Super-Hydrophobic, Stable, and Swelling Nanoporous Solid Strong Acid

Article abstract of DOI:10.1134/S0023158417060131

Super-hydrophobic solid strong acid with superior thermal stability and unique swelling properties has been synthesized by sulfonation of nanoporous polydivinylbenzene (PDVB) with super-acid of trifluoromethanesulfonic acid (TFMSA). The resultant PDVB–HOSO₂CF₃ has a well developed system of nanopores, superhydrophobic surface character and strong acid sites, and could be used as a highly efficient solid acid for catalyzing production of biodiesel and fine chemicals via transesterification, esterification and acylation. In terms of activity PDVB–HOSO₂CF₃ is superior to various solid acids such as SBA-15-SO₃CF₃, Nafion, Amberlyst 15, SBA-15-Ar-SO₃H and H form USY. The preparation of PDVB–HOSO₂CF₃ offers the way to develop new kind of porous solid acid with strong acid strength and regulated wettability.

Full text of DOI:10.1134/S0023158417060131

ISSN 0023-1584, Kinetics and Catalysis, 2017, Vol. 58, No. 6, pp. 816–824. © Pleiades Publishing, Ltd., 2017.
Super-Hydrophobic, Stable, and Swelling Nanoporous
Solid Strong Acid1
Qin Wua, Jinjun Penga, Weiping Konga, *, and Yongcun Zoub, **
aShaoxing University, Shaoxing, Zhejiang, P.R. China
bState Key Laboratory of Inorganic Synthesis and Preparative Chemistry and College of Chemistry, Jilin University,
Changchun, P.R. China
*e-mail: kongweiping0111@163.com
**e-mail: zouyc@jlu.edu.cn
Received October 19, 2016
Abstract—Super-hydrophobic solid strong acid with superior thermal stability and unique swelling properties
has been synthesized by sulfonation of nanoporous polydivinylbenzene (PDVB) with super-acid of trifluoro-
methanesulfonic acid (TFMSA). The resultant PDVB–HOSO2CF3 has a well developed system of
nanopores, superhydrophobic surface character and strong acid sites, and could be used as a highly efficient
solid acid for catalyzing production of biodiesel and fine chemicals via transesterification, esterification and
acylation. In terms of activity PDVB–HOSO2CF3 is superior to various solid acids such as SBA-15-SO3CF3,
Nafion, Amberlyst 15, SBA-15-Ar-SO3H and H form USY. The preparation of PDVB–HOSO2CF3 offers
the way to develop new kind of porous solid acid with strong acid strength and regulated wettability.
Keywords: nanoporous polymers, solid strong acid, superhydrophobicity, sulfonation, biodiesel
DOI: 10.1134/S0023158417060131
1. INTRODUCTION
widespread use of the polymeric solid acids such as
acidic resin Amberlyst 15 in a variety of reactions [29,
30]. Among these are a low BET surface area, poor
porosity [34, 35], an inadequate stability and poor
hydrophobicity.
In the area of green and sustainable chemistry, solid
acids have received considerable attention because of
their unique characteristics such as easy separation of
the catalyst from the reaction medium, corrosion resis-
tance, improved regenerating ability and enhanced cat-
alytic activities [1–28]. Accordingly, solid acids have
been widely used for catalyzing biomass transforma-
tions and fine chemicals synthesis. Conventional solid
acids usually include zeolites, heteropolyacids, sulfated
metal oxides, sulfonic groups functional mesoporous
materials, and strong acidic ion exchange resins [1–28],
which show excellent catalytic activities in various acid-
catalyzed reactions [29, 30].
Compared with the solid acids with inorganic
frameworks, organic polymeric networks show certain
hydrophobicity and easier grafting of functional
groups [24–31]. Therefore, the development of water-
tolerant solid strong acid catalysts is very important
because the acid sites could be severely poisoned by
water [32]. For example, in the etherification of vanil-
lyl alcohol with 1-hexanol, water, produced as a by-
product of the reaction, can be co-adsorbed near the
sulfonic acid centers and partially deactivate these sites
in competing with the alcohol reactant species [33].
