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Zeolites are three-dimensional crystalline solids of aluminium silicate. They are microporous in nature. They contain tiny, fixed-size holes that enable small molecules to pass through them easily. Zeolites are sometimes called molecular sieves because large molecules cannot pass through them.
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Keyterms: Crystalline Solid, Aluminium Silicate, Ions, Atoms, Germanium, Iron, Gallium, Boron, Zinc, Aluminium, Water
Nomenclature of Zeolites
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Axel Fredrik Cronstedt, a Swedish mineralogist, coined the term "zeolite" in the year 1756. He coined the word zeolite after observing the following events.
- Large quantities of steam were generated during rapid heating of stilbite.
- The steam was produced due to water that had been absorbed by the substance.
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Structure of Zeolite
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- In comparison to other crystalline solids, zeolite has a unique crystalline structure.
- Zeolite has a unique framework structure which includes cavities filled with ions and water molecules that have a lot of freedom of movement.
- The building block of zeolite is formed by the tetrahedral structure of SiO4 and AlO4.
Structure of Zeolite
Types of Zeolites
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Zeolites can be synthesised or manufactured industrially or can be generated naturally. There are now 50 distinct kinds of zeolites available.
Zeolite
Few Examples of Zeolites:
- Na2K2CaMg(AlO2)2(SiO2)2.6H2O (Erionite)
- Na2Ca(AlO2)2(SiO2)4.6H2O (Gemelinite)
- Hx[(AlO2)x(SiO2)96−x].16H2O (ZSM-5)
Natural Zeolites
Volcanic and sedimentary rocks are the most common places for the occurrence of Natural Zeolites.
One of the common examples of natural zeolite is Clinoptilolite. Its ratio of silica to alumina is 5 to 1.
Example: Na56(AlO2)56(SiO2)136.250H2O (Faujasite)
Structure of Faujasite
Synthesized Zeolites
Synthetic zeolites are made by slowly crystallising a silica-alumina gel in the presence of alkalis and organic templates. Zeolites can also be created from a variety of different atoms to make them chemically active. Germanium, iron, gallium, boron, zinc, tin, and titanium are examples of some heteroatoms.
Sol-gel processing is another essential step in the zeolite production process. Many other factors like the pH of the system, operating temperature, reaction mixture composition, pre-reaction seeding' time, reaction time, and the templates have an impact on the product's characteristics.
In general, the hydrothermal technique produces a fairly stable silicalite sol. The ratio of silica to alumina in synthesised zeolite is 1 to 1.
Example: [Na12(AlO2)12(SiO2)12.27H2O]8 (Linde-A)
Linda A
Chemical Composition of Zeolites
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The aluminosilicate framework, in which silicon and aluminium are tetrahedrally coupled, is found in all zeolites. Four oxygen anions (O2-) surround the silicon and aluminium cations. The tetrahedral structure of SiO4 and AlO4.
The following is a common zeolite formula:
Mx/n[AlO2]x.(SiO2)y.mH2O
Zeolites have a variety of chemical components in their composition. Where,
M = any one metal that could be magnesium, sodium, potassium, lithium, or calcium.
n = valence of the metal cation.
y = number of water molecules in the structure of the zeolite.
y/x = Atomic Si/Al ratio
Properties of zeolites
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- Zeolites are highly stable solids.
- Zeolite has an extremely high melting point of 1000?.
- Zeolites cannot be dissolved in water and other inorganic solvents.
- They do not oxidise in the presence of oxygen.
- The open cage-like structure of zeolite is a distinguishing feature that aids in the trapping of water and potassium and calcium ions.
- Synthetic zeolites are produced in a highly precise manner with uniform pore diameters, whereas natural zeolites exist in random forms with non-uniform pore diameters.
- Alumina-rich zeolites are attracted to polar molecules like water, whereas silica-rich zeolites are attracted to nonpolar molecules.
- Zeolites are non-reactive and are derived from naturally existing minerals.
- They have no negative environmental effects.
Applications of zeolites
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Exchange of ions
Zeolites' cage-like structure makes them extremely useful for ion exchange. For example, hard water is filtered via a column of sodium-containing zeolites. Calcium and magnesium ions are retained by zeolites, whereas sodium ions are released, resulting in water softening and sodium enrichment. Nowadays, zeolites are widely used in detergent to remove magnesium and calcium from the water, making it softer and improving the detergent's efficiency.
