Control of Wall Thickness and Extraordinarily High Hydrothermal Stability of Nanoporous MCM-41 Silica

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1 Journal of the Chinese Chemical Society, 2007, 54, Control of Wall Thickness and Extraordinarily High Hydrothermal Stability of Nanoporous MCM-41 Silica Chi-Feng Cheng* ( ), Shu-Hsien Chou ( ), Po-Wen Cheng ( ), Hsu-Hsuan Cheng ( ) and Hwa-Kwang Yak ( ) Department of Chemistry, Center of Nanotechnology and R&D Center for Membrane and Technology, Chung Yuan Christian University, 200, Chung Pei Rd.,Chung Li, Taiwan 32023, R.O.C. Wall thickness of siliceous MCM-41 could be controlled systematically up to 36.1 Å. A reasonable model explaining formation of thicker MCM-41 walls, not enlarging pore channel is proposed on the basis of TGA and 13 C MAS NMR data of samples. Thermal restructuring process under mild basic condition favors the silica redeposition on silica wall and building up thicker wall. Most mesostructure of calcined MCM-41 with thicker wall was retained even after hydrothermal treatment in boiling water for 14 days. To our best knowledge, the excellent hydrothermal stability of the MCM-41 silica reported herein has not been described before and facilitates practical applications of mesoporous molecular sieves in future. Keywords: MCM-41 silica; Mesoporous molecular sieve; Wall thickness. INTRODUCTION The synthesis of a new mesoporous molecular sieve, MCM-41, 1 using aggregates of cationic surfactants as the template instead of isolated organic molecules as with conventional microporous materials has drawn a great deal of attention due to their shape-selective catalysis 2 and separation 3 of bulky molecules, and application on nano-materials. 4 Low hydrothermal stability restricts practical applications of the mesoporous molecular sieve MCM-41. The introduction of Al on the surface or into the framework of MCM-41 results in a remarkable improvement of hydrothermal stability of MCM-41. 5,6 The most promising method for improving the hydrothermal stability of aluminosilicate mesoporous materials was using the aluminosilicate zeolitic nanoclusters as a raw material to build up mesostructure However, the improvement of mesoporous silica hydrothermal stability is still developing. Ryoo et al have covered the improvement of hydrothermal stability of mesoporous silica using salting solution. The salted and subsequently calcined sample showed no loss of hexagonal structure after heating in boiling water for 12 h. However, a sufficiently long salting period such as 10 days was essential for the improvement of the hydrothermal stability after tedious ph adjustment of gel solution. 15 It was reported by Kawi et al. 16,17 that mesoporous silica synthesized in the presence of fluoride anions had much better hydrothermal stability as compared with those synthesized in the presence of chloride anions. Part of the mesostructures was retained after treating samples in boiling water for 2 days. Cheng et al. 18 also reported that significant improvement in hydrothermal improvement can be achieved by adding different tetraalkylammonium or sodium cations to the synthesis gel. Most mesostructures of MCM-41 silica were retained after hydrothermal treatment in boiling water for 4 days. Pinnavaia et al. pointed out that entrapped framework sodium ions play a crucial role in limiting the hydrothermal stability of the mesostructure and catalyze the collapse of mesopores. 19 Thermal and hydrothermal stabilities of a wide range of mesoporous silica have been studied by Cassiers et al. 20 They concluded that the thermal and hydrothermal stability were strongly related to the wall thickness. Ryoo et al. 15 considered that the wall-thickening approach appears to be the simplest one among these techniques, but no completely synthetic strategies have yet been found for systematic control of the wall thickness. Here, we report that the wall thickness can be controlled systematically up to 36.1 Å simply by increasing crystallization time to 1~7 days at 150 ~ 180 C at mild basic conditions. A reasonable model explaining formation of a thicker MCM-41 wall, not en- * Corresponding author. Tel: ; Fax : ; chifeng@cycu.edu.tw

