혼합 계면활성제(양이온/중성)를 이용한 새로운 메조세공물질(OSU)의 합성

류재옥, 김대성, 장종산, 김성현*, 박상언

한국화학연구소, 분자활성공학연구실

*고려대학교 화학공학과

New Mesoporous Material (OSU) Synthesis Using Mixed Surfactants Composed

of Cationic and Nonionic Systems

J.O. Ryu, D.S. Kim, J.-S. Chang, S.-H. Kim* and S.-E. Park

Catalysis Center for Molecular Engineering, KRICT, Taejeon 305-600, Korea

*Dept. of Chemical Engineering, Korea University

Introduction

Since the discovery of the M41S in 1992 [1], the research interest focused on the characterization, the mechanism of formation, and the synthesis of new materials based on the MCM-41 synthesis approach. Especially, surfactants play a key role in the formation of these materials, with the self-assembled structures formed by surfactant and inorganic precursors during the stages of the reaction directing the structural properties of the final product. The concept of supramolecular templating of molecular aggregates of surfactants has expanded the possibility of the formation of various mesoporous structures. It gives us also new synthetic tools to engineer porous materials [2-4]. Stucky et al. have pointed out that organic co-solvents are particularly effective in control of the formed phase and interface geometry during the synthesis of mesoporous materials [3]. In line with this concept, Lee et al. has recently found a new mesoporous material with tetragonal structure using cetyltrimethyl ammonium chloride (CTACl) and co-surfactant and they called OSU (Ohio State University) [5]. The structure of OSU material is changed by the relative ratio of mixed surfactants (cationic/nonionic). Microwave induced heating technique has been newly emerged as a new tool for the zeolite synthesis considering several fascinating advantages in comparison with conventional hydrothermal heating. Our group has reported on crystallinity and morphology control of mesoporous material by the addition of ethylene glycol into the MCM-41 precursor gel under microwave irradiation [6,7]. An intermediate step in the microwave synthesis of this material has been monitored by ATR/FT-IR and photoluminescence spectroscopy [8]. Moreover, on the basis of the supramolecular templating mechanism, mesoporous materials in acidic condition by solvent evaporation method were also synthesized in very short period of time [9].

In this work we have tried to synthesize new mesoporous material (OSU) through the formation of micelles from (cationic/nonionic) mixed surfactants. And also we have examined the phase dependence of the OSU material according to the ratio of  CTACl/co-surfactant (decyl alcohol). Microwave method has been used for the synthesis of the material and compared with hydrothermal method.

Experimental

Chemicals. Cetyltrimethylammonium chloride (CTACl) and tetraethylorthosilicate (TEOS) were obtained from Aldrich Chemicals as 25 and 98 wt% aqueous solution, respectively. n-Decyl alcohol (C10OH) was used as a co-surfactant. And sodium hydroxide (Junsei Chem.) was used for adjusting pH of the synthetic solution.

Synthesis. A surfactant, CTACl was first added to the NaOH aqueous solution by dropwise with stirring. Then the mixture is maintained over Krafft point. Next, a co-surfactant, decyl alcohol was added to the mixture. In the following step, TEOS was drop by drop added to give the desired molar ratio of silicate to CTACl. The clear reaction gel became cloudy, and precipitation was observed. This solution was vigorously stirred for 1 h. The molar ratio of the mixture was  CTACl : Co-surfactant : TEOS : NaOH : H2O = 0.1∼1.0 : 0.1∼1.5 : 1.5  : 0.5-1.2 : 338. Reaction mixture was placed into a oven for 1d∼10days at 25℃∼100℃. In case of microwave synthesis, the mixture was heated with microwave for several minutes at 70∼120℃in a CEM microwave oven (MDS-2000) by using 50 - 100% of the maximum power of the oven. After this step, the white solid product was collected by filtration and dried in air at room temperature for 2∼3 days. The dried products (as-synthesized) were collected and analyzed. We obtained final product after calcination for 4 h at 550℃ with air or N2.

Characterization. Synthesized mesoporous materials were characterized by FT-IR (Nicolet). Bulk powders were directly peletized with dry KBr. In order to identify the mesostructure, the prepared mesoporous samples were analysed by Solid-state 29Si-NMR and X-ray diffraction (XRD). The unit used was a X-ray diffractometer (Rigaku 2155D6, Ni-filtered Cu K□ radiation with □=1.5418 Å). The morphology of the samples was examined with a SEM (Scanning electron microscope, 20 kV, Jeol JEM-300).

