Morphology and activity of zirconia-sulfate aerogels

Zirconia-sulfate aerogels obtained from the process with the molar hydrolysis ratio r = 4 are more porous and active than those synthesized with r = 2. Sulfate ions markedly hinder sintering of zirconia and structure consolidation during heat treatment and this process depends on their concentration. At a higher sulfate load (20 mol% of SO^2-₄ in ZrO₂) zirconia is amorphous upon calcination at 773 K, irrespective of the water content in the synthesis, while at a lower load it can be crystalline. XRD analysis indicates that the crystalline phase of calcined zirconia-sulfate is cubic and not tetragonal, as observed in conventional zirconia, and reported in previous studies. Acid strength of ZrO₂-SO^2-₄ system appears comparable to that of AlCl₃/Al₂O₃, but lower than that of SbF₅/Al₂O₃.