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Highly siliceous MCM-48 from rice husk ash for CO2 adsorption
International Journal of Greenhouse Gas Control 3 (2009) 545–549
Contents lists available at ScienceDirect
International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Highly siliceous MCM-48 from rice husk ash for CO2 adsorption Hyun Tae Jang a, YoonKook Park b,*, Yong Sig Ko c, Ji Yun Lee a, Bhagiyalakshmi Margandan a a
Chemical Engineering Department, Hanseo University, Seosan 360-706, South Korea Department of Chemical System Engineering, Hongik University, Yongi-gun 339-701, South Korea c Department of Advanced Material Chemistry, Shinsung University, Dangjin-gun 343-860, South Korea b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 August 2008 Received in revised form 17 February 2009 Accepted 25 February 2009 Available online 26 March 2009
Mesoporous MCM-48 silica was synthesized using a cationic-neutral surfactant mixture as the structure-directing template and rice husk ash (RHA) as the silica source. The MCM-48 samples were characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), N2 physisorption and SEM. X-ray diffraction pattern of the resulting MCM-48 revealed typical pattern of cubic Ia3d mesophase. BET results showed the MCM-48 to have a surface area of 1024 m2/g and FT-IR revealed a silanol functional group at about 3460 cm 1. Breakthrough experiments in the presence of MCM-48 were also carried out to test the material’s CO2 adsorption capacity. The breakthrough time for CO2 was found to decrease as the temperature increased from 298 K to 348 K. The steep slopes observed shows the CO2 adsorption occurred very quickly, with only a minimal mass transfer effect and very fast kinetics. In addition, amine grafted MCM-48, APTS-MCM-48 (RHA), was prepared with the 3aminopropyltriethoxysilane (APTS) to investigate the effect of amine functional group in CO2 separation. An order of magnitude higher CO2 adsorption capacity was obtained in the presence of APTS-MCM-48 (RHA) compared to that with MCM-48 (RHA). These results suggest that MCM-48 synthesized from rice husk ash could be usefully applied for CO2 removal. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: MCM-48 Rice husk ash CO2 adsorption Amine grafted
1. Introduction Owing to its high surface area and the ability to select its pore size and surface chemistry via functionalization, mesoporous silica (M41S) has attracted a great deal of attention since it was first reported by researchers at Mobil Oil (Beck et al., 1992). In particular, mesoporous silica has found a wide range of applications in separation (Lee et al., 2004) and catalytic reactions (Melero et al., 2006). One of the most investigated mesoporous silica compounds is MCM-48, which has a three-dimensional interconnected cubic pore structure. Presently, however, both the economic and environmental costs for the large-scale manufacture of these materials are high due to the cost and toxicities of both the templates and the preferred silica source. A variety of silica sources have been reported, including sodium silicate and silicon tetraethoxide (Kim and Stucky, 2000). Interestingly, the original work by Mobil was performed using synthetic precipitated silica and many researchers favor the use of solid fumed silica (Sayari et al., 1999). The industrial manufacture of mesoporous materials is likely to be economically prohibitive, when silicon alkoxides and
* Corresponding author. Tel.: +82 41 860 2296; fax: +82 41 866 6940. E-mail address: [email protected] (Y. Park). 1750-5836/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2009.02.008
fumed silica in particular were employed as silica source. Using the waste product like rice husk and fly ash from rice milling and coal combustion, respectively, as source of silica is quiet beneficial in industrial production of efficient mesoporous materials. Conventional use of coal combustion waste fly ash for synthesis of SBA-15, MCM-41 and MCM-48 have been reported (Chandrasekar et al., 2008; Endud and Wong, 2007; Kumar et al., 2001; Misran et al., 2007). The mesoporous silica Si-MCM-48 in the study was prepared by using rice husk ash (RHA), an agricultural waste of rice milling and is composed primarily of silica and carbon (Kalapathy et al., 2002; Proctor and Palaniappan, 1990). Total rice production in the United States for the crop year 2002 was 21.1 billion lbs of rough rice, and consequently somewhere in the region of 4 billion lbs of rice husk and 26 billion lbs of rice straw were produced (Marshall, 2004). Thus, when RHA is used, the preparation cost of the catalyst are much lower compared to other silica based porous catalyst obtained from fumed silica, organic silicate compounds and silica solution. Recently, preparation of mesoporous materials from agricultural waste such as rice husk, as silica source, has attracted researcher’s attention in economic view. In parallel, tailoring pore properties of mesoporous materials has been examined by many research groups (Kim et al., 2005; Xia et al., 2005; Huang et al.,
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H.T. Jang et al. / International Journal of Greenhouse Gas Control 3 (2009) 545–549
