Macroporous Dual Materials with Ordered

Article pubs.acs.org/Langmuir

Facile Synthesis of Meso/Macroporous Dual Materials with Ordered Mesopores Using Highly Concentrated Emulsions Based on a Cubic Liquid Crystal Jérémie Nestor, Alejandro Vílchez, Conxita Solans, and Jordi Esquena* Institute for Advanced Chemistry of Catalonia, Spanish Council for Scientific Research (IQAC−CSIC), and CIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Jordi Girona, 18-26, 08034 Barcelona, Spain S Supporting Information *

ABSTRACT: A new simple one-step method has been developed to obtain SiO2 monolithic materials with a bimodal meso- and macroporous pore-size distribution. Sol−gel reactions were carried out in the continuous phase of highly concentrated emulsions with a cubic liquid crystal in this external phase, using a polyoxyethylene alkyl ether surfactant and containing a novel glycolmodified silane, tetra(2-hydroxyethyl) orthosilicate (abbreviated as THEOS). The hydrolysis and condensation reactions of this precursor have been carried out in basic pH, between pH 8.8 and 11.4. Interestingly, the ethylene glycol released during condensation reactions does not affect significantly the structure of the cubic liquid crystalline phase, which was stable during the sol−gel reactions. As a result, the cubic phase based emulsions could template the formation of meso/macroporous dual materials, which possess interconnected polydisperse macropores, between 1 and 5 μm, and cubic-ordered mesopores, with a narrow pore size distribution around 4 nm. Monoliths with a specific surface area higher than 550 m2 g−1 and bulk density of 0.16 g cm−3 have been obtained.

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INTRODUCTION

one-step methods, has been recognized as a very useful method for controlling morphology of mesoporous materials with ordered pores. Many different surfactants have been used for this purpose, although most of the works focus on block copolymer surfactants.19−21 A more difficult task is the preparation of materials with pore sizes on different length scales, e.g. materials that combine the simultaneous presence of mesopores (between 2 and 50 nm) and macropores (bigger than 50 nm). Several authors have described the use of emulsions as templates, in simple singlestep processes, for the preparation of inorganic materials with dual meso-/macroporous structures.16,22−31 In these methods, sol−gel reactions are carried out in the external phase of the emulsions. Despite the numerous trials to obtain meso/ macroporous dual inorganic materials, only a limited number of examples are known from the literature which described

Monolithic meso/macroporous dual silica materials, with bimodal pore size distribution and well-controlled macroscopic morphology, have attracted a great deal of attention in the past decade because they combine the advantages of high specific surface with the accessible diffusion pathways associated with macroporous structures.1 The dual properties of these materials have stimulated research for applications that include catalytic supports, filters, chromatographic adsorbents, structural materials,2−7 and more recently enzyme-based biodiesel production,8 fuel cells9−11 or energy storage.12,13 However, despite increasing research effort, the synthesis of large meso/ macroporous monoliths, with ordered mesopores, remains a challenge not only because of inherent chemical incompatibilities in the precursor solution and the need for template removal, but also due to shrinkage during drying. Since mesoporous materials, such as hexagonal ordered MCM-41, were discovered by Mobil Corporation scientists in 1992,14,15 surfactant-templated synthetic procedures have been extensively studied.16−21 The use of surfactant self-assemblies, in © 2012 American Chemical Society

Received: September 21, 2012 Revised: November 22, 2012 Published: November 29, 2012 432

dx.doi.org/10.1021/la303801b | Langmuir 2013, 29, 432−440

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Figure 1 shows the presence of a discontinuous cubic liquid crystal in water (I1 phase).

