Formation of mesostructured silica in nonionic fluorinated surfactant systems

Microporous and Mesoporous Materials 92 (2006) 212–219 www.elsevier.com/locate/micromeso

Formation of mesostructured silica in nonionic fluorinated surfactant systems J. Esquena

a,*

, C. Rodrı´guez

b,c

, C. Solans a, H. Kunieda

b,z

a

c

Institut d’Investigacions Quı´miques i Ambientals de Barcelona (IIQAB/CSIC), Technology of Surfactants, Jordi Girona, 18-26, 08034 Barcelona, Spain b Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, Spain Received 1 July 2005; received in revised form 29 December 2005; accepted 9 January 2006 Available online 23 February 2006

Abstract The aqueous phase behavior and preparation of mesoporous silica by new fluorinated surfactants C8F17SO2(C3H7)N(C2H4O)nH (abbreviated C8F17(EO)n) is reported here. C8F17(EO)n forms elongated micelles and liquid crystals in water. Mesostructured silica was prepared by the cooperative self-assembly precipitation method and a systematic study was carried out, investigating the influence of surfactant and silica precursor (TEOS) concentrations, pH and the effect of poly(ethylene oxide) chain lengths. The resulting materials ˚ ) and the pore walls are thick were characterized by SAXS, nitrogen sorption and TEM. The pore inner diameters are small (635 A ˚ ). The materials possess high specific surface areas (1000 m2/g), which are achieved at very small surfactant concentrations (>20 A (2 wt.%), producing robust thick walls with no significant microporosity. The specific surface area is preserved during calcination despite a small shrinkage attributed to silica cross-linking. The d-spacing appeared invariable over a wide range of surfactant/SiO2 ratios, between 0.006 and 1 molar ratios. Hexagonal ordered (p6mm) mesopores were formed at HCl concentrations higher than 0.1 M, while disordered worm-like mesopores were obtained at HCl concentrations lower than 0.1 M. The optimum ethylene oxide chain length to obtain well-ordered mesoporous hexagonal silica corresponded to 10 ethylene oxide units.  2006 Elsevier Inc. All rights reserved. Keywords: Fluorinated surfactant; Self-aggregation; Mesostructured material; Mesoporous; Silica

1. Introduction The preparation of mesoporous materials, with pore diameters between 2 and 50 nm, as defined by IUPAC [1], is a field that attracts considerable interest, from both the scientific and technological points of view [2–4]. In 1992, Mobil scientists [5] reported the synthesis of new mesostructured materials denominated MCM, which are obtained by precipitation at low pH in the presence of qua*

Corresponding author. Tel.: +34 93 4006159; fax: +34 932045904. E-mail address: [email protected] (J. Esquena). z Prof. H. Kunieda died in Yokohama on 17 November 2005. He will always be remembered. 1387-1811/$ - see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.01.003

ternary ammonium salts as surfactants. Since then, much interest has been focused on the synthesis of mesoporous silica by combining sol–gel chemistry and different selfassembly procedures [2,4–8]. The main advantage of mesoporous materials in heterogeneous catalysis, respect to zeolitic materials with pore diameters smaller than 2 nm, is the possibility to use larger reactants and to obtain bigger molecules [2]. Large catalytic sites can be grafted to mesoporous supports, while retaining access that allows fast diffusion of reactants and products. Larger pore sizes, with enhanced hydrothermal stability, were obtained by coprecipitation with poly(ethylene oxide)–poly(propylene oxide)–poly(ethyelene oxide) block copolymers, (EO)n(PO)m(EO)n, surfactants [8,9]. Since then, the preparation

