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In the present work, we have concluded the synthesis of sulfonic acid functionalized mesoporous organosilicas with varied proportions of bridging etha...
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12250

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

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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.

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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

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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). References and Notes (1) Gallis, K. W.; Araujo, J. T.; Duff, K. J.; Moor, J. G.; Landry, C. C. AdV. Mater. 1999, 11, 1452. (2) Vallet-Regi, M.; Ra´mila, A.; Real, R. P.; Pe´rez-Pariente, J. Chem. Mater. 2001, 13, 308. (3) Pater, J. P. G.; Jacobs, P. A.; Martens, J. A. J. Catal. 1999, 184, 262.

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