2003; Son et al., 2008). The amine functional groups employed in modifying the mesoporous surface included aliphatic primary amines (Kim et al., 2008), aromatic primary amines (Kim et al., 2005) and polymeric amines (Son et al., 2008). Overall, it is well known that specific interaction between CO2 and amine functional group on the mesoporous surface plays a critical role in CO2 adsorption. CO2 sequestration has been evaluated as one of the major options for reduction of CO2 emission which in turn results in reduction of global warming. Mesoporous materials are found to have good CO2 adsorption capacity and many reported on amine modified MCM-41 and SBA-15 reported the beneficial adsorption (Hicks et al., 2008; Chandrasekar et al., 2008; Hiyoshi et al., 2005). To the best of the authors’ knowledge, the synthesis of MCM-48 from rice husks for CO2 adsorption has not previously been reported. In this study, we synthesized and characterized MCM-48 from RHA as silica source and amine modified MCM-48 using RHA and CO2 adsorption experiment was carried out at low temperature and atmospheric pressure to investigate the effect of amine grafting on the mesoporous surface. 2. Methods 2.1. Chemicals and starting materials Acetic acid, cetyltrimethylammonium bromide (CTAB: C16H33(CH3)3NBr), polyoxyethylene(23) lauryl ether (PLE), 3aminopropyltriethoxysilane (APTS), toluene and sodium hydroxide (98%) were all purchased from Aldrich and used with no further purification. Rice husks were obtained from a local farm, milled and calcined to 873 K for 12 h to obtain RHA. The chemical composition of the RHA was shown in Table 1. 2.2. Syntheses of MCM-48 (RHA) and APTS-MCM-48 (RHA) Mesoporous MCM-48 silica was synthesized using a cationicneutral surfactant mixture as the structure-directing template and RHA as the silica source (Endud and Wong, 2007; Nur et al., 2004). A sodium silicate solution was prepared by combining RHA (93% SiO2) with sodium hydroxide in H2O with stirring and heating at 350 K; while the surfactant mixture was prepared by dissolving CTAB, PLE, simultaneously in H2O with heating at 350 K. The sodium silicate solution and the surfactant solution were mixed and stirred vigorously for 30 min. The resulting mixture was maintained at 373 K under static conditions for 2 days in order to form a surfactant–silica gel mixture of MCM-48 mesophase. The pH of the gel mixture was then adjusted to 10 by addition of acetic acid (30 wt.%), followed by two further days at 373 K. The molar composition of the final gel mixture was 5 SiO2:1.25 Na2O:0.57 PLE:1.07 CTAB:510 H2O. The obtained product was recovered by filtration, washed repeatedly with deionized water and then dried at 373 K for overnight. The removal of the organic templates was done by calcination in air at 823 K for 4 h (heating rate 1 K/min and dwell times of 2 h each at 373 K, 473 K and 623 K). The mesoporous MCM-48 from RHA is designated as MCM-48 (RHA). The method proposed by Kim et al. (2005) was applied to synthesis amine grafted MCM-48, APTS-MCM-48 (RHA). A solution of 0.146 g of APTS in 50 mL of dry toluene was mixed with 1 g
MCM-48 (RHA). The solution was refluxed at 313 K for 24 h. After being filtered, the particles were washed with toluene, ethanol and diethyl ether and dried at 343 K under vacuum for 8 h. The product obtained was denoted as APTS-MCM-48 (RHA). 2.3. Characterization Powder X-ray diffraction patterns were recorded using a Rigaku Miniflex diffractometer with Cu Ka radiation (l = 1.54 A˚). The diffraction data were recorded in the 2u range of 1.5–108 at 18 step size and 1 s step time. The nitrogen adsorption–desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 volumetric adsorption analyzer. Prior to each adsorption measurement the samples were evacuated at 473 K. The specific surface area, SBET, was determined from the linear part of the BET equation. The calculation of the pore size distribution was performed using the Barrett–Joyner–Halenda (BJH) method. The elemental analysis of rice husk ash was performed with an inductively coupled plasma (ICP) spectrometer (Jobin Yuon, JY-38 VHR). Fourier transform infrared (FT-IR) spectra of samples were recorded at room temperature on a Nicolet 6700 spectrometer equipped with an ATR (attenuated total reflection) cell. Each sample was scanned 20 times at 4 cm 1 resolution over the range 4000–400 cm 1. The morphologies of the samples were studied by SEM after gold coating using a FEI Quanta 200 instrument operating at 30 keV and equipped with an EDX detector. 2.4. CO2 adsorption CO2 adsorption measurement was performed for both MCM-48 (RHA) and APTS-MCM-48 (RHA) on home-made fixed bed reactor (Scheme 1) at 298 K, 323 K and 348 K. The adsorbent weight of ca. 300 mg was loaded by porcelain sample pan inside the fixed bed setup. The initial activation of the MCM-48 was carried out at 383 K for 2 h in He atmosphere. The adsorption run was carried out using 1.04% CO2 gas diluted in He gas and feed flow was controlled to 10 mL/min by a mass flow controller to the fixed bed reactor in the experimental setup. The breakthrough curve of CO2 was obtained by analyzing the effluent gases using a gas chromatograph (GC) (Agilent series 6890 N) equipped with a Hayesep Q column (2 m) and a thermal conductivity detector (TCD). 3. Results and discussion 3.1. X-ray diffraction (XRD) The powder X-ray diffraction pattern of calcined MCM-48 (RHA) is shown in Fig. 1. The XRD pattern of the MCM-48 (RHA) exhibits a sharp d211 Bragg reflection, a weak d220 Bragg reflection shoulder and several unresolved peaks between 38 and 58 2u which
Table 1 Chemical composition of the rice husk ash. SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
