Synthesis of Spherical Silica Particles

PAH has been studied in detail for its role in silicification under ambient conditions and at neutral pH. It was demonstrated that PAH can facilitate the formation of nanometer and micrometer-size spherical silica particles imder mild conditions from an aqueous solution of a silica precursor (Fig. 6). It was shown by energy dispersive spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) that the PAH was incorporated into the final silica structures. In the absence of PAH the reaction mixture gelled in 1 day. These results indicate that PAH may act as a catalyst as well as a template or structure-directing agent in silicification. In this context, the behavior of this system is consistent with how Tacke described the role(s) of macromolecules that facilitate silica formation via scaffolding (see section II). 

In further investigations, various parameters that govern the silica synthesis and the morphology of the silicified products were studied. Among the important parameters were the reaction time, the precursor concentration, and the precursor prehydrolysis time. 

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Nanotubes of oxides of several transition metals, as well as of other metals, have been synthesized by employing different methodologies [24, 216-220]. Silica nanotubes were first produced as a spin-off product during the synthesis of spherical silica particles by the hydrolysis of tetraethylorthosilicate (TEOS) in a mixture of water, ammonia, ethanol and D,L-tartaric acid [216]. Since self-assembly reactions are not straightforward with respect to the desired product, particularly its morphology, templated reactions have been employed using carbon nanotubes to... 

A novel synthesis route was developed to produce spherical silica particles. The synthesis is based on a modified Stoeber method and the room-temperature synthesis of MCM 41S-materials applying tetraethoxysilane, alcohol, water, ammonia and homopolymers as template. The specific surface area, the specific pore volume and the average pore diameter were varied in the following ranges 5 - 1,000 m2/g 0.1 — 1.0 cm3/g and 2-50 nm. With respect to catalytic applications hetero-atoms e.g. Al were incorporated into the silica framework. 

We have combined these approaches of pore size engineering with another synthesis route where uniform spherical particles are obtained in the micron and submicron size range [7]. The latter procedure allows one to adjust the particle size and size distribution of the silica particles by the reaction conditions. 

Whatever the nature of the template, among possible applications such as catalysts, grafting supports, or filtration medium, the use of MTS for chromatography applications has been claimed for years. However, this latter application requires the ability to synthesize homogeneous batches of spherical particles with a mean diameter at least equal to 5 pm. Such particles were observed only once, in a pH-dependent synthesis of MCM-41 particles, performed by Yang, Ozin and co-workers [1], Most of the time, the synthesis of submicrometer-size particles of MCM-41 or MCM-48 materials was only reported [2-4], We present the synthesis of MSU-X silica with perfectly controlled size and shape suitable for chromatography applications. 

The main feature of this synthesis is the possibility to adjust the synthesis parameters in order to obtain dense spherical silica particles with a size range between 1 and 10 pm, as shown on the SEM picture (Fig. 3). Indeed, particle sizing shows that the size distribution of... 

The Mainz group, specialised in chromatography, developed a reproducible and rugged synthesis of monodispersed, spherical silica particles that exhibit mesoporosity. 

The materials which have been mentioned here so far are predominantly shaped in planar films of hierarchical order. However, the synthesis of hierarchically structured particles is also highly desirable, as they might be further processed and used for the preparation of composite porous materials. Wu et al. showed the synthesis of raspberry-like hollow silica spheres with a hierarchically structured, porous shell, using individual PS particles as sacrificial template [134]. In another intriguing approach by Li et al. [135], mesoporous cubes and near-spherical particles (Fig. 10) were formed by controlled disassembly of a hierarchically structured colloidal crystal, which itself was fabricated via PMMA latex and nonionic surfactant templating. The two different particle types concurrently generated by this method derive from the shape of the octahedral and tetrahedral voids, which are present in the template crystal with fee lattice symmetry. 

Making use of constrained polymerisation of divinylbenzene on surfactant-modified colloid silica, Jang and Lim prepared carbon nanocapsules and mesocellular foams. Later, they reported that mesoporous carbons with highly uniform and tunable mesopores were fabricated by one-step vapour deposition polymerisation using colloidal silica nanoparticles as template and polyacrylonitrile as carbon precursor. Hampsey et al. recently reported the synthesis of spherical mesoporous carbons via an aerosol-based, one-step approach using colloidal silica particles and/or silicate clusters as template. ... 

