Functionalization of periodic mesoporous organosilica with ureidopropyl groups by a direct synthesis method

Microporous and Mesoporous Materials 101 (2007) 381–387 www.elsevier.com/locate/micromeso

Functionalization of periodic mesoporous organosilica with ureidopropyl groups by a direct synthesis method Qi Wei, Li Liu, Zuo-Ren Nie *, Hui-Qiao Chen, Yan-Li Wang, Qun-Yan Li, Jing-Xia Zou College of Materials Science and Engineering, Beijing University of Technology, 100 Pingleyuan, Chaoyang District, Beijing 100022, PR China Received 1 June 2006; received in revised form 8 September 2006; accepted 14 September 2006 Available online 16 January 2007

Abstract Ureidopropyl groups were used to functionalize the pore channels of ethane-bridged periodic mesoporous organosilica by the co-condensation of 1,2-Bis(triethoxysilyl)ethane (BTESE) and ureidopropyltriethoxysilane (UPTES) in the presence of Poly(ethylene glycol)-BPoly(propylene glycol)-B-Poly(ethylene glycol) (P123) surfactants under acidic conditions. The final materials were investigated in detail by means of XRD, TEM, solid-state NMR, FT-IR and N2 adsorption, in order to study the effect of ureidopropyl groups concentration on their mesoscopic order and pore structure. The results show that bridging groups in the framework do not cleave and ureidopropyl groups are attached covalently to the pore wall of periodic mesoporous organosilica after functionalization. The mesoscopic order decreases with increasing amount of UPTES except for the sample with 20 mol% UPTES concentration, which exhibits a highly ordered two-dimensional hexagonal symmetry. The surface area and pore size decrease as the concentration of UPTES increases, but the materials with 20 mol% UPTES still preserve a desirable pore structure, with a surface area of 565 m2/g, a pore volume of 1.1 cm3/g and a mean pore size of 10.1 nm.  2006 Published by Elsevier Inc. Keywords: Periodic mesoporous organosilica; Terminal functionalization; Ureidopropyl groups; Mesoscopic order; Pore structure

1. Introduction As a novel class of organic–inorganic nanocomposites, periodic mesoporous organosilicas (PMOs) have drawn much attention due to potential applications such as host-guest inclusion, nanotechnology, chemoselective separation and adsorption, chemical sensing, and catalysis [1–8]. These PMOs materials are synthesized by the supramolecular–assembly route in the presence of structuredirecting agents using a silsesquioxane of the type (EtO)3Si–R–Si(OEt)3 as the sole precursor, therefore the organic groups R are located within the channel walls as bridges between Si centers. These homogenously distributed functional groups provide PMOs materials with many of the favorable properties associated with organic polymers, however, they are less reactive and accessible com*

Corresponding author. E-mail address: [email protected] (Z.-R. Nie).

1387-1811/$ - see front matter  2006 Published by Elsevier Inc. doi:10.1016/j.micromeso.2006.09.014

pared to the functional terminal organic groups protruding into the pores due to steric and electronic differences [1,2,9], which are generally incorporated to the surface of ordered mesoporous silica by both post-synthetic grafting and co-condensation of trialkoxyorganosilane with tetraethyl orthosilicate (TEOS) [10–13]. A combination of both bridging organic functional moieties in the framework and terminal organic functional groups protruding into the pores stands for a judicious alternative, conferring to PMOs materials additional functional functionalities [14]. Such approach was recently explored for various groups on periodic mesoporous organosilicas. Asefa et al. functionalized vinyl groups into periodic mesoporous organosilicas by the co-condensation of bis(triethoxysiyl)ethylene and triethoxyvinylsilane in the presence of cetyltrimethylammonium bromide under basic condition, leading to a so-called bifunctional periodic mesoporous organosilicas (BPMOs) in which the bridging ethylene plays a structural and mechanical role and the