2 36 J. Chin. Chem. Soc., Vol. 54, No. 1, 2007 Cheng et al. larging the pore channel is proposed. To our best knowledge, the excellent hydrothermal stability of the MCM-41 silica reported herein has not been described before. EXPERIMENTAL The siliceous MCM-41 was prepared as follows. 21 Tetramethylammonium hydroxide (TMAOH) and cetyltrimethylammonium bromide (CTABr) were added to deionized water with stirring at room temperature. Fumed silica was added to the solution with stirring and then aged for 24 hours at room temperature. The final gel of composition 1 SiO 2 : 0.19 TMAOH : 0.27 CTABr : 40 H 2 O was transferred to a Telflon-lined stainless steel autoclave at 150~ 180 C for 1~7 days. The reaction product was filtered, washed with distilled water, dried in air at 110 C and calcined at 550 C for 8 hours. Hydrothermal stability of the MCM-41 was studied by mixing 0.5 g of calcined samples with 50 g of deionized water, sealing in closed glass bottles and heating at 100 C for 1~28 days. The parent samples without hydrothermal treatment are designated MCM-x-yd where x, y respectively are the crystallization temperatures at C unit and time in days, i.e. MCM-165-7d indicated the sample prepared at 165 C for 7 days. Samples after hydrothermal treatment for z days are designated MCM-x-yd-zd, i.e. MCM-165-7d-14d indicated the calcined sample with a further water bath of 14 days at 100 C. The steam resistance of MCM-41 silica tested by some groups was not carried out in this study due to difficult comparison of data using different testing temperatures and water vapor pressures in different groups. However, soaking porous materials in boiling water is very general and a severe testing method for hydrothermal stability. The hydrothermal stability of our samples was studied by following intensity of X-ray diffractograms recorded on a Rigaku X-ray diffractometer with Cu K radiation. N 2 adsorption/desorption isotherms were measured at 196 C using a Micrometrics ASAP2010 analyzer. Prior to the experiments, samples were dehydrated at 350 C for 12 hours under vacuum. 29Si MAS NMR spectra were acquired at 79.4 MHz with a 30 pulse, 200 s recycle delay and 250 ~ 400 scans at 9.4 T using a Bruker AVANCE-400 spectrometer. Thermogravimetric analysis (TGA) experiments were performed on a Mettler Toledo TGA SDTA851 in which as-synthesized samples were heated under N 2 at a rate of 10 C/min. The specific surface area, A BET, was determined from the linear part of the BET plot (p/p o =0.05to0.30). Mesopore size was calculated by using the BJH model applied to adsorption data in this study because the structural parameters of this method were more consistent with TEM results. RESULTS AND DISCUSSION Powder X-ray patterns of MCM-41 samples prepared at 165 C for 1 to 7 days are shown in Fig. 1(a). Previous studies 21 have shown that the d 100 spacing increased sharply when crystallization time increased from 1 to 48 h at 165 C. XRD patterns of samples crystallized for 2 days show a very intense (100) diffraction peak and four additional (110), (200), (210), (300) peaks. This suggests that the MCM-165-2d sample has a long ordering of hexagonal pore arrays. However, the d 100 spacing increases slightly from 58.1 to 61.4 Å when crystallization time increases Fig. 1. XRD patterns of (a) calcined MCM-41 prepared at 165 C for 1 to 7 days and at 150, 180 C for 1 or 2 days without hydrothermal treatment and (b) after hydrothermal treatment at 100 C for 1 to 28 days.

3 Hydrothermal Stable MCM-41 Silica J. Chin. Chem. Soc., Vol. 54, No. 1, Table 1. Structural parameters of MCM-41 crystallized at 150, 165, 180 C for 1 to 7 days and after hydrothermal treatment at 100 Cfor1to28days Code of Samples d 100 (Å) D a /Å W b /Å Vt c /cm 3 g -1 A d BET /m 2 g -1 Q 4 /Q 3 MCM-150-2d MCM-150-2d-1d MCM-165-1d MCM-165-2d MCM-165-3d MCM-165-5d MCM-165-7d MCM-165-7d-4d MCM-165-7d-7d MCM-165-7d-14d MCM-165-7d-21d 63.1 e e MCM-165-7d-28d 63.6 e e MCM-180-1d MCM-180-1d-4d a BJH pore diameter calculated from the adsorption branches. b Wall thickness (W = a o D). c Total pore volume. d BET surface area. e The pore size distribution was too broad and irregular to determine the peak diameter and wall thickness. from 2 to 7 days. When the crystallization time increases gradually from 2 to 7 days, (110), (200) diffraction peaks of samples are gradually weakened and (210), (300) peaks progressively disappear. However, N 2 sorption isotherms of all the above samples without hydrothermal treatment exhibit a sharp adsorption/desorption hysteresis (not shown) and indicate that all samples possess good structural ordering and a narrow pore size distribution. Corresponding results are summarized in Table 1. It is shown that the increase of wall thickness results from a slight increase of