J. Esquena et al. / Microporous and Mesoporous Materials 92 (2006) 212–219

of mesostructured silica by precipitation in aqueous solution [5,8–10] has been extensively studied and a wide range of different mesostructures have been obtained [9,11]. An intensive research work has been devoted to the study of the self-assembly mechanisms during the formation of silica–surfactant aggregates [4,6,7,12–16]. Precipitation of mesostructured silica has been studied in aqueous systems containing nonionic block copolymer surfactants by in situ time-resolved experiments [14,17–20]. The formation of mesoporous silica has been studied as a function of time by NMR, EPR spectroscopy and synchrotron small angle X-ray scattering [20]. The results can be summarized to the following steps: hydrolysis of the silicon alkoxide, adsorption of hydrolyzed silica species to the polar copolymer chains, clustering of silica species and copolymer forming hybrid micelles, elongation and ordering of the micelles in two-dimensional hexagonal structures, and finally growth of such hexagonal domains [19]. Other different methods have been developed to control the mesopore organization and size and several reviews describe the current knowledge in this field [2–4,11,15,21]. The liquid crystal templating method described by Attard et al. [22], makes use of liquid crystalline phases in aqueous systems as reaction media for the sol–gel process. This method allows obtaining well-ordered mesoporous materials, but it has the disadvantages that it requires a high surfactant concentration to form the liquid crystalline phase, and that such phases are greatly affected by alcohols generated in alkoxysilane hydrolysis. More recently, Chmelka and coworkers [23] were able to correlate the aqueous phase behavior of block copolymer surfactants, with silica and titania mesostructures obtained by the evaporation-induced self-assembly method. In all the preparation methods, the crucial tools for controlling the mesostructure are the surfactant molecules. Tuning the hydrophilic and/or the lipophilic chain lengths allows determining the mesostructure of the materials. Nonionic surfactants have the advantage, compared to ionic surfactants, that they can be easily removed from the inorganic framework by solvent extractions or soft thermal treatments, because of weak hydrogen bond interactions between nonionic organic molecules and inorganic species, instead of stronger covalent or ionic interactions [4,11, 15,21]. For instance, copolymers consisting of blocks of poly(ethylene oxide) and poly(propylene oxide), which are hazard-free nonionic surfactants, have been extensively used in the preparation of mesoporous materials, with controlled pore size and morphology. Novel silica precursors and surfactant molecules have been used to obtain a more precise control of the mesostructure and properties of materials. The use of functionalized trialkoxysilanes, as precursors, allows the formation of inorganic–organic composite materials with controlled surface chemistry [24,25]. Chiral self-assembly of surfactants and inorganic precursors has been achieved by using aminosilanes or quarternized aminosilanes as structuredirecting agents [26]. Sattler and Hoffmann have reported

213

the use of ethylene glycol esters of silicic acid, instead of conventional alkoxysilanes such as TEOS [27]. Therefore, the hydrolysis of the inorganic precursor does not generate alcohols as byproducts, and the liquid crystalline phases are much less affected during the inorganic oxide synthesis. Recently, Ste´be´ and coworkers have reported the use of fluorocarbon surfactants in the preparation of ordered mesoporous materials [28,29]. The results showed that a higher degree of organization, compared to analogous hydrogenated surfactants, is obtained. The preparation of mesoporous silica in systems containing mixtures of fluorocarbon and hydrocarbon surfactants has been investigated by Antonietti and coworkers [30]. These two kinds of surfactants do not mix and they tend to form separate micelles. Therefore, silica can be prepared in systems with two types of self-organized templates at the same time. The resulting mesoporous materials possess a bimodal pore size distribution. Studies by Rankin et al. on the preparation of mesostructured silica, with fluorinated cationic surfactants, have produced materials with very small pore sizes (pore diameters approximately from 2.6 nm to 2.0 nm) [31,32]. However, these materials seem to have rather thin pore walls, which could lead to low mechanical and hydrothermal stability. Other recent studies, undertaken by Xiao and coworkers, with nonionic fluorinated surfactants F(CF2)n(EO)m, have shown that it is possible to obtain materials that simultaneously have small pore sizes (1.6–4.0 nm) and relatively thick pore walls (2.5– 2.9 nm) [33,34]. In that case, addition of 1,3,5-trimethylbenzene was used to improve the mesoscopic order [34]. However, no systematic study on the effect of preparation conditions, with fluorinated homologues, at low surfactant concentrations has been reported yet. Preparation of mesoporous silica, by using novel partially fluorinated surfactants, which possess a fluorocarbon chain and also a shorter hydrocarbon alkyl tail, is described in the present paper. Surfactants with different poly(ethyelene oxide) polar chain lengths were available, and therefore hydrophilic–lipophilic properties can be controlled to obtain well-organized supramolecular surfactant aggregates. These new surfactant molecules may provide a rich phase behavior, which could be used to obtain well-organized mesoporous materials. The main aim of the work was to study its self-aggregation properties and to correlate it with the pore size and morphology of mesostructured silica. Synthesis of silica, by hydrolysis of tetraethyl orthosilicate (TEOS), was carried out in aqueous media in the presence of hydrochloric acid. A systematic study was undertaken, investigating the influence of the poly(ethylene oxide) chain lengths, surfactant and TEOS concentrations, as well as pH. The resulting mesoporous silica materials have been characterized by small angle X-ray scattering (SAXS), by nitrogen sorption isotherms and by transmission electron microscopy. Relatively small pore sizes could be expected, because the surfactant fluorinated tails are strongly hydrophobic [35–37] and consequently, surfactants with short fluorocarbon chain lengths can be tested.

