Metal alkoxide precursors alcohols

A quite different approach from that of colloidal sols in the preparation of sol-gel derived membranes utilizes polymeric sols. In this category of sols the dispersed phase results from the hydrolysis and condensation of metal organic precursors in organic media. In most cases this process deals with the polymerization of metal alkoxides in alcohol according to the following reactions ... 

Tetramethoxysilane, Si(OCH3)4, abbreviated TMOS, and tetraethoxysi-lane, Si(OCH2CH3)4, abbreviated TEOS, are the most commonly used metal alkoxide precursors in sol-gel processing of silicates (24). Silicate gels are most often synthesized by hydrolyzing the alkoxides dissolved in their parent alcohols with a mineral acid or base catalyst. At the functional group level, three bimolecular nucleophilic reactions are generally used to describe the sol-gel process (24) ... 

A number of metal alkoxides are commercially available in high purity form. To make metal oxide powders from these organometallic precursors we start with a solution (a sol ) of the metal alkoxide in alcohol. (The alcohol is usually the same one that was used for alkoxide formation.) Water is added to the alcohol solution. Two reactions then occur, which, using aluminum isopropoxide as an example, may be written as follows ... 

It should be mentioned that the reaction of metal chlorides with alcohols could not be applied for the synthesis of metal alkoxides—precursors of oxide materials. Its products are usually quite complex mixtures of alkoxide chlorides and alcohol solvates of metal oxochlorides (Turova, 2002 Turevskaya, 1989). 

Chemical modification of transition metal alkoxides with alcohols, chlorides, acids or bases, chelating ligands, etc. is commonly employed to retard the hydrolysis and condensation reaction rates in order to control the condensation pathway of the evolving polymer [8,113]. In most cases, the modification occurs by an S v reaction between a nucleophilic reagent (XOH) and the metal alkoxide to produce a new molecular precursor [113] ... 

Alcohol exchange reactions occur readily with metal alkoxide precursors ... 

These condensation reactions work not only for silica alkoxides, but also for most of the metal alkoxide precursors. The photolysis of alcohols leads to bond... 

Sol-gel primers use inorganic or metal-organic precursors (generally aluminum, silicon or titanium alkoxides) whose chemistry is closely related to the silane coupling agents discussed previously. These precursors are dissolved in alcohol, then hydrolyzed by the addition of water ... 

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... 

Application of metal salts and well-defined metal complexes in ROP has enabled the exploitation of a three-step coordination-insertion mechanism, first formulated in 1971 by Dittrich and Schulz [17]. This proceeds through coordination of lactide by the carbonyl oxygen to the Lewis acidic metal center, leading to the initiation and subsequent propagation by a metal alkoxide species. This species can be either isolated or generated in situ by addition of an alcohol to a suitable metal precursor to result in the formation of a new chain-extended metal alkoxide, as shown in Scheme 3 [16]. 

Sol-gel processing can be used to prepare a variety of materials, including glass, powders, films, fibers, and monoliths. Traditionally, the sol-gel process generally involves hydrolysis and condensation of metal alkoxides. Metal alkox-ides are good precursors because they readily undergo hydrolysis that is, the hydrolysis step replaces an alkoxide with a hydroxide group from water and a free alcohol is formed. Once hydrolysis has occurred, the sol can react further... 

The electronegative metals usually form unstable alkoxides that tend to polymerize rapidly to form [-M(OR)2-0-] . Alkoxides are easily solubilized in alcohols. Alkoxide precursors must be kept fme of water to avoid hydrolysis. Hydrolysis is the first step in the reaction of alkoxides to form gel networks. This is difficult because alkoxide solutions easily absorb water finm the atmosphere. 

The reaction of metal chloride with alcohol is not the only way to synthesize alkoxides. Other inorganic compounds such as oxides can also be used as precursors for the synthesis of metal alkoxides ... 

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. 

From many possible precursors [e.g., metal nitrates or acetates (12), mono-dispersed metal hydrous oxides (549), oxides dissolved in alcohols (12), oxo-alkoxides (550), and alkoxides (12)], metal alkoxides were considered as specially suitable precursors (12, 31, 551) for the preparation of oxide ceramics since the 1950s, mainly due to the ease of their purification. This purification was generally distillation and in some cases crystallization, for example, Zr(0-i-Pr)4(i-Pr-OH), solubility in organic solvents such as the parent alcohols that are miscible with water, and their extremely facile hydrolyzability, which can be modulated effectively by substitution of some of the alkoxide groups with chelating ligands such as (3-diketonates (35, 536). The extraordinary future potential and possibilities of the SG process were reemphasized recently by Sakka (534) and Roy (552) in two review articles. 

Series of soluble bi- and tri-metallic species such as M [M(OR)6]n have been reported from reactions between the metal alkoxides or halides in alcohol solutions (e.g., see 284, 285 and refs, therein Section 4.5.2.4.4 below). They are of interest as precursors for sol-gel procedures. A number of oxo clusters have been described, apparently products of hydrolysis or oxidation. Pbn6Nb404(0Et)24 features an octahedral cluster [Pb6(//3-0)4(p3-0Et)4] whose faces are capped by the bridging ligands. The oxo ligands are also coordinated to Nb(OEt)5 groups.286... 

Well ordered mesoporous silicate films were prepared in supercritical carbon dioxide.[218] In the synthesis in aqueous or alcoholic solution, film morphology of preorganized surfactants on substrate cannot be fully prescribed before silica-framework formation, because structure evolution is coincident with precursor condensation. The rapid and efficient preparation of mesostructured metal oxides by the in situ condensation of metal oxides within preformed nonionic surfactants can be done in supercritical CCU- The synthesis procedure is as follows. A copolymer template is prepared by spin-coating from a solution containing a suitable acid catalyst. Upon drying and annealing to induce microphase separation and enhance order, the acid partitions into the hydrophilic domain of the template. The template is then exposed to a solution of metal alkoxide in humidified supercritical C02. The precursor diffuses into the template and condenses selectively within the acidic hydrophilic domain of the copolymer to form the incipient metal oxide network. The templates did not go into the C02 phase because their solubility is very low. The alcohol by-product of alkoxide condensation is extracted rapidly from the film into the C02 phase, which promotes rapid and extensive network condensation. Because the template and the metal oxide network form in discrete steps, it is possible to pattern the template via lithography or to orient the copolymer domains before the formation of the metal oxide network. 

Organic molecules can be readily mixed with M(0R)4 alkoxide precursors. Upon precursor hydrolysis, dye molecules, such as coumarines, rhodamines, pyranines, or others with nonlinear optical properties have been entrapped into silica, aluminosilica, or transition metal oxide matrices. Alkoxides can also be mixed with polymers dissolved in alcohols or... 

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