ONE-STEP SYNTHESIS AND CHARACTERIZATION OF ZIRCONIA-SULFATE AEROGELS AS SOLID SUPERACIDS

Zirconia-sulfate (ZrO2-SO42-) aerogels were prepared in a onestep synthesis by the sol-gel method followed by supercritical drying. Sulfuric acid was mixed with zirconium n-propoxide in n-propanol and reacted with water and nitric acid to form a zirconia-sulfate cogel. Supercritical drying with carbon dioxide removed the alcohol solvent forming a high surface area aerogel. This preparation method combined the two steps of zirconia support formation and sulfate promotion. The effect of changing sulfate content and activation temperature was systematically studied by nitrogen adsorption, X-ray diffraction, n-butane isomerization, diffuse reflectance infrared spectroscopy, pyridine adsorption, and thermogravimetry. Sulfate ions, in an ionic, inactive low temperature state, were initially trapped in the bulk of the aerogel. Coincident with crystallization of the zirconia support, sulfate was expelled onto the surface and transformed into an ''activated'' covalent sulfate species which helped to form strong Bronsted acidity. A minimum density of sulfate groups was required to create this Bronsted acidity which was necessary for n-butane isomerization. Increasing sulfate content retarded the support crystallization and increased the activation temperature required to generate maximum catalytic activity. This aerogel synthesis permitted unique preparative flexibility allowing for sulfate to be introduced into the bulk and for crystallization to be retarded to high temperatures, thus establishing the relationship between crystallinity, sulfate structure and content, and the acidity and activity of the catalyst. This work demonstrated the importance of the preparative parameters on the formation of a solid superacid. (C) 1994 Academic Press, Inc.