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2. Experimental 2.1. Materials Tetraethyl orthosilicate (TEOS), 99.999%, was supplied by Aldrich. Fluorinated surfactants, with the general structure CF3(CF2)7SO2–[CH3(CH2)2]N–(C2H4O)nH (abbreviated C8F17(EO)n), with the ethylene oxide chain length, n, equal to 3, 10 and 20, respectively named EF122C, EF122B and EF122A, were kindly provided by Mitsubishi Materials. They were purified by placing the samples at vacuum (pressure < 0.005 MPa) for 3 days, in order to remove volatile components, until weight remained constant. Hydrochloric acid (5 M) was purchased from Junsei (Japan) and Milli-Q purified water was used in all the experiments. 2.2. Determination of the phase diagram Various amounts of constituents were weighed and sealed in ampoules. Samples were mixed using a vortex mixer and homogeneity was attained by repeated centrifugation through a narrow constriction in the sample tubes. The phase equilibria were determined by visual observation. The optical isotropic nature of the samples was checked with crossed polarizers. The structural characterization of liquid crystal was determined by means of small-angle X-ray scattering (SAXS) measurements.

200 C, and weighed prior to sorption experiments. The specific surface area was determined by applying the multipoint BET model [38]. The pore size distribution was determined by the BJH method [39], applied to the desorption curve. The microporosity was assessed by extrapolation according to the t-plot method, as described by Lipens and Boer [40]. 2.6. Dynamic light scattering (DLS) DLS measurements were performed with a DLS-7000 (Otsuka Electronics Co., Ltd.) equipment, consisting of a goniometer, a 5 mW He–Ne laser (k = 632.8 nm) and a multiple Tau Digital Real Time Correlator (ALV-5000/ EPP, Germany). The field correlation function was fitted by the regularization CONTIN program, to determine the relaxation rate distribution function. 2.7. Transmission electron microscopy (TEM) Transmission electron micrographs were obtained using a Philips CM30 microscope, equipped with a CCD Multiscan Gatan camera. Samples were prepared by allowing ethanol suspensions of finely divided silicas to evaporate on holey copper grids coated with a carbon film. Observations were carried out in an electric field of 200 kV. The mesostructure was confirmed by the image analysis software, DigitalMicrograph, using Fourier Transforms.

2.3. Synthesis of mesoporous silica 3. Results and discussion Mesoporous silica was obtained by a procedure based in the well-known precipitation method [5]. Reactions were carried out by adding TEOS to an aqueous solution containing surfactant and hydrochloric acid. In a typical synthesis, the mass ratios were surfactant/HCl(aq.) 5 M/ TEOS = 2/93/5. The samples were kept at 60 C for 24 h, stirred by magnetic agitation. Afterwards, they were aged at 80 C, without agitation, for 24 h to increase the crosslinking of silica. Then, all samples were washed with ethanol for 24 h, filtered and dried at 80 C for 5 h. Finally, several silica samples were calcined at 500 C in the presence of air for 6 h, increasing the temperature at 2 C/min. 2.4. Small angle X-ray scattering (SAXS) measurements SAXS spectra were obtained in a Nanoviewer Instrument, from Rigaku Corporation (Japan), equipped with point collimation and a CCD Camera as detector, operated at 40 kV and 20 mA power beam. The silica samples were placed in a sample holder 1 mm thick and sealed by Mylar film. In typical experiments, X-ray irradiation was carried out for 3 min. 2.5. Nitrogen sorption determinations The instrument was an AUTOSORB-1, manufactured by QUANTACHROME. Samples were outgassed, at

