07) 1469–1473www.elsevier.com/locate/matlet

Materials Letters 61 (20

Preparation and characterization of vinyl-functionalized mesoporousSBA-15 silica by a direct synthesis method

Qi Wei, Hui-Qiao Chen, Zuo-Ren Nie ⁎, Ya-Li Hao, 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 21 June 2006; accepted 20 July 2006Available online 4 August 2006

Abstract

Vinyl groups were used to functionalize the pore channels of mesoporous SBA-15 materials by the co-condensation of tetraethyl orthosilicate(TEOS) and triethoxyvinylsilane (TEVS) in the presence of poly(ethylene glycol)–B-poly(propylene glycol)–B-poly(ethylene glycol) (P123)surfactants under acidic conditions. The final materials were investigated in detail by means of FT-IR, XRD, TEM, solid state NMR and N2

adsorption, in order to study the effect of vinyl concentration on their mesoscopic order and pore structure. The results show that vinyl groups areattached covalently to the pore wall of SBA-15 after modification. The mesoscopic architecture almost remains intact upon functionalization, withonly a minor decrease of the intensity of (110) diffractions for the 20 mol% TEVS-functionalized samples, which still preserve a desirable porestructure, with a surface area of 883.7 m2/g, a pore volume of 0.98 cm3/g and a mean pore size of 4.4 nm.© 2006 Published by Elsevier B.V.

Keywords: Vinyl-functionalized SBA-15 materials; Co-condensation; Mesoscopic order; Pore structure

1. Introduction

Ordered mesoporous silica materials become increasinglypopular in chemistry, physics and materials science since thediscovery of surfactant micelle-templated synthesis of meso-porous silica materials M41S [1,2], SBA-15 [3], KIT-1 [4] andFSM-16 [5]. Another more recent breakthrough in this field isattributed to the surface organic functionalization of mesopor-ous silica [6]. Such inorganic–organic hybrid material may playan important role in many applications such as adsorption, ionexchange, catalysis, and sensing due to their specific attributessuch as binding sites, stereochemical configuration, chargedensity, and acidity [7]. Generally speaking, there are twoapproaches to organic functionalization of mesoporous silicamaterials, i.e. post-grafting and direct synthesis. In the formermethod, organic functional groups are covalently attached to thepore surface by the reaction of a suitable organosilane bearingspecific functional groups with the silanol groups on the pore

⁎ Corresponding author. Tel./fax: +86 10 67391536.E-mail address: zrnie@bjut.edu.cn (Z.-R. Nie).

0167-577X/$ - see front matter © 2006 Published by Elsevier B.V.doi:10.1016/j.matlet.2006.07.056

surface using an appropriate solvent under reflux condition[8,9]. The direct synthesis, involving one-step co-condensa-tion of tetraalkoxysilanes and organosilanes, offers a higherand more uniform surface coverage of functional groups and abetter control over the surface properties of the resultantmaterials, compared to the post-grafting method [10]. Thismethod has widely been used to functionalize mesoporousmaterials and many functional groups including aliphatichydrocarbon [11], thiol [12,13], phenyl [14,15], amine[10,16–18] and sulfonic ligand [19,20] have been studied. Incontrast to the extensive investigation on the functionalizationof small pore silica such as HMS, MSU and MCM-41, lessattention has been paid to the surface modification of largepore silica, such as SBA-15, yet in many application fieldsfunctionalized SBA-15 materials are more desirable becausethey are more hydrothermally stable owing to their moreregular structure and much thicker pore wall [3]. Among thefunctional groups, the vinyl group is of great interest becausethe double bond is fairly stable under the synthetic conditionsof mesostructured materials, but sufficiently reactive in thespecific environments such as bromination [21] and hydro-boration [22,23]. Additionally, vinyl groups have important

Fig. 1. FT-IR spectra of the 20 mol% vinyl-functionalized SBA-15 materials (a)as-prepared; (b) ethanol-extracted.

Fig. 2. Solid state 29Si MAS NMR of vinyl-functionalized SBA-15 withdifferent molar concentrations of TEVS.

