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Polycondensation dehydrating

Silica gel is a polymer of silicic acid and has the general formula Si02 x H20. There are many ways to prepare silica gel. Let us consider only one preparation of silica gel to demonstrate how and where differences or variations in the silica may contribute to a different bonded (final) phase. A common way of making silica gel particles for LC is to use acidic hydrolysis of sodium silicate, followed by polycondensation dehydration of orthosilicic acid and grinding of the silica to make granular particles (e.g., 10 jam). If the reaction is not controlled (buffer, pH, temperature, etc.) the hydrogel will... [Pg.210]

The silanols formed above are unstable and under dehydration. On polycondensation, they give polysiloxanes (or silicones) which are characterized by their three-dimensional branched-chain structure. Various organic groups introduced within the polysiloxane chain impart certain characteristics and properties to these resins. [Pg.1023]

Theoretical Studies. Theoretical models for the Si(OR)4 hydrolysis, polycondensation, and dehydration reactions involved in sol—gel processes have been developed using semiempirical molecular orbital models. These have been reviewed (3,5). [Pg.252]

We examined several approaches for synthesizing polyanhydrides, including melt polycondensation, dehydrochlorination, and dehydrative coupling. Extensive details of these new polymer synthesis techniques and numerous polymerization conditions for a wide variety of polyanhydrides were previously described (1). [Pg.44]

Leong, K. W., Simonte, V., and Danger, R., Synthesis of polyanhydrides Melt-polycondensation, dehydrochlorination, and dehydrative coupling. Macromolecules, 20. 705-712, 1987. [Pg.68]

The sol is made of a stable suspended solution of metal salts or solvated metal precursors containing solid particles of nanometer diameter. Polycondensation or polyesterification results in the appearance of particles in a new phase called the gel . Aging, drying and dehydration are steps required to achieve solid-form ultra-fine particles. Coarsening and phase transformation occur simultaneously with aging. Gel drying is associated with the... [Pg.380]

The main reactions of the MTG/MTO process can be summarized as follows the first is the dehydration of methanol to DME on acidic zeolite catalysts. The equilibrium mixture of methanol, DME, and water is then converted to light alkenes, which react further to form higher alkenes, n- and Ao-alkanes, aromatics, and naphthenes by hydrogen transfer, alkylation, polycondensation, isomerization, and other secondary reactions. [Pg.205]

Benzyl acetate was prepared by addition of benzyl chloride (containing 0.6% pyridine as stabiliser) to preformed sodium acetate at 70°, followed by heating at 115°, then finally up to 135°C to complete the reaction. On one occasion, gas began to be evolved at the end of the dehydration phase, and the reaction accelerated to a violent explosion, rupturing the 25 mm thick cast iron vessel. This was attributed to presence of insufficient pyridine to maintain basicity, dissolution of iron by the acidic mixture, and catalysis by ferric chloride of a Friedel-Craft type polycondensation reaction to polybenzyls, with evolution of hydrogen chloride, which at 130°C would produce an overpressure approaching 100 bar. Previously the chloride had been supplied in steel drums containing 10% sodium carbonate or 3% sodium hydroxide solutions as... [Pg.968]

Some of these processes can be controlled and used for the preparation of chemicals of industrial interest, such as 5-(hydroxymethyl-2-furaldehyde, HMF). HMF is the product of a triple dehydration of fructose, which is itself one of the first degradation products of sucrose. The preparation and the uses of HMF have been extensively studied and reviewed by Lichtenthaler.342 It can be obtained under various degradative conditions, including acid catalysis and catalysis by lanthanide ions.343 Processes for the production of HMF on the multi-ton scale have been developed as well as many uses, notably polycondensations.344,345 The polymerization of HMF has been shown to produce complex... [Pg.257]

In principle, the polycondensation may also be regarded as a dehydration process caused by the increase in proton concentration. In the starting species of the polycondensation process, the fully hydrated monotungstate, the W H20 ratio is 1 4 (pH > 6.2). In the various polytungstates prevailing between pH 6.2 and 1, the ratio is somewhere between 1 4 and 1 2, and attains exactly 1 2 in the precipitated tungstic acid at pH 1. [Pg.120]

Polyheterocycles can even be made by polymer analogous reactions. A good example of this is the formation of oxadiazole-containing polymers. Thus, Janietz et al. [86] prepared the precursor polymers 89 by the polycondensation of 90 with 91 and then dehydrated it with phosphorus oychloride to give the phenylene oxadiazole copolymers 69 (Scheme 6.21). [Pg.233]

Several approaches were examined for synthesizing polyanhydrides melt polycondensation, dehydrochlorination, and dehydrative coupling. A report of polyanhydride synthesis techniques and polymerization conditions for polyanhydrides appears in Leong et al. (22). [Pg.12]

Methanol is first dehydrated to dimethylether (DME). The equilibrium mixture thereof is then converted to light olefins. In the final steps of the reaction path, the Cj-Cg olefins are converted to paraffins, aromatics, naphthenes and higher olefins by polycondensation and alkylation reactions. The importance of light olefins as intermediates in the conversion of methanol to gasoline was soon recognized. As a result, several attempts were made to selectively produce light olefins from methanol on zeolite catalysts, not only on medium-pore zeolites but also on small-pore... [Pg.1]


See other pages where Polycondensation dehydrating is mentioned: [Pg.289]    [Pg.243]    [Pg.173]    [Pg.189]    [Pg.864]    [Pg.99]    [Pg.49]    [Pg.57]    [Pg.289]    [Pg.265]    [Pg.265]    [Pg.1347]    [Pg.109]    [Pg.5]    [Pg.481]    [Pg.17]    [Pg.562]    [Pg.27]    [Pg.146]    [Pg.225]    [Pg.8]    [Pg.268]    [Pg.396]    [Pg.224]    [Pg.84]    [Pg.325]    [Pg.120]    [Pg.398]    [Pg.6]    [Pg.174]   
See also in sourсe #XX -- [ Pg.394 ]

See also in sourсe #XX -- [ Pg.283 ]




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