Concept and structure of molecular sieve.
I. Concept
Molecular sieve is a cubic lattice aluminosilicate compound. Molecular sieves have a uniform microporous structure. The diameters of their pores are uniform. These pores can adsorb molecules smaller than their diameters into the interior of the pore cavities and have a preferential adsorption ability for polar molecules and unsaturated molecules. Therefore, they can separate molecules with different degrees of polarity, saturation, sizes, and boiling points. That is, they have the function of "sieving" molecules, so they are called molecular sieves. Due to the advantages of high adsorption capacity and strong thermal stability that other adsorbents do not have, molecular sieves have been widely used.
II. Molecular sieve structure
In a narrow sense, molecular sieves are crystalline silicates or aluminosilicates. They are formed by silicon-oxygen tetrahedrons or aluminum-oxygen tetrahedrons connected by oxygen bridge bonds to form a pore and cavity system with molecular sizes (usually 0.3 to 2.0 nm), thus having the characteristic of sieving molecules. However, with the in-depth research on the synthesis and application of molecular sieves, researchers have discovered aluminophosphate molecular sieves. Moreover, the framework elements (silicon, aluminum, or phosphorus) of molecular sieves can also be replaced by B, Ga, Fe, Cr, Ge, Ti, V, Mn, Co, Zn, Be, and Cu. The sizes of their pores and cavities can also reach more than 2 nm. Therefore, molecular sieves can be divided into silica-alumina molecular sieves, aluminophosphate molecular sieves, and framework heteroatom molecular sieves according to their framework element composition. According to pore size division, molecular sieves with pore sizes less than 2 nm, 2 to 50 nm, and greater than 50 nm are called microporous, mesoporous, and macroporous molecular sieves respectively. Due to their relatively large pore diameters, they become good carriers for reactions of larger-sized molecules. However, the pore walls of mesoporous materials are amorphous, resulting in their hydrothermal stability and thermal stability still not being able to meet the harsh conditions required for petrochemical applications.
Because they contain metal ions with lower valence and larger ionic radii and combined water, water molecules are continuously lost after heating, but the crystal framework structure remains unchanged, forming many cavities of the same size. The cavities are connected by many micropores with the same diameter. These tiny pores have uniform diameters and can adsorb molecules smaller than the pore diameter into the interior of the pores, while repelling molecules larger than the pore diameter. Therefore, they can separate molecules with different shapes, sizes, degrees of polarity, boiling points, and saturation levels. That is, they have the function of "sieving" molecules, so they are called molecular sieves. At present, molecular sieves are widely used in industries such as metallurgy, chemical industry, electronics, petrochemical industry, and natural gas.
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