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terminal vinyl groups are readily accessible for chemical transformations [9]. Ethylene-bridged periodic mesoporous organosilicas have been further functionalized with a variety of functional groups, including amines, mercaptans, and simple aromatics in Markowitz’s group [15]. Propyland arene-sulfonic acid groups were used to functionalize periodic mesoporous ethanesilica over two different surfactant assemblies and it was found that their acid capacity and proton conductivity reached 1.38 meq/g and 1.6 · 10 2 S/cm, respectively [16]. More recently, Wahab et al. incorporated vinyl, ethyl, glycidoxypropyl and cyanopropyl groups into ethane-bridged periodic mesoporous organosilicas using binary surfactant mixtures under basic conditions [17]. Ureidopropyl groups contain two amine attached directly to a carbonyl and seem to be an attractive class of functional groups since the lone pair electron in oxygen and nitrogen may offer the functionality such as potential activity to form chelate complex with other reactant [18]. It is of considerable significance if ureidopropyl groups can be functionalized into periodic mesoporous organosilicas because they can be used in many application fields such as covalent coupling of protein to the surface of silica materials and selective adsorption of heavy metal ions such as Zn2+, Cr6+ and Ni2+ [19–21]. To the best of our knowledge, ureidopropyl groups have not yet been used to functionlize periodic mesoporous organosilicas in published literature. In the present paper, we report the functionalization of periodic mesoporous organosilicas with ureidopropyl groups by the co-condensation of bis(triethoxysilyl)ethane and ureidopropyltriethoxysilane under acidic conditions and the characterization of final materials by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), solid-state nuclear magnetic resonance (NMR) and N2 adsorption in detail. This paper aims to offer evidences that ureidopropyl groups have been successfully attached on the pore surface and discuss the effect of the funcionalization on the mesoscopic order and pore structure of periodic mesoporous organosilicas. 2. Experimental 2.1. Chemicals 1,2-Bis(triethoxysilyl)ethane (BTESE,96%) and poly(ethylene glycol)-B-poly(propylene glycol)-B-Poly(ethylene glycol)(P123) were obtained from Aldrich. Ureidopropyltriethoxysilane (UPTES, 97%) was purchased from Alfa Aesar. Hydrochloric acid (HCl) and ethanol (C2H5OH) were produced in China. All chemicals were used as received.

P123 as the structure-directing agent. For a typical synthesis, a molar ratio of BTESE, 1 minus x; UPTES x; P123, 0.034; HCl, 11.7; H2O, 326 was used, where x varies from 0% to 40%. In a typical procedure, a solution of P123, HCl and water was prepared at 40 C. To this solution was added dropwise a mixture of BTESE and UPTES. The mixture was allowed to stir for 24 h until white precipitates formed, and then the products were moved into Teflonlined autoclaves and aged for 72 h at 100 C. The white precipitates were recovered by filtration, washed with water until reaching a pH of 6–7, and then dried at atmosphere. The surfactant template was removed from organosilica materials through solvent extraction. An as-synthesized sample (0.5 g) was gently stirred for 6 h in a solution of HCl (36 wt.%, 5 g) and ethanol (1 0 0 g) in 50 C water bath. This procedure was repeated several times until the surfactants were totally removed. The powder was filtered, washed with ethanol, and air-dried at 60 C overnight to obtain final material. 2.3. Materials characterization Powder X-ray diffraction patterns were obtained on a Bruker D8/advance diffractometer using a high power ˚ ) source with a resoluNi-filtered Cu Ka radiation (1.541 A tion of 0.02 and scanning speed of 0.5/min. Nitrogen adsorption measurements were carried out at 77 K on a Micromeritics ASAP 2020 M volumetric adsorption analyzer. Before the measurements, the samples were outgassed under vacuum at 110 C for 5 h. The surface area was calculated according to BET equation at the relative pressures ranging from 0.05 to 0.20 and the pore size distribution was obtained from the adsorption branch of isotherms using BJH approach modified by Kruk–Jaroniec– Sayari (KJS) method. The morphology of the samples was observed by transmission electron microscopy (JEOL JEM-2010). The solid-state magic angle spinning 1H nuclear magnetic resonance (MAS NMR) experiments were performed at 9.4 T on a Varian Infinityplus-400 spectrometer using 4 mm probes under magic-angle spinning speed of 10 KHz. The resonance frequency is 400.1 MHz for 1H. 1H single-pulse experiment was used. The 1H 90 pulse width was measured to be 4.0 ls with repetition time of 4 s. The solid state cross-polarized magic angle spinning 29 Si and 13C nuclear magnetic resonance (CP MAS NMR) spectra were recorded on a Bruker AV300 spectrometer operating at a frequency of 59.62 MHz for 29Si and 75.47 MHz for 13C. Chemical shifts for 1H, 29Si and 13C were referenced to tetramethylsilane (TMS) at 0 ppm. Infrared spectra were acquired from KBr pellets with a Nicolet 5700 FT-IR spectrophotometer with a resolution of 4 cm 1 and a scan number of 32.

