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 SO2-4 in Zro2) zirconia is amorphous upon calcinations 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- SO2-4 system appears comparable to that of AlCl3/Al2O3, but lower than that of SbF5/Al2O3.