3.1. Phase behavior of binary C8F17(EO)10/water systems The binary phase diagram of C8F17(EO)10/H2O system, as a function of surfactant concentration and temperature, is shown in Fig. 1. The surfactant forms an aqueous micellar solution (Wm), a hexagonal liquid crystalline (H1) phase, a bicontinuous cubic liquid crystalline (V1) phase and a lamellar liquid crystalline (La) phase. The isotropic region at very high surfactant solution may consist of a reverse micellar solution or a melted surfactant phase. There is no sharp transition between these structures and the aqueous micellar solution, and they belong to the same region of the phase diagram. At high temperature, above the cloud point at 50 C, an excess water phase separates from the micellar solution. A gel region, with very high viscosity, was observed at lower surfactant concentrations than the hexagonal phase. This gel solution was isotropic and did not show any sharp diffraction peak in the SAXS spectra, suggesting that it may consist of worm-like micelles, which could produce the increase in viscosity. Preliminary rheological measurements on 5 wt.% aqueous surfactant solutions gave a high zero-shear viscosity (around 20 Pa s) and viscoelastic shear thinning behavior, which is typical of worm-like solutions. A more detailed rheological study will be published elsewhere [41].

J. Esquena et al. / Microporous and Mesoporous Materials 92 (2006) 212–219

However, it should be pointed out that Eq. (1) can only be applied to the translational diffusion of spherical particles. The radius, 12.5 nm, is much larger than the size of surfactant molecules and only represents an effective micellar radius. Hence, the micellar shape is most probably rod-like micelles at 5 wt.% surfactant.

100

II

Temperature / °C

75

50

V1

Wm

3.2. Preparation of mesoporous materials in C8F17(EO)n systems

Lα

Isotropic

25

gel H1 0

0

0.25 0.5 0.75 Weight fraction C8F17(EO)10

1

Fig. 1. Binary phase diagram of the C8F17(EO)10/water system. Wm: micellar solution, H1: hexagonal liquid crystalline phase, V1: bicontinuous liquid crystalline phase, La: lamellar liquid crystalline phase and II: twophase region.

The micellar structure was investigated by dynamic light scattering (DLS), as described in Section 2. The decay time, obtained from the autocorrelation function, is proportional to Dq2, where D is the translational diffusion coefficient, and q is the scattering vector. Fig. 2 shows, as example, the plot of the decay time as a function of the square of the scattering vector, for a 5 wt.% solution of C8F17(EO)10 in 5 M HCl, at 25 C. The linear fit indicates that the main contribution in the system relaxation is the micellar translational diffusion. The slope corresponds to the translational diffusion coefficient, 1.41 · 1011 m2 s1. The hydrodynamic radius, 12.5 nm, can be calculated by applying the Stokes–Einstein equation: RH ¼

215

kT 6pgD

Mesostructured silica was prepared according to the coprecipitation method described in Section 2. Initially, the effect of the hydrophilic chain length of surfactant was studied in order to select the most appropriate surfactant system. Since only three kinds of head groups, 3, 10 and 20 EO units were available, the average EO number was controlled by mixing different molecules, which had the same lipophilic tail group, C8F17SO2(C3H7)–. The average poly(ethylene oxide) chain length was varied between 6 and 20. The resulting mesoporous silica samples, non-calcined, were characterized by small angle X-ray scattering. The spectra (intensity vs. scattering vector) are shown in Fig. 3. The surfactant with very short ethylene oxide chain (C8F17(EO)3) is insoluble in water and no silica mesoporous particles could be obtained. The surfactant mixture with an average of six ethylene oxide units did also not produce any peak in the SAXS spectra, probably due to the

ð1Þ

C8F17(EO)6

Intensity / A.U.

