Gammaalumina

Gamma-alumina, or gamma-Al2O3, is a metastable polymorph of aluminum oxide that serves as a widely used catalyst support and adsorbent. It is characterized by a high specific surface area and a porous, poorly crystalline structure often described as a defective spinel, with aluminum cations in octahedral and tetrahedral coordination to oxide anions.

Formation and transformation: Gamma-alumina is typically produced by calcination of hydrated aluminum oxide precursors such as boehmite, gibbsite, or aluminum hydroxy-oxides. Upon dehydration and heating, these phases convert through gamma to delta, theta, and finally alpha-alumina at higher temperatures. Gamma is formed roughly in the range of 400–900°C, depending on precursor and conditions, and loses surface area with further heating.

Properties: Gamma-alumina exhibits a high surface area, often in the hundreds of square meters per gram, and a mesoporous structure. It shows a mix of Lewis and Brønsted acid sites, providing versatile catalytic behavior and the ability to anchor metal nanoparticles. It remains thermally robust relative to many other oxide supports, though it gradually transforms to lower-surface-area phases at elevated temperatures.

Applications: The principal use is as a catalyst support in petrochemical and fine-chemical processes, including hydrogenation, reforming, and oxidation catalysts. It is also an effective adsorbent for drying gases and separations, and it serves as a stationary phase in chromatography as well as a ceramic material in some membranes. Synthesis methods vary, including precipitation, hydrolysis, sol-gel, and templating approaches, with control over pore structure and surface area achieved through pH, aging, drying, and calcination conditions.