2.2. Materials synthesis 3. Results and discussion Functionalized mesoporous materials were prepared using BTESE as the source of bridging organic groups, UPTES as the source of terminal organic groups, and

Fig. 1 shows the XRD patterns of the PMOs materials functionalized with different molar ratio of UPTES. The

Intensity (a.u.)

Q. Wei et al. / Microporous and Mesoporous Materials 101 (2007) 381–387

0% (100)

5% 10%

(110) (200)

Á5 20% 30% 40% 0.5

1.0

1.5 2È/

2.0

2.5

3.0

o

Fig. 1. X-ray diffraction patterns of PMOs materials functionalized with different molar ratio of UPTES. The region where (1 1 0) and (2 0 0) peaks appear is scaled 5 times for the sample with 20 mol% UPTES.

pure PMOs sample displays only a sharp diffraction indexed to (100) at 2h value of 0.83, suggesting an uniform pore structure but not a highly ordered structure for the lack of the higher order diffractions. However, one very broad peak at 2h ranging from 0.8 to 0.9 is observed on the samples functionalized with 5 mol% UPTES, indicating a more amorphous structure. As the UPTES concentration further increases to 10 mol%, the intensity of this peak becomes stronger than that of the corresponding peak in the samples with 5 mol% UPTES, but the samples still remain an amorphous structure. The samples with 20 mol% UPTES exhibit a highly ordered pore system, with a distinct peak at the 2h value of 0.75 and two additional well-resolved diffractions at the 2h value between 1 and 2, which can be assigned to (1 0 0), (1 1 0) and (2 0 0) diffractions, respectively, and indexed according to a twodimensional P6mm hexagonal symmetry. It is particularly interesting that the samples with 20 mol% UPTES exhibit a better mesoscopic order than that of the pure PMOs materials, which is not expected as the presence of organosilane would be more or less disruptive to the structural order as reported previously in a similar synthetic system [15,17,22]. Such observation is further confirmed by the TEM images. As shown in Fig. 2, the pure PMOs material displays predominantly a wormhole-motif pore structure (Fig. 2b), but an uniform pore channel (Fig. 2a) and a small domain reflecting a more ordered structure in Fig. 2b are also observed. However, the materials functionalized with 20 mol% UPTES show perfect hexagonally packed pore arrangement (Fig. 2d) and straight pore channels (Fig. 2c). Such observation encourages us to further examine the structural order of functionalized samples prepared in the presence of higher concentration of UPTES. However, an absolutely opposite trend is observed with further increasing amount of UPTES. The presence of 30 mol% UPTES leads to a more disordered structure as evidenced by the pronounced decrease of the intensity of

383

the (1 0 0) reflection and the disappearance of the higher order reflections. It is also the case for the samples with 40 mol% UPTES. Therefore, in order to obtain a highly ordered functionalized PMOs material, the concentration of UPTES should be fixed to 20 mol%. The mesostructure of functionalized materials might be attributed to the behavior of UPTES and the assembly procedure in the mixture under different conditions during the sol-gel reaction. It is possible that under synthetic conditions most UPTES may react with BTESE through co-hydrolysis and condensation to form silica network, especially in the case of low UPTES concentration in the mixture, therefore ureidopropyl groups are attached to the surface of the final materials. However, ureidopropyl groups might have a strong influence on the self-assembly of polymer P123, leading to the destruction of the interface between silicates species and P123, which is formed by a combination of Hbonding, electrostatic, and van der Waals interactions [23], thus, resulting in the formation of a poor ordered mesostructure. However, functionalized materials have an excellent mesoscopic order as the concentration of UPTES is equal to 20 mol%, which seems to be incompatible to the above-mentioned mechanism. Such surprising observation may be related to the special nature of ureidopropyl groups in solution. As the concentration of UPTES increases gradually, a fraction of UPTES tends to form the so-called oligomerized silicate species through the intermolecular condensation of the hydrolysate of UPTES. The oligomerized silicate species, consisting of hydrophobic moieties and hydrophilic sections, have a similar behavior in solution as the organosilicon surfactants. The hydrophobic section of the oligomerized silicate species inserts into the hydrophobic block of P123 to form mixed surfactants, leading to a reduction of the charge density for the polar heads of mixed surfactants, and therefore it is more easy for the mixed surfactants to form micelles and to be organized into ordered assemblies. The competition between the formation of mixed surfactants and the disruptive effect of ureidopropyl groups compromises at the UPTES concentration of 20 mol%. The silicate species and the mixed surfactants are assembled together via the H-bonding, electrostatic and van der Waals interactions to form a highly ordered mesostructure. As mentioned above, the materials become structurally amorphous with further increase of UPTES concentration. This observation is attributed to the fact that UPTES precursors contain fewer hydrolysable groups (3 in UPTES vs. 6 in BTESE) and provide less sites (–OR or –OH groups, where R is referred to ethoxy groups) for condensation or interacting with triblock copolymer P123 in comparison with the BTESE precursors. Therefore, if large amount of UPTES substitutes for BTESE in the initial mixture, the degree of cross-linking will decrease and the silicate species will be poorly organized around P123 surfactants to such an extent that an ordered mesostructure does not form [9]. Additionally, the steric hindrance resulted from ureidopropyl groups in UPTES also prevents more or less the co-condensation of

