Molecular Sieve Type Selection: 3A, 4A, 5A, 13X, NaY and ZSM-5
Part of the Zeolite Molecular Sieves: The Complete Guide — this article is one of the detailed pages in the guide.
Direct answer: Molecular sieve selection follows the size and polarity of the molecules to be handled: 3A for drying small polar molecules, 4A for general drying and CO₂, 5A for straight-chain hydrocarbons, 13X and NaY for larger molecules and CO₂ at low partial pressure, ZSM-5 for VOC adsorption and catalysis. The rule is simple: the pore must admit the target while excluding what must not be adsorbed.
The type map
| Type | Effective pore size | Typical duties |
|---|---|---|
| 3A | ~0.3 nm | Deep drying of unsaturated streams (ethylene, propylene) without co-adsorption |
| 4A | ~0.4 nm | General gas drying, CO₂ removal from small streams |
| 5A | ~0.5 nm | n-paraffin separation, PSA oxygen/nitrogen, drying with larger molecules |
| 13X | ~0.7–1.0 nm | Air separation pre-purification, CO₂ at low partial pressure, larger polar molecules |
| NaY | ~0.7–0.9 nm | High-capacity CO₂ and polar VOC adsorption, catalysis supports |
| ZSM-5 | ~0.5–0.6 nm | Hydrophobic VOC adsorption, concentration wheels, catalytic applications |
How to choose
What is the target molecule? Pore size must admit it. Water (0.27 nm) fits everywhere; CO₂ (0.33 nm) fits 4A and larger; benzene (~0.6 nm) needs 13X/NaY or specialized structures; larger VOC molecules need the biggest pores or adsorbents other than zeolites.
What must be excluded? In drying unsaturated streams, 3A excludes the hydrocarbon while adsorbing water — protecting product yield. Choosing 5A for that duty would co-adsorb the olefin.
What is the concentration range? X-type zeolites (13X) hold high capacity at low partial pressure; at high humidity A-type sieves are the more economical drier.
Is the environment humid? High-humidity streams favour hydrophobic formulations (high-silica zeolites, ZSM-5 types) for VOC work, because hydrophilic sieves fill with water first.
Capacity, selectivity and regeneration
- Capacity is expressed as adsorption uptake (g/100g or wt%) at defined concentration and temperature — always condition-specific.
- Selectivity drives separation quality in PSA systems.
- Regeneration temperature varies by type: A-type driers typically 200–300°C; X/Y types need higher for strongly held species. See the regeneration article.
Manufacturer perspective
We ask for the complete stream composition before recommending a type — the unwritten rule of sieve selection is that the co-adsorbing components decide as much as the target. A stream with 3% water changes the answer for VOC adsorption completely.
Related articles
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