C8F17(EO)8 10

Γx10-3 / s-1

8

C8F17(EO)10

C8F17(EO)12

6

C8F17(EO)14

4

C8F17(EO)18

D = 1.41x10-11 m2s-1 C8F17(EO)20

RH = 12.5 nm

2

0 0 0

1

2

3

4

5

6

7

q2x104 / nm-2 Fig. 2. Analysis of DLS data of a 5 wt.% C8F17(EO)10 in HCl 5 M solution at 25 C, where C is the decay time, q is the modulus of the scattering vector, D is the translational diffusion coefficient and RH is the hydrodynamic radius.

0.1

0.2

0.3

0.4

q / Å-1 Fig. 3. Small angle X-ray scattering (SAXS) patterns of ethanol-washed mesoporous silicas, for different poly(ethylene oxide) lengths of surfactants. Samples were prepared with 5 wt.% surfactant and 5 wt.% TEOS concentrations, in presence of hydrochloric acid (5 M). For clarity of presentation, the intensity (arbitrary units, log scale) is shown regularly spaced, multiplied by arbitrary factors.

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formation of non-homogeneous pores. However, the surfactant with an average of 10 ethylene oxide units, C8F17(EO)10, produced well-ordered mesoporous silica, as demonstrated by the sharp p peaks observed in its SAXS pattern. Its peak sequence (1: 3:2:. . .) corresponds to a twodimensional hexagonal (p6mm) mesostructure. Its repeat ˚ , was calculated from distance (intercylinder spacing), 58 A the position of the first order peak: pffiffiffi 4p a ¼ 2= 3ðd-spacingÞ ¼ pffiffiffi ð2Þ q 3 All the surfactant mixtures with average ethylene oxide chains between 12 and 18 produced broad and smooth peaks, indicating that the mesopores are disordered. No peak was observed when using the surfactant with the longest ethylene oxide chain (C8F17(EO)20), probably because it was too hydrophilic and did not self-assemble with silica species. The mesostructures formed in C8F17(EO)12 and C8F17(EO)18 systems were studied by TEM. Some representative pictures are shown in Fig. 4. These TEM micrographs suggest that these materials consist of densely packed and connected worm-like mesopores, with narrow pore size distributions. These images strongly resemble those of wormhole-like mesoporous silica (MSU-1), which was first described by Pinnavaia and coworkers [42]. The formation of these mesoporous materials with no long-distance symmetry has also been described by Stucky and coworkers [43], using sodium silicate as precursor. The micrographs also show that the distance between adjacent mesopores is rather homogeneous, producing distance–correlation peaks in the SAXS spectra. These TEM images were studied with the image analysis software (DigitalMicrograph), to calculate the Fourier transforms, and the results were consistent to the spectra obtained by SAXS (Fig. 3). Therefore, it was concluded that the materials obtained with C8F17(EO)8, C8F17(EO)12, C8F17(EO)14 and C8F17(EO)18 possess densely packed worm-like mesopores. The average distance between worm-like mesopores increases with increasing the poly(ethylene oxide) chain length, as observed by the displacement of the main peak

to smaller q values (Fig. 3). This result is similar to that observed by Pinnavaia and coworkers [42] in the case of conventional alkyl ethoxylated surfactants. Obviously, longer surfactant chains mean that the average distance between aggregate cores increases. However, the longest poly(ethylene oxide) chain length, (EO)20, does not lead to any peak in the SAXS spectra. The surfactant EF122B (abbreviated C8F17(EO)10) was chosen as a model molecule for a more systematic study because produced well-ordered mesopores, with narrow pore size distributions. The effect of surfactant and TEOS concentrations, and pH were investigated. The resulting mesostructured materials were characterized by SAXS, transmission electron microscopy and by nitrogen sorption. The effect of the surfactant concentration on the mesostructure is shown in Fig. 5. At 5 wt.% surfactant concentration, the sequence of the p three visible peaks in the SAXS spectra (1: 3:2) clearly shows a two-dimensional hexagonal (p6mm) mesostructure. The SAXS spectra also demonstrate that well-ordered mesoporous silica can be obtained down to 1 wt.% surfactant concentration. These are concentrations lower than those used to obtain SBA-like materials by using nonionic block copolymer surfactants [9]. ˚ , is highly indeThe repeat distance, approximately 58 A pendent on surfactant concentration (Fig. 5), and also on TEOS concentration (results not shown). It remained approximately constant in all the range of surfactant/ SiO2 molar ratios that were studied (between 0.006 and 1, equivalent to mass ratios between 0.1 and 16.3). Therefore,

Intensity / A.U.

