THERMAL STABILITY AND HYDROLYSIS CATALYSTS EFFECT ON SOL-GEL ZIRCONIA IN THE PRESENCE OF HETEROPOLYACIDS

摘要

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, C_2H_4O_2 and NH_4OH, respectively. Fresh samples were thermally treated from 100℃ to 900℃, in a stepwise increments of 100℃, 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 1000℃; whereas in the lower content samples, TPA was surface dispersed, and dissociates to form several tungsten compounds near 700℃. Broensted 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.