Alkoxides hydrolysis

The effects of both pH and temperature of aluminum alkoxide hydrolysis on gelation is shown in Eigure 8. Addition of acid into the mixture hydrolyzed at 90°C, and by consequence reduction of pH, reduces the gelation time of the samples, whereas in mixtures hydrolyzed at room temperature, acidic addition increases gelation time. 

This chapter focuses on silica synthesis via the microemulsion-mediated alkoxide sol-gel process. The discussion begins with a brief introduction to the general principles underlying microemulsion-mediated silica synthesis. This is followed by a consideration of the main microemulsion characteristics believed to control particle formation. Included here is the influence of reactants and reaction products on the stability of the single-phase water-in-oil microemulsion region. This is an important issue since microemulsion-mediated synthesis relies on the availability of surfactant/ oil/water formulations that give stable microemulsions. Next is presented a survey of the available experimental results, with emphasis on synthesis protocols and particle characteristics. The kinetics of alkoxide hydrolysis in the microemulsion environment is then examined and its relationship to silica-particle formation mechanisms is discussed. Finally, some brief comments are offered concerning future directions of the microemulsion-based alkoxide sol-gel process for silica. 

These reactions are responsive to both acid and base catalysis, and can be manipulated to give a variety of silica products, e.g., discrete particles, monolithic gels, films, and fibers. This technique of materials synthesis via alkoxide hydrolysis has become known as sol-gel processing (17). It should be noted, however, that under certain conditions, gelation may be confined only to the interior of discrete particles (base-catalyzed systems), while the sol may consist of polymeric networks rather than individual particles (acid-catalyzed systems). 

Fig. 2.2.1 Silica synthesis by alkoxide hydrolysis in microcmulsion water pools.

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 ... 

Ba,La-hexaaluminate Aerogel formation alkoxides hydrolysis in alcoholic medium and supercritical drying. Alkoxides 1300-1400 Ba-P-Al203 or MP extraphases always detected, Sa=10-15m2/g Catalytic combustion 18... 

Titanium alkoxide hydrolysis, however, is more complex. Titanium differs from aluminum in that it does not have multiple forms of oxides [3]. The variability of oxide content in the polymerized alkoxide is described by the following equation ... 

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) ... 

With these two esters, the choice of base is important nucleophilic addition can occur at the ester carbonyl, which could lead to transesterification (with alkoxides), hydrolysis (with hydroxide), or amide formation (with amide anions). The best choice is usually an alkoxide Identical with the alkoxide component of the ester (that is, ethoxide for diethyl malonate methoxide for dimethyl malonate). Alkoxides (pKd 16) are basic enough to deprotonate between two carbonyl groups but, should substitution occur at C=0, there is no overall reaction. 

The most common method for preparing BaTiOj powder is the conventional solid state reaction [3], alternative methods have been developed [12], such as alkoxide hydrolysis [4], hydrothermal synthesis [5], colloid method [6], and so on. 

Bruneton, E., and Colomban, P, Influence of hydrolysis conditions on crystallization, phase transitions and sintering of zirconias prepared by alkoxide hydrolysis, J. Non-Crystl. Solids, 147, 201, 1992. 

Vendange, V., and Colomban, P., Determination of the hydroxyl gronp content in gels and porons glasses issued of alkoxide hydrolysis hy combined TGA and BET analysis, J. Porous Mater., 3, 193, 1996. 

Colomban, P, and Mazerolles, L., Nanocomposites in mullite-Zr02 and mullite-Ti02 systems synthesized through alkoxide hydrolysis gel routes. Microstructure and fractography, J. Mater. Sci., 26, 3503, 1991. 

Alumina is by far the most used inaterial for washcoating monolithic catalysts. In addition to alkoxide hydrolysis menti(j)ned above, Al-sol can be prepared from other... 

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. 

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