| Materials synthesis was used as a tool to study issues in materials preparation, characterization, and catalysis. Specifically, sol-gel chemistry and the formation of aerogels by supercritical drying was used to study the preparation and characteristics of zirconium oxide (ZrO{dollar}sb2{dollar}, zirconia) and its use as a catalytic support. A careful study of the formation of zirconia aerogels was undertaken to understand how sol-gel chemistry can be used to control the properties of the aerogel. The time of gel formation, which was controlled by the sol-gel parameters, determined the type of polymeric oxide network formed, which in turn dictated the macroscopic (texture, crystal structure) and microscopic (defect density) properties of the aerogel. These variable properties were used to examine the mechanism of the tetragonal-to-monoclinic phase transformation. Another of the sol-gel parameters studied was the choice of precursor. Pre-formed zirconia sols were used to study the effect of starting sol size on properties of the dried material. Comparison of the aerogel, sol-derived, and commercially available zirconia demonstrated the importance and usefulness of synthesis for preparing a material with desired properties.; This knowledge of zirconia synthesis was applied to the study of sulfate-promoted zirconia, a solid superacid. The flexibility of the aerogel synthesis allowed for sulfate introduction into the zirconia synthesis at three different points. A one-step synthesis of zirconia-sulfate aerogels allowed for sulfate to be included in the bulk. In addition, sulfate was impregnated on the aerogel after vacuum drying at 383 K and on the calcined aerogel after 773 K heat treatment. These three methods produced sulfated zirconia with different textural properties and activation profiles. From the one-step synthesis, a relationship was determined among crystallinity, sulfate structure and content, and the acidity and activity of the material. From impregnation of the calcined aerogel, the ability to form superacidity on a crystalline zirconia support was demonstrated. By impregnating a series of calcined zirconia supports prepared by different methods, the characteristics required for the generation of solid superacidity was determined. The hydroxyl content was the determining factor for superacidity generation. |