Results and discussion

Fig. 1 shows the XRD patterns of the as-synthesized materials by hydrothermal method. The material that was prepared by a CTACl surfactant at 70 oC for 5 days yields cubic mesoporous MCM-48 having with the cubic Ia3d symmetry (Fig. 1(a)). It has a d-spacing of approximately 39Å. Upon the addition of decyl alcohol, it is found that the structure was changed from cubic into tetragonal symmetry. Thus, when CTACl and decyl alcohol (1:1) are used as mixed surfactants, XRD pattern of the resultant material indicates the formation of 2-dimensional tetragonal phase having a d-spacing of approximately 48Å (Fig. 1(b)). This structure is equivalent to OSU having the tetragonal p4m symmetry.

When the microwave method was applied to the synthesis of the new phase with the same compositions as Fig. 1(b), the mesoporous silica material having the tetragonal symmetry and a d-spacing of 41Å was obtained (Fig. 2(a).). But all the peaks are shifted into higher 2 □ than those obtained from the hydrothermal method. In case of microwave method, the available time for the formation of the same structure is much shorter than hydrothermal method. Microwave irradiation seems to accelerate the formation of the mesoporous structure. The short crystallization time was ascribed to relatively fast dissolution of the gel upon microwave irradiation as compared to conventional hydrothermal heating. The shorter synthesis time by microwave heating could be explained by two different mechanisms, i.e., the rapid heat-up of the sample and superheating by a better heat transfer which results in rapid and sufficient heating of the synthetic mixture [6].

The structure of the OSU material were dependent on aging temperature, time and the ratio of CTACl/co-surfactant or Si/surfactant. For instance, even in case of dual surfactants a cubic  phase was obtained at lower synthesis temperature instead of the tetragonal symmetry, indicating temperature dependence on the mesophase formed.

The synthesis of the OSU material is based on the utilization of the surfactant synergism due to the delicate change of the interfacial area of micelles through nonionic/nonionic or nonionic/cationic mixed surfactant system. Sometimes, it will lead to the remarkably improved physical or chemical properties such as solubility, micellar stability, liquid crystallinity and so on. Based on these concepts, alkyl alcohol was adopted as a co-surfactant. In the self-assembling process of micelles, the higher surface area of the head group may favor the formation of curved bilayers, leading to the formation of the cubic mesophase through the periodic packing induced by the intermicellar silicate condensation. The spherical micelle by an assembling of CTACl is changed into cylindrical micelle by the addition of aliphatic alcohol (decyl alcohol). The reassembling of surfactants seems to yield a new mesoporous structure.

Further experiment is underway to obtain the information in detail about the role of each surfactant directing the new phase and self-assembling pathways during the synthesis.

References

[1] Beck, J.S., Vartuli, J.C., Roth, W.J., Leonowicz, M.E., Kresge, C.T., Schmitt, K.D., Chu, C.T.-W., Olson, D.H., Sheppard, E.W., McCullen, S.B., Higgins, J.B. and Schlenker, J.B. :  J. Am. Chem. Soc., 114, 10834 (1992).

[2] Vartuli, J.C. Shin, S.S. Kresge, C.T., and Beck, J.S. : Stud. Surf. Sci. Catal., 117  13 (1998).

[3] Stucky, G.D., Huo, Q., Firouzi, A., Chmelks, B.F., Schacht, S., Voigt-Martin, I.G. and Schuth, F. : Stud. Surf. Sci. Catal., 105, 3 (1997).

[4] Ryoo, R., Ko, C.H. and Howe, R.F. : Chem. Mater., 9, 1607 (1997).

[5] Lee, Y.S., Surjadi, D. and Rathman, J.F. : J. Amer. Chem. Soc. in press.

[6] Park, S.-E., Kim, D.S., Chang, J.-S. and Kim, W.Y. : Catal. Today, 44, 301 (1998).

[7] Kim, D.S., Chang, J.-S., Kim, W.Y., Kim, H.Y. and Park, S.-E. : Bull. Korean Chem Soc., 20, 408 (1999).

[8] Park, S.-E., Kim, D.S., Chang, J.-S. and Kim, W.Y. : Stud. Surf. Sci. Catal., 117, 265 (1998).

[9] Roh, H.-S., Chang, J.-S. and Park, S.-E. : Korean J. Chem. Eng., 16, 331 (1999).