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Application of activated alumina and molecular sieve

2022-04-19 09:19:11

Both 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 will use an article to introduce the application of activated alumina and molecular sieve.

First, the application of activated alumina

Activated alumina (γ-Al2O3) is a solid material with high porosity and high dispersity. It has the characteristics of large surface area, good adsorption performance, surface acidity and good thermal stability. Activated alumina belongs to transition form alumina. For powder, spherical or columnar white solid. Its crystal structure is different from that of industrial alumina. Activated alumina belongs to the tetragonal crystal system, and the structure of the lattice is very similar to that of spinel. The crystal of activated alumina is disordered. This disorder is mainly determined by the disorder of aluminum atoms. It is because of the disorder of aluminum atoms: controlling its preparation conditions, a variety of specific surface areas and pores can be obtained. It is a high-capacity activated alumina product, so it is most used in adsorption drying and catalysis.

1. Application of activated alumina in adsorption and 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 chromatographic separation processes.

Activated alumina drying gases mainly include: acetylene, cracking 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.

The dried liquids of activated alumina mainly include: 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, and at the same time, the dried liquid does not contain components that are easily adsorbed on the surface of alumina and are not easily removed during regeneration.

In terms of water purification and adsorption, in addition to being mainly used to remove fluoride in drinking water, activated alumina is also very 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 oils.

2. Application of activated alumina in catalysts and carriers

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

(2) Used as an active catalyst, activated alumina has obvious adsorbent characteristics and can activate many bonds, such as H-H bonds, C-H bonds, etc., so

In hydrocarbon cracking, alcohol dehydration to ether and other reactions, it can be directly added to the reaction system as an active catalyst. For example, ethanol is dehydrated to produce ethylene, due to the presence of both acidic and basic centers on the surface of activated alumina. Therefore, activated alumina itself is an excellent catalyst. However, there are not many active aluminas directly used as industrial catalysts at present.

(3) 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 centers provided by the carrier, such as the catalytic reforming reaction of gasoline fractions, The composition of the catalyst consisted of 0.35% Pb deposited on high-purity activated alumina with a surface area of 200 cm2/g, and 1% chloride was 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% of fluorine is added, and its cracking activity reaches 66%. This is due to the existence of four active centers in activated alumina. Therefore, activated alumina can provide active components for various catalytic reactions.

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

Second, the application of molecular sieve

The synthesized molecular sieve materials are generally powder, spherical or columnar. When natural minerals are used as raw materials or impurities are mixed in the synthesis process, the obtained molecular sieve product will be slightly colored, such as 13X molecular sieve, because it is added in the production process. Attapulgite natural minerals are pale yellow. Molecular sieves are porous materials with physical properties such as uniform pore distribution (which is the most fundamental difference from activated alumina), large specific surface area and large pore volume, making molecular sieve materials useful in adsorption, separation and other fields. value.

1. Application of molecular sieve in adsorption and separation

The most widely used microporous molecular sieve material is the silica-alumina zeolite molecular sieve. The earliest zeolite molecular sieves were discovered in the form of natural minerals, and their applications were limited to adsorption, ion exchange and gas separation. For example, the pore diameter of A-type zeolite is 0.4nm (4A molecular sieve), which is exactly between O2 and N2 (O2 is 0.38nm × 0.28nm, N2 is 0.4nm × 0.32nm), therefore, A-type zeolite membrane is suitable for The separation of nitrogen and oxygen in the air has a very good effect, and the b-axis directional growth of MFI molecular sieve can realize the separation of xylene isomers (o-xylene and p-xylene) with a kinetic radius difference of less than 0.1 nm. If all 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 the A-type molecular sieve can be obtained. After 8 β cages are connected, a sodalite structure is formed. If the γ cage is used as a bridge connection, the A-type molecular sieve structure is obtained. There is a large alpha cage in the center. There is an eight-membered ring window in the channel between the α cages, and its diameter is 4A, so it is called 4A molecular sieve. If 70% of Na+ on 4A molecular sieve is exchanged for Ca2+, the eight-membered ring can be increased to 5A, and the corresponding zeolite is called 5A molecular sieve. On the contrary, if 70% of Na+ is exchanged for K+, 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 initial successful application of Y-type zeolite in alkane catalytic cracking reaction, zeolite crystal materials have received rapid development and widespread attention in petroleum refining, petrochemical and other fields, and Y-type zeolite has also become an industrialized fluidized catalytic cracking (FCC) catalyst. The acid density and acid strength of molecular sieves can be regulated by adjusting the content of framework aluminum or using heteroatoms Ga, B, etc. to replace Al. As an important class of solid acid catalysts, silica-alumina zeolite molecular sieves make up for the disadvantage that catalysts and reactants cannot be separated in homogeneous catalytic reactions, and have been used in many acid-catalyzed reactions, such as catalytic cracking, hydrocracking, isomerization , methanol to olefins (MTO) should be and so on. In addition, by introducing transition metal elements into the framework of zeolite molecular sieves, its application in redox reactions has also been realized. The most typical example is the synthesis of titanium-silicon molecular sieves. The synthesized titanium-containing molecular sieves show quite high performance in catalytic oxidation reactions. Molecular sieves such as Ts-1, Ti-Beta, Ti-MWW, etc. have been widely used in phenol hydroxylation, olefin epoxidation, alkane oxidation and other reactions. After the exchange of Fe3 and Cu2+, zeolite molecular sieve can be used to degrade nitrogen oxide gas discharged from automobiles and acid-making plants, showing certain value in environmental protection and purification. As a catalyst or catalyst carrier, compared with other materials, zeolite molecular sieve has relatively obvious advantages: high thermal stability and hydrothermal stability enable the catalytic reaction to be carried out under harsh conditions, and the regular pore structure can realize a certain product. The high selectivity and tunable active center make it suitable for various reactions.

Mesoporous molecular sieves have the characteristics of highly ordered and uniformly distributed pore structure, wide pore size modulation range (2~5nm), large specific surface area (greater than 1000m2/g), and diversity of framework components. Many fields have good application prospects and have become a new hot spot in the field of porous materials research. Initially, mesoporous molecular sieves were mainly used in catalytic reactions, and later researchers continued to use their pore characteristics to expand, and successively developed into the fields of nanomaterials and biological adsorption and separation. The recent researches are mainly based on the previous research, the mesoporous molecular sieves are modified to support functional groups and metal ions to be applied in many fields.

Molecular sieve catalytic reaction tower

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


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