In 1992, a novel family of molecular sieves called M41S was launched by Mobil Corporation
[16]. The pore size of these mesoporous materials were 15–100Å. These
materials attracted interest due to their large pore size (>1200 m2 g–1) and tunable pore sizes.The M41S materials also provide a new approach in materials synthesis, supplanting the use of single molecules (zeolite) as templating structure-directing agents. In general, inorganic solids containing pores whose diameter range from 20 to 500 Å are considered mesoporous materials, according to IUPAC definition. Examples of mesoporous materials such as M41S, aerogels, and pillared layered structures as listed in Table 2.1.
Table 2.1. Pore size definition of the porous materials
Pore size Definition Examples
Macroporous (> 500 Å) Glasses
Mesoporous ( 20-500 Å) Aerogels, M41S
Microporous ( < 20 Å) Zeolites, Actived Carbon
On the most fundamental level, the formation of mesoporous materials from inorganic precursors and organic surfactants occurs in the presence of surfactants, in a solution from solubilized inorganic precursors. Surfactants contain a hydrophilic head group and a long hydrophobic tail group within the same molecule, and they will self-organize so as to minimize contact between the incompatible ends. The type of interaction between the surfactant and the inorganic precursor is seen as a significant difference among the various synthesis routes, the formation models, and the resulting classes of mesoporous materials [17].
A liquid crystal templating (LCT) mechanism has been proposed, based on the similarity (i.e.,
lyotropic phase) between liquid crystalline surfactant assemblies and M41S [18]. Common traits include the mesostructure dependence on the hydrocarbon chain-length of the surfactant tail groups, the effects of changing of the surfactant concentrations, and the influence of organic swelling agents [19].
Figure 2.1 Two possible pathways for the LCT mechanism
Self-assembled molecular aggregates or supermolecular assemblies are employed as structure-directing agents, co-assembling with the inorganic materials into sophisticated nanoscale structures through favorable molecular interaction. The resultant nanoscale materials have a delicate structural ordering; thus, not only scale ordering (1.5–30 nm), but both crystalline symmetry (i.e., those that are hexagonal, lamella, and cubic) and morphologies can be tailored.
Davis et al. found that the hexagonal LC phase did not develop during MCM-41 synthesis. They propose that the formation of MCM-41 begins with the deposition of two to three monolayers of silicate precursor onto isolated surfactant micellar rods [20]. In that study, the rods were randomly ordered, eventually packing into a hexagonal mesostructures.
Figure 2.2 Assembly of silicate rods.
Steels et al. found that surfactant molecules assembled directly into the hexagonal LC phase, upon the addition of a silicate species [21]. The silicates were organized into layers, with rows of cylindrical rods intercalating between layers.
Figure 2.3 Puckering of silicate layers in the direction.
Monnier et al. and Stucky et al. indicate a novel mechanism for a lamellar phase. In that study, the phase of the synthesis mixture formed as a result of the electrostatic attraction between the anionic silicates and the cationic surfactants [22, 23].
Figure 2.4 Curvature induced by charge density matching; the arrow indicates the reaction coordinate.
A lamellar-to-hexagonal phase motif accompanied by hydrated sodium silicate comprising single-layered silica sheets was found by Vartuli et al. [24]. They demonstrated that the layered structures were still retained in the kanemite-derived mesoporous materials.
Figure 2.5 Folding of silicate sheets around intercalated surfactant molecules. (a) Ion exchange, (b) calcination.
Under synthesis conditions that prevented the condensation of silicate species—such as low temperatures and high pH—a truly cooperative self-assembly of silicates and surfactants was found. Firouzi et al. showed that a micelle solution of
cetyltrimethylammonium bromide (CTAB) transformed to a hexagonal phase in the presence of silicate anions [25]. The silicate anions ion-exchanged with the surfactant halide counteranions to form a silicatropic liquid crystal (SLC) phase that involved silicate-encrusted cylindrical micelles. The SLC phase exhibited behavior very similar to those of typical lyotropic systems,
Figure 2.6 Formation of a silicatropic liquid crystal phase.
