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Materials Letters 62 (2008) 422 – 424 www.elsevier.com/locate/matlet
A facile way to synthesize mesoporous silica with Ia3d cubic symmetry Hong Ji Wang, Zheng Ying Wu, Yi Meng Wang, Jian Hua Zhu ⁎ Key Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China Received 23 March 2007; accepted 15 May 2007 Available online 25 May 2007
Abstract A facile synthesis route of the mesoporous silica with Ia3d cubic symmetry is reported for the first time, in which the MCM-48 type materials can be prepared at room temperature, instead of high or low temperature as usual, by using cetylpyridinium chloride (CPyCl) as template, tetraethyl orthosilicate (TEOS) as a silica source and HNO3 as the acid catalyst in the presence of phenol. The resulting samples exhibit a larger surface area than their analogues synthesized under alkaline conditions, which will be beneficial for their potential applications in adsorption and catalysis. Moreover, the influences of HNO3/TEOS molar composition and the amount of phenol on the formation of Ia3d cubic mesostructure are examined and the actual function of these is described in terms of g value. © 2007 Elsevier B.V. All rights reserved. Keywords: Mesoporous materials; Porosity; Ia3d cubic symmetry; MCM-48 type; Synthesis
1. Introduction Among ordered mesoporous materials MCM-48 is the versatile candidate for separation and catalysis , because of its cubic Ia3d symmetry possessing a bicontinuous structure centered on gyroid minimal surface that divides available pore space into two nonintersecting subvolumes. This special structure enables the mass-transfer to be faster in MCM-48 than that in MCM-41. However, MCM-48 is traditionally synthesized under alkaline conditions within a narrow range of compositions along with quite high or low temperatures , the restricted condition makes fewer reports published on the synthesis of MCM-48 than those related to MCM-41 . Hence, it is valuable to expand the synthesis domain of MCM48 type materials and thus a cationic alkylammonium surfactant , mixed cationic/anionic surfactants , or cationic/nonionic surfactants are tried as templates to develop several methodologies. In general the nonionic triblock copolymer-triblock poly (ethylene oxide)-b-poly (propylene oxide)-b-poly (ethylene oxide) copolymer (EO20PO70EO20, P123) is used as the template in acidic system . Until now, however, few are known on the synthesis of mesoporous silica with Ia3d cubic symmetry under room temperature by using cationic surfactant ⁎ Corresponding author. Tel.: +86 25 83595848; fax: +86 25 83317761. E-mail address: [email protected]
(J.H. Zhu). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.05.039
in acidic system. In this short paper, MCM-48 type material is synthesized, for the first time, by using cetylpyridinium chloride (CPyCl) as template in the solution of HNO3 under room temperature, and phenol as additive. Rather, the impact of HNO3/TEOS molar composition, along with the amount of phenol is examined to investigate the formation of Ia3d cubic mesostructure. 2. Experimental In a typical synthesis of cubic Ia3d mesoporous silica, 0.96 g CPyCl and 45 g H2O were mixed and then 0.28 g phenol was added under vigorous stirring. After phenol was dissolved, 4.25 g TEOS followed by different amount of HNO3 was added. The molar composition of the reaction mixture was 0.13 CPyCl:1 TEOS:x HNO3:122 H2O:0.13 phenol, where x varied in the range of 1–5. After stirring for 24 h, the obtained precipitation was filtered and air-dried, finally calcined at 773 K for 5 h. By changing the amount of phenol, mesoporous silicas with different structures were synthesized. In order to compare the impact of acid and template, HCl or H2SO4 was employed as acid catalyst and cetyltrimethylammonium bromide (CTAB) used as template. To characterize sample, X-ray diffraction (XRD) patterns were recorded on an ARL XTRA diffractometer in the 2-theta range of 0.5–8°. The Brunauer–Emmett–Teller (BET) specific
H.J. Wang et al. / Materials Letters 62 (2008) 422–424
Table 1 Impact of HNO3/TEOS molar ratio on the textural properties of MCM-48 type sample HNO3/TEOS
SBET (m2 g− 1)
Vp (cm3 g− 1)
1 2 3
1062 1479 1221
0.63 1.05 0.64
2.56 2.37 2.29
SBET: BET surface area, Vp: pore volume; D: pore diameter.
