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J. Phys. Chem. B 2005, 109, 12250-12256
Structural Relation Properties of Hydrothermally Stable Functionalized Mesoporous
Organosilicas and Catalysis
Jian Liu,† Qihua Yang,*,† Mahendra P. Kapoor,‡,§ Norihiko Setoyama,‡ Shinji Inagaki,‡
Jie Yang,† and Lei Zhang†
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
457 Zhongshan Road, Dalian, 116023, China; Toyota Central R&D Laboratories Inc., Nagakute,
Aichi 480-1192, Japan
ReceiVed: February 22, 2005; In Final Form: April 29, 2005
The surfactant assistant syntheses of sulfonic acid functionalized periodic mesoporous organosilicas with
large pores are reported. A one-step condensation of tetramethoxysilane (TMOS) with 1,2-bis(trimethoxysilyl)ethane (BTME) and 3-mercaptopropyltrimethoxysilane (MPTMS) in highly acidic medium was performed in
the presence of triblock copolymer Pluronic P123 and inorganic salt as additive. During the condensation
process, thiol (-SH) group was in situ oxidized to sulfonic acid (-SO3H) by hydrogen peroxide (30 wt %
H2O2). X-ray diffraction studies along with nitrogen and water sorption analyses reveal the formation of
stable, highly hydrophobic, and well-ordered hexagonal mesoscopic structures in a wide range of -CH2CH2concentrations in the mesoporous framework. The resultant materials were also investigated by 29Si MAS
and 13C CP MAS NMR, thermogravimetric analyses, UV-Raman spectroscopy, and FT-IR spectroscopy.
The role of the bridged organic group on the hydrothermal stability of the mesoporous materials was established,
which revealed an enhancement in hydrothermal stability of the materials with incorporation of the bridged
organic groups in the network. The catalytic performance of -SO3H functionalized mesoporous materials
was investigated in the esterification of ethanol with acetic acid, and the results demonstrate that the ethane
groups incorporated in the mesoporous framework have a positive influence on the catalytic behavior of the
materials.
1. Introduction
The diverse applications of mesoporous materials in the fields
of separation, chromatography, large molecular release systems,
and catalysis have stimulated the search for materials with new
structures and framework compositions.1-3 Recent discovery of
the periodic mesoporous organosilicas (PMOs) derived from a
number of bridged organosilane precursors represented by the
general formula (R′O)3Si-R-Si(R′O)3, where an organic group
is an integral part of the mesoporous network, opened up new
debates on the applications of mesoporous materials.4,5 Depending on the organic species in the network, the chemical and
physical properties of PMOs can be tuned for desirable
utilizations. Recent advances in this area have demonstrated that
PMOs can be synthesized with improved hydrothermal and
mechanical stability compared to the conventional MCM-41 type
mesoporous silicas.6 The approaches generally used to expand
the possibilities for tailoring the physical/chemical properties
of hybrid silica are the combination of organosilane precursors
with organic groups either bridging in the framework or dangling
into the channel pore. Such bifunctionalized periodic mesoporous organosilicas (BPMOs) are already reported in the
literature.7-9 Further, due to the limited choice and unavailability
of commercial bridged organosilane precursors, it is worthwhile
to explore the possible combination of the existing organosilane
precursors with alkoxysilanes for the synthesis of PMOs and
* Corresponding author. E-mail: [email protected].
† Chinese Academy of Sciences.
‡ Toyota Central R & D Laboratories Inc.
§ Present address: Taiyo Kagaku, 1-3 Takaramachi, Yokkaichi, Mie 5100844, Japan.
