Alkoxides hydrolysis/condensation

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 structures of sol-gel-derived inorganic polymers evolve continually as products of successive hydrolysis, condensation and restructuring (reverse of Equations 1-3) reactions. Therefore, to understand structural evolution in detail, we must understand the physical and chemical mechanisms which control the sequence and pattern of these reactions during gelation, drying, and consolidation. Although it is known that gel structure is affected by many factors including catalytic conditions, solvent composition and water to alkoxide ratio (13-141, we will show that many of the observed trends can be explained on the basis of the stability of the M-O-M condensation product in its synthesis environment. 

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... 

In principle, silica growth kinetics may be controlled by (1) slow release of monomer via alkoxide hydrolysis in the particle-free reverse micelles, (2) slow surface reaction of monomer addition to the growing particle, and (3) slow transport processes as determined by the dynamics of intermicellar mass transfer. There is strong experimental evidence to support the view that the rate of silica growth in the microemulsion environment is controlled by the rate of hydrolysis of TEOS (23,24,29). Silica growth kinetics can be analyzed in terms of the overall hydrolysis and condensation reactions ... 

Solgel Route Based on the Hydrolysis-Condensation of Metal Alkoxides... 

Another route for the production of materials involves the reaction of hydrolysis-condensation of metal alkoxides with water. We study here the important case of amorphous silica synthesis. In this case [38,39,44], silicic acid is first produced by the hydrolysis of a silicon alkoxide, formally a silicic acid ether. The silicic acids consequently formed can either undergo self-condensation, or condensation with the alkoxide. The global reaction continues as a condensation polymerization to form high molecular weight polysilicates. These polysilicates then connect together to form a network, whose pores are filled with solvent molecules, that is, a gel is formed [45],... 

Figure 3 Synthesis of mesoporous metal oxides (a) hexagonal assemblies of micellar systems are used as templates for the hydrolysis/condensation of M(OR)2 alkoxides (b) after calcination, the corresponding oxide exhibits an hexagonal array of mesopores...

The most widely used metal alkoxides are Si(OR)4 compounds, such as tetra-methoxysilane (TMOS) and tetraethoxysilane (TEOS). However, other alkoxides of Al, Ti, and B are also commonly used in the sol-gel process, often mixed with TEOS. For instance, aluminum silicates may be generated through hydrolysis/condensation of siloxides (Eq. 17), which proceed through an intermediate Al-O-Al network known as alumoxanes. Alumoxanes are important for applications in antiperspirants,... 

All samples were prepared by sol-gel synthesis in alkali-free medium via a polymeric gel route involving alkoxide hydrolysis and condensation, catalysed by base or acid. Similar reactant mixtures were used in all preparations and only the type of gelling agent was changed. 

Sols of Ti, Zr or Al with polymeric structure and defined fractal dimensions has not yet been reported in literature. Studies of hydrolysis-condensation behaviour of mixtures of Si and Ti alkoxides has been reported by de Lange [42,44,46] and Kamiyama et al. [50]. 

All initic zed membranes were pre-swollen (equilibrated) in stirred 2 1 (vol/vol) alcohol/water solutions for 5 hours at 26 C. The alcohols utilized were methanol, ethanol and 1- and 2-propanol. The pre-sorbed water serves to initiate the ux situ sol-gel reaction upon the subsequent uptake of TEOS which is introduced in amounts such that H,O TEOS = 4 1 (mole/mole). While this ratio theoretically provides for complete alkoxide hydrolysis, there is naturally additional water generated during the ensuing network condensation to support hydrolysis. Solutions consisting of TEOS/alcohol =... 

This chapter is a brief overview of silicon alkoxide hydrolysis and condensation and the resulting structures, emphasizing recent studies and unpublished work. Schmidt et al. (I) and more recently Brinker (2) have published excellent overviews of this chemistry this chapter attempts to amplify and complement these reports. 

An enormous number of possible multicomponent systems, makes it impossible to di.scuss them here in detail. However it should be mentioned that there are two general approaches in the sol-gel synthesis of multicomponent systems hydrolysis of mixed-alkoxide or metal organic precursors, and sequential addition of alkoxides to partially hydroly/.ed precursors (I9j. The first method was invented by Dislich [20], based on the idea to form complex via alcolation that contains all metals in proper stoichiometry. The second approach which is based on the sequential addition of alkoxides in the reverse order of their respective reactivities, was introduced by Thomas [18] and Yoldas [21,22]. The idea is that the newly added unhyhdrolyzed alkoxide will condense with partially hydrolyzed sites on the polymeric species fonned by the preceding hydrolysis and condensation, rather than reacting with themselves. The homogeneity of the product will depend on the size of the polymeric species to which the last component is added. T)ie most widely studied multicomponent systems are AI O,-SiO.. B.O,-SiO and TiO-SiO. ... 

Transition metal alkoxides are much more reactive toward hydrolysis and condensation than silicon alkoxides. This arises mainly from the larger size and lower electronegativity of transition metal elements. Coordination expansion becomes a key parameter that controls the molecular structure and chemical reactivity of these alkoxides. Hydrolysis and condensation rates of silicon alkoxides must be increased by acid or base catalysis, whereas they must be carefully controlled for the other metal alkoxides. The chemical modification of transition metal alkoxides leads to the development of a new molecular engineering. The chemical design of these new precursors allows the sol-gel synthesis of shaped materials in the form of fine powders, fibers, or films. 

The early stages of the hydrolysis-condensation of [M(OR)n]m can be characterized by x-ray diffraction for oxophilic metals (Table 4). These include centrosymmetric polynuclear oxoalkoxides of titanium, zirconium, and niobium. Their stmcture is often related to that of polyoxoanions TiyOalOEOao is isostructural with Mo7024, and Nb80io(OEt)20 has a cagelike structure that compares well with that of the paratungstate H2Wi2042 b-. These oxoalkoxides are soluble as a result of their closo structure, and NMR has been developed as a technique to follow the hydrolysis of titanium(IV) alkoxides with... 

Leaustic, A., Babonneau, F., and Livage, J., Structural investigation of the hydrolysis-condensation of titanium alkoxides Ti (OR)4 (R = iPr, Et) modified by acetylacetone 1. Study of the alkoxide modification, Chem. Mater., 1, 240 (1989). 

The preparation of nanocomposite membranes by intra-membrane growth within a proton exchange membrane was first described by Mauritz et al. [45-47]. The then novelty of this approach and the breadth and depth of these studies warrant the following discussion of the results, which in many ways laid the foundation for future work in this area. This group made use of the hydrophilic ionic cluster regions of Nafion for confined, sulfonic acid group catalysed, hydrolysis/condensation reactions of impregnated alkoxides. Nafion membranes were first swollen in ethanol/water, then tetraethoxy-silane (or aluminium, titanium and zirconium alkoxides) permeated from one side of the membrane. In addition to the concentration profile of in-... 

Although hydrolysis/condensation is the normal route leading to metal oxo-alkoxides, ether cleavage (Equation (21)) is another possibility for the formation of such species. 

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