Silicate dimers

Rectystallization of 7 from dichloromethane affords single crystals which are suitable for X-ray diffraction analysis. Unfortunately, Because of a twinning problem, the results of the crystal measurements were not satisfactory for the determination of precise geometric parameters. However, the molecular array of 7 has been elucidated and the results are shown in Fig. 1. The analysis revealed that 7 is a new type of silicate dimeric species. 

Dimer formation results in disappearance of OH functional groups, which prevents silicate dimer enlargement. This structure of low-modulus waterglasses is confirmed by numerous experimental data [128, 130, 131, 172]. 

Fig. 16.4. Silicate structures, (a) The Si04 monomer, (b) The Si207 dimer with a bridging oxygen. ( ) A chain silicate. (d) A sheet silicate. Each triangle is the projection of on Si04 monomer.

However, the free acid quickly starts to condense with itself, accompanied by the elimination of water to form dimers, trimers and eventually polymeric silicic acid. The polymer continues to grow, initially forming polymer aggregates and then polymer spheres, a few Angstroms in diameter. These polymeric spheres are termed the primary particles of silica gel and must not to be confused with the macro-particles of silica gel that are packed into the LC column. 

Silicagel. Silicagel is usually prepared from sodium silicates, which are produced as glasses with a stoichiometry of Si02/Na20 = 0.6-0.25. At the right pH they arc soluble in water and the solution contains mixtures of silicate anions monomeric, dimeric, trimeric species, etc. (Fig. 3.13). 

Various approaches to the analysis of dissolved silicon have been tried. Most of them are based on the formation of /J-molybdosilic acid [ 199-203 ]. Dissolved silicon exists in seawater almost entirely as undissociated orthosilicic acid. This form and its dimer, termed reactive silicate , combine with molybdosilicic acid to form a- and /I-molybdosilicic acid [180]. The molybdosilicic acid can be reduced to molybdenum blue, which is determined photometrically [206]. The photometric determination of silicate as molybdenum blue is sufficiently sensitive for most seawater samples. It is amenable to automated analysis by segmented continuous flow analysers [206-208]. Most recent analyses of silicate in seawater have, therefore, used this chemistry. Furthermore, reactive silicate is probably the only silicon species in seawater that can be used by siliceous organisms [204]. 

The incompletely condensed silsesquioxanes can be considered as intermediates in the synthesis of the whole family of silsesquioxanes, but at the same time they are the building blocks (e.g., initiators) for the new polymeric architectures.66 67 The importance of this class of materials in model studies of silica surfaces and silicate defects,68 and in creating various oligosilanols with novel frameworks,69 70 makes them the subject of extensive research.71 The use of the incompletely condensed silsesquioxanes in forming dimeric silsesquioxanes and metallasilsesquioxanes in attempts to design new routes to the well-defined Si-O assemblies has recently been reviewed in a well-illustrated research report.72... 

This, however, is an oversimplification, since these monomeric anions are in rapid equilibrium with oligomers (small polymers) such as the dimer (cf. Fig. 7.2b), cyclic trimer (7.2c), and other small silicate oligomers, 22 of which have been identified by silicon-29 magnetic resonance methods and many of which have no counterparts in solid silicate minerals9 ... 

Fortnum (4, 5), in his Raman spectrum study of aqueous ions, observed four distinct lines at 448, 607, 777, and 935 cm-1 which he attributed to the silicate ion. He adds that a fifth line is observed at 1040 cm 1 in solutions having little or no added sodium hydroxide however, this line disappears in solutions having large amounts of sodium hydroxide with an increase in sharpness and intensity of the 777- and 935-cm-1 lines. He says that there are two species which could have been present to give rise to the 1040-cm 1 lines either hydrolyzed silicate ion, SiO(OH)3 , or a dimer, H4Si2072. ... 

However, Fortnum concludes that the 1040-cm-1 line is generated by the dimeric species of the silicate ion. Fortnum s silicate solutions ranged from 3% to 19.2% Si02 with Si02/Na20 ratios from 0.33 to 1.0 permitting a comparison of his spectra with those made in this investigation. 

The largest silicate species in the extracts of the concentrated TEA system after 45 min, detected with GPC as a shoulder on the main peak, has a retention volume characteristic for dimer of tetracyclic undecamer (Fig.2). This indicates that the first steps of the TEOS polycondensation processes till the formation of (4) and (4 ) occur with TEA as well. This is in agreement with the proposed model in which TEOS hydrolyses at the TEOS-aqueous interfaces in the vicinity of the alkyl groups of the tetraalkylammonium cations (Fig.l). These alkyl groups favor the formation of the hydrophobic silica surfaces encountered already in the smallest species (l)-(3). The absence of trimers... 

The aggregation of the silica species in solution continues upon dilution with water [3], With TPA, the main final products at room temperature are nanoslabs (8), counting 396 Si atoms and composed of twelve precursors (5) (Fig.l). With TPA, it was experienced that the timing of the water addition was not very critical. The yield of Silicalite-1 nanoslabs on silica basis is typically 80% [1]. With TBA, there is a violent hydrolysis and gel formation when the water is added from the beginning. To avoid this, it is preferred to add water after complete hydrolysis. The MEL nanoslabs with occluded TBA rapidly dimerize at room temperature to result in the formation of slabs with dimensions of 1.3 x 4.0 x 8.0 nm [3], The product yield on silica basis is similar to the TPA system. With TEA, the amount of extractable silicate was much smaller. 1R spectra of the extracts recorded after 24 h stirring of the solutions are shown in Fig.3. 

The chemical state of dissolved silica (if Si02 is supplied as silica sol) must influence the nucleation and growth of crystalline silicates. Cary et al. (213) dissolved isotopically enriched silica in H20/D20 and, using 29Si NMR, concluded that tetrahedral dimers corresponding to pyrosilicic acid, H6Si207, built of two Q1 units were present in addition to monomers (Q°). The former species resonated at —9.26 ppm from TMS and accounted for up to 6 % of total spectral intensity. 

Figure 3.11 Schematic of the formation of ITQ-29, a purely siliceous version of the widely used zeolite A, using a large cationic dimer as a template [55],...

O dimers may appropriately be called peroxy anions, 022 . In a silicate matrix they appear to occur as peroxy links or, more generally, peroxy entities, X/00 Y where X,Y = Si4+,A13+, Fe3+... Chemically a peroxy entity represents an extra oxygen it may be called an interstitial oxygen atom though this does not correctly convey the covalent binding in the 0-0 bond. 

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