Oxide formation from metal alkoxides

Over the last three decades an intensive development of the synthetic approaches to various oxide materials has been based on the application of metal alkoxides, M(OR) . In principle, a number of ways leading to the formation of oxides from metal alkoxides are possible — for example,... 

The process of formation of oxide from metal alkoxide can be presented by the following scheme ... 

IR spectra measurements as well as variation of the film thickness, shrinkage, and refractive index demonstrated substantial differences in the mechanisms of thermal decomposition of films prepared from the exclusively metal alkoxide precursor and from the metal alkoxides modified by 2-ethylhexanoic acid. These differences affect the evolution of film microstructure and thus determine the different dielectric properties of the obtained films. The dielectric permittivity of the films prepared from metal alkoxide solutions was relatively low (about 100) and showed weak dependence ofthe bias field. This fact may be explained by the early formation of metal-oxide network (mostly in the... 

The formation of oxides from metal alkoxides generally involves the following steps, which tend to overlap. 

Due to the high reaction rates of hydrolysis and condensation reactions of metal alkoxides, information on the progressive structural evolution in transition metal systems is difficult to obtain. The 0x0 alkoxo clusters are therefore interesting models to assist in understanding the growth of three-dimensional oxide structures from the alkoxides. Some oxo-alkoxo oligomers may even be intermediates in the formation of the gel networks. 

Hierarchically porous metal oxide networks can be formed via a spontaneous self-formation phenomenon from metal alkoxides in aqueous solution [113]. Two chemical processes, hydrolysis and condensation, are involved in this spontaneous self-formation procedure to target hierarchically porous structures [114,115]. In fact, the hydrolysis and condensation rates are generally comparable for metal alkoxides [116]. The condensation rate is directly proportional to the rapid hydrolysis rate of reactive metal alkoxides [117,118]. It is well known that the rapid reaction rate of metal alkoxides plays the key role in the formation of hierarchically porous metal oxides [119,120]. The self-formation procedure to form hierarchically porous materials can be achieved by dropping liquid metal alkoxide precursors into an aqueous solution. In this section, the features of self-formation procediu-e and the resulting hierarchically porous materials are summarized. 

It is found that hierarchically meso/macroporous metal oxides can be synthesized even without the use of any external macrotemplate. In fact, great efforts have been made by scientists to promote development of hierarchically porous materials via the spontaneous self-formation phenomenon from metal alkoxides during the past decade. In this section, we will review the history of self-formation phenomenon to target hierarchically porous materials based on metal alkoxides. 

Another challenge faced by sol-gel technologies involves controUing the dispersion of different metals within a mixed metal (e.g., sihcon and titanium) matrix. The solvolysis and condensation steps for metal alkoxide precursors involved in sol-gel reactions can be quite different from that of orthosilicates, which often leads to the loss of dispersion and formation of separate silica and other metal oxide domains [54]. 

Monodisperse spherical oxide particles were prepared by the hydrolysis of metal alkoxide in homogeneous alcohol in an emulsion state. The formation mechanism from homogeneous alcohol and emulsion state was discussed by chronomal analysis and in situ observation using laser photo scattering. Two types of continuous systems for the industrial production of monodispersed oxide powders were also offered. 

These methods should be clearly distinguished from the formation of metal oxides by the pyrolysis of the corresponding alkoxide vapors (64-66). 

Sometimes, reactions in which an alcohol is oxidized by hydride transfer to a metallic cluster, resulting in the formation of a metallic hydride that subsequently transfers a hydride to a sacrificial aldehyde or ketone, are described as Oppenauer oxidations.44 In the opinion of the authors, the name Oppenauer oxidation should be reserved for oxidation of alcohols in which a hydride is directly transferred from a metallic alkoxide to an aldehyde or ketone acting as oxidant. 

Poly(bisphenol-A-carbonate) under pseudoideal reaction conditions was investigated, and the cyclic polycarbonate was obtained as the main product. In the system, the interface of the water/toluene mixture might have favored the cyclization reaction between the polar end groups [88]. Cyclic carbonates during the (Salen)CrCl catalyzed CCh/cyclohexene oxide copolymerization process in the presence of ionic initiators was also obtained [89]. The cyclic carbonate is produced via the backbiting mechanism, and the process is assumed to take place via a metal alkoxide (polymer chain) intermediate. Subsequent ring-opening of the cyclic carbonate with concomitant formation of polyether and CO2 was fast at the reaction temperatures from 80 to 100 °C). 

In aerobic oxidations of alcohols a third pathway is possible with late transition metal ions, particularly those of Group VIII elements. The key step involves dehydrogenation of the alcohol, via -hydride elimination from the metal alkoxide to form a metal hydride (see Fig. 4.57). This constitutes a commonly employed method for the synthesis of such metal hydrides. The reaction is often base-catalyzed which explains the use of bases as cocatalysts in these systems. In the catalytic cycle the hydridometal species is reoxidized by 02, possibly via insertion into the M-H bond and formation of H202. Alternatively, an al-koxymetal species can afford a proton and the reduced form of the catalyst, either directly or via the intermediacy of a hydridometal species (see Fig. 4.57). Examples of metal ions that operate via this pathway are Pd(II), Ru(III) and Rh(III). We note the close similarity of the -hydride elimination step in this pathway to the analogous step in the oxometal pathway (see Fig. 4.56). Some metals, e.g. ruthenium, can operate via both pathways and it is often difficult to distinguish between the two. 

Solvothermal reactions in alcohols are sometimes called alcohothermal reactions this word is derived from alcoholysis based on the similarity between hydrothermal and hydrolysis. Alcohols are the most common solvents for sol-gel synthesis. Primary alcohols are fairly stable at higher temperatures (up to 360°C) and therefore are widely used for the solvothermal reactions." For example, amorphous gel derived by hydrolysis of metal alkoxides can be crystallized by solvothermal treatment in alcohols. Since lower alcohols (methanol, ethanol, and 1-propanol) are completely miscible with water, water molecules present in the precursor gel may be replaced with the solvent alcohols. Therefore the precursor gel is easily dispersed in the solvent, where crystallization takes place. Detailed mechanisms for the formation of crystals are not yet fully elucidated. Crystallization of metal oxides is usually reported to take place by dissolution-recrystallization mechanisms, but the mechanism seems to depend on the gel structure. Moreover, water molecules dissolved from the gel in the reaction medium may facilitate crystallization of the product. More discussion is given in Section III.D of this chapter. 

Well ordered mesoporous silicate films were prepared in supercritical carbon dioxide.[218] In the synthesis in aqueous or alcoholic solution, film morphology of preorganized surfactants on substrate cannot be fully prescribed before silica-framework formation, because structure evolution is coincident with precursor condensation. The rapid and efficient preparation of mesostructured metal oxides by the in situ condensation of metal oxides within preformed nonionic surfactants can be done in supercritical CCU- The synthesis procedure is as follows. A copolymer template is prepared by spin-coating from a solution containing a suitable acid catalyst. Upon drying and annealing to induce microphase separation and enhance order, the acid partitions into the hydrophilic domain of the template. The template is then exposed to a solution of metal alkoxide in humidified supercritical C02. The precursor diffuses into the template and condenses selectively within the acidic hydrophilic domain of the copolymer to form the incipient metal oxide network. The templates did not go into the C02 phase because their solubility is very low. The alcohol by-product of alkoxide condensation is extracted rapidly from the film into the C02 phase, which promotes rapid and extensive network condensation. Because the template and the metal oxide network form in discrete steps, it is possible to pattern the template via lithography or to orient the copolymer domains before the formation of the metal oxide network. 

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