Activated alumina is a non-stoichiometric alumina (Al₂O₃·nH₂O) with a high specific surface area and abundant surface hydroxyl groups. Its main crystal form is γ-Al₂O₃. Due to its excellent adsorption, catalytic activity and thermal stability, activated alumina is widely used in petrochemical, environmental protection, gas drying and catalyst carrier fields. However, its active state is affected by many factors, such as preparation process, heat treatment conditions, surface acidity, impurity content and degree of hydration. Therefore, a deep understanding of the impact of these factors on the performance of activated alumina is of great significance to optimize its industrial application.
1. Effect of preparation method on the activity of activated alumina
The preparation method of activated alumina directly affects its specific surface area, pore structure and surface chemical properties, thus determining its active state. Common preparation methods include:
(1) Sol-gel method
This method hydrolyzes aluminum salts (such as aluminum nitrate, aluminum isopropoxide) to form a sol, which is then gelled, dried and calcined to obtain γ-Al₂O₃. Activated alumina prepared by the sol-gel method usually has a high specific surface area (300–500 m²/g) and a controllable pore size distribution, which is suitable for high-activity catalyst carriers.
(2) Precipitation method
Aluminum hydroxide is precipitated by adjusting the pH value of the aluminum salt solution, and then activated alumina is obtained by washing, drying and calcining. The key control parameters of the precipitation method include the precipitant (ammonia, NaOH, etc.), pH value and aging time. Optimizing these conditions can increase the specific surface area and surface acidity of alumina.
(3) Hydrothermal method
Under high temperature and high pressure hydrothermal conditions, aluminum precursors (such as boehmite) can be converted into high-crystallinity γ-Al₂O₃. The alumina prepared by this method has high thermal stability and regular pore structure, and is suitable for high-temperature catalytic reactions.
Activated alumina obtained by different preparation methods have significant differences in specific surface area, pore structure and surface hydroxyl content, which in turn affects its adsorption and catalytic performance.
2. Effect of heat treatment conditions on the active state
Heat treatment (calcination) is a key step in regulating the structure of activated alumina, which mainly affects its crystal form, specific surface area and surface acidity.
(1) Calcination temperature
• Low temperature calcination (300–500℃): Formation of γ-Al₂O₃ with high specific surface area, rich surface hydroxyl groups, suitable for adsorption and low-temperature catalysis.
• Medium temperature calcination (500–800℃): Part of the hydroxyl groups are removed, the specific surface area decreases slightly, but the acidity and thermal stability are improved, suitable for catalytic reactions such as petroleum cracking.
• High temperature calcination (>1000℃): γ-Al₂O₃ gradually transforms into θ-Al₂O₃ and α-Al₂O₃ with low specific surface area, and the activity is significantly reduced.
(2) Calcination atmosphere
• Air calcination: Promotes the retention of surface hydroxyl groups, suitable for applications requiring high surface activity.
• Calcination in inert atmosphere (N₂, Ar): reduces surface oxidation and is suitable for controlling surface acidity.
• Calcination in reducing atmosphere (H₂): may form low-valent aluminum species, affecting catalytic performance.
3. Effect of surface properties on activity
(1) Specific surface area and pore structure
• High specific surface area (>200 m²/g) provides more active sites, improving adsorption and catalytic efficiency.
• Appropriate pore size (2–50 nm) facilitates the diffusion of reactants and avoids pore blockage.
(2) Surface acidity
The surface acidity of activated alumina includes Lewis acid (coordinated unsaturated Al³⁺) and Brønsted acid (surface hydroxyl):
• Lewis acid: promotes olefin polymerization, isomerization and other reactions.
• Brønsted acid: suitable for proton catalytic reactions such as hydrolysis and esterification.
The surface acidity distribution can be optimized by adjusting the preparation method and doping modification (such as introducing SiO₂, F⁻, etc.).
4. Effect of impurity doping
Certain impurities can significantly change the catalytic performance of activated alumina:
• Promoting impurities (such as Fe, Ni, Co): can act as active centers to enhance redox performance.
• Poisoning impurities (such as Na⁺, K⁺): neutralize surface acidity and reduce catalytic activity.
• Structural stabilizers (such as La₂O₃, SiO₂): improve thermal stability and prevent high-temperature sintering.
5. Effect of hydration state
Activated alumina contains a large number of hydroxyl groups (-OH) on its surface, and its hydration state affects its adsorption and catalytic behavior:
• Moderate hydration (3–10% H₂O): maintain surface hydroxyl groups, improve hydrophilicity and catalytic activity.
• Excessive dehydration: leads to a decrease in surface hydroxyl groups and reduces activity.
• Excessive hydration: may block pores and affect the diffusion of reactants.
6. Influence of storage conditions
Activated alumina may reduce its activity during storage due to moisture absorption or CO₂ adsorption. Therefore, it needs to be stored in a dry inert environment or passivated on the surface to improve stability.
The active state of activated alumina is affected by many factors, including preparation method, heat treatment conditions, surface properties, impurity doping and hydration state. By optimizing these factors, its specific surface area, pore structure and surface acidity can be adjusted, thereby improving its application performance in catalysis, adsorption and other fields.