successful preparation of large monoliths with macropores and ordered mesopores.16,23,24,29 The main challenges are to achieve long-range periodic order of the mesopores,23 and to obtain monoliths, with a length of several centimeters and controlled shape,16,23,24,29 since powder materials may have limitations in certain applications.24 In addition, for industrial production, it is also very important to obtain such materials by simple and cost-effective methods. In this context, highly concentrated emulsions, also referred in the literature as high-internal-phase ratio emulsions (HIPRE or HIPE), represent a promising template for the preparation of meso/macroporous dual materials.32,33 For example, it is an attractive alternative to the method consisting in phase separation of silica sols induced in poly(ethylene oxide) or methanol solutions.34−36 Highly concentrated emulsions are characterized by an internal phase volume fraction larger than 0.74, which is the maximum packing of monodisperse spherical droplets.37,38 Consequently, these emulsions have a compact foamlike structure, which consist in deformed and/or polydispersed droplets, separated by a thin film of continuous phase.39,40 As a result, highly concentrated emulsions have a viscoelastic rheological behavior, that provides a gel appearance. The continuous phase of highly concentrated emulsions can possess a nanostructure (e.g., a microemulsion39,40 or a liquid crystalline phase41) which could template the formation of ordered mesopores. Therefore, highly concentrated emulsions can be regarded as dual systems: micrometer-sized droplets surrounded by nanometer-scale aggregates. The use of emulsion droplets as templates for macroporosity22 rather than more rigid structure-directing agents such as polystyrene spheres42,43 is a key element for the preparation of large monoliths because they are deformable and macroscopic samples are able to accommodate stresses that arise during gelation and shrinkage.44 In a previous study, the preparation of dual meso/ macroporous materials, with ordered mesopores and wellcontrolled macroscopic morphology, was achieved by using highly concentrated emulsions as templates, in a two-step process. First, styrene was polymerized in the continuous phase of the emulsions, to obtain polystyrene macroporous foams,45 which were later impregnated with sol−gel precursor solutions.46 Block copolymer surfactants were added to the inorganic precursor solutions to direct the formation of ordered mesophases during inorganic oxide gelation, which was induced by ethanol evaporation.46 However, this method has the drawback of complexity and time-consuming preparation. In a more recent work, we described the use of highly concentrated emulsions for the synthesis of meso/macroporous silica materials in a simple one-step method.47 The materials exhibit a porous network structure with a bimodal pore size distribution consisting of macropores in the range between 200 and 800 nm and mesopores of 4 nm.47 Tetraethyl orthosilicate (TEOS) was used as silica precursor and a polyoxyethylene alkyl ether surfactant (C12(EO)8) as structure-directing agent. This surfactant allowed the formation of a cubic liquid crystalline phase in aqueous solution at concentrations between 35 and 45 wt %. Nevertheless, the cubic phase was not stable in the presence of ethanol released during the alkoxysilane hydrolysis, and consequently, no ordered mesopores were obtained. Highly concentrated emulsions, with a continuous phase consisting of cubic liquid crystals, were described in detail by Kunieda and co-workers.48 The phase diagram reproduced in

Figure 1. Phase diagram of water/C12(EO)25/decane ternary system at 25 °C. Wm is an aqueous micellar solution phase, I1 is a discontinuous micellar cubic phase, O is an excess-oil phase, and S is a solid phase. (Reproduced with permission from ref 48. Copyright 2000 by Academic Press.)

Highly hydrophilic nonionic surfactants, such as C12(EO)25, tend to form this kind of cubic phases, in which micelles are packed in a cubic array.48 This I1 cubic phase is highly viscous, ensuring a very good emulsion stability. Therefore, the system displayed in Figure 1 can be very promising as a template for the preparation of porous foams with cubic-ordered mesopores. Surfactant composition can easily be tuned, since the I1 liquid cubic phase appears in a wide range (above 30 wt %) of surfactant concentrations in the C12(EO)25/water binary system. O/W emulsions with a considerable amount of oil (even higher than 90 wt % n-decane) can be prepared in the I1+ O region, obtaining highly concentrated emulsions having an I1 cubic phase as the external phase. The preparation of large monoliths of meso/macroporous silica that display periodic structural order is generally achieved by making use of electrostatic interactions between protonated species at low pH. However, successful synthesis of monolithic meso/macroporous silica, under neutral or basic pH, has not been reported yet. In this regard, the reports by Voegtlin and co-workers49,50 are of considerable interest, since they describe the synthesis of ordered mesoporous silica that provides an improved X-ray diffraction pattern by using nonionic surfactants under moderate basic conditions (pH between 8.8 and 12). The formation of ordered mesopores, in the presence of nonionic surfactants at high pH, can be achieved by a combination of electrostatic and hydrogen bonding interactions, causing the cooperative self-assembly and templating the formation of mesopores,27,51 as described by Pinnavaia and co-workers27 and Stébé and co-workers.25 In this context, the use of a precursors, with faster sol−gel reactions at high pH, could allow to obtain silica materials in basic conditions. In the present work, a novel sol−gel precursor, tetra(2hydroxyethyl) orthosilicate (abbreviated as THEOS),52−54 is used for the preparation of meso/macroporous materials at high pH. O/W highly concentrated emulsions with the glycolmodified silane (THEOS) in the continuous phase have been gelled (by sol−gel hydrolysis and condensation) to produce porous three-dimensional silica oxides. A great advantage of THEOS, compared to classical silica precursors such as tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS), is that the hydrolysis of THEOS produces ethylene 433


