Zeolites

What Are Zeolites?

Zeolites are microporous, three-dimensional crystalline solids 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 sieves.

Table of Contents

• Types of Zeolites
• Chemical Composition and Structure of Zeolites
• Properties of Zeolites
• Occurrence and Production
• Application of Zeolites
• Frequently Asked Questions on Zeolites

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 that 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 the Greek words, ζέω (zéō), meaning “to boil”, and λίθος (líthos), meaning “stone”.

Types of Zeolites

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

A Few Examples of Zeolites:

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

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

3. Hx[(AlO₂)x(SiO₂)96−x].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 is broadly used in the agricultural industry as a feed additive and soil amendment.

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

Synthesised Zeolites

Synthetic zeolites are usually synthesised by the process of slow crystallisation of a silica-alumina gel in the presence of alkalis and organic templates. Interestingly, many different structures could be made using this process. Apart from variations in structures, zeolites can be manufactured or made from a variety of other atoms, making them chemically interesting and active. For instance, so-called heteroatoms include germanium, iron, gallium, boron, zinc, tin, and titanium.

Another important process used to carry out zeolite synthesis is sol-gel processing. Here, the properties of the product largely depend on several factors, such as the pH of the system, operating temperature, reaction mixture composition, pre-reaction seeding time, reaction time and the templates used. In this process, a few other elements (metals, metal oxides) can be easily incorporated. Generally, the silicalite sol produced by the hydrothermal method is very stable. This process for zeolite synthesis is usually preferred by many as it can be easily scaled up.

Several synthetic zeolites have been synthesised for a specific purpose, a 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: 

Also Read: Alkali Metals

Chemical Composition and Structure of Zeolites

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-). The tetrahedral structure of Si0₄ and Al0₄ forms the building block of zeolite.

The zeolite formula is often given as follows:

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

Structure

The crystalline structure of zeolite is very different as compared to other crystalline solids. Zeolite can be considered as a 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 solids under different environmental conditions. The melting point of zeolite is very high, i.e., 1000^oC.
• 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 its open cage-like framework structure that helps zeolite trap water and ions of potassium and calcium.
• The natural form of zeolite occurs in random forms, and pore sizes are not uniform, whereas synthetic zeolites are synthesised in a very precise manner with uniform pore sizes.
• Zeolites that are rich in alumina are attracted to polar molecules like water, whereas zeolites rich in silica are attracted towards nonpolar molecules.
• Since zeolites are not reactive and are obtained from naturally occurring minerals, 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 volcanic rocks and ash layers react with alkaline groundwater. According to reports, 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.

On the other hand, 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. To date, over 200 synthetic zeolites have been synthesised. This has been done using a process of slow crystallisation involving silica-alumina gel, as well as adding alkalis and organic templates.

Nonetheless, synthetically produced zeolites have a 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, 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 the 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 released, which results in softening of water, making water rich in sodium. Nowadays, zeolites are also 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, isomerisation 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 also 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 applications 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.

Commercial and Domestic

• Zeolites, as a molecular sieve, are often used in cryosorption style vacuum pumps.
• Non-clumping cat litter is often made of zeolite.
• Synthetic zeolites are used as an additive in the production process of warm-mix asphalt concrete.

Gemstones

One of the rarer zeolite minerals, known as Thomsonites, is collected as gemstones.

Biological

• Zeolite-based oxygen concentrator systems are widely used to manufacture medical-grade oxygen.
• In agriculture, clinoptilolite (a naturally occurring zeolite) is used for soil treatment.

Solar Energy Storage and Use

Zeolites are used to thermochemically store solar heat captured from solar thermal collectors. It is also used for adsorption refrigeration. In such cases, zeolites’ high heat of adsorption and ability to hydrate and dehydrate while maintaining structural stability is used extensively. This hygroscopic property, when matched with an inherent exothermic reaction during the transition from a dehydrated form to a hydrated form, makes natural zeolites very useful in harvesting solar heat and waste heat energy.