Self-Assembly of Mesoporous Silicas

External templates were used to structure silicates and generate porous materials for the first time in the early 1990s by independent groups in Waseda University in Japan [7] and at Mobil in the United States [8]. These syntheses took place under hydrothermal conditions similar to those for the synthesis of zeolites basic media and in the presence of cationic surfactants such as cetyltrimethylam-monium bromide. After this pioneer work, numerous silicates have been synthesized using a wide variety of surfectants. They have allowed the preparation of many mesoporous silicates such as MCM-41 with a hexagonal pore structure, MCM-48 with a cubic structure, and MCM-50 with a lamellar structure. 

The synthesis of these ordered materials involves a structuring agent, which is a surfactant (cationic, anionic, or nonionic). The surfactant molecules form aggregates whose interactions with the silicate precursors direct the condensation toward an ordered solid containing the surfectant Removal of the surfactant by extraction or calcination leads to the mesoporous material. The surfectant plays the role of the template in the construction of the porous structure of the solid. 

After the discovery of the first mesoporous silicas through external templating, a lot of work has been done to understand and rationalize the formation mechanisms of these materials. Numerous research groups employed a variety of techniques (e.g., NMR spectroscopy. X-ray diffraction, cryo-TEM, electron paramagnetic resonance, and fluorescence) toward this objective. Several models have been proposed [10] and two of them are generally accepted the liquid crystal templating approach and the cooperative self-assembly approach. In both models, the interactions between the surfactant molecules and the inorganic species direct the formation of the ordered solid. 

The liquid crystal templating approach has been suggested by Beck et al. [8]. This hypothesis resulted from the observation that the crystallographic phase of 

MCM-41 mimicked the surfactant s liquid crystal mesophase in aqueous solutions. As a consequence, it was advanced that the siufactant forms a liquid crystal mesophase and the siUca species organize in the space around the surfactant micellar rods interacting with the polar heads of the surfactant molecules. The silicate condensation occurs in the second step once this organization has been set up and traps it. Alternatively, they have postulated that the liquid crystal phase does not pre-exist but forms upon the addition of the inorganic species. It was later evidenced that the MCM-41 synthesis takes place at concentrations lower than that necessary to form the liquid crystal phase, so the second pathway is more plausible when working in dilute media while the first one is true in more concentrated syntheses [11]. 

---

Walcarius, A., E. Sibottier, M. Etienne, and J. Ghanbaja, 2007. Electrochemically assisted self-assembly of mesoporous silica thin films. Nat Mater 6 602-608. 

Porous materials with chemically modified surfaces have been extensively studied as adsorbents for heavy metal ions from water (see the review by Jal et al.2 and references therein). There is a continuously growing demand for adsorbents which are non-swelling, thermally and hydrothermally stable, exhibit large adsorption capacity, fast kinetics and high affinity towards heavy metal ions. Discovery of self-assembled ordered mesoporous silicas (OMSs)3 opened enormous opportunities for the design and synthesis of highly selective and efficient adsorbents for heavy metal ions. 

Time-resolved in situ Small Angle Neutron Scattering (SANS) investigations have provided direct experimental evidence for the initial steps in the formation of the SBA-15 mesoporous material, prepared using the non-ionic tri-block copolymer Pluronic 123 and TEOS as silica precursor. Upon time, three steps take place during the cooperative self-assembly of the Pluronic micelles and the silica species. First, the hydrolysis of TEOS is completed, without modifications of the Pluronic spherical micelles. Then, when silica species begin to interact with the micelles, a transformation from spherical to cylindrical micelles takes place before the precipitation of the ordered SBA-15 material. Lastly, the precipitation occurs and hybrid cylindrical micelles assemble into the two-dimensional hexagonal structure of SBA-15. 

For instance, H.Yang, N. Coombs, G.A. Ozin, Morphogenesis of shapes and surface patterns in mesoporous silica, Nature 386 (1997) 692-695 N. Bowden, A. Ter-fort, J. Carbeck, G. Whitesides, Self-assembly of mesoscale objects into ordered two-dimensional arrays, Science TIB (1997) 233-235. 

The current methods for the synthesis of mesoporous silica materials can be divided into three main types bottom-up, based on the polymerization of orthosilicic acid derivatives top-down, based on the reorganization of silica and evaporation-induced self-assembly,5 6 all of them promoted by surfactant templation. 

Curved structures are not only limited to carbon and the dichalcogenides of Mo and W. Perhaps the most well-known example of a tube-like structure with diameters in the nanometer range is formed by the asbestos mineral (chrysotil) whose fibrous characteristics are determined by the tubular structure of the fused tetrahedral and octahedral layers. The synthesis of mesoporous silica with well-defined pores in the 2-20 nm range was reported by Beck and Kresge [215]. The synthetic strategy involved the self-assembly of liquid crystalline templates. The pore size in zeolitic and other inorganic porous solids is varied by a suitable choice of the template. However, in contrast to the synthesis of porous compounds, the synthesis of nanotubes is somewhat more difficult. 

The synthesis of mesoporous silica films typically begins with the preparation of precursor solutions. These solutions contain a silica source (typically an alkoxide, although chloride and colloidal precursors can be used), a surfactant molecule used to template the mesostructure, an acid or base catalyst, and solvents. The nanoscale structure is then formed by a cooperative self-assembly of monomeric or partially... 

There has been extensive research on the synthesis of mesoporous silica thin films since the first reports in 1996. However, most research has focused on two general synthesis techniques evaporation induced self-assembly and spontaneous film growth from solution. 

The chemistry involved in the formation of mesoporous silica thin films is qualitatively well understood. However, specific reaction mechanisms of the individual steps are still debated. In addition, owing to the complexity of the sol-gel reaction pathways and cooperative self-assembly, full kinetic models have not been developed. From the time of mixing, hydrolysis reactions, condensation reactions, protonation and deprotonation, dynamic exchange with solution nucleophiles, complexation with solution ions and surfactants, and self-assembly, all occur in parallel and are discussed here. Although the sol-gel reactions involved may be acid or base catalyzed, mesoporous silica film formation is carried out under acidic conditions, as silica species are metastable and the relative rates of hydrolysis and condensation reactions lead to interconnected structures as opposed to the stable sols produced at higher pH. Silicon alkoxides are the primary silica source (tetramethyl orthosilicate, tetraethyl orthosilicate, tetrapropyl orthosilicate, etc.) and are abbreviated TMOS, TEOS, and TPOS, respectively. Starting from the alkoxide, Si(OR)4, in ROH and H2O solution, some of the general reactions are ... 

The above discussion concerned some general trends and principles governing the formation of mesoporous materials. On a practical level the formation/synthesis of products with uniform mesopores by self-assembly, especially the silica-based ones, appears quite facile... 

---