No abstract

The steam treatment effect on the structure stability and acidity of ultrastable Y (USY) and Cr(III)-exchanged USY zeolites is investigated in the presence and absence of impregnated vanadium at 1000 up to 4000 ppm concentrations. Under steam-treatment conditions and in the presence of vanadium, the obtained results give evidence of the complete structure breakdown of an USY zeolite sample, which is nearly proportional to the percent dealumination; however, the structure breakdown of a Cr(III)-exchanged USY zeolite sample is limited even though the percent dealumination is still high. In addition, it is proved that the impregnated vanadium attacks selectively the strong acid sites, i.e., removes isolated framework Al atoms; however, although this action comes to completeness, the zeolite structure of the Cr(III)-exchanged USY sample is strong enough to keep its crystallinity at a high percentage. Finally, the acidity measurements give evidence that the Cr(III)-exchanged USY zeolite possesses medium strength acid sites that are absent in the pure USY zeolite material, which suggests that Cr may isomorphously replace Al of the zeolitic framework structure; these sites have vanished in samples with high vanadium content after steam treatment.

No abstract

No abstract

No abstract