Self-assembly synthesis of a high-content sulfonic acid group functionalized ordered mesoporous polymer-based solid as a stable and highly active acid catalyst

Article abstract of DOI:10.1039/c2jm32894a

A stable and highly active ordered mesoporous polymer-based acid catalyst has been prepared via a simple surfactant templating approach and oxidation treatment. The composition and nanostructure are characterized by XRD, NMR, XPS, TEM, nitrogen sorption, elemental and chemical analysis. The sulfonic acid groups have been anchored within the well-arranged channels of the polymer-based matrix. Even with a high -SO₃H group loading (up to about 27.4 wt%) on the mesoporous polymer-based material, the ordered mesostructure and high surface area (～400 m² g^-1) can be retained and the functional moieties are highly chemically accessible. With the large number of acid sites (0.93-2.38 H⁺ mmol g^-1 determined by acid-base titration) and the hydrophobic character, the mesoporous polymer-based solid exhibits unique catalytic performance in acid-catalyzed reactions such as condensation and acetalization, not only high activity (per site yield of bisphenol-A is over 45 in the condensation of phenol and acetone) but also excellent stability. Loss in acidic loading and activity is negligible even after the catalyst is reused 20 times in the acetalization of butanediol and aldehyde. The stability is most likely attributed to the hydrophobic nature of the mesoporous polymer-based solids, which favors the diffusion of water and thereby inhibits the poisoning of acidic sites caused by water generating in the reaction. Moreover, with large mesopores, the diffusion of reactants and products can be promoted and hence the catalytic activity can be further increased.

Full text of DOI:10.1039/c2jm32894a

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PAPER
Self-assembly synthesis of a high-content sulfonic acid group functionalized
ordered mesoporous polymer-based solid as a stable and highly active acid
catalyst†
*
Wei Wang, Xin Zhuang, Qingfei Zhao and Ying Wan
Received 8th May 2012, Accepted 3rd June 2012
DOI: 10.1039/c2jm32894a
A stable and highly active ordered mesoporous polymer-based acid catalyst has been prepared via a
simple surfactant templating approach and oxidation treatment. The composition and nanostructure
are characterized by XRD, NMR, XPS, TEM, nitrogen sorption, elemental and chemical analysis. The
sulfonic acid groups have been anchored within the well-arranged channels of the polymer-based
matrix. Even with a high –SO3H group loading (up to about 27.4 wt%) on the mesoporous polymer-
based material, the ordered mesostructure and high surface area (ꢀ400 m2 gꢁ1) can be retained and the
functional moieties are highly chemically accessible. With the large number of acid sites (0.93–2.38 H+
mmol gꢁ1 determined by acid–base titration) and the hydrophobic character, the mesoporous polymer-
based solid exhibits unique catalytic performance in acid-catalyzed reactions such as condensation and
acetalization, not only high activity (per site yield of bisphenol-A is over 45 in the condensation of
phenol and acetone) but also excellent stability. Loss in acidic loading and activity is negligible even
after the catalyst is reused 20 times in the acetalization of butanediol and aldehyde. The stability is most
likely attributed to the hydrophobic nature of the mesoporous polymer-based solids, which favors the
diffusion of water and thereby inhibits the poisoning of acidic sites caused by water generating in the
reaction. Moreover, with large mesopores, the diffusion of reactants and products can be promoted and
hence the catalytic activity can be further increased.
for carbonyl compounds. Bisphenol A is a significant raw
1. Introduction
material for the synthesis of resins and polymers. Both acetali-
zation and condensation of acetone and phenol are typically
acid-catalyzed processes using homogenous catalysts (para-
toluenesulfonic acid, triflic acid, or pyridinium salts, etc.).1,2 The
increasing concerns of environment protection and safety issues,
together with the principle of green chemistry, have stimulated
the development of stable, reusable, and highly active solid acid
alternatives to replace the hazardous and corrosive mineral acid
catalysts.3,4 Insoluble acidic solids, which can be readily sepa-
rated from the products and repeatedly used, thereby minimizing
the energy consumption for the production of chemicals, are one
of the promising candidates, such as sulfated zirconia, hetero-
polyacids, zeolites, and acidic polymers.5,6
Acetalization is an important reaction, especially in a variety of
organic syntheses in which acetals are used as protecting groups
Key Laboratory of Resource Chemistry of Ministry of Education, Shanghai
Key Laboratory of Rare Earth Functional Materials, Department of
Chemistry, Shanghai Normal University, Shanghai 200234, P. R. China.