TiO2
MnO
P2O5
Loss on ignition
93.2
0.13
0.07
1.23
0.25
0.78
0.08
0.006
0.33
0.15
3.66 Scheme 1. Diagram of fixed bed reactor for CO2 adsorption.
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547
Fig. 1. X-ray powder diffraction pattern for MCM-48 (RHA). Fig. 3. FT-IR spectrum of MCM-48 (RHA).
Fig. 2. The N2 adsorption–desorption isotherm of MCM-48 (RHA) powder at 77 K.
indicates the Ia3d bicontinuous cubic phase which resembles same as that of conventional siliceous MCM-48 from tetraethylorthosilicate (TEOS) (Kim et al., 2005). 3.2. Nitrogen adsorption–desorption isotherms Fig. 2 shows nitrogen adsorption–desorption of the calcined MCM-48 (RHA) exhibited a typical type IV curves, indicating that the pore size of the resulting cubic structure was in the range of mesopore (Vermeulen et al., 1984). The isotherm of the MCM-48 (RHA) also exhibits the type H1 hysteresis loop associated with the open-ended cylindrical channel with uniform size and shape. Table 2 shows the specific BET surface
areas, pore volumes, pore diameters for the MCM-48 (RHA) as well as those for the MCM-48 (TEOS). The BET surface area was 1290 m2/g for the MCM-48 from TEOS and 1024 m2/g for the MCM-48 from rice husk ash. The corresponding pore sizes were 2.58 and 4.02 nm and pore volumes were 1.15 and 2.58 cm3/g for the MCM-48 (TEOS) and MCM-48 (RHA), respectively. The specific surface area, pore volume, pore diameter are smaller for amine grafted mesoporous material, APTS-MCM-48 (RHA), than those for pristine counterpart, MCM-48 (RHA). Such significant decrease of textural properties for small-pore materials upon amine grafting was in good agreement with the literature (Kim et al., 2005). The nitrogen content obtained from the elemental analysis was used to determine the surface coverage of 3-aminopropyl groups. As can be seen in Table 2, 2.4 mmol/g of amine functional groups were attached to the MCM-48 (RHA) pore surface. When TEOS was used as a source to prepare the MCM-48, similar results were obtained by Kim et al. (2005). 3.3. FT-IR spectrum Fig. 3 shows the FT-IR spectrum of an MCM-48 (RHA) sample over the range of 4000–400 cm 1. The IR spectrum for MCM-48 (RHA) closely matches with MCM-48 (TEOS) that reported in the literature (Beck et al., 1992). The Si–OH peak appears at about 3460 cm 1, while peaks for the weak single Si–OH groups derived from the germinal Si–OH groups were observed at 3742 cm 1. The sample also exhibited peaks at 805 cm 1, attributed to the Si–O–Si stretching vibration, and 957 cm 1, which was assigned to the symmetric stretching vibration of the Si–OH groups. These results confirm that the surface of MCM-48 (RHA) does indeed possess silanol groups, as expected.
Table 2 Structural properties of MCM-48 (RHA) and APTS-MCM-48 (RHA). Sample
BET surface area (m2/g)
BJH ads. pore volume (cm3/g)
BJH ads. pore diameter (nm)
N (wt.%)
Amine group content (mmol/g)
MCM-48 (RHA) APTS-MCM-48 (RHA)
1024 102
2.58 0.098
4.02 10.24
3.34
2.4
MCM-48 (TEOS)a APTS-MCM-48 (TEOS)a
1290 505
1.15 0.13
2.58 Microporous
3.42
2.45
a
Ref. Kim et al. (2005).
H.T. Jang et al. / International Journal of Greenhouse Gas Control 3 (2009) 545–549
548
Fig. 6. Breakthrough curves for CO2 on APTS-MCM-48 (RHA).
Fig. 4. SEM images of (a) RHA and (b) MCM-48 (RHA).
