The structural characteristics of molecular sieves can be divided into four aspects and three different structural levels. The first structural level is the most basic structural unit silicon oxytetrahedron (SiO4) and aluminum oxytetrahedron (AlO4), which constitute the skeleton of the molecular sieve. Adjacent tetrahedrons are connected in a ring by oxygen bridges. The ring is the second level of the molecular sieve structure. It is divided according to the number of oxygen atoms forming a ring. There are four-membered oxygen rings, five-membered oxygen rings, six-membered oxygen rings, eight-membered oxygen rings, ten-membered oxygen rings, and twelve-membered oxygen rings. Wait. The ring is the channel orifice of the molecular sieve, which plays a sieving role for passing molecules. The oxygen rings are connected to each other through an oxygen bridge to form a polyhedron having a three-dimensional space. Various polyhedra are the third level of the molecular sieve structure. Polyhedrons have hollow cages, which are an important feature of the molecular sieve structure. The cages are divided into alpha cages, faujasite cages, beta cages and gamma cages.

Alpha cage: It is the main pore of the A-type molecular sieve skeleton structure. It is a hexahedron composed of 12 four-membered rings, eight six-membered rings and six eight-membered rings. The average pore diameter of the cage was 1.14 nm and the cavity volume was 7603. The largest window hole of the α cage is an eight-membered ring with a pore diameter of 0.41 nm. Faujasite cage: It is the main pore of X-type and Y-type molecular sieve framework. It is composed of 18 four-membered rings, four six-membered rings and four twelve-membered rings. The pore size is 1.25 nm and the cavity volume is 8503. The largest aperture window is a twelve-membered ring with a pore diameter of 0.74 nm. Faujasite cages are also called supercages. β cage: It is mainly used to form the framework structure of A-type, X-type and Y-type molecular sieves. It is the most important type of pore. Its shape is like a regular octahedron with a chipped cavity. 0.66nm, only small molecules such as NH3, H2O are allowed to enter.

In addition, there are hexagonal column cages and γ cages. These two cages are relatively small in size and generally cannot enter the cage. Cages of different structures are then connected to each other through oxygen bridges to form molecular sieves of various structures, mainly A-type, X-type and Y-type.

Reference "catalysis principle"