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Application of activated alumina and molecular sieve

2021-02-05
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Activated alumina and molecular sieves are solid materials with internal hollow pore structure, which have the characteristics of large specific surface area and adjustable pore size. Both have good adsorption capacity and are commonly used adsorbents in industrial production. Then we use an article to introduce the application of activated alumina and molecular sieve.


1. Application of activated alumina


Activated alumina (γ-Al2O3) is a porous, high-dispersion solid material with the characteristics of large surface area, good absorption performance, surface acidity, and good thermal stability. Activated alumina is a transitional alumina. It is powdery, spherical or columnar white solid. Its crystal structure is different from industrial alumina. Activated alumina belongs to the tetragonal crystal system, and the structure of the crystal lattice is very similar to that of spinel. The crystals of activated alumina are disordered. This disorder is mainly determined by the disorder of aluminum atoms. It is precisely because of the disorder of aluminum atoms: controlling its preparation conditions, a variety of different specific surface areas and pores can be obtained. Therefore, it is widely used in adsorption drying and catalysis field.


Activated alumina pore distribution


1. Application of activated alumina in adsorption drying


Activated alumina adsorption drying equipment


The main industrial applications of activated alumina as adsorbents include gas drying, liquid drying, water purification, selective adsorption in the petroleum industry, and chromatography.


Activated alumina drying gas mainly includes: acetylene, cracked gas, coke oven gas, hydrogen, oxygen, air, ethane, hydrogen chloride, propane, ammonia, ethylene, hydrogen sulfide, propylene, argon, methane, sulfur dioxide, carbon dioxide, Natural gas, helium, nitrogen, chlorine, etc.


Activated alumina drying liquid mainly includes: aromatic hydrocarbons, polymer olefins, gasoline, kerosene, cyclohexane, propylene, butene and many halogenated hydrocarbons. When these liquids are in contact with alumina, the two will not react or polymerize. At the same time, there are components in the dry liquid that are easily adsorbed on the alumina surface and are not easily removed during regeneration.


In terms of water purification and adsorption, activated alumina is mainly used to remove fluoride in drinking water, and it is also effective in eliminating the color and odor of industrial sewage. In addition, activated alumina is also widely used in the recovery and selective adsorption of carbohydrates and the maintenance of power system oil.


2. Application of activated alumina in catalysts and carriers


(1) Activated alumina is used as a catalyst carrier. In a simple catalytic reaction, activated alumina does not directly participate in the catalytic process. Its role is to dilute, support and disperse precious metals. More than 70% of activated alumina is used as a catalyst carrier. In addition to the above functions, activated alumina also has the function of enhancing thermal and mechanical stability in some reactions, such as pd/Al2O3, Cu/ The catalysts for r-Al2O3 and petroleum cracking reactions belong to this type. The nickel-supported activated alumina catalyst used in the hydrogenation of olefins has a larger thermal stability range than that of the diatomite-supported nickel catalyst.


(2) It is used as an active catalyst, the adsorbent characteristics of activated alumina, and it can activate many bonds, such as HH bonds, CH bonds, etc. Therefore, it can be directly used as an active catalyst in reactions such as hydrocarbon cracking and alcohol dehydration to ether. Add to the reaction system. Such as ethanol dehydration to produce ethylene, because of the presence of acidic centers and basic centers on the surface of activated alumina. Therefore activated alumina itself is a kind of catalyst. However, there are not many activated aluminas directly used as industrial catalysts.


(3) Used as an active component, some catalytic reactions require the catalyst to have dual functions, not only the active center provided by the active component, but also the acid and alkali center provided by the carrier, such as the catalytic reforming reaction of gasoline fractions. The composition of the catalyst consists of 0.35% pb deposited on high-purity activated alumina with a surface area of 200 cm2/g, and 1% chloride is added to increase its acidity. For the cracking reaction of n-heptane, chlorinated activated alumina with a sodium content of less than 50×10-6 is selected, and 1% fluorine is added, and the cracking activity reaches 66%. This is because activated alumina has four active centers. Therefore, activated alumina can provide active components for a variety of catalytic reactions.


The application of activated alumina is mainly based on the catalyst carrier, and the fields involved are: organic chemical, petrochemical, polymer chemistry, etc. Therefore, the market for activated alumina is very extensive.


