Metal alkoxides hydrolysis

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. 

The main principles of metal alkoxide hydrolysis were formulated by Livage in his reviews [1020]. On the addition of water or a water-alcohol mixture to the solutions of alkoxides in organic solvents, nucleophillic attack of H20 molecule at the positively charged metal atom occurs (a), which leads to an intermediate state (b) ... 

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] ... 

Sol-gel methods have been used to produce a coating of additives on ferrite particles prior to sintering, in an attempt to achieve a good dispersion (Brooks Amarakoon, 1991). The coating was a boro-silicate obtained from metal alcoholates or metal alkoxides. Hydrolysis was catalysed by an acid (HF) to obtain a polymeric chain and... 

FIGURE 8.1 Basic steps of a typical sol-gel process from a metal alkoxide. Hydrolysis of aUcylorthosilicates affords silicon hydroxides that undergo condensation reactions to form a silicate network (reproduced from [76] with permission from Wiley). 

For most metal alkoxides, hydrolysis with excess water yields insoluble oxide or hydroxide precipitates that are useless for further polymerization reactions [Eqs. (5.21)-(5.23)]. However, if small amounts of water are added slowly to a sufficiently dilute solution, it is possible to form polymerizable molecular species from these alkoxides also. For example, when a dilute solution of boron alkoxide in alcohol is exposed to water, soluble transient molecular species such as B(0R)2(0H) and B(OR)(OH)2, representing various degrees of hydrolysis, form initially, e.g.. 

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. 

The most outstanding property of the metal alkoxides is ease of hydrolysis. 

Partial hydrolysis of a metal alkoxide to form reactive monomers. 

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 health ha2ard presented by metal alkoxides reflects the toxicity of the metals they contain and the metallic hydroxides and alcohols they form on hydrolysis. 

The ease of hydrolysis of metal alkoxides makes metal analysis a comparatively simple task. In many cases, the metal may be estimated by hydrolysis of a sample in a cmcible, and ignition to the metal oxide. Alternatively, the metal ion may be brought into solution by hydrolysis of a sample with dilute acid, followed by a standard analytical procedure for a solution of that particular metal. If the alcohol Hberated during the hydrolysis is likely to cause interference, it may be distilled from the solution by boiling. 

Besides direct hydrolysis, heterometaHic oxoalkoxides may be produced by ester elimination from a mixture of a metal alkoxide and the acetate of another metal. In addition to their use in the preparation of ceramic materials, bimetallic oxoalkoxides having the general formula (RO) MOM OM(OR) where M is Ti or Al, is a bivalent metal (such as Mn, Co, Ni, and Zn), is 3 or 4, and R is Pr or Bu, are being evaluated as catalysts for polymerization of heterocychc monomers, such as lactones, oxiranes, and epoxides. An excellent review of metal oxoalkoxides has been pubUshed (571). 

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

Sol-gel chemistry offers a unique advantage in the creation of novel organic-inorganic hybrids. The sol-gel process begins with a solution of metal alkoxide precursors [M(0/f) ] and water, where M is a network-forming element, and R is typically an alkyl group. Hydrolysis and... 

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... 

Further examination has shown that the acid content should be small in order for the solution to become spinnable in the course of hydrolysis and polycondensation. It has been found (4 ) that very large concentrations at more than 0.15 in the [HCl]/[Metal alkoxide] ratio of acid catalyst produce round-shaped particles in the tetra-ethoxysilane (7) and tetramethoxysilane solutions, and so no spinnability appears. 

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. 

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... 

Hirano S. I. Kato, K. 1988. Formation of LiNb03 films by hydrolysis of metal alkoxides./. Non-Cryst. Sol. 100 538-541. 

Stober A process for making metal oxides in the form of small spheres of uniform diameter by the controlled hydrolysis of metal alkoxides. First used in 1968 to make silica spheres from alkyl silicates. The products can be used to make high quality oxide ceramics. See also Sol-Gel. 

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