Three-Dimensional Silicon-oxygen Structures

From the chemical standpoint a nucleus of an HDS particle is a three-dimensional polymer whose structural units are silicon-oxygen tetrahedra bonded by disiloxane bridges Si—O—Si. On the surface of HDS particles there are groups O—H chemically bonded with silicon atoms (silanol groups SiOH). The hydroxylic cover of HDS gives rise to a high hydrophilicity of its surface and, correspondingly, to its ability to sorb polar molecules. 

Pure silica contains no metal ions and every oxygen becomes a bridge between two silicon atoms giving a three-dimensional network. The high-temperature form, shown in Fig. 16.3(c), is cubic the tetrahedra are stacked in the same way as the carbon atoms in the diamond-cubic structure. At room temperature the stable crystalline form of silica is more complicated but, as before, it is a three-dimensional network in which all the oxygens bridge silicons. 

Many mineral species have the same or similar chemical basic units within their atomic structure. All common silicate minerals, for example, are characterized by the association of four large oxygen ions (0 ) bonded to a small silicon ion (Si ). The shape of the complex ion is a tetrahedral unit, with the composition (Si04) . The two- and the three-dimensional expressions of the silicate ion are presented in Fig. 2.1, parts A and B, respectively. The three-dimensional figures emphasize the potential variations in orientation between the ions as they have been observed in minerals. 

The basic structural element in both vitreous and crystalline silica is the Si04-4 tetrahedron, which arises from the sp hybrid orbitals of the silicon. Each silicon atom sits in the center of the tetrahedron surrounded by four oxygen atoms that hold the comer positions. Tetrahedrons bond together by comer sharing. In a properly developed structure, each oxygen is shared by only two tetrahedrons. This bonding scheme can produce a large variety of three-dimensional structures and is the reason that silica has a number of crystalline phases. 

Zeolite molecular sieves are composed of silicon and aluminum and can be natural or manmade minerals. Molecular sieves are crystalline, hydrated aluminosilicates of (most commonly) sodium, calcium, potassium, and magnesium. The alumininosilicate portion of the structure is a three-dimensional open framework consisting of a network of A104 and Si04 tetrahedra linked to each other by sharing all of the oxygens (Sherman, 1978). Zeolites may be represented by the empirical formula... 

Draw structures of (SiQi)4 IShOjJ. [SiOx U- (StiOn 1 ISLjOtn I,.. and [SiO- . Enclose the repeating units in brackets and show that these empirical formulas are cor rect. How do the ratios of oxygen to silicon correlate with the degree of polymerization m silicates (i.e., discrete ions compared to chains compared to double chains compared to infinite sheets compared to three-dimensional frameworks) ... 

Teciosilicates involve the sharing of all four oxygens in each tetrahedral unit with adjacent tetrahedrons to form a three-dimensional framework of SiOj units linked together. The product is a strongly bonded structure with a silicon-oxygen ratio of 1 2. The greater portion of the earth s crust is composed of minerals found within this classification. 

This compoundexists in at least eleven distinct crystalline forms. Several of them are obtained by heating a-quartz, which has a number of transition points, to produce 0-quartz, and to give various forms of tndymite and crystobalite. The unit of structure is the tetrahedron in which each silicon atom is covalently bonded to four oxygen atoms, and the variation is in the ways these tetrahedra are interconnected (by oxygen atoms) to form a three-dimensional system. 

The molecular structure depends on the oxygen-to-silicon ratio. If O/Si = 2, each oxygen is covalently bonded to two tetrahedra so a three-dimensional network is formed. At the other extreme, if the O/Si = 4, none of the four oxygen atoms is shared by another tetrahedra, and isolated molecules are formed. Figure 17.2 shows many of the possibilities. 

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],... 

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