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Langmuir 2004, 20, 9130-9137
CO2 Adsorption over Si-MCM-41 Materials Having Basic Sites Created by Postmodification with La2O3 S. C. Shen, Xiaoyin Chen, and S. Kawi* Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117576, Republic of Singapore Received January 7, 2004. In Final Form: May 20, 2004 Moderate basic sites could be created onto mesoporous Si-MCM-41 materials by postsynthesis modification with highly dispersed La2O3. The La2O3-modified MCM-41 materials (designated here as LaM) have been characterized by Fourier transform infrared spectroscopy, temperature-programmed desorption, X-ray photoelectron spectroscopy, X-ray diffraction (XRD), and N2 adsorption/desorption and have been tested as model adsorbents for CO2 adsorption. XRD and N2 adsorption results showed that all LaM materials still maintained their uniform hexagonal mesoporous structure even after postsynthesis modification with La2O3 loading up to 20 wt %. Although the surface area, pore size, and pore volume of LaM materials decreased with increasing La2O3 loading, their capacity for CO2 storage could be significantly improved when La2O3 loading was increased from 0 to 10 wt %. Unidentate and bidentate carbonates have been identified by in situ FTIR as the two types of CO2 species adsorbed on LaM surface. The LaM material also possesses good thermal stability, allowing the model adsorbent to be regenerated at high temperature and recyclable.
Introduction Surface acidity and basicity of an oxide material are important properties for the material to have practical application as a catalyst or an adsorbent. Up to now, the acidity of MCM-41 material has been investigated in detail.1-9 The type and strength of acid sites on MCM-41 have been studied by Fourier transform infrared (FTIR) spectroscopy and temperature-programmed desorption (TPD) using basic probe molecules or catalytic techniques.10-12 The surface acidity of MCM-41 was reported to be improved by incorporation of a heterometal, especially Al, or by postmodification.13,14 However, the investigation on the basicity of MCM-41 material is limited, although its basicity is also an important property for catalysts and adsorbents.15 Since Si-MCM-41 and Alincorporated MCM-41 have strong acid sites on the surface, postmodification is required to create basic sites on MCM-41. Some organic amino compounds were re* Corresponding author: e-mail,
[email protected]; telephone, 65-68746312. (1) Mokaya, R.; Jones, W.; Luan, Z. H.; Alba, M. D.; Klinowski, J. Catal. Lett. 1996, 37, 113. (2) Corma, A.; Fornes, V.; Navarro, M. T.; Perez-Pariente, J. J. Catal. 1994, 148, 569. (3) Di Renzo, F.; Chiche, B.; Fajula, F.; Viale, S.; Garrone, E. Stud. Surf. Sci. Catal. 1996, 101, 851. (4) Mokaya, R.; Jones, W. Chem. Commun. 1996, 983. (5) Liepold, A.; Roos, K.; Reschetilowski, W. Chem. Eng. Sci. 1996, 51, 3007. (6) Taouli, A.; Reschetilowski, W. Stud. Surf. Sci. Catal. 2002, 142, 1315. (7) Linssen, T.; Mees, F.; Cassiers, K.; Cool, P.; Whittaker, A.; Vansant, E. F. J. Phys. Chem. B 2003, 107, 8599. (8) Guo, W. P.; Kong, L. D.; Ha, C. S.; Li, Q. Z. Stud. Surf. Sci. Catal. 2003, 146, 307. (9) Twaiq, F. A.; Mohamed, A. R.; Bhatia, S. Microporous Mesoporous Mater. 2003, 64, 95. (10) Hunger, M.; Schenk, U.; Breuninger, M.; Gla¨ser, R.; Weitkamp, J. Microporous Mesoporous Mater. 1999, 27, 261. (11) Weglarski, J.; Datka, J.; He, H. Y.; Klinowski, J. J. Chem. Soc., Faraday Trans. 1996, 92, 5161. (12) Corma, A.; fornes, V.; Navarro, M. T.; Perez-Pariente, J. J. Catal. 1994, 148, 569. (13) Shen, S. C.; Kawi, S. Chem. Lett. 1999, 1293. (14) Mokaya, R.; Jones, W. Chem. Commun. 1998, 1839. (15) Triantafillou, N. D.; Gates, B. C. Langmuir 1999, 15, 2595.