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Fig. 2. TEM images of PMOs materials with UPTES molar ratio of (a,b) 0 mol% and (c,d) 20 mol% taken perpendicular to the direction of the pore axis (a,c) and along the pore axis (b,d).

UPTES and BTESE, leading to a reduction of the density and an increase of the defects for the hybrid frameworks, and furthermore resulting in a disruptive local and long range order. From the exact positions of the (1 0 0) peaks, the d1 0 0 spacings of the samples can be calculated and the results are listed in Table 1. It can be seen from Table 1 that increasing the concentration of UPTES in the mixture causes an increase of d1 0 0 spacings from 10.6 nm for the pure PMOs to 13.1 nm for the sample with 30 mol% UPTES. This indicates an expansion of the framework, resulting from the lower degree of the condensation of UPTES and BTESE. Solid state NMR experiments are used to determine whether the functional groups have been attached to pore

channels of PMOs and whether surfactant species have been removed after ethanol extraction. The 1H MAS NMR spectra of the pure and functionalized samples are shown in Fig. 3. The resonance of H2 protons (as shown in the inset in Fig. 3) almost overlaps with that of H3 protons, resulting in a broad peak at 7.9 ppm in the functionalized samples. A signal at 1.3 ppm is related to the methylene within the silicate framework and in the attached ureidopropyl groups (H1), whereas the intensity of the signal in functionalized materials is larger than that in pure PMOs samples. This observation, together with the presence of resonances related to amine proton, indicates that ureidopropyl groups have been functionalized into PMOs materials. The peak at 4.8 ppm is assigned to phys-

Table 1 Textural data of the functionalized PMOs with different molar ratio of UPTES UPTES concentration (mol%)

Surface area (m2 g 1)

Pore volume (cm3 g 1)

Mean pore diameter (nm)

d1 0 0 (nm)

0 5 10 20 30

1071 1004 876 565 539

1.5 1.4 1.3 1.1 0.9

8.2 10.1 8.3 10.1 12.1

10.6 10.3 11.3 11.7 13.1

Q. Wei et al. / Microporous and Mesoporous Materials 101 (2007) 381–387 4.8

O O Si CH 2 OH

O O Si CH2 CH2 OH

O CH2

Si O O H OH

H1

O

O

Si CH2 CH2 CH2 NH C NH2 O

H1

H2

H1

H3

H4

H4 4.1

1.6 1.3

7.9 4.1

(b)

1.3

(a) 15

10

5 0 Chemical shift/ppm

-5

Fig. 3. Solid state 1H MAS NMR spectra of the surfactant-removed samples: (a) pure PMOs and (b) PMOs functionalized with 20 mol% UPTES.

isorbed water while the signal at 1.6 ppm is resulted from isolated silanol species (H4) [24]. The signal at 4.1 ppm may be attributed to the impurities in the precursors. Fig. 4 shows a comparison of the 13C CP MAS NMR spectra for the as-synthesized or solvent-extracted materials, including the pure and 20 mol% UPTES-functionalized PMOs. A prominent peak is observed at the chemical shift of 5.0 ppm for all samples, which is attributed to ethanecarbon atoms covalently linked to Si atoms in the framework [25,26]. The presence of such resonance indicates that no Si–C bond cleavage occurs during either the synthesis or the surfactant extraction process. For the functionalized materials, the peaks occurring at ca. 22, 24, 42 and 161 ppm are assigned to the C1, C2, C3 and C4 carbon atoms (as shown in the inset of Fig. 4) contained in ureido-