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functionalities in many applications such as enzyme immobi-lization or selective adsorption of heavy metal ions. Therefore,it is of great significance if vinyl groups are functionalized intothe pore of SBA-15 materials, however, scarce publicationshave been devoted to this work. Y. Wang et al. reported that aless ordered material was obtained upon the functionalizationof 20 mol% TEVS into SBA-15 without the addition of MgCl2[24]. In another paper, they reported that 20 mol% TEVS led tothe formation of well-ordered Ia3d material in the presence ofNaCl, but the maximum content of vinyl functional groups thatcould be incorporated into SBA-15 without observabledisruption of the hexagonal mesostructure was relatively low(only 10%) [25]. In the work of Zhao's group, a concentrationof TEVS as high as 17 mol% was proved not to pronouncedlyperturb the structural order of SBA-15, but the reflections ofthe higher order (110) and (200) completely disappeared [10].It can be found from the limited related literatures that highlymesoscopically ordered functionalized SBA-15 materials withhigh concentration of TEVS, no matter in hexagonal or cubicsymmetry, can only be yielded by the help of inorganic saltsaddition. In the present paper, however, we functionalize theSBA-15 materials with TEVS under synthetic conditionsdissimilar to those previously reported, that is, allowing TEOSto pre-hydrolyze for several hours before the addition of TEVSat the absence of inorganic salts, and investigate whether ahighly ordered structure can be achieved and the effect ofTEVS concentration on the pore structure of the functionalizedmaterials.

2. Experimental

2.1. Chemicals

Tetraethyl orthosilicate (TEOS, 98%) and poly(ethyleneglycol)–B-poly(propylene glycol)–B-Poly(ethylene glycol)(P123) were obtained from Aldrich. Triethoxyvinylsilane(TEVS, 97%) was purchased from Alfa Aesar. Hydrochloric

acid (HCl) and ethanol (C2H5OH) were produced in China. Allchemicals were used as received.

2.2. Materials synthesis

The synthesis of SBA-15 material was performed accordingto the following procedure with a molar ratio of TEOS:surfactant:HCl:H2O of 1:0.017:5.854:162.681. Triblock copolymer P123was used as the surfactant template. Organosilane TEVS, as thesource of vinyl groups, was introduced into the mixture with aTEVS/(TEVS+TEOS) molar ratio ranging from 5% to 20%. Acertain amount of P123 was dissolved in a mixture of water and2 M HCl aqueous solution with strong stirring at 40 °C, and thenTEOS was added dropwise into the mixture, followed by theaddition of TEVS 4 h later. After stirring for another 20 h, themixture was moved into Teflon-lined autoclaves and aged for24 h at 100 °C. The product was filtered and air-dried, followedby surfactant removal by Soxhlet extraction with ethanol for24 h. The final material was obtained after drying at 60 °C atatmosphere overnight.

2.3. Materials characterization

Infrared spectra were acquired fromKBr pellets with a Nicolet5700 FT-IR spectrophotometer with a resolution of 4 cm−1 and ascan number of 32. Solid state 29Si NMR measurement wasperformed on a Bruker AV300 spectrometer operating at a fre-quency of 59.62 MHz with the following experimental condi-tions: magic-angle spinning at 5 kHz; π/2 pulse, 7 μs; a repetitiondelay of 600 s; 200 scans. The chemical shift is referenced totetramethylsilane. Powder X-ray diffraction patterns wereobtained on a Bruker D8/advance diffractometer using a highpower Ni-filtered Cu-Kα radiation (1.541 Å) source with aresolution of 0.02° and scanning speed of 0.5°/min. The mor-phology of the samples was observed by transmission electronmicroscopy (JEOL JEM-2010). The samples were dispersed inacetone until a suspension was obtained, and a drop of thesuspension was deposited and dried on a Cu grid. A low

1471Q. Wei et al. / Materials Letters 61 (2007) 1469–1473

exposure technique was used to reduce the effect of beamdamage and sample drift. N2 adsorption was measured withMicromeritics ASAP 2020 at −196 °C. Before analysis, thesamples were first degassed at 110 °C for 5 h. The surface areawas calculated according to the BET equation at the relativepressure ranging from 0.05 to 0.20 and the pore size distributionwas obtained from the desorption branch of isotherms using theBJH approach. The pore volume was obtained by the amountadsorbed at saturated pressure.