ported to be grafted on the surface of MCM-41,16-18 producing basicity on its surface and rendering selective catalytic activity for the desired reactions.19,20 A solid Brφnsted basic catalyst with a uniform distribution of basic sites with high base strength has been reported by anchoring tetraalkylammonium hydroxide on the surface of MCM-41.21 A novel “molecular basket” CO2 adsorbent has been reported to be prepared by synthesizing and modifying MCM-41 with polyethylenimine (PEI),22 and the CO2 storage capacity was found to be significantly increased by postmodification with the basic PEI. However, these organic amine groups are not stable at high temperature since organic amino species on the surface start to decompose at 200 °C, limiting its application as recyclable adsorbents which require high-temperature regeneration. Due to its high surface area and mesoporous structure, MCM-41 has received great interest for potential application as catalysts23-30 or adsorbents.31-34 The improved surface basicity provides potential for adsorption of acidic (16) Lau, S. H.; Caps, V.; Yeung, K. W.; Wong, K. Y.; Tsang, S. C. Microporous Mesoporous Mater. 1999, 32, 279. (17) Lin, X. H.; Chuah, G. K.; Jaenicke, S. J. Mol. Catal. A: Chem. 1999, 150, 287. (18) Rodriguez, I.; Iborra, S.; Corma, A.; Rey, F.; Jorda, J. Chem. Commun. 1999, 593. (19) Climent, M. J.; Corma, A.; Garcia, H.; Guil-Lopez, R.; Iborra, S.; Fornes, V. J. Catal. 2001, 197, 385. (20) Climent, M. J.; Corma, A.; Iborra, S.; Velty, A. J. Mol. Catal. A: Chem. 2002, 182, 327. (21) Rodriguez, I.; Iborra, S.; Rey, F.; Corma, A. Appl. Catal., A 2000, 194, 241. (22) Xu, X. C.; Song, C.; Andre´sen, J. M.; Miller B. G.; Scaroni, A. W. Microporous Mesoporous Mater. 2003, 62, 29. (23) Jentys, A.; Schiesser, W.; Vinek, H. Catal. Today 2000, 59, 313. (24) Corma, A.; Navarro, M. T.; Pe´rez-Pariente, J. J. Chem. Soc., Chem. Commun. 1994, 147. (25) Kosslick, H.; Lischke, G.; Walther, G.; Storek, W.; Martin, A.; Fricke, R. Microporous Mater. 1997, 9, 13. (26) Lee, C. W.; Lee, W. J.; Park, Y. K.; Park, S. E. Catal. Today 2000, 61, 137. (27) Long, R.; Yang, R. Catal. Lett. 1998, 52, 91. (28) Stoylkova, T. Y.; Chanev, D.; Lechert, H. T.; Bezouhanova, C. P. Appl. Catal., A 2000, 203, 121. (29) Wingen, A.; Anastasievic, N.; Hollnagel, A.; Werner, D.; Schuth, F. J. Catal. 2000, 193, 248.
10.1021/la049947v CCC: $27.50 © 2004 American Chemical Society Published on Web 09/14/2004
CO2 Adsorption
pollution components and controls their emission. Since fossil fuel is still the main primary energy for all over the world, the emission control for the combustion of fossil fuel has been intensively studied.35,36 This is because the serious warning of global warming has been identified to be caused by “greenhouse” gases, such as CO2, NOx, and N2O.37-39 A continuous increase in CO2 emission and its link to global climate changes call for the development of effective approaches to address the CO2 emission from large stationary plants. Thus, CO2 adsorption and its downstream processing have attracted significant attention recently and are believed to be a significant option to mitigate the danger of global warming. Although CO2 has been identified as one of the culprits causing global warming; however a concentrated CO2 stream can be used as an important source for synthetic clean fuels and fine chemicals.40,41 In this regard, honeycomb zeolites,42-46 carbon molecular sieves,47 activated carbon,48-51 carbon nanotubes,52 and modified mesoporous materials53 have been investigated as CO2 storage substrates. Although zeolite-based adsorbents and activated carbons show high adsorption capacity at temperatures below room temperature, their adsorption selectivity to CO2 in the presence of other gases (such as O2 and N2) is low and their CO2 storage capacities decrease quickly with increasing temperature above 30 °C, requiring an intensive cooling system for scrubbing and concentrating CO2 from large power plants. Moreover, carbon-based materials with large surface areas and organic-grafted materials may limit their applications in air due to their combustion with O2 at high temperature during recycling. Thus, MCM-41 mesoporous materials show potential advantages as adsorbents since they possess thermal stability and their large surface area and tunable uniform pore size can be modified with characteristic groups for various adsorption purposes. Using a postmodification method, La(NO3)3 has been used in this paper as a precursor to generate basic La2O3 (30) Tsumura, R.; Higashimoto, S.; Matsuoka, M.; Yamashita, H.; Che, M.; Anpo, M. Catal. Lett. 2000, 68, 101. (31) Llewellyn, P. L.; Grillet, Y.; Schuth, F.; Reichert, H.; Unger, K. K. Microporous Mater. 