to study their structural relation properties. In addition, the
incorporation of transition metals in the framework of mesoporous organosilicas was also attempted for their catalytic
application in epoxidation and ammoximation reactions.10,11
In recent years, the sulfonic acid functionalized mesoporous
materials were reported to be efficient catalysts for acid
catalyzed reactions.12 For practical applications, the hydrothermal stability of the materials is very important because most of
the acid catalyzed reactions such as esterification, hydration,
and condensation always involve water. The previously reported
sulfonic acid modified mesoporous materials are hydrothermally
unstable, and the mesostructure tends to collapse when they are
subjected to prolonged water contact. Incorporating the sulfonic
acid functionality into mesoporous organosilicas is of particular
interest because the bridged organic groups in the mesoporous
network would make the materials hydrophobic and thus provide the materials with improved hydrothermal stability for
catalytic applications. In this regard, earlier we focused on the
synthesis of hydrophobic sulfonic acid functionalized benzene-,
biphenylene-, and ethane-bridged mesoporous organosilicas for
possible use as solid acid catalysts, adsorbents for chromatography, and fillers for the nanocomposites for fuel cell applications.13,14
In the present work, we have concluded the synthesis of
sulfonic acid functionalized mesoporous organosilicas with
varied proportions of bridging ethane groups in the network.
The materials were synthesized by co-condensation of 1,2-bis(trimethoxysilyl)ethane (BTME), 3-mercaptopropyltrimethoxysilane (MPTMS), and tetramethoxysilane (TMOS) in the presence of Pluronic P123 and KCl as additive in acidic medium.
10.1021/jp0509109 CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/04/2005
Multifunctional Periodic Mesoporous Organosilicas
J. Phys. Chem. B, Vol. 109, No. 25, 2005 12251
TABLE 1: Initial Concentration Ratios of BTME, TMOS,
and MPTMS Precursors for the Synthesis of Sulfonic Acid
Functionalized Mesoporous Materials
material
BEME (mol %)
TMOS (mol %)
MPTMS (mol %)
B0T-SO3H
B25T-SO3H
B50T-SO3H
B75T-SO3H
B100T-SO3H
0
23.7
47.4
71.1
94.8
94.8
71.1
47.4
23.7
0
5.2
5.2
5.2
5.2
5.2
We have attempted to create structural variation via incorporating different concentrations of bridging ethane groups in the
mesoporous framework to tailor the surface and structural
properties of the resultant sulfonic acid functionalized mesoporous hybrids. Ordered two-dimensional hexagonal mesostructures were obtained in a wide range of BTME/TMOS ratios.
The resultant -SO3H functionalized mesoporous materials were
found to be effective catalysts in the esterification of ethanol
with acetic acid. An additional advantage of this approach is to
demonstrate the cost-effective solution to synthesize mesoporous
organosilicas with reduced consumption of the expensive
(R′O)3Si-R-Si(R′O)3 precursors. The structural relation properties analyzed from the combined characterization and catalytic
results are also presented.
2. Experimental Section
Chemicals and Reagents. 1,2-Bis(trimethoxysilyl)ethane
(BTME), triblock copolymer HO(CH2CH2O)20(CH2CH2(CH3)O)70(CH2CH2O)20H (Pluronic 123), and 3-mercaptopropyltrimethoxysilane (MPTMS) were purchased from SigmaAldrich Company Ltd. (USA). Tetramethoxysilane (TMOS) and
other reagents were obtained from ShangHai Chemical Reagent.
Inc., of Chinese Medicine Group. All materials were analytical
grade and used without any further purification.
Synthetic Procedure. In a typical synthesis, P123 (0.55 g)
and KCl (3.49 g) were first dissolved in aqueous acidic solution
[HCl (2 M), 16.5 g; H2O, 3.75 g] at 45 °C under vigorous
stirring. A pre-prepared mixture of BTME, TMOS, and MPTMS
precursors was then introduced to this solution, H2O2 (4 g, ∼30
wt %) was quickly added, and the resulting mixture was further
stirred in a closed vessel at 45 °C for 20 h and subsequently
aged at 100 °C under static conditions for an additional 24 h.
The solid product was recovered by filtration and air-dried at
room temperature overnight. The molar ratio of the original gel
was 1.0 Si/6.9 KCl/158.5 H2O/4.9 HCl/0.013 P123. The
resulting -SO3H functionalized mesoporous materials were
denoted as BnT-SO3H, where n is the mole percent ratio of
BTME/(BTME + TMOS) in the initial gel mixture. The details
on the molar ratios of silica precursors used for the synthesis
are listed in Table 1. Finally, the surfactant was extracted by
refluxing 0.5 g of as-synthesized material in 150 mL of ethanol
for 24 h as described elsewhere.15
Hydrothermal Stability. Hydrothermal stability of the
materials was estimated by refluxing the material in deionized
water. Typically, 1 g of the sample was taken in 1 L of deionized
water and after 48 h treatment the materials were isolated by
filtration followed by overnight drying at 100 °C.
