Siliceous framework, oxygen

X-Rav Diffraction. Borosilicate molecular sieves have been studied by X-ray diffraction (1.16-20 ). X-ray diffraction techniques have been developed to determine the degree of substitution of the silicate framework by borate tetrahedra (291. The boron-oxygen bond is shorter than the the silicon-oxygen bond, which leads to a contraction of the unit cell for a borosilicate molecular sieve as boron substitution increases. The unit cell volume determined from peak positions in the ranges 2O°<20<35< and 45°<20<5O° or an empirical parameter termed ST (sum of four d spacings) correlated with structural boron content (29). 

When a molecule adsorbs on the siliceous part of the micropore of a zeolite, the main interaction it experiences is a dispersive van der Waals-type interaction. This is due to the dominant interaction with the large polarizable oxygen atoms that make up the zeolite framework. For example, the interaction between a hydrocarbon CH3 or CH2 group and the siliceous framework typically results in an interaction energy that is on the order of 5-10 kJ/mol. These electrostatic interactions are small. 

Coordination— The coordination numbers of the bulk sodium silicate glass are listed in Table 21.5. It is clear from these numbers that the glass is made up of a silicate framework in which the sodium ions are dispersed. The majority of the silicon atoms form four-coordinate structures. Two-thirds of the oxygen ions form bridging atoms between the silicate tetrahedra, which can be seen by their twofold... 

The complete process for synthesizing such species using this approach would entail the acquisition of an appropriate natural silicate or the preparation of an appropriate synthetic silicate and then the conversion of this silicate into the alkyl silicate or organosiloxane by suitable substitution reactions. In terms of bond cleavage, this process could entail no destruction and reformation of framework silicon-oxygen bonds, and, in terms of oxidation number, it would entail no reduction and reoxidation of the silicon. 

Feldspar is a framework silicate in which tetrahedra containing A1 or Si are linked to one another by shared oxygens in all directions rather than in chains or sheets (3) The plagioclase solid-solution series... 

As well as occurring naturally, alumino-silicates are manufactured. Their structure is that of a framework of silicon, aluminium and oxygen atoms. If the framework contains water, then this may be driven off by heating, leaving a porous structure, access to which is controlled by windows of precise molecular dimensions. Larger molecules are excluded, hence the description molecular sieve as discussed in Chapter 17. 

When all four oxygens of the tetrahedra are shared (Si 0 = 1 2), a fully polymerized ion results, and a three-dimensional framework is formed. Quartz, an example of this type of silicate array, is discusssed in the section on silica minerals. 

If one single element divides the modern world from that before the Second World War, it is the unassuming grey solid called silicon. This element is everywhere, and always has been. Silicon is the second most abundant element in the Earth s crust, since most common rocks have crystalline frameworks made from silicon and oxygen they are silicates. Quartz and sand are composed of silicon and oxygen alone silicon dioxide, or silica. 

Three-dimensional or tecto-silicates Three-dimensional frameworks of [Si04] tetrahedra sharing all four oxygen atoms Feldspars, zeolites, ultramarines... 

The rule reqiiires that in silicates the silicon tetrahedra share no elements with each other if the oxygen-silicon ratio is equal to or greater than four. This is found t o be vrue in general (topaz, zircon, olivine, other orthosilicates), Most of the few exceptional substances now known contain extra oxygen atoms in the form of hydroxide ions. These include the clay minerals, micas, and chlorites, discussed above, and also the mineral hemimorphite, Zn[Pg.561]

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) ... 

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