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Several representative molecular sieves

Sep. 22, 2022

Several representative molecular sieves

Type A molecular sieve

Similar to the cubic crystal structure of NaCl. If all Na+ and Cl- in the NaCl lattice are replaced with β cages, and adjacent β cages are connected with γ cages, the crystal structure of A-type molecular sieves can be obtained. After 8 β cages are connected, a sodalite structure is formed. If a γ cage is used as a bridge connection, an A-type molecular sieve structure is obtained. There is a large alpha cage in the center. There is an eight-membered ring window in the channel between the α cages, and its diameter is 4Å, so it is called 4A molecular sieve. If 70% of Na+ on 4A molecular sieve is exchanged for Ca2+, the eight-membered ring can be increased to 5Å, the corresponding zeolite is called 5A molecular sieve. On the contrary, if 70% of Na+ is exchanged for K+, the pore size of the eight-membered ring is reduced to 3Å, and the corresponding zeolite is called 3A molecular sieve.

X-type and Y-type molecular sieves

Diamond-like close-packed hexagonal structure. If the β cage is used as the structural unit to replace the carbon atom nodes of diamond, and the two adjacent β cages are connected by hexagonal column cages, that is, 5 β cages are connected together with 4 hexagonal column cages, and one β cage is used. In the center, the remaining four β cages are located at the vertices of the regular tetrahedron, forming an octahedral zeolite crystal structure. Continuing to link with this structure, X-type and Y-type molecular sieve structures are obtained. In this structure, the large cage formed by the beta cage and the hexagonal column cage is a faujasite cage, and their interconnected window holes are twelve-membered rings, and their average effective pore size is 0.74nm, which is the X-type and Y-type zeolite cages. The pore size of the molecular sieve. The difference between these two types is mainly the Si/Al ratio, X-type is 1~1.5; Y-type is 1.5~3.0.

Mordenite Molecular Sieve

The structure of this zeolite has no cage but a layered structure. The structure contains a large number of five-membered rings, which are linked together in pairs, and each pair of five-membered rings is linked to another pair through an oxygen bridge. The junction forms a four-membered ring. Such structural units are further joined to form a layered structure. There are eight-membered rings and twelve-membered rings in the layer, and the latter is oval with an average diameter of 0.74 nm, which is the main channel of mordenite. Such channels are one-dimensional, ie straight channels.

High silica zeolite ZSM (Zeolite Socony Mobil) molecular sieve

This zeolite has a series, ZSM-5 is widely used, ZSM-8 and ZSM-11 have the same structure; another group is ZSM-21, ZSM-35 and ZSM-38. ZSM-5 is often referred to as high-silica type zeolite, and its Si/Al ratio can be as high as 50 or more, and ZSM-8 can be as high as 100. This group of molecular sieves also shows hydrophobic properties. Their structural unit is similar to that of mordenite, consisting of pairs of five-membered rings, without cage-like cavities, and only with channels. ZSM-5 has two sets of intersecting channels, one is straight through and the other is zigzag perpendicular to each other, all formed by ten-membered rings. The channel is elliptical with a window diameter of (0.55-0.60) nm. The zeolite belonging to the high silicon family also includes the all-silicon Silicalite-1, which has the same structure as ZSM-5, and Silicalite-2 is the same as ZSM-11.

Aluminum Phosphate Molecular Sieve

This series of zeolites is the third generation of new molecular sieves that appeared in the 1980s after Y-type molecular sieves in the 1960s and ZSM-5 high-silica molecular sieves in the 1970s. Including macroporous AlPO-5 (0.1-0.8nm), mesoporous AlPO-11 (0.6nm) and small pore AlPO-34 (0.4nm) and other structures and MAPO-n series and AlPO diameters are chemically modified by Si into the SAPO series, etc.

 

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