Silica, structure

Polmanteer, K.E. and Lentz, C.W., Reinforcement studies. Effect of silica structure on properties and crosslink density. Rubber Chem. Technol., 48(5), 795-809 (1975). 

Various additives and fillers may be employed. Calcium carbonate, talc, carbon black, titanium dioxide, and wollastonite are commonly used as fillers. Plasticizers are often utilized also. Plasticizers may reduce viscosity and may help adhesion to certain substrates. Thixotropes such as fumed silica, structured clays, precipitated silica, PVC powder, etc. can be added. Adhesion promoters, such as silane coupling agents, may also be used in the formulation [69]. 

An important difference between Protein-Pak columns and other size exclusion columns is the silica backbone of the Protein-Pak columns. Because the silica structure is unaffected by the solvent, these columns do not swell or shrink as a function of the solvent. This is a general advantage compared to other size exclusion columns. However, silica-based columns can only be used up to pH 8, which limits their applicability. Also, surface silanols are accessible for interaction with the analytes, but this phenomenon has been minimized by proper derivatization techniques. Generally, a small amount of salt in the mobile phase eliminates interaction with silanols. 

Many of the high-pressure forms of ice are also based on silica structures (Table 14.9) and in ice II, VIII and IX the protons are ordered, the last 2 being low-temperature forms of ice VII and III respectively in which the protons are disordered. Note also that the high-pressure polymorphs VI and VII can exist at temperatures as high as 80°C and that, as expected, the high-pressure forms have substantially greater densities than that for ice I. A vitreous form of ice can be obtained by condensing water vapour at temperatures of — 160°C or below. 

Cha JN, Stucky GD, Morse DE, Deming TJ (2000) Biomimetic synthesis of ordered silica structures mediated by block copolypeptides. Nature 403 289-292... 

We have noted earlier that aluminium is unusual in forming alumino-phosphate complexes in phosphoric acid solution which may be of a polymeric nature. Bearing in mind the analogies between aluminium phosphate and silica structures, it may well be that during cement formation an aluminium phosphate hydrogel is formed. Its character may be analogous to that of silica gel, where a structure is built up by the... 

Wilson, N. G., McGreedy, T, Microporous silica structures for the immobilization of catalysts and enhancement of electroosmotic flow (EOF) in micro reactors, in Ehreeld, W. (Ed.), Microreaction Technology 3rd International Conference on Microreaction Technology, Proc. of IMRET 3,... 

The mechanical incorporation of active nanoparticles into the silica pore structure is very promising for the general synthesis of supported catalysts, although particles larger than the support s pore diameter cannot be incorporated into the mesopore structure. To overcome this limitation, pre-defined Pt particles were mixed with silica precursors, and the mesoporous silica structures were grown by a hydrothermal method. This process is referred to as nanoparticle encapsulation (NE) (Scheme 2) [16] because the resulting silica encapsulates metal nanoparticles inside the pore structure. 

Pt particles remain highly dispersed in the reaction mixture during mesostructure formation. All measurements including XRD, SAXS, and TEM indicate a well-ordered silica structure. N2 physisorption measurement indicated high surface areas (523-661 m g ) and meso-sized pores (112-113 A) for the silica supports produced in the presence of different Pt particles. 

Certain azacrown-appended cholesterol derivatives can form unique vesicular or lamellar structures in the absence and the presence of metal salts in aqueous solution.187-190 These superstructures created from the azacrown-appended cholesterol derivatives are useful as a template for the transcription into the silica structure. Azacrown-appended cholesterol gelator creates the novel multilayered spherical structure in acetic acid, whereas this gelator results in the fluffy globular aggregates in the presence of Pd(N03)2.191... 

One of the most promising applications of enzyme-immobilized mesoporous materials is as microscopic reactors. Galameau et al. investigated the effect of mesoporous silica structures and their surface natures on the activity of immobilized lipases [199]. Too hydrophilic (pure silica) or too hydrophobic (butyl-grafted silica) supports are not appropriate for the development of high activity for lipases. An adequate hydrophobic/hydrophilic balance of the support, such as a supported-micelle, provides the best route to enhance lipase activity. They also encapsulated the lipases in sponge mesoporous silicates, a new procedure based on the addition of a mixture of lecithin and amines to a sol-gel synthesis to provide pore-size control. 

D. Y. Zhao, Q. S. Huo, J. L. Feng, B. F. Chmelka, and G. D. Smcky, Nonionic triblock and star diblock copolymer and oligomeric surfactant syntheses of highly ordered, hydrothermally stable, mesoporous silica structures, J. Am. Chem. Soc. 120(24), 6024-6036 (1998). 

Another way to produce acetic acid is based on a carbonylation of methanol in the so called Monsanto process, which is the dominant technology for the production of acetic acid today [15]. Acetic acid then is converted to VAM by addition of ethylene to acetic acid in the gas phase using heterogeneous catalysts usually based on palladium, cadmium, gold and its alloys (vinylation reaction 3 in Fig. 2) [16] supported on silica structures. 

Nakamura and Matsui [71] prepared silica nanotubes as a spin-off product of sol-gel synthesis wherein tetraethylorthosilicate (TEOS) was hydrolyzed in the presence of ammonia and D, L-tartaric acid. Ono et al. [72] showed that certain cholesterol derivatives can gelate tetraethyl orthosilicate (TEOS) to obtain tubular silica structures. Using cholesterol based gelators nanotubes of transition metal (Ti, V and Ta) oxides can also be prepared. The organogelators used in these processes are chiral diamino... 

Jung JH, Ono Y, Hanabusa K, Shinkai S (2000) Creation of both right-handed and left-handed silica structures by sol-gel transcription of organogel fibers comprised of chiral diaminocyclohexane derivative. J Am Chem Soc 122 5008-5009... 

Since the main peaks are 3.25A, 4.2-4.6A and 7.0A, the most reasonable unit structure of the sodium aluminosilicate hydrogel is the 4-member chain illustrated in Fig, 11. Many 4-member chains easily form sodalite cages as shown in Fig. 12. In the case of a simpler silica structure, the double 4-member ring (cube) was recognized by Sakka et al C5). 

The industrial catalyst consists of a mixture of V2Os and K2S207 supported on silica. Under technical reaction conditions (>440°C), this mixture forms a viscous molten phase on the surface of the porous silica structure. Apparently a redox model can also be applied to such a system [154],... 

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