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Langmuir 2002, 18, 4720-4728
MCM-41 with Improved Hydrothermal Stability: Formation and Prevention of Al Content Dependent Structural Defects S. C. Shen and S. Kawi* Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Republic of Singapore Received October 29, 2001. In Final Form: March 6, 2002
The effects of Si/Al ratio of Al-containing MCM-41 materials prepared by either aluminum incorporation or postsynthesis alumination on the formation or prevention of structural defects and improvement of hydrothermal stability of MCM-41 have been investigated in detail. The states of Al and Si in the Alcontaining materials are analyzed by MAS NMR, and the detailed properties of the materials subjected to hydrothermal treatment under different conditions are characterized by ICP, XRD, N2 adsorption, and FTIR. XRD and N2 adsorption results indicate that the amount of structural defects formed during calcination increases with the amount of aluminum in Si-Al-MCM-41. After calcination, Si-MCM-41 and Si-AlMCM-41 having high Si/Al ratio (g50) still show uniform pore structures, but structural defects are formed on the calcined Al-rich MCM-41 (Si/Al ) 25 or 10). Structural defects are formed on all Si-Al-MCM-41 materials during hydrothermal treatment in boiling water. Although the mesoporous framework of SiAl-MCM-41 is still found to be somewhat preserved after treatment in boiling water for 1 month, serious blockage of the pore channel is observed. To prevent the formation of structural defects, postsynthesis alumination has been found to be an effective method to prepare Al-containing MCM-41 with uniform pore structure. Moreover, the resulting uniform mesoporous structure of alumina-modified MCM-41 could be maintained, without the formation of structural defects, upon hydrothermal treatment in boiling water for 1 week.
Introduction The discovery of M41S family of mesoporous materials with uniformly sized unidimensional pore structure has widened the range of heterogeneous catalysis.1 One member of the series having hexagonally arranged uniform pores, i.e., MCM-41, has attracted considerable attention for potential application as catalyst supports or adsorbents because of its high surface areas and large pore volumes. MCM-41 or other mesoporous materials have been reported to be a catalyst or a catalyst support having outstanding catalytic activity for some reactions and as an adsorbent.2-10 In addition, the incorporation of aluminum species into the mesoporous framework of MCM-41 is of special interest because it can produce moderate acidity11,12 which is useful for application in * Corresponding author: Tel (65)8746312, Fax (65)7791936, e-mail
[email protected]. (1) Kresge, C. T.; Leonowicz, M. G.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature (London) 1992, 359, 710. (2) Davis, M. E. Stud. Surf. Sci. Catal. 1999, 121, 23. (3) Corma, A.; Grande, M. S.; Gonzalez-Alfaro, V.; Orchilles, A. V. J. Catal. 1996, 159, 375. (4) Junges, U.; Jacobs, W.; Voigt-Martin, I.; Krutzsch, B.; Schu¨th, F. J. Chem. Soc., Chem. Commun. 1995, 2283. (5) Mokaya, R.; Jones, W.; Luan, Z. H.; Alba, M. D.; Klinowski, J. Catal. Lett. 1996, 37, 113. (6) Koh, C. A.; Nooney, R.; Tahir, S. Catal. Lett. 1997, 47, 3. (7) Liu, A. M.; Hidajat, K.; Kawi, S. J. Mol. Catal. A: Chem. 2001, 168, 303. (8) Long, R.; Yang, R. Catal. Lett. 1998, 52, 91. (9) Anwander, R.; Palm, C.; Gerstberger, G.; Groeger, O.; Engelhardt, G. J. Chem. Soc., Chem. Commun. 1998, 1811. (10) Liu, A. M.; Hidajat, K.; Kawi, S.; Zhao, D. Y. Chem. Commun. 2000, 1145. (11) Corma, A.; Fornes, V.; Navarro, M. T.; Perz-Periente, J. J. Catal. 1994, 148, 569. (12) Weglarski, J.; Datka, J.; He, H. Y.; Klinowski, J. J. Chem. Soc., Faraday Trans. 1996, 92, 5161.
catalysis13 and adsorption.14-17 However, the mesoporous structure of MCM-41 has been reported to be influenced by the amount and type of surfactant,18,19 pH,2 shear and salt effect,20,21 or metal incorporation during the synthesis procedure.22 The incorporation of aluminum in the framework of MCM-41 has been reported to produce mesoporous channels shorter than that of Si-MCM-41, and the pore structure of Si-Al-MCM-41 was found to be distorted and somewhat irregularly arranged as observed by XRD and TEM measurements.22 In addition, different aluminum sources were found to influence the level of crystallinity of Si-Al-MCM-41.23 Furthermore, the incorporation of aluminum is generally accompanied by Na+ or H+ to balance the charge, and because of the difference of these ion sizes, the formation of local structural defects is inevitable upon incorporation of aluminum in the mesoporous framework of MCM-41. (13) Funamoto, G.; Tamura, S.; Segawa, K.; Wan, K. T.; Davis, M. E. Res. Chem. Intermed. 1998, 24, 449. (14) Xia, Q. H.; Hidajat, K.; Kawi, S. Chem. Commun. 2000, 2229. (15) Busio, M.; Ja¨nchen, J.; Van Hoff, J. H. C. Microporous Mater. 1995, 5, 211. (16) Boger, T.; Roesky, R.; Gla¨ser, R.; Ernt, S.; Eigenberger, G.; Weitkamp, J. Microporous Mater. 1997, 8, 19. (17) Branton, P. J.; Hall, P. G.; Treguer, M.; Sing, K. S. W. J. Chem. Soc., Faraday Trans. 1995, 91, 2041. (18) 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.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (19) Tusji, K.; Davis, M. E. Microporous Mater. 1997, 11, 53. (20) Edler, K. J.; Reynolds, P. A.; Brown, A. S.; Slwecki, T. M.; White, J. W. J. Chem. Soc., Faraday Trans. 1998, 94, 1287. (21) Ryoo, R.; Jun, S. J. Phys. Chem. B 1997, 101, 317. (22) Luan, Z. H.; He, H. Y.; Zhou, W. Z.; Cheng, C. F.; Klinowski, J. J. Chem. Soc., Faraday Trans. 1995, 91, 2955. (23) Reddy, K. M.; Song, C. S. Catal. Lett. 1996, 36, 103.