3.4. Morphology The morphology of the RHA and MCM-48 (RHA) was studied by SEM. Fig. 4a shows SEM images of RHA after calcination at 873 K the ash retains the serrated structure of rice husk and consists mainly of fragments of loose flakes with a skeleton like inner structure (Kordatos et al., 2008). Fig. 4b shows the SEM image of MCM-48 (RHA) consisted of fine spherical particles of 1–1.5 mm in diameter which is similar to already reported (Endud and Wong, 2007). 3.5. Adsorption behavior study In order to test the CO2 adsorption capacity of MCM-48 (RHA), breakthrough experiments were carried out in fixed bed reactor and the results are shown in Fig. 5. Based on these breakthrough curves, it is evident that the breakthrough time for CO2 decreased
as the temperature increased from 298 K to 348 K. The steep slope observed for all the temperatures tested indicates that the CO2 adsorption occurred very quickly, with minimal mass transfer and very fast kinetics. The CO2 adsorption capacity can be calculated to be about 0.069 mmol of CO2/g of sorbent at 323 K on a molar basis based on these breakthrough curves. Though mesoporous silica has uniform and large pores as well as high surface area, a large number of active sites or adsorption sites, its deprived basic sites might be the cause for low CO2 adsorption. Fortunately it carries plenty of defective Si–OH groups as evidenced from FT-IR spectrum can be functionalized with amine compound. Such functionalized mesoporous silica (Chandrasekar et al., 2008) has been reported to have beneficial CO2 adsorption property and the same study is to be undertaken with MCM-48 (RHA) in due course. After mesoporous MCM-48 (RHA) surfaces were modified with amine functional groups, we performed the CO2 adsorption capacity experiments in the presence of APTS-MCM-48 (RHA) to see the effect of amine modification on the surface. In terms of breakthrough curve shape, the experimental results with APTS-MCM-48 (RHA) were similar to that of MCM-48 (RHA), as can be seen in Fig. 6. However, the CO2 adsorption capacity in the presence of APTS-MCM-48 (RHA) is almost an order of magnitude higher than that of MCM-48 (RHA). This may be attributed to the fact that the amine functionality containing mesoporous material has a specific interaction with CO2, which result in higher CO2 adsorption capacity. 4. Conclusions These results clearly demonstrate the feasibility and utility of employing rice husk ash as a source of silica to produce MCM-48 (RHA) mesoporous material. XRD analysis of the product exhibited typical pattern of cubic Ia3d mesophase. There were type IV adsorption isotherm and H1 hysteresis loop in nitrogen adsorption–desorption curves. The BET surface area of the MCM-48 (RHA) was 1024 m2/g. FT-IR spectrometry showed an absorbance peak at 3460 cm 1, which corresponds to the silanol functional group on MCM-48 (RHA). Breakthrough experiments confirmed the suitability of the MCM-48 produced from RHA, a new and abundant source for CO2 capture applications. Further, amino group containing mesoporous material, APTS-MCM-48 (RHA), showed higher CO2 separation capacity than untreated mesoporous material, MCM-48 (RHA). Acknowledgements
Fig. 5. Breakthrough curves for CO2 on MCM-48 (RHA).
This research was supported by a grant (code CD3-201) from Carbon Dioxide Reduction & Sequestration Research Center, one of
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the 21st Century Frontier funded by the Ministry of Science and Technology of Korean Government. References 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., Huggins, J.B., Schlenker, J.L., 1992. A new family of mesoporous molecular sieves prepared with liquid crystal templates. J. Am. Chem. Soc. 114, 10834–10843. Chandrasekar, G., Son, W.J., Ahn, W.S., 2008. Synthesis of mesoporous materials SBA-15 and CMK-3 from fly ash and their application for CO2 adsorption. J. Por. Mater. doi:10.1007/s10934-008-9231-x (online first). Endud, S., Wong, K., 2007. Mesoporous silica MCM-48 molecular sieve modified with SnCl2 in alkaline medium for selective oxidation of alcohol. Micropor. Mesopor. Mater. 101, 256–263. Hicks, J.C., Drese, J.H., Fauth, D.J., Gray, M.L., Qi, G., Jones, C.W., 2008. Designing adsorbents for CO2 capture from flue gas-hyperbranched aminosilicas capable of capturing CO2 reversibly. J. Am. Chem. Soc. 130, 2902–2903. Hiyoshi, N., Yogo, K., Yashima, T., 2005. Adsorption characteristics of carbon dioxide on organically functionalized SBA-15. Micropor. Mesopor. Mater. 84, 357–365. Huang, H.Y., Yang, R.T., Chinn, D., Munson, C.L., 2003. Amine-grafted MCM-48 and silica xerogel as superior sorbents for acidic gas removal from natural gas. Ind. Eng. Chem. Res. 42, 2427–2433. Kalapathy, U., Proctor, A., Shultz, J., 2002. An improved method for production of silica from rice hull ash. Bioresour. Technol. 85, 285–289. Kim, J.M., Stucky, G.D., 2000. Synthesis of highly ordered mesoporous silica materials using sodium silicate and amphiphilic block copolymers. Chem. Commun. 1159–1160. Kim, S., Ida, J., Guliants, V.V., Lin, Y.S., 2005. Tailoring pore properties of MCM-48 silica for selective adsorption of CO2. J. Phys. Chem. B 109, 6287–6293. Kim, S., Son, W.J., Choi, J.S., Ahn, W.S., 2008. CO2 adsorption using amine-functionalized mesoporous silica prepared via anionic surfactant-mediated synthesis. Micropor. Mesopor. Mater. 115, 497–503.
