Synthesis silica

SCHEME 3.3 Overall reaction for silica synthesis from tetraethoxysilane (TEOS). 

Scheme 2. Encapsulation of size- and shape-controlled Pt nanoparticles under neutral hydrothermal synthesis conditions of SBA-15. Silica templating block copolymers and silica precursors were added to PVP-protected Pt nanoparticle solutions and subjected to the standard SBA-15 silica synthesis conditions. Neutral, rather than acidic pH conditions were employed to prevent particle aggregation and amorphous silica formation [16j. (Reprinted from Ref. [16], 2006, with permission from American Chemical Society.)...

Roth, K.M., Zhou, Y., Yang, W. and Morse, D.E. (2005) Bifunctional small molecules are biomimetic catalysts for silica synthesis at neutral pH. Journal of the American Chemical Society, 127, 325-330. 

This chapter focuses on silica synthesis via the microemulsion-mediated alkoxide sol-gel process. The discussion begins with a brief introduction to the general principles underlying microemulsion-mediated silica synthesis. This is followed by a consideration of the main microemulsion characteristics believed to control particle formation. Included here is the influence of reactants and reaction products on the stability of the single-phase water-in-oil microemulsion region. This is an important issue since microemulsion-mediated synthesis relies on the availability of surfactant/ oil/water formulations that give stable microemulsions. Next is presented a survey of the available experimental results, with emphasis on synthesis protocols and particle characteristics. The kinetics of alkoxide hydrolysis in the microemulsion environment is then examined and its relationship to silica-particle formation mechanisms is discussed. Finally, some brief comments are offered concerning future directions of the microemulsion-based alkoxide sol-gel process for silica. 

Fig. 2.2.1 Silica synthesis by alkoxide hydrolysis in microcmulsion water pools.

Table 2.2.2 Surfactants Used in Microemulsion-Mediated Silica Synthesis...

Silica Gels. The acid-catalyzed alkoxide sol-gel process produces gels (17). Frib-erg and coworkers (40-50) pioneered the extension of this process to silica synthesis in microemulsions both aqueous and nonaqueous microemulsions were used. For aqueous microemulsions, experiments were conducted mostly with the SDS/ pentanol/water/acid system. A representative flow diagram is shown in Figure 2.2.9. The nonaqueous microemulsion systems utilized included CTAB/decanol/ decane/formamide and AOT/decane/glycerol (44-46,49,50). The experimental approach followed the sequence nonaqueous microemulsion preparation, water addition, and then TEOS addition. 

Sol-gel silica synthesis is based on the controlled condensation of Si(OH)4 entities. These may be formed by hydrolysis of soluble alkali metal silicates or of alkoxysilanes. The commonly used compounds are sodium silicates and tetraethoxysilane (TEOS). 

Figure 2.15 General concept of the syntheses of mesoporous materials (a) mesoporous silica synthesis with soft template (b) mesoporous carbon synthesis with hard template.

George, J., Shylesh, S. and Singh, A. P. Vanadium-containing ordered mesoporous silicas Synthesis, characterization and catalytic activity in the hydroxylation of biphenyl, Appl. Catal., A, 2005, 290, 148-158. 

Another route for the production of materials involves the reaction of hydrolysis-condensation of metal alkoxides with water. We study here the important case of amorphous silica synthesis. In this case [38,39,44], silicic acid is first produced by the hydrolysis of a silicon alkoxide, formally a silicic acid ether. The silicic acids consequently formed can either undergo self-condensation, or condensation with the alkoxide. The global reaction continues as a condensation polymerization to form high molecular weight polysilicates. These polysilicates then connect together to form a network, whose pores are filled with solvent molecules, that is, a gel is formed [45],... 

The investigation of silica materials has been a field of huge interest for a long time, because this is an inert and very stable material [68-76], In Chapter 2, the amorphous morphology of silica and, in Chapter 3, the method for silica synthesis were described. 

II. OVERVIEW OF SILICA SYNTHESIS IN THE EXPERIMENTAL SYSTEMS A. Diatoms... 

Furthermore, diatoms and radiolaria exert strict species-specific control over the patterns and pore sizes of the silica, which is precipitated at ambient conditions. Controlled industrial synthesis of silica, on the other hand, requires extreme pHs and/or temperatures. Biomimetic silica synthesis has, therefore, attracted much interest among materials scientists and phycologists for potential industrial applications, such as components of industrial separators, catalysts, and electronic devices.64-67... 

Sahai, N. and Tossell, J.A., 29Si NMR shifts and relative stabilities calculated for hypercoordinated silicon-polyalcohol complexes Role in sol-gel and biogenic silica synthesis, Inorg. Chem., 41, 748, 2002. 

Molecular nitrogen will see a significant surface area due to its small size comparable to the dimensions of the surface roughness, while bigger molecules such as pyrene will not be able to see all ridges and valleys and will see a significantly lower effective surface area. These factors have been studied extensively [17] for silica, and authors have found fractal factors to vary between 2 and 3, depending on the silica synthesis, treatment, and so on. 

Modification of the base silica synthesis by addition of methylthrietoxysi-lane allows the introduction of methyl groups on the silica surface. A schematic representation of the expected surface is shown in Figure 3-6. Surface concentration of methyl groups is dependent on the reagent ratio. 

Table 1 Effect of amount of porogen on the pore structural parameters of particulate silicas. Synthesis conditions 500 ml of PES (viscosity 75 mP s). Cyclohexane 0, 150, 300, 450 and 600 ml, 3,000 ml of 2-propanol, 250 ml of ammonia, stirring speed 1,500 rpm.

Table 8.1 Common chemicals (or groups) for mesoporous silica synthesis...

F. Kleitz, D.N. Liu, G.M. Anilkumar, I.S. Park, L.A. Solovyov, A.N. Shmakov, and R. Ryoo, Large Cage Face-centered-cubic Fm3m Mesoporous Silica Synthesis and Structure. J. Phys. Chem. B, 2003, 107, 14296-14300. 

F. Kleitz, S.H. Choi, and R. Ryoo, Cubic Ia3d Large Mesoporous Silica Synthesis and Replication to Platinum Nanowires, Carbon Nanorods and Carbon Nanotubes. Chem. Commun., 2003, 2136-2137. 

A.N. Volkova, A.A. Malygin, S.I. Koltsov, N.N.Stafeeva and V.B. Aleskovskii, Method of Molibdenium-containing Silica Synthesis, USSR patent No. 440338... 

A. Tuel, L. G. Hubert-Pfalzgraf, Nanometric monodispersed titanium oxide particles on mesoporous silica synthesis, characterization, and catalytic activity in oxidation reactions in the liquid phase, /. Catal. 217 (2003) 343. 

Another innovative approach to improve silica support is the development of hybrid particles.3,16 In the silica synthesis of the first commercial hybrid... 

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