Materials Letters 109 (2013) 70–73
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Inﬂuence of CTAB molar ratio in tuning the texture of rice husk silica into MCM 41 and SBA-16 N.K. Renuka a,n, A.K. Praveen a, K. Anas b a b
Department of Chemistry, University of Calicut, Kerala 673635, India R&D Division, Sud-Chemie India Pvt. Limited, Binanipuram, Kerala 683502, India
art ic l e i nf o
a b s t r a c t
Article history: Received 18 March 2013 Accepted 20 July 2013 Available online 26 July 2013
An interesting observation of achieving both MCM 41 and SBA-16 from rice husk ash, by varying the CTAB molar ratio is reported. The work furnishes the ﬁrst report on achieving SBA-16 from rice husk ash derived silica using CTAB as the single surfactant. The meso phase has been characterized by low angle XRD, SEM, TEM, and N2 adsorption study. A reduction in CTAB to SiO2 molar ratio by 0.005 altered the mesoporous texture of silica from MCM 41 to SBA-16 under the prevailing conditions. MCM 41 possessed honeycomb like pores with thin walls, and exhibited surface area 807 m2/g. SBA-16, with 226 m2/g surface area exhibited 3D cubic cage structured pores with thick microporous walls of size 8.9 nm. The decisive role of surfactant concentration in fabricating the mesoporous texture of silica is demonstrated in this study. & 2013 Elsevier B.V. All rights reserved.
Keywords: Rice husk ash MCM 41 SBA-16 Electron microscopy Texture
1. Introduction MCM 41 has drawn much attention in the ﬁeld of heterogeneous catalysis due to its high surface area and excellent pore features. The material is characterized by unidirectional cylindrical pores arranged in a honeycomb like structure (space group: p6mm). The pore size can be tailored between 1.5 and 8 nm, and the pore walls are quite thin, ranging from 0.6 to 1.5 nm . The literature is rich with studies concentrated on the synthesis and characterization of this material [2–6]. Researchers have identiﬁed rice husk ash silica as an economic source to yield MCM 41 [7–9]. In an attempt to synthesize MCM 41 from rice husk ash in presence of CTAB (cetyltrimethyl ammonium bromide) following the reported procedure , accidentally we noticed that a minor change in the surfactant concentration has dramatically altered the texture of the meso form, which ended up in a SBA-16 material. SBA-16 is characterized by 3D cubic arrangement of mesopores (Im3m space group), and shows a broad hysteresis loop which closes around P/P0 ¼ 0.45, typical of ink bottle type mesopores. The material is also marked by very high thermal stability and thick inherent microporous walls of size 3.1–10 nm. It has been stated that SBA-16 can be obtained only in a narrow range of dilute surfactant concentration . Here we support this observation by concluding that a slight decrease in the CTAB concentration has altered the MCM 41 porous texture into SBA-16 type under the prevailing conditions. Except as a co-surfactant for
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modifying the pore geometry [11–13], CTAB alone has never been used as surfactant for the fabrication of SBA-16. Since the ordered mesoporous nature of the system is to be unveiled, N2-physisorption study, low angle XRD, and TEM analysis have been used as characterization tools, along with SEM analysis. 2. Experimental section Rice husk selected as silica precursor was purchased from a local industry unit in Malappuram District, Kerala. The synthesis procedure adopted is as follows. Rice husk was washed to remove the impurities, followed by digestion with 10% HCl for 3 h. The residue was washed, dried at 110 1C for 12 h, and calcined at 600 1C for 6 h to yield the amorphous silica. The synthesis of MCM 41 was in accordance with the earlier report . Silica derived from rice husk ash was added to NaOH (E-Merck) solution, and stirred for 3 h, followed by the addition of CTAB (Sigma-Aldrich) solution maintaining the molar ratio 4SiO2:1Na2O:1CTAB:200H2O, and the solution pH 10. The solution was stirred for 48 h, and thoroughly washed with ethanol–water mixture. The solid was ﬁltered, and subjected to calcination at 550 1C for 5 h to obtain MCM 41 system. During the synthesis of SBA-16 material, dilution was affected to CTAB solution by enhancing the mole ratio of water. A difference in CTAB concentration by 0.6 M (amounts to CTAB to SiO2 molar ratio difference 0.005) was made under the existing conditions, maintaining all the other experimental parameters the same. The microstructure of the samples was obtained by transmission electron microscopy (TEM) image, by using Philips CM 200 TEM operating at 20–200 kV range. SEM-JSM
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848 instrument was adopted to study the morphology of the particles. Brucker Nanostar instrument served to obtain the low angle X-ray diffraction (XRD) pattern of the materials. The Brunauer– Emmett–Teller (BET) surface area and Barret–Joyner–Halenda (BJH) pore distribution data were determined using Micromeritics Gemini Surface Area analyzer by the N2 adsorption method at 77 K. The mesopore wall thickness was calculated according to the equation, wall thickness, t¼((√3a0/2) D), where a0 is the lattice parameter and ‘D’ the mesopore diameter.