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Kordatos, K., Gavela, S., Ntziouni, A., Pistiolas, K.N., Kyritsi, A., Kasselouri, R.V., 2008. Synthesis of highly siliceous ZSM-5 zeolite using silica from rice husk ash. Micropor. Mesopor. Mater. 115, 189–196. Kumar, P., Mal, N., Oumi, Y., Yamana, K., Sano, T., 2001. Mesoporous materials prepared using coal fly ash as the silicon and aluminium source. J. Mater. Chem. 11, 3285–3290. Lee, J.W., Cho, D.L., Shim, W.G., Moon, H., 2004. Application of mesoporous MCM-48 and SBA-15 materials for the separation of biochemicals dissolved in aqueous solution. Kor. J. Chem. Eng. 21, 246–251. Marshall, W.E., 2004. Rice hull and rice straw. In: Champagne, E.T. (Ed.), Rice Chemistry and Technology. 3rd ed. AACC Press, Washington, DC, pp. 611– 630. Melero, J.A., van Grieken, R., Morales, G., 2006. Advances in the synthesis and catalytic applications of organosulfonic-functionalized mesostructured materials. Chem. Rev. 106, 3790–3812. Misran, H., Ramesh, S., Shahida, B., Mohd, A.Y., 2007. Processing of mesoporous silica materials (MCM-41) from coal fly ash. J. Mater. Process. Technol. 186, 8–13. Nur, H., Lau, C.G., Endud, S., Hamdan, H., 2004. Quantitative measurement of a mixture of mesophases cubic MCM-48 and hexagonal MCM-41 by 13C CP/MAS NMR. Mater. Lett. 58, 1971–1974. Proctor, A., Palaniappan, S., 1990. Adsorption of soy oil free fatty acids by rice hull ash. J. Am. Oil Chem. Soc. 67, 15–17. Sayari, A., Yang, Y., Kruck, M., Jaroniec, M., 1999. Expanding the pore size of MCM-41 silicas: use of amines as expanders in direct synthesis and postsynthesis procedures. J. Phys. Chem. B 103, 3651–3658. Son, W.J., Choi, J.S., Ahn, W.S., 2008. Adsorptive removal of carbon dioxide using polyethyleneimine-loaded mesoporous silica materials. Micropor. Mesopor. Mater. 113, 31–40. Vermeulen, T., Le Van, M.D., Hiester, N.K., Klein, G., 1984. Adsorption and ion exchange. In: Perry, R.H., Green, D. (Eds.), Perry’s Chemical Engineers’ Handbook. 50th ed. McGraw-Hill, New York. Xia, Q.H., Su, K.X., Ma, X.T., Ge, H.Q., Zhu, H.B., 2005. Efficiently tailoring the pore diameter of mesoporous MCM-48 to micropore. Mater. Lett. 59, 2110–2114.
Contents lists available at ScienceDirect
International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Highly siliceous MCM-48 from rice husk ash for CO2 adsorption Hyun Tae Jang a, YoonKook Park b,*, Yong Sig Ko c, Ji Yun Lee a, Bhagiyalakshmi Margandan a a
Chemical Engineering Department, Hanseo University, Seosan 360-706, South Korea Department of Chemical System Engineering, Hongik University, Yongi-gun 339-701, South Korea c Department of Advanced Material Chemistry, Shinsung University, Dangjin-gun 343-860, South Korea b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 August 2008 Received in revised form 17 February 2009 Accepted 25 February 2009 Available online 26 March 2009
Mesoporous MCM-48 silica was synthesized using a cationic-neutral surfactant mixture as the structure-directing template and rice husk ash (RHA) as the silica source. The MCM-48 samples were characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), N2 physisorption and SEM. X-ray diffraction pattern of the resulting MCM-48 revealed typical pattern of cubic Ia3d mesophase. BET results showed the MCM-48 to have a surface area of 1024 m2/g and FT-IR revealed a silanol functional group at about 3460 cm 1. Breakthrough experiments in the presence of MCM-48 were also carried out to test the material’s CO2 adsorption capacity. The breakthrough time for CO2 was found to decrease as the temperature increased from 298 K to 348 K. The steep slopes observed shows the CO2 adsorption occurred very quickly, with only a minimal mass transfer effect and very fast kinetics. In addition, amine grafted MCM-48, APTS-MCM-48 (RHA), was prepared with the 3aminopropyltriethoxysilane (APTS) to investigate the effect of amine functional group in CO2 separation. An order of magnitude higher CO2 adsorption capacity was obtained in the presence of APTS-MCM-48 (RHA) compared to that with MCM-48 (RHA). These results suggest that MCM-48 synthesized from rice husk ash could be usefully applied for CO2 removal. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: MCM-48 Rice husk ash CO2 adsorption Amine grafted
1. Introduction Owing to its high surface area and the ability to select its pore size and surface chemistry via functionalization, mesoporous silica (M41S) has attracted a great deal of attention since it was first reported by researchers at Mobil Oil (Beck et al., 1992). In particular, mesoporous silica has found a wide range of applications in separation (Lee et al., 2004) and catalytic reactions (Melero et al., 2006). One of the most investigated mesoporous silica compounds is MCM-48, which has a three-dimensional interconnected cubic pore structure. Presently, however, both the economic and environmental costs for the large-scale manufacture of these materials are high due to the cost and toxicities of both the templates and the preferred silica source. A variety of silica sources have been reported, including sodium silicate and silicon tetraethoxide (Kim and Stucky, 2000). Interestingly, the original work by Mobil was performed using synthetic precipitated silica and many researchers favor the use of solid fumed silica (Sayari et al., 1999). The industrial manufacture of mesoporous materials is likely to be economically prohibitive, when silicon alkoxides and