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Afterward, the gel samples were extracted from the cylindrical molds, using a syringe plug to eject the samples, since they were very fragile at this stage. Then, they were washed with acetone three times to remove the hexadecane and the surfactant, and three times with water, before drying. The volume of solvent was approximately 40 mL, per each washing. This process may allow removing most of surfactant and hexadecane molecules, as well as hydrophilic substances such as ethylene glycol, electrolytes, and so forth. Due to the fragility of the gel framework, the drying process is a crucial step to prevent the collapse of the porous architecture. For this reason, samples were freeze-dried (in an Alpha 2-4 CDplus CHRIST freeze-drier) in order to remove water while preserving the monolithic shape. This method was already successfully used by Mukai et al.60 to obtain large monoliths with dual macro/mesoporous structures. Finally, the samples were calcined in a furnace (Heron, model 12/ 200) at 500 °C during 6 h, at 2 °C min−1 heating rate. Three highly concentrated emulsions, having different THEOS concentration and pH values, were used for the synthesis of the siliceous meso/ macroporous materials. Compositions are summarized in Table 1.

glycol instead of alcohols. Therefore, cubic liquid crystalline (LC) phases can be stable in larger concentration ranges. It is known that diols and polyols are more compatible with liquid crystalline phases, in comparison to short-chain alcohols, such as ethanol or methanol.55−58 The last two solvents reduce the solvophobic effect, meaning that self-organization of surfactants into micelles is hindered. However, diols and polyols, including ethylene glycol, are much less unfavorable for the formation of micelles.56 Furthermore, in the presence of THEOS, the whole process occurs in a single homogeneous phase since this precursor is completely water-soluble. For all these reasons, THEOS can be a good candidate for the preparation of dual meso/macroporous materials with ordered mesopores, by a true liquid crystal templating mechanism. Herein, the first successful preparation at high pH, of meso/ macroporous large silica monoliths with ordered mesopores, is described. These materials were prepared by a new single-step process, using highly concentrated emulsions stabilized by polyoxyethylene alkyl ether surfactants forming a cubic liquid crystal, in the presence of a novel glycol-modified silica precursor. Small angle X-ray scattering, nitrogen sorption, and electron microscopy have been used for a detailed structural characterization of samples. The results show that high specific surface areas can be obtained and that a minimum concentration of silica precursor is required to obtain meso/ macroporous silica monoliths with very high pore volume and ordered mesopores.

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Table 1. Summary of the Compositions of the Highly Concentrated Emulsions THEOS [wt %]

C12(EO)25 [wt%]

hexadecane [wt %]

