Silica lattice structure

Zeolite is sometimes called molecular sieve. It has a well defined lattice structure. Its basic building blocks are silica and alumina tetrahedra (pyramids). Each tetrahedron (Figure 3-1) consists of a silicon or aluminum atom at the center of the tetrahedron, with oxygen atoms at the four comers. 

The variation in the lattice vibration of the solid products was examined by utilizing the FT-IR technique at successive DGC process times and the results are presented in Fig. 5. The absorption bands at 550 cm and 450 cm" are assigned to the vibration of the MFI-type zeolite and the internal vibration of tetrahedral inorganic atoms. The band 960 cm" has been assigned to the 0-Si stretching vibration associated with the incorporation of titanium species into silica lattice [4], This indicates that the amorphous wall of Ti-MCM-41 was transformed into the TS-1 structure. 

A unique way of identifying acid sites in amorphous silica-alumina was tried by Bourne et al. (128). These authors decided to synthesize, then characterize, two extreme types of acid site structures that they felt existed in commercial silica-aluminas. The two catalyst types consisted of low concentrations (<1.4% wt) of aluminum atoms incorporated (a) on the surface of silica gel (termed aluminum-on-silica) and (b) within the silica lattice (termed aluminum-in-silica). From infrared measurements of pyridine chemisorbed on the two materials, they conclude that dehydrated aluminum-on-silica contains only Lewis acid sites and that dehy-... 

Zeolite is a crystalline, porous aluminosilicate mineral with a unique interconnecting lattice structure. This lattice structure is arranged to form a honeycomb framework of consistent diameter interconnecting channels and pores. Negatively charged alumina and neutrally charged silica tetrahedral building blocks are stacked to produce the open three-dimensional honeycomb framework. 

An important question in characterization of high-silica molecular sieves regards die location of trivalent species, whether these are part of the lattice structure or just present as an oxide which is not part of the lattice. A number of methods are available for determining the lattice concentration, each with its own advantages and disadvantages. 

A silica-alumina is formed upon isomorphous substitution of a trivalent aluminum atom for a tetrava-lent surface silicon atom in the silica lattice. The commensurate incomplete coordination of the aluminum atom generates charged structural sites that, in the absence of surface hydroxyl groups, behave as Lewis acid and base sites(7). The presence of a proton as a counterion or a hydroxyl group as occurs upon hydrolysis can produce strong Bronsted acid sites(2 15, 16). Removal of the hydroxyl groups may convert Bronsted sites to Lewis sites reversibly(9, 15). Similar complexities as alluded to previously with the alumina surface arise in a detailed discussion of the nature of the active sites on silica-alumina so that several kinds of sites are usually invoked in interpretations of experimental measurements of surface acidity and catalytic activity(14, 15). 

Silica-titania may also be cogelled in the presence of chromium to form a "tergel" of Cr203, Si02, and Ti02. However, Cr2C>3 does not fit into the silica lattice, and exists only as a separate but highly dispersed phase. Upon activation, the Cr(III) is oxidized to Cr(VI) and it then resides on the surface as in other Cr/silica catalysts. There is no evidence that chromium in either oxidation state ever becomes part of the bulk silica structure. It can be easily removed from the catalyst by dissolution. 

The catalysts used in the aforementioned studies were always titanium silicates of MFI structure prepared by hydrothermal synthesis. Ti can, however, be inserted in the silica lattice by post-synthesis treatment of a dealuminated H-ZSM-5 with TiCl4 vapor [11]. Titanium silicalite-2 (TS-2), with the MEL structure of ZSM-11, was prepared shortly after the first synthesis of TS-1 [15]. Both catalysts have been used for the hydroxylation of phenol. Kraushaar-Czarnetzki and van Hooff showed that no major catalytic differences resulted from the method of synthesis of TS-1 [11]. The slow rate of reaction they observed was probably the result of large crystal size and low titanium content [7]. Tuel and Ben Taarit demonstrated there was no perceptible difference between the catalytic activity of TS-2 and TS-1 [8]. This was predictable, because of the close similarity of the Ti-site structure, chemical composition, and pore dimensions of the two titanium silicates. 

Silica, Si02, is an involatile solid and occurs in many different forms, nearly all of which possess lattice structures constructed of tetrahedral Si04 building blocks, often represented as in structure 13.17. Each unit is connected to the next by sharing an oxygen atom to give Si—O—Si bridges. At atmospheric pressure, three polymorphs of silica exist each is stable within a characteristic temperature... 

Bronsted or the Lewis type, owe their existence in silica-alumina catalysts to an isomorphous substitution of trivalent aluminum for tetravalent silicon in the silica lattice (Hansford, 42 Thomas, 18 Tamele, 35). Such an isomorphous substitution would lead to a structure somewhat as follows ... 

The theory of structural acids is largely due to Pauling (22). In any crystal lattice involving both n ative and positive ions, a net negative charge can be created by the isomorphous substitution of a positive ion of a valence lower than that of the substituted podtive ion. Thus, if an aluminum ion is substituted for a silicon ion in a silica lattice made up of silica tetrahedra, a tiivalent ion has been substituted for a quadrivalent ion and there results a positive valence deficiency of one for each aluminum ion so isomorphously introduced. In many naturally-occurring silica-alumina structures, this type of substitution has taken place. In all these systems the valence deficiency or net n ative charge in the crystal lattice is made up or satisfied by a positive ion at or near the point in the structure at which the substitution has taken place. Materials typical for these structural characteristics are natrolite and other natural zeolites, montmorillonites, and feldspars. 

The nature of the hydrocarbon and sulfur content is still not clear. However, calculations based on density data would seem to support earlier suggestions that the sulfur must be present as SO, or H O within the silica lattice. The optical characteristics of the mineral show that the organic matter occurs in films between the faces of the crystals. On the other hand, calculations based on the difference in densities of the original mineral and the pyrolyzed silica crystals show that the sulfur compounds at least must be within the crystal lattice. Kamb offers evidence (69) that the silica structure is a clathrate with SO,. HjO, and CH in the lattice analogous to the known 12 A gas hydrates of water, 6X 46HjO, where X is CH , H S, CO SOi, Cl, etc., and in fact the structure is the complete analogue of 6CI - H 0. 

Despite the fact that clathrate hydrate science can be considered to be a mature field, numerous challenges remain. In part, this is because hydrates exist in nature and hence exhibit the complexity of mineral phases, where the structure and composition reflect not only the conditions of synthesis but also subsequent exposure to changing conditions after synthesis. Another part is that the principal component of these materials is water, likely the most important material on earth, and the clathrates offer a possibility to study the diverse ways in which water interacts with other materials. In this respect, it is interesting to note that new structures continue to be found. Structurally related families of materials such as the sUica-based zeosils and clathrasils tend to show a far greater structural diversity. In part, this is because of the covalent nature of the silica lattices that give far more robust frameworks than the water-based ones however, each structure in these classes of materials could be considered as a candidate for clathrate synthesis. 

Just like the lattice structures in atomic crystals, colloidal particles can form equilibrium crystal structures by minimizing their interparticle potentials. Opals are naturally occurring colloidal crystals. These semiprecious stones consist of a close-packed structure of silica spheres... 

Comparing the structure of kaolin and montmorillonite, kaolin has 1 1 layer lattice and montmorillonite has 2 1 lattice structure. Kaolin consists of successive layers of alumina and silica mineral (silica-alumina), whereas montmorillonite has two layers of silica with a layer of alumina sandwiched between them (silica-alumina-silica). 

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