Enhancement of Hydrothermal Stability and Hydrophobicity of a Silica

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Ind. Eng. Chem. Res. 2001, 40, 6105-6110

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MATERIALS AND INTERFACES Enhancement of Hydrothermal Stability and Hydrophobicity of a Silica MCM-48 Membrane by Silylation Dong-Huy Park, Norikazu Nishiyama,* Yasuyuki Egashira, and Korekazu Ueyama Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan

An MCM-48 membrane was prepared on a porous R-alumina support by hydrothermal synthesis. The silylation with trimethylsilane and triethylsilane was used to enhance hydrothermal stability and hydrophobicity of the MCM-48 membrane. The studies of X-ray diffraction, Fourier transform infrared (FTIR), nitrogen adsorption, and water adsorption were carried out using an MCM-48 powder to characterize the pore surface of MCM-48. The FTIR study indicates that the silyl group was substituted for the Si-OH group on the pore surface of MCM-48. The pore sizes of the nonsilylated and the silylated MCM-48 were 2.4 (nonsilylated), 1.9 (trimethylsilylated), and 1.8 nm (triethylsilylated). The water adsorption study showed that the pore surface of MCM-48 significantly changed to hydrophobic by silylation. The nonsilylated and the trimethylsilylated MCM-48 membranes were used for the separation of an ethanol/water mixture. The separation factor was 16 through the silylated membrane, although that of the nonsilylated membrane was 0.3, indicating that the pore of the silylated membrane hardly adsorbed water. Furthermore, the hydrothermal stability of the MCM-48 membrane was improved by the silylation. 1. Introduction Inorganic membranes made of ceramics or metals have been of interest for separation processes because of their superior characteristics of thermal stability, structural stability, and chemical resistance. In the field of microporous membranes, Gavalas’s group1,2 prepared a microporous silica membrane within the pores of Vycor tubes by silane oxidation. Further, zeolite membranes such as ZSM-5,3-5 ferrierite,6 A type,7 and mordenite8 have been extensively studied. Mesoporous inorganic membranes have been studied mainly with respect to silica, titania, and zirconia membranes prepared by a sol-gel method. However, the mesoporous membrane with more uniform pores is in demand for high-performance applications because the conventional membranes such as SiO2 and TiO2 synthesized by the sol-gel method have a wide pore size distribution. Mobil scientists discovered a new family of mesoporous molecular sieves designated as M41S in 1992.9,10 M41S molecular sieves have uniform pore structures of hexagonal (MCM-41), cubic (MCM-48), and lamellar (MCM-50) symmetry. In this method, surfactant liquidcrystal structures serve as an organic template for the polymerization of silicate. Using the liquid-crystal templating strategy, it is possible to adjust the pore diameter between 2 and 10 nm, which is achieved by using surfactants with different chain lengths of carbon. MCM-41 has a one-dimensional, hexagonal pore system, while MCM-48 contains two independent three* Corresponding author. Phone: +81-6-6850-6256. Fax: +81-6-6850-6256. E-mail: [email protected].

dimensional pore systems. The one-dimensional pore system seems to be hard to utilize as a membrane material for separation because the axis of the pore system should be arranged perpendicularly to the membrane surface. Therefore, MCM-48 is thought to be a promising material as films or membranes, which can be applied to chemical sensor, filtration, pervaporation, membrane reactor, etc. However, the range of application of silica materials is limited because of the low structural stability against moisture and compression. The structure of MCM-48 collapses after long exposure to water vapor.11 Koyano et al.12 reported that the trimethylsilylation is effective in improving the stability of porous silica materials to moisture and compression. They stated that the trimethyl group capping the hydrophilic silanol protects the Si-O-Si bonds from hydrolysis. Enhancement of hydrophobicity of a silicalite membrane was reported by Sano et al.,13 who silylated a silicalite membrane with methyltrichlorosilane, dimethyldichlorosilane, and trimethylchlorosilane. We have synthesized MCM-48 membranes on a porous stainless steel support14 and a porous alumina support.15 The MCM-48 particles densely existed in the pores of the alumina support, with an average pore size of 100 nm forming the composite of silica MCM-48 and alumina.15 The improvement of hydrothermal stability is required for the development of the high-performance silica MCM-48 membranes. We have studied the effect of silylation on the hydrothermal stability and hydrophilic-hydrophobic properties of the MCM-48/alumina composite membrane as well as the powder. The hydrothermal stability of the nonsilylated and the silylated products was characterized by X-ray diffraction (XRD).