aqueous phase pH

1 2 3

3 3 4

10 10 10

80 80 80

8.8 11.4 11.4

Characterization of the Materials. SAXS measurements were performed in a S3-Micro instrument (Hecus X-ray Systems GMBH Graz, Austria) operating with point focalization. The scattering was detected with a linear position sensitive detector OED-50M. The temperature controller was a Peltier device, and samples were placed in cells of Kallebrat film (Kalle GmbH, Austria) and measured at 25 °C for 30 min. Nitrogen adsorption−desorption isotherms were determined using an adsorption porosimeter (AUTOSORB-1, manufactured by QUANTACHROME). Samples were outgassed, at 200 °C for 6 h under vacuum, and weighed prior to sorption experiments. The specific surface area was determined by applying the multipoint BET model61 in a relative pressure range (p/p0) from 0.05 to 0.3. The mesopore size distribution was calculated using the BJH model,62 applied to the desorption curve. The microporosity was assessed by extrapolation according to the t-plot method, as described by de Boer et al.63 The macroporous structure of the calcined materials was observed by scanning electron microscopy (SEM), with two instruments: a Hitachi S-4100 at 20 kV and a Hitachi TM-1000 tabletop at 15 kV. The mesoporous structure was investigated by transmission electron microscopy (TEM) using a Philips CM30 microscope, equipped with a CCD Multiscan Gatan camera and operated at 200 kV. Samples for TEM were prepared by allowing ethanol suspensions of finely divided silicas to evaporate on holey copper grids coated with a carbon film. The mechanical properties were determined in a MT-LQ texture analyzer (Stable Micro Systems, U.K.) equipped with a 50 N load cell. The sample was placed between compression plates, moving at 0.2 mm/s, until a displacement of half the original sample height was reached. The crush strength (kPa) value was obtained from the maximum value of the stress−strain curve, at the end of the initial linear region. The toughness (J/g), defined as the absorbed energy during compression, was determined by measuring the area underneath the curve.

EXPERIMENTAL SECTION

Materials. The nonionic ethoxylated surfactant octaethylene glycol mono-n-dodecyl ether, abbreviated as C12(EO)25, was purchased from Tokyo Chemical Industry (Japan), and it was used without further purification. The O/W emulsions were prepared with hexadecane, purchased from Sigma-Aldrich, and Milli-Q purified water. Tetra(2hydroxyethyl) orthosilicate (abbreviated as THEOS) was synthesized in our laboratory from tetraethyl orthosilicate (TEOS), which was purchased from Merck, following the procedure of Mehrotra and Narain59 via a transesterification reaction of ethylene glycol with tetraethyl orthosilicate in the stoichiometric ratio of 4:1, according to eq 1. Si(OCH 2CH3)4 + 4HOCH 2CH 2OH → Si(OCH 2CH 2OH)4 + 4CH3CH 2OH

formulation

(1)

Ethylene glycol was predried with Na2SO4, and it was subsequently distilled prior to the reaction, since THEOS is very sensitive toward hydrolysis reactions in the presence of water. The reaction requires no additional solvent; however, the synthesis has to be conducted in inert atmosphere avoiding H2O presence.59 Ethanol released was removed during reaction by simple distillation. The hydrolysis and polycondensation of THEOS leads to the synthesis of silica and the release of ethylene glycol. These reactions were carried out at basic pH, in the presence of sodium hydroxide, purchased from Carlo Erba. Synthesis of Meso/Macroporous Silica. In a typical synthesis procedure, highly concentrated emulsions were prepared by slow drop-by-drop addition of hexadecane to the mixture of surfactant, sodium hydroxide aqueous solution, and THEOS, under vigorous stirring. The hexadecane mass fraction was fixed at 0.8, and the surfactant/aqueous solution mass ratio was 1/2. The pH of the surfactant/THEOS/NaOH/H2O mixture was either 8.8 or 11.4. Due to the high viscosity of the cubic liquid crystalline phase, hexadecane was added to the continuous phase at approximately 95 °C, above the melting point of the cubic phase, to facilitate the mixing. The final emulsions were cooled down and then allowed to react for 1 week at 25 °C.

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RESULTS AND DISCUSSION Gelling Properties of THEOS Precursor. Figure 2 presents the gelation kinetics of the THEOS precursor as a function of the pH in aqueous solution. The gelling time, at 434


