Micro-, Meso-, and Macroporous Materials Obtained ... - CSJ Journals

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Sep 22, 2012 - drops in the dispersed phase acting as templates for macropores .... 12 V. Meynen, P. Cool, E. F. Vansant, Microporous Mesoporous · Mater.
doi:10.1246/cl.2012.1041 Published on the web September 22, 2012

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Micro-, Meso-, and Macroporous Materials Obtained from a Highly Concentrated Emulsion of Decane/Brij 35/Water and Decane/Brij 700/Water Esther Santamaría, Marta Cortés, Alicia Maestro, Montserrat Porras, Jose Maria Gutiérrez, and Carmen González* Department of Chemical Engineering, Faculty of Chemistry, University of Barcelona, Barcelona 08028, Martí i Franqués 1-11, Spain (Received May 14, 2012; CL-120490; E-mail: [email protected]) Mesoporous materials have high potential for use as catalyst supports. Different formation mechanisms for mesostructured materials have already been described. In the present study, different micro-, meso-, and macroporous materials were obtained from highly concentrated emulsions, and the possibility of raw materials recovery was investigated. The materials, including that synthesized from a recovered surfactant, were tested as Nafion catalyst supports. Moreover, ordered mesoporous materials were obtained through another formation mechanism, namely, cooperative autoassembling, and the influence of the preparation and composition variables on the specific area of the materials was studied. A number of applications for mesoporous materials have already been described in the literature, such as catalyst supports, and adsorption and encapsulation of proteins or biomolecules. Some of these applications require a combination of mesopores (e.g., those described for mesostructured materials such as MCM-411,2) and macropores that improve the diffusion of reactants and products (favoring mass transfer while reducing transport limitations). Such a combination of different pore sizes may be obtained from highly concentrated emulsions, with drops in the dispersed phase acting as templates for macropores and structures afforded by surfactant molecules acting as templates for mesopores. According to IUPAC,3 materials are considered as microporous when their pore sizes are less than 2 nm; mesoporous, 2­50 nm; and macroporous, bigger than 50 nm. The obtaining of meso- and macroporous3­7 materials has already been reported, but the obtaining of micro-, meso-, and macroporous materials has been barely studied.8 In order to obtain mesostructured materials, two formation mechanisms have been employed. The first one involves the use of a liquid crystal template (LCT)3,9 in which an inorganic precursor is added over a liquid crystal. The precursor polymerizes around the liquid crystal; thus, when the surfactant is removed, the liquid crystal structure becomes the net of the pores structure. The second mechanism is cooperative self-assembly (CSA)10­12 in which an inorganic precursor is added to a micellar solution and both interact to from the mesostructured material. In the present study, different micro-, meso-, and macroporous materials are obtained from two different surfactants. Because of the high formation cost of materials when an LCT is used, the possibility of raw materials recovery and their reuse is studied. Mesostructured materials are formed via the CSA mechanism, and the influence of preparation and composition variables is studied. Brij 35 (C12EO23), Brij 700 (C18EO100), a sodium silicate solution (26.50% SiO2), and a Nafion polymer were supplied by Sigma Aldrich. Decane was used as the dispersed phase and was

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purchased from Quimidroga. Tetraethyl orthosilicate (TEOS) was used as the silica source and was supplied by Sigma Aldrich. Ethanol (96% v/v) and HCl (37%) were purchased from Panreac. Deionized water was used in all the samples. All chemicals were used without further purification. Small-angle X-ray diffraction scattering (SAXS) measurements were performed to determine the type of the liquid crystal phase. Measurements were performed using a small-angle X-ray camera (Hecus X-ray Systems GmbH Graz) equipped with a Siemens Kristalloflex 760 (K-760) generator. The temperature of the samples was controlled using a Peltier Anton Paar (25­ 300 °C) controller. The radiation wavelength was 1.54 nm. In this study, scanning electron microscopy (SEM) (Hitachi S-4100 microscope; operating voltage: 15 keV; carbon-coated samples were used) was used to observe the morphology of the samples. Furthermore, the samples were examined using transmission electron microscopy (TEM) (JEOL JEM-2100 microscope; acceleration voltage: 200 kV). In order to prepare samples for TEM analysis, the samples were suspended in ethanol by sonicating them for 5 min. The suspension was dropped onto a copper grid coated with a carbon film and dried at room temperature. The specific surface areas of the samples were determined by N2 sorption analysis performed using a Micromeritics TriStar 300 instrument at ¹196 °C. Prior to each measurement, the samples were degassed at 120 °C for 6 h. The specific surface areas were estimated by the BET method. The pore diameter and pore size distribution were determined from the desorption branch of the isotherm by the BJH method.13 The pore volume was reported as mesopore (2­50 nm) and macropore (>50 nm) volume. The material was obtained from an emulsion (O/W), in which the continuous phase comprised a HCl solution (10%):Brij 35, 0.70:0.30. The continuous phase (W) was introduced in a jacketed vessel at 70 °C. Decane (O) was added at a constant addition speed (q = 7.5 mL min¹1) and an agitation speed (N) of 1150 rpm until reaching 80% of the dispersed phase (decane). A strong acidic medium was needed in order to hydrolyze the TEOS; therefore, HCl was chosen because it does not break the liquid crystal structure. Van Grieken et al.14 reported lower the pH, higher the specific surface area obtained, and narrower the distribution of the material. After it was obtained, the emulsion was added dropwise to the material precursor and TEOS in a TEOS:water ratio of 1:6; the precursor polymerized around the decane droplets forming the SiO2 structure. The emulsion acted as the template to obtain the macropores, and the emulsion droplets (decane) were removed through calcination. The mesopores were formed by the surfactant liquid crystal structure.

