silicaalumina

Silicaalumina, often written as silica-alumina, refers to a class of amorphous materials composed of silicon dioxide (SiO2) and aluminum oxide (Al2O3) with a variable silica-to-alumina ratio (SAR). It is commonly used as a catalyst support and adsorbent in chemical processing due to its adjustable acidity, surface area, and thermal stability.

Structure and properties: Silica-alumina systems are typically amorphous rather than crystalline. The SAR controls acidity and hydrophilicity: higher Al2O3 content generally increases Brønsted and Lewis acid sites, while higher SiO2 content tends to reduce acidity and enhance thermal stability. Surface areas for practical supports often range from a few hundred square meters per gram, and the materials can exhibit strong resistance to high temperatures, depending on composition and treatment. The network is porous, with porosity and pore size distribution tunable through synthesis conditions.

Synthesis and forms: Silica-alumina is prepared by methods such as co-precipitation, sol-gel processing, or impregnation of alumina on silica or vice versa. Calcination and drying steps consolidate the network and remove volatiles. Resulting materials may be amorphous gels, xerogels, or engineered supports with controlled porosity. Doping or pillaring with other species can modify acidity, hydrophilicity, or mechanical properties.

Applications: The primary use is as a catalyst support in refinery and petrochemical processes, including fluid catalytic cracking, hydrocracking, isomerization, and alkylation. Its acid sites facilitate cracking and selective reactions, while the large surface area provides active sites. Silica-alumina is also employed as adsorbents and desiccants, and as components in high-temperature ceramics and refractories.

Compared with zeolites, silica-alumina typically lacks well-defined microporosity, offering broader pore structures and different diffusion characteristics while providing robust acidity and versatility.