Alkoxides based precursors

Maneeratana, V., Sigmund, W.M. Continuous hollow alumina gel fibers by direct electrospinning of an alkoxide-based precursor. Chem. Eng. J. 137(1), 137-143 (2008)... 

There are various instances in which crystalline alumina phases, with special attention to a-Al20s (corundum), have been synthesized as a function of calcination temperature from alkoxide-derived alumina gels. Advantage of seeding with corundmn powders has also been taken to obtain the a-phase at a relatively low temperature. Interesting new developments have been reported by Kamiya et al. (1996, 2001) and Takeda et al. (2002) in this direction. These authors have reported synthesis of a-alumina at a temperature as low as 500-600°C under ambient pressure, using conirolled alkoxide-based precursor... 

The suitable alternatives to alkoxide-based precursor systems for titania is the oxo-lactato-titanate of ammonium, the so-called TiBALDH or TALH, stable at neutral pH and capable to produce Ti02 nanoparticles in solution equilibrium at room temperature, and for sihca - the water glass - is the water-soluble sodium silicate that can be converted into pH neutral colloid by action of a buffer [146,147]. 

Depending on the desired materials properties and structure, these approaches have been explored for sol-gel processing of alkoxide-based precursors and further developed in combination with many experimental parameters that can also be adjusted to control the sol composition and particle size, including alkoxide/ water ratio, concentration of the precursors, reaction temperature, reaction time, drying methods, choice and concentration of the catalyst, and the solvent used [42]. Besides the well-known hydrolytic sol-gel processes, even nonhydro-lytic approaches have been reported as a powerful method in the synthesis of mixed oxide materials and will be discussed in more detail subsequently [43]. Another processing option to tailor the material properties is the application of soft or hard templates [44]. In addition to the chemical parameters, drying and thermal treatments or firing processes determine the final architecture, chemical... 

These workers examined the mechanism of this reaction in detail (258). Burst kinetics were observed suggestive of the formation of an initial species from the catalyst precursor with subsequent slow turnover. The reaction was found to be pH sensitive, with a break point at pH 7.4, indicating a change in mechanism under these conditions. This pH corresponds to the expected value for secondary Cu(II) alkoxides. Based on this evidence, a formulated mechanism was advanced for this reaction, illustrated in Scheme 29. 

Alkoxide-based silicas are typically derived from letramethoxysilane [Si(OCH,)4, TMOS1 and tetracthoxysilane [Si(OCH2CH )4, TEOS] as precursors (17). The silica... 

Alternatively, action of alkoxide base on the unsaturated monobromo epoxide 100 or action of sodium iodide on the unsaturated dibromo epoxide 101 also produces 86 96.54 Methyl and other substituted benzene oxides have been prepared in a similar way from the bromo precursors.8... 

Using preformed sols instead of metal alkoxides as precursors is an attractive alternative in sol-gel preparation because recent advances in inorganic colloidal dispersions allow some control over the characteristics of the starting sols [11]. Often a colloidal suspension of sol particles is stabilized (i.e. prevented from flocculation) by pH adjustment. Thus, pH of the solution, which can be changed by the addition of either acid or base, is the single most important parameter in obtaining a gel from preformed sols. Other parameters that can impact on gel quality are the size and concentration of the starting sol particles. 

Sommelet-Hauser product 678. Small amounts of the para rearrangement product (679) and the direct displacement product (680) are also present. The ammonium salt precursor to the ylid (674) can generate either ylid (675 or 681), hut formation of 681 is sterically inhibited relative to 675. The Stevens product of 681 (amine 682) is more sterically crowded than 676 or 677. In polar aprotic solvents the major product is the Sommelet-Hauser product 678, hut in nonpolar solvents (hexane) the Stevens product 676 predominates. In DMSO, the enhanced nucleophilicity of the base causes the displacement product 680 (base attacks the methyl carbon and displaces the amine) to be formed in high yield. Alkoxide bases are not strong enough to generate the ylid from 674, and the displacement reaction (to give 680) dominates in those cases. 

Development of xanthate and dithiocarhamate derivatives overcomes several drawbacks of the sulfonium monomer (Scheme 7.2b and c). Xanthates and dithiocarbamates are easily prepared by the reaction of bis(halomethyl)benzene with alkylxanthate and dialkyldithiocarbamate salts respectively. Both precursors are stable at room temperature and soluble in organic solvents. This means the polymerization of these monomers can be performed in organic solvents e.g. THE) with the addition of alkoxide base e.g. potassium tert-butoxide). For the dithiocarhamate precursor, lithium bis(trimethylsilyl)amide can be used as the base and the polymerization proceeds at 35 The elimination temperature of these precursor polymers is typically lower than that of the sulfonium polymers with xanthate elimination at 160-250 °C and dithio-carbamate at 180 °C. It has been found that elimination of dithiocarbamate gave materials with reduced structural defects. Both xanthate and dithiocarbamate routes avoid the corrosive acid byproducts (HCl) present in the sulfonium elimination. This is particularly advantageous in device fabrication as adds have a negative impact on indium tin oxide electrodes and interfaces. ... 

In the case of alkoxide-based route (non-ionized precursors), the reactions that occur are the following ... 

Different strategies in the solution are described in the literature for the preparation of CSD precursors, where the prominence of the type of chemicals transcends the former grouping of solution methods. In general, but especially in the case of complex oxide compositions, neither of them can be used alone to obtain the liquid precursor due to difficulties in finding available metal reagents of the same family (e.g., alkoxides, carboxylates, and p-diketonates) for all the cations involved in the system. Therefore, a combination of several approaches is used to stabilize all the metal reagents in a common and stable solution. Some of these methods are shown here. They are divided considering the type of precursor used to obtain a stable solution, more than the traditional classification of solution methods shown above. Seven major solution processes are explained next alkoxide-based methods, carboxylate- and p-diketone-based methods, Pechini methods, diol routes, amine-based methods, polymer-assisted methods, and aqueous solution-gel methods. 

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