Catalytic Measurement. The catalytic performance of the
materials in the esterification of ethanol with acetic acid was
carried out in a 100 mL three-necked bottle equipped with a
condenser. Ethanol (46 g, 1 mol), acetic acid (6 g, 0.1 mol),
and catalyst (0.5 g) were continuously stirred at 60 °C for 6 h.
Reaction products were collected by a syringe at fixed time
intervals and were analyzed using a precalibrated gas chromatograph (Agilent 6890) equipped with an FID detector and
HP-5 capillary column (30 m × 0.25 mm × 0.25 µm). The
tetradecane (0.5 g for each analysis) was employed as an internal
standard. All catalysts were pretreated at 140 °C for 10 h under
evacuation at 10-2 mmHg before measurements of catalysis.
Characterization. X-ray diffraction (XRD) patterns were
recorded on a Rigaku RINT D/Max-2500 powder diffraction
system using Cu KR radiation of 0.15406 nm wavelength. The
nitrogen sorption experiments were performed at 77 K on an
ASAP 2000 system. Prior to the measurement, the samples were
outgassed at 100 °C for 4 h. The pore size distribution curve
was estimated from the adsorption branch using the BarrettJoyner-Halenda (BJH) method. The adsorption isotherms of
water were recorded using an automatic vapor adsorption
apparatus, BELSORP-18, BEL Japan Inc. Prior to the measurements, all surfactant-free samples were evacuated at room
temperature below 2 × 10-3 mmHg. Transmission electron
microscopy (TEM) was performed using a JEOL JEM-2010 at
an acceleration voltage of 100 kV. 13C (100.5 MHz) crosspolarization magic angle spinning (CP-MAS) and 29Si (79.4
MHz) MAS solid-state NMR experiments were recorded on a
Bruker DRX-400 spectrometer equipped with a magic angle
spin probe in a 4-mm ZrO2 rotor. UV Raman spectra were
recorded on a homemade UV Raman spectrometer. The 244
nm laser lines from Kimmon were chosen as an excitation
source. The power of the UV laser lines was below 4.0 mW.
The wavenumber of Raman spectra was corrected, and the
spectral error was (2 cm-1. The thermogravimetric analysis
(TGA) was performed on a Perkin-Elmer Pyris Diamond TG
instrument at a heating rate of 10 °C min-1 under a flow of
nitrogen. The acid exchange capacity was determined by titration
with NaOH. In a typical procedure, 0.1 g of solid was suspended
in 20 g of 2 M aqueous NaCl solution. The resulting suspension
was stirred at room temperature for 24 h until equilibrium was
reached. The filtrate was potentiometrically titrated by 0.1 M
NaOH.
3. Results and Discussion
Synthesis of Sulfonic Acid Functionalized Mesoporous
Materials. Previously, the synthesis of mesoporous hybrids
using BTME and TEOS in the presence of cationic surfactant
in basic medium was reported with only up to 30 mol %
framework incorporation of BTME.4a Later, Char and coworkers reported the synthesis of mesoporous organosilicas via
co-condensation of BTME and TEOS precursors using F127
or LGE76 as surfactant in acidic medium.15 However, the
mesostructure obtained with 60 mol % BTME in TEOS was
highly disordered. This is likely due to the different hydrolysis
rates of BTME and TEOS during the synthesis. The match of
hydrolysis and condensation rate of different types of alkoxysilanes is one of the most crucial factors for the formation of
ordered mesostructure with uniform pore size and homogeneous
distributions of active functionalities. In the presented synthesis,
we have chosen TMOS instead of TEOS because it has a faster
hydrolysis rate and could approximately match the hydrolysis
rate of ethane-bridged organosilane precursor. In addition, KCl
salt was also used as an additive to help the formation of the
ordered mesostructure.16 The -SH groups were oxidized to
-SO3H functionalities by simple addition of H2O2 during the
hydrolysis and condensation process of silica precursors.17 It is
worth mentioning that, compared to the postsynthesis oxidation,
this in situ oxidation method serves better and could also avoid
the tedious synthesis process and possible damages to the
mesostructure of the materials. The structural relation properties
of the materials are discussed as follows.