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(b)

Figure 1. Phase behavior of (a) the C12EO23/water system and (b) C18EO100/water. V1 indicates a micellar cubic phase, Wm a micellar solution, Om a inverse micellar solution, S a solidpresent region. As the first step, it is mandatory to condition the support; thus, for ensuring that the support has no water, it is washed with methanol. It is then dried in an oven for 2 h at 90 °C and then left overnight in a vacuum oven at 110 °C and 10 mbar. During the soaking, the active phase of the catalyst settles on the support. In order to obtain it, the material was mixed with Nafion (13%) in an isopropanol and water mixture. The resultant mixture was mixed for 20 h and then evaporated. The material was dried overnight in an oven. A wide bicontinuous cubic liquid crystal phase region (V1) was found in the C12EO23/water and C18EO100/water systems (Figure 1). This behavior is typical of very hydrophilic surfactants and amphiphilic polymers.15,16 The SAXS analysis confirmed bicontinuous structures Pn3m for the system with Brij 35 and Im3m for the system with Brij 700. The obtained material was characterized through SEM in order to observe the presence of macropores (Figure 2). The specific area of the material was obtained through the BET technique. It was observed that in the material formed from Brij 700, the diameter of the macropores was approximately double that of the ones obtained from Brij 35 (this finding is consistent with the results of a previous study on emulsions characterization). Both materials provide a high density of macropores due to the highly concentrated emulsion. TEM images clearly show that an ordered structure was obtained in none of the cases. The adsorption­desorption isotherms show a shoulder shape in the first part that indicates the presence of micropores. The central part shows a typical hysteresis loop corresponding to the mesoporous materials. In both materials, the size of the mesopores is around 4 nm. The isotherm for the material obtained from Brij 700 showed an upward trend in the final part that indicates the presence of macropores, though the macroporosity of the samples was determined from the SEM images. The BET areas obtained are 907 and 670 m2 g¹1 for the materials obtained from Brij 35 and Brij 700, respectively. In order to develop a more economical process with high viability in the chemical industry, the recovery of a part of the used raw materials has been studied. In the present study, EtOH used in washing and the surfactant extracted with it were recovered. The recovered ethanol was analyzed through gas chromatography, revealing a purity of 95.06%. Furthermore, the residual surfactant was also analyzed through Raman spectroscopy (Figure 3) to identify whether the recovered residue corresponded to the surfactant specter. In the figure, both specters are shown and it is easy to determine that both samples are very similar. Thus, the recovered residue was

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(a)

(b)

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Figure 2. (a, b) TEM and (c, d) SEM images at different magnifications and (e, f) N2 adsorption­desorption isotherms for (a, c, e) Brij 35 and (b, d, f) Brij 700.