However, several obstacles still stand on the way of the
In recent years, solid strong acids have been widely
used in various acid-catalyzed reactions such as Frie-
del–Crafts reactions, condensation, polymerization,
transesterification, and Koch carbonylation [1, 12, 14,
34–39]. To date, much investigation has been done on
inorganic solid super acids such as sulfated metal
oxides [1], super acid functionalized ordered meso-
porous silica [35]. However, there are few reports on
nanoporous polymeric solid strong acid. Although
commercial nafion NR50 shows strong acid strength,
its poor porosity (0.02 m2/g), lower concentration of
acid sites (0.8 mmol/g), and limited hydrophobicity
still constrain its wide applications. Therefore, the
synthesis of polymeric solid acids with a large BET
surface area, strong acid strength, well developed
porosity and high degree of hydrophobicity might be
favorable for the enhancement of their catalytic per-
formances.
Liu et al. reported the preparation of the solid strong
acid by treating porous polydivinylbenzene (PDVB)
functionalized with sulfonic groups with trifluorometh-
anesulfonic acid (TFMSA) and stated that the intro-
duction of electron-withdrawing group of –SO2CF3
1
The article is published in the original.
816

SUPER-HYDROPHOBIC, STABLE, AND SWELLING NANOPOROUS SOLID
817
largely enhances acid site strength. However, their 20 mL of THF, followed by addition of 2 mL of H2O.
complicated synthetic procedures and limited concen-
tration of –SO2CF3 suppress their wide applications
After stirring at room temperature for 3 h, the mixture
was hydrothermally treated at 100°C for 2 days. After
[12]. We report here a novel super-hydrophobic solid evaporation of the solvents at room temperature, the
strong acid based on the functionalized mesoporous mesoporous PDVB sample was obtained.
polydivinylbenzene (PDVB–HOSO2CF3) with stable
network, high acid concentration, large BET surface
area, abundant nanopores, and unique swelling prop-
erties. The synthesis involves direct sulfonation of
PDVB with trifluoromethanesulfonic acid (TFMSA),
which endows the acid with enhanced acid strength
(H0 up to –11.35) [34]. In terms of the catalytic activity
PDVB–HOSO2CF3 is superior in a variety of acid-
catalyzed reactions (transesterification, esterification
and acylation) to various solid acid catalysts such as
acidic resin of Amberlyst 15, Nafion NR50, sulfonic
groups functional porous polydivinylbenzenes
(PDVB–SO3H), trifluoromethanesulfonic acid func-
tional mesoporous silicas (SBA-15-SO3CF3), arene-
sulfonic groups functional mesoporous silicas (SBA-
15-Ar-SO3H) and H-USY zeolite. It is well known
that transesterification reaction is an efficient tool for
the production of biodiesel. Combined the unique
properties such as stable and super-hydrophobic net-
work, high BET surface area, high concentration of
acid sites, PDVB–HOSO2CF3 could be used as highly
efficient solid acids for catalyzing production of biofu-
els and useful chemicals in industry.
Sulfonation. Sulfonation of swelling mesoporous
PDVB was performed by TFMSA in CCl4 or toluene.
Typically, 3.0 g of PDVB was outgassed at 100°C in a
three-necked round flask for 3 h. Then, 50 mL of CCl4
or toluene containing TFMSA (5 mL) was slowly
added into the flask at 80°C, stirring for 12 h in nitro-
gen atmosphere. Finally, the products were filtered,
washed with a large amount acetone to remove the
TFMSA residue and dried at 80°C for 1 h. The sam-
ples were denoted as PDVB–HOSO2CF3.
For comparison, PDVB–SO3H, SBA-15-Ar-
SO3H and SBA-15-SO3CF3 with molar ratios of S/Si =
0.1 were synthesized according to the literature [33,
35, 36]. The acid strength of PDVB–HOSO2CF3 was
tested by Hammett indicator method [1]. Typically,
0.3 g of indicator (benzalacetophenone, an anthraqui-
none, and p-nitrophenylmethane) was dissolved in
30 g of toluene, forming indicator solution. Then, the
indicator solution was slowly dropped onto the surface
of the samples until the change in the sample color
could be observed.
2.3. Characterizations
Nitrogen adsorption–desorption isotherms were
measured using a Micromeritics ASAP 2020M system.