E-mail: ywan@shnu.edu.cn; Fax: +86 21 6432 2511; Tel: +86 21 6432
2511
† Electronic supplementary information (ESI) available: synthesis of
carbon precursors, preparation conditions for sulfonic acid
functionalized mesoporous polymer-based materials; conditions for
acetalization reactions; t-plot analysis of sulfonic acid functionalized
mesoporous polymer-based materials; XPS spectra of the
functionalized mesoporous polymer-based materials before oxidation;
FT-IR spectrum, and small-angle XRD pattern for the mesoporous
carbon-based product MP-SO3H-22.7(600) obtained by heating
MP-SO3H-22.7 at 600 ꢂC under protection of nitrogen; small-angle
XRD pattern and N2 sorption isotherms for the mesoporous
polymer-based acidic solid MP-SO3H-11.4* synthesized via the
evaporation induced self-assembly of triblock copolymer F127,
low-polymerized phenolic resins, TEOS and MPTMS (initial 8.11
mmol TEOS and 1.89 mmol MPTMS): 1.0 g phenolic resins), and the
sample heated at 600 ꢂC under protection of nitrogen
MP-SO3H-11.4*(600); small-angle XRD pattern, N2 sorption isotherms
and pore-size distribution for the mesoporous carbon product after
dissolution of silica component from MP-SO3H-11.4*(600). See DOI:
10.1039/c2jm32894a
Insoluble acidic solids can be described in terms of their
Brønsted/Lewis acidity, the strength and number of acidic sites,
and the textural property and surface chemistry of the support.3,7
High product yield can depend on the fine-tuning of these
properties. For example, esterification and acetalization reac-
tions often require Brønsted acidity.8 The synthesis of pure
Brønsted solid acids is a particularly important challenge.3
Sulfonic modified solid materials as analogues for sulfonic acids
have been recently reported and show great promise as solid
Brønsted acids.3,9–11 An organic compound with sulfonic acid
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groups (SO3H) was reported to act as an efficient solid acid
catalyst by carbonization of D-glucose at 573–723 K and then
sulfonation.12 The material consists of a three-dimensional (3-D)
network of partially carbonized organic compounds containing
SO3H and phenolic hydroxyl (OH) groups, and exhibits high
catalytic activity in acetalization.13 Lu and co-workers synthe-
sized sulfonated poly(styrene-co-divinylbenzene) which was
encapsulated with Fe3O4 nanoparticles, and found that the
catalyst showed high activity and good reusability in the
condensation reaction of ethylene glycol and benzaldehyde.14
However, these materials are nonporous and non-ordered, and
exhibit low surface areas. It has been reported that the fast
deactivation of acid catalysts is possibly related to the slow
diffusion of water generated in acetalization and water adsorp-
tion on acidic sites.6,15 As a result, the solid acid catalysts with
low surface areas and/or small pore sizes always undergo a fast
deactivation in acetalization. For example, the conversion of
1,4-butanediol in acetalization over acidic polymer resins with a
low surface area (<20 m2 gꢁ1) is limited to 2.8% with a feeding
mole ratio of acetone : diol ¼ 1 : 1.6
molecular-weight pre-cured epoxy resin to an ordered PEO-
containing block copolymer may result in an ordered meso-
structure. Unfortunately, the block copolymer templates in the
polymeric resins are not removed, leading to nonporous poly-
mers. Ikkala and co-workers found that the self-assembling
thermosetting resin can be derived from a system containing a
strongly hydrogen-bonded amphiphilic block copolymer and
thermosetting precursor Novalac resin.39 Dai and coworkers
synthesized ordered mesoporous polymers and carbons with
enhanced hydrogen-bonding interactions between the block
copolymer and the organic precursor (resorcinol–formaldehyde
resins).38 An organic–organic self-assembly approach was
proposed by Zhao and co-workers to fabricate a family of
mesoporous phenolic resins and carbons with various ordered
mesostructures.34,35 Owing to the difference in chemical and
thermal stability between the resin and triblock copolymer, the
template can be removed either by calcination at 350–450 ꢂC (ref.
35) or by extraction with sulfuric acid solution,40 leaving the
partially carbonaceous bakelite framework with ordered aligned
voids. After a careful sulfonation under the vapor of 50% SO3/
H2SO4 at 60 ꢂC, a sulfonic acid group functionalized meso-
structured polymer can be obtained.41 However, the operation
involves harsh conditions using hazardous and corrosive mineral
acids, which may pollute the environment and destroy the
mesostructure to some extent, and the acid amount cannot be
well controlled. Therefore, a green synthesis method for ordered
mesoporous acidic polymers with large pore diameters, amphi-
philic nature, and tunable, large acidic numbers is desired.
