Zeolites

What are Zeolites?

Zeolites are microporous, three dimensional crystalline solid of aluminium silicate. Zeolites have small openings of fixed size in them which allow small molecules to pass through them easily but larger molecules cannot pass through them; that is why they are sometimes called molecular sieve.

Types Of Zeolites

Zeolites are either formed naturally or can be synthesized. There are 50 different types of zeolites are available nowadays.

Examples:

1. Na2K2CaMg(AlO2)2(SiO2)2.6H2O (Erionite)

2. Na2Ca(AlO2)2(SiO2)4.6H2O (Gemelinite)

3. Hx[(AlO2)x(SiO2)96x].16H2O (ZSM-5)

Natural zeolites:  They are mostly found in volcanic and sedimentary rocks.

Examples:

1. Clinoptilolite (Clino) zeolite have silica to alumina ratio of 5 to 1 ratio.

Clino zeolite does not break down in the middle of an acid environment and broadly used in the agricultural industry as a feed additive and soil amendment.

2. Na56(AlO2)56(SiO2)136.250H2O (Faujasite)

Synthesized zeolites: They are manufactured by energy-consuming process. Several synthetic zeolites have also been synthesized for a specific purpose, the well-known example is zeolite A which is commonly used in laundry detergent. SYnthesised zeolite will have a silica to alumina ratio of 1 to 1.

Example: 

[Na12(AlO2)12(SiO2)12.27H2O]8 (Linde-A)

Nomenclature

The term “zeolite” was first used in the year 1756 by Axel Fredrik Cronstedt, a Swedish mineralogist. During a process of rapidly heating a material which was observed to be stilbite, it produced large amounts of steam. This was due to the water that the material had absorbed earlier. After witnessing such developments, he coined the term zeolite. It has been derived from Greek words, ζέω (zéō), meaning “to boil” and λίθος (líthos), meaning “stone”.

Also Read: Alkali Metals

Chemical Composition

All zeolites are made up of the aluminosilicate framework where silicon and aluminium are tetrahedrally coordinated. Silicon cation and aluminium cations are enclosed by four oxygen anions (O2-). Tetrahedral structure of Si04 and Al04 forms the building block of zeolite.

Zeolite formula is often given as

Mx/n[AlO2]x.(SiO2)y.mH2O

However, zeolites tend to have different chemical elements in their composition. The formula for zeolite is given in the ratio 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

Zeolites Structure

The crystalline structure of zeolite is very different as compared to other crystalline solid. Zeolite can be considered as special crystalline solid having a characteristic framework structure with cavities occupied by ions and water molecules that have considerable freedom of movement.

Properties Of Zeolites

  • Zeolites are very stable solid under different environmental conditions. The melting point of zeolite is very high, i.e.1000oC.
  • They are insoluble in water or other inorganic solvents.
  • They do not undergo oxidation in the presence of air.
  • The characteristic property of zeolite is their open cage-like framework structure that helps zeolite to trap water and ions of potassium and calcium.
  • The natural form of zeolite occurs in random forms and pore size are not uniform whereas synthetic zeolites are synthesized in a very precise manner with uniform pore size.
  • Zeolites which are rich in alumina are attracted to polar molecules like water whereas zeolite rich in silica are attracted towards nonpolar molecules.
  • Since zeolites are not reactive and are obtained from naturally occurring minerals, therefore, they do not have any harmful environmental effects; although skin contact or inhalation may have a carcinogenic effect.

Occurrence and Production

As stated above, zeolites are found naturally in places where the volcanic rocks and ash layers react with alkaline groundwater. As far as the reports go around 245 unique zeolite frameworks have been discovered, and there are about 40 naturally occurring zeolite frameworks that are known. Whenever a new zeolite structure is found it is carefully examined by the International Zeolite Association Structure Commission. Upon identification, the material receives a three-letter designation. Generally, open-pit mining techniques are applied to mine natural zeolites.

Meanwhile, zeolites that are industrially important are synthetically produced. Some of the most common procedures that are used include heating aqueous solutions of alumina and silica with sodium hydroxide. Equivalent reagents also include sodium aluminate and sodium silicate. Some other variations include making changes in the cations to add quaternary ammonium cations. Till date, over 200 synthetic zeolites have been synthesized. This has been done using a process of slow crystallization involving silica-alumina gel as well as adding alkalis and organic templates.

Nonetheless, synthetically produced zeolites have few advantages over natural zeolites. Synthetic zeolites are produced in a uniform and phase-pure state. Further, unique zeolite structures can also be manufactured industrially. For example, Zeolite A. Furthermore, as silica and alumina are the most abundant mineral components on earth, therefore zeolites can be produced and supplied in an unlimited manner.

Application Of Zeolites

Some of the most common uses of zeolites are as follows.

Ion Exchange

The cage-like structure of zeolites makes them very useful in exchange of ions. For example, hard water is passed through a column filled with sodium-containing zeolites. In this process, calcium and magnesium get trapped by zeolites and sodium ions get release which results in softening of water making water rich in sodium. Nowadays zeolites are also being used in detergent for the removal of magnesium and calcium thereby making the water softer and increasing the effectiveness of the detergent.

As Catalyst

Zeolites are used as a catalyst in several important reactions such as cracking, isomerization and hydrocarbon synthesis. The porous structure of zeolite makes it a highly effective catalyst. Moreover, pores in a particular zeolite are of fixed shape and size making zeolite selective on certain molecules, therefore zeolite is sometimes referred to as shape-selective catalysis.

Read More: Catalysts

Adsorbent

zeolites have a very high adsorbing capacity and are used to adsorb a variety of materials. They have a wide range of application in the field of purification, drying and separation.

Separation of Harmful Substances

Zeolites can effectively remove radioactive particles from nuclear waste. It can also be used to clean water or soil contaminated with heavy toxic metals.

Frequently Asked Questions

1. Are zeolites dangerous?

Answer: 

Some zeolites such as natural fibrous zeolites (Erionite) can cause cancer when it is been inhaled.

2. Are zeolites are water-soluble?

Answer:

Zeolites are insoluble in water so they can be separated from aqueous solution by filtration.

3. How water is purified from zeolites?

Answer:

Zeolites have high porosity and large surface area, due to this reason it is used for biological treatment of water. Also, zeolites remove undesired cations, radioactive elements, heavy metals from water.