10.1021/ie0103761 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/22/2001

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The separation of an ethanol/water mixture was carried out using the nonsilylated and the silylated MCM-48 membranes. 2. Experimental Section 2.1. Synthesis of MCM-48 Powder and Membrane. An MCM-48 membrane was prepared as follows: A porous R-alumina support (NGK Insulators, Ltd.) with an average pore diameter of 0.1 µm was placed in tetraethyl orthosilicate (TEOS) (Wako Pure Chemical Industries Co., Ltd.). Then, a solution which consists of the quaternary ammonium surfactant, C16H33(CH3)3NBr (Wako Pure Chemical Industries Co., Ltd.), NaOH (Wako Pure Chemical Industries Co., Ltd.), and deionized water was added to TEOS containing the R-alumina support. The molar ratio of the mixture was 0.4:1.0:0.5:61 C16H33(CH3)3NBr/TEOS/NaOH/H2O. Although a clear solution was obtained after mixing, the solution became inhomogeneous after 30 min of stirring. After the mixture was stirred for 90 min more, the mixture and support were transferred to an autoclave. The alumina support was placed horizontally in the bottom of the autoclave. The reaction was carried out without stirring at 363 K for 72 h. The product was rinsed with deionized water and calcined at 773 K for 7 h. An MCM-48 powder was synthesized by the same method. 2.2. Silylation of MCM-48 Powder and Membrane. The silylation of the calcined MCM-48 powder or the membrane was carried out at 453 K for 24 h in the liquid phase of a silylation reagent of trimethylchlorosilane or triethylchlorosilane. After silylation, the product was rinsed with deionized water and dried at 473 K. 2.3. Characterization. The product was identified by XRD with Cu KR radiation on Philips X’s Pert-MPD. The nitrogen adsorption and desorption isotherms were measured using a Quantachrome Autosorb Gas Sorption System (Quantachrome Co.) at 77 K. The Fourier transform infrared (FTIR) spectra were recorded on an FTIR-8200PC spectrometer (Shimadzu Co.) at 2 cm-1 resolutions. Thermogravimetric analyses (TGA and DTA) were obtained on a DTG-50 (Shimadzu Co.). The hydrothermal stability of the nonsilylated and the silylated MCM-48 membranes was evaluated by the XRD patterns before and after a hydrothermal treatment, in which the membranes were kept in water at 453 K for 24 h in a closed vessel. 2.4. Gas Permeation Measurements. Gas permeation measurements using as-synthesized and calcined membranes were carried out using N2, He, and H2. The feed side of the membrane was kept at a constant pressure between 200 and 300 kPa. The permeate side of the membrane was set at atmospheric pressure. The flow rate of gas through the membrane was measured at 373 K with a soap flowmeter. 2.5. Separation of an Ethanol/Water Mixture. The separation of ethanol from an ethanol/water mixture was carried out at 293 K using the nonsilylated and the trimethyl- and triethylsilylated MCM-48 membranes. The feed solution was an ethanol/water mixture with 10 wt % ethanol. Helium with a flow rate of 40 mL min-1 was used as a sweep gas on the permeate side. The amount of ethanol and water passed through the membrane was measured by gas chromatography.

Figure 1. XRD patterns of MCM-48 membranes (a) before and (b) after calcination.

Figure 2. Permeances of N2, He, and H2 gases through a MCM48 membrane after calcination.

3. Results and Discussion 3.1. Structure and Gas Permeation Properties of a Nonsilylated MCM-48 Membrane. Figure 1 shows the XRD patterns of MCM-48 membranes before and after calcination at 773 K. The XRD pattern has four reflections of (211), (220), (420), and (332), which is consistent with the reported XRD pattern of MCM48.9 Four peaks shifted to higher angle after calcination at 773 K because of structural shrinkage by removing the surfactant inside the MCM-48 pores. However, the intense reflection peaks of MCM-48 indicate that the MCM-48 membrane has high thermal stability up to 773 K. A permeation test through the MCM-48 membrane was carried out using N2 gas before calcination with a pressure drop of 3 kg cm-2. No permeation flux through the MCM-48 membrane was detectable before calcination, which means that there are no pinholes or cracks in the MCM-48 membrane. Figure 2 shows the permeances of N2, He, and H2 gases through the MCM-48 membrane after calcination. The permeance of all gases

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Figure 3. FTIR spectra of the silylated MCM-48 powder. Silylation time: 0, 24, 48, and 100 h.