Very recently, our group reported that the partially carbona-
ceous mesoporous framework shows a hydrophobic nature, and
the incorporation of silica nanoparticles can endow the material
with a unique amphiphilic feature, showing high catalytic activi-
ties.42–44 Here we present the facile synthesis of sulfonic acid group
functionalized ordered mesoporous polymer-based solids with
high concentrations of SO3H groups and their application as
stable and highly active acid catalysts. The process involves the
direct templating synthesis of thiol-group-containing mesoporous
polymers using triblock copolymer F127 as a structure directing
agent, low-polymerized phenolic resins as polymer precursors,
3-mercaptopropyltrimethoxysilane as a functional monomer and
TEOS as an additive, and further oxidation by low-concentration
H2O2. The SO3H-group modified polymers have ordered meso-
structures, highsurface areas (275–448m2 gꢁ1), large porevolumes
(0.40–0.81 cm3 gꢁ1), and uniform pore sizes (about 8.0 nm). The
framework consists of both hydrophobic polymer matrix and
hydrophilic functional group and silicate, and the acidic equiva-
lent weight can be tuned from 0.96 to 2.38 mmol gꢁ1. The polymer-
based acid catalysts are highly active in the condensation of
phenol and acetone, and acetalization reactions of butanediol and
butyraldehyde, etc. They are stable and can keep high catalytic
activity after being reused more than 20 times. This significantly
high activity and stability will be discussed in detail.
Pore constraints and surface nature may also influence
product selectivity as a result of the sizes of substrates, inter-
mediates and products. Corma and co-workers found that the
confinement effects of the reactants and products inside the voids
of the microporous materials would lead to a fast deactivation of
the catalysts, compared to that for larger pore materials.16,17
Clark and co-workers found that mesoporous silica served as a
better catalyst carrier compared to larger pore materials to
support aluminum chloride with the improved selectivity to
monoalkylaromatics using long chain alkenes.18 Mesoporous
materials have also enabled supported catalysts to be used in the
reactions of much bulkier substrates than that could be consid-
ered for zeolites.7,8,19 On the other hand, periodic mesoporous
organosilicas (PMOs) with more hydrophobic frameworks show
higher conversion for bromobenzene coupling in water than
pristine mesoporous silicates.20
Integration of SO3H groups into ordered mesoporous solids
has been explored for promising solid Brønsted acids, such as
MCM-n,21 SBA-m series silicas,22–24 PMOs,15,25 carbons,12,26–28
carbon–silica hybrids,29 etc. The synthesis includes post-grafting
and co-condensation of sulfur-containing organosilanes on silica
surfaces (with/without subsequent oxidation)30 and covalent
attachment of orthosubstituted aryl groups on carbon surfaces.
Hexagonal tube like mesoporous carbon CMK-5 modified with
sulfonic acid groups exhibits a much higher catalytic activity in
condensation of phenol with acetone (per site of bisphenol-A can
reach 23.0%) than sulfonated SBA-15 mesoporous silica,
possibly due to the high acid amount, and amphiphilic nature
originating from the hydrophobic carbon substrate and the
hydrophilic functional group.27 However, CMK-5 mesoporous
carbon is prepared by nanocasting. After infiltration of furfuryl
alcohol, carefully nanocasting on the surface into the channels of
mesoporous silicate and etching of Al-substituted silica scaffold,
ordered tubular mesoporous carbon replicas of CMK-5 can be
prepared.27 The sacrifice of mesoporous silica makes the process
uneconomic, and the nanocasting product shows a relatively
wide pore-size distribution.31
2. Experimental
2.1. Chemicals
Several groups have reported the self-assembly of mesoporous
polymers and carbons.32–38 Adding a PEO compatible low-
Poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene
oxide) triblock copolymer F127 (EO106PO70EO106, Mw ¼ 12 600),
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and 3-mercaptopropyltrimethoxysilane (MPTMS, 85 wt%) were
purchased from Acros Chemical Inc. Phenol (C6H5OH, 99.98
wt%), formalin (HCHO, 37.0–40.0 wt%), tetraethylorthosilicate
(TEOS, minimum 98 wt%), ethanol (C2H5OH, minimum 99.7
wt%), acetone (CH3COCH3, minimum 99.5 wt%), hydrogen
chloride (HCl, 36.0–38.0 wt%), sodium hydroxide (NaOH,
minimum 96.0 wt%), sodium chloride (NaCl, minimum 99.5 wt
%), hydrogen peroxide (H2O2, 30 wt%), sulfuric acid (H2SO4,
95–98 wt%), butyraldehyde (C3H10CHO, minimum 98.5 wt%),
propionaldehyde (C2H5CHO, minimum 98 wt%), cyclohexanone
(C6H10O, minimum 99.5 wt%), ethylene glycol ((CH2OH)2,
minimum 99.9 wt%) and acetophenone (C8H8O, minimum 99.9
wt%) were obtained from Shanghai Chemical Company. 1,4-
butanediol (C4H10O2, 98 wt%) was obtained from Aladdin
Chemical Company. All chemicals were used as received without
any further purification. Water used in all syntheses was distilled
and deionized.
