Abstract: The ultrafine NaY zeolite with 200 nm grain size was dealuminated deeply by the optimized treatment of (NH₄)₂SiF₆ solution, steamed at 600 ^oC, silicon sol + oxalic acid, and steamed at 800 ^oC, respectively. The first treatment of (NH₄)₂SiF₆ solution is very important, which can fix the framework defects of zeolite and strengthen its stability. The amount of (NH₄)₂SiF₆ and treated number need to be carefully controlled according to the grain size of NaY zeolite. For 200 nm NaY zeolite, the best molar ratio of (NH₄)₂SiF₆ to framework aluminum of zeolite is 0.16 and once treatment is relatively appropriate. The final dealuminated Y zeolite has the high framework Si/Al ratio of 27.3 and surface area of 581.9 m²/g, as well as the crystallinity of 65%. The key factor for the successful deep dealumination of ultrafine Y zeolite is to repair promptly its crystal defects resulting from the dealumination process. The treatments of (NH₄)₂SiF₆ solution, silicon sol, and steamed at 800 ^oC appear such effect.

Key words: ultrafine Y zeolite, dealumination, substitution of silicon, high Si/Al ratio, stability