A variant of the methods described above was reported by Jafelicci et al. [272] who prepared a microemulsion by addition of water or 3M nitric acid solution into a solution of NaAOT in heptane under sonication. A dilute solution of sodium silicate was added to this microemulsion and the system further sonicated. The product particles were gathered by centrifugation. The spherical silica particles (hollow under specific conditions) thus obtained had a size range of 1-10 pm. It is noteworthy that though the authors claim to have prepared particles via microemulsions, the large size of the particles does not indicate them to be a product of microemulsion-mediated synthesis in nanoreactors (see Section 5.3.1). 

Kobayashi, Y. Saeki, S. Yoshida, M. Nagao, D. Konno, M. (2008), Synthesis of spherical submicron-sized magnetite/silica nanocomposite particles. J. Sol-Gel Sci. TechnoL, 45 35 1. 

A. van Blaaderen and A. Vrij Synthesis and characterization of colloid dispersions of fluorescent, monodispersed silica spheres, Langmuir, 8 (1992) 2921-2931 J.D. Wells, L.K. Koopal, and A. de Keizer Monodisperse, nonporous, spherical silica particles, Colloids Surf. A Physicochem. Eng. Asp., 166 (2000) 171-176 Howard A. Ketelson, Robert Pelton, and Michael A. Brook Surface and colloidal properties of hydrosilane-modifledStOber silica. Colloids Surf. A Physicochem. Eng. Asp., 132 (1998) 229-239... 

A seeding technique has been developed by Yoshizawa et al. (1988) for increasing the average particle size if necessary. The so-called Stbber sihca has been used recently for incorporation of a variety of dyes (Shibata et al., 1997). In addihon to silica gel micro-spheres, the method with modified parameters has been utihzed for the synthesis of other oxide particles in spherical morphology. 

Important work on microemulsion synthesis of silica nanoparticles has been carried out by Arriagada and Osseo-Asare (1995, 1999) using different systems like NaAOT (an anionic surfactant)/decane/benzyl alcohol/ammonia solution and TEOS the w value varied in the range 2.0-9.5. As pointed out above, low w values (less than 4 in this case) caused all the water to get bound to the surfactants, and no particles were obtained. With increased water content, spherical silica particles in the size range of 10-60 nm (depending on experimental conditions) were obtained. When the selected w value was 9 or more, stable microemulsions were not obtained. In a system like cyclohexane/NP-5 (a non-ionic surfactant)/ammonia solution and TEOS, on the other hand, low values of w ( 0.05-2.0) could cause formation of particles 40-60 nm in size when w increased to 5.5, the particle size increased to 75 nm. The general trend was, thus, an increase in the particle size with increase in w when other conditions remained the same. 

Andersson et al. [55] recently demonstrated the synthesis of spherical, mesoporous silica particles using an approach that combines emulsion-based precipitation methods [56,57] with the EISA process. This synthetic route, termed the emulsion and solvent evaporation (ESE) method, produced well-ordered two-dimensional (2D) hexagonal mesoporous silica microspheres. Because the emulsions were prepared in bulk using vigorous stirring, the droplets, and therefore the resulting particles, were produced with a broad particle size distribution. 

Thiols have also been used in an elaborate fashion to prepare spherical, hoUow Pd nanoparticles. Hyeon s group reported the fabrication of uniform hollow Pd spheres using thiols as stabilizers and sihca particles as templates [22]. The template-assisted method used (see Scheme 9.1) first involves the synthesis of uniform silica spheres which were then refluxed with mercaptopropyltrimethoxysilane in toluene in order to functionahze the sihca surface with mercaptopropylsilyl (MPS)... 

In the wide field of possible applications for MTS, the use of their properties of adsorption and steric selectivity is still to be explored. However, such applications require well-defined particles, especially spherical particles in the micrometric range. The synthesis of MSU-X silica that exhibits these shapes allowed us to test their properties in adsorption HPLC. Non polar solvent such as hexane are suitable to allow a significant separation. Further analyses and testing for size exclusion separation processes are under progress. 

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