As Catalyst
Cracking, isomerization, and hydrocarbon synthesis are all processes in which zeolites are utilised as a catalyst. Zeolite is a highly efficient catalyst due to its porous nature. Zeolite is also referred to as shape-selective catalysis because it has fixed shape and size.
As Adsorbent
Zeolites are used to adsorb a wide range of compounds due to their high adsorption capability. In the fields of purification, drying, and separation, they have a wide variety of uses.
In getting rid of harmful substances
Radioactive particles can be easily removed from nuclear waste using zeolites. It can also be used to clean heavy hazardous metal-contaminated water or soil.
Commercial and domestic use
- Zeolites are used in cryosorption vacuum pumps as molecular filters.
- Zeolite is commonly used in non-clumping cat litter.
- Synthetic zeolites are used as an ingredient in the warm mix asphalt concrete manufacturing process.
Gemstone
Thomsonites are a rare type of zeolite mineral. It is collected as a gemstone.
Biological
- Medical-grade oxygen is commonly produced using zeolite-based oxygen concentrator devices.
- A naturally occurring zeolite known as Clinoptilolite is used as a soil treatment in agriculture.
Things to Remember
- Zeolites are three-dimensional crystalline solids of aluminium silicate.
- Zeolites are sometimes called molecular sieves because small molecules can easily pass through them but large molecules cannot.
- In comparison to other crystalline solids, zeolite has a unique crystalline structure.
- Zeolite has cavities filled with ions and water molecules that have a lot of freedom of movement.
- The molecular structure of zeolite include tetrahedral structure of SiO4 and AlO4
- Zeolites are of two types: natural zeolites and synthesized zeolites.
- They are highly stable solids and have an extremely high melting point of 1000?.
- Zeolites cannot be dissolved in water and other inorganic solvents.
- The open cage-like structure of zeolite is a distinguishing feature that helps in the holding of water and potassium and calcium ions.
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Sample Questions
Ques: Give an example of ‘shape-selective catalyst’. (Delhi 2010, 1 Mark)
Ans: Zeolites are shape-selective catalysts.
Ques: Are zeolites harmful? (1 Mark)
Ans: When inhaled, some zeolites, such as natural fibrous zeolites (Erionite) can cause cancer.
Ques: Are zeolites soluble in water? (1 Mark)
Ans: Zeolites are insoluble in water. They can be filtered out of an aqueous solution.
Ques: What is meant by ‘shape-selective catalysis’ of reactions? Give an example of selective absorbents or shape-selective catalysts. (All India 2011, 2 Marks)
Ans: Shape-selective catalysis is a catalyst reaction in which small sized molecules are absorbed in the pores and cavities of selective adsorbents.
Zeolite is an example of shape-selective catalyst.
Ques: How are zeolites used to purify water? (2 Marks)
Ans: Zeolites are used for biological treatment of water because of their high porosity and wide surface area. Zeolites also used to filter out unwanted cations, radioactive elements, and heavy metals from water.
Ques: In chemistry, what is zeolite? (2 Marks)
Ans: The tectosilicate category includes zeolites, which are crystalline aluminosilicate molecular sieves. They are porous materials having organised, interconnecting microporous channels with diameters ranging from 0.2 to 2 nm, about equivalent to the size of many organic molecules.
Ques: What kind of heavy metals can be filtered out by zeolites? (2 Marks)
Ans: Natural zeolites have the ability to filter out heavy metals such as lead and cadmium from certain solutions under different values of pH.
The efficiency of natural zeolites in the removal of these toxic metals is high under low pH values or acidic medium.
Ques: Describe the following :
(i) Tyndall effect
(ii) Shape-selective catalysis (All India 2010, 2 Marks)
Ans: (i) Tyndall effect : When a beam of light is passed through a colloidal solution, the path of light becomes visible like a bright streak. This bright path is called the Tyndall cone and the phenomenon behind this effect is called the Tyndall effect.
(ii) Shape-selective catalysis : It is a catalyst reaction in which small sized molecules are absorbed in the pores and cavities of selective adsorbents like zeolites.
Ques: Are zeolites toxic and harmful for human health? (3 Marks)
Ans: Few zeolites are toxic and harmful for human health. Some zeolites like erionite are classified as carcinogens or potential carcinogens.
Erionite is a naturally occurring fibrous zeolite. It has the ability to cause the development of cancers.
There are some other zeolites that have extremely toxic heavy metals including arsenic, mercury, lead, nickel, cadmium, copper, and chromium. These zeolites are mostly sold in Europe. However, there is no proof these types of zeolites can cause cancer.
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