다공성 나노구조 무기박막의 제조와 투과특성

Theories and Application of Chem. Eng., 2002, Vol. 8, No.1

화학공학의 이론과 응용 제8권 제1호 2002년

김영석, 양승만

한국과학기술원 생명화학공학과

Preparation of nano-structured inorganic thin film on a porous support and separation characteristics

Young-Seok Kim, Seung-Man Yang

Department of Chemical & Biomolecular Engineering, KAIST, Taejon 305-701, Korea

Introduction

Inorganic membranes have been of practical significance for separation processes owing to their superior properties of thermal and chemical stability as well as chemical resistance.[1,2] Mesoporous inorganic membranes have been widely studied mainly using □-alumina, titania and zirconia. However, these membranes are x-ray amorphous materials and generally have a wide pore size distribution whereas uniform pore channels have been in demand for high performance application in nanofiltration, ultrafiltration, pervaporation and membrane reactors. Mesoporous silica with three-dimensionally accessible pores is a promising material for membrane-based separation processes, catalysts and chemical sensors.[3,4] Silica membranes fabricated from the self-assembled surfactant template have macrocrystalline structure and a potential advantage in the separation process with tunable pores. Recently, a new family of mesoporous molecular sieves (designated as M41S) was discovered by using micellar aggregates as templates.[5] However, many of them are in granular structure and have a difficulty in thin-film processing. More recently, it was reported that the continuous mesoporous silica thin films were formed on a dense nonporous support such as mica or polished silicon wafer,[6,7] or free standing film[8] by sol-gel method.

Here we report a method for the formation of ordered mesoporous silica thin film (~1m) with narrow pore size distribution on the porous alumina support. First, the porous alumina support was pretreated with poly(vinyl alcohol) as a pore filling substance. The water soluble PVA reduced the support surface roughness, and at the same time successfully prevented the coating solution from infiltrating into the support pores. After the silica thin film was formed on the modified support surface, the pore filling substance PVA was removed by thermal decomposition during calcination. The prepared films exhibited high thermal stability upon calcination at 823K and were crack-free until the thickness increased up to 1□m. Separation of small molecules such as sucrose or glucose are performed with prepared membrane and showed high performance.

Experimental

The coating silica sol solution prepared by mixing tetraethylorthosilicate (TEOS), cetylpyridinium chloride (CpCl), distilled water and a small amount of hydrochloric acid in ethanol (EtOH). A typical molar ratio was TEOS:CpCl:H2O:HCl:EtOH= 5:1:15:0.027:50 for which the solution looked clear. The mixture was stirred for 24 hours at room temperature. Before the dip-coating process, the prepared solution was concentrated up to 50wt% in a vacuum oven. Poly(vinyl alcohol) (PVA; average m.w.:9,000-10,000) was used as a support filling material. The alumina support tube was 2.1 mm in outer diameter and 1.9mm in inner diameter. The support has a porosity of 0.39 and an average pore size of 150 nm. First, the alumina support was soaked with a 10wt% PVA solution and dried. This process was repeated for several times to fill the void sites of the porous support with PVA. The support was then dipped in the prepared sol for several minutes, pulled out at the rate of 10 cm/min and dried for a day. The prepared membrane was then calcined in air flow at 823K for 3 hours with a heating rate of 1 K/min to remove PVA, and to form a mesoporous silicate. Fig.1 shows an experimental setup for separation process. Glucose and sucrose were tested to verify membrane performance.

Fig.1 Experimental setup for separation process.

Results and Discussion

Mesoporous silica was synthesized through the sol-gel route based on the self-organization of surfactant molecules (cetylpyridinium chloride) and silicate. The unsupported mesoporous silica samples are ordered into the hexagonal (p6mm) arrangement, which was consistent with the X-ray diffraction (XRD) patterns and the TEM image reproduced in Fig.2. The mesopores of the partially disordered structure have the same diameter as the ordered state. In addition, the channels in the partially disordered phase are interconnected in a three dimensional disordered structure.[9] The Ar adsorption and desorption isotherms of the calcined silica exhibited a narrow pore size distribution around 1.8 nm estimated by using the Barrett-Joyner-Halenda (BJH) analysis and Brunauer-Emmett-Teller (BET) surface area was in the range of 980 m2g-1 at 87K. The weight loss and thermal evolution of silica during heat treatment in air was measured by TGA/DTA with a heating rate of 1℃/min. The exothermic peak around 330℃ was accompanied by a drastic weight loss about 50%, which was due to the pyrolysis of the surfactant and residual organics. This considerable weight loss is responsible for the relatively high porosity compared to amorphous membranes that have been obtained with organic templates.

