What is Alumina and How Is It Made?
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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.