Before measurements the samples were outgassed at
2. EXPERIMENTAL SECTION
2.1. Chemicals
All reagents were of analytical grade and used as 150°C for 10 h. The pore size distribution for meso-
purchased without further purification. Amberlyst 15, pores was calculated using the Barrett–Joyner–Hal-
2-(4-chlorosulfonylphenyl)-ethyltriethyltrimethoxy
enda (BJH) model. CHNS elemental analysis was
silane (CSPTMS), divinylbenzene (DVB), and non- performed with a Perkin-Elmer series II CHNS ana-
ionic block copolymer surfactant poly(ethyleneox- lyzer 2400. FTIR spectra were recorded by using a
ide)-poly(propyleneoxide)-poly(ethyleneoxide) block Bruker 66V FTIR spectrometer. Thermogravimetric
copolymer (Pluronic 123, molecular weight of about analysis (TG) were performed on a Perkin-Elmer
5800), TFMSA were purchased from Sigma-Aldrich TGA7 and a DTA-1700 in flowing air with a heating
Company, Ltd. (USA). DVB, azobisisobutyronitrile rate of 20°C/min. Contact angle (CA) tests were per-
(AIBN), tetrahydrofuran (THF), carbon tetrachloride formed on a Contact angles DSA10MK2G140, Kruss
CCl4, tetraethyl orthosilicate (TEOS), chlorosulfonic Company, Germany. SEM images were performed on
a JSM-6700F electron microscope. TEM images were
performed on a JEM-3010 electron microscope
(JEOL, Japan) with an acceleration voltage of 300 kV.
acid, dichlormethane CH2Cl2, cyclohexanol, acetic
acid, benzene, toluene, anisole, benzoyl chloride,
resorcinol, ethyl acetoacetate, dodecane were
obtained from Tianjin Guangfu Chemical Reagent.
H-forms of Beta zeolite and ultrastable Y zeolite
(USY) were supplied by Sinopec Catalyst Co.
2.4. Catalytic Reactions
Transesterification of tripalmitin/soybean oil with
methanol, esterification of acetic acid with cyclohexa-
nol, acylation of benzene/toluene/anisole with ben-
zoyl chloride and Pechmann condensation were used
as model reactions.
2.2. Synthesis of Samples
Preparation of mesoporous PDVB. Swelling meso-
porous PDVB were hydrothermally synthesized by
polymerizing DVB making use of the initial reaction
mixture of DVB/AIBN/THF/H2O with the composi-
The transesterification of tripalmitin with metha-
nol (TTM) was performed by melting 1.67 g of tripal-
tion 1/0.02/16.1/7.23 [36]. Typically, 2 g of DVB was mitin at 65°C, and then 10 mL of methanol and 0.2 g
added into a solution containing 0.05 g of AIBN and of catalysts were quickly added, the reaction time was
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818
WU et al.
12 h. In this reaction, the molar ratio of methanol/tri- (FID). The temperature program was as follows: the
palmitin was 90 : 1 and the weight ratio of catalyst was initial temperature 100°C, the ramping rate
0.0208. The product was methyl palmitate with selec- 20°C/min, the final temperature 280°C; the tempera-
tivity close to 100%. Transesterification of soybean oil ture of the FID detector 300°C. Product concentra-
with methanol was performed by mixing 2.35 mL of oil tions were calculated using dodecane as an internal
and 5 mL of methanol at 65°C, and then 0.2 g of cata- standard.
lysts was quickly added, the reaction was continued for
10 h. In this reaction, the molar ratio of methanol/oil
3. RESULTS AND DISCUSSION
was 90 : 1 and the weight ratio of catalyst was 0.0317.
The main products were long chain esters including
Figure 1a shows the photograph of sulfonated
PDVB–HOSO2CF3, which exhibits gray color caused
by the introduction of the acidic groups in the sample.
Notably, PDVB–HOSO2CF3 shows very good swell-
ing property in variously organic solvents. For exam-
ple, when PDVB–HOSO2CF3 was brought in contact
with organic solvent such as acetone, the volume of
PDVB–HOSO2CF3 was significantly expanded (Fig.
