SYNTHESIS, STABILITY, AND CATALYTIC PROPERTIES OF HIGH SURFACE-AREA SILICON OXYNITRIDE AND SILICON-CARBIDE

The aim of this work is to extend the range of inorganic materials which find application in heterogeneous catalysis, as catalysts or catalyst supports, through use of progress in the field of advanced ceramics. Developments in synthetic routes to high surface area non-oxides (nitrides, carbides, borides) are briefly reviewed. Work on the synthesis, stability and catalytic properties of high surface area silicon oxynitride and silicon carbide is described. For the former compound, three synthetic routes were investigated: the reaction of an amorphous silica with ammonia proved to be the preferred route, giving a silicon oxynitride of ca. 150 m2 g-1. The surface composition of the material obtained by the latter method showed good stability, as indicated by retention of surface nitrogen under reducing or oxidising conditions at about 700-degrees-C. The oxynitride surface was unreactive towards butene-1, causing no double-bond isomerisation at 400-degrees-C. However, the material was also found to be capable of functioning as a solid basic catalyst, catalysing Knoevenagel condensations in solution at 50-degrees-C. The use of nitrides as solid basic catalysts appears to be novel, and this result suggests new applications for such materials. In the case of silicon carbide, a mesoporous form was obtained by polymer pyrolysis. The texture shows improved thermal (but not hydrothermal) stability compared to silica. The surface is less reactive than that of the oxynitride, catalysing neither the double-bond isomerisation of butene-1 nor the Knoevenagel condensation. This compound, and the oxynitride, may be useful as inert support materials for reactions involving alkenes.