* Corresponding author. Tel.: +82 41 860 2296; fax: +82 41 866 6940. E-mail address: [email protected] (Y. Park). 1750-5836/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2009.02.008
fumed silica in particular were employed as silica source. Using the waste product like rice husk and fly ash from rice milling and coal combustion, respectively, as source of silica is quiet beneficial in industrial production of efficient mesoporous materials. Conventional use of coal combustion waste fly ash for synthesis of SBA-15, MCM-41 and MCM-48 have been reported (Chandrasekar et al., 2008; Endud and Wong, 2007; Kumar et al., 2001; Misran et al., 2007). The mesoporous silica Si-MCM-48 in the study was prepared by using rice husk ash (RHA), an agricultural waste of rice milling and is composed primarily of silica and carbon (Kalapathy et al., 2002; Proctor and Palaniappan, 1990). Total rice production in the United States for the crop year 2002 was 21.1 billion lbs of rough rice, and consequently somewhere in the region of 4 billion lbs of rice husk and 26 billion lbs of rice straw were produced (Marshall, 2004). Thus, when RHA is used, the preparation cost of the catalyst are much lower compared to other silica based porous catalyst obtained from fumed silica, organic silicate compounds and silica solution. Recently, preparation of mesoporous materials from agricultural waste such as rice husk, as silica source, has attracted researcher’s attention in economic view. In parallel, tailoring pore properties of mesoporous materials has been examined by many research groups (Kim et al., 2005; Xia et al., 2005; Huang et al.,
546
H.T. Jang et al. / International Journal of Greenhouse Gas Control 3 (2009) 545–549
2003; Son et al., 2008). The amine functional groups employed in modifying the mesoporous surface included aliphatic primary amines (Kim et al., 2008), aromatic primary amines (Kim et al., 2005) and polymeric amines (Son et al., 2008). Overall, it is well known that specific interaction between CO2 and amine functional group on the mesoporous surface plays a critical role in CO2 adsorption. CO2 sequestration has been evaluated as one of the major options for reduction of CO2 emission which in turn results in reduction of global warming. Mesoporous materials are found to have good CO2 adsorption capacity and many reported on amine modified MCM-41 and SBA-15 reported the beneficial adsorption (Hicks et al., 2008; Chandrasekar et al., 2008; Hiyoshi et al., 2005). To the best of the authors’ knowledge, the synthesis of MCM-48 from rice husks for CO2 adsorption has not previously been reported. In this study, we synthesized and characterized MCM-48 from RHA as silica source and amine modified MCM-48 using RHA and CO2 adsorption experiment was carried out at low temperature and atmospheric pressure to investigate the effect of amine grafting on the mesoporous surface. 2. Methods 2.1. Chemicals and starting materials Acetic acid, cetyltrimethylammonium bromide (CTAB: C16H33(CH3)3NBr), polyoxyethylene(23) lauryl ether (PLE), 3aminopropyltriethoxysilane (APTS), toluene and sodium hydroxide (98%) were all purchased from Aldrich and used with no further purification. Rice husks were obtained from a local farm, milled and calcined to 873 K for 12 h to obtain RHA. The chemical composition of the RHA was shown in Table 1. 2.2. Syntheses of MCM-48 (RHA) and APTS-MCM-48 (RHA) Mesoporous MCM-48 silica was synthesized using a cationicneutral surfactant mixture as the structure-directing template and RHA as the silica source (Endud and Wong, 2007; Nur et al., 2004). A sodium silicate solution was prepared by combining RHA (93% SiO2) with sodium hydroxide in H2O with stirring and heating at 350 K; while the surfactant mixture was prepared by dissolving CTAB, PLE, simultaneously in H2O with heating at 350 K. The sodium silicate solution and the surfactant solution were mixed and stirred vigorously for 30 min. The resulting mixture was maintained at 373 K under static conditions for 2 days in order to form a surfactant–silica gel mixture of MCM-48 mesophase. The pH of the gel mixture was then adjusted to 10 by addition of acetic acid (30 wt.%), followed by two further days at 373 K. The molar composition of the final gel mixture was 5 SiO2:1.25 Na2O:0.57 PLE:1.07 CTAB:510 H2O. The obtained product was recovered by filtration, washed repeatedly with deionized water and then dried at 373 K for overnight. The removal of the organic templates was done by calcination in air at 823 K for 4 h (heating rate 1 K/min and dwell times of 2 h each at 373 K, 473 K and 623 K). The mesoporous MCM-48 from RHA is designated as MCM-48 (RHA). The method proposed by Kim et al. (2005) was applied to synthesis amine grafted MCM-48, APTS-MCM-48 (RHA). A solution of 0.146 g of APTS in 50 mL of dry toluene was mixed with 1 g