under vacuum overnight. The sulfonic-acid-functionalized
polymers were designated as MP-SO3H-x, wherein x is the
theoretical weight percentage of SO3H groups in the solids. The
theoretical calculation is based on the S molar content deter-
mined by elemental analysis, and assuming that all elemental S is
originating from the SO3H group. An alternative oxidation
procedure was also carried out. 2.04 g of 30 wt% H2O2 was mixed
with 1.0 g of MP-SO3H-27.4-BO sample, followed by the addi-
tion of 6.12 g of methanol. This sample was denoted as
MP-SO3H-27.4-w.
For comparison, the mesoporous polymer-based material
without sulfonic acid functional groups was synthesized, desig-
nated as MP, by the surfactant self-assembly of triblock
copolymer, low-polymerized phenolic resol and TEOS.36. The
mesoporous MP-SO3H-11.4* material was synthesized from the
initial solution containing 3.458 g of TEOS, 0.654 g of MPTMS,
2.0 g of phenolic resin and 3.2 g of F127. Carbonization heating
programs were taken from ambient temperature to 350 ꢂC with a
rate ꢂof 1 ꢂC minꢁ1, keeping this temperature for 1 h, from 350 to
2.2. Synthesis of mesoporous sulfonic acid functionalized
polymer-based solids
600 C with a rate of 1 C minꢁ1, and keeping this temperature
ꢂ
for 4 h to carbonize. The carbonized product MP-SO3H-
11.4*(600) was repeatedly washed with a NaOH solution (4 M)
The precursors were low-polymerized phenolic resins, and
3-mercaptopropyltrimethoxysilane, the coupling agent was
TEOS and the structure directing agent was triblock copolymer
F127.44 The low-polymerized phenolic resins were prepared
according to the established procedure (see details in ESI†). In a
typical synthesis, 3.458 g of TEOS and 0.654 g of MPTMS was
firstly pre-hydrolyzed in the presence of 2.0 g of HCl (0.2 M) and
10 g of ethanol for 30 min. At the same time, 2.0 g of Pluronic
F127 and 10 g of ethanol were mixed together to obtain a clear
solution. To it, the mixture of pre-hydrolyzed TEOS and
MPTMS and a solution containing 0.32 g of low-polymerized
phenolic resins and 1.28 g of ethanol were added in sequence.
After stirring for 2 h, the mixture was poured into multiple
dishes. The dishes were placed in a hood to evaporate ethanol at
ambient temperature for 6 h, and then in an oven to thermo-
polymerize at 100 ꢂC for 24 h. The as-made films were scratched
from the dishes. The triblock copolymer templates were removed
from as-made mesostructured composites by sulfuric acid
extraction under mechanical stirring (typically 1.0 g of mercapto-
functionalized polymers per 100 mL of 48 wt% sulfuric acid) at
90 ꢂC for 24 h.40,45 The resulting materials were filtered off,
washed with 100 mL of distilled water to remove the remainiꢂng
surfactant located on the material surface, and dried at 100 C
under vacuum. The extraction experiment was repeated by twice.
The organic functional thiol group contents could be tuned by
adjusting the added amounts of TEOS and MPTMS. It should be
noted that the optimal value for F127 varies with the ratio of
TEOS : MPTMS. The details for the synthesis are listed in ESI†,
Table S1. The functionalized mesoporous polymers were desig-
nated as MP-SO3H-x-BO (the x value is described below for
simplification, BO means before oxidation). Then an oxidation
step was carried out to oxidize thiol groups to sulfonic acid
groups. Typically, 1.0 g of MP-SO3H-x-BO was suspended in
30 g of aqueous 30% H2O2. The suspension was stirred at room
temperature for 24 h. The solid was filtered, then washed with
water and ethanol. The wet material was suspended again in
20 mL of H2SO4 (1 M) solution for 2 h. Finally, the material was
washed several times with water and ethanol and dried at 100 ꢂC
ꢂ
at 40 C to remove the silica component.
