“Molecular sieve” is not a single product — it is a family of synthetic zeolite adsorbents, each with a specific pore size that determines which molecules it can capture. Picking the wrong type wastes money and can fail the application entirely. This guide explains the four most common grades — 3A, 4A, 5A and 13X — and where each one belongs.
How molecular sieves work
A molecular sieve adsorbs molecules that are smaller than its pore opening and rejects molecules that are larger. The pore size is measured in angstroms (Å), and it is built into the crystal structure of the zeolite. This “size exclusion” is what makes molecular sieves far more selective than general desiccants such as silica gel. The name of each grade — 3A, 4A, 5A — refers directly to its approximate pore diameter.
3A molecular sieve (3 Å pore)
3A adsorbs water but excludes almost everything larger, including nitrogen, oxygen and CO2. That narrow selectivity makes it the standard for insulating glass units, and for drying reactive or valuable streams where you do not want to co-adsorb the product — for example ethanol and other solvents, and refrigerants. If the only thing you need to remove is water, 3A is often the safest choice.
4A molecular sieve (4 Å pore)
4A is the general-purpose workhorse. Its 4 Å pores adsorb water plus CO2, SO2, H2S, ethylene and ethane. It is widely used for static drying (packaged desiccant), drying of natural gas and compressed air, and as a broad desiccant in manufacturing. If you need strong, economical moisture removal and are not worried about co-adsorbing small gases, 4A is usually the answer.
5A molecular sieve (5 Å pore)
5A opens the pore far enough to admit larger molecules, including normal (straight-chain) paraffins while excluding branched and cyclic ones. This makes it the choice for PSA hydrogen purification, oxygen concentration, natural gas treatment, and the separation of n-paraffins from iso-paraffins. 5A does more than dry — it separates.
13X molecular sieve (10 Å pore)
13X has the largest pore of the common grades and the highest adsorption capacity. It captures water, CO2, sulfur compounds (H2S, mercaptans) and larger molecules, which is why it is the standard for air pre-purification before cryogenic air separation and for gas sweetening. A high-nitrogen-capacity variant, 13X APG, is formulated specifically for PSA oxygen generation.
What about activated alumina?
Not every drying job needs a zeolite. Activated alumina is a lower-cost desiccant used for compressed air and gas drying and, in water treatment, for fluoride and arsenic removal. It is often paired with a molecular sieve in a two-layer bed: alumina takes the bulk of the water, and the sieve polishes the stream to a very low dew point.
Quick selection summary
- Insulating glass, solvent/refrigerant drying: 3A
- General drying, natural gas, static desiccant: 4A
- PSA gas separation, n-paraffin separation: 5A
- Air pre-purification, gas sweetening, high capacity: 13X
- PSA oxygen generation: 13X APG
- Low-cost bulk drying, fluoride removal: activated alumina
Still unsure which grade fits your process? Tell us the stream, the contaminant you need to remove and the target dew point or purity, and we will recommend a grade. Request specifications, a sample or an export quote and our team will help you select.