Fig.2 TEM images of the unsupported mesoporous silica with different viewing directions.

The mesoporous silica film was deposited by dip-coating on an -alumina support tube. In this case, the key idea to control a uniform, thin film formation was the pretreatment of the support with a pore filling substance (PVA). The cross-sectional SEM images of the untreated bare alumina support and the PVA-filled alumina support are shown in Figures 4a and 4c, respectively. Also included in Figure 4b for comparison is the SEM image of the silica film formed by dip coating on an unfilled alumina support. As noted, the silica film on the untreated support was structurally nonuniform and its surface morphology was very poor. Meanwhile, Figure 4c shows that most of the alumina support pores were filled with PVA, which facilitated the subsequent film coating process. The cross-sectional SEM images of the silica coated membranes were taken before and after the calcination and the results were shown in Figures 4d and 4e, respectively. Indeed, the skin layer was formed successfully. During the calcination, PVA was decomposed thermally and removed. It can be seen that the skin layer of silica membrane is very thin and about 500 nm in thickness. The thickness of the film varied from 300 nm to several micrometers by adjusting the concentration or by changing the dip-coating speed. The SEM images of the calcined silica film surfaces at different magnifications are shown in Figure 4f. Crack-free thin film with large area was successfully coated without any additional treatment. As noted from Figure 4f, when the support pore was filled with PVA before the dip coating, the film surface was smoothened by the pretreatment of the support. This is due to the fact that when the support pores were filled with PVA, the capillary stress development on the silica film was suppressed during the coating process. It was observed that PVA filled the support pores without channeling and successfully prevented the infiltration of the coating silica sol solution into the support pore. The XRD results showed (not presented here), there was no remarkable change in the mesoporous structure when small amount of PVA was added into prepared sol. The crack formation could be avoided by repeating the dip-coating process or by adopting the pore-plugging method.[10] Prepared membrane is under the  test for separation of glucose and sucrose which have nano-size diameter.

Fig.3 Cross-sectional scanning electron micrographs (SEM) of various samples.

Conclusion

The present study showed that a continuous mesoporous silica thin film with large crack-free area could be successfully formed on a porous support by prefilling the support pores and smoothening the support surface with the water soluble PVA. Prepared membrane was tested for the separation of glucose and sucrose to elucidate the porous structure of coated top layer.

References

[1] A. J. Burggraaf, L. Cot, Fundamentals of Inorganic Membrane Science and Technology, Elsevier, Amsterdam 1996.

[3] Y. S. Kim, K. Kusakabe, S. Morooka, S. M. Yang, Korean J. Chem. Eng. 2001, 18, 106.

[3] Y. Sakamoto, M. Kaneda, O. Terasaki, D. Y. Zhao, J. M. Kim, G. D. Stucky, H. J. Shin, R. Ryoo, Nature 2000, 408, 449.

[4] I. A. Aksay, M. Trau, I. Honma, N. Yao, L, Zhou, P. Fenter, P. M. Eisenberger, S. M. Gruner, Science 1996, 273, 892.

[5] C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature 1992, 359, 710.

[6] Y. Lu, R. Ganguli, C. A. Drewien, M.T. Anderson, C. J. Brinker, W. Gong, Y. Guo, H. Soyez, B. Dunn, M. Huang, J. I. Zink, Nature 1997, 389, 364.

[7] M. Ogawa, J. Am. Chem. Soc. 1994, 116, 7941.

[8] M. Ogawa, T. Kikuchi, Adv. Mater. 1998, 10, 1077.

[9] R. Ryoo, J. M. Kim, C. H. Ko, C.H. Shin, J. Phys, Chem. 1996, 100, 17718.

[10] J. H. So, S. M. Yang, S. B. Park, J. Membr. Sci. 1998, 147, 147.