Definition and Basic Properties of Molecular Sieves

Molecular sieves are a class of insoluble aluminosilicates with uniform pore sizes. Traditional molecular sieves are zeolite-type aluminosilicates with a microporous structure (pore diameter < 2 nm). However, with the development of molecular sieve science, new types of molecular sieves and porous compounds such as aluminophosphates and silicoaluminophosphates have emerged. By 2003, there were already 145 types of molecular sieves with unique framework structures, and the framework elements that make up the micropores exceeded 30 kinds.

Molecular sieves have uniform micropores with a size on the same order of magnitude as the molecules of general substances, an open framework structure. The pore volume accounts for 30% - 50% of the total volume, a large specific surface area (300 - 1000 m²/g), a small external surface area (about 1% of the total surface area), and high thermal stability.

After being artificially synthesized, molecular sieves are white crystalline powders. When natural minerals are used as raw materials or impurities are mixed in during the synthesis process, the products sometimes have a slight color. Their particle size range is 1 - 10 μm, and the average particle size is 1 - 5 μm. They are insoluble in water and organic solvents and are generally soluble in strong acids. To meet the needs of industrial applications, the original molecular sieve powder is usually bonded with binders such as clay to form a certain shape. Common shapes include granular, strip-shaped, block-shaped, honeycomb-shaped, etc. The basic properties of molecular sieves are mainly reflected in: ion exchangeability, adsorption performance, and catalytic performance.