How do molecular sieves separate molecules with linear and bulky shapes?

Molecular sieves separate molecules based on their size and shape through the use of porous solids, typically synthetic or natural zeolites. These materials possess a well-defined crystalline structure containing internal cavities and channels of a specific, uniform size.123

The separation process functions as follows:

• Size Exclusion: Because the pores have a definite, fixed diameter, only molecules small enough to pass through these openings can enter the crystal lattice.12
• Selective Adsorption: Linear molecules, such as normal paraffins, are often small enough to enter the channels and be selectively retained within the internal cavities. Conversely, bulky molecules that exceed the pore dimensions are physically excluded from the lattice and remain outside the crystal.2
• Polarity: If multiple molecules are small enough to enter the pores, the sieve can further differentiate them based on polarity, with more polar molecules being preferentially adsorbed.1

This mechanism is widely utilized in industrial applications, such as improving the combustion properties of gasoline by removing normal paraffins and in the production of high-purity solvents.2