Hydrolysis reactions metal alkoxides

Figure 17 summarizes the avadable sol—gel processes (56). The process on the right of the figure involves the hydrolysis of metal alkoxides in a water—alcohol solution. The hydrolyzed alkoxides are polymerized to form a chemical gel, which is dried and heat treated to form a rigid oxide network held together by chemical bonds. This process is difficult to carry out, because the hydrolysis and polymerization must be carefully controlled. If the hydrolysis reaction proceeds too far, precipitation of hydrous metal oxides from the solution starts to occur, causing agglomerations of particulates in the sol. 

Nucleophilic metal alkoxide reacts rapidly with electrophilic water to generate the several hydrolysis products with complex intermediate species containing some residual alkoxy groups. The concentration ratio of the starting reagents and the nature of alkoxide and the solvent influence the morphology of hydrolysis products. Overall, the formation reaction for particles consists of hydrolysis of metal alkoxide and condensation of the hydrolysis species. The hydrolysis reaction of metal alkoxide... 

Hydrolysis of metal alkoxides is the basis for the sol-gel method of preparation of oxide materials therefore, reactions of metal alkoxides with water in various solvents, and primarily in alcohols, may be considered as their most important chemical properties. For many years the sol-gel method was mosdy associated with hydrolysis of Si(OR)4, discussed in numerous original papers and reviews [242, 1793,243]. Hydrolysis of M(OR) , in contrast to hydrolysis of Si(OR)4, is an extremely quick process therefore, the main concepts well developed for Si(OR)4 cannot be applied to hydrolysis of alcoholic derivatives of metals. Moreover, it proved impossible to apply classical kinetic approaches successfully used for the hydrolysis of Si(OR)4 to the study of the hydrolysis of metal alkoxides. A higher coordination number of metals in their alcoholic derivatives in comparison with Si(OR)4 leads to the high tendency to oligomerization of metal alkoxides in their solutions prior to hydrolysis step as well as to the continuation of this process of oligomerization and polymerization after first steps of substitution of alkoxide groups by hydroxides in the course of their reactions with water molecules. This results in extremely complicated oligomeric and polymeric structures of the metal alkoxides hydrolysis products. 

Sol Preparation. The synthesis of oxide sols via the sol-gel process involves the controlled hydrolysis of metal alkoxides and/or metal salts. However, the properties of the resulting sol, including the size of the sol particles, are determined by the interactions of several variables which must be carefully controlled. Such factors as the solvents used, the ratios of alkoxide or salt to water and/or alcohol, the amount of added acid or base, the reaction temperature, the stirring speed, and the rate at which reactants are added to the... 

The initial species used in sol-gel processing are metal alkoxides (M(OR)y). Hydrolysis of metal alkoxides involves nucleophilic reactions with water as follows ... 

Preparation of metal oxides by the sol—gel route proceeds through three basic steps (/) partial hydrolysis of metal alkoxides to form reactive monomers (2) the polycondensation of these monomers to form coUoid-like oligomers (sol formation) and (i) additional hydrolysis to promote polymerization and cross-linking leading to a three-dimensional matrix (gel formation). Although presented herein sequentially, these reactions occur simultaneously after the initial processing stage. 

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. 

One major problem in producing gels containing homogeneous mixtures of a variety of oxides is that the precursors may not all hydrolyze at the same rate. In particular, transition metal alkoxides hydrolyze much more rapidly than silicon alkoxides. The controlled hydrolysis of low-molecular-weight homometallic species described in the previous section can be adapted to prepare mixed alkoxides. For example, pre-hydrolysis of metal alkoxide followed by reaction with the silicon alkoxide gives a mixed dimeric species such as ... 

The normal way to obtain colloidal sols from oxide precursors is therefore a two-step process. In the first step, a precipitate of hydroxylated condensed species is formed from hydrolysed precursors. As described below, it can be seen that hydroxylated species capable of further condensation and precipitation in aqueous media can also be obtained from hydrolysis of metal alkoxides with excess water. In the second step this precipitate is transformed into a stable sol through a peptization reaction using basic or acid electrolytes. After adding appropriate organic binders, if requested, this sol can be directly used to form supported membranes. 

The fundamental reaction in the hydrolysis of metal alkoxides is a nucleophilic substitution (Sn ) that involves a nucleophilic addition of OH group onto the electrophilic metal center (M ). As a result, the coordination number (CN) of the metal atom is increased in the transition state, which leads to the migration of proton toward an alkoxy group, consequently knocking off the protonated ROH ligand (Figure 12). The chemical reactivity of metal alkoxides toward nucleophilic reactions mainly depends on the strength of the nucleophile, the electrophilic character of the metal atom, and its ability to increase the CN. 

Hydrolysis reactions of alkoxides proceed by nucleophilic attack of the metal [Eqs. (7)], and polymerization occurs via the reactive hydroxoalkoxides [Eqs. (7b) and (7c)]. Hydrolysis-polycondensation reactions are governed by numerous factors (Table 3). The metals involved in the composition of electrooptical ceramics are electropositive and oxophilic, and thus the hydrolysis of their ho-... 