1b). After the adsorbed solvent evaporated, the volume
of PDVB–HOSO2CF3 shrank to the initial state con-
firming that PDVB–HOSO2CF3 exhibits shape mem-
ory ability (Fig. 1c). The images of a water droplet
lying on the surface of various samples are shown in
Fig. 1d–f, the contact angles (CA) of PDVB–
HOSO2CF3, Amberlyst 15, and SBA-15-SO3CF3 are
150, 48 and 34.8°, respectively. Compared with
Amberlyst 15 and SBA-15-SO3CF3, PDVB–
HOSO2CF3 showed much higher hydrophobicity, this
is presumably due to its unique combination of nano-
structure, hydrophobic network and acid sites. On the
contrary, the hydrophilicity of SBA-15-SO3CF3 is
mainly due to its hydrophilic framework. Although
Amberlyst 15 has pure organic network, the hydro-
philicity of -SO3H group and poor porosity result in its
lower hydrophobicity. Super hydrophobicity is a very
important property for the solid acids since water usu-
ally acts as by-products in various reactions [29, 30]
and its presence can inhibit reaction by deactivating
acid sites.
Notably, elemental analysis (Table 1) shows that
PDVB–HOSO2CF3 contains acid sites in a high con-
centration (3.1 mmol/g). In contrast, SBA-15-SO3CF3
shows relatively low concentration of acid sites
(1.41 mmol/g). These results indicate that the swelling
property plays an important role in generating an
enhanced acidity of PDVB–HOSO2CF3. Possibly, the
excellent swelling property of PDVB enables the com-
pletely exposed framework to react with TFMSA in the
solvent; the similar result has been reported earlier [36].
Figure 2a shows nitrogen adsorption–desorption
isotherms of PDVB and PDVB–HOSO2CF3. Before
and after sulfonation by TFMSA in CCl4, the samples
show typical type-IV isotherms, giving a steep increas-
ing at a relative pressure 0.7 < P/P0 < 0.95, which is
characteristic of the presence of mesoporosity [35,
C
14:0, C18:0, C18:1, C18:2, and C18:3.
The esterification of acetic acid with cyclohexanol
(EAC) was performed by mixing and stirring 0.2 g of
catalyst and 11.5 mL (0.11 mol) of cyclohexanol in a
three-necked round flask equipped with a condenser
and a magnetic stirrer. When the mixture was heated to
100°C by oil bath, 17.5mL (0.305 mol) of acetic acid
was rapidly added into the mixture, the reaction time
was 5 h. In this reaction, the molar ratio of ace-
tic/cyclohexanol acid was 2.6 and the mass ratio of
catalyst/cyclohexanol was 0.018. The product was
cyclohexyl acetate with selectivity close to 100%.
Pechmann condensation. The interaction of resor-
cinol with ethyl acetoacetate (PRA) to produce 7-
hydroxy-4-methylcoumarin was used as a model reac-
tion. Typically, 0.1 g of catalyst and 1.1 g (10 mmol) of
resorcinol were placed in a three-necked round flask
equipped with a condenser and a magnetic stirrer, after
adding 1.3 g of (10 mmol) ethyl acetoacetate, the mix-
ture was heated to 100°C, and the reaction time was
12 h. In this reaction, the molar ratio of resorci-
nol/ethyl acetoacetate was 1.0, and the mass ratio of
catalyst/resorcinol was 0.091; the selectivity for 7-
hydroxy-4-methylcoumarin was close to 97.1%.
The acylation of benzene/toluene with benzoyl
chloride (ABB/ATB) was carried out in a three-
necked round flask equipped with a condenser and a
magnetic stirrer. To perform the reaction of benzene
with benzoyl chloride 0.2 g of fresh catalyst, 4.587 mL
of benzene (51.6 mmol) and 1.0 mL of benzoyl chlo-
ride (8.6 mmol) were added into the reactor while stir-
ring at 80°C. The reaction time was 12 h and samples
of reaction mixture reacted for different reaction times
were taken by a syringe. The analysis indicated that
benzophenone was a dominant product with selectiv-
ity close to 100%. To carry out the reaction of toluene
with benzoyl chloride 0.2 g of fresh catalyst, 5.484 mL
of toluene (51.6 mmol) and 1.0 mL of benzoyl chloride
(8.6 mmol) were added into the reactor under vigorous
stirring, the reaction temperature was maintained at
110°C. The reaction time was 12 h and samples of
reaction mixture reacted for different reaction times
were taken by a syringe. Analyze indicated that the
main product was 4-methyl benzophenone with selec-
tivity close to 94.1–98.4%.
