Spherical molecular sieves are a type of selective adsorbent with specific pore sizes and structures, which are widely used in various fields such as gas separation, catalysis, and ion exchange.

I. Basic Information

Molecular sieves are silicon – aluminum compounds with a cubic lattice structure. They are usually white or grayish – white spherical granules, and can also be in the form of bars, beads, flakes or powders. They are odorless and hygroscopic. Chemically, they are a type of multi – hydrate silicon – aluminum acid salt, also known as synthetic zeolites. They can dissolve in strong acids and alkalis and decompose, but are insoluble in water and organic solvents.

The chemical composition general formula of molecular sieves is (Mn+)2/nO⋅Al2O3⋅xSiO2⋅pH2O(Mn+)2/n​O⋅Al2​O3​⋅xSiO2​⋅pH2​O, where MM represents metal ions (usually NaNa in artificial synthesis), nn represents the valence of metal ions, xx represents the molar number of SiO2SiO2​ (also called the silicon – aluminum ratio), and pp represents the molar number of water.

II. Structure and Function

The silicon and aluminum atoms in molecular sieves connected by oxygen bridges to form an open framework structure. There are many pore channels with uniform pore sizes and large – surface – area cavities arranged in an orderly manner. After heating, the water molecules continuously lost, but the crystal framework structure remains unchanged, forming many cavities of the same size, connected by many micropores of the same diameter.

These tiny pores have a uniform diameter, which can adsorb molecules smaller than the pore channel diameter into the interior of the cavities, while excluding larger molecules. This property enables them to separate molecules of different shapes, sizes, polarities, boiling points and saturation degrees, thus having the function of “sieving” molecules.

III. Applications of Spherical Molecular Sieves

A. Gas Separation

1. 3A Molecular Sieves: With an effective pore size of about 0.32 nm, they are mainly for the drying of petroleum cracking gases, olefins (such as ethylene, butadiene), and liquid substances (such as ethanol). They also used for drying the air in hollow glass, N2−H2N2​−H2​ mixed gases, and refrigerants.

2. 4A Molecular Sieves: They can adsorb normal – structure hydrocarbons and alcohol gases. They widely used for the deep drying of gases and liquids such as air, natural gas, and refrigerants, as well as the preparation and purification of argon. They can also be as dehydrating agents in paints, dyes, and coatings.
3. 13X Molecular Sieves: They can adsorb molecules with a relatively large critical diameter. They mainly used for gas drying and purification, especially for the purification of raw gas in air separation devices, effectively adsorbing carbon dioxide, water, and organic sulfides in the air.

B. Catalysis

Most molecular sieve catalysts exchanged with polyvalent metal cations or H+H+. They have acidity and selectivity for molecular size, so they can be as catalysts or carriers. For example, Y – type molecular sieves with twelve – membered oxygen rings can be as cracking catalysts and bifunctional catalysts, while mordenite can be as a toluene disproportionation catalyst.

IV. Manufacturing Process of Spherical Molecular Sieves

A. Raw Material Preparation

The raw materials for synthesizing spherical molecular sieves usually include silicon sources, aluminum sources, templating agents, water, and alkalis. The proportion and type of these raw materials will affect the performance and structure of the final product.

B. Mixing and Kneading

The prepared raw materials mixed in a certain proportion and fully kneaded to ensure uniform distribution of the raw materials. This step is crucial for subsequent molding and product performance.

C. Molding

1. Selecting a Suitable Molding Method: Depending on the required particle size and shape of the spherical molecular sieves, appropriate molding methods such as extrusion molding, spray – drying molding, or oil – ammonia column molding can select.
2. Controlling Molding Conditions: During the molding process, conditions such as temperature, pressure, and humidity controlled to ensure product quality and performance.

D. Drying

The molded spherical molecular sieves dried to remove water and solvents. The temperature and humidity also controlled during the drying process to avoid product cracking or deformation.

E. Calcination

Finally, the dried spherical molecular sieves calcined to remove the templating agent and stabilize the product structure. The calcination temperature and time need to set according to the specific requirements of the product.

In conclusion, spherical molecular sieves, with their unique structure and excellent performance, play an important role in many industrial fields, and their production process requires strict control to ensure product quality.