The mesoporous silica-based MSiO2-SO3H-19.4 material
was synthesized by the surfactant self-assembly of triblock
copolymer, MPTMS and TEOS, and all other procedures are
exactly the same as the MP-SO3H-x catalyst. The mesoporous
sulfonic acid functionalized silica-based solids were also prepared
according to the established methods of post-grafting.46,47 Firstly,
mesoporous silicates MCM-41 and SBA-15 were prepared. 1.5 g
of mesoporous silica support MCM-41 or SBA-15 was dispersed
in 50 mL of toluene. 3.6 g (0.018 mol) of MTPMS was added and
the mixture was refluxed at 110 ꢂC for 12 h. The solids were
collected by filtration, washed with toluene and air-dried. The
further oxidation step was exactly the same as that for the mes-
oporous polymer-based materials. These two samples were
denoted as MCM-SO3H and SBA-SO3H.
2.3. Characterization and measurements
The small-angle X-ray diffraction (XRD) measurements were
taken on a Rigaku D/max B diffractometer using Cu Ka radia-
tion (40 kV, 20 mA). The d-spacing values were calculated by the
formula of d ¼ 0.15408/2sin q, and the unit cell parameters were
calculated from the formula of a0 ¼ 2d100/O3. N2 adsorption–
desorption isotherms were measured at 77 K with a Quantach-
rome NOVA 4000e analyzer. The Brunauer–Emmett–Teller
(BET) method was utilized to calculate the specific surface areas
(SBET). By using the Barrett–Joyner–Halenda (BJH) model, the
pore volumes and pore size distributions were derived from the
adsorption branches of isotherms. The micropore volumes (Vm)
were calculated from the V–t plot method. The t values were
calculated as a function of the relative pressure using the de Boer
equation, t (nm) ¼ [0.1399/(log(p0/p) + 0.0340)]1/2. Vm was
obtained using the equation of Vm (cm3 gꢁ1) ¼ 0.001547I, where I
represents the Y intercept in the V–t plot. Transmission electron
microscopy (TEM) experiments were conducted on a JEOL 2011
microscope operated at 200 kV. The samples for TEM
measurements were suspended in ethanol and supported onto a
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holey carbon film on a Cu grid. Weight losses and the associated
temperature were determined by thermogravimetric analysis
with a Mettler Toledo TG/SDTA 851e apparatus. Samples were
heated from room temperature to 1000 ꢂC at a rate of 10 ꢂC
minꢁ1 in an air flow. 29Si solid state nuclear magnetic resonance
(NMR) experiments were performed on a Bruker DSX300
spectrometer (Germany) under the condition of magic angle
spinning (MAS), and the spectra were collected at room
temperature at a frequency of 59.6 MHz, a recycling delay of
600 s, a radiation frequency intensity of 62.5 kHz, and a reference
sample of Q8M8 ([(CH3)3SiO]8Si8O12). Energy dispersive X-ray
spectroscopy (EDX) was performed on a Philips EDAX instru-
ment. The S content was measured on a Vario EL III elemental
analyzer (Germany). X-ray photoelectron spectroscopy (XPS)
using a GC-MS instrument (Agilent 6890n-5973i equipped with a
JW DB-5, 95% dimethyl 1-(5%)-diphenylpolysiloxane capillary
column). Biphenyl in acetonitrile was used as an external stan-
dard for the analysis of bisphenol-A, and the analysis was
repeated at least three times for all tests, and the experimental
errors were within ꢃ5%. Catalytic results are shown either in
terms of absolute yield bisphenol-A/conversion of aldehydes
(ketone) or in terms of initial rate values (mmol of reacted
butyraldehyde per g of catalyst per minute), calculated at the
beginning of the reaction. The mass balance was determined for
some selected reactions.48 At the end of the reaction, the catalyst
was filtered and submitted to continuous solid–liquid extraction
with dichloromethane using a micro-Soxhlet equipment. After
removal of the solvent, the residue was mixed with the organic
solution. The mixture was weighed and analyzed by GC-MS. The
recovered material accounted for more than 90% of the starting
material.
measurements were performed on
a Perkin-Elmer PHI
5000CESCA system with a base pressure of 10ꢁ9 Torr.