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For this reason, hexadecane was preferred instead of decane, decreasing Tmax from 107 to 87 °C. Stable highly concentrated cubic phase-based emulsions were obtained with a dispersed phase volume fraction of 0.8. As illustration, an image captured by optical microscopy is available in the Supporting Information (Figure S1), showing the typical aspect of a highly concentrated emulsion. The three different formulations, indicated in Table 1, produced very similar emulsions. The emulsion is polydispersed and polyhedral droplets can be observed, since the volume fraction of the internal phase (hydrocarbon) exceeds the maximum value for a packing of homogeneous spheres, 0.74. The continuous phase, which contains the hydrolyzed precursor, constitutes the thin film between droplets. The emulsions were kept at 25 °C for 1 week, to ensure almost complete sol−gel reactions. Afterward, the gels were extracted and washed, freeze-dried, and calcined, as described in detail in the Experimental Section. Material Characterization at the Macroscopic Length Scale. After freeze-drying and calcination, different materials were recovered depending on THEOS concentration and pH. Photographs of the final monoliths, obtained after calcination, are shown in Figure 3. In the case of formulation 1, prepared at pH 8.8, only a white powder was recovered (Figure 3a). In contrast, formulation 2, in which pH was increased to 11, allowed to obtain a chalky white monolith (Figure 3b). Regarding formulation 3, with the continuous phase containing 4 wt % THEOS and pH 11.4, a white monolith was also obtained (Figure 3c). In this case, the initial shape of the emulsion container was well reproduced, and thus, the container was a true mold, allowing the control of the macroscopic morphology of the monolith. The macroscopic shrinkage was very small, and could not be quantified, indicating that the solvent extraction and drying steps were not detrimental to the macrostructure of the silica monolith. Size (2.6 cm per 1 cm) and weight (0.3280 g) of the monolith, resulting from formulation 3, were used to calculate the approximate bulk density of this material, 0.16 g mL−3. The total pore volume fraction, approximately 92%, was also

Figure 2. Gelation time of a 50/50 mixture (weight ratio) of THEOS/ water as a function of pH, which was controlled with either HCl or NaOH.

room temperature, was visually determined as the point when the solution did not flow after turning the test tubes upsidedown. As expected, the slowest gelation time is observed at pH 2−3 that coincides with the isoelectric point for silica, in which the condensation rate is minimum. It is well-known from literature that the pH affects the gelation time of TEOS and TMOS, due to its impact on hydrolysis and condensation rates.64 The fast gelation of THEOS, found at neutral and basic pH, can be partly explained by the much higher water miscibility of the ethylene-glycol-modified derivatives when compared to TEOS. The gel formation reactions are much faster above pH = 8, and therefore, the experiments were carried out between pH 8 and 12. Preparation and Properties of Highly Concentrated Emulsions in the Water/C12EO25/Hexadecane System. Highly concentrated emulsions were prepared, as previously described by Kunieda and co-workers48 in the water/ C12(EO)25/decane system. The extremely high viscosity of the cubic phase provided very good stability to the emulsion, preventing creaming and coalescence. Kunieda and co-workers described that the oil has an effect on the maximum melting temperature of the cubic phase (Tmax) as long as it is incorporated in the micelles forming the cubic structure.65 The Tmax decreases by increasing the molar volume of the oil.48

Figure 3. Photographs (top) and SEM micrographs (bottom) of the final calcined silicas, obtained from the three formulations. The coin, with a diameter of 2.3 cm, is shown to illustrate the monolith size. Scale bars are shown in the SEM images, indicating 20 μm. 435


