Study of the structure and composition of decationated dealuminated Y zeolites

The effect of the modification of Y zeolite dealuminated with an (NH4)(2)SiF6 solution on its structure and composition was studied by IR spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The modification of the dealuminated Y zeolite was carried out by successive thermal treatments at 500-550 degrees C and cation exchanges with NH4+. The thermal treatment of a high-silica Y zeolite under deep-bed conditions caused its further dealumination due to the hydrolysis of both the framework aluminum in Si-O-Al groups and subframework aluminum with the formation of new terminal Si-OH and Al-OH groups and nonframework species of octahedrally coordinated aluminum as hydroxyaluminum cations. The dealumination of the zeolite was also accompanied by the formation of siloxane bonds. As the number of treatments increased, the formation of new Si-O-Si bonds was intensified, the zeolite structure was stabilized, the degree of crystallinity increased, and the unit cell parameter decreased. A part of the nonframework aluminum species was removed during the cation exchange with NH4+. Repeated treatments of the zeolite resulted in a more uniform distribution of aluminum (virtual constancy of the true Si/Al ratio) in the bulk of zeolite crystallites. The external zeolite surface remained enriched in silicon despite possible migration of nonframework aluminum to the outer surface.