Properties of modified zirconia used as Friedel-Crafts-acylation catalysts

Both precipitated and aerogel synthesized zirconia were modified employing gaseous and solid compounds, respectively, as well as concentrated mineral acids. The sulfur and phosphorous-containing samples were characterized by nitrogen adsorption, Xray diffraction, FTIR pyridine adsorption, ammonia TPD, and 1-butene isomerization. Modification of zirconia samples results in enhanced Bronsted acidity and catalytic activity. Especially ammonium sulfate and sulfite generate strong acidic centers at the surface in high concentrations. As was shown on the basis of XPS measurements the final calcination step in air converts the sulfite into surface sulfate, resulting finally in the same, or at least very similar, surface states of both differently treated samples. Some few degrees enhanced catalytic activities were obtained using sulfated zirconia aerogels. Applied to the benzoylation of anisole, a slurry of these catalytically active zirconia samples yields similar high conversion degrees as nonafluorobutanesulfonic acid known as an excellent catalyst for this reaction in homogeneous solution. There is evidence to assume that the benzoylation reaction under the conditions used occurs exclusively at Bronsted acid sites but not at Lewis sites. A hypothetical path for the action of a sulfated zirconia surface in the catalytic step is proposed. (C) 1998 Academic Press.