12252 J. Phys. Chem. B, Vol. 109, No. 25, 2005
Liu et al.
Figure 2. Transmission electron micrograph (TEM) of representative
sulfonic acid functionalized mesoporous material B50T-SO3H: (A)
perpendicular to the channel axis; (B) parallel to the channel axis.
Figure 1. X-ray diffraction patterns of sulfonic acid functionalized
mesoporous materials (A) before and (B) after hydrothermal treatment: (a) B100T-SO3H; (b) B75T-SO3H; (c) B50T-SO3H; (d) B25T-SO3H;
(e) B0T-SO3H.
TABLE 2: Physicochemical Properties of Sulfonic Acid
Functionalized Mesoporous Materials before and after
Hydrothermal Treatmenta
sample
d100
(nm)
BET
surf. area
(m2/g)
B0T-SO3H
9.8
517 (626)
B25T-SO3H
9.4 (9.6) 564 (943)
B50T-SO3H 10.1 (9.8) 588 (953)
B75T-SO3H
9.0 (8.8) 683 (785)
B100T-SO3H 10.3 (9.6) 694 (713)
pore
diam
(nm)
total
pore vol
(cm3/g)
wall
thicknessb
(nm)
8.1
8.1 (7.5)
8.1 (8.4)
6.8 (7.4)
6.8 (7.4)
0.88 (0.37)
1.00 (1.28)
0.94 (1.21)
0.90 (0.92)
0.85 (0.83)
3.2
2.7 (3.6)
3.6 (2.9)
3.6 (2.8)
5.1 (3.7)
a Data in parentheses represent the materials after the hydrothermal
treatment. b Calculated by 2d/x3 - pore diameter.
Structure and Hydrothermal Stability. Powder X-ray
diffraction patterns of surfactant-free materials are shown in
Figure 1. For each sample, three diffraction peaks were observed
in the lower angle range of 0.8-2°, which are indexed as (100),
(110), and (200) reflections of a hexagonal symmetry lattice
(p6mm). The intensity of diffraction peaks decreased with an
increasing amount of BTME in the initial gel mixture. The
ordering degree was high for B0T-SO3H with no bridging ethane
groups in the framework. The results are consistent with the
lower electron contrast as expected between the walls and the
channels of the silica networks. A similar trend was also noticed
for the mesoporous materials derived using cetyltrimethylammonium bromide surfactant in basic medium with different
proportions of bridging ethylene or methylene groups in the
silica framework.4a
X-ray diffraction patterns of -SO3H functionalized mesoporous materials after hydrothermal treatment are also presented
in Figure 1. After boiling in water for 48 h, the lower angle
diffraction peaks are completely absent in the XRD pattern of
B0T-SO3H that has no bridging ethane groups in the mesoporous
network, indicating that the hydrothermal stability of this
material is very low. Interestingly, the lower angle d100, d110,
and d200 reflections were clearly observed in the XRD patterns
of BnT-SO3H (n ) 25, 50, 75, 100) with bridging ethane groups
in the mesoporous framework. This evidently indicates that no
structural degradation occurred in any of these materials upon
hydrothermal treatment. However, all materials were accompanied by an increase in the intensities of d110 and d200
reflections, while a shift of the d100 reflection toward higher 2θ
values was noticed upon hydrothermal treatment (Table 2). The
decrease of d100 spacing can be ascribed to the lattice contraction
during the hydrothermal treatment.
