Supported amorphous silica structure

Figure 34.10 (Left) FIB TEM of recent supported amorphous silica structure the silica is just visible as a 100 nm (light gray) layer on top of a 1-mm (darker gray) mesoporous y-alumina layer. The support is made by partial sintering of dense-packed 300-nm a-Al203 particles. (Right) Three layers (left) are all formed from a colloidally stable dispersion by the filtration as shown.

Amorphous silica, with pore sizes in the range 1-10 nm is a common support for base catalysts, whilst more structured pore sizes can be made by what is known as the sol-gel method. In this method a micelle is... 

Our emphasis here is not on catalyst preparation and structure, but we need to describe briefly the preparation and properties of several major catalysts amorphous silica, y-alumina, zeolites, activated carbon, and supported metals. 

It was mentioned that when a piece of SC-155 was calcined at 1000 "C in oxidizing atmosphere, the obtained product was a white porous amorphous silica material with the same shape and external volume as the starting one, and when a piece of the same material was submitted to acid attack by 20 % HF aqueous solution, a self supported carbon network was obtained, also with the same shape and external volume as the original These experiences show that the silica network and carbon network are self supported structures and independent one from the other. 

Amorphous silicas play an important role in many different fields, since siliceous materials are used as adsorbents, catalysts, nanomaterial supports, chromatographic stationary phases, in ultrafiltration membrane synthesis, and other large-surface, and porosity-related applications [16,150-156], The common factor linking the different forms of silica are the tetrahedral silicon-oxygen blocks if the tetrahedra are randomly packed, with a nonperiodic structure, various forms of amorphous silica result [16]. This random association of tetrahedra shapes the complexity of the nanoscale and mesoscale morphologies of amorphous silica pore systems. Any porous medium can be described as a three-dimensional arrangement of matter and empty space where matter and empty space are divided by an interface, which in the case of amorphous silica have a virtually unlimited complexity [158],... 

Amorphous silica, silica gel, can be made by hydrolysis of alkoxides such as Si(OEt)4 it is used, when dehydrated, as a drying agent, and chromatographic and catalyst support material. It appears to contain Si(OSi=)4, Si(OSi=)3OH, and Si(OSi=)2(OH)2 groups. The nmr studies on MSi indicate that silica found in plants, flagellates, and other biological systems has the same type of structure as silica gel. 

In the case of solid supports having crystalline structures, like quartz, zirconium dioxide and o-alumina, the shape of the enthalpic curve is quite different and exhibits a distinct exothermic peak in the vicinity of the cmc (e.g., Figs. 6a and 6b). At first sight, the phenomenon seems strange, especially if all other curves are similar to those for the amorphous silica (see for example Figs. 2b, 3a and 7). An explanation based on the specific model of surface topography is advanced in this paragraph. 

A comparison of the spectra of steamed La.NH.Y and Ce.NH.Y suggests considerably stronger dealumination in the latter material, in spite of the poor quality of spectra obtained with cerium containing zeolites. The lower unit cell size of steamed CeNH.Y with the same rare earth content supports this conclusion. The strong signal at -110 ppm in the spectrum of steamed Hi-CeY is indicative of the presence of amorphous silica, resulting from partial structural collapse (33). 

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