Abstract: Ultrafine zeolite NaYwas hydrothermally crystallized with silica sol and Al₂(SO₄)₂ as source materials via optimizing synthesis conditions and adding light rare earth ions(Ln ³⁺) to the synthesis batch. The crystallization performance of NaYcould be affected by the aging time of nucleation Si-Al gel and the crystallization temperature of reaction Si-Al gel. With increasing the aging time(2—7 d) of nucleation gel, the crystallization rate of zeolite NaYincreased whereas the average crystal size of zeolite NaYdecreased. When elevating the crystallization temperature(363—383 K) of reaction gel, both the crystallization rate and the average crystal size of zeolite NaYincreased. Nevertheless, the crystallization performance of zeolite NaYwas more remarkably affected by the addition of Ln³⁺ ions in the synthesis batch. The crystallization rate and the n(SiO₂)/n(Al₂O₃) ratio obviously increased while the average crystal size greatly reduced when zeolite NaYwas crystallized in the presence of Ln³⁺ ions, comparing with that in the absence of Ln³⁺ ions. It was found that, with the amount of the added Ln ³⁺ ions increasing, the average crystal size of zeolite NaYdecreased followed by increasing, while the crystallization rate and the SiO₂/Al₂O₃ ratio in framework changed in the opposite direction. The optimal amount of Ln ³⁺ additive appears to be in the range of n(Ln³⁺)∶n(Al ³⁺)=0.05∶1—0.1∶1 in mole. In the case, the as-synthesized NaYzeolites had the smallest average crystal sizes(<100 nm), the highest n(SiO₂)/n(Al₂O₃) ratio(>5.7) and the shortest crystallization time(<36 h). The role of the added Ln³⁺ ions was proposed to be that there formed the micro-crystalline Ln(OH)₃ under the synthesis conditions, which may present additional area to initiate the nucleation of zeolite NaY.

Key words: Ultrafine zeolite, NaY, Crystal size, Synthesis, Rare earth ions