Molecular Sieve Structure And Common Types

- Sep 09, 2019-

Definition of molecular sieve

Chinese name: molecular sieve English name: molecular sieve

Definition: A natural or synthetic chemical with a network structure. For example, cross-linked dextran, zeolite, etc., when used as a chromatographic medium, the mixture can be fractionated by molecular size.

Subject: Biochemistry and Molecular Biology (a subject); methods and techniques (two subjects)

Molecular sieve concept

In a narrow sense, China Molecular Sieve is a crystalline silicate or aluminosilicate composed of a silicon oxytetrahedron or an aluminoxy tetrahedron connected by an oxygen bridge to form a molecular sieve molecular size (usually 0.3 to 2.0 nm). The pores and cavity system have the characteristics of sieving molecules. However, with the deepening of molecular sieve synthesis and application research, researchers have discovered aluminophosphate molecular sieves, and the molecular framework of the molecular sieve (silicon or aluminum or phosphorus) can also be from B, Ga, Fe, Cr, Ge, Ti, V. Substituting Mn, Co, Zn, Be and Cu, the size of the pores and cavities can reach more than 2 nm. Therefore, the molecular sieve can be divided into silica-alumina molecular sieve, phosphorus-aluminum molecular sieve and skeleton heteroatomic molecular sieve according to the skeleton element composition. According to the size of the channel, molecular sieves with pore sizes less than 2 nm, 2 to 50 nm and greater than 50 nm are called microporous, mesoporous and macroporous molecular sieves, respectively. Due to its large pore size, it becomes a good carrier for larger size molecular reactions, but the pore walls of mesoporous materials are amorphous, which makes its hydrothermal stability and thermal stability still not meet the demanding conditions required for petrochemical applications. .

Due to the inclusion of metal ions with lower ionic radii and water of mixed state, water molecules are continuously lost after heating, but the crystal skeleton structure is unchanged, and many cavities of the same size are formed, and the cavity has many diameters. The same micropores are connected. These tiny pores have a uniform diameter and can adsorb molecules smaller than the pore diameter into the interior of the pores, and the molecules larger than the pores are repelled, so that molecules having different diameters and diameters can be Molecules with different polarities, molecules with different boiling points, and molecules with different degrees of saturation are separated, that is, they have the function of "sieving" molecules. At present, molecular sieves are widely used in industries such as metallurgy, chemical, electronics, petrochemical, and natural gas.

Common molecular sieve

Molecular sieve models commonly used in the gas industry;

Type A: potassium A (3A), sodium A (4A), calcium A (5A),

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

Y type:, sodium Y, calcium Y, ZSM35 Molecular Sieve

         Refrigerant Molecular Sieve factory

        China CMS Carbon Molecular Sieve

Molecular sieve characteristics

Molecular sieves are extremely hygroscopic and are used for the purification of gases. Avoid direct exposure to air during storage. Molecular sieves that have been stored for a long period of time and have been hygroscopically hydrated should be regenerated before use. Molecular sieves avoid oil and liquid water. Avoid contact with oil and liquid water when using. The gas to be dried in industrial production is air, hydrogen, oxygen, nitrogen, argon, etc. Two adsorption dryers are connected in parallel, one is working, and the other can be regenerated. Work alternately and regenerate to ensure continuous operation of the equipment. The dryer is operated at 8-12 ° C and is regenerated by heating to 350 ° C. The molecular sieve regeneration temperatures of different specifications are slightly different. Molecular sieves have a good catalytic effect on certain organic gas phase reactions.

Also known as zeolite or zeolite, it is a crystalline aluminosilicate with a regular and uniform pore structure in the crystal structure. The pore size is on the molecular sieve size of the molecular size. It only allows molecules with a smaller diameter than the pore size to enter. It is therefore possible to sieve the molecules in the mixture by size. Therefore, it is called molecular sieve. As early as more than 200 years ago, B. Kronstedt first named the aluminosilicate as zeolite, the chemical composition of which M and n are metal ions and their valence; x is silica The number of molecules; 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 substances adsorbed by them can be desorbed, and the molecular sieves can be regenerated after use. It is also used for the drying, purification, separation and recovery of gases and liquids. Since the 1960s, as a cracking catalyst in the petroleum refining industry, a variety of molecular sieve catalysts have been developed for different catalytic processes.

