Acid properties of pure and sulfated mixed oxides of Al2O3-TiO2 and Al2O3-ZrO2 prepared by sol-gel

Mixed oxides of Al and Ti or Zr were prepared by sol-gel with a ratio Ti(Zr)/Al = 5 to 20 mol.%, and compared to the products of a one step technique with sulfuric acid as catalyst in the sol-gel. The insertion of Zr or Ti results in a higher surface area after calcination at 650degreesC. The products of one step sol-gel synthesis retain sulfur after calcination, and show a better stability of their surface. The pure mixed oxides are Lewis acids of moderate strength. Sulfation increases the number of acid sites for both Ti and Zr materials; the effect is larger for Zr/Al mixed oxides and nearly independent of the Zr (Ti) content above 5%. The sulfated materials prepared by one step procedure show protonic acidity absent on the non sulfated materials. This suggests that the protonic acidity is linked to sulfates born by Zr cations inserted in the lattice of alumina. The Lewis acidity of these materials is consistent with their activity for isomerisation of alpha-pinene epoxide to campholenic aldehyde.