The chemical synthesis of metal oxide nanocrystals based on hydrolysis falls into two major groups hydrolysis of metal alkoxides hydrolysis of metal halides, and other inorganic salts. Metal alkoxide compounds are defined as compounds that have metal-oxygen-carbon bonds. Si(OC2H5)4 (tetraethyl orthosUicate or TEOS), for instance, is an alkoxide compound. This class of compound is highly reactive with water. Because the hydroxyl ion (OH ) becomes bonded to the metal of the organic precursor, this reaction is called hydrolysis. Equation (50) shows a typical hydrolytic reaction of an alkoxide compound [100] ... 

Joint decomposition of titanium and barium alkoxides makes it possible to obtain high-purity powders of very fine BaTiOs [187, 188]. For this purpose, Ba and Ti alkoxides are merged at temperature between 50 and 100 °C to obtain mixed precursor. Then the mixture is hydrolyzed at room temperature by adding water with a controlled rate. In general, the flow diagram of the process is shown in Fig. 5.9 and the reaction occurring during hydrolysis of metal alkoxides can be presented [188] in the form of chemical equation ... 

The generation of silica in situ, in the polymer matrix, has been performed by using the sol-gel process. The typical aqueous sol-gel process consists of the hydrolysis of metal alkoxides M(OR)x followed by condensation reaction. Reactions involved to prepare silica through the sol-gel process are reported as follows ... 

Since most metal alkoxides containing lower aliphatic alkyl groups are actually coordinated complexes and not single molecules (Fig. 5.8), the reactions described by Eqs. (5.18) to (5.20) must be considered to be somewhat simplified. The rate of hydrolysis of metal alkoxides depends on the characteristics of the metal and those of the alkyl group. In general, silicon alkoxides are among the slowest to... 

A detailed mechanistic study of hydrolysis reactions was made by Bradley who had isolated a number of metal oxo-alkoxides and assigned plausible strucmres for such products. There has been a renewal of interest in the chemistry of metal oxo-alkoxides (Chapter 5), some of which have been isolated under anhydrous conditions also. Further, these hydrolysis reactions have assumed unprecedented technological significance in view of their importance in the preparation of ceramic materials by the sol-gel process (Chapter 7) which involves controlled hydrolysis of metal alkoxides (or some other precursors). 

The projects discussed in Section 20.2 have been centred around the spectroscopic probing of the structure and dynamics of surfactant systems and lyotropic mesophases, in particular hydrocarbon gels. Three related investigations not yet discussed in detail due to limitations in space shall be briefly mentioned. These are the study of the conformation of surfactant molecules by surface enhanced Raman spectroscopy, the development of a lipophilic dye probe for surfactant systems, and an outlook onto a different class of reaction gels created by hydrolysis of metal alkoxide precursors. 

Synthesis is typically described by two steps first hydrolysis of metal alkoxides to produce hydroxyl groups, followed by polycondensation of the hydroxyl groups and residual alkoxyl groups to form a three-dimensional network. These reactions are as follows ... 

While the sol-gel method offers a significant improvement over conventional solid state reactions, its utility for processing multimetallic oxides suffers from a major drawback. Rates of hydrolysis of metal alkoxides can differ by orders of magnitude (3-5). Differences in hydrolytic reactivities of different metal alkoxides often leads to preferential hydrolysis of one metal alkoxide over another, leading to segregation in the final ceramic material (6). Furthermore, sol-gel processing commonly involves the use of expensive, highly reactive metal alkoxides. Attempts to alter the hydrolytic reactivity of metal alkoxides by chemical modifications (e.g. 

This reaction is important in the manufacture of long-chain alcohols by means of hydrolysis of the aluminum alkoxide. Examples of oxidation of metal alkoxides (40,42) include ... 

The alcohol-based process involves reactions with metal alkoxides. The reactions are hydrolysis,... 

SCHEME 3.1 Hydrolysis and condensation reactions of metal alkoxide forming metal oxide. 

Either particulate sol or polymeric sol has been used for thin film coatings. The polymeric sol was fabricated by partial hydrolysis of corresponding metal alkoxide. If the rate of hydrolysis or condensation is very fast, then some kinds of organic acids, beta-dicarbonyls, and alkanolamines have been used as chelating agent in sol-gel processes to control the extent and direction of the hydrolysis-condensation reaction by forming a strong complex with alkoxide. [2]. 

The most common sol-gel process employs metal alkoxides of network forming elements (M(0R) where M is Si, B, Ti, Al, etc. and R is often an alkyl group) as monomeric precursors. In alcohol/water solutions the alkoxide groups are removed stepwise by hydrolysis reactions, generally employing acid or base catalysts, and are... 

The basic sol-gel reaction can be viewed as a two-step network-forming polymerization process. Initially a metal alkoxide (usually TEOS, Si(OCIl2CH )4) is hydrolyzed generating ethanol and several metal hydroxide species depending on the reaction conditions. These metal hydroxides then undergo a step-wise polycondensation forming a three-dimensional network in the process. The implication here is that the two reactions, hydrolysis and condensation, occur in succession this is not necessarily true (8.9). Depending on the type of catalyst and the experimental conditions used, these reactions typically occur simultaneously and in fact may show some reversibility. 

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