unit cell and gradual decrease of pore size. Wall thickness increases systematically from 23.3 to 36.1 Å as a result of increasing crystallization time from 1 to 7 days at 165 C. To our best knowledge, the 36.1 Å wall thickness of siliceous MCM-41 reported herein has not been described before. Wall thicknesses of MCM-41 prepared at 180 and 150 C for one or two days are 20.3, 25.3 Å, respectively, and much less than those of samples prepared at 165 C for 7 days. If the pore size is calculated by -plot method 22 on the basis of the geometric model, resulting wall thickness will be smaller by about 10 Å than that calculated by using the BJH model of the adsorption data. However, it is important to note that, whatever method is used to calculate textural parameters (pore size, wall thickness), the wall thickness can be controlled systematically by prolonged crystallization time at 165 C. MCM-41 with the wall thickness of 36.1 Å prepared at 165 C for 7 days shows outstanding hydrothermal stability as compared to those prepared at 180 and 150 C for one or two days from the X-ray data in Fig. 1(b). Relative XRD intensity of MCM-41 synthesized for different times at 165 C after hydrothermal treatment for 1 to 28 days and relative surface area of those samples are shown in Fig. 2. MCM-165-7d after 7 days of hydrothermal treatment at 100 C reserves 62% of (100) X-ray peak intensity and 90% of surface area as compared to the original MCM d in Fig. 2. After further hydrothermal treatment for one week, MCM-165-7d-14d still has 52% of (100) X-ray peak intensity and 90% of surface area. The sharp hysteresis loop of adsorption/desorption and narrow pore size distribution for MCM-165-7d-14d in Fig. 3 illustrates that most of the mesostructure of calcined MCM-41 with ultra-thick walls is retained even after hydrothermal treatment at 100 C for 14 days. However, the mesostructure of MCM-165-7d after 3 or 4 weeks of hydrothermal treatment disintegrates gradually. It is interesting that 26% of the (100) X-ray peak intensity is retained, but the surface area is only 6.6 m 2 /g for MCM-165-7d-28d. The excellent hydrothermal stability of MCM-41 obviously is related to its ultra-thick wall thickness as a result of greater silica condensation after prolonged crystallization time from 2 to 7 days at 165 C. This is evidenced by the 29Si MAS NMR spectra in Fig. 4, which show the Q 4 /Q 3 ratios are gradually enhanced from 2.2 to 4.6. Further evidence for the shrinkages of pore and increase in wall thickness is obtained from thermogravimetric analysis

4 38 J. Chin. Chem. Soc., Vol. 54, No. 1, 2007 Cheng et al. of the as-synthesized samples in Fig. 5. Weight percentages of the occluded template in as-synthesized samples crystallized from 2 to 7 days at 165 C descend from 38 to 22%. Scheme I shows the model for the thickening pore wall of mesoporous MCM-41 silica under mild basic conditions at high temperature. The increase in wall thickness is most likely because some of occluded template of cetyltrimethylammonium (CTMA) cations gradually lose one methyl group to become cetyldimethyl amine (CDMA) neutral molecules. This is evidenced by the 13C MAS NMR spectra, which indicate both CTMA and CDMA in the as-synthesized sample prepared at 165 C for 7 days in Fig. 6. The neutral CDMA molecules gradually withdraw from the micellar pore due to lack of electrostatic interaction with the anionic silicate wall. This seepage of CDMA molecules creates voids that would be filled by the silica source, which results in greater silica condensation and wall thickness under mild basic concentration (0.26 m TMAOH). It seems that the thermal restructuring process involves silica dissolution, transport, redeposition in inner pore wall and facilitates pore wall thickening under mild basic conditions (see supplements). Ozin et al. and Corma et al. 23,24 showed that crystallization of MCM-41 silica at a high temperature Fig. 2. Relative XRD intensity of MCM-41 synthesized for different times at 165 C and relative surface area of MCM-41 prepared for 7 days at 165 C after hydrothermal treatment for 1 to 28 days compared to samples without hydrothermal treatment. Fig. 3. (A) N 2 adsorption/desorption and (B) pore size distribution of MCM-41 silica (a) prepared at 165 C for 7 days and after hydrothermal treatment for different times. (b) 4, (c) 7, (d) 14, (e) 21, (f) 28 days. Fig Si MAS NMR spectra of MCM-41 silica at 165 C for different reaction times. (a) 1, (b) 2, (c) 3, (d) 5, (e) 7 days.

