Nonsilica Inorganic Oxide Gels

Titania gels and aerogels are promising catalysts owing to their high chemical stability [64-67] and intrinsic catalytic activity for a number of reactions, such as phenol amination [68], olefin epoxidation [69], cumene dealkylation [70], 2-propanol dehydration [69], and the decomposition of 1,2-dichloroethane [71]. More 

Epoxide-Initiated Metal Oxide Gels and Composites 

The organic epoxide reagent (ethylene oxide, propylene oxide, and epichloro-hydrin) serves as proton scavenger and is stoichiometrically consumed in this sol-gel reaction. Given the higher acidity of water ligands of solvated metal cations (salts) in solution, they tend to spontaneously form hydroxide ligands (see Eq. (17.5)). The proton that is released can protonate the epoxide  

Nucleophilic attack by the conjugate base A (the counterion of the metal salt) on the protonated epoxide causes irreversible ring opening and the proton is irreversibly trapped in the substituted alcohol side product, as shown in Eq. (17.6)  

The resulting metal hydroxide species can then condensate with other metal cations, forming M—O—M bonds. In analogy to other metal oxide gels such as 

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The key property required of the inorganic species is ability to build up (polymerize) around the template molecules into a stable framework. As is already evident in this article, the most commonly used inorganic species are silicate ions, which yield a silica framework. The silica can be doped with a wide variety of other elements (heteroatoms), which are able to occupy positions within the framework. For example, addition of an aluminium source to the synthesis gel provides aluminosilicate ions and ultimately an aluminosilicate mesoporous molecular sieve. Other nonsilica metal oxides can also be used to construct stable mesoporous materials. These include alumina, zirconia, and titania. Metal oxide mesophases, of varying stability, have also been obtained from metals such as antimony (Sb), iron (Fe), zinc (Zn), lead (Pb), tungsten (W), molybdenum (M), niobium (Nb), tantalum (Ta), and manganese (Mn). The thermal stability, after template removal, and structural ordering of these mesostructured metal oxides, is far lower, however, than that of mesoporous silica. Other compositions that are possible include mesostructured metal sulfides (though these are unstable to template removal) and mesoporous metals (e.g., platinum, Pt). 

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