MCM-48 (RHA). The solution was refluxed at 313 K for 24 h. After being filtered, the particles were washed with toluene, ethanol and diethyl ether and dried at 343 K under vacuum for 8 h. The product obtained was denoted as APTS-MCM-48 (RHA). 2.3. Characterization Powder X-ray diffraction patterns were recorded using a Rigaku Miniflex diffractometer with Cu Ka radiation (l = 1.54 A˚). The diffraction data were recorded in the 2u range of 1.5–108 at 18 step size and 1 s step time. The nitrogen adsorption–desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 volumetric adsorption analyzer. Prior to each adsorption measurement the samples were evacuated at 473 K. The specific surface area, SBET, was determined from the linear part of the BET equation. The calculation of the pore size distribution was performed using the Barrett–Joyner–Halenda (BJH) method. The elemental analysis of rice husk ash was performed with an inductively coupled plasma (ICP) spectrometer (Jobin Yuon, JY-38 VHR). Fourier transform infrared (FT-IR) spectra of samples were recorded at room temperature on a Nicolet 6700 spectrometer equipped with an ATR (attenuated total reflection) cell. Each sample was scanned 20 times at 4 cm 1 resolution over the range 4000–400 cm 1. The morphologies of the samples were studied by SEM after gold coating using a FEI Quanta 200 instrument operating at 30 keV and equipped with an EDX detector. 2.4. CO2 adsorption CO2 adsorption measurement was performed for both MCM-48 (RHA) and APTS-MCM-48 (RHA) on home-made fixed bed reactor (Scheme 1) at 298 K, 323 K and 348 K. The adsorbent weight of ca. 300 mg was loaded by porcelain sample pan inside the fixed bed setup. The initial activation of the MCM-48 was carried out at 383 K for 2 h in He atmosphere. The adsorption run was carried out using 1.04% CO2 gas diluted in He gas and feed flow was controlled to 10 mL/min by a mass flow controller to the fixed bed reactor in the experimental setup. The breakthrough curve of CO2 was obtained by analyzing the effluent gases using a gas chromatograph (GC) (Agilent series 6890 N) equipped with a Hayesep Q column (2 m) and a thermal conductivity detector (TCD). 3. Results and discussion 3.1. X-ray diffraction (XRD) The powder X-ray diffraction pattern of calcined MCM-48 (RHA) is shown in Fig. 1. The XRD pattern of the MCM-48 (RHA) exhibits a sharp d211 Bragg reflection, a weak d220 Bragg reflection shoulder and several unresolved peaks between 38 and 58 2u which
Table 1 Chemical composition of the rice husk ash. SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
TiO2
MnO
P2O5
Loss on ignition
93.2
0.13
0.07
1.23
0.25
0.78
0.08
0.006
0.33
0.15
3.66 Scheme 1. Diagram of fixed bed reactor for CO2 adsorption.
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Fig. 1. X-ray powder diffraction pattern for MCM-48 (RHA). Fig. 3. FT-IR spectrum of MCM-48 (RHA).
Fig. 2. The N2 adsorption–desorption isotherm of MCM-48 (RHA) powder at 77 K.
indicates the Ia3d bicontinuous cubic phase which resembles same as that of conventional siliceous MCM-48 from tetraethylorthosilicate (TEOS) (Kim et al., 2005). 3.2. Nitrogen adsorption–desorption isotherms Fig. 2 shows nitrogen adsorption–desorption of the calcined MCM-48 (RHA) exhibited a typical type IV curves, indicating that the pore size of the resulting cubic structure was in the range of mesopore (Vermeulen et al., 1984). The isotherm of the MCM-48 (RHA) also exhibits the type H1 hysteresis loop associated with the open-ended cylindrical channel with uniform size and shape. Table 2 shows the specific BET surface
areas, pore volumes, pore diameters for the MCM-48 (RHA) as well as those for the MCM-48 (TEOS). The BET surface area was 1290 m2/g for the MCM-48 from TEOS and 1024 m2/g for the MCM-48 from rice husk ash. The corresponding pore sizes were 2.58 and 4.02 nm and pore volumes were 1.15 and 2.58 cm3/g for the MCM-48 (TEOS) and MCM-48 (RHA), respectively. The specific surface area, pore volume, pore diameter are smaller for amine grafted mesoporous material, APTS-MCM-48 (RHA), than those for pristine counterpart, MCM-48 (RHA). Such significant decrease of textural properties for small-pore materials upon amine grafting was in good agreement with the literature (Kim et al., 2005). The nitrogen content obtained from the elemental analysis was used to determine the surface coverage of 3-aminopropyl groups. As can be seen in Table 2, 2.4 mmol/g of amine functional groups were attached to the MCM-48 (RHA) pore surface. When TEOS was used as a source to prepare the MCM-48, similar results were obtained by Kim et al. (2005). 3.3. FT-IR spectrum Fig. 3 shows the FT-IR spectrum of an MCM-48 (RHA) sample over the range of 4000–400 cm 1. The IR spectrum for MCM-48 (RHA) closely matches with MCM-48 (TEOS) that reported in the literature (Beck et al., 1992). The Si–OH peak appears at about 3460 cm 1, while peaks for the weak single Si–OH groups derived from the germinal Si–OH groups were observed at 3742 cm 1. The sample also exhibited peaks at 805 cm 1, attributed to the Si–O–Si stretching vibration, and 957 cm 1, which was assigned to the symmetric stretching vibration of the Si–OH groups. These results confirm that the surface of MCM-48 (RHA) does indeed possess silanol groups, as expected.
