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Molecular Sieve

Natural aluminosilicates are found in nature and they have molecular screening, adsorption, ion exchange and catalytic properties. This natural substance is called zeolite, and synthetic zeolite is also called molecular sieve. The general chemical composition formula of molecular sieve is: (M)2/nO·Al2O3·xSiO2·pH2O, M represents the metal ion (usually Na when artificially synthesized), n represents the valence of the metal ion, x represents the number of moles of SiO2, also known as is the silicon-aluminum ratio, and p represents the number of moles of water.

The most basic structure of the molecular sieve skeleton is SiO4 and AlO4 tetrahedrons, which form a three-dimensional network structure crystal through the combination of shared oxygen atoms. This combination forms cavities and channels with molecular level and uniform pore size. Due to different structures and forms, "cage"-shaped space pores are divided into "cage" structures such as α, β, γ, hexagonal prism, faujasite, etc. The crystal structures of type A, type X and type Y molecular sieves are shown in Figure 1 and Figure 2.

Since the AlO4 tetrahedron has a negative charge, it can combine with sodium ions and become electrically neutral. In aqueous solutions, Na exchanges readily with other cations. Most molecular sieve catalysts are exchangers of multivalent metal cations or H. Molecular sieves have acidity and selectivity for molecular size and can be used as catalysts or carriers. High silica zeolites exhibit a high affinity for organic groups, in contrast, low silica zeolites exhibit hydrophilicity due to Lewis and Bronsted acid properties.

Silicon and aluminum atoms form an oxygen ring through oxygen, and the size of the oxygen ring determines the pore size of the zeolite. The number of oxygen atoms in each oxygen ring is 4 to 12. Usually, there are eight-membered rings (0.4~0.5nm), ten-membered rings (0.5~0.6nm) and twelve-membered rings (0.7~0.9nm) that function as molecular sieves.

Those with twelve-membered oxygen rings include Y-type molecular sieves (x= 3.1~6.0) and mordenite (x=9~11). The former can be used as a cracking catalyst and bifunctional catalyst, and the latter can be used as a disproportionation catalyst for toluene.

The ten-membered oxygen ring includes ZSM-5, ZSM-11 and other ZSM series molecular sieves.

Eight-membered oxygen rings include A-type molecular sieve (x=2), T-type molecular sieve and ZSM-34, etc. Their pores are so small that only straight-chain hydrocarbons can enter the pores. Catalysts that use molecular sieve as the catalytically active component or main active component are called molecular sieve catalysts. Molecular sieves have ion exchange properties, uniform molecular size pores, excellent acid catalytic activity, and good thermal and hydrothermal stability. Catalysts with high activity and selectivity can be made for many reactions.