except that the surfactant concentrations were much lower and the silicate counteranions were reactive
[26]. Additionally, Firouzi et al. also demonstrated the charge-balance requirement
(electrostatic interaction) [25, 26]. There was a preferential bonding of the ammonium head group to multi-charged D4R [(Si8O20)8–] silicate anions, under high pH conditions; the interaction was so strong that an alkyltrimethylammouonium surfactant solution could force a silicate solution that did not contain D4R oligomers to re-equilibrate and form a D4R species.Furthermore, Fyfe and Fu were able to prepare mesostructured silicates with D4R silicates
[27]; they tried to combine D4R precursors with cetyltrimethylammonium chloride (CTAC)
surfactant-produced mesostructured materials. Control of the condensation of the silicates within the mesostructure by acidic vapor treatment led to the observation of cubic, lamellar, and hexagonal phases as intermediate transformation phases. The M41S family comprises made up of three well-defined mesostructures: MCM-41, MCM-48, and MCM-50 [28].Illustrations of the M41S family are provided in Figure 2.7. MCM-41 has a hexagonally packed array of noninterconnecting cylindrical pores,
Figure 2.7 Illustrations of M41S materials: (a) MCM-41, (b) MCM-48, and (c) MCM-50.
while the structure of MCM-48 can be thought of as two intertwined networks of spherical cages that are separated by a continuous silicate framework. MCM-50, meanwhile, contains a lamellar structure in the uncalcined form.
Through the acidic route, other species considered to be in the SBA series have been synthesized
[29]. These materials have thicker pore walls and a framework charge that is
different from those in M41S materials, due to their different precipitation conditions and charge-balance requirements. Zhao et al. have reported the synthesis of highly ordered hexagonal mesoporous silica structures named SBA-15, by using an amphiphilic block copolymer as an organic-templating agent [30]. Poly(alkylene oxide) triblock copolymers triblock copolymers ((EOx-POy-EOx))—where EO is ethylene oxide and PO is propyleneoxide—were good candidates, owing to their mesostructural ordering properties, commercial availability, biodegradability, and cost-effectiveness. The hydrothermal stability and mechanism properties of SBA-15 were superior to those of MCM-41, because the walls of SBA-15 are thicker than those of MCM-41 with cationic surfactants. The EO species had greater hydrophilicity, causing it to interact more strongly than the hydrophobic PO species (Figure 2.8). Several morphologies of mesoporous materials—such as films, spheres, hollow spheres, and fibers—have been synthesized [31–34].
Figure 2.8 A schematic diagram of SBA-15 synthesized with triblock copolymer.
Zhao et al. and Lin et al. have each synthesized hollow mesoporous silicate spheres with hierarchically ordered structures by using bitanol as a co-surfactant [35–39]. Moreover, Zhao et al. have also demonstrated a variety of SBA-15 morphologies by using a co-surfactant, co-solvent, or electrolytes [40]. Herein, the morphology of SBA-15 strongly depended on the surface curvature energy at the interface of inorganic silica and organic copolymer species.
The large pores of mesoporous materials are formed directly by copolymers called mesostructured cellular foams (MCFs). Prior to their development, large-pore molecular sieves were much in great demand, for the separations of large molecules; the development of
SBA-15 and MCF has extended the range of mesoporous materials with pore sizes larger than 10 nm [41]. On the other hand, SBA-15 and MCF materials were synthesized with triblock copolymer (EO-PO-EO) templates and are related by phase transition from a hexagonally ordered cylindrical mesoporous structure to the mesocellular foam structure.
To date, there has been increased interest in the fabrication of nanometer-sized fine structures, because of their potential utilization in electronics and optical and micromechanical devices. There have been numerous reports of the preparation of inorganic mesoporous films, due to their potential use in separation membranes, chemical sensors, optical devices, and electronic devices such as low-k dielectric films [42–44]. Most of all, the mesoporous silica of MCM-41 and SBA-15 have conventionally been fabricated by surfactant-silicate composites from a liquid phase, under acidic or basic conditions [45–47]. A simple way to synthesize mesoporous silica films has been developed using the spin-coating and dip-coating methods [48, 49]. Recently, Kim and Ryoo have synthesized MCM-48 crystals (Ia3d) with a cubic structure [50], and Che et al. have reported the synthesis of SBA-1 (Pm3n) with a large number of facets by adjusting temperature, synthesis time, acidity, and surfactant concentrations [51]. It should be noted that ionic surfactants have been used in all reports; this is because the specific interaction between copolymer and silica is much weaker than that of ionic surfactants and inorganic species in solution.
In recent years, much work has been done in which there was use of ordered mesoporous carbon, CMK-3, and mesoporous metal oxide that were synthesized through a template method from SBA-15 mesoporous silica; in much of this work, these films were used to derive electric double-layer capacitors (EDLCs) and sensing materials [52–56].