Figure 3. Raman spectra of the commercial Brij 35 (black lines) and for the recovered Brij 35 (blue line). almost identical to a fresh commercial surfactant. 76.0% of the ethanol and 77.9% of the surfactant used initially in the first experiment were recovered. Figure 4 shows an SEM image of the material synthesized with recovered Brij 35. In this figure, macropores could be clearly observed, and the shape of the adsorption­desorption isotherm is the same as that of the isotherm for fresh materials. The obtained area is 630 m2 g¹1; this is slightly lower than the area for the nonrecovered material. The material was soaked as shown in the diagram in Figure 1. The results were compared with those for soaking on commercial MCM-41. 5.9% of MCM-41 was soaked while for

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Figure 6. Influence of the composition variables in the specific area of the mesostructured material. Figure 4. SEM image and N2 adsorption­desorption isotherm with corresponding pore size distribution of a micro­meso­ macroporous recovered material.

Figure 7. Influence of the preparation variables in the specific area of the mesostructured material. Financial support from CYCYT No. CTQ2011-29336-C0302 is gratefully acknowledged. Figure 5. TEM image confirming the ordered structure of the porous and the typical mesoporous material isotherm. The composition of the sample was silicate solution 8.8 g/HCl 9.7 g/ Water/57.7 g/Brij 700 3 g.

Paper based on a presentation made at the International Association of Colloid and Interface Scientists, Conference (IACIS2012), Sendai, Japan, May 13­18, 2012.

the micro-, meso-, and macroporous materials, 10.4% soaking was observed in the material obtained from Brij 35, 11.6% soaking was observed in the material obtained from Brij 700, and 9.5% soaking was observed in the material obtained from the recovered raw materials. Ordered mesoporous materials were obtained using the CSA method. Their order was determined through TEM (Figure 5); the isotherms show the typical shape of the mesoporous material. Moreover, the influence of the composition (Figure 6) and preparation variables (Figure 7) on the specific area of the samples was also studied. It was shown that among the composition variables, the amount of silicate is very important. The preparation variables (a minimum of 12 h in the oven at 80 °C) did not show a significant influence because otherwise the mesopores did not adopt an ordered structure. Micro-, meso-, and macroporous materials were obtained from highly concentrated emulsions via the LCT method. Raw materials were recovered and reused to obtain a material with characteristics similar to those of fresh raw materials. When soaked with a Nafion polymer, the obtained materials showed a soaking capability twice that of commercial MCM-41 because of the presence of macropores. The mesoporous materials obtained using the CSA method had ordered pores, and through the use of emulsions, they open the possibility of obtaining a structured meso- and macroporous material. This is one of the investigations that will be pursued in the future. The mesostructured materials did not show significant differences in their properties when the preparation variables changed. However, the composition variables influenced the specific area of the obtained material.

References 1 Z.-Y. Yuan, B.-L. Su, J. Mater. Chem. 2006, 16, 663. 2 S.-H. Kim, C.-K. Shin, C.-H. Ahn, G.-J. Kim, J. Porous Mater. 2006, 13, 201. 3 G. J. de A. A. Soler-Illia, C. Sanchez, B. Lebeau, J. Patarin, Chem. Rev. 2002, 102, 4093. 4 S.-T. Wang, M.-L. Chen, Y.-Q. Feng, Microporous Mesoporous Mater. 2012, 151, 250. 5 A. Khaleel, S. Al-Mansouri, Colloids Surf., A 2010, 369, 272. 6 G.-S. Shao, L. Liu, T.-Y. Ma, F.-Y. Wang, T.-Z. Ren, Z.-Y. Yuan, Chem. Eng. J. 2010, 160, 370. 7 A. Lemaire, B.-L. Su, Microporous Mesoporous Mater. 2011, 142, 70. 8 A. Vantomme, A. Léonard, Z.-Y. Yuan, B.-L. Su, Colloids Surf., A 2007, 300, 70. 9 G. S. Attard, J. C. Glyde, C. G. Göltner, Nature 1995, 378, 366. 10 F. Zhang, Y. Meng, D. Gu, Y. Yan, Z. Chen, B. Tu, D. Zhao, Chem. Mater. 2006, 18, 5279. 11 Y. Wan, D. Zhao, Chem. Rev. 2007, 107, 2821. 12 V. Meynen, P. Cool, E. F. Vansant, Microporous Mesoporous Mater. 2009, 125, 170. 13 E. P. Barrett, L. G. Joyner, P. P. Halenda, J. Am. Chem. Soc. 1951, 73, 373. 14 R. van Grieken, J. A. Melero, G. Calleja, An. R. Soc. Esp. Fis. Quim. 2003, 31. 15 K. Shigeta, U. Olsson, H. Kunieda, Langmuir 2001, 17, 4717. 16 N. Kanei, K.-i. Watanabe, H. Kunieda, J. Oleo Sci. 2003, 52, 607.

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