Figure 4. TGA/DTA curves for silylated MCM-48 heated to 973 K with a heating rate of 10 K min-1: (a) TMCS silylation; (b) TECS silylation.

was independent of the pressure drop, indicating that there is no effect of viscous flow, which occurs in large pinholes. The permeation of these gases must be governed by the Knudsen diffusion mechanism. 3.2. Characterization of a Silylated MCM-48 Powder. Figure 3 shows the variation in the FTIR spectra of the MCM-48 powders silylated by trimethyland triethylsilylation with silylation times of 0, 24, 48, and 100 h. The sharp adsorption band at 1100 cm-1 observed in all spectra is ascribed to the Si-O-Si framework of MCM-48. The broad band at 3500 cm-1 is assigned to the Si-OH group. The peak intensity at 3500 cm-1 decreased with increasing silylation time, indicating that the Si-OH group was replaced with trimethylsilyl and triethylsilyl groups. After trimethyland triethylsilylation, the bands at 850 and 780 cm-1 appeared, respectively. These two bands can be assigned to the attached trimethylsilyl and triethylsilyl groups. In this study, silylation seems to be completed after 48 h, judging from the band at 3500 cm-1. Figure 4 shows the TGA/DTA curves for the silylated MCM-48 powder heated to 973 K with a heating rate of 10 K min-1. The decomposition of the trimethylsilyl group started at 793 K. The decomposition of the triethylsilyl group started at 553 K. This means that the thermal stability of the triethylsilyl group is lower than that of the trimethylsilyl group. Figure 5 A shows the adsorption and desorption isotherms of nitrogen at 77 K on the nonsilylated MCM48 and the silylated MCM-48 samples. Figure 5B shows the pore size distributions calculated by the Barrett-

Joyner-Halenda (BJH) method using an adsorption isotherm. The nonsilylated and the silylated MCM-48 powders have sharp pore size distributions. Table 1 lists the pore volume, the pore size, and the BrunauerEmmett-Teller (BET) surface area of the nonsilylated and the silylated MCM-48 powders. The average pore size and the pore volume of the nonsilylated MCM-48 samples were about 2.4 nm and 0.99 cm3 g-1, respectively. After trimethyl- and triethylsilylation, the pore volume was reduced to 0.81 and 0.78 cm3 g-1, respectively, and the average pore sizes for both of the samples were reduced by ca. 0.5 nm. The BET surface areas of MCM-48 and trimethyl- and triethylsilylated MCM-48 powders are 1030, 1196, and 1099 m2 g-1, respectively. There is no large difference in the BET surface area between the nonsilylated and the silylated MCM-48 powders. Moreover, it is clear that the silyl group uniformly dispersed on the surface of pores of MCM-48 from Figure 5B. Parts a and b of Figure 6 show the adsorption isotherms of ethanol and water vapor on the MCM-48 and the silylated MCM-48 powders at 293 K. The amount of adsorbed ethanol of the silylated MCM-48 powders at relative vapor pressure (p/p0) ) 1 was 5060% of the nonsilylated one. This difference is consistent with the results of nitrogen adsorption as shown in Figure 5. This can be explained by the difference in the pore volume between the silylated and the nonsilylated MCM-48 powders. The amount of adsorbed water of the nonsilylated MCM-48 powder rose with increasing p/p0 and was ca.

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Figure 5. (A) N2 adsorption and desorption isotherms: 0, MCM48 (Ads); 9, MCM-48 (des); 4, TM-MCM-48 (ads); 2, TM-MCM48 (des); O, TE-MCM-48 (ads); b, TE-MCM-48 (des). (B) Pore size distribution: (a) MCM-48; (b) TM-MCM-48; (c) TE-MCM-48. TM: trimethylsilylated. TE: triethylsilylated. Table 1. Pore Volume, Pore Size, and BET Surface Area of Nonsilylated MCM-48 and Silylated MCM-48 Powdersa sample

pore volume [cm3 g-1]

BET surface area [m2 g-1]

pore size [nm]

MCM-48 TM-MCM-48 TE-MCM-48

0.99 0.81 0.78

1030 1196 1099

2.4 1.9 1.8

a

Figure 6. Adsorption isotherms of (a) ethanol and (b) water: ], nonsilylated MCM-48; O, TM-MCM-48; 3, TE-MCM-48. TM: trimethylsilylated. TE: triethylsilylated.

