3A、4A、5A and 13X Molecular Sieves: Differences in Pore Size and Main Adsorbents
Briefly introduces the differences in pore size and main adsorption targets of four molecular sieve models.
3A、4A、5A and 13X Molecular Sieves: Differences in Pore Size and Main Adsorbents
Molecular sieves are a class of alkali metal or alkaline earth metal aluminosilicate crystals with a uniform microporous structure. Their pore sizes are comparable to the diameters of typical molecules, thus enabling selective adsorption and sieving based on differences in molecular size. 3A, 4A, 5A, and 13X are the four most typical molecular sieve types used in industrial applications. Their pore sizes increase sequentially from small to large, and the adsorbable targets also exhibit a clear progressive pattern.
Physical Basis
Model | Crystal Type | Cation | Nominal Pore Size |
3A | KA Type | K⁺ | 3Å (0.3nm) |
4A | NaA Type | Na⁺ | 4Å (0.4nm) |
5A | CaA Type | Ca²⁺ | 5Å (0.5nm) |
13X | NaX Type | Na⁺ | 10Å (1.0nm) |
3A, 4A, and 5A all belong to type A molecular sieves, with the same framework structure, differing only in effective pore size due to different exchangeable cations; 13X belongs to type X molecular sieves, with a different framework structure and larger pores.
Adsorption targets of various models
3A Molecular Sieve
Adsorbable Molecules: H₂O (2.65Å)
Non-Adsorbable Molecules: All hydrocarbons (ethylene 3.9Å, propylene 4.5Å, etc.)
Applications: Dehydration of cracked gas/olefins (preventing polymerization and coking), methanol dehydration, NH₃/H₂S separation
4A Molecular Sieve
Adsorbable Molecules: H₂O, CO₂, NH₃, H₂S, C1-C2 hydrocarbons
Non-Adsorbable Molecules: C3+ hydrocarbons (propylene, n-butane, etc.)
Applications: Natural gas dehydration and decarbonization, hollow glass drying, air pre-purification
5A Molecular Sieve
Adsorbable Molecules: All 4A targets + n-Alkanes (n-butane, n-hexane), straight-chain alkenes
Non-adsorbable molecules: Branched-chain hydrocarbons (isobutane, isopentane), cyclic hydrocarbons, aromatic hydrocarbons
Applications: Separation of n- and iso-alkanes (simulated moving bed), PSA oxygen production, removal of trace straight-chain impurities
13X Molecular Sieve
Adsorbable molecules: All 3A/4A/5A molecules + branched-chain hydrocarbons, cyclic hydrocarbons, aromatic hydrocarbons, thiophene/pyridine, etc. (diameter <10Å)
Non-adsorbable molecules: Macromolecules with diameter >10Å (heavy aromatic hydrocarbons, etc.)
Applications: Air purification (de-H₂O/CO₂), natural gas desulfurization and decarbonization, aromatic hydrocarbon separation, VOCs adsorption, liquid paraffin refining
Adsorption Capacity Comparison Summary
Model | Pore Size | Typical Adsorbable Targets | Main Non-Adsorbable Targets | Core Applications |
3A | 3Å | H₂O | All Hydrocarbons | Olefin Dehydration, Drying of Polar Small Molecules |
4A | 4Å | H₂O、CO₂、C1-C2 | C3+ Hydrocarbons, Branched Hydrocarbons | Natural Gas Dehydration and Decarbonization, Static Drying |
5A | 5Å | Straight-Chain C1-C4 Hydrocarbons | Branched Hydrocarbons, Cyclic Hydrocarbons | Separation of Normal and Isomeric Hydrocarbons, PSA Oxygen Production |
13X | 10Å | H₂O, CO₂, Aromatic Hydrocarbons, Branched Hydrocarbons | Large Molecules with Diameter >10Å | Air Purification, Desulfurization, VOCs Adsorption |
Selection Principles
Target Adsorbate Size: Select a model with a pore size slightly larger than the target molecule based on molecular dynamics diameter to achieve efficient adsorption.
Competitive Adsorption Control: If multiple adsorbates coexist, the selectivity of pore size for competitive adsorption must be considered. For example, 3A must be used for dehydration of olefin-containing gas streams to avoid olefin co-adsorption.
Desorption and Regeneration Requirements: The smaller the pore size, the more drastic the temperature or pressure change required for desorption.
Adsorption Capacity and Rate: 13X has a higher adsorption capacity for most molecules than type A, but 4A and 5A, with their moderate pore size, have lower diffusion resistance and faster adsorption rates.
Summary
3A, 4A, 5A, and 13X constitute a complete adsorption spectrum from polar small molecules to aromatic macromolecules. 3A maintains the boundary between water and hydrocarbons, 4A handles basic small molecule purification, 5A achieves precise sorting of straight and branched chains, and 13X, with its large pore size, undertakes broad-spectrum adsorption tasks. Understanding the differences in pore size and adsorbates among these four models is the logical starting point for selecting molecular sieves for industrial applications. We are a Chinese industrial ceramics manufacturer. For more information, please contact us via email at annayu@169chem.net or WhatsApp at +8618909016373.