Table 2 Structural properties of MCM-48 (RHA) and APTS-MCM-48 (RHA). Sample
BET surface area (m2/g)
BJH ads. pore volume (cm3/g)
BJH ads. pore diameter (nm)
N (wt.%)
Amine group content (mmol/g)
MCM-48 (RHA) APTS-MCM-48 (RHA)
1024 102
2.58 0.098
4.02 10.24
3.34
2.4
MCM-48 (TEOS)a APTS-MCM-48 (TEOS)a
1290 505
1.15 0.13
2.58 Microporous
3.42
2.45
a
Ref. Kim et al. (2005).
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Fig. 6. Breakthrough curves for CO2 on APTS-MCM-48 (RHA).
Fig. 4. SEM images of (a) RHA and (b) MCM-48 (RHA).
3.4. Morphology The morphology of the RHA and MCM-48 (RHA) was studied by SEM. Fig. 4a shows SEM images of RHA after calcination at 873 K the ash retains the serrated structure of rice husk and consists mainly of fragments of loose flakes with a skeleton like inner structure (Kordatos et al., 2008). Fig. 4b shows the SEM image of MCM-48 (RHA) consisted of fine spherical particles of 1–1.5 mm in diameter which is similar to already reported (Endud and Wong, 2007). 3.5. Adsorption behavior study In order to test the CO2 adsorption capacity of MCM-48 (RHA), breakthrough experiments were carried out in fixed bed reactor and the results are shown in Fig. 5. Based on these breakthrough curves, it is evident that the breakthrough time for CO2 decreased
as the temperature increased from 298 K to 348 K. The steep slope observed for all the temperatures tested indicates that the CO2 adsorption occurred very quickly, with minimal mass transfer and very fast kinetics. The CO2 adsorption capacity can be calculated to be about 0.069 mmol of CO2/g of sorbent at 323 K on a molar basis based on these breakthrough curves. Though mesoporous silica has uniform and large pores as well as high surface area, a large number of active sites or adsorption sites, its deprived basic sites might be the cause for low CO2 adsorption. Fortunately it carries plenty of defective Si–OH groups as evidenced from FT-IR spectrum can be functionalized with amine compound. Such functionalized mesoporous silica (Chandrasekar et al., 2008) has been reported to have beneficial CO2 adsorption property and the same study is to be undertaken with MCM-48 (RHA) in due course. After mesoporous MCM-48 (RHA) surfaces were modified with amine functional groups, we performed the CO2 adsorption capacity experiments in the presence of APTS-MCM-48 (RHA) to see the effect of amine modification on the surface. In terms of breakthrough curve shape, the experimental results with APTS-MCM-48 (RHA) were similar to that of MCM-48 (RHA), as can be seen in Fig. 6. However, the CO2 adsorption capacity in the presence of APTS-MCM-48 (RHA) is almost an order of magnitude higher than that of MCM-48 (RHA). This may be attributed to the fact that the amine functionality containing mesoporous material has a specific interaction with CO2, which result in higher CO2 adsorption capacity. 4. Conclusions These results clearly demonstrate the feasibility and utility of employing rice husk ash as a source of silica to produce MCM-48 (RHA) mesoporous material. XRD analysis of the product exhibited typical pattern of cubic Ia3d mesophase. There were type IV adsorption isotherm and H1 hysteresis loop in nitrogen adsorption–desorption curves. The BET surface area of the MCM-48 (RHA) was 1024 m2/g. FT-IR spectrometry showed an absorbance peak at 3460 cm 1, which corresponds to the silanol functional group on MCM-48 (RHA). Breakthrough experiments confirmed the suitability of the MCM-48 produced from RHA, a new and abundant source for CO2 capture applications. Further, amino group containing mesoporous material, APTS-MCM-48 (RHA), showed higher CO2 separation capacity than untreated mesoporous material, MCM-48 (RHA). Acknowledgements
Fig. 5. Breakthrough curves for CO2 on MCM-48 (RHA).
This research was supported by a grant (code CD3-201) from Carbon Dioxide Reduction & Sequestration Research Center, one of
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