 to 650 °C at heating rate of 10 °C/min
under nitrogen atmosphere. All organic reagents were
commercial products of highest purity available (98%) and
used for the reaction without further purification. Solid acid
was synthesized and purified as in our previous report [17].
PAN was synthesized according to the literature [18].
0.5
1.0
1.5
2.0
2.5
Time/h
Fig. 3 Time dependence of solid acid/PAN mat-catalyzed acetal-
ization reaction of cyclohexanone and ethylene glycol
Table 2 Effect of substrate molar ratio on the reaction
Cyclohexanone:ethylene glycol
Conversion/%
1.0:1.0
96.00
1.0:1.2
96.21
1.0:1.5
98.48
Yield/%
95.16
95.71
98.45
123

1
202
L. Shao et al.
Table 3 Acetalization of various carbonyl compounds and diols
Conversion
Yield
1
00
Entry Substrate
Product
Conversion/% Yield/%
1
99.80
99.84
96.56
97.85
99.84
96.56
8
6
0
0
O
Cl
CHO
CHO
Cl
O
2
O
40
Cl
Cl
O
2
0
0
3
O
O
O
1
2
3
4
5
6
Run
Fig. 4 Reuse of the solid acid/PAN mat
4
97.09
98.09
99.08
98.48
99.23
99.58
95.90
98.09
99.08
98.45
99.23
98.06
99.49
O
O
O
(DMF) at PAN concentration of 8 wt%. Electrospinning
procedure was carried out using a self-made electrospinning
3
equipment [15]. A sample of 20 cm polymer solution was
3
transferred into a plastic syringe (20 cm ) and delivered to
5
6
7
8
9
O
O
O
O
the tip of the syringe needle (0.8 mm inner diameter) by a
micro-infusion pump (WZ-502C; Zhejiang University
Medical Instrument Co., LTD, China) at constant feeding
O
O
3
rate of 1.0 cm /min. The syringe needle was positioned at a
distance of 12 cm from the collector and attached to a high-
voltage supply (GDW-a; Tianjin Dongwen High-voltage
Power Supply Plant, China) at 15 kV. A sheet of aluminum
foil connected to ground was adopted as the collector. The
resultant mats were dried under reduced pressure at room
temperature to remove residual solvent, and then used as
catalyst directly.
O
O
O
O
O
General procedure for hybrid fiber mat-catalyzed
acetalization reaction
O
Typical procedure (Scheme 1): aldehyde or ketone (0.020
3
mol), 5 cm cyclohexane, 0.024 mol diol, and 0.020 g
O
O
CHO
catalyst were mixed together in a three-necked round-
bottomed flask equipped with a magnetic stirrer and a
thermometer, and a Dean–Stark apparatus with manifold
and condenser was used to remove the water continuously
from the reaction mixture.
10
100
O
CHO
O
O
Preparation of solid acid/PAN hybrid fiber mat
O
O
Solid acid
Reflux
+
R1
R2
OH OH
R1
R2
Solid acid and PAN were mixed in certain weight ratio (1:2).
The mixture was then dissolved in N,N-dimethylformamide
Scheme 1
1
23

Polyacrylonitrile fiber mat supported solid acid catalyst
1203
Recovery and recycling of catalyst
6. Devi BLAP, Gangadhar KN, Kumar KLNS, Shanker KS, Prasad
RBN, Prasad PSS (2011) J Mol Catal A: Chem 345:96
7
. Mirkhani V, Moghadam M, Tangestaninejad S, Mohammadpoor-
Baltork I, Mahdavi M (2009) Synth Commun 39:4328
After reaction completion, the mat was removed from the
reaction solution by filtration, and then was directly used in
the next acetalization reaction. The procedure of the fol-
lowing catalyzing cycles was the same as the first cycle.
8. Xiao HQ, Guo YX, Liang XZ, Qi CZ (2010) Monatsh Chem
141:929
9
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1
1
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1
Acknowledgments This work was supported by the Shaoxing Key
Discipline Foundation, China.
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