Dealumination of USY Zeoiites by Thermal Treatment (Part 2) Effect of H_2O in the Atmosphere

摘要

The effect of H_2O int he atmosphere on the dealumination of USY zeolite by thermal treatment was studied by ~(29)Si-MASNMR, ~(129)Xe-NMR spectroscopy, ~1H-MASNMR, XPS and XPD. Structral analysis of two samples, which were prepared by thermal dealumination of USY zeolite and had equal unit cell parameter, showed that the mechanism of dealumination by thermal treatment varies with the presence of H_2O in the atmosphere. The thermal treatment in an atmosphere containing H_2O resulted in the formation of hydroxyl nests resulting from the removal of framework Al and replacement by Si from amorphous SiO_2, and migration of the extra-framework Al to outer surface of the zeolite crystallites. And smaller crystallites were associated with a greater degree of dealumination. On the other hand, thermal treatment in an atmosphere not containing H_2O resulted in smaller crystallites and more irregular arrangement of the supercages due to a greater increase in the hydroxyl nests. And it is suggested that cationic extra-framework Al generated by dealumination remain in the crystallites.