Silica synthesis route

Scheme 2. The synthesis route used in preparation of silica nanotube...

Taramasso et al. (5) had originally reported two methods for the hydrothermal synthesis of TS-1. The first method (mixed alkoxide method) involves the preparation of a solution of mixed alkoxides of titanium and silica (preferably ethoxides) followed by hydrolysis with alkali-free solution of tetrapropylammonium hydroxide (TPAOH), distillation of the alcohol and crystallization of the resulting gel at 448 K. In the second method (dissolved or hydrolyzed titanium method) a soluble tetrapropylammonium peroxo-titanate species was prepared initially and then colloidal SiC>2 (Ludox AS-40) was added. This entire operation had to be carried out at 278 K. The TS-1 samples obtained by these two synthesis routes differed, particularly because of the presence of impurities such as Al3+ usually present in colloidal silica (33). 

Various metal and metal oxide nanoparticles have been prepared on polymer (sacrificial) templates, with the polymers subsequently removed. Synthesis of nanoparticles inside mesoporus materials such as MCM-41 is an illustrative template synthesis route. In this method, ions adsorbed into the pores can subsequently be oxidized or reduced to nanoparticulate materials (oxides or metals). Such composite materials are particularly attractive as supported catalysts. A classical example of the technique is deposition of 10 nm particles of NiO inside the pore structure of MCM-41 by impregnating the mesoporus material with an aqueous solution of nickel citrate followed by calicination of the composite at 450°C in air [68]. Successful synthesis of nanosized perovskites (ABO3) and spinels (AB2O4), such as LaMnOs and CuMn204, of high surface area have been demonstrated using a porous silica template [69]. 

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. 

Starting in the 1950s a process was developed that leads from small-molecule silicon alkoxides such as tetraethoxysilane (tetraethyl orthosilicate), to organosiloxane oligomers and low polymers, and eventually to silica via a low temperature synthesis route.14 24 A simplified outline of the basic chemistry is shown in reactions (l)-(4), where R is an ethyl or higher alkyl unit. Any or all of the Si-OR bonds can be... 

The conventional industrial method for the synthesis of a-silicon carbide is to heat silica (sand) with coke in an electric furnace at 2,000-2,500 °C. However, because of the high melting point of the product, it is difficult to fabricate by sintering or melt techniques. Thus, the discovery of a lower temperature fabrication and synthesis route to silicon carbide by Yajima and coworkers in 197526,27 proved to be an important technological breakthrough. This is a preceramic polymer pyrolysis route that has been developed commercially for the production of ceramic fibers. 

The traditional synthesis route involves the direct reaction of silicon with nitrogen at temperatures above 1,300 °C, or by heating silica with carbon (coke) in a stream of nitrogen and hydrogen at 1,500 °C.41 However, as in the case of silicon carbide, the high processing and fabrication temperatures focused attention on the need for alternative access routes based on preceramic polymers. 

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],... 

Breck has reviewed the early literature where Ga3+, P5+, and Ge4 were potentially incorporated into a few zeolite structures via a primary synthesis route (2). Evidence has also been presented to show that the small amounts of Fe3+, typically present in both natural and synthetic zeolites, are located in framework tetrahedral positions (3). A more recent review of "isomorphic substitution" in zeolites, via primary synthesis methods, speculates on the potential Impact of such substitutions on catalysis (4). The vast majority of work has been related to the high silica zeolites, particularly of the ZSM-5 type. Another approach to substitution of metal atoms into the open frameworks of zeolite structures has been to replace the typical silica alumina gel with gels containing other metal atoms. This concept has resulted in numerous unique molecular sieve compositions containing aluminum and phosphorus 5 silicon, aluminum and phosphorus (6) and with... 

This paper describes a new synthesis route for the formation of silica and titania nanotubes filled with high amounts of Pt metal clusters up to 24.5 wt.%. To some extent, the mechanism of the synthesis has been explored. 

