Zeolite

Zeolites are aluminosilicate minerals with microporous pores that are widely used as adsorbents and catalysts in industry. Axel Fredrik Cronstedt, a Swedish mineralogist, coined the term zeolite in 1756 after discovering that rapidly heating a material, assumed to be stilbite, produced large quantities of steam from water adsorbed by the material.

This article will study zeolite chemistry, zeolite uses, zeolite properties, and types of zeolites.

Zeolite Chemistry

All zeolite formulas are made up of an aluminosilicate matrix with a tetrahedral arrangement of silicon (Si⁴⁺) and aluminum (Al³⁺) cations surrounded by four oxygen anions (O^2-). Each oxygen ion in the Si-O and Al-O bonds binds two cations and is shared by two tetrahedrons (see figure 1.1), resulting in a macromolecular three-dimensional structure of SiO₂ and AlO₂ tetrahedral building blocks. Each tetrahedron in this arrangement of atoms has four O atoms surrounding Si or Al cation, yielding a three-dimensional structure of silicate tetrahedra with a Si:O ratio of 1:2.

Zeolite Structure

A zeolite mineral is a crystalline material with a structure consisting of connected tetrahedra, each of which contains four O atoms surrounding a cation. Open cavities in the form of channels and cages can be found in this system. These are normally occupied by H₂O molecules and typically exchangeable extra-framework cations. Guest organisms can move through the channels because they are wide enough. Dehydration occurs in the hydrated phases at temperatures predominantly below 400 °C and is essentially reversible. (OH, F) groups will break the structure by occupying a tetrahedron apex that is not shared with neighboring tetrahedra.

Zeolites Properties

1. Zeolites have a porous structure that allows a wide range of cations to pass through, including Na⁺, K⁺, Ca²⁺, Mg²⁺, and others. In a contact solution, these positive ions are held loosely and can easily be exchanged for others. Analcime, chabazite, clinoptilolite, heulandite, natrolite, phillipsite, and stilbite are some of the more common types of zeolites(zeolite mineral). The formula for natrolite is Na₂Al₂Si₃O₁₀2H₂O, which is an example of a zeolite mineral. These cation exchanged zeolites have varying acidities and catalyze a variety of acid catalysis reactions.

2. Zeolites are aluminosilicate members of the "molecular sieves" class of microporous solids, which are predominantly composed of silicon, aluminum, oxygen, and metals such as titanium, tin, and zinc. The word "molecular sieve" refers to a property of these materials that allows them to sort molecules selectively using a size exclusion procedure. This is due to a molecular-scale pore structure that is very regular. The measurements of the channels specify the maximum size of molecular or ionic species that can penetrate the pores of a zeolite.

3. These are usually defined by the aperture's ring size, with the term "eight-ring" referring to a closed-loop made up of eight tetrahedrally coordinated silicon (or aluminum) atoms and eight oxygen atoms, for example. Owing to a number of factors, such as strain caused by unit bonding or coordination of some of the rings' oxygen atoms to cations within the structure, these rings are not always perfectly symmetrical. As a result, many zeolites' pores are not cylindrical.

Zeolite Uses

Because of their versatility and adaptability, zeolites have drawn a lot of interest from researchers and scientists over the years. Zeolites have been discovered to be strong adsorbents, ion exchangers, and molecular sieves since their discovery in 1756 by Axel Fredrik Cronstedt. In particular, the molecular sieve properties of zeolites are widely used in industry.

1. Treatment of Industrial Wastewater: The growing demand for high-quality drinking water has necessitated the purification of water from a variety of sources, including natural, commercial, agricultural, and municipal wastewaters. As a result, the use of natural zeolites as agents in the removal of wastewater pollutants has sparked a lot of interest, leading to a lot of research.

2. Industry: Synthetic zeolites, like other mesoporous materials (such as MCM-41), are commonly used as catalysts in the petrochemical industry, like fluid catalytic cracking and hydrocracking. Molecules are confined in small spaces by zeolites, causing structural and reactivity changes. Acidic zeolites are also strong solid-state solid acids, allowing for a number of acid-catalyzed reactions including isomerization, alkylation, and cracking.

3. Zeolites are also used as sorbents and catalysts. They are highly active in a wide range of reactions due to their well-defined pore structure and adjustable acidity. 

4. Zeolites have the ability to have precise and specific gas separation, such as the removal of water, carbon dioxide, and sulfur dioxide from low-grade natural gas streams. Noble gases, N₂, O₂, freon, and formaldehyde are examples of other separations.

5. Zeolites were discovered to assist silver in emitting light naturally, which could compete with fluorescent lights or LEDs.

6. Zeolites can be used to thermochemically store solar heat collected from solar thermal collectors, as well as for adsorption refrigeration.  Their high heat of adsorption and ability to hydrate and dehydrate while preserving structural integrity are used in these applications. Natural zeolites are useful in extracting waste heat and solar heat energy because of their hygroscopic property and an intrinsic exothermic (energy-releasing) reaction when transitioning from a dehydrated to a hydrated state.

7. In cryosorption vacuum pumps, zeolites are used as a molecular sieve.

8. The global laundry detergent market is the largest single use for zeolite.

9. Zeolite or diatomite are widely used in non-clumping cat litter

10. In the manufacture of warm mix asphalt concrete, synthetic zeolites are used as an additive. They aid by lowering the temperature during the manufacturing and laying of asphalt concrete, resulting in less fossil fuel consumption and therefore less carbon dioxide, aerosols, and vapors being released. Synthetic zeolites in hot mixed asphalt make compaction simpler and, to some extent, allow for cold weather paving and longer hauls.

Did You Know?

Clinoptilolite (a naturally occurring zeolite) is used as a soil treatment in agriculture. It is a source of potassium that is released slowly. If the zeolite has been pre-loaded with ammonium, it may perform a similar role in the slow release of nitrogen. Zeolites can also be used as water moderators, absorbing up to 55 percent of their weight in water and slowly releasing it as required by the plant. Root rot can be avoided, and drought cycles can be moderated, thanks to this house.

Clinoptilolite has also been applied to chicken feed: since the zeolite absorbs water and ammonia, the birds' droppings are drier and less odoriferous, making them easier to treat.