Here is a detailed overview of the synthesis of NAY molecular sieve (NaY zeolite) in English:

1. Hydrothermal Synthesis Method (Most Common)

The hydrothermal method is the traditional and widely used approach for synthesizing NaY zeolite. It involves high-temperature and high-pressure reactions in an aqueous solution to form the zeolite's crystalline framework.

Key Steps:

1. Raw Material Preparation:
   • Silica Source: Sodium silicate (Na₂SiO₃), colloidal silica, or fumed silica.
   • Alumina Source: Sodium aluminate (NaAlO₂), aluminum sulfate (Al₂(SO₄)₃), or pseudoboehmite.
   • Alkali Source: Sodium hydroxide (NaOH) to adjust pH and provide Na⁺ ions.
   • Water: Deionized water is typically used to avoid impurities.
2. Preparation of Structuring Agent (Optional):
   • A small portion of the raw materials may be pre-reacted to form a "structuring agent" or "seed gel" to guide crystallization. This step is optional but can improve crystal quality and reduce synthesis time.
3. Reaction Mixture Preparation:

   • The silica source, alumina source, and alkali are mixed in specific molar ratios. The typical composition for NaY synthesis is:

• The mixture is stirred thoroughly to ensure homogeneity.

4. Hydrothermal Crystallization:
   • The reaction mixture is transferred to a stainless steel autoclave (high-pressure reactor) lined with Teflon or other inert materials.
   • The autoclave is heated to 80–120°C under autogenous pressure (typically 0.1–0.5 MPa) for 10–72 hours, depending on the desired crystal size and purity.
   • Crystallization may occur in static conditions or with gentle stirring to enhance mass transfer.
5. Product Recovery and Washing:
   • After crystallization, the autoclave is cooled to room temperature.
   • The solid product (NaY zeolite) is separated by filtration or centrifugation.
   • The zeolite is washed repeatedly with deionized water to remove residual Na⁺ ions and unreacted precursors.
6. Drying and Calcination (Optional):
   • The washed zeolite is dried at 80–120°C overnight to remove moisture.
   • If needed, calcination at 400–600°C for 2–6 hours can be performed to remove any trapped organic molecules or improve crystallinity.

2. In-Situ Crystallization from Kaolin (Alternative Method)

This method is cost-effective and widely used in industrial-scale production. It involves converting kaolin (a clay mineral) into NaY zeolite through spray-drying and hydrothermal treatment.

Key Steps:

1. Kaolin Activation:
   • Kaolin is calcined at 700–900°C to convert it into metakaolin (a reactive amorphous phase).
2. Slurry Preparation:
   • The metakaolin is mixed with sodium silicate and sodium hydroxide solutions to form a homogeneous slurry.
3. Spray-Drying:
   • The slurry is spray-dried to form microspherical particles (typically 20–100 μm in diameter), which serve as precursors for zeolite synthesis.
4. Hydrothermal Treatment:
   • The microspheres are transferred to an autoclave and subjected to hydrothermal crystallization at 80–100°C for 10–48 hours to form NaY zeolite crystals within the microspheres.
5. Post-Treatment:
   • The product is washed, dried, and optionally calcined as described in the hydrothermal method.

3. Microwave-Assisted Hydrothermal Synthesis (Emerging Method)

Microwave heating can accelerate the crystallization process and reduce energy consumption compared to conventional heating.

Key Steps:

1. Reaction Mixture Preparation: Similar to the hydrothermal method.
2. Microwave Irradiation:
   • The mixture is placed in a microwave-transparent vessel (e.g., Teflon) and irradiated at 2.45 GHz under controlled temperature (80–120°C) and pressure for 1–10 hours.
3. Product Recovery: Follows the same steps as the hydrothermal method.

Key Factors Affecting NaY Synthesis

1. Molar Composition: The SiO₂/Al₂O₃ ratio determines the framework Si/Al ratio, which affects acidity and thermal stability.
2. Alkali Concentration: High NaOH concentrations promote nucleation but may lead to smaller crystals.
3. Temperature and Time: Higher temperatures and longer times favor larger crystals but may increase impurity formation.
4. Additives: Organic templates (e.g., tetramethylammonium hydroxide) can be added to modify pore size or shape, though they are rarely used for NaY.

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