2. Application of molecular sieve


Synthetic molecular sieve materials are generally powder, spherical particles or columnar. When natural minerals are used as raw materials or impurities are mixed in the synthesis process, the resulting molecular sieve product will have a slight color, such as 13X molecular sieve, because it is added in the production process. Attapulgite natural minerals are pale yellow. Molecular sieve is a porous material, with uniform pore distribution (this is the difference from activated alumina), large specific surface area and large pore volume and other physical properties, making molecular sieve materials have important application value in adsorption, separation and other fields.


Molecular sieve structure


1. Application of molecular sieve in adsorption separation


Microporous molecular sieve materials are widely used as silica alumina zeolite molecular sieve. Early zeolite molecular sieves are found in the form of natural minerals, and their applications are also in adsorption, ion exchange and gas separation. For example, the pore diameter of A-type zeolite is 0.4nm (4A molecular sieve), which is just between O2 and N2 (O2 is 0.38nm×0.28nm, N2 is 0.4nm×0.32nm), therefore, A-type zeolite membrane The separation of nitrogen and oxygen in the air has a very good effect. The b-axis directional growth of MFI molecular sieve can realize the separation of diisomers (ortho-two and pair-two) with a dynamic radius difference of less than 0.1nm. If the Na+ and Cl- in the NaCl lattice are replaced with β cages, and the adjacent β cages are connected with γ cages, the crystal structure of A-type molecular sieve is obtained. After 8 β cages are connected, a sodalite structure is formed. If the γ cage is used as a bridge connection, a type A molecular sieve structure is obtained. There is a large alpha cage in the center. The channel between the alpha cages has an eight-membered ring window with a diameter of 4A, so it is called 4A molecular sieve. If 70% of the Na+ on the 4A molecular sieve is Ca2+ exchange, the eight-membered ring can be increased to 5A, and the corresponding zeolite is called 5A molecular sieve. Conversely, if 70% of Na+ is K+ exchanged, the pore size of the eight-membered ring is reduced to 3A, and the corresponding zeolite is called 3A molecular sieve.


2. Application of molecular sieve in catalytic reaction.


Since the successful application of Y-type zeolite in the catalytic cracking reaction of alkanes, zeolite crystal materials have been rapidly developed and widely concerned in the fields of petroleum refining and petrochemical industry. Y-type zeolite has also become an industrialized fluidized catalytic cracking (FCC) catalyst. The acid density and acid strength of molecular sieve can be controlled by adjusting the content of framework aluminum or using heteroatoms Ga and B to replace Al. As an important type of solid acid catalyst, silica alumina zeolite molecular sieve makes up for the shortcoming that the catalyst and reactants cannot be separated in the homogeneous catalytic reaction. It has been used in many acid-catalyzed reactions, such as catalytic cracking, hydrocracking, and isomerization. , Methanol to olefins (MTO) should wait. In addition, the introduction of transition metal elements into the framework of the zeolite molecular sieve also realizes its application in redox reactions. A typical example is the synthesis of titanium silicate molecular sieve. The synthesized titanium-containing molecular sieve exhibits a high degree of catalysis in the catalytic oxidation reaction. Active, such as Ts-1, Ti-Beta, Ti-MWW and other molecular sieves have been widely used in phenol hydroxylation, olefin epoxidation, alkane oxidation and other reactions. Zeolite molecular sieves can be used to degrade nitrogen oxide gas discharged from automobiles and acid plants after Fe3 and Cu2+ exchange, which shows value in environmental protection and purification. As a catalyst or catalyst carrier, compared with other materials, zeolite molecular sieves have relatively obvious advantages: high thermal stability and hydrothermal stability make the catalytic reaction can be carried out under harsh conditions, and the regular pore structure can realize the effect of certain products. Its high selectivity and adjustable active center enable it to be used in various reactions.


Mesoporous molecular sieves have a highly ordered and uniformly distributed pore structure, a wide range of pore size adjustment (2~5nm), a large specific surface area (greater than 1000m2/g), and the diversity of their skeleton components. Many fields have good application prospects and have become a new hot spot in the field of porous materials research. At the beginning, mesoporous molecular sieves were mainly used in catalytic reactions. Later, researchers used its pore characteristics to expand and gradually developed into the fields of nanomaterials and biological adsorption and separation. Recent studies are mainly based on previous studies, modified mesoporous molecular sieves to load functional groups and metal ions, for applications in many fields.


Molecular sieve catalytic reaction tower


From the application of activated alumina and molecular sieves, we can clearly realize that they are widely used in adsorption, drying, and gas separation in industrial production, but the most important thing is their application in the field of catalytic reactions. Therefore, the two functions complement each other, and their application in the production of chemical energy is important.

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