As the requirements for catalysis in fields such as refining, coal chemical industry, fine chemical industry, chemical medicine, environmental catalysis, etc., continue to increase, pure alumina carriers can no longer meet the requirements of catalysts, leading to a growing interest in the research of active alumina-based composite material preparation technology. Alumina materials can be divided into alumina-based modified materials and alumina-based composite materials. The former does not change the structure of Al2O3, while the latter changes the framework structure. Alumina-based modified materials mainly change the surface properties of Al2O3 by introducing minor additives such as silica (SiO2), titanium dioxide (TiO2), rare earth elements, phosphorus, etc. The modified material still has alumina as the main body structure, and the content of modified elements generally does not exceed 10%. Alumina-based composite materials, on the other hand, introduce higher contents of additives such as silica (SiO2), titanium dioxide (TiO2), rare earth elements, etc., through specific preparation methods, resulting in significant differences in the framework structure of alumina materials compared to single alumina materials.
Both alumina-based modified materials and alumina-based composite materials possess many unique physicochemical properties, such as the ability to adjust the interaction between the carrier and the active component, change the morphology of active sites, improve the activity or selectivity of the catalyst, etc.
Alumina-based modified materials:
Effect of TiO2 modification on the properties of alumina:
Introducing TiO2 into alumina carriers can not only affect the pore structure and surface acidity of the carrier but also influence the electronic structure of the active components after loading, as well as the interaction between the active components and the carrier and the activity of the catalyst.
Effect of TiO2 modification on pore structure:
Introducing titanium into alumina materials for modification results in a decrease in specific surface area and pore volume of the modified material with the increasing amount of titanium introduced, mainly due to the occupation of titanium atoms in the pores of alumina.
Structural effects of TiO2 modification:
It can improve the interaction between metal active components and the carrier.
Electronic effects of TiO2 modification:
In hydrogenation reactions, TiO2 can act as an electron promoter, facilitating the transfer of electrons from the carrier to the metal, thus helping to generate more coordinatively unsaturated sites and enhance the hydrogenation activity of the catalyst.
Common methods of TiO2 modification:
TiO2 modification is usually introduced through methods such as similar additives or introduction of metal ions. Typical preparation methods include coprecipitation (precipitation of TO2 on γ-Al2O3, aluminum sol, etc.), vapor deposition, impregnation, etc.
Effect of SiO2 modification on the properties of alumina:
SiO2 is the most commonly used modifier for alumina. It itself has almost no acidity, but when combined with Al2O3 as an additive or when forming SiO2-Al2O3 composite oxides, it can greatly enhance the weak acidity of Al2O3 surfaces and generate Bronsted acid. In addition, it can also improve the interaction between the carrier and the active metal component.
Compared with Al2O3 materials, SiO2 materials have larger specific surface areas and weaker interactions with metal active components. Introducing SiO2 for modification of Al2O3 can help improve the dispersion of active components on the carrier. Introducing an appropriate amount of SiO2 into Al2O3 can effectively reduce the surface Al3+ of Al2O3, weakening the strong interaction between the active component and the carrier in the catalyst.
Effect of SiO2 modification on pore structure:
Introducing SiO2 into alumina can significantly increase the pore volume and pore size of alumina materials. The pore volume and specific surface area of SiO2-modified series pseudo-boehmite materials prepared by Sasol Company gradually increase as the SiO2 content increases from 1% to 10%.
Effect of SiO2 modification on surface acidity:
Different acid-base catalytic reactions require different acidic properties of materials: in isomerization, hydrogen exchange, and other reactions, catalytic active sites are concentrated in strong acid sites; in normal octane cracking, propylene polymerization, and other reactions, catalytic active sites are concentrated in weaker acid sites; while in dehydration reactions, both strong acid sites and weak acid sites can play a role. Therefore, selecting a carrier with appropriate acid strength, distribution, and type is crucial to ensuring the activity of the catalyst.
Common methods of SiO2 modification:
In the preparation of pseudo-boehmite, SiO2 can be introduced during aging by introducing inorganic salts such as sodium silicate as a silicon source, or by introducing them before aging. Since the pore structure of pseudo-boehmite has been formed before aging, whether introduced before or during aging, inorganic salts such as sodium silicate are easily in contact with pseudo-boehmite in the form of aggregates, resulting in uneven contact between SiO2 and AlO, thereby affecting the surface properties of SiO2-Al2O3 materials. Therefore, the most common way to introduce SiO2 for modification is to introduce it during the molding process, that is, mixing and molding pseudo-boehmite and amorphous silica-alumina, or molding after treating commercially available pseudo-boehmite. Using amorphous silica-alumina for modification during the molding process of reforming pre-hydrogenation catalyst carriers can significantly increase the specific surface area and surface acidity of the carrier and catalyst, and the catalyst prepared with silicon-modified alumina carrier also significantly improves the hydrodesulfurization activity.
Other modifications and their effects on alumina properties:
In order to improve the thermal stability, mechanical strength, pore structure, and surface properties of alumina, commonly used inorganic compound modifications include magnesium oxide modification, rare earth oxide modification, barium oxide modification, borate modification, phosphate modification, surfactant modification, carbon black modification, and molecular sieve modification, etc.
Rare earth oxide modification:
Alumina has at least eight crystal types, some of which are homogeneous but some are transitional, but when the temperature is above 1200°C, they all transform into the same stable final product, α-Al2O3. In the field of catalysis, in order to improve the thermal stability and catalytic activity of active alumina, it is necessary to inhibit the phase transition of alumina at room temperature. Rare earth elements have special outer electron distributions, larger ionic radii, higher melting points, and higher chemical activity. A small amount added to alumina can significantly improve the thermal stability of alumina.
Rare earth metal oxide modification can be added during the preparation of alumina carriers or after the preparation of alumina carriers is completed, such as by impregnation modification or coating modification.
Phosphorus modification:
Phosphorus, as an important additive for hydrogenation catalysts, can improve the surface electrochemical properties and surface acidity of the catalyst, reduce the rate of carbon deposition on the catalyst, and help the catalyst operate stably for long periods. Introducing phosphorus into alumina can not only change the activity of the hydrogenation catalyst but also change the selectivity of the hydrogenation catalyst.
Modification of molecular sieves
Molecular sieves possess a well-ordered crystalline structure, uniformly sized micropores, enormous specific surface area, the capability for exchange of cations with catalytic properties due to balanced framework negative charges, and the presence of special structural properties such as non-framework components that may exist within the framework structure, making molecular sieves effective catalysts and catalyst carriers. In comparison to molecular sieves, aluminum oxide, as one of the important components of catalyst carriers, exhibits characteristics such as high specific surface area, large pore volume, and a wider pore size distribution. Embedding molecular sieves into the structure of aluminum oxide enables the preparation of catalysts with superior catalytic performance
Boron modification
The effect of boron on Al2O3 carriers mainly manifests in two aspects: 1) reducing the number of strong acid centers in the catalyst and increasing the number of weak acid and medium-strong acid centers; 2) altering the structure and morphology of the Al2O3 carrier, affecting the dispersion and stacking of active components on the carrier.