CHIMIE DOUCE APPROACHES TO THE SYNTHESIS OF METASTABLE OXIDE MATERIALS

While thermodynamically stable metal oxides and other solids could be prepared by conventional high-temperature (ceramic) methods, syntheses of the more important metastable oxide materials of current interest, such as the zeolites and other microporous solids as well as layered and framework oxides of various kinds require mild, chemistry-based routes that occur at relatively low temperatures. In this review, we present a brief survey of the emerging chimie douce (soft-chemical) routes which enable synthesis of a wide variety of metastable oxide materials. A distinction is made between topochemical and onomatopoetically routes. Ion exchange, intercalation, redox insertion/extraction, and pillaring are examples of topochemical routes; syntheses using sol-gel, hydrothermal conditions, molten salt media, and acid leaching of appropriate precursor oxides belong to the nontopochemical prefabricated by a high-temperature route, which is then transformed into a metastable phase by an appropriate soft-chemical route. Several examples of synthesis of metastable oxides, drawn from the recent literature as well as from the author's work, are discussed.