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calculated assuming that the density for nonporous amorphous silica is 2.2 g mL−3. Therefore, this material possesses a very high porosity. The mechanical strength was evaluated for the material synthesized from formulation 3, which was tested under compression to obtain the corresponding stress−strain curve. Despite the material being robust enough to resist calcination, the crush strength and the toughness were small, with values of 20 kPa and 1.6 × 10−2 J/g, respectively. Probably, these low values are due to the extremely high pore volume and thin silica walls. The mechanical properties (compression tests) of similar materials (silica with around 90% pore volume) have not been found in the literature, and therefore, no comparison can be made. It should be pointed out that the meso/macroporous materials were obtained by hydrolysis−condensation reactions at high pH, which is uncommon for conventional alkoxysilanes as precursors. The sol−gel reactions are complex, with many parameters affecting the interactions between the polymerizable siliceous precursor and the surfactant as structure-directing agent. In the case of the present glycol-modified silane, the hydrolysis reactions are very fast at high pH, and consequently, network formation relies on the rate of condensation only.49 As a result, macroscopic phase separation, in the form of particle precipitation, is inhibited due to rapid gel formation. Another advantage of using THEOS at high pH is that gelation may occur by the union of larger and highly cross-linked silica clusters,64 allowing to reduce the shrinkage of the material when drying.49 The SEM micrographs of the three calcined samples are also presented in Figure 3, revealing the macroporous texture of the materials. A close observation of these SEM images allows the estimation of the average cell size, which is roughly between 1 and 5 μm. This is consistent with the droplet size of the highly concentrated emulsions used as templates (Supporting Information, Figure S1). Depending on both the pH and the precursor concentration, it can be observed that macroporosity is dramatically affected. At pH 11.4, at 3 and 4 wt % THEOS concentrations, complete mineralization of the continuous aqueous phase occurred, preserving the typical texture of highly concentrated emulsions, with polydisperse and interconnected macropores (Figure 3b and c). In contrast, at pH 8.8 and 3 wt % THEOS, silica condensation occurred mainly at the oil−water interface, leading to a “hollow spheres” looking texture (Figure 3a), with thin macropore walls. In this case, large monoliths were not obtained, and a thick powder (consisting of particles with sizes in the order of milimeters) was formed, since the thin macropore walls could not resist calcination. It is likely that the material may not have enough cohesive strength, due to such thin pore walls. Characterization at the Mesoscopic Length Scale. The presence of the I1 cubic liquid crystal in the continuous phase of the highly concentrated emulsion was confirmed by small angle X-ray scattering (SAXS), as presented in Figure 4a. The spectra of the emulsion showed four well-defined Bragg diffraction peaks, which appeared at ratios (q values) approximately equal to 1, 1/√2, 1/√3, and √16/√3. These can be associated to the Miller indices (110), (200), (211), and (220), corresponding to a body-centered cubic phase (Im3m space group), as shown in Figure 4b. Interestingly, the presence of ethylene glycol (a maximum of 3.6 wt %, assuming 100% conversion for 4 wt % THEOS) did

Figure 4. (a) SAXS diffraction spectra, of formulation 2 (3 wt % THEOS and pH 11.4). The graph shows the spectra of the initial highly concentrated emulsion, observed at zero time, the gelled sample kept 1 week at 25 °C after THEOS addition, and the final silica monolith obtained after freeze-drying and calcining. (b) Crystallographic assignations of calcined silicas obtained from formulations 1 and 2, where h, k, and l are the Miller indices. The results of the initial emulsion (indicated as HIPE) are also shown for comparison. Silica 3 is not plotted, since diffraction peaks were not observed in its SAXS spectra.

not greatly modify the phase behavior of the continuous phase and the liquid cubic crystalline microstructure was preserved. The evolution of the mesostructure, for formulation 2, was studied in more detail, since it produced the highest degree of mesopore ordering. Figure 4a shows the changes in the X-ray spectra during material formation. It should be pointed out that the peak intensity decreased, which could be attributed to a reduction in the number of repetitions within the ordered domains. Consequently, the release of ethylene glycol could produce a decrease in the degree of ordering. However, no shift in the position of diffraction peaks was observed during THEOS addition and calcination. This result may indicate that the addition of THEOS, and its gelation did not greatly distort the liquid crystalline structure, since the distance of the unit cubic cell did not vary. Therefore, one could conclude that the formation of the mesostructured domains in the final material proceeds via a mechanism of true liquid crystal templating. The crystallographic assignation of the SAXS peaks of the two calcined samples, which may possess ordered mesopores (formulations 1 and 2), is shown in Figure 4b. It was possible to 436


