Molecular sieve concept

Molecular sieve, also known as zeolite or zeolite, is a crystalline aluminosilicate with regular and uniform pores in its crystal structure. The pore size is of the order of several molecular sieves of molecular size. It only allows molecules with a smaller diameter than the pore size to enter. Therefore, the molecules in the mixture can be screened according to the size, so it is called molecular sieve. As early as more than 200 years ago, B Cronstett was the first to name aluminosilicate as zeolite, with a chemical composition of (m) 2/nO · Al2O3 · xSiO2 · pH2O. In the formulaM and n are metal ions and their valence numbers; x is the number of molecules of silicon dioxide; y is the number of molecules of water; p is the number of atoms of aluminum; q is the number of atoms of silicon. Molecular sieves are used as solid adsorbents in the chemical industry, and the adsorbed substances can be desorbed, and the molecular sieves can be regenerated after use. It is also used for drying, purification, separation and recovery of gases and liquids. Since the 1960 s, it has been used as a cracking catalyst in the petroleum refining industry, and now a variety of molecular sieve catalysts suitable for different catalytic processes have been developed.

In a narrow sense, the molecular sieve is a crystalline silicate or aluminosilicate, which is formed by a silicon-oxygen tetrahedron or an aluminum-oxygen tetrahedron connected by an oxygen bridge bond to form a molecular sieve molecular size (usually 0.3-2.0nm) pore and cavity system, thus having the characteristic of sieving molecules. However, with the in-depth research on the synthesis and application of molecular sieves, researchers have found that aluminophosphate molecular sieves, and the framework elements (silicon or aluminum or phosphorus) of molecular sieves can also be replaced by B, Ga, Fe, Cr, Ge, Ti, V, Mn, Co, Zn, Be and Cu, and the size of their pores and cavities can reach more than 2nm, therefore, molecular sieves can be divided into silicon-aluminum molecular sieves, phosphorus-aluminum molecular sieves and framework heteroatom molecular sieves according to the composition of framework elements. According to the pore size, molecular sieves with pore size less than 2nm, 2 ~ 50nm and greater than 50nm are called microporous, mesoporous and macroporous molecular sieves, respectively. Because of its large pore size, it becomes a good carrier for the reaction of larger size molecules, but the pore wall of mesoporous materials is amorphous, so its hydrothermal and thermal stability can not meet the harsh conditions required for petrochemical applications.

Because it contains metal ions with lower electricity price and larger ionic radius and combined water, water molecules are continuously lost after heating, but the crystal skeleton structure is unchanged, forming many cavities with the same size, and the cavities are connected with many micropores with the same diameter. These tiny holes have uniform diameter and can adsorb molecules smaller than the diameter of the channel into the interior of the holes, the molecules larger than the pore are excluded, so that molecules with different shapes and diameters, molecules with different degrees of polarity, molecules with different boiling points and molecules with different degrees of saturation can be separated, I .e.The role of "sieve" molecules, so called molecular sieve. At present, molecular sieves are widely used in metallurgy, chemical industry, electronics, petrochemical industry, natural gas and other industries.

commonly used molecular sieve

Types of molecular sieves commonly used in adsorption industry:

Type A: Potassium A(3A), Sodium A(4A), CalciumA(5A)

Type X: Calcium X(10X), SodiumX(13X)

Types of molecular sieves commonly used in the catalytic industry:

Y type: sodium Y, hydrogen Y

ZSM-5、BETA、MOR、MWW、FER等。

Molecular sieve characteristics

  Molecular sieve has two kinds of natural zeolite and synthetic zeolite.① Natural zeolite is mostly formed by the reaction of volcanic tuff and tuffaceous sedimentary rocks in marine or lacustrine environment. At present, more than 1000 kinds of zeolite ore have been found, 35 of which are more important, including clinoptilolite, mordenite, erionite and chabazite. It is mainly distributed in the United States, Japan, France and other countries. A large number of mordenite and clinoptilolite deposits are also found in China. Japan is the country with the largest amount of natural zeolite mining. Because natural zeolite is limited by resources, synthetic zeolite has been widely used since the 1950 s.

