What are zeolites? Why are they called shape selective catalysts?


Zeolites are crystalline microporous materials with well-defined features. They usually have silicon, aluminium, and oxygen in their frameworks, as well as cations, water, and other molecules in their pores. Many are often found naturally as minerals and are widely mined throughout different parts of the world. Others are synthetic, and are either manufactured commercially for specialised purposes or created by researchers interested in learning more about their chemistry. Zeolites are essentially employed in a range of applications due to their distinctive porous qualities, with an increasing global market of several million tonnes per year. Petrochemical cracking, ion-exchange (water softening and purification), and the separation and removal of gases and solvents are it’s common industrial uses. Agriculture, animal husbandry, and construction are some of the other applications. Molecular sieves are a term that is frequently used to describe them.

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Structure And Framework Of Zeolites:

The framework of zeolites is formed with four connected networks of atoms, which is a divergent property. Tetrahedra, with a silicon atom in the middle and oxygen atoms in the corners, is one example. These tetrahedra can then be linked together by their corners to create a wide range of complex structures. The framework structure may include interconnected cages, cavities, or channels of the right size to allow tiny molecules to pass through. The pore sizes are typically between 3 and 10 microns in diameter.

Over 130 distinct framework structures have now been identified. Other compositions, such as the emerging category of microporous aluminophosphates known as ALPOs, have been synthesised in addition to silicon or aluminium as the tetrahedral atom.

Zeolites in the Environment:

Zeolites help to make the environment cleaner and safer in a variety of ways. In retrospect, practically every application of zeolites is motivated by environmental concerns and helps to reduce toxic waste and energy consumption. Zeolites replaced toxic phosphate builders in powder detergents, which are now banned in many parts of the world due to water pollution concerns.

Catalysts, by definition, improve the efficiency of a chemical reaction, saving energy and decreasing pollution indirectly. Furthermore, procedures can be completed in fewer steps, resulting in less waste and by-products. Zeolites act as solid acids, reducing the need for corrosive liquid acids, and as redox catalysts and sorbents, they can remove pollutants from the atmosphere, such as exhaust gases from engines and ozone-depleting CFCs. Zeolites can also be used to separate hazardous substances from water, such as heavy metal ions created by nuclear fission.

Zeolites As Shape Selective Catalysts

Zeolites are capable of acting as catalysts for chemical reactions that occur within the interior cavities. The reactions catalysed by hydrogen-exchanged zeolites, whose framework-bound protons produce extremely high acidity, are an important class of chemical reactions. Many organic reactions, such as crude oil cracking, isomerization, and fuel synthesis, utilise this. After metals have been introduced into the framework, zeolites can be used as oxidation or reduction catalysts.

The distinctive microporous nature of zeolites underpins all of these sorts of reactions, where the form and size of a particular pore system has a steric influence on the reaction, restricting reactant and product access. Therefore, zeolites are frequently referred to as shape-selective catalysts. The characteristics of zeolite catalysts are increasingly being fine-tuned in order to carry out very specialised syntheses of high-value compounds, such as medicines and cosmetics.

Separation and Adsorption:

Zeolites are used in molecular adsorption because of their shape-selective characteristics. The capacity to selectively adsorb specific molecules while rejecting others has opened up a whole new world of molecular sieving possibilities. It’s sometimes only a matter of the size and shape of the pores that govern zeolite access. In other circumstances, like in the purification of para-xylene, multiple types of molecules enter the zeolite, but some diffuse down the channels faster than others, leaving others stranded behind. The form of para-xylene, allows it to diffuse freely in silicalite channels whereas the other isomers get caught.

Because of their high affinity for water, cation-containing zeolites are widely used as desiccants. They also have applications in gas separation, where molecules are separated based on their electrostatic interactions with metal ions. Hydrophobic silica zeolites, on the other hand, tend to absorb organic solvents. Therefore, zeolites may segregate molecules based on their size, shape, and polarity.

Exchange of ions:

Extra-framework metal ions can easily be exchanged for other forms of metal when in an aqueous solution due to their loosely bound nature. This is used extensively in water softening, where alkali metals like sodium and potassium prefer to exchange out of the zeolite, with calcium and magnesium ions from the water replacing them. As a result, zeolite is found in a lot of commercial washing powders. Zeolites can also be used to clean up commercial wastewater containing heavy metals and nuclear effluents carrying radioactive isotopes.

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