Formation mechanism mesoporous silica

Figure 15.13 Pictorial representation of proposed mechanism for Pt nanowire formation in mesoporous silica templates such as FSM-16 and HMM-1 under UV-irradiation of H2PtCl6/ FSM-16 and H2PtCl6/HMM-l in the presence of methanol and water vapor.

Figure 16.2 Possible mechanisms of the formation of mesoporous silica materials, by generation of hexagonal silica-surfactant mesophases horn the interaction of silica precursor with (a) spherical micelles, (b) rod-like micelles, or (c) a preformed liquid crystalline phase of the surfactant.

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 ... 

Figure 2.6 Scheme of the mechanism for the formation of mesoporous silica. Silica polymers formed initially from silica monomers, and associated with surfactant monomers, which form composite self-organised primary particles which can either continue to grow via monomer addition (path 1) or themselves aggregate in a directional fashion (path 2) to form the final mesophase composite. Nondirectional aggregation would cause formation of disordered pore structures. Reprinted with permission from Nooney, R.I. Thirunavukkarasu, D. Chen, Y. Josephs, R. Ostafin, A.E., Synthesis of Nanoscale Mesoporous Silica Spheres with Controlled Particle Size, Chem. Mater., 14, 4721—4728. Copyright (2002) American Chemical Society... 

Figure 1. Scheme for the liquid crystalline templating mechanism proposed by Kresge et al 1 for synthesis of mesoporous silica MCM-41. Formation of a hexagonal array of cylindrical micelles possibly mediated by silicate anions followed by condensation of the silicate anions from the silicate source (tetraethylorthosilicate) leads to templated framework structure. Calcination or extraction of the template produces hexagonally ordered mesoporous silica. 

This chapter deals with the selective preparation, TEM/EXAFS/XPS characterization and catalysis of mono- and bimetallic nanowires and nanoparticles highly ordered in silica FSM-16, organosilica HMM-1 and mesoporous silica thin films. The mechanism of nanowire formation is discussed with the specific surface-mediated reactions of metal precursors in the restraint of nanoscale void space of mesoporous silica templates. The unique catalytic performances of nanowires and particles occluded in mesoporous cavities are also reviewed in terms of their shape and size dependency in catalysis as well as their unique electronic and magnetic properties for the device application. 

Mechanism for Formation of p( Nanowires in Mesoporous Silica Templates... 

Following similar principles of combining the aggregate-forming properties of bifunctional molecules with low cost and low toxicity, a mesoporous silica material with a three-dimensional worm-like pore system was reported. Triethanol amine (TEA) was used as the SDA and TEOS as the silica source in this mesoporous silica, TUD-1.[48] The formation mechanism is depicted in Figure 1.3(a). The properties of the material can be easily tuned by modifications in the synthesis procedure, for example, the pore size of the material was found to be proportional to... 

Pinnavaia etal. used neutral alkylamines as templates to form disordered mesoporous silica, named hexagonal molecular sieves (HMS). The S°I° formation mechanism was proposed between neutral amine micelles (S°) and neutral inorganic precursors (1°). The interactions between S° and 1° were assumed to be hydrogen bonding. The resulting HMS has a worm-like pore structure, with thicker framework walls and smaller X-ray scattering domain sizes compared to M41S. 

Figure 1. Schematic pathway for preparing surfactant-templated mesoporous silicas, illustrating a formation mechanism based on preformed liquid crystal (LC) mesophase (route A) or a cooperative process (route B). Reprinted from [20], Copyright (2008) WILEY-VCH Verlag GmbH Co.

One-step direct synthesis of the highly stable mesoporous silica-based material is also possible. MMS-H[209] has a structure analogous to that of MCM-48 but which contains zeolite building units. A mixture of CTAB and Brij30 was used as template for the mesopores. The use of TPAOH without the assistance of NaOH helps to introduce zeolite secondary building units, as well as the direct formation of acidity after removal of the template. This material was also found to possess superior thermal, hydrothermal, steam, and mechanical stabilities. 

Grosso, D. Balkenenda, A.R. Albouy, P.A. Ayral, A. Amenitsch, H. Babonneau, F. Two-dimensional hexagonal mesoporous silica thin films prepared from block copolymers detailed characterization and formation mechanism. Chem. Mater. 2001, B (5), 1848-1856. 

Zana, R. et al., Fluorescence probing investigations of the mechanism of formation of organized mesoporous silica, Langmuir, 15, 2603, 1999. 

Mesostructured thin silica films can be prepared as freestanding films or supported by a variety of different substrates. In the following, a short description of the different synthesis methods is given the synthesis and the formation mechanism of mesoporous silica films has been reviewed in detail elsewhere. " ... 

FIGURE 20.7 Mechanism of formation of Ag NPs in mesoporous silica (a) at a low dose rate and (b) at a high dose rate. (From Hornebecq, V., Antonietti, M., Cardinal, T., and Treguer-Delapierre, M., Stable silver nanoparticles immobilized in mesoporous silica, Chem. Mater., 15,1993-1999. Copyright 2003, with permission from American Chemical Society.)... 

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