In these reactions, the products were analyzed by 36]. Correspondingly, the two samples show similar
gas chromatography using an Agilent 7890A instru- pore-size distribution with maxima at about 22.0–
ment equipped with a flame ionization detector 26.0 nm (Fig. 2b). Notably, PDVB has surface area at
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SUPER-HYDROPHOBIC, STABLE, AND SWELLING NANOPOROUS SOLID
(b) (c)
819
(a)
m
cm
5 mm
m
cm
5 mm
m
cm
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
mm
mm
mm
5 mm
(d)
(e)
(f)
Fig. 1. Microphotographs of PDVB–HOSO CF (a), swollen PDVB–HOSO CF in acetone (b), dried PDVB–HOSO CF
3
2
3
2
3
2
after evaporation of acetone (c); and a drop of water on PDVB–HOSO CF (d), Amberlyst 15 (e), and SBA-15-SO CF (f).
2
3
3
3
702 m2/g, while the surface area of PDVB–HOSO2CF3
fairly different for these solid acids. It was found that
commercial acid resin of Amberlyst 15 has the highest
concentration of acid site (4.7 mmol/g), and its sur-
face area is as low as 45 m2/g. Conventional SBA-15-
is 662 m2/g. A small reduction in the surface area
found for PDVB–HOSO2CF3 is attributed to a suc-
cessful introduction of trifluoromethanesulfonic acid
group in the mesoporous PDVB, which cannot bring Ar-SO3H has a high surface area (626 m2/g) with a rel-
about a significant enlargement of the surface area.
Similar phenomenon has also been reported earlier
[29, 36].
atively lower concentration of acid sites (1.70 mmol/g).
Compared with conventional solid acids, PDVB–
HOSO2CF3 has a high BET surface area (662 m2/g)
and relative high concentration of acid sites (3.1 mmol/g)
These characteristics are expected to be very favorable
The textural parameters of various solid acids are
also summarized in Table 1. It is evident that surface
areas, pore volumes, and densities of acid sites are for the enhancement of its catalytic activity in various
Table 1. The textural and acidic parameters of various solid acid catalysts
SBET, m2/g
Vp, cm3/g
Dp, nmb
Acid site density, mmol/ga
Samples
PDVB
–
3.1
702
662
380
45
579
626
723
447
1.35
1.20
0.90
0.31
0.48
0.72
0.32
0.33
22.0
21.6
23.5
40.0
5.8
PDVB–HOSO2CF3
PDVB–SO3H
Amberlyst 15
SBA-15-SO3CF3
SBA-15-Ar-SO3H
USY
3.72
4.30
1.41
1.38
–
6.3
–
FDU-15-SO3Hc
1.80
2.5
a
Measured by elemental analysis.
Pore size distribution estimated from BJH model.
Parameters from [38].
b
c
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820
WU et al.
(a)
(b)
2.4
2.0
1.6
1.2
0.8
0.4
1500
1200
900
600
1
2
1
2
300
0
0.2
0.4
0.6
0.8
1.0
10
Pore diameter, nm
100
Relative pressure p/p0
Fig. 2. Nitrogen isotherms (a) and pore size distribution (b) of PDVB (1), PDVB–HOSO CF (2). The isotherm and pore size
2
3
3
curve for (1) are offset by 500 and 1.0 cm /g along the vertical axis for clarity, respectively.
(a)
(b)
100 nm
General
SEI
5 kV
×60
100 nm WD 6 mm
Fig. 3. SEM (a) and TEM (b) images of PDVB–HOSO CF .
2
3
acid-catalyzed reactions because an appropriate com- exhibits rough surface characteristics with abundant
cavities, which form hierarchical nanopores (meso-
macropores) with the pore size range from 10 to
50 nm. Meanwhile, TEM image further confirms a
well-developed system of meso- and macropores (10–
50 nm) in PDVB–HOSO2CF3, in agreement with N2
adsorption–desorption isotherms.
bination of porosity and acidic properties is an import-
ant prerequisite for the successful performance of the
heterogeneous acid catalyst.
Figure 3 shows SEM and TEM images of PDVB–
HOSO2CF3. It can be seen that PDVB–HOSO2CF3
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SUPER-HYDROPHOBIC, STABLE, AND SWELLING NANOPOROUS SOLID
821
Figure 4 shows XPS spectra of PDVB and PDVB– TFMSA hydroxyl group is added to benzene ring and
HOSO2CF3. Porous PDVB shows the signals of C1s
and O1s lines indicating that in addition to carbon and
hydrogen atoms the remaining OH groups are present.
