Application of Alumina in the Field of Catalysis
Alumina is one of the most commonly used industrial raw materials and has extremely important applications in ceramics, refractory materials, medicine, catalysis and other fields. Because alumina has the advantages of good mechanical strength, good thermal and chemical stability, suitable isoelectric point, adjustable surface acidity and alkalinity, and many different crystal phase structures, alumina has become a chemical industry And the most widely used catalyst carrier in the petroleum industry, in the reaction process of cracking of petroleum components, hydrorefining, hydrodesulfurization, reforming of hydrocarbons, purification of gas phase oil components, purification of automobile exhaust gas, etc. Both play an important role.
Catalyst carrier morphology
There are many structural forms of alumina. We know that ceramic materials are easy to form structural variants. In the crystal structure, the phenomenon of the same chemical composition but different crystal structure is often called homogeneity and polymorphism, also known as polymorphism. And alumina ceramics have more than 8 isomorphs, for example, α-Al2O3, γ-Al2O3, η-Al2O3, θ-Al2O3 and so on. After careful investigation of the most commonly used anomalies, it is generally believed that γ-Al2O3 is the alumina with the most development potential. Compared with other forms of alumina, γ-Al2O3 is a porous, highly dispersed solid material with the characteristics of large specific surface area, good adsorption performance, surface acidity and good thermal stability. In addition, the preparation conditions can be controlled to produce a variety of γ-Al2O3 products with different specific surface areas and pore volumes, so it is often called "activated alumina", not only in alumina-based supports, but also in the entire field of catalyst supports. Wide variety.
1. Use adsorption as an active catalyst
Activated alumina has obvious adsorbent characteristics, such as H-H bond, C-H bond, etc. Therefore, it can be directly added as an active catalyst to the reaction system in the reaction of hydrocarbon cracking and alcohol dehydration to ether. For example, the dehydration of ethanol to produce ethylene, because there are both acidic centers and basic centers on the surface of activated alumina, activated alumina itself is a good catalyst. However, there are not many activated aluminas directly used as industrial catalysts at present.
2. As a catalyst carrier
Alumina carrier refers to a white powdered or shaped alumina solid. It is one of the most widely used catalyst carriers, accounting for about 70% of industrial supported catalysts. The commonly used shapes of the catalyst carrier include: columnar, ring-shaped, spherical, sheet-like, granular, extruded, and so on. The process can also be roughly divided into two categories. To put it simply, one is: firstly, the activated alumina is made into the required shape and then the catalyst component is loaded by soaking; the second: the activated alumina is now combined with the catalyst component After mixing, it is made into the required shape by an instrument to form a new type of catalyst. Generally it is divided into the following categories:
(1) High-temperature alumina carrier: this type of alumina has a small specific surface area, has high temperature resistance, chemical resistance and high mechanical strength, so it can withstand harsh operating conditions. Due to the inertness of alumina, the high-temperature alumina carrier will not become a potential source of activity that causes side reactions and a decrease in selectivity, nor will it become a potential source of toxicity for the catalyst system.
(2) Interaction type carrier: This type of alumina is the most widely used. It can react with the active components of the catalyst to disperse the active components of the catalyst into the carrier, and provide effective specific surface area and suitable pore structure for the active components. To improve the thermal stability and anti-toxicity of the catalyst.
(3) Synergistic or dual-functional carrier: In addition to acting as the framework of the active component, this type of alumina also provides a gain effect for the catalytic effect of the catalyst.
1. Application in automobile exhaust catalyst carrier
The main pollutants emitted by automobile exhaust are: CO, NOx, CxHy and Pb, etc. After NOx and hydrocarbons are irradiated by strong ultraviolet rays, they will also produce new secondary pollution—photochemical smog, which is directly or directly caused by photochemical smog. The indirect economic losses are huge. An important way to avoid the production of photochemical smog is to carry out pre-treatment of automobile exhaust. At present, the best method is to install a catalytic device on the exhaust pipe of the automobile to convert unsaturated hydrocarbons and nitrogen oxides into saturated compounds. Alumina is an excellent carrier for the catalyst in this conversion process.
2. Application in oxalate synthesis catalyst
The key technology of CO gas-phase coupling synthesis of oxalate is the development of high-efficiency catalysts. Alumina is one of the most widely used catalyst support materials in this reaction. The preparation of a suitable alumina carrier is the core performance of the catalyst for the development of CO gas phase coupling synthesis of dimethyl oxalate. Excellent alumina can improve the activity and selectivity of the catalyst.
3. Application in catalytic cracking (FCC) catalyst
Activated alumina is an important matrix material. As a solid acid added to the cracking catalyst matrix, it can not only improve the activity of the matrix, but also make full use of the binding effect on kaolin and molecular sieve, and the preparation has matrix activity and good anti-wear The catalyst of sex.