Highly siliceous MCM-48 from rice husk ash for CO2 adsorption

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Abstract

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 3-aminopropyltriethoxysilane (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.

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 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., 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.

Section snippets

Chemicals and starting materials

Acetic acid, cetyltrimethylammonium bromide (CTAB: C16H33(CH3)3NBr), polyoxyethylene(23) lauryl ether (PLE), 3-aminopropyltriethoxysilane (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.

Syntheses of MCM-48 (RHA) and APTS-MCM-48 (RHA)

Mesoporous MCM-48 silica was synthesized using a cationic-neutral surfactant mixture as the

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 3° and 5° 2θ which indicates the Ia3d bicontinuous cubic phase which resembles same as that of conventional siliceous MCM-48 from tetraethylorthosilicate (TEOS) (Kim et al., 2005).

Nitrogen adsorption–desorption isotherms

Fig. 2 shows nitrogen adsorption–desorption of the calcined MCM-48 (RHA) exhibited a

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

Acknowledgements

This research was supported by a grant (code CD3-201) from Carbon Dioxide Reduction & Sequestration Research Center, one of the 21st Century Frontier funded by the Ministry of Science and Technology of Korean Government.

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