After sulfonation with TFMSA to give PDVB–
HOSO2CF3, the product exhibits the signals of S2p,
C1s, O1s, and F1s. During sulfonation process
alkyl chain of the polymer molecule to form a pair of
cation radical and trifluoromethanesulfonate anion.
The resultant active site is a strong acid due to the
presence of the strong electron-withdrawing group of
–SO2CF3, which effectively promotes removal of H+
(Scheme) a strongly acidic proton that represents the from acid sites [12, 29, 36].
CH CH2 CH2 CH
OH2
CH CH2 CH2 CH
n
n
HSO3CF3
OH2
OSO2CF3
OSO2CF3
H2C
C
HC CH
H2O
OSO2CF3
Scheme 1. The scheme for sulfonation porous PDVB with trifluoromethanesulfonic acid.
H2C
C
HC CH
OH
n
n
n
n
H
H
Furthermore, the acid strength of PDVB–
C1s
HOSO2CF3 was tested by Hammett indicator method
[1, 34, 40]. In our experiments, benzalacetophenone,
an anthraquinone, and p-nitrophenylmethane were
chosen as indicators. The pKa values of these indica-
tors are –5.6, –8.2, and –11.35, respectively. After the
indicator solutions were brought into contact with
PDVB–HOSO2CF3 for 30 min, gray color of PDVB–
HOSO2CF3 gradually turned into orange-yellow sug-
gesting that their acidic strength is less than –11.35.
These results imply that an enhanced acid strength of
PDVB–HOSO2CF3 is comparable with that of a super
acid [1, 34].
F1s
O1s
2
1
S2p
800 700 600 500 400 300 200 100
Binding energy, eV
TG results showed that PDVB–HOSO2CF3 is
more stable than PDVB–SO3H and Amberlyst 15
(Fig. 5). For the samples of PDVB–SO3H and
Amberlyst 15 the loss in weight is registered at 35–150,
250–360, and 450–580°C assignable to desorption of
water, decomposition of sulfonic groups, and destruc-
tion of polymer framework, respectively [36, 41–44].
On the contrary, for PDVB–HOSO2CF3 the first loss
in weight occurs at 355–420°C and the final loss
occurs at 510–610°C. These changes are assigned to
decomposition of functional groups, and destruction of
polymer framework. Contrary to PDVB–SO3H and
Amberlyst15, no significant weight loss was observed
below 200°C for PDVB–HOSO2CF3, which is due to
its hydrophobic network. Interestingly, the destruc-
tion temperature for functional groups of PDVB–
HOSO2CF3 (385°C) is considerably higher than those
of PDVB–SO3H (315°C) and Amberlyst 15 (265°C).
This observation can be explained by the presence of
electron-withdrawing group of –SO2CF3 [12]. More-
over, the network of PDVB–HOSO2CF3 collapses at
543°C whereas those of PDVB–SO3H and Amberlyst 15
undergo the degradation at lower temperatures (528
Fig. 4. XPS spectra of porous PDVB (1) and PDVB–
HOSO CF (2).
2
3
100
80
60
40
20
2
4
1
3
0
–20
100 200 300 400 500 600 700 800 900
Temperature, °C
Fig. 5. Thermogravimetric (TG) curves of Amberlyst 15 (1),
PDVB–SO H (2), PDVB–HOSO CF (3) and Nafion
3
2
3
NR50 resin (4).
KINETICS AND CATALYSIS Vol. 58 No. 6 2017

822
WU et al.
and 500°C respectively). The above results demon-
strate that the stability of both functional group and
network are improved following the introduction of
‒SO2CF3 group because –SO2CF3 shows stronger
electron-withdrawing properties than –SO3H entity.
12
10
8
Acetone
Ethanol
Hexane
Toluene
Water
Figure 6 shows adsorption capacities of solid acids
for different solvents measured under liquid or gas
phase conditions. Compared to Amberlyst 15 and
SBA-15-Ar-SO3H the sample of PDVB–HOSO2CF3
exhibits higher adsorption ability for various solvents
under liquid phase conditions. For example, the
adsorption capacity of PDVB–HOSO2CF3 for toluene
is determined to be 12.82 g/g, while the relevant
adsorption capacities of Amberlyst 15 and SBA-15-Ar-
SO3H are as low as 1.33 and 2.5 g/g. Interestingly, for
adsorption of water, PDVB–HOSO2CF3 exhibits
much lower adsorption capacity (0.04 g/g) compared
with that of Amberlyst 15 (0.508 g/g) and SBA-15-Ar-
SO3H (0.502 g/g). A lower adsorption affinity for
water is due to super-hydrophobic properties PDVB–
HOSO2CF3, which is in agreement with results of con-
tact angle measurements.