TM: trimethylsilylated. TE: triethylsilylated.

400 cm3 g-1 STP at p/p0 ) 1. This value is comparable to that of N2 (ca. 500 cm3 g-1 STP) at p/p0 ) 1 shown in Figure 5A. This indicates that the pore surface of the nonsilylated MCM-48 is hydrophilic and the pores of the MCM-48 powder are almost fully filled with water molecules at p/p0 ) 1. However, the trimethyl- and triethylsilylated MCM-48 powders did not show a remarkable increase in the amount of adsorbed water with p/p0. The amount of adsorbed water at p/p0 ) 1 on the silylated powders was about 20 cm3 g-1 STP and much less than that of the nonsilylated one, indicating that the silylation substituted the silyl group for the SiOH group on the pore surface and then the pore surface effectively became hydrophobic. 3.3. Hydrothermal Stability of a Silylated MCM48 Membrane. The XRD patterns of MCM-48 membranes after trimethyl- and triethylsilylation are shown in Figure 7. No change in the peak intensity and no peak shift to the higher angle were observed. This means that the structural collapse of the MCM-48 membrane did not occur after silylation. Generally, porous silica materials do not have enough structural stability under hydrothermal conditions.

Figure 7. XRD patterns of nonsilylated and silylated MCM-48 membranes: (a) MCM-48 membrane for trimethylsilylation; (a)-1 TM-MCM-48 membrane; (b) MCM-48 membrane for triethylsilylation; (b)-1 TE-MCM-48 membrane. TM: trimethylsilylated, TE: triethylsilylated.

Figures 8-10 show the XRD patterns of the nonsilylated and the trimethyl- and triethylsilylated MCM-48 membranes before and after hydrothermal treatment. The

Ind. Eng. Chem. Res., Vol. 40, No. 26, 2001 6109 Table 2. Separation Results of an Ethanol/Water Mixturea flux [kg m-2 h-1] water ethanol MCM-48 membrane TM-MCM-48 membrane TE-MCM-48 membrane

16.2 0.11 0.02

0.54 0.19 0.04

separation factor (ethanol/water) 0.3 16 24

a Feed: 10 wt % ethanol. TM: trimethylsilylated. TE: triethylsilylated.

Figure 8. XRD patterns of a nonsilylated MCM-48 membrane (a) before and (b) after hydrothermal treatment.

Figure 9. XRD patterns of a TM-MCM-48 membrane (a) before and (b) after hydrothermal treatment. TM: trimethylsilylated.

Si bonds that were caused by the water adsorbed on the silanol groups. As suggested by Koyano et al.,12 the improvement of the hydrothermal stability of the MCM48 membrane shown in Figures 9 and 10 is probably due to the decrease in the concentration of the water adsorption site near the Si-O-Si bonds constituting the silica wall. 3.4. Separation of an Ethanol/Water Mixture. Table 2 lists the separation results for an ethanol/water mixture using the nonsilylated and trimethyl- and triethylsilylated MCM-48 membranes. The separation factor REtOH of the nonsilylated MCM-48 membrane was 0.3. This means that water is the faster component in an ethanol/water mixture. The fluxes of water and ethanol were 16.2 and 0.54 kg m-2 h-1, respectively. On the other hand, REtOH of the trimethyl- and triethylsilylated MCM-48 membranes were 16 and 24, which were 53 and 80 times larger than that of the nonsilylated MCM-48 membrane. The large increase in the separation factor is due to the remarkable decrease in the flux of water after trimethyl- and triethylsilylation although the pore size of the silylated MCM-48 is not much smaller than that of the nonsilylated MCM-48 as shown in part b of Figure 5B. This clearly shows that the silylation effectively changed the properties of pore surfaces and affected the ethanol/water adsorption properties. These results indicate that the silylation is a useful method to improve the separation performance of porous silica membranes. The difference in the separation performance between the trimethyl- and triethylsilylated membranes cannot be discussed because of the difference in the membrane quality. However, the separation performance is considered to be comparable to each other because no large difference in the adsorption isotherms of ethanol and water between the trimethyl- and triethylsilylated MCM-48 powders was observed in Figure 6. 4. Conclusions

Figure 10. XRD patterns of TE-MCM-48 membrane (a) before and (b) after hydrothermal treatment. TE: triethylsilylated.

