Molecular sieves belong to the group of zeolites. These materials, consisting of aluminum silicates, are used to separate molecules according to their size or shape. The term "molecular sieve" is derived from the ability of these materials to sieve out molecules based on their molecular dimension.

A key feature of molecular sieves is their defined pore size, which is typically in the range of 3 to 10 Å (0.3 to 1 nm). This pore size determines which molecules are adsorbed or excluded, making them an ideal choice for the selective separation of gases and liquids.

Molecular sieves are divided into type A and type X, differing mainly in their structure and composition:

• A-type: The A-type is based on a cubic crystal structure and has pore sizes that typically vary in a narrower range of 3 Å to 5 Å. This structure is particularly efficient for drying and selective separation of small molecules.

• X-Type: The X-type has a faujasite-like structure with larger pores, which are around 10 Å. This larger pore size allows the adsorption of larger molecules, which is why X-type molecular sieves are often used for the capture of CO₂ or the separation of larger organic molecules.

In the context of molecular sieves, the term "Module" the specific way in which the zeolite structure (the framework of aluminum silicates) is constructed. A module describes the ratio of silicon to aluminum atoms in the structure. This ratio influences the chemical and physical properties of the molecular sieve, such as:

• Acid strength: A higher silicon content leads to lower acid strength.

•Hydrophilicity/hydrophobicity: Zeolites with a higher aluminum content are more hydrophilic, while those with a higher silicon content are more hydrophobic.

• Thermal stability: Molecular sieves with a higher silicon content are more thermally stable and more resistant to high temperatures.

By adapting the module, the performance of the molecular sieve can be optimized for specific applications, such as drying, hydrocarbon separation or CO₂ capture.

It is evident that molecular sieves play a crucial role in various industries. Some typical representatives and their applications are:

• 3A: Pore ​​size of 3 Å. Suitable for drying gases in the chemical industry or as a condensation inhibitor in double-glazed windows.

• 4A: Pore ​​size of 4 Å. Ideal for removing water from organic solvents or gas streams.

• 13X: Pore ​​size of about 10 Å. Commonly used for CO₂ capture.