Molecular sieve production method

China 5A Molecular Sieve has hydrothermal synthesis, hydrothermal conversion and ion exchange methods:

1 Hydrothermal synthesis For the production of higher purity products, as well as the synthesis of molecular sieves that are not found in nature. A silicon-containing compound (water molecular sieve glass, silica sol, etc.), an aluminum-containing compound (hydrated alumina, aluminum salt, etc.), a base (sodium hydroxide, potassium hydroxide, etc.) and water are mixed in an appropriate ratio in an autoclave. Heating for a certain period of time, that is, the molecular sieve crystals are precipitated. The synthesis process can be expressed as:

Na-molecular sieves are generally synthesized in industrial processes, such as the synthesis of 13X and 10X molecular sieves (see figure). The addition of certain additives during the hydrothermal synthesis can change the structure of the final product, such as the addition of quaternary ammonium salts to give ZSM-5 molecular sieves.

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

3 Ion exchange method The Na-molecular sieve is usually converted into a molecular sieve containing a desired cation in an aqueous solution, and the general formula is as follows:

Wherein Z- represents an anion skeleton, Me+ represents a cation to be exchanged, such as NH嬃, Ca2+, Mg2+, Zn2+, etc., and the raw material is usually a hollow glass molecular sieve chloride, a sulfate, a nitrate. The difficulty of exchanging cations of different properties in the solution onto the molecular sieve is different, which is called the selection order of the cations by the molecular sieve. For example, the order of selection of the 13X molecular sieve is Ag+, Cu2+, H+, Ba2+, Au3+, Th4+, Sr2+, Hg2+, Cd2+, Zn2+, Ni2+, Ca2+, Co2+, NH嬃, K+, Au2+, Na+, Mg2+, Li+. The following parameters are commonly used to indicate the exchange result: the degree of exchange, that is, the percentage of Na+ exchanged in the molecular sieve in the original Na+; the exchange capacity, the number of milligrams of cation exchanged per 100 g of molecular sieve; the exchange efficiency, indicating the exchange of cations in the solution The mass percentage on the molecular sieve. In order to prepare a suitable molecular sieve catalyst, it is fashionable to formulate the exchanged product with other components, which may be other catalytically active components, cocatalysts, diluents or binders, etc., and the formulated materials are formed. Activation of the catalyst can be carried out.

Molecular sieve structure

Molecular Sieve factory mentioned that The proteoglycan polymer thus constituted is zigzagly wound to form a microporous sieve-like structure called a molecular sieve. Molecular sieves only allow passage of substances smaller than their micromolecular sieve pores, and have a barrier effect on macromolecular substances, bacteria and the like larger than their micropores. Make the matrix a defensive barrier that limits the spread of harmful substances such as bacteria. Hemolytic streptococci and cancer cells can produce hyaluronidase, decompose proteoglycans, destroy matrix structure, and spread. The proteoglycan polymer also binds a number of hydrophilic groups and can bind a large number of water molecules to form an extracellular "reservoir".

Introduction to molecular sieves

3A Molecular Sieve factory told us that molecular sieve is a cubic aluminosilicate compound. The molecular sieve has a uniform microporous structure, and its pore diameter is uniform. These pores can adsorb molecules smaller than its diameter into the interior of the pore cavity, and have preferential adsorption ability for polar molecules and saturated molecules, thereby enabling polarity. Different degrees, different degrees of saturation, molecules with different molecular sizes and different boiling points are separated, that is, they have the function of "sieving" molecules, so they are called molecular sieves. Because molecular sieves have the advantages of high adsorption capacity, strong thermal stability and other adsorbents, molecular sieves have been widely used.