6
4
2
0
a
b
c
Fig. 6. Saturated adsorptive capacities for different solvents in
liquid phase over PDVB–HOSO CF (a), Amberlyst 15 (b),
and SBA-15-SO CF (c).
2
3
3
3
and USY gave 77.5, 58.1, 65.4, 44.6 and 36.8% conver-
sions, respectively. Considering that the structures of
PDVB–HOSO2CF3 and PDVB–SO3H are similar,
higher catalytic activities over PDVB–HOSO2CF3
could be assigned to its stronger acidity, which
decreases the activation energy of the reaction [34,
35]. Compared with Amberlyst 15, PDVB–
HOSO2CF3 shows higher BET surface area, improved
hydrophobicity and stronger acidity. These properties
Table 2 presents catalytic activities in the transes-
terification of tripalmitin with methanol, esterifica-
tion of acetic acid with cyclohexanol, acylation of ben-
zene/toluene/anisole with benzoyl chloride, and
Pechmann reaction of resorcinol with ethyl acetoace-
tate over various solid acid catalysts. Notably, PDVB– appear to ensure an increased catalytic activity.
HOSO2CF3 showed much higher activities than
PDVB–SO3H, Amberlyst 15, SBA-15-SO3CF3, SBA-
15-Ar-SO3H, and commercial zeolite USY. For exam-
ple, in the esterification of acetic acid with cyclohex-
anol, PDVB–HOSO2CF3 as much as 86.42% of
cyclohexanol was converted while PDVB–SO3H,
Although SBA-15-SO3CF3 and SBA-15-Ar-SO3H
possess high surface areas, large pore volumes, low
concentrations of acid sites and hydrophilic nature of
frameworks might be the reasons why their these solids
are inferior to PDVB–HOSO2CF3 in terms of catalytic
activities [35]. When the samples with the same acidity
Amberlyst 15, SBA-15-SO3CF3, SBA-15-Ar-SO3H were tested, the activity of PDVB–HOSO2CF3 (86.7%)
Table 2. Catalytic performances of various solid acids in esterification, acylation and transesterification reactions
PREa
EACb
ABBc
ATBd
TTMe
Catalysts
selectivity, % conversion, % conversion, % conversion, % conversion, % selectivity, %
yield, %
PDVB–HOSO2CF3
98.3
97.9
84.8
84.1
84.6
82.5
86.7
84.4
97.8
96.4
98.4
97.5
99.3
98.7
f
PDVB–HOSO2CF3
PDVB–SO3H
Amberlyst-15
SBA-15-SO3CF3
SBA-15-Ar-SO3H
USY
98.2
97.9
96.9
98.2
99.1
77.3
49.7
77.0
60.8
15.9
77.5
58.1
73.4
44.6
36.8
78.1
62.8
63.2
54.7
47.8
83.2
84.8
86.9
79.8
65.7
96.5
97.1
98.2
98.1
94.4
83.4
61.8
48.6
46.3
38.5
a
Pechmann reaction of resorcinol with ethyl acetoacetate.
Esterification of acetic acid with cyclohexanol.
Acylation of benzene with benzoyl chloride.
Acylation of toluene with benzoyl chloride.
Transesterification of tripalmitin with methanol.
The sample has been recycled for 5 times.
b
c
d
e
f
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SUPER-HYDROPHOBIC, STABLE, AND SWELLING NANOPOROUS SOLID
823
results can also be found in the acylation reaction. In
the interaction of benzene with benzoyl chloride,
PDVB–HOSO2CF3 86.4% of benzoyl chloride was
converted while in the presence of Amberlyst 15, SBA-
15-SO3CF3, SBA-15-Ar-SO3H and USY conversions
were 62.8, 74.5, 54.7, and 47.8% (Table 2). The results
outlined above show that PDVB–HOSO2CF3 func-
tions as a novel solid acid catalyst, the use of which
could be extended beyond the catalytic esterification
to many other acid catalyzed transformations such as
acylation and Pechmann reaction.