216

5.0 wt%

1.0 wt% 0.73 wt%

0.14 wt%

0

Fig. 4. TEM images of ethanol-washed silica, obtained at 5 wt.% surfactant concentration, 5 wt.% TEOS concentration and 5 M HCl.

0.1

0.2 q / Å-1

0.3

0.4

Fig. 5. SAXS patterns of silica samples, washed with ethanol, at different surfactant concentration, at constant [TEOS] = 5 wt.%, obtained with C8F17SO2(C3H7)(EO)10H, in the presence of HCl 5 M. For improved clarity, the intensity (expressed in arbitrary units in log scale) is shown multiplied by arbitrary factors.

J. Esquena et al. / Microporous and Mesoporous Materials 92 (2006) 212–219

the preparation of mesostructured materials using these fluorinated surfactants demonstrated to be highly insensitive to concentration effects. The influence of hydrochloric acid concentration was also investigated, because it was observed that it changes the viscosity of the aqueous surfactant solution and therefore it could influence the phase behavior. Hexagonal mesostructured silica was obtained at HCl concentrations between 0.17 and 5 M HCl. Reducing the HCl concentration induced the formation of less-ordered materials, producing smooth peaks in the SAXS spectra (not shown here). It was concluded that worm-like mesoporous materials were obtained at HCl concentration lower than 0.1 M. The d-spacing of the first order peak, as a function of HCl concentration (before and after calcinations), is displayed in Fig. 6. The repeat distance of the hexagonal silica ˚ ) during prepared at 5 M HCl decreased (from 58 to 54 A calcination at 500 C. This shrinkage is also observed in mesoporous silica prepared by other means [4], which is

60

d-spacing /Å-1

55

Non-calcined

50

45

40 0.001

Calcined

0.01

0.1 [HCl] / M

1

10

Fig. 6. (d-spacing) Corresponding to the first order peak, as a function of HCl concentration in the aqueous surfactant solution, observed by SAXS. Open circles indicate silica samples purified by washing in ethanol for 24 h. Closed circles refer to silicas washed with ethanol and calcined at 500 C. [C8F17(EO)10] = 2 wt.%, [TEOS] = 5 wt.%.

217

generally attributed to an enhancement of cross-linking at very high temperatures. However, it is noteworthy that the morphology of mesopores did not change during the calcinations. Mesopores remained with hexagonal order above 0.1 M HCl and worm-like below that level. The structure of the two types of mesoporous silicas (hexagonal obtained at high-acid and worm-like prepared at low-acid) were studied by transmission electron microscopy. Representative images as shown in Fig. 7. The TEM observations confirmed that hexagonal mesoporous silica was obtained at hydrochloric acid concentrations higher than 0.1 M and disordered mesoporous silica was formed at HCl concentrations lower than 0.1 M. Moreover, the TEM images were studied with the DigitalMicrograph image analysis software, to calculate the Fourier transforms. They were consistent to the spectra obtained by SAXS and consequently it was concluded that the broad SAXS peaks observed at low hydrochloric acid concentrations were certainly the result of densely packed disordered mesopores, which most probably are elongated worm-like. The effect of the HCl concentration can be attributed to several factors. The point of zero charge (isoelectric point) of silica is pH = 2 and silica species are protonated (positively charged) at higher hydrochloric acid concentrations [44]. Moreover, the influence of pH on surfactant phase behavior should be taken into account. It was visually observed that the addition of HCl greatly decreased the viscosity of surfactant solutions in water. This fact could be explained by a reduction in the length and/or the entanglement of elongated micelles. Therefore, hydrochloric acid may induce an increase in the curvature of surfactant micellar aggregates. These effects are complex and its evaluation is not in the scope of the present paper. The mesopores were characterized by nitrogen sorption isotherms, as described in Section 2. The main properties of some selected materials are summarized in Table 1. High specific surface areas (1000 m2/g), which are comparable to those of MCM-41 materials, were obtained. There is almost no reduction in specific surface area during calcinations, despite the fact that the pore size decreased slightly