Another important parameter that has to be deeply considered is the pH. It is well known the role of pH on silica chemistry it affects dissolution and polymerization rate, gel or precipitate formation and the textural properties of the final silica (11). Also for surfactant micelle or cluster templated syntheses of silica-aluminas, the effect of pH on porosity remains relevant and it is strongly influenced by the kind of material and by the synthesis route selected for its preparation. 

Figure 1. Schematic representation of the templated synthesis route using mesoporous silicas.

Silica gels with different particle size were synthesized by the SFB process[l2], a base-catalyzed synthesis route. In the present study, the particle size was controlled by changing the volume ratio of H2O to EtOH and the addition amount of NH3, a base-catalyst. In the synthesis process, tetraethoxysilane(TEOS) was used as a silica source material. 

Scheme 13 Synthesis route to silica-supported guanidinium chloride catalyst...

In the following, the synthesis of the most often employed stationary phase is discussed spherical silica with an n-octadecyl modification. The synthesis route has been chosen because all synthesis steps are well characterized and documented in standard operation procedure (SOP) protocols. The objective of this work was to develop a manufacturing process for a reversed phase C18-bonded silica column for HPLC according to standardized and validated procedures and to perform certification of the column, the tests and the mobile phases (du Fresne von Hohenesche et al., 2004). Figure 3.13 shows a scheme of the whole manufacturing process, and Table 3.7 summarizes the main steps. 

Effects of composition ratios and process parameters in silica precursors The synthesis route of silica membranes is schematically given in the upper part of Fig. 8.25. Tetraethylorthosilicate (TEOS) is not hydrolysed directly in water. To obtain a better control, the hydrolysis and condensation reaction rates were decreased by first diluting the TEOS in alcohol (ethanol) and then adding to this mixture a water-acid (HNO3) mixture dropwise under vigorous stirring. The mixture was kept for 3 h at 86°C under reflux conditions. Note that even with this procedure locally and for short times a relative large water excess exists in the reaction zone. 

Interest in the synthesis and processing of mesoporous silica materials has grown extensively since their discovery in 1992, and the exciting potential that these films hold in low-k dielectrics, sensors, nanowire fabrication, catalysis, membrane separations, and many other applications will continue to fuel academic and industrial interest in these films. While there are many new synthesis routes for processing mesoporous silica thin films, spin coating and dip coating remain the most facile methods available. These methods deliver high quality reproducible films that can be used for any of the variety of applications. 

Obviously, the structure of the produced carbon material is controlled by the pore structure of the matrix or the template. Mesoporous sihcas are attractive templates as they are available in a large variety of structures the thickness of their pore walls can be tailored [16] they exhibit a high structural order and methods for their cost-effective synthesis have been developed [17, 18]. In some cases, the OMC is a replica of the matrix pore system. This was proven by synthesis of a mesoporous sihca in the pore system of an OMC. The structure of the silica initially used for the synthesis of the OMC and of the silica synthesized in the pore system of the OMC was identical, proving that no structural changes occurred during the entire synthesis route [19]. In other cases, changes of the structure have been observed. For example, the pore system of the mesoporous silica MCM-48 consists of two interwoven but unconnected three-dimensional pore systems [20]. It is evident that upon removal of the MCM-48 matrix the structure of the OMC, known as CMK-1, changes [11]. 

The similar structural and catalytic properties of the SiOj-supported and unsupported samples prepared from the same precursor suggest that the same active surface is formed on both types of samples. The higher conversions obtained with the supported samples could be attributed to higher dispersions of the VPO compounds. The slightly lower maleic anhydride selectivity observed for catalyst A than B or the bulk catalyst could be due to some phosphorus atoms interacting with the silica surface rather than with vanadium atoms, such that the P/V ratio is less than two in the VPO compounds. Addition of phosphorus to catalyst B replenished this lost phosphorus. Previous studies of supported vanadium-phosphorus oxides have shown that some phosphorus atoms can be associated with the silica [2,8]. The catalytic properties of the supported samples as well as the LRS are similar to the SiOj-supported PA =2 VPO samples prepared previously [2,3]. These earlier samples were prepared by adding H3PO4 to PA =1 samples synthesized by various synthesis routes. Thus, for the supported samples, the method of preparation is much less important than the composition. 

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