5 Hydrothermal Stable MCM-41 Silica J. Chin. Chem. Soc., Vol. 54, No. 1, Scheme I The model for thickening pore wall of mesoporous MCM-41 silica under mild basic conditions at high temperature of 150 C for a longer reaction time could increase pore size up to 66 Å but the wall thickness is less than 10 Å at high basic concentration of 1.7 m 23 and 0.6 m. 24 It is probable that the crystallization in higher basic conditions favors dissolution of the silica wall, enlargement of pore channels, and disfavors condensation of silica on inner pore wall in Scheme II. CONCLUSION These studies suggest that wall thickness of siliceous MCM-41 can be controlled systematically up to 36.1 Å simply under mild TMAOH concentration at 165 C for 1 to 7 days. A reasonable model explaining formation of a thicker MCM-41 wall, not enlarging the pore channel, is proposed on the basis of TGA and 13C MAS NMR data of samples. The increase in wall thickness is most likely because the thermal restructuring process involves silica dissolution, transport, redeposition in inner pore wall and facilitates pore wall thickening under mild basic conditions. It demonstrates the remarkable hydrothermal stability of MCM-41 synthesized in this study and facilitates practical Fig. 5. Thermogravimetric analysis of as-synthesized MCM-41 prepared at 165 C for (a) 2, (b) 5 and (c) 7 days. Fig C MAS NMR spectra of as-synthesized sample prepared at 165 C for different reaction times. (a) 2, (b) 7 days.

6 40 J. Chin. Chem. Soc., Vol. 54, No. 1, 2007 Cheng et al. Scheme II Affect of base concentration in reaction gel on the pore size and wall thickness of MCM-41 silica applications of mesoporous molecular sieves in the future that most of them mesostructure of calcined MCM-41 with a thick wall is retained even after hydrothermal treatment at 100 C for 14 days. ACKNOWLEDGMENT The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC M & NSC M urd and the Center-of-Excellence Program on Membrane Technology, the Ministry of Education, Taiwan, R.O.C. Received May 19, REFERENCES 1. Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, Corma, A.; Fornés, V.; Navarro, M. T.; Pérez-Pariente, J. J. Catal. 1994, 148, Zhou, J. W.; Gao, F.; Fu, Y. L.; Yang, P. Y.; Zhou, D. Y. Chem. Commun. 2002, Ying, J.; Mehnert, C. P.; Wong, M. S. Angew. Chem. Int. Ed. 1999, 38, Shen, S. C.; Kawi, S. J. Phys. Chem. B. 1999, 103, Shen, S. C.; Kawi, S. Langmuir 2002, 18, Zheng, J.; Zhang, Y.; Li, Z.; Wei, W.; Wu, D.; Sun, Y. Chem. Phys. Letters 2003, 376, Li, Y.; Shi, J.; Hua, Z.; Chen, H.; Ruan, M.; Yan, D. Nano. Letters 2003, 3, Han, Y.; Wu, S.; Sun, Y.; Li, D.; Xiao, F.-S. Chem. Mater. 2002, 14, Zhang, Z.; Han, Y.; Xiao, F.-S.; Qiu, S.; Zhu, L.; Wang, R.; Yu, Y.; Zhang, Z.; Zou, B.; Wang, Y.; Sun, H.; Zhao, D.; Wei, Y. J. Am. Chem. Soc. 2001, 123, Liu, Y.; Pinnavaia, T. J. Chem. Mater. 2002, 14, Li, D.; Su, D. S.; Song, J.; Guan, X.; Hofmann, K.; Xiao, F. S. J. Mater. Chem. 2005, 15, Ryoo, R.; Kim, J. M.; Shin, C. H. J. Phys. Chem. B. 1996, 100, Ryoo, R.; Jun, S. J. Phys. Chem. B. 1997, 101, Kim, J. M.; Jun, S. R.; Ryoo, J. Phys. Chem. B. 1999, 103, Xia, Q.-H.; Hidajat, K.; Kawi, S. Mater. Letter 2000, 42, Xia, Q.-H.; Hidajat, K.; Kawi, S. Chem. Letters, 2001, Das, D.; Tsai, C.-M.; Cheng, S. Chem. Commun. 1999, Pauly, T. R.; Petkov, V.; Liu, Y.; Billinge, S. J. L.; Pinnavaia, T. J. J. Am. Chem. Soc. 2002, 124, Cassiers, K.; Linssen, T.; Mathieu, M.; Benjelloun, M.; Schrijnemakers, K.; Van Der Voort, P.; Cool, P.; Vansant, E. F. Chem. Mater. 2002, 14, Cheng, C. F.; Zhou, W.; Klinowski, J. Chem. Phys. Lett. 1996, 263, Sayari, A.; Liu, P.; Kruk, M.; Jaroniec, M. Chem. Mater. 1997, 9, Khushalani, D.; Kuperman, A.; Ozin, G. A.; Tanaka, K.; Graces, J.; Olken, M. M.; Coombs, N. Adv. Mater. 1995, 7, Corma, A.; Kan, Q.; Navarro, M. T.; Pérez-Pariente, J.; Rey, F. Chem. Mater. 1997, 9, 2123.