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could be explained by the thin walls between macropores, which do not allow large repetition domains of mesopores. Regarding the silica sample obtained from formulation 3, prepared with 4 wt % THEOS at pH 11.4, no sharp diffraction Bragg’s peaks could be observed in the X-ray spectra. It could be explained by destabilization of the cubic liquid crystal, induced by ethylene glycol release, due to penetration of polyol molecules into the surfactant palisade layer. Such an effect has already been described in the literature.55,56 This penetration induces changes in curvature and flexibility of the surfactant layer that cause the I1 phase to turn into Wm. The mesopore morphology was observed by transmission electron microscopy, to confirm the mesopore order. Figure 6 shows the TEM images of silicas synthesized in the three formulations. The images provide the evidence of narrow pore size distributions. At the lower pH (Figure 6a), most of the sample presents a wormhole-like mesoporous structure; however, a close observation of the micrograph shows the presence of small domains with some mesopore order. Using formulation 2, more organized mesopores were obtained (Figure 6b). The Fourier transform confirmed the periodicity in this TEM image. However, it should be pointed out that it is difficult, by TEM, to observe the symmetry of the ordered lattice, due to possible imperfect alignment with respect to the incident electron beam. However, the symmetry is unambiguously established by the X-ray diffraction spectra of the material, which appeared body-centered cubic (Im3m space group), as discussed above. Given that TEM images only show a very small region of the entire sample, many pictures were obtained, comparing different regions of the sample prepared using formulation 2. Similar images to that displayed in Figure 6b were also observed, and it could be concluded that mesopore organization occurs in the entire sample. Several TEM images are available in the Supporting Information (Figure S2). Regarding the micrograph of the monolith from formulation 3 (Figure 6c), it shows a complete wormhole-like disordered structure, with the absence of any long-range mesopore order. Several authors have tried to obtain ordered meso/ macroporous dual materials at neutral or basic pH but failed to produce materials with ordered mesopores. In previous works, TEOS has been used as silica precursor and ethanol release can explain the disordering of mesopores. However, ethylene glycol is produced by hydrolysis of THEOS, affecting much less the liquid crystalline structures. Another reason for the formation of relatively ordered mesostructures, using THEOS at basic pH, is probably the very fast hydrolysis reaction of the silica species, and thus, network formation depends mainly on the rate of condensation. It could be postulated that increasing the pH resulted in faster rate of silica condensation, providing shorter time for mesophase evolution, and therefore, the cubic structure of the initial emulsion is quickly “frozen”, with less probabilities for disorganization of the structures. Therefore, mesoscopic and macroscopic phase separation, in the form of precipitation, could be inhibited due to the rapid gel formation. However, it should be pointed out that the mechanism of the silica condensation is not clear at such a basic pH. Voegtlin et al. suggested a combination of electrostatic and hydrogen bonding interactions causing the cooperative self-assembly and templating the formation of mesopores.49

calculate the cubic cell size as the inverse of the slope when plotting 1/d, as a function of (h2 + k2 + l2)0.5, where d is the dspacing and h, k, and l are the Miller indices. The crystallographic assignation of the initial highly concentrated emulsion (indicated as HIPE) was also plotted for comparison. The results of the lineal fits are displayed in Table 2, assuming (110), (200), (211), and (220) Miller indices, which Table 2. Results of Linear Fit, Plotting 1/d as a Function of (h2 + k2 + l2)0.5, Where d is the d-Spacing and h, k, and l are the Miller Indicesa formulation

slope [nm−1]

R2

d [nm]

HIPE 1 2

0.110 0.145 0.111

0.9985 0.9999 0.9999

9.1 6.9 9.0

a

The slope, the regression coefficient (R2) and the size of the cubic cell (d) are shown, assuming an Im3m cell type.

correspond to a cubic Im3m space group. The regression coefficients are rather good for silicas 1 and 2, confirming the presence of a cubic symmetry, formed by the ordering of mesopores. In formulation 1, prepared at pH 8.8, the cubic cell size greatly decreased from 9.1 nm (in the emulsion) to 6.9 nm (in the final silica), as indicated in Table 2. Interestingly, it was not the case for the sample obtained from formulation 2 (pH 11.4). In this system, there was a very good control of the mesopore structure, with almost no shrinkage of the cubic cell during reactions and calcination, and the cubic cell size remained approximately constant at ≈9.0 nm. Figure 5 shows the SAXS spectra of the 3 calcined materials.

Figure 5. SAXS spectra of the calcined materials. For clarity, an expanded SAXS profile of the silica sample obtained from formulation 2 (with 3 wt % THEOS, pH 11.4) is shown in the inset. Arrows indicate the high order peaks.

