What is Alumina

What is Alumina and How Is It Made?

Activated Alumina is an aluminum oxide that is highly porous and exhibits tremendous surface area. Activated Alumina is resistant to thermal shock and abrasion and will not shrink, swell, soften nor disintegrate when immersed in water.

Alumina oxide is an inorganic substance made by the dehydration of alumina hydroxide Al(OH)3 at high temperatures. The active sites used for separation consist of acidic alumina ions, basic oxide ions and polar Al-OH groups. The unsaturated alumina ions act as Lewis acids, and the Al-OH groups act as dipoles. The hydroxyl groups at the surface are responsible for the selective retention of adsorbed materials.

Surface area are in the range of 150-350 m2/gram . Because it has a higher capacity for water than Silica Gel at elevated temperatures it has replaced silica gel as a desiccant for warm gases including air.

The alumina used for chromatographic and separation applications is gamma alumina. Alumina is manufactured with 3 pH ranges – acid, neutral and basic. The goal in providing three essential alumina products assures the best means for separating different types of compounds.

Here is a rough guide for the utilization of activated alumina oxide for  compounds in an aqueous solution:

Acidic alumina Adjusted to a pH of 4.5±0.5 Used to separate phenols, sulphonic, carboxylic and amino acids
Basic alumina Adjusted to pH of 10.0±0.5 Used to separate amines, imines and basic dyes
Neutral alumina Adjusted to pH of 8.0±0.5 Used to separate aldehydes, ketones and lactones

In a non aqueous eluent alumina is used to purify aromatic hydrocarbons, carotenoids, porphorins, alkaloids and steroids. It is essential to control the amount of adsorbed water that can block the active separating sites on the alumina surface. Alumina is such an active adsorbent that it adsorbs water from ambient air if given the opportunity. Indeed, the ability to create a water-alumina barrier on the surface of aluminum products is what prevents aluminum from corroding.


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