Sol-Gel Formation of Reticular Methyl-Silicate Materials by Hydrogen Peroxide Decomposition

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Journal of Sol-Gel Science and Technology 13, 189–193 (1998) c 1998 Kluwer Academic Publishers. Manufactured in The Netherlands. °

Sol-Gel Formation of Reticular Methyl-Silicate Materials by Hydrogen Peroxide Decomposition J. GUN AND O. LEV Fredy and Nadine Herrmann School of Applied Science, The Hebrew University of Jerusalem, Jerusalem 91904, Israel O. REGEV AND S. PEVZNER Chemical Engineering Department, Ben Gurion University of Negev, Beer Sheva 84105, Israel A. KUCERNAK Department of Chemistry, Imperial College of Science, Technology and Medicine, London, SW7 2AZ, UK

Abstract. A new method for the formation of reticular silicate and organically modified silicate is introduced. Monoliths were prepared by incorporating a few percent hydrogen peroxide in the sol-gel starting solution. For example, incorporation of 6–10% (v) hydrogen peroxide in base catalyzed sol-gel precursors of methyl-Ormosil yielded macroporous monoliths with a bi-modal pore size distribution. The average characteristic pore diameters were approximately 1.2 nm and 0.7 µm, depending on the sol-gel precursors used and the preparation protocol. The specific surface area was approximately 160 m2 /g, contributed mainly by the microporous structure. A similar preparation procedure without hydrogen peroxide yielded only fractured or powdery materials. Presumably, the decomposition of the hydrogen peroxide yielded microbubbles, which formed templates for the polycondensation reaction. SEM, nitrogen adsorption isotherms and small angle X-ray spectroscopy were used to characterize the reticular materials. Keywords:

chromatography, foams, macroreticular

Introduction There is considerable scientific and technological interest in macroreticular ceramic materials, which exhibit a bi-modal pore size distribution. Such materials benefit from the large specific surface area contributed by the micro- or mesopores and from the high accessibility and permeability contributed by the macroporous structure. These materials are especially useful as supports for chemical catalysts and for separation and chromatographic applications. Additionally, the large void fraction of such materials results in low bulk density and low thermal expansion coefficient, which are favorable for dielectric materials.

The most useful method of preparation of such materials involves impregnation of organic polymer sponges (e.g., poly(urethane), poly(styrene) or latex) with a slurry containing the ceramic precursors [1]. Macroreticular or foamy materials are formed after drying, sintering and evaporation or thermal degradation of the organic material. Another synthesis pathway includes the production of inorganic or organically modified aerogels by freeze-drying or supercritical drying of sol-gel derived metal oxide gels [2]. These methods are quite expensive and involve high pressure or extreme temperature conditions. The preparation of chromatographic media in the form of monoliths or thick films, suitable for thin layer chromatography,

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is a newly developed area and several teams are pursuing sol-gel methodologies for preparation of such media [3–5]. The ability to produce organically modified surfaces (Ormosils) by single step sol-gel processing is especially attractive for the formation of reversed phase chromatographic media. Thick macroporous Ormosil films were recently prepared [4] by incorporation of cluster-forming materials along with the precursors of sol-gel films. After film formation and leaching the organic compounds out of the film a macroporous thick film (ca. 10–60 µm), suitable for thin layer chromatography was produced. Nakanishi and co-workers introduced sol-gel derived monoliths for high pressure liquid chromatography [5]. These were prepared by incorporation of poly(ethylenglycol) during the sol-gel process and subsequent derivatization of the macroporous xerogels with alkyltrimethoxysilane [6]. This paper introduces, for the first time, a one-step preparation method for the production of organically modified ceramic monoliths with a bi-modal pore size distribution. The material is prepared by a base catalyzed sol-gel process in the presence of hydrogen peroxide. Decomposition of the hydrogen peroxide

forms miniature bubbles, which serve as templates for the inorganic polymerization. The characteristics of these materials are presented in the following sections. Experimental Details Four type of materials were compared: A, base catalyzed methyl silicate; B, base catalyzed methyl silicate prepared in the presence of hydrogen peroxide; C, acid catalyzed methyl silicate; D, acid catalyzed methyl silicate prepared in the presence of hydrogen peroxide. Preparation conditions of base catalyzed methyl silicate in the presence of hydrogen peroxide (Sample B) are as follows: 2.0 ml methyl trimethoxysilane, 2.0 ml methanol and 0.10 ml 3M NaOH catalyst, were mixed, then 2 ml hydrogen peroxide was added, properly mixed and allowed to dry to reach a constant weight (ca. 1.0 g) at 41◦ C for approximately one week. The same preparation protocol was used for the preparation of sample A but with addition of distilled water instead of hydrogen peroxide. Preparation conditions of the acid catalyzed methyl silicate in the presence of hydrogen peroxide

Figure 1. Base catalyzed sol-gel monoliths prepared with hydrogen peroxide (Sample B) and fractured material prepared under similar conditions but without hydrogen peroxide (Sample A).

