Silica crystalline structure

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

The zeolites with applications to FCC are Type X, Type Y, and ZSM-5. Both X and Y zeolites have essentially the same crystalline structure. The X zeolite has a lower silica-alumina ratio than the Y zeolite. The X zeolite also has a lower thermal and hydrothermal... 

Glasses are oxides which have hardened and become rigid without crystallizing. The glassy structure consists of silica tetra-hedra or other ionic groups that provide a solid, non-crystalline structure. 

In addition to the crystalline clays described earlier, there are some materials that act like clays but do not have crystalline structure. Amorphous clays do not have a definite X-ray diffraction pattern and are differentiated from the crystalline clays on this basis. They are composed of mixtures of alumina, silica, and other oxides and generally have high sorptive and cation exchange capacities. Few soils contain large amounts of amorphous clays [2],... 

The same principles that are valid for the surface of crystalline substances hold for the surface of amorphous solids. Crystals can be of the purely ionic type, e.g., NaF, or of the purely covalent type, e.g., diamond. Most substances, however, are somewhere in between these extremes [even in lithium fluoride, a slight tendency towards bond formation between cations and anions has been shown by precise determinations of the electron density distribution (/)]. Mostly, amorphous solids are found with predominantly covalent bonds. As with liquids, there is usually some close-range ordering of the atoms similar to the ordering in the corresponding crystalline structures. Obviously, this is caused by the tendency of the atoms to retain their normal electron configuration, such as the sp hybridization of silicon in silica. Here, too, transitions from crystalline to amorphous do occur. The microcrystalline forms of carbon which are structurally descended from graphite are an example. 

Self-assembly of highly charged colloidal spheres can, under the correct conditions, lead to 3D crystalline structures. The highly charged spheres used are either polystyrene beads or silica spheres, which are laid down to give the ordered structures by evaporation from a solvent, by sedimentation or by electrostatic repulsion (Figure 5.34). The structures created with these materials do not show full photonic band gap, due to their comparitively low relative permittivity, although the voids can be in-filled with other materials to modify the relative permittivity. 

Amorphous silica, ie, silicon dioxide [7631-86-9]9 Si02, does not have a crystalline structure as defined by x-ray diffraction measurements. Amorphous silica, which can be naturally occurring or synthetic, can be either surface-hydrated or anhydrous. Synthetic amorphous silica can be broadly divided into two categories of stable materials (1) vitreous silica or glass (qv), which is made by fusing quartz at temperatures greater than approximately 1700°C (see Silica, vitreous silica), and microamorphous silica, which is discussed herein. 

Zeolitic Catalyst—Since the early 1960s. modern cracking catalysts contain a silica-alumina crystalline structured material called zeolite. This zeolite is commonly called a molecular sieve. The admixture of a molecular sieve in with the base clay matrix imparts desirable cracking selectivities. 

Ya.B. considered that the most important adsorbents—porous coal, silica gel, and the powdered manganese dioxide which he had studied experimentally—are amorphous substances, i.e., they do not have clearly articulated crystalline structure. Only thus is it possible to obtain a large developed surface—the most important feature of an adsorbent. In this case, it is natural to consider all the possible values of adsorption activity and a smooth distribution function of surface sectors according to their level of activity. 

The catalytic activity of titanium silicates must be ascribed Tilv sites, because pure crystalline silicas are totally inactive. As was discussed in Section III, Tilv is present in the crystalline structure at random. Very likely, the random distribution that is obtained in the precursor reagents is maintained in the solid. Being dilute, each Tilv is expected to be surrounded by OSiIV groups and isolated from other Tiiv ions by long O—Si—O—Si—O sequences. It has been... 

To each of these systems a measured amount (s 5g) of crushed (< 125p) quartz was added in a stainless-steel beaker, and the systems were heated to their boiling points at atmospheric pressure for 5 hr. Quartz was chosen for these experiments because of its tight crystalline structure and relative insolubility in relation to other silica species (2). Solubility values for this mineral would represent a minimum for a given solvent system with respect to silica. 

A characteristic of the early neutron reflectivity studies of nonionic surfactant adsorption was some variability in the pattern of adsorption. This was investigated in more detail and more systematically by McDermott et al. [55], who compared the adsorption of Ci2E6 onto a range of different substrates, amorphous silica, crystalline quartz, and the oxide layer on a silicon single crystal. The adsorbed surfactant was found to form a bilayer with an overall thickness 49 4 A, with a structure similar to that determined in the previous studies (see Fig. 4). 

Zeolites are combinations of A1203 and Si02 that are crystalline structures with precisely defined pore structures in the molecular size range (0.4—1.5 nm or 4-15 A). A related group of materials known as mesoporous silica-alumina has extended the range of pore sizes attainable in ordered Si02-Al203 supports to 4 nm (40 A). They are commonly used in the chemical and petroleum industry due to their surface acidity and ability to exclude molecules... 

A brand new area should conclusively be mentioned, that of the PP-nanocomposites made of single-walled carbon nanotubes [157] or silica nanoparticles [158] which were reported to facilitate the growth of the -crystalline structure. However, no published data support their supposed outstanding performance. 

The short range order and long range disorder lead to the model of the continuous random network, introduced by Zachariasen (1932) to describe glasses such as silica. The periodic crystalline structure is replaced by a random network in which each atom has a specific number of bonds to its immediate neighbors (the coordination). Fig. 

Ceramics are primarily compounds. Ceramics other than glasses generally have a crystalline structure, while silica-based glasses, a subclass of ceramic materials, are noncrystalline. In crystalline ceramic compounds, stoichiometry dictates the ratio of one element to another. Nonstoichiometric ceramic compounds, however, occur frequently. Some important ceramic materials are listed in Table... 

Ordered macroporous materials (OMMs) are a new family of porous materials that can be synthesized by using colloidal microspheies as the template. - The most unique characteristics of OMMs are their uniformly sized macropores arranged at micrometer length scale in three dimensions. Colloidal microspheres (latex polymer or silica) can self assemble into ordered arrays (synthetic opals) with a three-dimensional crystalline structure. The interstices in the colloidal crystals are infiltrated with a precursor material such as metal alkoxide. Upon removal of the template, a skeleton of the infiltrated material with a three-dimensionally ordered macroporous structure (inverse opals) is obtained. Because of the 30 periodicity of the materials, these structures have been extensively studied for photonic applications. In this paper, the synthesis and characterization of highly ordered macroporous materials with various compositions and functionalities (silica, organosilica, titana, titanosilicate, alumina) are presented. The application potential of OMMS in adsorption/separation is analyzed and discussed. 

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