Mechanism of action of molecular sieves

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Molecular sieve catalysts and their mechanism of action 1. Concept of molecular sieves Molecular sieves are crystalline silicoaluminates with a uniform pore structure. Zeolites contain a large amount of crystalline water, which can be vaporized and removed when heated; hence they are also known as zeolites. Those that exist in nature are commonly called zeolites, while those synthesized artificially are called molecular sieves. Their chemical composition can be expressed as Mx/n ·ZH2O, where M is the 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. Since AlO2 carries a negative charge, the presence of metal cations helps to maintain the electrical neutrality of the molecular sieve. When the valence of the metal ion is n = 1, the number of M atoms is equal to the number of Al atoms ； If n = 2, the number of atoms of M is half the number of Al atoms. The commonly used molecular sieves mainly include: sodium square zeolites, such as Type A molecular sieves ； Octahedral zeolites, such as X-type, Y-type molecular sieves ； Mordenite-type zeolite (-M type) ； High-silica zeolites, such as ZSM-5, etc. Molecular sieves can provide high activity and unusual selectivity in various acidic catalysts, and the majority of reactions are driven by the acidity of the molecular sieves, which also belong to the category of solid acids. It has been widely used in industry over the past 20 years, holding an important position as an industrial catalyst, especially in the petroleum refining industry and petrochemicals. 2. Structural characteristics of molecular sieves (1) Four aspects, three levels: The structural characteristics of molecular sieves can be divided into four aspects and three different structural levels. The first structural level, namely the most basic structural units, are the silicate tetrahedra (SiO4) and aluminate tetrahedra (AlO4), which form the framework of molecular sieves. Adjacent tetrahedra are connected into rings by oxygen bridges. The ring is the second level of the molecular sieve structure; based on the number of oxygen atoms in the ring, there are tetraoxo rings, pentaoxo rings, hexaoxo rings, octaoxo rings, decaoxo rings, and dodecaoxo rings, among others. The rings are the channel openings in molecular sieves, serving to sieve the molecules that pass through them. Oxirings are connected to each other through oxygen bridges, forming polyhedra with a three-dimensional structure. Various types of polyhedra represent the third level of molecular sieve structures. Polyhedra have hollow cages, which are an important feature of the molecular sieve structure. Cages are divided into α cages, octahedral zeolite cages, β cages, and γ cages, etc. (2) Molecular sieve cages: α cage: It is the main pore in the framework structure of Type A molecular sieves; it is a icosahedron composed of 12 tetraatomic rings, 8 hexaatomic rings, and 6 octaatomic rings. The average pore size of the cage is 1.14 nm, and the cavity volume is 7603. The largest window in the α cage is an octagonal ring with a pore diameter of 0.41 nm. Octahedral zeolite cage: It is the primary pore that constitutes the framework of X-type and Y-type molecular sieves; it is a 26-sided polyhedron composed of 18 tetrahedral rings, 4 hexagonal rings, and 4 dodecahedral rings. The average pore size of this cage is 1.25 nm, with a cavity volume of 8503. The largest pore window is a dodecahedral ring with a pore diameter of 0.74 nm. Octahedral zeolite cages are also known as supercages. β cage: It is primarily used to form the skeletal structure of A-type, X-type, and Y-type molecular sieves. It is the most important type of pore, having a shape similar to a truncated octahedron. Its cavity volume is 1603, and the pore diameter is approximately 0.66 nm; it allows only molecules of small size such as NH3 and H2O to pass through. In addition, there are hexagonal prismatic cages and gamma cages; these two types of cages have small volumes, and molecules generally cannot enter them. Cages with different structures are then connected to each other through oxygen bridges to form molecular sieves of various structures, mainly of the A-type, X-type, and Y-type. (3) Several representative molecular sieves: Type A molecular sieves have a cubic crystal structure similar to that of NaCl. If all the Na+ and Cl- ions in the NaCl lattice are replaced by β cages, and adjacent β cages are connected by γ cages, the crystal structure of the A-type molecular sieve is obtained. Eight β cages connected together form a natrolite structure; if γ cages are used for bridging, an A-type molecular sieve structure is obtained. There is a large alpha cage in the center. There is an octagonal-ring window in the channel between the α cages, with a diameter of 4 Å; hence it is called 4A molecular sieve. If 70% of the Na+ ions on the 4A molecular sieve are exchanged for Ca2+, the octagonal ring can increase to 5Å, and the resulting zeolite is called 5A molecular sieve. Conversely, if 70% of the Na+ is exchanged for K+, the pore size of the octahedral structure decreases to 3 Å, and the resulting zeolite is called 3A molecular sieve. X-type and Y-type molecular sieves have a hexagonal close-packed structure similar to that of diamond. If β-cages are used as structural units to replace the carbon atom nodes of diamond, and hexagonal prismatic cages are employed to connect two adjacent β-cages – that is, 4 hexagonal prismatic cages are used to bind together 5 β-cages, with one β-cage at the center and the remaining 4 at the vertices of a regular tetrahedron – an octahedral zeolite-type crystal structure is formed. By continuing to link in this structure, X-type and Y-type molecular sieve structures are obtained. In this structure, the larger cages formed by β-cages and hexagonal prismatic cages are octahedral zeolite cages; their interconnected windows are dodecagonal rings, with an average effective pore size of 0.74 nm, which corresponds to the pore size of X-type and Y-type molecular sieves. The main difference between these two models lies in their Si/Al ratio, which is 1–1.5 for the X-type ； The Y-type is 1.5~3.0. Mordenite-type molecular sieve: This type of zeolite has a layered structure rather than a cage structure. The structure contains a large number of pentagonal rings that are connected in pairs, with each pair of pentagonal rings being linked to another pair via oxygen bridges. A tetrahydrocycle is formed at the junction. These structural units further connect to form a layered structure. The layers contain octagonal and dodecagonal rings; the latter are oval in shape with an average diameter of 0.74 nm, and they constitute the main pore channels of mordenite. Such channels are one-dimensional, that is, straight channels. High-silica zeolites of the ZSM (Zeolite Socony Mobil) type are molecular sieves. There is a series of such zeolites, with ZSM-5 being the one that is widely used; ZSM-8 and ZSM-11 have similar structures to it ； The other group includes ZSM-21, ZSM-35, and ZSM-38, etc. ZSM-5 is often referred to as a high-silica zeolite, with a Si/Al ratio that can exceed 50; in the case of ZSM-8, this ratio can reach 100. These molecular sieves also exhibit hydrophobic properties. Their structural units are similar to mordenite, consisting of paired pentagonal rings; they have no cage-like cavities, only channels. ZSM-5 has two sets of intersecting channels, one being straight and the other zigzag-shaped and perpendicular to each other, both formed by decagonal rings. The channel is oval in shape, with a window diameter of (0.55–0.60) nm. Other zeolites belonging to the high-silica group include the all-silica type Silicalite-1, which has a structure similar to that of ZSM-5, and Silicalite-2, which has a structure similar to that of ZSM-11.