Hydrolysis of alkoxide

Oo, and Sr ferrites mixed sulfides such as Zn—OdS and Pb—OdS and coated particles such as Ee O with Al(OH)2 or Or(OH)2. OontroUed hydrolysis of alkoxides has also been used to produce submicrometer Ti02, doped Ti02, Zr02, doped Zr02, doped Si02, SrTiO, and even cordierite powders (1,3). 

The sol-gel process involves hydrolysis of alkoxide precursors under acidic or basic conditions, followed by condensation and polycondensation of the hydroxylated units, which lead to the formation of porous gel. Typically a low molecular weight metal alkoxide precursor molecule such as tetramethoxy silane (TMOS) or tetra ethoxysilane (TEOS) is hydrolyzed first in the presence of water, acid catalyst, and mutual solvent... 

Traditionally, the sol-gel process has been used for the preparation of silica nanoparticles via the hydrolysis of alkoxides in organic solvents [52,53]. Similar hydrolysis and condensation carried out in w/o microemulsion offers robust control over the synthesis process. W/o emiflsion-mediated sol-gel synthesis is currently used for the fabrication of pure sihca, as well as inorganic and organic dye-doped silica nanoparticles. The synthesis of sihca and dye-doped nanoparticles is classified in the following sections on the basis of the classification of the head group fimctionahty of the major surfactant used. 

Hydrolysis of Alkoxides - Although it is simple and economical, the solid state reaction route has the following major drawbacks for the preparation of catalysts (1) very high temperatures are required to complete the formation of the final phase unless ball milling is performed before firing (2) industrial catalysts often exhibit complex chemical composition that is difficult to obtain uniformly through solid state reaction and (3) the low surface areas, that are typically achieved by solid state reaction, may affect the catalytic performances. 

Figure 2 Main steps of the preparation route via hydrolysis of alkoxides.

Hydrolysis of Alkoxides - A different thermal evolution of the precursor phase was observed for a BaAli20i9 sample prepared via hydrolysis of alkoxides.8,9 For this sample no XRD diffraction lines were detected after heating at 1000°C. At 1200°C Ba-hexaaluminate formed, and it was the only phase detected in the XRD spectra. Calcination at 1450 °C resulted in higher sharpness and intensity of the XRD reflection of Ba-hexaaluminate. No further changes were observed upon calcination at 1600°C. Also in this case, the appearance of Ba-(3-Al203 was accompanied by a marked drop of surface area. However, the... 

Arai and co-workers investigated the effects of cation composition in the mirror plane of Mn-substituted hexaaluminates.5,15 Investigation of a series of rare-earth-based hexaaluminates (AMnAluOi9, A=La, Pr, Nd, Sm) prepared via hydrolysis of alkoxides and calcined at 1200 °C for 5 h showed that surface area increases with the ionic radius of A. The La-substituted sample, the largest cation of the series, showed a surface area of 15 m2/g, which compared with 5 m2/g for the Sm-substituted one, i.e., the smallest cation of the series. These data are consistent with the mechanism of sintering resistance reported above. Indeed, it is expected that the larger the cation, the more difficult its diffusion along the c axis. 

La-hexaaluminates showed higher catalytic activity for all the M-substituted systems with respect to Ba-hexaaluminates.15 27 A similar promoting effect was also found for the composition Sro.gLao.aMn lnO, 15,16,26 which also exhibits the maximum surface area, but only when prepared via hydrolysis of alkoxides. 

Effective preparation methods of hexaaluminates for catalytic applications, such as the hydrolysis of alkoxides and the co-precipitation in aqueous medium, ensure high interspersion of the constituents in the precursor. This allows the formation of single phase materials with layered-alumina structure at reasonably low temperature (1100-1200 °C) and with high surface area. The hydrolysis of alkoxides was extensively studied and used for the industrial scale-up in the production of catalysts in the monolith shape. However, the co-precipitation in aqueous medium has much potential in view of the possible commercialization of these materials due to its simplicity and low cost. 