All spectra show a peak at 0.88 nm−1, a strong indication of the presence of mesoporosity. This peak is quite narrow for formulations 1 and 2, which contain 3% (w/w) THEOS, indicating a certain degree of long-range ordering. Furthermore, in the case of formulation 2 (with 3 wt % THEOS and pH 11.4), the SAXS spectrum clearly shows a sequence of three peaks, corresponding to the Im3m body-centered cubic phase. The low intensity of the diffraction peaks (inset in Figure 5) 437


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Figure 6. TEM images of silica prepared with (a) 3 wt % THEOS at pH 8.8, (b) 3 wt % THEOS at pH 11.4, and (c) 4 wt % THEOS at pH 11.4. The insets show the Fourier transforms.

4 wt % decreases the molar ratio surfactant/SiO2 to 0.52 and also increases the amount of glycol released during hydrolysis of the precursor, which may not allow obtaining ordered mesostructures. Nevertheless, monoliths with well-controlled shape and size, which are robust enough to resist calcination, are obtained at such surfactant/SiO2 ratio. In any case, the use of pH 11 may be useful in industrial synthesis because of easy handling and chemical compatibility with metals. The resulting materials, with dual meso- and macroporous structures, may have promising applications as adsorbents and supports for catalysts.

The nitrogen adsorption−desorption isotherms, for formulation 2, shown in Figure 7, are consistent with the presence

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CONCLUSIONS Using a simple single-step method, we successfully prepared monolithic silica with dual meso/macroporosity and controlled macroscopic size and shape. Highly concentrated emulsion droplets template the formation of macropores, whereas the Im3m external cubic phase controls the mesoporosity via a true liquid crystal templating mechanism, producing ordered mesopores and negligible presence of micropores. This is the first report of synthesis of such dual meso/macroporous silica monoliths using nonionic surfactants at basic pH. Monoliths with a specific surface area higher than 550 m2 g−1, bulk density of 0.16 g cm−3, and total pore volume higher than 90% were obtained. This achievement has been possible using a novel ethylene-glycol-modified silane precursor, which takes advantage of the good compatibility of ethylene glycol with lyotropic cubic liquid crystalline phases, up to a certain concentration.

Figure 7. N2 adsorption and desorption isotherms, and BJH pore-size distribution (inset) of a silica material synthesized with 3 wt % THEOS at pH 11.4 (formulation 2).

of mesopores with homogeneous size. The figure shows the expected type IV adsorption isotherm with a H1 hysteresis loop (IUPAC notation).66 The BET surface area is 576 m2 g−1, and the most frequent pore size (peak in the BJH pore size distribution, shown as inset in Figure 7) is approximately 4 nm, consistent with X-rays data. The SAXS spectra of this material showed a cubic cell distance of 9 nm, and therefore, it can be deduced that the silica wall thickness between mesopores is around 5 nm. This distance is also consistent with qualitative observations by TEM (Figure S2e, Supporting Information). The H1-type hysteresis loop is typical of mesoporous materials displaying interconnected pores with open-ended cylindrical mesopores.66 The t-plot (not presented here) shows a straight line passing through the origin, meaning that the sample has a negligible presence of micropores. As a conclusion, it is interesting to remark that dual meso/ macroporous monoliths can be prepared with homogeneous and ordered mesopores, by a simple method, at high pH (11.4). The results have revealed that well-organized mesoporosity is formed with 3 wt % of THEOS, corresponding to a molar ratio surfactant/SiO2 of 0.7. Increasing the THEOS concentration to

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ASSOCIATED CONTENT

S Supporting Information *

Images by optical microcopy and TEM. An example of a highly concentrated emulsion, in the water/C12(EO)5/hexadecane system with 10 wt % surfactant and 80 wt % hexadecane, observed by optical microscopy (Figure S1). TEM images of a silica material prepared with 3 wt % THEOS at pH 11.4 (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 438


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ACKNOWLEDGMENTS The authors acknowledge the Spanish Ministry of Economy and Competitiveness for the CTQ2011-23842 project and Generalitat de Catalunya for the 2009SGR961 grant.

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