The ion-exchange reaction for mesoporous sulfonic-acid-
functionalized organic–inorganic composites was achieved by
treating 1.0 g of sample with 50 mL of aqueous sodium chloride
solution (2 M) at room temperature for 1 h. The resulting solids
were washed with 25 mL of water. The filtration containing the
generated HCl was colorimetrically titrated with a sodium
hydroxide solution (0.0102 M).
For comparison, a blank reaction (without addition of any
catalyst) and a reaction using MP as a catalyst, and reactions
using mesoporous silica-based acidic catalysts were also carried
out.
For the kinetics study, 0.01 g of catalyst was used with the
maintenance of all other conditions. For the recycling study, the
reactions were maintained with the same reaction conditions as
described above, except using the recovered catalyst. Each time,
after the completion of reaction, the catalyst was recovered by
centrifuging (an insulation unit was settled for each tube filled
with reaction liquor), and then washed thoroughly with ethanol
followed by a copious amount of water to remove the interme-
diates, solvents, etc. present in the used catalyst. The liquor
temperature after centrifugation and separation was about
80 ꢂC. This operation can avoid the re-deposition of any soluble
species on the support during cold centrifꢂugation. The recovered
catalyst was dried under vacuum at 100 C overnight, weighed,
and reused in the next run. Several parallel reactions were carried
out to ensure the catalyst amount was the same in each run. The
reused catalyst was designated as MP-SO3H-x-yrun, wherein x
was the weight percentage of SO3H groups in the fresh catalyst
and y was the reaction times. For example, MP-SO3H-x-10run
represented the catalyst after 10 catalytic runs.
Adsorption equilibrium measurements were performed using a
digital microbalance (CAHN Instrument, model D-200) with the
sensitivity of 0.1 mg. Isotherm measurements were carried out by
introducing a dosed amount of high purity water vapor directly
to the sample chamber and recording the weight change after a
stable equilibrium pressure was reached. Consecutive measure-
ments were made by incrementing the vapor pressure by steps.
Before carrying out isotherm measurements, the sample was
heated up to 100 ꢂC for at least 3 h in a high vacuum system. The
temperature was then gradually raised to 250 ꢂC, and maintained
constant for at least 6 h. Once the sample weight remained
constant, the sample was cooled down to the required experi-
mental temperature.
2.4. Catalytic tests
The condensation of phenol and acetone to bisphenol-A and
acetalization reactions were carried out in a 50 mL round-bottled
flask with refluxing.
3. Results and discussion
In a typical condensation reaction, 4.70 g (50 mmol) of phenol
was dissolved in 0.58 g (10 mmol) of acetone and placed in the
reactor. To the solution, 0.05 g of sulfonic-acid-functionalized
mesoporous polymer-based catalyst was added. The mixture was
3.1. Identification of SO3H acid group, compositions and
ordered mesostructure with large, uniform mesopores for the
catalyst
ꢂ
then heated at 85 C for 24 h under stirring.
The molar ratio of aldehydes (ketone) and diol in acetalization
reactions was kept to 1 : 1.5. In a typical reaction, 6.6 mL
(75 mmol) of 1,4-butanediol, 4.5 mL (50 mmol) of butyralde-
hyde, and 0.05 g of solid sulfonicꢂacid functionalized catalyst
were mixed together, heated to 90 C and kept at this tempera-
ture for 2 h under stirring. In one series of experiments, the
amount of the catalyst used was different for each reaction in
order to keep the same total content of acid sites in the reactor.
The reaction was carried out in the absence of solvent. The
detailed reaction conditions are listed in ESI†, Table S2. After
reaction, the suspension was centrifuged (an insulation unit was
settled for each tube filled with reaction liquor), and analyzed
The mesoporous sulfonic-acid-functionalized polymer-based
catalysts were prepared using the surfactant self-assembling
approach. The as-made membrane is continuous, clear and
opaque, implying no macrophase separation. Fig. 1 shows the
small-angle XRD patterns for the mesoporous polymer-based
solids with different sulfonic acid group contents (MP-SO3H-
13.5, MP-SO3H-22.7, and MP-SO3H-27.4). Three distinct
diffraction peaks can be detected in the small-angle range. These
peaks can be assignable to the 100, 110, and 200 diffractions
belonging to the 2D hexagonal mesostructure (spacing group of
p6mm). The cell parameters are calculated to be about 12.5 nm,
similar to the mesoporous polymer-based materials without
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 15874–15886 | 15877