Comparison of different commercially available Zr02 samples in sintering of PZT ceramics has demonstrated that crystallization of PZT occurs in one step only if samples prepared by hydrolysis of alkoxides are used [1697],... 

Precursors useful in the surface sol-gel process are not restricted to alkoxides. The requirements as precursors are chemisorption on surface hydroxyl groups and regeneration of the hydroxyl groups after hydrolysis. For example, TiO(acac)2 repeatedly adsorbs, when acid and alkali are added to the adsorption and hydrolysis media, respectively. Except for the case of Nb(0"Bu)5, all the compounds listed in Table 6.1 show linear frequency shifts. Adsorption conditions such as concentration, temperature, and immersion time are dependent on the solubility, reactivity, and the ease of hydrolysis of alkoxides. These conditions are varied as the structure of alkoxide units is changed. For example, Ti(01Pr)4, which exists as a monomer in solution [17], requires conditions different from Ti(0"Bu)4, which tends to form oligomer species. 

Amorphous silica, silica gel, can be made by hydrolysis of alkoxides such as Si(OEt)4 it is used, when dehydrated, as a drying agent, and chromatographic and catalyst support material. It appears to contain Si(OSi=)4, Si(OSi=)3OH, and Si(OSi=)2(OH)2 groups. The nmr studies on MSi indicate that silica found in plants, flagellates, and other biological systems has the same type of structure as silica gel. 

These compounds are most readily obtained by hydrolysis of organotin hahdes, especially under basic conditions, although hydrolysis of alkoxides and amides, etc., can also be used. Hydrolysis of di- and trihalides occurs in a stepwise fashion and various of the partial hydrolysis products have been characterized (Scheme 19). 

The most extensive studies in this field have focused on Si02 and are reflected in this chapter. Typically, sol-gel materials are produced via controlled hydrolysis of alkoxides, as shown with silicon ... 

Colomban, P, and Vendange, V., Sintering of alumina and mullite prepared by slow hydrolysis of alkoxides the role of the protonic species and of pore topology, J. Non-Crystl. Solids, 147, 245, 1992. 

Formation of alkoxide from hydroxide is a reverse reaction of hydrolysis of alkoxide, which proceeds easily at room temperature and is a highly exothermic reaction (therefore Equation 2.2 has a positive reaction enthalpy). However, metal hydroxide is usually solid and has a polymeric M-(OH)-M network, while metal alkoxide usually has oligomeric structure. Therefore the former compound has lesser freedom (lower entropy). Consequently the unfavorable enthalpy term is overcome by the entropy term at high temperatures and equilibrium is attained. 

Recipe from [2125] 0.5 M solution of titanium ethoxide, isopropoxide or tert-butoxide in corresponding alcohol was added dropwise to alcohol-water mixture. The precipitate was washed with water, redispersed in 0.075 M HNO3, and refluxed for 8 h at 80 C. Reference [2013] was also cited for the recipe, but no specific recipe (based on hydrolysis of alkoxide) is reported there. 

Conventionally hydrolysis of alkoxide is performed by adding alkoxide to excess water under vigorous stirring conditions. It was observed that the uniform rate of alkoxide addition cannot be maintained due to the formation of hydrolysed product at the tip of the outlet during addition. Hence, reverse addition of water... 

Acid or base is used as a catalyst for the hydrolysis of silicon alkoxide. Acid or base with a mole ratio to alkoxide as low as 0.01 or less is usually enough for the catalytic effect. On the other hand, water with a mole ratio to alkoxide as high as 10 or more is used for the complete hydrolysis of alkoxide. We have found, however, that monolithic opaque gels composed of micrometer-sized silica particles can be prepared from tetramethoxysi-lane (TMOS) when a limited amount of water and a large amount of... 

In the hydrolysis of alkoxides, generally more vigorous conditions are required than for halo analogues. In highly activated systems, however, hydrolysis may be rapid acidic conditions were used for the hydrolysis of the 4-methoxy group in (131) <68JA709>. 

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