3. Results and discussion Rice husk ash derived silica was rather amorphous, as obvious from the XRD pattern (not included in the article). The features of both the mesostructures, MCM 41 and SBA-16, are presented simultaneously in order to provide a better comparison of the texture. X-ray diffraction at low angle (Fig. 1) conﬁrmed the mesoporous character of rice husk derived MCM 41 (Fig. 1a) and SBA-16 (Fig. 1b) systems yielded from rice husk ash silica. The former showed major reﬂections at 2θ values, 2.441 and 4.101, along with a low intense one at 4.741 corresponding to (100), (110), and (200) planes, signifying 2D hexagonal pores belonging to P6mm space group. Lattice parameter (a0) of this meso form was obtained as 4.17 nm. Diffraction peaks at d spacings 10.77, 7.61, and 6.01 nm (2θ values 0.821, 1.171, and 1.471), corresponding to (110), (200), and (211) planes were displayed by SBA-16. These reﬂections could be indexed to the cubic body centered space group structure of SBA-16 (Im3m space group) [2,3,6], and the lattice parameter (nanometer) derived from the XRD pattern was 15.14. Both the meso forms were devoid of XRD wide angle peaks, showing the amorphous nature of meso silicas. Electron microscopy images displayed in Fig. 2 enable the morphological differentiation of the two mesoporous forms. Wheat like species (length∼200 nm and width∼70 nm), in interconnected fashion
were noticed in SBA structure (Fig. 2b), while MCM 41 showed highly agglomerated spherical particles with size 100–200 nm (Fig. 2a). MCM41, as expected, exhibited hexagonally arranged mesopores of 3–4 nm, and possessed thin pore walls as shown in Fig. 3a. The other one had ordered pores of diameter ∼4 nm, which were separated from each other by highly thick amorphous walls of silica, as is expected for SBA-16 materials (Fig. 3b). Silica derived from rice husk ash possessed low BET surface area and pore volume: 154 m2/g and 0.313 cm3/g respectively, and the distribution of pore size covered a wide spectrum (not shown here). Fig. 4a and b present the nitrogen adsorption isotherm and associated pore size distribution of MCM 41 and SBA-16, respectively. The isotherms were of type IV according to the IUPAC classiﬁcation . For MCM 41, the characteristic hysteresis loop was observed at 2.5 oP/P0 o 3.8, which indicated a mesoporous material with narrow range of uniform cylindrical pores. These results agree with the BJH pore size distribution patterns (inset Fig. 4a), where one sharp peak appeared in the range of 3–3.8 nm with average pore diameter of 3.6 nm. Wall thickness, achieved by combining the nitrogen physisorption result and XRD data was 0.94 nm, which agrees well with the wall features of MCM 41. The surface area and pore volume values of the system were 602 m2/g and 0.49 cm3/g respectively. Noticeably different adsorption features were exhibited by SBA species, which exhibited H2 type hysteresis loop that closes at P/P0 4.5; a characteristic feature of SBA-16, typical of ink bottle type pores [15–17]. The inset in Fig. 4b shows the distribution of frame work pore diameter within 3–5 nm, with average pore width 4.2 nm. However, the surface area of SBA system was rather low (226 m2/g) when compared with other reported values of the corresponding material [18,19]. Total pore volume achieved via nitrogen adsorption was observed to be 0.37 cm3/g. Presence of intra-wall micrporosity (o 2 nm), another characteristic trait of SBA-16 is also evident from the same ﬁgure. The αs-plot, which compares the adsorption data with a standard isotherm of adsorption on some non-porous solid, yields
6000 5000 4000 3000 2000
2500 2000 1500 1000 (200) (211)
Fig. 1. Low angle XRD pattern of (a) MCM 41 and (b) SBA-16.
Fig. 2. Scanning Electron Microscopy images of (a) MCM 41 and (b) SBA-16.
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Fig. 3. Transmission Electron Microscopy images of (a) MCM 41 and (b) SBA-16.
0.8 0.6 0.4 0.2 0.0
Pore width (nm) 0.4
180 160 140 120 100
Volume adsorbed (cm 3 /g)
Amount adsorbed (cm3/g)
Volume adsorbed (cm3/g)
Amount adsorbed (cm3/g)
0.5 0.4 0.3 0.2 0.1 0.0
2 4 6 8 Pore width (nm)
80 60 40 20 0.0
Relative pressure (P/P0)
Relative pressure (P/P0)
Fig. 4. N2 adsorption/desorption isotherms and pore size distribution proﬁle of (a) MCM 41 and (b) SBA-16.
the amount of micropores within the system. The adsorption in a certain region can be described by a straight line in which the y-intercept describes saturated adsorption isotherm on micropores, whereas the slope includes the external surface area of the solid, which is signiﬁed by adsorption on non-microporous part. Micropore volume of the present system determined via the αs-plot method was 0.17 cm3/g for our SBA type oxide. Presence of intrawall microporosity in SBA-16 is attributed to the partial occlusion of hydrophilic polymeric part into silica walls during synthesis, in presence of triblock copolymer surfactants . Lowmolecular-weight alkyl quaternary ammonium ions also create microporosity in SBA materials, though to a lesser extent , which substantiates the observation made here. A highly thick pore wall was noticed for this oxide (8.9 nm), that enhances the signiﬁcance of the report. Traditionally, strong acidic conditions were followed for the synthesis of SBA structures. The present study deserves attention in the sense that it offers a new versatile, greener route to achieve these materials. This investigation demonstrates that it is possible to tune the texture of silica in such a way to yield two meso forms from the same precursor, adopting the same synthetic route and surfactant. A tentative suggestion to justify this surveillance can be made by quoting Zimny et al.  who suggested that higher concentration of metal oxide precursor leads to squeezing of surfactant cylinders leading to thick pore walls. However, a detailed research in this regard is necessary to completely explain this observation.
4. Conclusions Mesoporous silica materials have been synthesized in presence of CTAB from rice husk ash silica. The characterization revealed
that a minimal variation of surfactant concentration during the synthesis drastically changed the porous feature of the material. A decrease in molar concentration of CTAB by 0.6 in the synthesis mixture, where the difference in the molar ratio between CTAB and SiO2 was 0.005, altered the meso phase from MCM 41 to SBA-16, proving the decisive role of surfactant concentration in deﬁning the texture of mesopores.
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