Molecular sieve concept

A molecular sieve is an aluminosilicate compound with a cubic lattice. Molecular sieves have a uniform microporous structure, and their pore diameters are uniform. These pores can adsorb molecules smaller than their diameters into the interior of the pore cavity, and have preferential adsorption ability for polar molecules and unsaturated molecules. Different degrees of sex, different degrees of saturation, molecules with different molecular sizes and different boiling points are separated, that is, they have the function of "sieving" molecules, so they are called molecular sieves. Because molecular sieves have advantages such as high adsorption capacity and strong thermal stability, other sorbents do not make molecular sieves widely used.

In a narrow sense, molecular sieves are crystalline silicates or aluminosilicates. Silicon oxytetrahedrons or aluminooxytetrahedrons are connected by oxygen bridges to form channels and voids of molecular size (usually 0.3 to 2.0 nm). Cavity system, which has the characteristics of screening molecules. However, with the deepening of molecular sieve synthesis and application research, researchers have discovered aluminophosphate molecular sieves, and the skeleton elements of the molecular sieve (silicon or aluminum or phosphorus) can also be composed of B, Ga, Fe, Cr, Ge, Ti, V , Mn, Co, Zn, Be and Cu, etc., the size of its pores and cavities can also reach more than 2 nm, so the molecular sieve can be divided into silicon aluminum molecular sieve, phosphorus aluminum molecular sieve and framework heteroatom molecular sieve according to the composition of the framework element. According to the size of the pores, molecular sieves with pore sizes less than 2 nm, 2-50 nm and more than 50 nm are called micropores, mesopores and macropores. Because of its larger pore size, it becomes a good carrier for larger-scale molecular reactions, but the pore wall of the mesoporous material is amorphous, so that its hydrothermal stability and thermal stability cannot meet the harsh conditions required for petrochemical applications. .

Because it contains metal ions with low electricity prices and large ionic radius and water in a combined state, water molecules are continuously lost after heating, but the crystal skeleton structure is unchanged, forming many cavities of the same size, and the cavities have many diameters. The same micropores are connected. The diameter of these tiny pores is uniform. It can adsorb molecules smaller than the diameter of the pores into the interior of the pores, and exclude molecules larger than the pores. Therefore, molecules with different shapes and diameters can be removed. Molecules with different polarities, molecules with different boiling points, and molecules with different saturation levels are separated, that is, they have the function of "sieving" molecules, so they are called molecular sieves. Currently molecular sieves are widely used in metallurgy, chemical, electronics, petrochemical, natural gas and other industries.

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