Fig. 7. Images obtained by TEM of calcined silica particles obtained at 2 wt.% surfactant concentration and 5 wt.% TEOS concentration: (A) 5 M HCl, image of a plane parallel to the mesopores; (B) 5 M HCl, image of the mesopore cross-section; (C) 0.0027 M HCl.

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Table 1 Preparation conditions (surfactant, HCl concentration and calcination), d-spacing, specific surface area (BET area), pore diameter (BJH pore size), repeat distance (hexagonal cell parameter) and pore wall thickness Sample name

Surfactant

[HCl] (mol dm3)

Calcination

Mesostructure

BET area (m2 g1)

BJH pore ˚) size (A

d-Spacing ˚) (A

Repeat ˚) distance (A

Pore wall ˚) thickness (A

YNU-1 YNU-2 YNU-3 YNU-4

C8F17(EO)10 C8F17(EO)10 C8F17(EO)18 C8F17(EO)10

5 5 5 0.0027

No Yes No Yes

Hexagonal Hexagonal Wormhole Wormhole

911 898 520 1135

35 33 33 35

50 46 63 51

58 54 – –

23 21 – –

Samples were prepared as described in Section 2. TEOS concentration was 5 wt.% in all these samples.

˚ . These pores are relatively small, narrower from 35 to 33 A than those of SBA-15 silica (obtained with block copolymer surfactants) and similar to the pores of MCM-41 materials, prepared with conventional cationic surfactants [2–5]. The pore size distributions, as calculated by the BJH method applied to the desorption isotherm, are shown in Fig. 8. The well-ordered hexagonal and the disordered wormlike mesostructures, obtained respectively at high HCl and low HCl concentrations, have very similar pore size ˚ . Moreover, in distributions, with the maximum at 35 A both cases the pore size distributions are very narrow, demonstrating that the mesopores are homogeneous. It is noteworthy that the materials seem to be robust, because there is almost no change in the width of the distribution during calcinations, despite the fact that the pore size decreased ˚ to 33 A ˚. slightly from 35 A The thickness of the pore walls can be determined with accuracy in the case of two-dimensional hexagonal silica, by subtracting the pore diameter from the repeat distance (intercylinder spacing). The pore wall thickness of sample YNU-1 (non-calcined silica that was washed with ethanol) ˚ , respectively. It is and YNU-2 (calcined) are 23 and 21 A very interesting to point out that these pore walls are thicker that those of MCM-41 materials, which are gener-

0.2

Desorption ΔV / cm-3Å-1g-1

HCl = 5 M, Calcined 0.15

HCl = 5 M, Washed HCl = 0.0027 M, Calcined

0.1

ally very thin [5]. In addition, calcination reduces very little ˚ ), indicating that the the wall thickness (approximately 2 A porous mesostructure is robust, which is difficult to achieve by using conventional hydrocarbon surfactants with low molecular weight. The pore walls are also thicker than those described when using fluorinated cationic surfactants [31,32], which resemble MCM materials, or when using nonionic surfactants with partially fluorinated alkyl tails [29]. The main characteristics of the materials described here are similar to those obtained by using F(CF2)5(EO)10 or F(CF2)6(EO)14 fluorinated nonionic surfactants [33,34]. ˚ ) increases when Table 1 also shows that d-spacing (63 A using a mixed surfactant system containing C8F17(EO)10 and C8F17(EO)20, with average molecular structure C8F17(EO)18. The pore size distribution when using this surfactant mixture (C8F17(EO)18) is rather narrow (result not shown). This indicates that the different surfactant molecules are well mixed and do not form segregate micelles. The pore diameter has remained approximately the same than for C8F17(EO)10, and it can be concluded that pore wall thickness can be increased by using fluorinated surfactants with longer (EO)n chains. The pore diameter is not enlarged probably because the fluorinated tails are identical in both surfactants. The volume of micropores (those smaller than 2 nm) was analyzed by the t-plot method, as described by Lipens and Boer [40]. The pore volume, extrapolated to zero thickness of adsorbed nitrogen, indicated that the volume of micropores was very small, in all the samples. This result is the opposite than that described for the SBA-15 hexagonal silica, obtained with block copolymer surfactants [45], in which micropores are present if the material is prepared at 60 C. The microporosity of SBA-15 is attributed to extended long poly(ethylene oxide) chains, which penetrate deeply into the silica matrix and micropores appear after its removal [45]. Consequently, the fluorinated surfactants described in the present work would penetrate much less into the hydrophilic silica matrix.

