Zeolite molecular sieve catalyst and its mechanism of action

Molecular sieve catalyst and its mechanism of action
1. The concept of molecular sieve
Molecular sieves are crystalline aluminosilicates with a uniform pore structure. The molecular sieve contains a large amount of crystal water, which can be vaporized and removed when heated, so it is also called zeolite. The zeolites that are often called in nature are synthetically called molecular sieves. Their chemical composition can be expressed as
Mx/n[(AlO2)x?(SiO2)y] ?ZH2O
Where M is a metal cation, n is its valence, x is the number of AlO2 molecules, y is the number of SiO2 molecules, and Z is the number of water molecules, because AlO2 has a negative charge, and the presence of metal cations keeps the molecular sieves electrically neutral . When the valence of the metal ion is n = 1, the number of atoms of M is equal to the number of atoms of Al; if n = 2, the number of atoms of M is half of the number of Al atoms.
The commonly used molecular sieves mainly include: Sodium-zeolite type zeolites, such as A-type molecular sieves; Octahedral-type zeolites, such as X-type, Y-type molecular sieves; Mercer-type zeolites (-M type); High-silicon type zeolites, such as ZSM-5, etc. . Molecular sieves can provide very high activity and unusual selectivity in various acidic catalysts, and most of the reactions are caused by the acidity of molecular sieves and also belong to solid acids. In the past 20 years, it has been widely used in industry, especially in the refining industry and petrochemical industry as an industrial catalyst occupies an important position.
2. Structural features of molecular sieves
(1) Four aspects and three levels:
The structural characteristics of molecular sieve can be divided into four aspects, three different levels of structure. The first structural level is also the most basic structural units: silicon tetrahedron (SiO4) and aluminum oxide tetrahedron (AlO4), which constitute the framework of the molecular sieve. The adjacent tetrahedrons are linked by oxygen bridges into rings. The ring is the second level of the molecular sieve structure. It is divided into 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 opening of the molecular sieve and acts as a sieve for passing molecules. The oxygen rings are linked to each other through an oxygen bridge to form a polyhedron having a three-dimensional space. A variety of polyhedrons are the third level of molecular sieve structure. The polyhedron has a hollow cage, which is an important feature of the molecular sieve structure. The cages are divided into alpha cages, octahedral cages, beta cages and gamma cages.
(2) Molecular sieve cage:
Alpha cage: It is the main pore of the framework structure of A-type molecular sieves. It is a hexahedron consisting of 12 four-membered rings, 8 six-membered rings and 6 eight-membered rings. The cage had an average pore size of 1.14 nm and a cavity volume of 760 [Å]3. The largest window of the α cage is an eight-membered ring with a pore diameter of 0.41 nm.
An octahedral cage is the main cavities forming the skeleton of X-type and Y-type zeolites. It consists of 18 tetrahedrons consisting of four-membered rings, four six-membered rings and four 12-membered rings. The aperture is 1.25 nm and the volume of the cavity is 850 [Å]3. The largest hole window is a twelve-membered ring with an aperture of 0.74 nm. Octahedral cages are also called super cages.
Beta cage: Mainly used to form the skeleton structure of A-type, X-type and Y-type molecular sieves, and is the most important kind of cavity. Its shape resembles that of the regular octahedron related to top-cutting, and the volume of the cavity is 160 [Å]3 The window aperture is approximately 0.66 nm, allowing only small molecules such as NH3 and H2O to enter.
There are also hexagonal cages and gamma cages, which are smaller in size and generally do not reach the cage.
The cages of different structures are connected to each other through oxygen bridges to form various molecular sieves of different structures, mainly A-type, X-type and Y-type.
(3) Several representative molecular sieves
A type molecular sieve
Similar to the cubic structure of NaCl. If the Na+ and Cl- in the NaCl lattice are all replaced by β cages, and the adjacent β cages are linked by a γ cage, the crystal structure of the A-type molecular sieve is obtained. Eight beta cages form a sodalite structure after being joined. If a gamma cage is used for bridge connection, A-type molecular sieve structure is obtained. The center has a large cage of alpha. There is an eight-ring window in the passage between the α cages. Its diameter is 4 & Aring; it is called 4A molecular sieve. If 70% of the Na+ on the 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 K+ exchanged, the eight-membered ring pore size is reduced to 3Å the corresponding zeolite is called 3A molecular sieve.
X-type and Y-type molecular sieves
Close-packed hexagonal crystal structure resembling diamond. If β-cage is used as a structural unit to replace the carbon atom of diamond, and two adjacent β-cage are connected by a hexagonal cage, that is, four β cages are connected by four hexagonal cages, one of the β cages. At the center, the remaining 4 β cages are located at the apex of the tetrahedron, forming an octahedral zeolite-type crystal structure. With this structure continuing to join, X-type and Y-type molecular sieve structures are obtained. In this structure, the large cages formed by the cages of β cages and hexagonal cages are cages of faujasite. The windows that communicate with each other are twelve-membered rings with an average effective pore size of 0.74 nm. This is the X-type and Y- Molecular sieve pore size. The difference between the two models is mainly Si/Al ratio is different, 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. Each pair of five-membered rings is linked to the other by an oxygen bridge. A four-membered ring is formed at the junction. This structural unit is further linked to form a layered structure. There are eight-membered and twelve-membered rings in the layer, the latter being elliptical with an average diameter of 0.74 nm, which is the main pore of the mordenite. This channel is one-dimensional, that is, straight channels.
Zeolite Socony Mobil Zeolite
This zeolite has a series of widely used ZSM-5, ZSM-8 and ZSM-11 with the same structure; ZSM-21, ZSM-35 and ZSM-38 are another group. ZSM-5 is often referred to as a high-silicon zeolite. Its Si/Al ratio can be as high as 50 or more and ZSM-8 can be as high as 100. This group of zeolites also exhibits hydrophobic properties. Their structural units are similar to those of mordenite. They consist of pairs of five-membered rings with no cage-like cavity and only channels. The ZSM-5 has two sets of intersecting channels, one of which is straight-through, and the other is that the zig-zag type is perpendicular to each other, all formed by a ten-membered ring. The channel is elliptical with a window diameter of (0.55-0.60) nm. The zeolites belonging to the high-silicon family also have Silicalite-1, which is an all-silicon type, and has the same structure as ZSM-5. Silicalite-2 is the same as ZSM-11.