Aluminum 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. 

Hydrolysis. Aluminum alkoxides are hydrolysed using either water or sulfuric acid, usually at around 100°C. In addition to the alcohol product, neutral hydrolysis gives high quaUty alumina (see Aluminum compounds) the sulfuric acid hydrolysis yields alum. The cmde alcohols are washed and then fractionated. 

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 ... 

Hydrolysis of aluminum alkoxides is also used commercially to produce precursor gels. This approach avoids the introduction of undesirable anions or cations so that the need for extensive washing is reduced. Although gels having surface area over 800 m /g can be produced by this approach, the commercial products are mosdy pseudoboehmite powders in the 200 —300 m /g range (28). The forming processes already described are used to convert these powders into activated alumina shapes. 

Tlie amphoteric behavior of aluminum hydroxide, wliich dissolves readily in strong acids and bases, is shown in Figure 4. In the pH range of 4 to 9, a small change in pH towards the neutral value causes rapid and voluminous precipitation of colloidal hydroxide wliich readily fomis a gel. Gels are also fomied by the hydrolysis of organoaluminum compounds such as aluminum alkoxides (see Alkoxides, metal). 

In preparing the membrane, a clear sol was obtained by the addition of acid into the aluminum sec-butoxide sol to peptise the sol and obtain a stable colloid solution. Aluminum monohydroxides formed by the hydrolysis of aluminum alkoxides, which are peptisable to a clear sol. Peptisation was performed by the addition of acid and heat treatment for a sufficient time. It was found that stable sols cannot be obtained when the concentration of the peptisation acid is too low. The critical range for inorganic acids such as nitric, hydrochloric and perchloric acids is 0.03-0.1 mole/mole of hydroxide. In this study, nitric acid was used as the peptising agent. The resulting sols are poured into Petri dishes and dried in an oven at a controlled drying rate to obtain a gel layer. The molar ratio of zirconia salt... 

The products are liberated by hydrolysis of the aluminum alkoxide at the end of the reaction. Lithium aluminum hydride reduction of esters to alcohols involves an elimination step in addition to hydride transfers. 

FIGURE 1.2. Formation of nanoparticles of metal oxide by reverse micelle method. A solution of inverse micelles is first formed by adding a long-chain alkylamine to a toluene solution. A small amount of water is trapped in the reverse micelle core. Mixing the reverse micelle solution with an aluminum alkoxy amine adduct results in hydrolysis of the aluminum alkoxide adduct and formation of nano-sized particles of aluminum oxyhydroxide after drying. These particles are shown in the SEM picture above. 

A variation of this method was used to control the surface properties of aluminum oxide particles, particularly the surface density of Lewis acid sites. Instead of using a long-chain amine surfactant, the solution of aluminum alkoxide precursor was mixed with a small amine to convert the alkoxide dimer (or oligomer in general) into monomeric alkoxide-amine adduct. Controlled hydrolysis of this adduct produces an aluminum oxyhydroxide in which the surface A1 ions are coordinated to amine... 

The procedure described in this experiment exemplifies a general method [225] for the reduction of propargylic alcohols to -allylic alcohols. The first step in the reaction is the formation of the aluminium alkoxide -C=C-C-OAlH3. Subsequently one of the three hydrogen atoms attached to aluminum is transferred to the triple bond with formation of a 5-membered cyclic aluminum compound. Hydrolysis affords the -allylic alcohol. In the present case an -enyne alcohol is formed. 

Instead of the displacement reaction, aluminum alkyls may be oxidized with dry air to produce aluminum alkoxides that, after hydrolysis with dilute sulfuric acid or water, yield linear primary alcohols with an even number of carbon atoms (fatty... 

Reduction then proceeds by successive transfers of hydride ion, H e, from aluminum to carbon. The first such transfer reduces the acid salt to the oxidation level of the aldehyde reduction does not stop at this point, however, but continues rapidly to the alcohol. Insufficient information is available to permit very specific structures to be written for the intermediates in the lithium aluminum hydride reduction of carboxylic acids. However, the product is a complex aluminum alkoxide, from which the alcohol is freed by hydrolysis ... 

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 ... 

First investigations of hydrolysis of aluminum alkoxides were performed in the 1950s and 1960s [1252, 285]. Stolarek [1536] studied hydrolysis reactions with the aim of the preparation of aluminum oxide as a carrier for catalysts. 

Aluminum oxide sols are successfully used for application of films [1794, 1793], such as high area films for hybrid circuits [1236]. The same technique was used for the preparation of J-A1203 (NaAlnO,7) in this case alcoholic solutions of NaOR were added to aluminum alkoxide solutions [1795, 1038]. To increase the stability of sols, aluminum alkoxides are frequently modified with acetic acid [1570] or (3-diketones [1659]. Alumina sheets are commercially produced by the sintering of high-purity fine A1203 powders prepared by hydrolysis of modified Al(OPr )3 [1236]. To prepare powders with spherical species from dilute solutions, hydroxopropylcellulose was added to prevent... 

For living ROP with the ability to control molecular architecture and weight, aluminum alkoxides can be used, the propagation being characterized by the almost total absence of side-reactions. The reaction is usually performed in solution at low temperatures. The sensitivity towards hydrolysis is however a limitation. 

The final step is hydrolysis of aluminum alkoxides to free the product alcohols. 

Production of alumina aquosols from alumina has been well researched in sol-gel science [19]. Yoldas [20] was the first one to show that monolithic alumina gels could be formed by hydrolysis and condensation of aluminum alkoxide. As discussed in Chapter 5, formation of aquosols and their gel is an intermediate step in the formation of chemically bonded phosphate ceramics. Condensation of the hydrated alumina sols by reaction with phosphoric acid to form A1(H2P04)3-H20 gel is the first step toward synthesis of a berlinite-bonded alumina ceramic. When this gel is heated to 150°C, this gel reacts with additional alumina and releases water, and crystalline berlinite is produced. This Chapter... 

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