TEM images of the representative material are also consistent
with the powder X-ray diffraction results, showing the two-
Figure 3. (A) Nitrogen adsorption-desorption isotherms and (B) pore
size distribution curves of sulfonic acid functionalized mesoporous
materials: (a) B100T-SO3H; (b) B75T-SO3H; (c) B50T-SO3H; (d) B25TSO3H; (e) B0T-SO3H.
dimensional hexagonal symmetry mesopores throughout the
sample (Figure 2).
Nitrogen sorption isotherms of surfactant-free materials are
of classical type IV according to IUPAC classification with a
sharp capillary condensation step, which is typical of the wellordered mesoporous material with narrow pore size distributions
(Figure 3). The capillary condensation step was found to shift
to lower P/P0 values with increasing concentration of BTEB in
the materials. However, all materials exhibited an H1 hysteresis
loop at relative pressures (P/P0) in the range of 0.55-0.80,
which is usually indicative of materials with pore diameters
larger than 4 nm.18 The BET specific surface area of the
materials increased from 517 to 694 m2 g-1 with increasing
concentration of BTME in the initial gels (Table 2). The pore
diameter remained almost the same (8.1 nm) for the samples
up to 50 mol % BTME concentrations, while a further increase
in BTME concentration resulted in a drastic decrease in the pore
diameters. The smaller pore diameters of the mesoporous
organosilicas compared to the conventional mesoporous silicas
synthesized under identical conditions are due to the relatively
thick pore walls of PMOs with ethane groups bridging in the
mesoporous network. These results also suggest that the textural
properties of the materials completely inherit the characteristics
of the mesoporous organosilicas as the BTME concentration
reaches above 50 mol %.
The nitrogen sorption analyses of -SO3H functionalized
mesoporous materials after hydrothermal treatment are given
in Figure 4. The materials BnT-SO3H (n ) 25, 50, 75, 100)
also exhibit type IV isotherms with a large capillary condensation step in the mesoporous range similar to those observed prior
to the hydrothermal treatments. The sharp BJH pore size
distributions were also observed. B0T-SO3H material shows the
type I isotherm after hydrothermal treatment, and this result is
consistent with XRD measurement, suggesting the degradation
of the mesostructure of B0T-SO3H upon hydrothermal treatment.
Multifunctional Periodic Mesoporous Organosilicas
Figure 4. (A) Nitrogen adsorption-desorption isotherms and (B) pore
size distributions of sulfonic acid functionalized mesoporous materials
after hydrothermal treatment: (a) B100T-SO3H; (b) B75T-SO3H; (c) B50TSO3H; (d) B25T-SO3H; (e) B0T-SO3H.
J. Phys. Chem. B, Vol. 109, No. 25, 2005 12253
Figure 6. Water sorption isotherms of sulfonic acid functionalized
B50T-SO3H mesoporous material: (a) first adsorption; (b) second
adsorption; (c) third adsorption.
Figure 7. Log plots of water sorption isotherms (adsorption branch)
normalized to specific surface area of sulfonic acid functionalized
mesoporous materials.
Figure 5. Water sorption isotherms of sulfonic acid functionalized
mesoporous materials: (a) B100T-SO3H; (b) B75T-SO3H; (c) B50T-SO3H;
(d) B25T-SO3H; (e) B0T-SO3H.
The BET surface area and pore diameter of the materials are
also listed in Table 2.
The above results demonstrate that the hydrothermal stability
of the materials is increased with the increasing incorporation
of bridging ethane groups in the mesoporous network. Such an
increase in hydrothermal stability is due to the increase in
hydrophobic character of the materials upon incorporation of
bridging ethane groups in the mesoporous network. We further
investigate the surface hydrophobic/hydrophilic properties of
the materials by water sorption analyses.
Ion Exchange Properties and Surface Hydrophobicity/
Hydrophilicity. The acid exchange capacity of BnT-SO3H was
measured by potentiometric titration with NaOH (Table 3). The
acidity of BnT-SO3H is in the range of 0.51-0.37 mmol/g. With
increasing amounts of ethane groups in the framework, the
acidity was found to be varied and comparable to the sulfur
content in the materials.
