Shape-selective Catalysis in Zeolites

Zeolites are microporous three-dimensional crystalline solids of aluminium silicate. Zeolites contain small, fixed-size pores in them that allow small molecules to pass through but not larger molecules; this is why they are sometimes referred to as molecular sieves. 

Zeolites occur naturally, but they are also mass-produced on a large scale in the industrial sector. Zeolites are known for their ion exchange and reversible dehydration properties. They are composed of a framework structure that encloses interconnected cavities populated by massive metal cations (positively charged ions) and water molecules.

What are Zeolites?

A zeolite's primary structural feature is a three-dimensional tetrahedral framework in which two tetrahedra share each oxygen atom. The framework would be neutral if all tetrahedra contained silicon; substituting aluminium for silicon generates a charge imbalance and necessitates the presence of other metal ions in relatively significant cavities of the framework. 

These metal ions are typically mono- or divalent ions found in naturally occurring zeolites, such as sodium, potassium, magnesium, calcium, and barium. Zeolites are similar to feldspar minerals, except that the cavities in zeolites are bigger and there is usually water present. Zeolites are classed structurally based on the sorts of structural elements that make up the framework, such as rings or polyhedra. The cavities generated by the framework units have widths varying from around 2 to 8 angstroms, allowing ions to travel relatively easily across cavities.

Properties Of Zeolites:

1. Under many environmental circumstances, zeolites are relatively stable solids.
2. Zeolite has an extremely high melting point.
3. They cannot be dissolved in water or other inorganic solvents.
4. In the presence of air, they do not oxidise.
5. The open cage-like framework structure of zeolite aids in the trapping of water as well as potassium and calcium ions.
6. Natural zeolites occur in random shapes with non-uniform pore diameters, but synthetic zeolites are made in a very precise manner with uniform pore size.
7. Alumina-rich zeolites are attracted to polar molecules such as water, whereas silica-rich zeolites are attracted to nonpolar molecules.
8. Because zeolites are not reactive and are derived from naturally existing minerals, they have no negative environmental consequences; nonetheless, skin contact or inhalation may have a carcinogenic effect.

Occurrence and Production of Zeolites

As previously noted, zeolites occur naturally in areas where volcanic rocks and ash deposits react with alkaline groundwater. According to sources, over 245 distinct zeolite frameworks have been identified, and approximately 40 naturally occurring zeolite frameworks are known. When a new zeolite structure is discovered, the International Zeolite Association Structure Commission thoroughly examines it. The substance is assigned a three-letter designation after identification. Natural zeolites are typically mined using open-pit mining techniques.

Meanwhile, industrially important zeolites are being synthesised. Heating aqueous solutions of alumina and silica with sodium hydroxide is one of the most popular techniques. Sodium aluminate and sodium silicate are also equivalent reagents. Other variants include changing the cations to include quaternary ammonium cations. Over 200 synthetic zeolites have been created to date. This was accomplished by the use of a gradual crystallisation technique utilising silica-alumina gel, as well as the addition of alkalis and organic templates.

Despite this, manufactured zeolites have limited advantages over natural zeolites. Synthetic zeolites are created in a phase-pure and uniform state. Furthermore, one-of-a-kind zeolite structures can be produced industrially. For instance, Zeolite A. Furthermore, because silica and alumina are the most prevalent mineral components on the planet, zeolites can be created and supplied indefinitely..

Types of Zeolites

Zeolites can be created naturally, synthesised, or manufactured industrially. Zeolites are found in around 50 different kinds.

Natural Zeolites

They are found primarily in volcanic and sedimentary rocks. Examples:

• The silica to alumina ratio of Clinoptilolite (Clino) zeolite is 5 to 1. Clino zeolite does not degrade in an acidic environment and is widely used in agriculture as a feed additive and soil amendment.
• Na₅₆(AlO₂)₅₆(SiO₂)136.250H₂O (Faujasite).

Synthesized Zeolites

Synthetic zeolites are typically created through the gradual crystallisation of a silica-alumina gel in the presence of alkalis and organic templates. Surprisingly, this procedure can be used to create a wide range of structures. Aside from structural changes, zeolites can be created or synthesised from a variety of different atoms, making them chemically fascinating and active. Germanium, iron, gallium, boron, zinc, tin, and titanium are examples of so-called heteroatoms.