structure of the nonsilylated MCM-48 membrane collapsed after hydrothermal treatment at 363 K for 24 h. However, the structures of the trimethyl- and triethylsilylated MCM-48 membranes were maintained after hydrothermal treatment. The reason for the collapse of the ordered structure must be the hydrolysis of Si-O-

The Si-OH groups on the pore surface of MCM-48 were replaced by trimethyl and triethyl groups by silylation. After silylation, the pore diameter of MCM48 reduced by ca. 0.5 nm. The amount of adsorbed water vapor on silylated MCM-48 powder was extremely small, indicating that silylation effectively made the pore surface of MCM-48 hydrophobic. The MCM-48 membranes silylated with trimethyl- and triethylsilane separated an ethanol/water mixture with a separation factor of 16 and 24. Furthermore, the silylation enhanced the hydrothermal stability of the MCM-48 membrane. Acknowledgment We thank NGK Insulators, Ltd., for supporting R-alumina supports. We also thank GHAS at the Department of Chemical Engineering at Osaka University for XRD

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measurements and Dr. T. Takahashi and K. Suzuki (NGK Insulators, Ltd.) for N2 adsorption measurements. Literature Cited (1) Gavalas, G. R.; Megiris, C. E.; Nam, S. W. Deposition of H2-permselective SiO2 films. Chem. Eng. Sci. 1989, 44, 1827. (2) Tsapatsis, M.; Kim, S.; Nam, S. W.; Gabalas, G. Synthesis of hydrogen permselective SiO2, TiO2, Al2O3 and B2O3 membranes from the chloride precursors. Ind. Eng. Chem. Res. 1991, 30, 2152. (3) Bakker, W. J. W.; Kapteijn, F.; Poppe, J.; Moulijn, J. A. Permeation characteristics of a metal supported silicalite-1 zeolite membrane. J. Membr. Sci. 1996, 117, 57. (4) Bai, C.; Jia, M. D.; Falconer, J.; Noble, L. R. D. Preparation and separation properties of silicalite composite membranes. J. Membr. Sci. 1995, 105, 79. (5) Kusakabe, K.; Yoneshige, S.; Murata, A.; Morooka, S. Morphology and gas permeance of ZSM-5 type zeolite membrane formed on porous a-alumina support tube. J. Membr. Sci. 1996, 116, 39. (6) Nishiyama, N.; Matsufuji, T.; Ueyama, K.; Matsukata, M. FER membrane synthesized by vapor-phase transport method: its structure and separation characteristics. Microporous Mater. 1997, 12, 293. (7) Kita, K.; Horii, K.; Ohtoshi, Y.; Tanaka, K.; Okamoto, K. Synthesis of a zeolite NaA membrane for pervaporation of water/ organic liquid mixtures. J. Mater. Sci. Lett. 1995, 14, 206. (8) Nishiyama, N.; Ueyama, K.; Matsukata, M. A defect-free mordenite membrane syntehsized by vapor phase transport method. J. Chem. Soc., Chem. Commun. 1995, 1967.

(9) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature 1992, 359, 710. (10) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmidt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. A new family of mesoporous molecular sieves prepared with liquid crystal template. J. Am. Chem. Soc. 1992, 114, 10834. (11) Kim, J. M.; Ryoo, R. Disintegration of Mesoporous Structures of MCM-41 and MCM-48 in Water. Bull. Korean Chem. Soc. 1996, 17, 66. (12) Koyano, K. A.; Tatsumi, T.; Tanaka, Y.; Nakata, S. Stabilization of mesoporous molecular sieves by trimethylsilylation. J. Phys. Chem. B 1997, 101, 9436. (13) Sano, T.; Yamada, K.; Eire, S.; Hasegawa, M.; Kawakami, Y.; Yanagishita, H. Silylation of silicalite membrane and its pervaporation performance. Stud. Surf. Sci. Catal. 1997, 105, 2179. (14) Nishiyama, N.; Koide, A.; Egashira, Y.; Ueyama, K. Mesoporous MCM-48 membrane synthesized on a porous stainless steel support. Chem. Commun. 1998, 2147. (15) Nishiyama, N.; Park, D. H.; Koide, A.; Egashira, Y.; Ueyama, K. A new mesoporous silica membrane with periodic structure; Preparation and characterization. J. Membr. Sci. 2001, 182, 235.

Received for review April 27, 2001 Revised manuscript received September 24, 2001 Accepted October 8, 2001 IE0103761