3. Results and discussion

FT-IR measurement is conducted on the samples with 20 mol%TEVS to examine whether the surfactants have been removed by Soxhletextraction with ethanol and the results are shown in Fig. 1. The bands at2973, 2929 and 1463 cm−1 can be attributed to CH2 absorption of P123surfactants [26], however, these bands are not observed for ethanol-extracted samples, which confirms that solvent extraction with ethanol isreasonably effective at removing the templates. The vibrations of C2H4

in the 1620–1680 cm−1 [23] and 3000–3100 cm−1 [26] regions cannotbe separately detected in the sample due to the overlap of the former peak

Fig. 3. XRD spectra of the vinyl-functionalized SBA-15 with different molar concenttimes.

with that of hydroxy groups (1628 cm−1) and the considerably weak IRsignal of the latter vibration, but the presence of vinyl groups in SBA-15silica can be demonstrated by the solid state 29Si MAS NMR spectra(Fig. 2). In the functionalized materials, the resonances at around −70and −78 ppm represent silicon atoms T2(RSi(OSi)2(OH)) and T3(RSi(OSi)3), respectively, where R is referred to the vinyl group, however,these peaks are not observed in the pure SBA-15 materials, whichconfirms that functional organic groups have been attached to the surfaceof SBA-15 materials by covalent bond. The three peaks at about−94 ppm due to Q2(Si(OSi)2(OH)2), −101 ppm due to Q3(Si(OSi)3OH),and −110 ppm due to Q4(Si(OSi)4) silicon sites indicate that theframeworks are mainly composed of siloxane [10].

Fig. 3 shows the X-ray diffraction patterns of the pure and the vinyl-functionalized SBA-15 materials. The pure SBA-15 sample displays threewell-resolved peaks, which can be indexed as (100), (110) and (200)diffractions, respectively, suggesting a two-dimensional hexagonal P6mmsymmetry. There exists a slight difference between the higher order (110)and (200) diffractions depending on the concentration of TEVS infunctionalized materials. The intensity of the (110) diffraction decreaseswith the increasing amount of TEVS, but the (200) diffraction remains aconstant intensity and a sharp peak of (100) diffraction is still clearlyobserved, indicating that the incorporation of TEVS into the initial mixture

rations of TEVS. The region where the (110) and (200) peaks appear is scaled 5

Fig. 4. TEM image of the vinyl-functionalized SBA-15 with different molarconcentrations of TEVS.

Fig. 5. Isotherms of the vinyl-functionalized SBA-15 with different molarconcentrations of TEVS.

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does not considerably disrupt the long-range mesoscopic order except aslight local adjustment for the samples with larger concentration of TEVS.The mesoscopic order and symmetry inferred from the XRD data can befurther confirmed by the TEM images shown in Fig. 4. The two-dimensional P6mm hexagonal symmetry can be clearly observed on boththe pure and the 20 mol% TEVS-functionalized SBA-15 samples. Ourresult is quite different from that of the previous report [24], where theaddition of 20% TEVS leads to a disordered structure with only a ratherbroad peak in the XRDpattern. Themoremesoscopically ordered structure

Table 1Textural data of the vinyl-functionalized SBA-15 with different molarconcentration of TEVS

TEVSconcentration (mol%)

Surface area(m2 g−1)

Pore volume(cm3 g−1)

Mean poresize (nm)

d100(nm)

0 733.4 0.96 5.2 9.45 777.6 0.99 5.1 9.615 841.9 0.91 4.3 10.120 883.7 0.98 4.4 10.3

in our work may be attributed to the different synthetic conditions. In ourwork, TEOS was allowed to pre-hydrolyze for 4 h prior to the addition ofTEVS, however, in the case of the above literature, TEOS and TEVS wereadded simultaneously into the aqueous solution. It can be expected thatP123 has already self-assembled and reacted strongly with the silicatespecies derived fromTEOShydrolysis for 4 h before the addition of TEVS,so the addition of TEVS might have less effect on the self-organization ofthe surfactants. Furthermore, the addition of TEVS 4 h later may lead to anintact silicate framework because TEVS may not disrupt the relativelycontinuous silicate networks derived from hydrolysis and condensation ofTEOS. From the exact positions of the (100) peaks, the d100 spacings of thesamples can be calculated and the results are listed in Table 1. It can be seenin Table 1 that increasing the concentration of TEVS in the mixture causesan increase of d100 spacings from 9.4 nm for the pure SBA-15 sample to10.3 nm for the sample with 20 mol% TEVS. The evolution of d100spacings of our work is contrary to that of the previous work, where adecrease of d100 spacings is observed with an increasing amount of TEVS.This indicates that the variation of synthetic conditions may cause differentsilicate networks, and the expansion of the framework in the case of our