Fig. 1A illustrates the low angle XRD patterns of the as-synthesized mesoporous silica prepared with different molar ratio of HNO3/TEOS. When the molar ratio is 1, two characteristic peaks (211 and 220) emerge with the 2-theta value of 2.22° and 2.54°, and the corresponding interplanar distances (d) are 3.98 and 3.48 nm respectively. The reciprocal of interplanar distances (1/d) have a linear relationship with (h2 + k2 + l2)1/2, which satisfies with qualification of Ia3d cubic symmetry. In addition, four other diffraction peaks appear in the 2-theta range from 3.0° to 4.5°. All of these peaks can be indexed as (211), (220), (321), (400), (420) and (332) of space group Ia3d . Increasing the molar ratio of HNO3/TEOS to 1.5 or 2, similarly results are gained to indicate the resulting cubic Ia3d phase. However,
calcination makes the sample synthesized with the HNO3/TEOS molar ratio of 2 to show the lower lattice parameter while the intensity of the main peak is distinctively improved, indicating the condensation of the mesostructure and the improved long range ordering of the sample (Fig. 1B). When the molar ratio of HNO3/TEOS rises to 2.5, a new shoulder peak appears and it is difficult to discern. Once the molar ratio of HNO3/TEOS exceeds 3.5, less ordered mesostructures form as shown in Fig. 1 (curves g–h). The presence of acid is in favor of the condensation but the excess acid makes the silica condensation too quick to form the ordered mesostructure . Fig. 2 illustrates the N2 adsorption–desorption isotherms of the calcined samples synthesized with different molar ratio of HNO3/TEOS. All of the isotherms are type IV, the same as that of MCM-48 with a sharp capillary condensation step at relative pressures between 0.20 and 0.35. Narrow pore size distributions are also observed in the samples (Fig. 2B). The materials prepared with phenol additive possess larger surface area (Table 1, 1060–1470 m2 g− 1) than the MCM-48 synthesized with CTAB template under alkaline conditions (660–1010 m2 g− 1) . Phenol is the necessary ingredient in this synthesis. If the molar composition of the reaction mixture was 0.13 CPyCl:1 TEOS:1 HNO3:122 H2O:0.13 phenol, the mesoporous silica with Ia3d cubic symmetry is obtained. However, only hexagonal p6mm mesoporous silica forms when phenol is absent in the system (Fig. 3A). In case too much phenol is used (0.13 CPyCl:1 TEOS:1 HNO3:122 H2O:0.26 phenol), mesoporous silica with lamellar structure is produced (Fig. 3A). HNO3 is another necessary ingredient in this synthetic system. When HCl or H2SO4 replaces HNO3, only hexagonal p6mm mesophase forms (Fig. 3B). CPyCl plays a crucial role in the structure formation of mesoporous silica with Ia3d cubic. When CTAB is used as the template, hexagonal p6mm mesoporous silica is received (Fig. 3B).
Fig. 2. (A) Nitrogen adsorption–desorption isotherms, and (B) pore size distribution of the calcined samples synthesized with different molar ratio of HNO3/TEOS.
Fig. 3. Low angle XRD patterns of the sample (A) synthesized with the HNO3/ TEOS molar ratio of 1 with (a) 0.26 and (b) no phenol and (B) as-prepared samples synthesized in different systems of (c) CPyCl–phenol–HNO3, (d) CTAB–phenol–HNO3, (e) CPyCl–phenol–HCl, and (f) CPyCl–phenol– H2SO4.
Fig. 1. (A) Low angle XRD patterns of the as-synthesized samples prepared with different HNO3/TEOS molar ratio of (a) 1/1, (b) 1.5/1, (c) 2/1, (d) 2.5/1, (e) 3/1, (f) 3.5/1, (g) 4/1, and (h) 5/1. (B) The sample as-synthesized (c) and calcined (c′).
surface area of sample was calculated using 77 K nitrogen adsorption data in the relative pressure range from 0.04 to 0.2, and the total pore volume was determined from the amount adsorbed at a relative pressure of about 0.99. The pore size distribution (PSDs) curves were calculated from the analysis of the desorption branch of the isotherm, using the Barret–Joyner– Halenda (BJH) algorithm. 3. Results and discussion
H.J. Wang et al. / Materials Letters 62 (2008) 422–424
To understand the formation mechanism of the special cubic structure, the effective surfactant ion pair packing parameter g is introduced as g =V /a0l , where V is the effective volume of the surfactant tail region, a0 refers to the effective head group area at the micelle surface, and l is the surfactant tail length. In the acid condition hexagonal p6mm mesophase can be prepared as the g value is between 1/3 and 1/2, cubic Ia3d mesophase forms as the g value is between 1/2 and 2/3 and lamellar mesophase emerges as the g value exceeds 1 . Phenol, as a polar organic additive reducing the repulsive interaction of hydrophilic groups, results in the decrease of a0; and it inserts into the micelle palisade  to increase the value of V. The decrease of a0 and the increase of V induce to the increase in the g value. This is the reason why absence of phenol relates to formation of hexagonal p6mm mesoporous silica with a small g value. The same reason can explain why excess phenol causes the appearance of mesoporous silica with lamellar structure with a large g value. HNO3 is another influencing element of g value. In the surfactant solution the anions are more or less hydrated. More strongly hydrated ions have, in general, bigger ionic radii and bind less closely and strongly on the head group of the surfactant. The small anions contribute to the partial reduction in the electrostatic repulsion between the charged surfactant head groups and the decrease in the effective head group area of surfactant a0, therefore resulting in a significant increase in the g value . The ionic radii were − − − reported to decrease in the following order: 1/2SO2− 4 N Cl N Br N NO3 . Consequently HNO3 leads to the formation of the Ia3d cubic mesophase with a large g value and HCl or H2SO4 favors the formation of the hexagonal p6mm mesophase with a small g value. CTAB is commonly used as template in acid condition to direct the formation of hexagonal p6mm mesoporous silica. However CPyCl has a large hexahydric ring that can be folded in the surfactant tail region to enlarge the value of V, therefore the g value is enlarged. So CPyCl cannot be replaced by CTAB in the synthesis of cubic Ia3d mesophase under the acid condition. Since CPyCl, phenol and HNO3 all have the character to increase the g value, mesoporous silica with Ia3d cubic symmetry can thus be synthesized in acidic condition at room temperature.
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