Fluorosilicates structures

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The zwitterionic A5S7-fluorosilicates 4-22 were isolated as crystalline solids. Compounds 4-8, 13, and 17-22 were structurally characterized in the solid state by single-crystal X-ray diffraction. In contrast to the achiral zwitterions 4-16, the zwitterions 17-22 are chiral, the respective crystals consisting of pairs of enantiomers [(A)- and (C)-enantiomers]. In all cases, the /-coordination polyhedron was found to be a somewhat distorted trigonal bipyramid, with fluorine atoms in the two axial sites. This is illustrated for 6 and 19 in Fig. 1. Selected geometric parameters for compounds 4-8, 13, and 17-22 are listed in Table I. As can be seen from these data, the axial Si-F distances [1.647(2)-1.743(1) A] are significantly longer than the equatorial ones [1.589(2)-1.638(1) A]. The Si-Cl distances amount... 

Glass-Ceramics Based on Silicate Crystals. The principal commercial glass-ceramics fall into this category. These can be grouped by composition, simple silicates, fluorosilicates, and aluminosilicates, and by the crystal structures of these phases. 

Fluorosilicates. Compared to the simple silicates, these crystals have more complex chain and sheet structures. Examples from nature include hydrous micas and amphiboles, including hornblende and nephrite jade. In glass-ceramics, fluorine replaces the hydroxyl ion fluorine is much easier to incorporate in glass and also makes the crystals more refractory. Four commercial fluorosilicate glass-ceramic compositions and their properties are listed in Table 2. 

FIGURE 5. X-ray crystallographic structure of 9 (zwitterionic fluorosilicate). Reproduced from Reference 31a by permission of VCH Verlagsgesellschaft... 

The 29 Si NMR (Table 20) supports a hexacoordinate structure in solution which is similar to that in the solid state. (529Si for the hexacoordinate fluorosilicates is ca 50 ppm shifted upheld relative to the pentacoordinate precursors. The multiplicity of the 29 Si resonances indicates the nonequivalence of fluorine ligands at low temperatures (between 25 and — 60 °C). These become equivalent as the temperature is raised, such that a symmetrical 29Si multiplet is observed. 

The reaction of zeolites with an aqueous fluorosilicate solution under relatively mild conditions has been shown to yield zeolites with silicon enriched frameworks which are essentially free of structural defects (1). As a result of the treatment, the framework topologies of the respective zeolites are relatively unchanged, but the zeolite compositions which are produced either do not occur naturally or are not synthesized directly. The fluorosi1icate treatment process has been termed "Secondary Synthesis". 

Table III shows XRD and porosimetry data for calcined USY and AFS zeolites. All samples show shrinkage of the unit cell to comparable values following calcination. As a result, calcined samples are compared at similar silica-alumina framework ratios. All calcined samples have well developed microporous structures and comparable total pore volumes. These porosimetry data confirm that the hydrothermally dealuminated materials contain a significant fraction of mesopores relative to chemically dealuminated materials. The extensive washing given to AFS-1 results in higher micropore surface area and volume compared to AFS-2 and suggest that AFS-2 contains occluded fluoroaluminate and fluorosilicate compounds within the microporous structure.

Careful purification of solutions of fluorosilane and fluorosilicate anions and a decrease in concentration and temperature result in a fine structure of F NMR spectra due to a non-equivalent arrangement of ligands (Table 16). For example, in the F NMR spectra of octahedral complexes of the type RSiFj" a doublet (equatorial atoms) and a quintuplet are observed whose mutual arrangement depends on the nature of the ligand In fluorine derivatives of penta- or hexa-coordinate silicon with slow exchange, the Hgip values (if they can be determined) are greater for the fluorine atoms with chemical shifts in low field. 

The main objective of this work is to characterise the textural properties of a series of Y zeolites dealuminated by ammonium hexafluorosilicate treatment. It was observed that the fluorosilicate treatment produced a highly crystalline product with a contracted unit cell. Both textural and XRD analysis confirmed the samples to be at least 95% crystalline for dealumination degrees <50%. According to N2 adsorption only a minimal contribution of mesopores was observed. However, a notable loss of micropores accompanied by the formation of mesopores was noted for severe dealumination levels, along with a considerable structural degradation. 

With silicates and fluorosilicate Mn2Si04 300---75 5.93 -175 Sucks olivine structure 66X16, 66S4,... 

LXIX. Steric hindrance in pentacoordinated fluorosilicates - synthesis and molecular-structure of the diphenyl-1-naphthyldifluorosilicate anion and the phenylmethyltrifluorosilicate anion, Inorg. Chem. 26 (5), 760-765 (1987). 

Dixon, D.A., Farnham, W.B., Heilemann, W., Mews, R. and Noltemeyer, M., Structural studies of tris(diallq laniino)sulfoniuni (TAS) fluorosilicates, Heteroat. Chem. 4 (2-3), 287-295 (1993). 

In summary, we suggest that the process of fibrous fluorosilicate formation from mining rocks involved a sequence of complex phase transformations and chemical reactions in combination with structural transformations and recrystallization. 

Synthesized FFS by their chemical composition (Table 5), structure (Table 6, Fig 11), and physical-chemical properties (Table 7) related to NaMg-arfvedsonite asbestos (Hodgson, 1965 Chigareva, 1967 Grigor eva et al., 1975). Crystallochemical formulas of fibrous fluorosilicates synthesized in serpentinite-, dunite-, peridotite- and p)rroxnite-containmg blends corresponded respectively to ... 

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