O

Si CH 2 CH2 Si

Si CH2 CH2 CH2 NH C NH2 C1

C2

C3

385

propyl groups [27,28]. It can be confirmed that ureidopropyl groups remain intact and have successfully been attached to the pore channels of PMOs materials due to the presence of resonances that are characteristic of these moieties. The resonance peak at ca. 60 ppm for the extracted materials is derived from ethanol, which may be trapped during the surfactant extraction. It is shown that surfactants have been removed by ethanol extraction since the signals at 17 and 73 ppm assigned to remnant P123 surfactants are not observed in the extracted samples [12]. Fig. 5 depicts the 29Si CP MAS NMR spectra of the surfactant-removed pure silica PMOs and 20 mol% UPTES-functionalized samples. Both samples display T2 and T3 resonances ascribed to Si(OSi)2OHR and Si(OSi)3R framework silicon sites [29], respectively, with only slight differences in chemical shifts. The absence of Qn[Si(OSi)n(OH)4 n] Si sites indicates that the Si–C bonds remain intact under our synthesis and subsequent treatment conditions. Since there are no signals of –OC2H5 groups in the 1H MAS NMR and 13C CP MAS NMR spectra, we can deduce that all UPTES precursors introduced in the mixture have involved in the co-condensation reaction, and that all ureidopropyl groups except those mixed with P123 have been integrated in the final materials. The presence of ureidopropyl groups in functionalized samples can also be confirmed by the FT-IR spectra in Fig. 6. The absorption bands at 2924 cm 1 are attributed to the –CH2 asymmetric stretching vibration within both the framework and ureidopropyl groups. The C–N–H bending absorption can be found in functionalized PMOs materials at a wavenumber of 1550 cm 1, but this vibration is not observed in pure POMs samples. The H-banded C@O absorption occurs at about 1643 cm 1, overlapped with that of silanol groups, but the presence of H-banded urea C@O in functionalized materials can be confirmed by the observation that the 1643 cm 1 absorption in functionalized samples is much stronger than that of pure

C4

T2

C2H 5OH C3 C2

C4 (d)

OSi SiO Si C OH

T T

3

2

OSi

C1

T3

SiO

(c)

Si C OSi

P123 (b) P123 (b)

(a)

(a) 200

150

100

50

0

Chemical shift/ppm

0 13

Fig. 4. Solid state C CP MAS NMR spectra of the samples: (a) pure PMOs, before surfactant-removal; (b) pure PMOs, after surfactantremoval; (c) PMOs functionalized with 20 mol% UPTES, before surfactant-removal and (d) PMOs functionalized with 20 mol% UPTES, after surfactant-removal.

-20

-40

-60

-80

-100

-120

Chemical shift/ppm

Fig. 5. Solid state 29Si CP MAS NMR spectra of the surfactant-removed samples: (a) pure PMOs and (b) PMOs functionalized with 20 mol% UPTES.

Absorbance

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-CH2 asymmetric stretching vibration -OH and H-banded urea C=O 1643 2924

(b) C-N-H bending 1555

(a)

3375 -NH2 and physisorbed water

4000 3600 3200 2800 2400 2000 1600 1200

800

400

Wavenumber/cm-1 Fig. 6. FT-IR spectra of the PMOs materials functionalized with different molar ratio of UPTES. (a) Pure PMOs and (b) PMOs functionalized with 20 mol% UPTES.

PMOs samples. Although the stretching vibrations of NH2 in the 3380–3280 cm 1 region cannot be detected in the sample due to the broad band centered at 3430 cm 1 corresponding to adsorbed water, the presence of H-banded urea C@O, –CH2 and C–N–H demonstrates that ureidopropyl groups are successfully bonded to the pore surface of PMOs. Nitrogen adsorption isotherms of the samples functionalized with different molar ratio of UPTES are generally of type IV (Fig. 7), with a clear H1-type hysteresis loop at relative pressure in the range from 0.6 to 0.9, which is characteristic of large-pore mesoporous materials with cylindrical channels. It is well known that the pore size distribution of materials with type IV isotherms can be calculated by using the Barrett–Joyner–Halenda (BJH) method [30]. This approach is based on the Kelvin equation and the correction for the statistical film thickness. However, computer simulations and other theoretical approaches suggested

