Silicates cyclic

There is an interesting analogy between cyclosiloxanes and the cyclic metasilicates [Si 03 ] " n = 3-6), e.g. (Me2SiO)3 (10.42) and [SisOg] " (10.43) the methyl groups in the dimethylsiloxanes are replaced by the formally iso-electronic in the silicates. Cyclic metasilicates occur naturally in certain minerals. For example, the trimer (n = 3) is found in CasSisOg (a-wollastonite), whereas the hexamer (n = 6) is a constituent of Be3Al2Si60ig (beryl). 

Magnesium silicate Cyclic imines from diamines... 

Today n-paraffms are exclusively produced from the corresponding distillation cuts of paraffin-rich oils with the use of molecular sieves. Molecular sieves are synthetically manufactured aluminum silicates of the zeolite type, which after dehydration have hollow spaces of specific diameters with openings of specific diameters. The molecules are then able to penetrate the openings in the correct size and form and are held in the hollow spaces by electrostatic or van der Waals forces. The diameter of the zeolite type used for the production of paraffins is 5 A and is refined so that the n-paraffins (C5-C24) can penetrate the hollow spaces while the iso- and cyclic paraffins are unable to pass through [15]. 

The oxidation of both linear and cyclic ethers to the corresponding acids and lactones by aqueous H202 as catalyzed by TS-1 and TS-2 was reported by Sasidharan et al. (241) (Scheme 17 and Table XXXV). The titanosilicates exhibited significantly better activity (about 55% conversion) and selectivity (98%) than chromium silicates, although vanadium silicates totally failed to catalyze the reaction. Such conversions are usually accomplished using either stoichiometric amounts of chromium trioxide, lead tetraacetate, or ruthenium tetroxide as oxidants (242) or catalytic amounts of Ru04 in the presence of... 

Titanium silicate molecular sieves not only catalyze the oxidation of C=C double bonds but can be successfully employed for the oxidative cleavage of carbon-nitrogen double bonds as well. Tosylhydrazones and imines are oxidized to their corresponding carbonyl compounds (243) (Scheme 19). Similarly, oximes can be cleaved to their corresponding carbonyl compounds (165). The conversion of cyclic dienes into hydroxyl ketones or lactones is a novel reaction reported by Kumar et al. (165) (Scheme 20). Thus, when cyclopentadienes, 1,3-cyclohexadiene, or furan is treated with aqueous H202 in acetone at reflux temperatures for 6 h in the presence of TS-1, the corresponding hydroxyl ketone or lactone is obtained in moderate to good yields (208). 

Cyclic Salt Carbonates Silicates Evaporites Pollution ... 

In 1992, Hari Prasad Rao and Ramaswamy reported on the oxyfunctionalization of alkanes with H2O2 using a vanadium silicate molecular sieve s . With this catalyst acyclic and cyclic alkanes were oxidized to a mixture of the corresponding alcohols (primary and secondary ones), aldehydes and ketones. Unfortunately, most of the early attempts were of rather limited success due to low turnover frequencies and radical producing side reactions as observed, for example, by Mansuy and coworkers in 1988. ... 

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 ... 

Polymeric chain, band- and sheet silicate structures have been discussed previously (page 742), and it should not be surprising to learn that cyclic silicate anions, such as [Si3Oy]ft and [Si601B]l2 (Fig. 16.28) are known. These anions are sometimes referred to as metasilicates in line with the older system of nomenclature, which assigned vrtlto to the most fully hydrated species [as in orthosilicic acid." Si(QH)J and metu to the acid (and anion) from which one mole of water has been removed [either in fact or formally for example, "metasilicic acid," OSi(OH)2]. 

The cyclic silicates arc considered later in the section on inorganic ring systems. 

The ether extract containing the catalyst and neutral products was fractionally distilled (130°-160°C at 0.01 mm Hg). The soluble catalyst was concentrated in the pot residues. The distillation fractions were then chromatographed through a silicic acid column. Monoesters and cyclic ketones were eluted successively with 5 95 and 10 90 diethyl ether petroleum ether, and more polar material was eluted with 15 85 diethyl ether. -petroleum ether followed by pure diethyl ether. 

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