5
100
80
60
40
20
1
2
3
4
An additional aspect deserves attention. Contrary
to Amberlyst 15, the catalyst PDVB–HOSO2CF3 did
not show apparent external diffusion limitation in the
acylation of toluene with benzoyl chloride over various
stirring rates. For example, with stirring rates of 60 and
900 rpm, PDVB–HOSO2CF3 gave 97.1 and 97.8%
conversions of benzoyl chloride respectively. No sig-
nificant indications of external diffusion limitation
could be observed and this can be attributed to
improved swelling properties and high adsorption
capacity for many organic compounds shown by
PDVB–HOSO2CF3. At the same time, with
Amberlyst 15 different conversions of benzoyl chloride
(84.8 and 72.8%) were recorded when the stirring rate
was reduced from 900 to 60 rpm.
Very good reusability of PDVB–HOSO2CF3 also
could be noted. For example, after 5 operating cycles
in the Friedel–Crafts acylation of toluene with ben-
zoyl chloride, PDVB–HOSO2CF3 still showed high
catalytic activity (96.9%) comparable with that of the
fresh PDVB–HOSO2CF3 (97.8%).
0
2
4
6
8
10
Reaction time, h
Fig. 7. Dependences of catalytic activities on the time in
the transesterification of soybean oil with methanol over
PDVB–HOSO CF . (Where 1—C , 2—C , 3—C ,
18:1
2
3
18:3
14:0
18:0
4—C , and 5—C
stand for biodiesel products of
18:2
methyl myristate, methyl stearate, methyl oleate, methyl
linoleate, and methyl γ-linolenate).
proved to be higher than that of SBA-15-SO3CF3
(75.8%). Given the similar BET surface area and the
close concentration of acid sites, the reason for an
improved catalytic performance of PDVB–HOSO2CF3
can be sought in its super-hydrophobic properties.
Other important effects have been observed.
PDVB–HOSO2CF3 showed better catalytic activities
in the transesterification of tripalmitin/soybean oil
with methanol than conventional acidic catalysts such
as Amberlyst 15, SBA-15-SO3CF3, and heteropolyac-
ids. The transesterification is a very promising way to
manufacture biodiesel. We were able to observe that in
the transesterification of tripalmitin with methanol,
PDVB–HOSO2CF3 showed much better yields
(99.3%) as compared with 61.8% for Amberlyst 15 and
48.6% for SBA-15-SO3CF3. Noteworthy that PDVB–
HOSO2CF3 could be reused five times without notice-
able loss of catalytic activity with 98.7% yield of bio-
diesel. Interestingly, PDVB–HOSO2CF3 also shows
excellent catalytic activity in the transesterification of
soybean oil (a kind of very cheap oil get from plant)
with methanol under normal pressure and mild condi-
tion (Fig. 7). For instance, nearly all the soybean was
transformed into corresponding biodiesel for 6 h incu-
bation. An easy transformation of low cost plant oil
into biodiesel will be potential important for the wide
applications of biodiesel in industrial scale.
CONCLUSIONS
Efficient and stable solid strong acid catalyst of
PDVB–HOSO2CF3 with super-hydrophobic network
was successfully synthesized from sulfonation of swell-
ing mesoporous PDVB with trifluoromethanesulfonic
acid (TFMSA). Compared with commercial pure
organic network solid acids, this solid catalyst exhibits
novel super-hydrophobic property, high stability, better
swelling property, large surface area (up to 662 m2/g),
well developed system of mesopores, and high content
of functional groups (3.1 mmol/g). The novel catalyst
exhibited much higher activities in esterifications,
Pechmann reaction, and acylation than conventional
solid catalysts such as Amberlyst 15, SBA-15-SO3CF3,
SBA-15-Ar-SO3H and USY. Cycling experiments indi-
cated very good reusability of PDVB–HOSO2CF3.
Except for the transesterification or esterification
reaction, PDVB–HOSO2CF3 also showed better cata-
lytic activities in other acid catalytic reactions such as
Pechmann and acylation reactions. In Pechmann
reaction, PDVB–HOSO2CF3 showed 84.6% conver-
sion of ethyl acetoacetate, while Amberlyst 15, SBA-
15-SO3CF3, SBA-15-Ar-SO3H and USY gave 49.7,
77.0, 60.8, and 47.8% conversions (Table 2). Similar
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