Water sorption results provide the evaluation of the surface
hydrophilic/hydrophobic properties of FSM-16 mesoporous
silica and phenylene bridged mesoporous organosilicas.19 The
surface properties of BnT-SO3H determined by water adsorption
experiments are described in this article. Due to the presence
of hydrophilic propylsulfonic acid groups in the materials, the
evaluations of surface hydrophobic/hydrophilic properties are
complicated. Like the phenylene-bridged mesoporous organosilicas and freshly calcined FSM-16, the water sorption isotherms of BnT-SO3H are of typical type V (Figure 5), indicating
the hydrophobic nature mainly derived from the incorporation
of ethane groups in the framework. The sharp capillary
condensation step of BnT-SO3H (n ) 100, 75, 50, 25) begins
at P/P0 ) 0.8. Such a rapid increase suggests the reasonable
adsorbent-substrate interaction usually seen for the hydrophilic
silica surfaces. The position of a capillary condensation step of
B0T-SO3H (without ethane groups) was at P/P0 ) 0.74, which
was lower than those of BnT-SO3H (n ) 100, 75, 50, 25). The
relative pressure position suggests that the affinity of water to
the surface is higher on B0T-SO3H compared with BnT-SO3H
(n ) 100, 75, 50, 25). Water sorption isotherms were also
measured three times on the same B50T-SO3H at 25 °C. The
first, second, and third isotherms of water vapor are shown in
Figure 6. The adsorption branch of the first isotherm is
characterized by a small adsorption amount at the low P/P0
region under 0.8. At the high P/P0 region over 0.8, capillary
condensation of water occurred and water molecules filled in
the mesopores. This also suggests the weak interaction between
surface and water molecules in the ethane-bridged mesoporous
materials. The isotherms change slightly after the measurements
of the first isotherms. The second and third isotherms show little
increase in the adsorption amount at the low P/P0 region and a
minor shift of the capillary condensation step toward the lower
P/P0 region. The change is caused by the increase in hydrophilicity due to the hydration of silica surface with adsorbed
water during the measurements of isotherms. A similar hydration
was observed for FSM-16; however, the degree of hydration is
smaller for B50T-SO3H.19a The hydration did not change the
framework structure, and the mesoscopically ordered structure
was completely preserved even after the hydration.
From the first observation, the surface hydrophobicity/
hydrophilicity appeared almost the same for all the BnT-SO3H
(n ) 100, 75, 50, 25) materials (Figure 5). However, to gain
more insight in the understanding of the structural relation
properties of hydrophobicity/hydrophilicity of materials, we have
compared the water adsorption at a fixed relative pressure value
normalized to surface areas which explains the variation in the
local water density over the surface with some accumulation
of the molecules in the most accessible hydrophilic portion such
as silanol groups of the material. Figure 7 shows the adsorption
12254 J. Phys. Chem. B, Vol. 109, No. 25, 2005
Liu et al.
Figure 8. (A) 13C CP-MAS NMR and (B) 29Si MAS NMR spectra of the representative sulfonic acid functionalized mesoporous material B50TSO3H.
branch of water vapor sorption normalized to specific surface
areas of the materials studied. The results are in good agreement
with the content of the hydrophobic ethane groups present in
the framework of the hybrid materials.
The results of water sorption analyses suggest that the
hydrophobic character of -SO3H functionalized materials
decreased in the following order: B100T-SO3H > B75T-SO3H
> B50T-SO3H > B25T-SO3H > B0T-SO3H. This also supports
the observation that the materials with higher hydrophobic nature
exhibit higher hydrothermal stability. For example, B25T-SO3H,
which contains only 25 mol % bridging ethane groups, exhibits
a much higher hydrothermal stability than B0T-SO3H without
any bridging ethane groups in the mesoporous network. Overall,
the result implies that even a small fraction of bridging ethane
groups in the framework can lead to materials with improved
hydrothermal stability.
