Molecular sieve

There is a natural aluminosilicate in nature that has sieving molecules, adsorption, ion exchange and catalysis. This natural substance is called zeolite, and the synthetic zeolite is also called molecular sieve. The chemical composition of molecular sieves is: (Mn+)2/nO· Al2O3·xSiO2·pH2O, M represents metal ions (usually Na when artificial synthesis), n represents the valence of metal ions, and x represents the number of moles of SiO2, also called For the silicon to aluminum ratio, p represents the number of moles of water. The most basic structure of the molecular sieve framework is SiO4 and AlO4 tetrahedra, which are combined by a common oxygen atom to form a three-dimensional network structure. This combination form a cavity and a pore having a molecular level and a uniform pore size. Due to different structures and different forms, the "cage" shaped space holes are divided into "cage" structures such as α, β, γ, hexagonal columns, and faujasite. The crystal structures of the A, X and Y molecular sieves are shown in Figure 1. Figure 1 shows the main structure of the A-type, X-type and Y-type molecular sieves. Since the AlO4 tetrahedron has a negative charge, it can be combined with sodium plasma to become electrically neutral. In aqueous solution, Na+ is easily exchanged with other cations. Most molecular sieve catalysts are exchanges of polyvalent metal cations or H+. The molecular sieves are acidic and molecularly selective and can be used as a catalyst or carrier. Silicon and aluminum atoms form an oxygen ring through oxygen, and the size of the oxygen ring determines the pore diameter of the zeolite. The number of oxygen atoms per oxygen ring is 4 to 12. Generally, there are an eight-membered ring (0.4 to 0.5 nm), a ten-membered ring (0.5 to 0.6 nm), and a twelve-membered ring (0.7 to 0.9 nm) having a molecular sieve action. There are Y-type molecular sieves (x = 3.1 to 6.0) and mordenite (x = 9 to 11) having a twelve-membered oxygen ring. The former can be used as a cracking catalyst or a bifunctional catalyst, and the latter can be used as a disproportionation catalyst for toluene. The ten-membered oxygen ring has some ZSM series molecular sieves such as ZSM-5 and ZSM-11. The eight-membered oxygen ring has a type A molecular sieve (x=2), a T-type molecular sieve, and a ZSM-34. Their pores are small and only linear hydrocarbons can enter the pores. A catalyst having a molecular sieve as a catalytically active component or a main active component is referred to as a molecular sieve catalyst. Molecular sieves have ion exchange properties, uniform molecular size pores, excellent acid catalytic activity, and good thermal stability and hydrothermal stability. It can be made into a catalyst which has high activity and high selectivity for many reactions. Molecular sieves are widely used as solid adsorbents in the chemical industry, the petroleum industry, and other related sectors and experiments. The proper adsorption (such as heating) can desorb the adsorbed material. This process is called "regeneration" and the molecular sieve can be reused. Molecular sieves are used for the drying of gases and liquids and for the dehydration of compounds (de-crystallization water, or hydroxyl and hydrogen atoms on nearly two atoms of a compound molecule in the form of water). The molecular sieve of the appropriate pore size can be selected to adsorb specific substances in the mixture for separation or purification purposes. A catalyst having a molecular sieve as an active component is called a molecular sieve catalyst. The most widely used molecular sieve catalyst in the industry is a cracking catalyst in the petroleum refining industry. There are also catalysts that catalyze organic synthesis and other properties. Molecular sieve catalysts usually contain only 5 to 15% molecular sieves, with the remainder being the matrix. The matrix is often composed of a refractory inorganic oxide or a mixture thereof and a clay. M in the molecular sieve formula is usually Na+, K+, Ca2+, and these ions may be partially or completely replaced by metal ions having a small radius and a large charge, so that the skeleton structure is basically unchanged, but the performance of the molecular sieve is greatly affected. It has a specific catalytic property. In recent years, other atoms (such as gallium, germanium, iron, boron, phosphorus, chromium, vanadium, molybdenum and arsenic) have been introduced into the silicon-aluminum framework of the molecular sieve to replace (or partially replace) silicon or aluminum, and the hetero atom molecular sieve formed has a certain Some special catalytic properties.