600

0% 5% 10% 20% 30%

5 Pore volume/cm3g-1

Quantity adsorbed/cm3g-1 STP

6

0% 5% 10% 20% 30%

800

that the Kelvin equation underestimates the pore sizes in the range up to ca.7.5 nm, even when it is corrected for the statistical film thickness [31]. Therefore, a more reliable method of the pore size evaluation is necessarily required. In 1997 Kruk, Jaroniec and Sayari (KJS) proposed a simple method to modify the BJH algorithm by using X-ray diffraction measurements and nitrogen adsorption isotherms for a series of MCM-41 materials having the pore width between 2 and 6.5 nm to estimate accurate relations for the statistical film thickness as a function of the equilibrium pressure and for the pore width as a function of the capillary condensation pressure [32]. More recently, KJS method is improved to give a more accurate pore size evaluation for the ordered silica with a lager pore width up to 12 nm [33]. So the KJS-modified BJH approach is used to determine the pore size distribution of the samples in the present paper. Since some samples may have pore blocked, the desorption branch of the isotherms cannot be used because it gives an artifact at low values of pore widths. Therefore adsorption branch is used instead to determine the pore size distribution and the result is shown in Fig. 8. All the samples exhibit a predominant peak centered at a pore diameter ranging from 8.2 to 12.1 nm depending on the UPTES concentration, and these peaks shift to larger pore size and become wider as the amount of UPTES increases. It can be observed from Table 1 that the surface area and pore volume decrease, whereas mean pore size increases with increasing UPTES concentration. It seems to be more reasonable for a shrinkage of the pore size because the functional ureidopropyl groups are expected to protrude into the pore and reduce the pore size. However, the expansion of the pore in this case may be due to the release of the electrostatic interaction with the surfactant [34] since there is a repulsion between the ureidopropyl groups and the positively charge-associated EO block of the P123 surfactants [23]. Anyway, the materials functionalized with UPTES still preserve desirable pore structure, for example, the materials with 20 mol% UPTES possess

400

4 3 2 1

200

0 10 Pore diameter/nm

0 0.0

0.2

0.4

0.6

0.8

100

1.0

Relative pressure

Fig. 7. N2 adsorption–desorption isotherms of the PMOs materials functionalized with different molar ratio of UPTES.

Fig. 8. Pore size distributions calculated from adsorption branch of nitrogen adsorption isotherm by using Kruk–Jaroniec–Sayari (KJS) method modified BJH approach for the PMOs materials functionalized with different molar ratio of UPTES.

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a surface area of 565 m2/g, a pore volume of 1.1 cm3/g and a mean pore diameter of 10.1 nm. 4. Conclusion On the basis of the co-condensation of bis(triethoxysilyl)ethane and ureidopropyltriethoxysilane, ureidopropyl groups are terminally functionalized to periodic mesoporous organosilicas without any damage to the ethane bridging groups in the framework. The functionalized materials become structurally disordered with increasing of ureidopropyl group except for the sample with 20 mol% UPTES, which displays a highly ordered two-dimensional hexagonal structure, with a better mesoscopic order than that of pure PMOs. This material still possesses a desirable pore structure with a surface area of 565 m2/g, a pore volume of 1.1 cm3/g, and a mean pore size of 10.1 nm, although it shows a trend that the surface area and pore volume decrease with increasing amount of UPTES for functionalized materials. Acknowledgements The financial support of National Natural Science Foundation of China (Grant Nos. 50502002, 50525413) and Scientific Research Common Program of Beijing Municipal Commission of Education (Grant No. KM200610005016) is gratefully acknowledged. References [1] T. Asefa, M.J. MacLachlan, N. Coombs, G.A. Ozin, Nature 402 (1999) 867–871. [2] B.J. Melde, B.T. Holland, C.F. Blanford, A. Stein, Chem. Mater. 11 (1999) 3302–3308. [3] S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 121 (1999) 9611–9614. [4] B. Hatton, K. Landskron, W. Whitnall, D. Perovic, G.A. Ozin, Acc. Chem. Res. 38 (2005) 305–312. [5] O. Olkhovyk, M. Jaroniec, J. Am. Chem. Soc. 127 (2005) 60–61. [6] G. Kickelbick, Angew. Chem. Int. Ed. 43 (2004) 3102–3104. [7] K. Landskron, B.D. Hatton, D.D. Perovic, G.A. Ozin, Science 302 (2003) 266–269. [8] X.Y. Bao, X.S. Zhao, J. Phys. Chem. B 109 (2005) 10727–10736.

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