Metal alkoxides chemically controlled condensation

Because of coordination expansion, most metal alkoxides, other than silicon, are highly reactive toward hydrolysis and condensation. Precipitation occurs as soon as water is added. Therefore their chemical reactivity has to be decreased in order to avoid uncontrolled precipitation. This can be performed via the chemical modification of the molecular precursor prior to hydrolysis. Nucleophilic chemical additives are currently employed in order to stabilize highly reactive metal alkoxides and control the formation of condensed species. 

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 chemical reactivity of metal alkoxides strongly depends on their molecular structure. Oligomeric titanium alkoxides, in which Ti has a higher coordination number, are less reactive, allowing better control of hydrolysis and condensation reactions. Monodispersed TiOa powers are usually obtained from Ti(OEt)4 rather than Ti(OiPr)4 [9,10]. 

Most metal alkoxides are very reactive toward hydrolysis and condensation. They must be stabilized to avoid precipitation. The chemical control of these reactions is currently performed by adding complexing reagents that react with metal alkoxides at a molecular level, giving rise to new molecular precursors of different structure, reactivity, and functionality. Chemical modification is usually performed with hydroxylated nucleophilic ligands, such as carboxylic acids or P-diketones. In most cases complexation by XOH species can be described as a nucleophilic substimtion, as follows ... 

Titania membranes show excellent chemical resistance, and can be used in both acidic and basic pH and moreover, they show interesting photocatalytic activity. Titania UF membranes have been commercialized by several companies. At present, extensive efforts have focused on the preparation of porous membranes having small pore sizes in the NF range (1-2 nm). Since pore sizes are believed to be controlled by the sizes of the packed particles, controlling the sol size is a crucial process for membrane processing. Anderson et al. [23] prepared nanosized TiOi and Zr02 particles (3-5 nm) by carrying out hydrolysis and condensation reactions of metal tcrt-amyloxide with a small amount of water (molar ratio of H20/Ti = 3—5) in tcrt-amyl alcohol solutions. Hydrolysis and condensation reactions of metal alkoxides are written as follows ... 

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] ... 

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