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

Zirconia-sulfate (ZrO2-SO42-) aerogels were prepared in a onestep synt hesis by the sol-gel method followed by supercritical drying. Sulfuric acid was mixed with zirconium n-propoxide in n-propanol and reacted w ith water and nitric acid to form a zirconia-sulfate cogel. Supercriti cal drying with carbon dioxide removed the alcohol solvent forming a h igh surface area aerogel. This preparation method combined the two ste ps 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 isomeriza tion, diffuse reflectance infrared spectroscopy, pyridine adsorption, and thermogravimetry. Sulfate ions, in an ionic, inactive low temperat ure state, were initially trapped in the bulk of the aerogel. Coincide nt with crystallization of the zirconia support, sulfate was expelled onto the surface and transformed into an ''activated'' covalent sulfat e species which helped to form strong Bronsted acidity. A minimum dens ity of sulfate groups was required to create this Bronsted acidity whi ch was necessary for n-butane isomerization. Increasing sulfate conten t retarded the support crystallization and increased the activation te mperature required to generate maximum catalytic activity. This aeroge l synthesis permitted unique preparative flexibility allowing for sulf ate to be introduced into the bulk and for crystallization to be retar ded to high temperatures, thus establishing the relationship between c rystallinity, sulfate structure and content, and the acidity and activ ity of the catalyst. This work demonstrated the importance of the prep arative parameters on the formation of a solid superacid. (C) 1994 Aca demic Press, Inc.