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Molecular Sieve

There is a natural aluminosilicate in nature, which has the functions of screening molecules, adsorption, ion exchange and catalysis. This natural material is called zeolite, and the synthetic zeolite is also called molecular sieve. The general chemical composition formula of molecular sieve is: (M) 2/nO · Al2O3 · xSiO2 · pH2O. M represents metal ions (usually Na in artificial synthesis), n represents valence number of metal ions, x represents mole number of SiO2, also known as silicon aluminum ratio, and p represents mole number of water. The most basic structure of the molecular sieve framework is the tetrahedron of SiO4 and AlO4, which combines with common oxygen atoms to form a three-dimensional network structure crystal. This kind of combination form forms voids and channels with molecular level and uniform pore size. Due to different structures and forms, "cage" shaped space holes are divided into α、β、γ、 Hexagonal column, octahedral zeolite and other "cage" structures. The crystal structures of A, X and Y zeolites.


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Because AlO4 tetrahedron has a negative charge, it can combine with sodium plasma and become electrically neutral. In aqueous solution, Na can easily exchange with other cations. Most molecular sieve catalysts are multivalent metal cation or H exchanger. Molecular sieve has acidity and selectivity for molecular size, and can be used as catalyst or carrier. High silica zeolite shows high affinity for organic groups. In contrast, low silica zeolite shows hydrophilicity due to its Lewis and Bronsted acid characteristics. [2] Silicon and aluminum atoms form an oxygen ring through oxygen. The size of the oxygen ring determines the pore size of zeolite. The number of oxygen atoms in each oxygen ring is 4-12. Generally, eight membered ring (0.4 ~ 0.5 nm), ten membered ring (0.5 ~ 0.6 nm) and twelve membered ring (0.7 ~ 0.9 nm) have the role of molecular sieve.

Y-type molecular sieves (x=3.1~6.0) and mordenites (x=9~11) have twelve membered oxygen rings. The former can be used as cracking catalyst and bifunctional catalyst, while the latter can be used as toluene disproportionation catalyst.

Some ZSM series molecular sieves, such as ZSM-5 and ZSM-11, are available for the ten membered oxygen ring.

The eight membered oxygen ring includes A-type molecular sieve (x=2), T-type molecular sieve and ZSM-34. Their pores are very small, and only straight chain hydrocarbons can enter the pores. The catalyst with molecular sieve as catalytic active component or main active component is called molecular sieve catalyst. Molecular sieves have ion exchange properties, uniform molecular size channels, excellent acid catalytic activity, and good thermal and hydrothermal stability. It can be made into a catalyst with high activity and selectivity for many reactions.