Catalyst support is a special part of solid. It is the dispersant, binder and carrier of its active components. Sometimes it also plays the role of cocatalyst or cocatalyst. It is also called support, which is one of the components of load type. It is usually a porous material with a certain specific surface area, and its active components are often attached to it. The carrier is mainly used to support the active components and make them have specific physical properties, while the carrier itself generally does not have catalytic activity. It has a large content in the catalyst component.
requirement:
1. The density of active components, especially precious metals, can be diluted
2. Can be prepared into a certain shape
3. Sintering between active components can be avoided to a certain extent
4. It can resist poison
5. It can interact with the active components and act together with the main catalyst.
effect
1. Reduce costs
2. Improve mechanical strength
3. Improve thermal stability
4. Increase activity and selectivity
5. Extended life
Introduction to several main carriers
1. Activated alumina: the most widely used carrier in industry. Low price, high heat resistance and good affinity of active components.
2. Silica gel: the chemical composition is SiO2. It is usually prepared by acidification of water glass (Na2SiO3). Water glass reacts with acid to form silicic acid; Silicic acid polymerizes and condenses to form polymers with uncertain structure.
SiO2 is a widely used carrier, but its industrial application is less than that of Al2O3. This is due to the disadvantages of difficult preparation, weak affinity with active components, easy sintering under the coexistence of water vapor and so on.
3. Diatomite: natural SiO2. It contains a small amount of metal oxides and organics, and its pore structure and specific surface change with the origin. Acid treatment should be used before use. First, to improve the content of SiO2 and increase the specific surface, specific pore volume and main pore radius; Second, in order to improve the thermal stability, the specific surface area can be further increased after acid treatment. Diatomite is mainly used to prepare fixed bed catalyst.
4. Activated carbon: the main component is C, containing a small amount of H, O, N, s and ash. Activated carbon has irregular stone structure, and there are carbonyl, quinone, hydroxyl and carboxyl functional groups on the surface. Activated carbon is characterized by developed pores, large surface area and high thermal stability.
5. TiO2: it has three crystal forms: anatase, brookite and rutile. Brookite is difficult to synthesize because of its instability; Anatase is formed at lower temperature, with a relative density of 3.84 and a large specific surface area; Anatase becomes rutile when heated at 600 1000 C. the relative density of rutile is 4.22 and the specific surface area is small.
6. Silicon carbide: the melting point of carbide ceramics is higher than 2000 C. It has high thermal conductivity, high hardness, strong heat resistance and impact resistance, but it is easy to be oxidized in oxygen atmosphere. Therefore, SiC is often used as the carrier of catalyst in high temperature environment.
7. Molecular sieve: it is a crystalline silicate or aluminosilicate. It is a pore and cavity system formed by silicon oxygen tetrahedron or aluminum oxygen tetrahedron connected by oxygen bridge bond. It has high thermal stability, hydrothermal stability and acid and alkali resistance.