Thermal stability and hydrolysis catalysts effect on sol-gel zirconia in the presence of heteropolyacids

Zirconia catalysts with 15 and 25 wt% of tungstophosphoric acid (TPA) were prepared by the sol gel method. The gels were obtained by catalytic hydrolysis of zirconium n-butoxide and TPA, at pH= 3, 5 and 7, using as catalysts HCl, C2H4O2 and NH4OH, respectively. Fresh samples were thermally treated from 100degreesC to 900degreesC, in a stepwise increments of 100degreesC, during 20 hours per step. The crystallization behavior was followed by powder X-ray diffraction. From the x-ray data, it was clear that the 25% TPA samples formed a solid solution with tetragonal zirconia, since the presence of tungsten compounds was not detected up to 1000degreesC; whereas in the lower content samples, TPA was surface dispersed, and dissociates to form several tungsten compounds near 700degreesC. Bronsted and Lewis acidity was measured by FTIR pyridine adsorption. Lewis sites were detected for both TPA compositions, whereas Bronsted sites were present only in the 25% samples. A. model is proposed to explain the stabilization of TPA in zirconia.