Compositional and Functional Properties. The incorporation of both ethane and propylsulfonic acid groups in BnT-SO3H
were confirmed by NMR spectroscopy, and results on a
representative B50T-SO3H are shown in Figure 8. In the 13C
CP-MAS NMR spectrum of B50T-SO3H, the resonance at 4.7
ppm can be assigned to ethane carbons, while the resonances
at 11.4, 17.7, and 54.2 ppm correspond to the 3C, 2C, and 1C
carbons, respectively, of tSi-1CH22CH23CH2-SO3H.13 The
peaks centered at 69.3 and 16.1 ppm [labeled with an asterisk
(/)] are probably due to the carbons of O-CH2CH3 formed
during the surfactant extraction process.20 The 29Si NMR
spectrum of B50T-SO3H shows the existence of both nQ and nT
sites as expected (Figure 8). The characteristic resonances in
the range of -90 to -110 ppm can be assigned to (HO)2Si(OSi)2
(2Q δ -91), (HO)Si(OSi)3 (3Q δ -102), and Si(OSi)4 (4Q δ
-109) silicon species. The signal at -60.4 ppm is derived from
the mixture of Si [3T, SiC(OSi)3] attached with propylsulfonic
acid functionalities and Si [2T, (OH)SiC(OSi)2] bridged by the
ethane groups. The sharp signal at -67.6 ppm could be
attributed to Si [3T, SiC(OSi)3] bridged by ethane groups. The
T/(T + Q) ratio calculated from the normalized peak areas is
0.54, which is well estimated and comparable with the amount
of BTME incorporated.
The UV-Raman spectra of the surfactant-free materials are
displayed in Figure 9. No vibrations attributable to S-H were
found, which validates the NMR findings. The symmetric and
asymmetric vibration modes of -SO3H were clearly observed
at 1046 and 1104 cm-1, respectively. The results confirm the
complete oxidation of -SH to -SO3H groups. The surfactantfree materials with bridging ethane groups in the framework
show strong C-H vibration bands of -CH2CH2- at 2936 and
2909 cm-1, while B0T-SO3H exhibits only a weak vibration at
2936 cm-1, which is assignable to the CH vibrations of
-CH2CH2CH2SO3H species.
Figure 9. UV-Raman spectra of sulfonic acid functionalized mesoporous materials: (a) B100T-SO3H; (b) B75T-SO3H; (c) B50T-SO3H; (d)
B25T-SO3H; (e) B0T-SO3H.
The thermogravimetric analyses of surfactant-free materials
were performed under nitrogen atmosphere (Figure 10). The
weight loss below 120 °C is mainly due to the removal of
physically adsorbed water from the materials. The decomposition of the propylsulfonic acid moieties starts at about
380 °C, and the corresponding weight loss from 480 to 800 °C
can be related to the partial decomposition of bridging ethane
groups. This weight loss was varied depending on the BTME
concentration in the materials, suggesting that appropriate
amounts of ethane groups were incorporated in the mesoporous network. Obviously, this weight loss was absent for B0TSO3H.
The results of solid-state NMR spectroscopy, UV-Raman
spectral analyses, and thermogravimetric analyses confirmed the
incorporation and integrity of ethane and propylsulfonic acid
groups in the materials.
Catalytic Properties. The details of catalytic setup are
already described in the Experimental Section. The catalytic
performance of the sulfonic acid functionalized mesoporous
materials was assessed in the esterification of ethanol with acetic
acid (Table 3 and Figure 11). The sulfonic acid functionalized
materials showed much higher catalytic conversion in esterification reaction compared to those performed without catalyst.
This clearly indicates the involvement of sulfonic acid functionalities in the esterification (Figure 11). The materials with
varied ion exchange capacities showed the enhanced turnover
numbers (TONs), which were calculated at the fixed reaction
conditions. The overall esterification activity followed the trend
B75T-SO3H = B100T-SO3H > B50T-SO3H = B25T-SO3H > B0TSO3H. The results evidently support that the catalysts with
bridging ethane moieties in the framework generally are more
active compared to the materials without bridging ethane
moieties. Combining the results of hydrothermal stability and
Multifunctional Periodic Mesoporous Organosilicas
J. Phys. Chem. B, Vol. 109, No. 25, 2005 12255
TABLE 3: Acid Exchange Capacity, Sulfur Content, and
Catalytic Activities of Sulfonic Acid Functionalized
Mesoporous Materialsa
sample
S (mmol/g)
H+ (mmol/g)
ethyl acetate
yield (mmol)
TON
B0T-SO3H
B25T-SO3H
B50T-SO3H
B75T-SO3H
B100T-SO3H
Nafion
0.407
0.435
0.429
0.457
0.425
0.51
0.37
0.41
0.42
0.39
0.8
41.6
43.0
49.2
55.6
49.1
53.5
163
232
240
264
252
334
a
Reaction conditions: ethanol, 1 mol; acetic acid, 0.1 mol; catalyst,
0.5 g (Nafion, 0.2 g); reaction temperature, 60 °C; TON ) mmol of
acetate/mmol of H+.