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Reticular Methyl-Silicate Materials

Figure 2.

191

SEM micrographs of sample A and sample B (bar = 1 µm).

(Sample D) were similar to that of sample B with one exception: 0.1 ml of concentrated HCl was added instead of NaOH. The same preparation protocol was used for the preparation of sample C but with addition of distilled water instead of hydrogen peroxide.

Results and Discussion Table 1 summarizes the composition and pertinent physical parameters of the four types of materials (A–D) described above. Figure 1 presents photographs

of typical products of base catalyzed xerogels, that were prepared with and without hydrogen peroxide addition (Samples B and A, respectively). Large monoliths (e.g., 20 cm long × 0.5 cm in diameter rods) that almost did not shrink or crack during sol-gel processing could be prepared in the presence of hydrogen peroxide. The material maintained the original dimension of the reaction beaker and did not shrink during aging. In contrast, sample A was highly fractured during the drying step. The materials produced by acid catalysis were rather similar in appearance to that of sample A and therefore are not shown in Fig. 1. However, these

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Table 1.

Physical characteristics of samples A–D. A

Sample Composition (Si : H2 O : H2 O2 ) (molar)

B

C

Basic/H2 O

Basic/H2 O2

Acid/H2 O

Acid/H2 O2

1:8:0

1 : 5 : 2.5

1:8:0

1 : 5 : 2.5

Bulk density (g/cm3 )

0.6

0.22

0.34

0.31

Skeletal density (g/cm3 )

2.2

2.1

2.2

2.1

0.90

0.85

0.85

1.7

6.4

Void fraction Surface area (BET) (m2 /g)

Figure 3.

D

0.73 90

158

Radius of gyration (nm)

1.7

1.9

2.0

2.1

Fractal mass dimension

2.3

2.5

2

2

Average macroparticle size (µm)

—

0.7

3

3

Average micropore size (nm)

0.9

1.1

1.2

1.1

Pore size distribution, based on nitrogen adsorption isotherm of sample A and B.

materials shrank to approximately 85% of the dimension of the casting beaker. Figure 2 depicts SEM micrographs of samples A and B. It is clear that the macrostructure of the two materials is very different. Sample B is comprised of approximately 0.7 µm spheres that often agglomerate to form tubular structures or more complex networks. Sample A exhibits a rather featureless structure, at least on the 0.01–100 µm scale that can be discerned by SEM. The microporous structures of all four materials were examined by nitrogen adsorption isotherms and small angle X-ray scattering (SAXS). The average pore size of the two samples A and B were comparable (1.1 and

1.2 nm) and the specific surface area determined by multipoint nitrogen adsorption BET calculation are 90 and 158 m2 /g for samples A and B, respectively. However, Fig. 3 and the corresponding BET result should be taken with caution, since the adsorption isotherm did not form a closed loop even when long (15 min/N2 addition) equilibration times were attempted. Such nonequilibrium behavior is usually encountered in microporous materials. In these cases the actual surface area far exceeds the area estimated by nitrogen adsorption tests [6]. SAXS spectra of the four samples A–D are depicted in Fig. 4. The mass fractal dimension was rather similar

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Reticular Methyl-Silicate Materials

Figure 4.

193

SAXS spectra of samples A–D.

for samples A and B, with fractal dimensions D = 2.3 and 2.5, respectively. The intensity (I ) dependence of the scattering vector (q) obeyed the Guinier law over a large range (0.002–0.005) and permitted estimation of the radius of gyration, R, by Eq. (1), I (q) ∼ exp(−R 2 q 2 /3)

(1)

The radius of gyration was found to be very similar for samples prepared with and without hydrogen peroxide (R = 1.8 ± 0.2 and R = 2.0 ± 0.1 for the base and acid catalysis, respectively). Conclusions Addition of hydrogen peroxide during the catalyzed sol-gel synthesis of methyl silicate materials yielded macroreticular materials with a bi-modal pore size distribution. The addition of hydrogen peroxide did not affect the micro- and mesoporosity of the materials.

Acknowledgment We thankfully acknowledge the financial help of the Scientific Infrastructure Program of the Ministry of Science, Israel. J.G. thankfully acknowledges the Junior Scientists Scholarship endowed by the British Council. References 1. J. Saggio-Woyansky, C.E. Scott, and W.P. Minnear, American Ceramic Society Bulletin 71, 1682 (1992). 2. J. Brinker and G.W. Scherer, Sol-Gel Science (Academic Press, San-Diego, 1990). 3. M. Tsionsky, A. Vanger, and O. Lev, J. Sol-Gel Sci. Tech. 2, 595, (1994). 4. O. Lev and M. Tsionsky, US Patent application no. 08/376,646 (1995). 5. K. Nakanishi, H. Minakuchi, N. Soga, and N. Tanaka, J. Sol-Gel Sci. Tech. 8, 547 (1997). 6. Y. Polevaya, J. Samuel, M. Ottolenhi, and D. Avnir, J. Sol-Gel Sci. Tech. 5, 65 (1995).