0.05

4. Conclusions 0 10

100 Porediameter / Å-1

Fig. 8. Pore size distribution, determined applying the BJH equations to the desorption curve of the nitrogen isotherms.

Mesoporous silica has been obtained using the new fluorinated surfactants C8F17SO2(C3H7)N(C2H4O)nH as templating systems. The silica was prepared by the cooperative self-assembly precipitation method in acidic aqueous solutions, and the resulting silica materials were character-

J. Esquena et al. / Microporous and Mesoporous Materials 92 (2006) 212–219

ized by small angle X-ray scattering, transmission electron microscopy and nitrogen sorption isotherms. The results showed that the optimal poly(ethylene oxide) chain length, to obtain well-ordered mesostructured materials was 10 units of ethylene oxide. The concentration of surfactant required to obtain well-ordered mesoporous materials was very low (1 wt.%), and the pore walls were thick ˚ ). Two-dimensional hexagonal (p6mm) mesostruc(>20 A ture was obtained at HCl concentrations higher than 0.1 M, whereas disordered worm-like mesostructured silica was formed at lower HCl concentrations. The nitrogen sorption determinations showed high specific surface areas (around 1000 m2/g) and very homogeneous inner pore diameters, in both hexagonal and worm-like materials. The mesostructure was robust and the specific surface area was not affected by calcination at high temperature, despite ˚ ) in d-spacing. All a small reduction (approximately 2 A these results demonstrated that fluorinated surfactants are very appropriate for the preparation of mesostructured materials with relatively small pore diameters and thick pore walls. Acknowledgments This work was supported by CREST of JST (Japan Science and Technology Corporation). C.R. is grateful to the Japan Society for the Promotion of Science (JSPS) for a research grant. We greatly acknowledge the people offering the adsorption equipment (Profs. T. Meguro and J. Tatami, Dr. T. Wakihara and Mr. T. Hirasaki) and the furnaces for calcinations (Prof. T. Tatsumi and Dr. Y. Kubota). We also acknowledge Serveis Cientı´fico-Te`cnics of the University of Barcelona, for the Transmission Electron Microscopy (Dr. J. Portillo and J. Mendoza). References [1] A.D. McNaught, A. Wilkinson (Eds.), IUPAC Compendium of Chemical Terminology, second ed., Blackwell Science, 1997. [2] U. Ciesla, F. Schu¨th, Micropor. Mesopor. Mater. 27 (1999) 131. [3] J. Patarin, B. Lebeau, R. Zana, Curr. Opin. Colloid Interf. Sci. 7 (2002) 107. [4] G.J. Soler-Ilia, E.L. Crepaldi, D. Grosso, C. Sanchez, Curr. Opin. Colloid Interf. Sci. 8 (2003) 109. [5] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [6] A. Firouzi, F. Atef, A.G. Oertli, G.D. Stucky, B.F. Chmelka, J. Am. Chem. Soc. 119 (1997) 3596. [7] N.A. Melosh, P. Lipic, F.S. Bates, F. Wudl, G.D. Stucky, G.H. Fredrickson, B.F. Chmelka, Macromolecules 32 (1999) 4332. [8] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [9] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [10] P. Tanev, T.J. Pinnavaia, Science 267 (1995) 865.

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