Zeolite – Catalyst and Adsorbent
Zeolite (Catalyst and Adsorbent) belongs to the family of hydrated aluminosilicate minerals containing alkali and alkaline-earth metals.
They are well-known for their lability towards reversible dehydration and ion-exchange. The main roles of Zeolite are Odour removal, Filtering, Ion exchange, Gas absorption, Catalysts, and Cation Exchange. They are widely popular as Catalyst and Adsorbent.
Their framework structure encloses interconnected cavities which are occupied by molecules of water and large metal cations (i.e., positively charged ions).
Chemical Structure of Zeolite
A three-dimensional tetrahedral framework is the essential structural feature of zeolite in which each oxygen atom is shared by two tetrahedra. If all tetrahedra contained silicon, then the framework would be neutral.
A charge imbalance is created by the substitution of aluminum for silicon and it requires other metal ions to be present in relatively large cavities of the framework.
These metal ions are typically mono- or di-valent ions in naturally occurring zeolites such as potassium, sodium, calcium, magnesium, and barium.
Zeolites and feldspar minerals are almost the same except that cavities are larger in zeolites and water is generally present.
Structurally, zeolites are characterized by the sorts of structural units that form the framework, such as polyhedra types or rings.
The cavities have diameters ranging between 2 to 8 angstroms shaped by the framework units, which generally allows an easy movement of ions between cavities.
This ease of movement of water and ions inside the framework permits cation exchange and reversible dehydration, properties that shift significantly with structural and chemical contrasts.
Various Origins of Zeolite
Mafic volcanic rocks contain natural zeolites as cavity fillings, probably as a result of deposition by vapors or fluids. Zeolites occur as alteration products of volcanic glass in sedimentary rocks and fill in as solidifying material in detrital rocks; they additionally are found in marine origin’s chemical sedimentary rocks.
All oceans contain extensive deposits of zeolites. A sequence of zeolite minerals is found in Metamorphic rocks, useful for appointing relative metamorphic grade; these minerals form at the detriment of volcanic glass and feldspars.
Dehydration occurs at relatively high temperatures for those zeolites in which water is tightly bound; some of the water can be released at low temperatures by contrast, in certain zeolites with large cavities.
The ratio of ion exchange depends on the connections between cavities and their size. Because of specific structural properties, some ions are excluded.
Commercial production of zeolites exploits zeolite properties with particular chemical and structural features.
Some commercial uses include drying of liquids and gases; separation of hydrocarbons, such as in petroleum refining; and pollution control by selective molecular adsorption.