1994, 3, 345. (32) Choudhary, V. R.; Mantri, K. Langmuir 2000, 16, 7031. (33) Yamazaki, T.; Watanabe, M.; Saito, H. Bull. Chem. Soc. Jpn. 2000, 73, 1353. (34) Kimura, T.; Sugahara, Y.; Kuroda, K. Microporous Mesoporous Mater. 1998, 22, 115. (35) Shen, S. C.; Kawi, S. J. Catal. 2003, 213, 241. (36) Shen, S. C.; Kawi, S. Appl. Catal. B: Environ. 2003, 45, 63. (37) Environmental Challenges and Greenhouse Gas Control for Fossil Fuel Utilization in the 21st Century; Maroto-Valer, M. M., Song, C., Soong, Y., Eds.; Kluwer Academic/Plenum Publishers: New York, 2002; p 447. (38) CO2 Conversion and Utilization; Song, C., Gaffney, A. M., Fujimoto, K., Eds.; ACS Symposium Series 809; American Chemical Societ: Washington, DC, 2002; p 448. (39) Azar, C.; Rodhe, H. Science 1997, 276, 1818. (40) Song, C. Chem. Innovat. 2001, 31, 21. (41) Song, C. ACS Symp. Ser. 2002, No. 809, 2. (42) Kamiuto, K.; Abe, S.; Ermalina Appl. Energy 2002, 72, 555. (43) Triebe, R. W.; Tezel, F. H. Gas Sep. Purif. 1995, 9, 223. (44) Hayhurst, D. T. Chem. Eng. Commun. 1980, 4, 729. (45) Choudhary, V. R.; Mayadevi, S.; Singh, A. P. J. Chem. Soc., Faraday Trans. 1995, 91, 2935. (46) Katoh, M.; Yoshikawa, T.; Tomonama, K.; Tomida, T. J. Colloid Interface Sci. 2000, 226, 145. (47) Burchell, T. D.; Judkins, R. R.; Rogers, M. R.; Williams, A. M. Carbon 1997, 35, 1279. (48) Montoya, A.; Mondrago´n, F.; Truong, T. N. Carbon 2003, 41, 29. (49) Dreisbach, F.; Staudt, R.; Keller, J. U. Adsorption 1999, 5, 215. (50) Heuchel, M.; Davies, G. M.; Buss, E.; Seaton, N. A. Langmuir 1999, 15, 8695. (51) Foeth, F.; Bosch, H.; Sjostrand, A.; Aly, G.; Reith, T. Sep. Sci. Technol. 1996, 31, 21. (52) Cinke, M.; Li, J.; Bauschlicher, C. W.; Jr., Ricca, A.; Meyyappan, M. Chem. Phys. Lett. 2003, 376, 761.
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sites on the surface of siliceous MCM-41. The La2O3modified MCM-41 has moderate basicity and thermal stability and it shows good performance for dynamic adsorption-desorption of CO2. Experimental Section Materials. Purely siliceous MCM-41 (designated here as PSM) was synthesized as follows. Two grams of NaOH was first dissolved in 90 g of deionized water. Silicate gel was prepared by adding 6 g of silica aerosol to the NaOH solution under stirring and heating until the aerosol was completely dissolved. A CTMABr solution (prepared by dissolving 9.1 g of CTMABr in 50 g of deionized water) was added dropwise to the silicate gel under stirring at room temperature. The pH value of the solution was adjusted to 11.5 using 2 N of HCl solution. After being stirred continuously for an additional 6 h at room temperature, the gel mixture was then transferred into a polypropylene bottle and statically heated at 100 °C for 72 h. The resulting solid product was recovered by filtration, washed with deionized water, and dried at 50 °C for 24 h. The as-synthesized samples were calcined in air at 600 °C for 10 h, using a heating rate of 1 °C/min. For postmodification, 1.0 g of PSM was impregnated in 5 mL of La(NO3)3 solution. The slurry was heated at 50 °C under stirring and then dried at 100 °C. Finally, the sample was calcined at 550 °C for 5 h, using a heating ramp of 1 °C/min. By variation of the concentration of La(NO3)3 solution, four La2O3-modified MCM41 materials (designated here as LaM) were prepared to have La2O3 loading of 1 wt % (LaM-1), 5 wt % (LaM-5), 10 wt % (LaM-10), and 20 wt % (LaM-20). N2 Adsorption. Nitrogen adsorption-desorption isotherms were obtained at 77 K by AutoSorb-1-C (Quantachrome). Prior to measurement, the samples were outgassed at 300 °C for 3 h. The specific surface areas of the samples were determined from the linear portion of the BET plots. Pore size distribution was calculated from the desorption branch of N2 desorption isotherm using the conventional Barrett-Joyner-Halenda (BJH) method, as suggested by Tanev and Vlaev,54 because the desorption branch can provide more information about the degree of blocking than the adsorption branch. X-ray Diffraction (XRD). Powder X-ray diffraction patterns of PSM and LaM samples were recorded using a Shimadzu XRD6000 powder diffractometer, where Cu target KR X-rays were used as the X-ray source. X-ray Photoelectron Spectroscopy (XPS). The surface La species was characterized by XPS using a Kratos AXIS analytical instrument. A Mg KR X-ray source (hv ) 1253.6 eV) with an analyzer pass energy of 80 eV was operated at 10 mA and 15 kV. The vacuum in the XPS analysis chamber was less than 10-9 Torr. FTIR of Hydroxyl Groups and CO2 Adsorption. Fifteen milligrams of sample was pressed (at 2 ton/cm2 pressure for 30 min) into a self-supported wafer (16 mm in diameter). Prior to the collection of FTIR spectra characterizing the surface hydroxyl groups, the wafer was heated at 120 °C under vacuum (
