Hydrothermally resistant high-silica Y zeolites stabilized by covering with non-framework aluminum species

The hydrothermal stability of high-silica Y zeolites dealuminated by Al/Si substitution may be increased by alumination of the crystal surface. The externally introduced non-framework aluminum species — alkali aluminosilicates or alumina — cover the surface where the water molecules attack the terminal OH groups and energy-rich Si-O-Si bonds. An alkaline and combined acid/alkaline pathway of alumination are suggested. A reinsertion of aluminum into the zeolite lattice could not be observed.