Formation Mechanism

In their early studies, Mobil scientists noticed that the formation of mesoporous materials is closely related to the assembly of traditional liquid crystals and biomineralization [12, 13]. The surfactants employed in the synthesis of meso-porous materials usually possess one or more polar head-groups and one long hydrophobic tail, whereas the lipid generally studied in life science has two hydrophobic tails. Traditional liquid crystals usually consist of one tail with a specific group (usually benzene groups). To explain the formation mechanism of MCM-41, the Mobil researchers first proposed a liquid-crystal-templating (LCT) mechanism [12, 13]. Based on high-resolution transmission electron

Figure 1. Schematic drawing of the pore structures of (a) MCM-41; (b) MCM-48; and (c) MCM-50.

Figure 2. Representative XRD patterns of mesoporous materials. (a) Highly ordered MCM-41 prepared with cationic surfactant under basic conditions. (b) MCM-48 prepared with Gemini surfactant C22_12_22 as a template at 100 °C

microscopy (HRTEM) and XRD studies of MCM-41 materials [12, 13], which obtained results similar to those for liquid crystals formed in aqueous solutions, they proposed that the liquid crystal phase formed by the surfactants in aqueous solution was a template for the formation of an MCM-41 mesostructure. Mobil researchers suggested that the formation of liquid crystal phases of surfactants may occur (1) before and (2) after the addition of inorganic precursors [12, 13]. In this context, the LCT mechanism is "errorless," since there will be no other choices considering the formation of the liquid crystal phase.

When pathway (1) was utilized to explain the formation of mesoporous materials in aqueous solutions [12, 13], some conflicts occurred. In aqueous solutions, liquid crystal phases can only be observed at very high surfactant concentration. In the case of cetyltrimethyl ammonium bromide (CTAB), the hexagonal liquid crystal phase is formed when the concentration of CTAB is higher than 28 wt%, and the cubic phase occurs when the concentration of CTAB is higher than 80 wt%. On the other hand, MCM-41 can be synthesized at a very low CTAB concentration (~2 wt%) [24-28]. Even for the synthesis of cubic MCM-48 materials, the concentration of CTAB in starting compositions can be as low as 10 wt%. Later, Attard et al. prepared a liquid crystal phase by using surfactants and/or inorganic precursors under a nonaqueous phase [30]. With the use of this liquid crystal phase as a template, mesoporous materials were obtained. This observation supported the proposed pathway (1) by Mobil scientists; however, the formation of the liquid crystal phase prior to synthesis of mesoporous materials is somewhat difficult, and the synthesis of mesoporous materials strictly according to liquid crystal templating is not common in the literature [30].

Several formation mechanisms have been proposed to describe in more detail the synthesis pathway (2) where the liquid crystal phases are formed after the inorganic species were added. M. E. Davis et al. [25] suggested that surfactant molecules self-assembled into micelle rods first in aqueous solutions; these micelle rods could interact with inorganic silicate precursors and resulted in rod-like micelles of organic-inorganic hybrid composites that contained two or three layers of silicate oligomers (Fig. 3). These coated micelles agglomerated spontaneously to give a long-range ordered hexagonal mesostructure. After further condensation of silicates, the MCM-41 mesophase was obtained.

G. D. Stucky et al. proposed that the cooperative self-assembly of organic and inorganic species gave long-range ordered mesostructures [24, 27, 29]. Polymeric silicate anions interacted with the headgroups of cationic surfactants (Fig. 4) at the organic-inorganic interface; the charge density of the inorganic layer changed with the condensation of silicates and resulted in close packing of the long hydrophobic chain of the surfactants. The charge density matching between organic and inorganic species controlled the array of surfactant-inorganic hybrid materials. The pre-arrangement of organic surfactant molecules was not necessary in this mechanism. With the condensation of inorganic species, the charge density of the inorganic layer changed and the solid phase of the organic/inorganic composites was transferred. The final structure of the composite phase was governed by the extent of the reaction (the condensation of inorganic species) and by the charge density matching of surfactant/inorganic assemblies (Fig. 4).

Some questions may be argued with respect to Davis's mechanism. The length of the micelle rods is not uniform in solution; furthermore, long micelle rods must overcome a large energy gap to assemble into ordered organic-inorganic hybrid mesostructures. These facts contradict many observations; for example, the mesochannel of MCM-41 synthesized in the solution can be as long as several micrometers [31, 32]. In fact, at the surfactant concentration used to synthesize the mesoporous materials, the global micelles as well as rod-like micelles can be observed in solution; in this case, the composite phases other than hexagonal structures should coexist in the final products. Furthermore, this mechanism cannot be used to interpret the formation of cubic MCM-48 and lamellar mesostructured materials [24, 26]. Obviously, the limitation of Davis's mechanism restricted their general use in the synthesis of mesoporous materials. However, during the synthesis of mesostructured silica or germanium oxides, Yuan and Zhou [33] and Zhao et al. [34] observed that single rod-like inorganic or organic/inorganic materials could be obtained, suggesting that in some cases Davis's mechanism might be applicable.

The cooperative self-assembly mechanism proposed by G. D. Stucky et al. is quite general and thus has been accepted by most scientists. This mechanism can be widely used in different synthesis systems, and the general principle can be applied to the design of new materials.

Figure 3. Self-assembly of silicate-encapsulated micelle rods into hexagonal mesophase. Reprinted with permission from [4], J. Y. Ying et al., Angew. Chem. Int. Ed. Engl. 38, 56 (1999). © 1999, Wiley.

Figure 3. Self-assembly of silicate-encapsulated micelle rods into hexagonal mesophase. Reprinted with permission from [4], J. Y. Ying et al., Angew. Chem. Int. Ed. Engl. 38, 56 (1999). © 1999, Wiley.

Figure 4. Cooperative templating model (left) and the synthesis approach (right) for the synthesis of mesoporous materials. Reprinted with permission from [4], J. Y. Ying et al., Angew. Chem. Int. Ed. Engl. 38, 56 (1999). © 1999, Wiley.

Japanese scientists have proposed a lamellar transformation mechanism to explain the synthesis of mesoporous FSM-16 materials from kanemites [35]. However, this mechanism is restricted to specific conditions. From silicate oligomer precursors to the final hexagonal ordered meso-porous materials, the lamellar intermediate occurs only in a narrow pH range. Actually, it is not necessary to form the lamellar intermediate when the hexagonal mesostructure is assembled [24]. Furthermore, what is the difference in the structures between FSM-16 and MCM-41 materials has not been fully understood [36].

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