Figure 11. Esterification of ethanol with acetic acid catalyzed by
sulfonic acid functionalized mesoporous materials. Reaction conditions: ethanol, 1 mol; acetic acid, 0.1 mol; catalyst, 0.5 g (Nafion, 0.2
g); reaction temperature, 60 °C.
Figure 10. Thermogravimetric analysis of sulfonic acid functionalized
mesoporous orgaosilicas:[(a) B100T-SO3H; (b) B75T-SO3H; (c) B50TSO3H; (d) B25T-SO3H; (e) B0T-SO3H.
Figure 12. Recyclablity studies of B75T-SO3H in esterification of
ethanol with acetic acid. Reaction conditions are analogous to those in
Figure 11. (b) Fresh; (9) recycle 1; (2) recycle 2; ([) recycle 3.
water adsorption, it is supposed that the higher catalytic activity
of BnT-SO3H (n ) 100, 75, 50, 25) materials mainly contributes
to the improved surface hydrophobicity of the materials arising
from the hydrophobic ethane groups bridging in the framework.
On the other hand, the catalytic activity of BnT-SO3H is
somewhat lower compared to the commercial Nafion, which is
probably due to the weak acidity of BnT-SO3H materials.
However, the use of Nafion is limited because the diffusion
limitation of Nafion can restrict the participation of some of its
acidic sites due to the swelling of Nafion beads in the course
of reaction.
In addition, the reusability of B75T-SO3H was also assessed.
After the reaction, the catalyst was filtered off and recycled for
further reactions (Figure 12). About 50% loss in the catalytic
activity was observed after the first recycle. However, the second
and third recycles gave similar catalytic activities, indicating
that the catalyst was stable after the first recycle. The acidity
of B75T-SO3H was also measured after the third recycle and
12256 J. Phys. Chem. B, Vol. 109, No. 25, 2005
was almost the same (0.41 mmol of H+/g) as that of the fresh
catalysts. These results demonstrate that leaching of the active
sites from the catalyst is not evident and is not a major
contributing factor for the loss of activity of the catalyst after
the first recycle. One of the possible reasons for the deactivation
of the catalyst after the first recycle could be the blockage of
micropores existing in the wall surface of BnT-SO3H material
(SBA-15 type) by reaction product during the catalysis.
4. Conclusion
In summary, sulfonic acid functionalized mesoporous
organosilicas with different concentrations of ethane groups
bridged in the framework were successfully synthesized by a
one-step condensation method using different silica precursors
in the acidic medium. Acid-catalyzed synthesis allows slow and
moderate condensation of the organosilica units and facilitates
the ordering of the mesoporous materials. The presence of
hydrophobic ethane groups in the framework is very important
and positively affects the hydrothermal stability and catalytic
activity of the materials. The results show that even a small
fraction of bridging ethane groups in the framework can lead
to materials with greatly improved hydrothermal stability. The
results of water adsorption and catalytic activity demonstrate
that the catalytic activity of the materials depends largely on
the surface hydrophobic/hydrophilic properties of the sulfonic
acid functionalized mesoporous organosilicas.
Acknowledgment. The authors thank Prof. Can Li for the
UV Raman analysis. This work was supported by the National
Natural Science Foundation of China (20303020), the National
Basic Research Program of China (2003CB615803), and the
Talent Science Program of the Chinese Academy of Sciences.
This work was partially supported by a Core Research for
Evolutional Science and Technology (CREST), Japan Science
and Technology Agency (JST).
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