Fig. 6. Pore size distribution of the vinyl-functionalized SBA-15 with differentmolar concentrations of TEVS.

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work may result from the lower degree of the condensation of TEOS andTEVS.

Fig. 5 shows the N2 adsorption–desorption isotherms of the pureand functionalized SBA-15 materials. All samples display a type IVisotherm, with characteristics of monolayer–multilayer adsorptionfollowed by capillary condensation at a relative pressure of 0.6,indicating that the samples are mesostructured materials. In addition,the sharp H1 hysterisis at P /P0=0.6 is an indication of the presence ofuniform mesopores in the samples, which can be confirmed by the poresize distribution (PSD) curves shown in Fig. 6. It is interesting that theshape of the isotherms is almost not affected by the functionalization ofTEVS, which is not necessarily expected since the incorporation oforganosilanes has a more or less effect on the pore structure of thematerials. The nitrogen sorption data discussed above further confirmthat the addition of TEVS at a certain stage of the sol preparation hasminor disruptive effects on the formation of SBA-15, which is in goodagreement with the XRD patterns shown in Fig. 3. This is also due tothe formation of ordered assembly of silicate species and P123surfactants prior to the addition of TEVS. All samples have a sharppore size distribution centered at around 6 nm (Fig. 6). This is quitedifferent from that of the previous report, where an increase of TEVSconcentration leads to a pronounced shift of the PSD peaks to a smallerpore size. The surface area, pore volume and mean pore size determinedfrom the isotherms are summarized in Table 1. As the TEVSconcentration in the initial synthesis mixtures increases from 0 to20 mol%, the surface area, pore volume and mean pore size showdifferent variation trends. The BET surface areas increase gradually,appearing to have an opposite trend compared to that of the samplesfunctionalized with other organosilanes such as aminopropyltriethox-ysilane (APTES), and mercaptopropylmethoxysilane (MPTMS) [13].The very weak disruptive effect of TEVS on SBA-15 mesostructureshould mainly account for this observation. In most cases, the porevolume of the functionalized materials decreases with increasingconcentration of organosilane because functional organic groups areexpected to protrude into the pores and occupy the space of the channels.In the case of our work, however, the pore volume remains almostconstant, which may be ascribed to the nature of the vinyl groups whosesmall and compact molecular configuration causes less blockage of thepores [10]. Therefore, it seems to be reasonable that only a minorreduction of the pore size is observed upon the functionlization of SBA-15 materials with vinyl groups. It can be observed in Table 1 that thesamples functionalized with TEVS concentration as high as 20 mol%still preserve a desirable pore structure, with a surface area of 883.7 m2/g,a pore volume of 0.98 cm3/g and a mean pore size of 4.4 nm.

4. Conclusion

On the basis of the co-condensation of tetraethyl orthosilicate(TEOS) and triethoxyvinylsilane (TEVS), vinyl groups areterminally functionalized to the ordered mesoporous SBA-15materials. The functionalized materials preserve a mesoscopi-cally ordered two-dimensional P6mm hexagonal structure withthe concentration of TEVS as high as 20 mol% in the initialmixtures. The BETsurface areas increase gradually and the porevolume remains almost constant with the increase of TEVScontent. The materials with 20 mol% TEVS remain a desirable

pore structure, with a surface area of 883.7 m2/g, a pore volumeof 0.98 cm3/g and a mean pore size of 4.4 nm.

Acknowledgements

The financial support of the National Natural ScienceFoundation of China (grant no. 50502002, 50525413) and theScientific Research Common Program of Beijing MunicipalCommission of Education (grant no. KM200610005016) isgratefully acknowledged.

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