Sol-gel processing is another major method for zeolite production. The product's qualities are heavily influenced by various aspects, including the pH of the system, the operating temperature, the composition of the reaction mixture, the pre-reaction seeding period, the reaction time, and the templates utilised. Few other elements (metals, metal oxides) can be easily integrated in this method. The silicalite sol formed by the hydrothermal technique is often quite stable. Many people prefer this method of zeolite production since it is easily scaled up.

Several synthetic zeolites have been created for various applications, the most well-known of which being zeolite A, which is often used in laundry detergent. The silica to alumina ratio of the synthesised zeolite will be one to one. An example is [Na₁₂(AlO₂)₁₂(SiO₂)₁₂.27H₂O]₈ (Linde-A).

Chemical Composition and Structure Of Zeolites

The aluminosilicate framework, in which silicon and aluminium are tetrahedrally coordinated, is shared by all zeolites. Four oxygen anions surround silicon and aluminium cations (O₂^-). Zeolite is made up of the tetrahedral structure of SiO₄ and AlO₄. However, zeolites tend to have different chemical elements in their composition. The formula for zeolite is given in the ratio:

M_x/n[AlO₂]_x.(SiO₂)_y.mH₂O

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

Shape selective catalysis

Shape-selective catalysis is a type of catalysis in which the size of the reactant and product molecules, as well as the pore structure of the catalyst, influence the reaction. Because of their honeycomb-like architecture, zeolites are excellent shape-selective catalysts. 

They are microporous aluminosilicates having a three-dimensional silicate network in which certain silicon atoms are replaced by aluminium atoms, resulting in an Al–O–Si framework. The size and form of the reactant and product molecules, as well as the pores and cavities of the zeolites, determine the reactions that occur in them. They can be found in nature or synthesised for catalytic selectivity.

Zeolites are frequently utilised as catalysts in the petrochemical industry for hydrocarbon cracking and isomerisation. ZSM-5 is a popular zeolite catalyst in the petroleum sector. It converts alcohols directly into gasoline (petrol) by dehydrating them to produce a hydrocarbon combination Here are several examples:

• The catalysis reaction utilising zeolite is an example of shape-selective catalysis. Molecules having pore sizes greater than 260-740 pm are unable to pass through the zeolite and undergo the reaction as a result.
• Zeolites, a crystalline aluminosilicate mineral, are a frequent shape-selective catalyst and adsorbent. Zeolite is a porous substance that can endure temperature and pressure extremes.
• Zeolite is used as a shape-selective catalyst due to the consistent form and size of the micropores. In a catalytic process employing zeolite, the molecules of the reactant are distributed in the pores of the mineral, i.e. the molecule is trapped inside it and then adsorbed in the active sites of the chemical reaction.
• ZSM-5 is a nanometer-scale shape-selective zeolite catalyst that can be used for isomerisation, polymerisation, and dehydration. It is used to dehydrate methanol and a mixture of aromatic and aliphatic hydrocarbons directly into synthetic gasoline.

Sample Questions

Question 1: Define electrophoresis.

Answer:

Positively charged particles migrate to the cathode when an electric current is passed through a colloidal solution, whereas negatively charged particles move to the anode, where they lose their charge and coagulate. This phenomenon is referred to as electrophoresis.

Question 2: Give an example of a catalyst that is shape-selective.

Answer:

Small molecules are absorbed in the pores and cavities of selective adsorbents such as zeolites, which is known as shape-selective catalysis.

Question 3: What is the contact process?

Answer:

The contact process is a sophisticated industrial method for manufacturing concentrated sulfuric acid. During this process, sulphur dioxide and oxygen are transferred across a heated catalyst. Sulphur trioxide is formed when they combine, and it combines with water to generate sulfuric acid.

Question 4: What is autocatalysis?

Answer:

In the autocatalytic reaction, no special catalyst is used. Instead, one of the products serves as a catalyst, speeding up the product production process.

Question 5: How can a positive catalyst alter the reaction?

Answer:  

A positive catalyst accelerates a reaction by altering the reaction's route and lowering the activation energy base. As a result, a large number of reactant molecules become products.