10.1021/la0116140 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/10/2002
MCM-41
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While the incorporation of a substantial amount of aluminum in the framework of MCM-41 may inevitably introduce some deformation of mesopores or the formation of structural defects, the use of aluminum species has been used in recent years to increase not only the acidity but also the hydrothermal stability of the mesoporous materials. For example, we have reported that aluminum substitution in the mesoporous framework can remarkably improve the hydrothermal stability (in boiling water up to 1 week) of Si-Al-MCM-41.24 Postsynthesis alumination has also been used to enhance the hydrothermal, chemical, and mechanical stabilities of alumina-containing MCM-41.25-27 The dependence of hydrothermal stability (in boiling water for up to 48 h) on the amount of Al incorporated in the framework of MCM-41 has also been reported.28 However, because of the potential application of these aluminum-containing MCM-41 materials as catalysts, it is of utmost importance to fully understand the fundamental issues of the effects of aluminum content on the formation of mesoporous structure (with and without structural defects) and the improvement of hydrothermal stability of the mesoporous framework of aluminumcontaining MCM-41 under various hydrothermal treatment conditions. We believe that the knowledge gained from this study can help in the design and synthesis of new aluminum-containing or other metal oxide-containing mesoporous materials possessing uniform mesoporous structure and good hydrothermal stability. In this paper we report the results of our further investigation on the effects of different Si/Al ratios, prepared by direct aluminum incorporation and postsynthesis alumination, on the formation of structural defects on MCM-41 after calcination and hydrothermal treatment in boiling water as well as on the improvement of the hydrothermal stability of MCM-41 in boiling water for long duration up to 1 month. Experimental Section Materials. To investigate the effects of Al substitution on the formation of structural defects and hydrothermal stability of MCM-41, all the samples were synthesized in this study by using only cetyltrimethylammonium hydroxide (CTMAOH) as a surfactant and without any pH adjustment by acid; thus, the synthesis was carried out in the absence of the influence of salt. The detailed synthesis procedure of aluminum-incorporated MCM-41 (i.e., Si-Al-MCM-41) and purely siliceous MCM-41 (i.e., Si-MCM-41) has been described elsewhere.24 Al-containing MCM-41 material having surface Al species was prepared by postsynthesis alumination of calcined Si-MCM-41 with Al(NO3)3. 1.0 g of Si-MCM-41 was impregnated with 5 mL of Al(NO3)3 solution. The slurry was stirred at 50 °C and then dried at 100 °C. Finally, the resulting solid sample was calcined in an airflow at 550 °C for 5 h using a heating rate of 1 °C/min. By changing the concentration of Al(NO3)3 solution, three alumina-modified MCM-41 materials were prepared to have the alumina loading of 0.1, 1.0, and 5.0 wt %. Hydrothermal Treatment. The hydrothermal stability study of MCM-41 was investigated by exposing the samples in a stainless steel reactor (o.d. 1/2 in.) to air streams containing various water vapor concentrations at 600 °C. The flow rate of the air stream was kept at 50 mL/min during hydrothermal treatment. The concentration of water vapor was controlled by the temperature of the water bubbler. In addition, MCM-41 samples were also treated in boiling water in polypropylene bottles and retained at 100 °C for different periods in order to evaluate the (24) Shen, S. C.; Kawi, S. J. Phys. Chem. B 1999, 103, 8870. (25) Kawi, S.; Shen, S. C. Stud. Surf. Sci. Catal. 2000, 129, 227. (26) Mokaya, R.; Jones, W. J. Chem. Commun. 1998, 1839. (27) Mokaya, R. Chem. Commun. 2001, 633. (28) Mokaya, R. J. Phys. Chem. B 2000, 104, 8279.
Figure 1. XRD patterns of calcined Si-MCM-41 and Si-AlMCM-41 with different Si/Al ratios. durability of the mesoporous structure of MCM-41 under severe hydrothermal conditions. Characterization. The powder X-ray diffraction patterns of MCM-41 samples were recorded using a SHIMADZU XRD-6000 powder diffractometer, where Cu target KR-ray was used as the X-ray source. The surface areas and pore properties of MCM-41 materials before and after hydrothermal treatments were analyzed by nitrogen physisorption at 77 K using a Quantachrome AutoSorb1 analyzer. Prior to measurements, each MCM-41 sample was heated at 300 °C under vacuum for 2 h. The pore size distribution was calculated from the desorption branch of N2 isotherms using the conventional Barrett-Joyner-Halenda (BJH) method.29 The MAS 27Al and 29Si NMR spectra were obtained at 300 K with a Bruker FT NMR, DRX-400 MHz instrument operating at 104.3 MHz and 0.5 s of recycle delays. External Al(H2O)3+ was used as a reference for 27Al NMR, and the chemical shift of 29Si (in ppm) was based on the external tetramethylsilane as a reference. The IR spectra were measured using a Shimadzu FTIR-8700 spectrometer having a resolution of 2 cm-1. 15 mg of sample was pressed (under a pressure of 2 ton/cm2 for 30 min) into a selfsupported wafer (16 mm in diameter). Prior to measurement, the wafer was heated at 150 °C under vacuum (