A, X type molecular sieve moisture absorption ability is very strong, used for gas purification treatment, storage should avoid direct exposure to the air. The molecular sieve that has been stored for a long time and has absorbed moisture should be regenerated before use. Molecular sieve bogey oil and liquid water. Use should try to avoid contact with oil and liquid water. Drying gas in industrial production: air, hydrogen, oxygen, nitrogen, argon and so on. Two adsorption dryers are connected in parallel, one can work while the other can be regenerated. Alternate work and regeneration to ensure continuous operation of the equipment. The dryer was operated at 8-12°C and gas regeneration was carried out by heating to 350°C. Different specifications of molecular sieve regeneration temperature is slightly different. Molecular sieves have good catalytic effect on some organic gas phase reactions.

The molecular sieve is a powder crystal with metallic luster, hardness of 3~5, relative density of 2~2.8, natural zeolite has color, synthetic zeolite is white, insoluble in water, thermal stability and acid resistance increase with the increase of SiO2/Al2O3 composition ratio. Molecular sieve has a large specific surface area, up to 300~1000 m2/g, the inner crystal surface is highly polarized, for a class of efficient adsorbent, but also a class of solid acid, the surface has a high acid concentration and acid strength, can cause positive carbon ion type catalytic reaction. When the metal ions in the composition are exchanged with other ions in the solution, the pore size can be adjusted to change its adsorption and catalytic properties, thereby preparing molecular sieve catalysts with different properties.

Molecular sieve production method

There are hydrothermal synthesis, hydrothermal conversion and ion exchange methods:

The hydrothermal synthesis method is used to produce high purity products and to synthesize molecular sieves that do not exist in nature. The silicon-containing compound (water molecular sieve glass, silica sol, etc.), aluminum-containing compound (hydrated alumina, aluminum salt, etc.), alkali (sodium hydroxide, potassium hydroxide, etc.) and water are mixed according to an appropriate ratio, and heated in a autoclave for a certain period of time, that is, the molecular sieve crystal is precipitated.

In the industrial production process, Na-molecular sieves are generally synthesized first, such as the synthesis of 13X and 10X molecular sieves. The addition of certain additives in the hydrothermal synthesis process can change the structure of the final product, such as the addition of quaternary ammonium salts can be obtained.ZSM-5 molecular sieve.

Hydrothermal conversion method in the presence of excess alkali, the solid aluminosilicate hydrothermal conversion into molecular sieve. The raw materials used are kaolin, bentonite, diatomite, etc., and synthetic silica-alumina gel particles can also be used. This method has low cost, but the product purity is not as good as the hydrothermal synthesis method.

③ Ion exchange method usually converts Na-molecular sieve into molecular sieve containing required cations in aqueous solution, such as NH4-, Ca2, Mg2, Zn2, etc. The raw materials are usually chloride, sulfate and nitrate. The degree of difficulty in exchanging cations with different properties in solution to molecular sieves is called the selection order of cations by molecular sieves. For example, the selection order of 13X molecular sieves is Ag, Cu2, H, Ba2, Au3, Th4, Sr2, Hg2, Cd2, Zn2, Ni2, Ca2, Co2, NH, K, Au2, Na, Mg2, Li. The following parameters are commonly used to express the exchange results: exchange degree, that is, the percentage of the amount of Na exchanged in the original amount of Na in the molecular sieve; Exchange capacity, which is the number of milligrams of cations exchanged per 100 grams of molecular sieve; Exchange efficiency, which indicates the mass percentage of cations in solution exchanged to molecular sieve. In order to prepare a suitable molecular sieve catalyst, it is necessary to prepare the product obtained by the exchange with other components, which may be other catalytically active components, cocatalysts, diluents or binders, etc., and the prepared material can be activated by molding.

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