Discoid Bicelles as Efficient Templates for Pillared Lamellar Periodic Mesoporous Silicas at pH 7 and Ultrafast Reaction Times
© Mohanty et al. 2010
Received: 18 August 2010
Accepted: 15 September 2010
Published: 6 October 2010
We report the first synthesis of periodic mesoporous silicas templated by bicelles. The obtained materials form novel pillared lamellar structures with a high degree of periodic order, narrow pore size distributions, and exceptionally high surface areas.
KeywordsBicelle Mesoporous silica Ultrasonication Templating
Hardly any other field of materials chemistry has experienced as much growth over the last 20 years as the field of periodic mesoporous materials . Numerous applications have been found for periodic mesoporous materials in catalysis , separation , chemical storage , and delivery . Periodic meosporous materials can be produced by either soft-templating  or hard-templating  methods. Soft-templating is the dominating technique because hard-templating requires additional undesirable reaction steps. With only one exception , all soft-templated periodic mesoporous materials are produced by templation of micelles. Most micellar templates are produced from cationic  or non-ionic  surfactants. Typical cationic surfactants are quaternized ammonium salts, for example cetyltrimethyl ammonium bromide (CTAB). Amphiphilic block-copolymers such as block-copolyethers have been extensively used as non-ionic surfactants. More recently, micelles from anionic surfactants have been used as templates in the presence of co-structure directing agents . The use of new micellar templates has substantially driven the field of periodic mesoporous materials because new templates have frequently led to the discovery of new mesostructures with new pore systems . With the exception of folic acid , no other templates but surfactant-containing micelles are known today for the production of periodic mesoporous materials. It has been attempted to use other soft templates, for example polyoxometallates , but to our best knowledge in no case periodic mesoporous materials have been obtained.
Structural parameters of BMS materials (extracted)
Lattice constants (nm)
BET surface area (m2/g)
Pore size (nm)a
Pore volume (cm3/g)
To understand the relationships between the reaction conditions and the structural properties of the BMS materials in more depth, we performed two additional experiments with bicelle concentrations of 5% (w/w) and 20% (w/w), respectively. The TMOS/bicelle ratio was kept constant by variation in the volume of the bicelle solution (see additional file 1 for details). For both as-synthesized materials (BMS-3 and BMS-4), strong reflexes at d = 3.80 and 4.50 nm, with two shoulders at 4.01 and 5.38, respectively, were observed by SAXS (Figure 2a). One higher-order peak was observed at 2.59 nm for BMS-4. However, no clear higher-order peak was observed for the sample BMS-3.
While BMS-3 retained mesoscopic periodicity upon extraction (d = 4.348, 3.75, 2.12, and 1.85 nm, a = 5.02, c = 7.35), the mesostructure was lost for BMS-4 (Figure 2b). This suggests that BMS-4 is isotypic to BMS-1. N2 sorption of BMS-3 produced a type IV isotherm, which confirms the presence of a mesostructure in the material (Figure 3a). Similar to BMS-2, the capillary condensation takes place at pressures between 0.20 and 0.35 p/p0. The BJH pore size distribution calculated from the desorption branch of the isotherm is narrow and centred around 2.7 nm (Figure 3b). This pore size is very similar to the pore size of BMS-2 and suggests a pillared lamellar structure, which is isotypic to BMS-2. The pore volume of BMS-3 was found to be very high (1.28 cm3 g-1). The BET surface area of BMS-3 is exceptionally high for a periodic mesoporous silica material (1,701 m2 g-1). The much higher surface area of BMS-3 in comparison with BMS-2 can be explained by the decreased thickness of the silica layers. According to SAXS and sorption data, the thickness of the silica layers are estimated to be only 1.0 nm compared with 1.4 nm (BMS-2). TEM and FFT of BMS-3 confirm the mesostructural order of BMS-3 (Figure 4b). The lattice spacings of the TEM images (3.7 nm) are half of the lattice parameter c and thus in excellent accordance with SAXS data.
In a further series of experiments, we changed the TMOS/bicelle ratio by variation in the bicelle concentration in the aqueous solution. Compared to the synthesis of BMS-1 (12% (w/w) bicelles), 20 and 5% (w/w) bicellar solutions were used, yielding the materials BMS-5 and BMS-6, respectively. In both cases, well-ordered mesostructures were observed by SAXS, showing d-spacings at 4.80 nm (001), 4.50 nm (002), and 2.59 (103) for BMS-5 and 5.04, 3.91, and 2.25 nm for BMS-6 nm, respectively (Figure 2a). As expected, the mesostructural order vanished after extraction of BMS-5 due to the very low TMOS/bicelle ratio (same ratio as BMS-1). For BMS-6, with a higher TMOS/bicelle ratio, the mesostructural periodicity was preserved (Figure 2b), and the lattice constants of could be determined (a = 5.82 nm, c = 7.51 nm). A well-defined type IV N2 isotherm was obtained that was similar to BMS-2 and BMS-3 (Figure 3a). The BJH pore size distribution of BMS-6 was centred around 2.4 nm, which is similar to BMS-2 and BMS-3. The pore volume of BMS-6 (0.82 cm-3 g-1) is similar to BMS-3. The BET surface area of BMS-6 is 1,191 m2 g-1. TEM clearly confirms the presence of a periodic mesostructure, showing lattice fringes with spacings of ca. 3.8 nm which is in excellent accordance with SAXS data (Figure 4c).
In summary, bicelles are very efficient templates for periodic mesoporous silicas. The synthesis can be done in pure water (containing catalytic amounts of NH4F) at room temperature within a reaction time of 2 min. The material structures derive from lamellar structures. For BMS-1, BMS-4, and BMS-5, non-pillared hexagonal lamellar structures are formed, while BMS-2, BMS-3, and BMS-6 form a novel hexagonal pillared lamellar structure. The material BMS-3 shows a very high surface area of 1,701 m2 g.
We gratefully acknowledge Lehigh University for supporting this work.
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