There are many types of molecular sieve adsorbents, with their core differences stemming from their crystal structure, pore size, silicon-to-aluminum ratio, and cation type. These differences directly determine their adsorption selectivity, stability, and application scenarios. The following are the characteristics and key differences of common types:

1、Type A Molecular Sieve

Structure: Cubic crystal system (LTA structure), composed of silicon-oxygen tetrahedra and aluminum-oxygen tetrahedra forming a cage-like structure, with uniform and small pore size.

Key Features: Small pore size (3-5 Å), strong polarity, high selectivity, and low cost.

2、Type X and Type Y Molecular Sieve (Type X & Y)

Structure: Both are cubic (FAU) crystals composed of supercages with large pore sizes (8-10 Å). The core difference lies in the silicon-aluminum ratio (SiO₂/Al₂O₃).

3. ZSM-5 Molecular Sieve

Structure: Orthorhombic/monoclinic (MFI structure), composed of intersecting 10-membered ring channels, a high-silica molecular sieve.

Key Features:

Pore diameter approximately 5-6 Å (suitable for adsorption of small C5-C10 molecules), shape selectivity (allowing only linear or small cyclic molecules to enter);

Very wide adjustable Si/Al ratio range (20-2000); high-silica versions exhibit weak hydrophilicity, excellent acid resistance, and thermal stability (≤1000°C);

Not only an adsorbent, it is also commonly used as a catalyst (due to adjustable surface acidity).

Typical Applications:

Catalysis: Petroleum cracking (gasoline production), methanol to olefins (MTO), aromatics isomerization;

Adsorption: Separation of xylene isomers (such as p-xylene and o-xylene); Removal of trace organic matter from water.

Mordenite (MOR)

Structure: Orthorhombic crystal system, composed of 12-membered and 8-membered ring channels, pore diameter approximately 6-7 Å. Core Features:

High thermal stability (≤800°C), strong acid resistance (can be used in strong acid environments);

Weak water adsorption (low hydrophilicity), preferring to adsorb non-polar or weakly polar molecules (such as aromatic hydrocarbons and amines).

Typical Applications:

Gas separation (such as separation of C2-C3 hydrocarbons);

Wastewater treatment (adsorption of ammonia nitrogen and organic amines);

Catalysis (such as alkylation reactions).

V. SAPO Series Molecular Sieves (Silicoaluminophosphate Molecular Sieves)

Structure: Synthetic heteroatom molecular sieves with a framework containing Si, Al, and P, and diverse structures (such as SAPO-34, which has a CHA structure).

Core Features:

Adjustable pore size (3-10 Å), low polarity (due to the presence of P, less hydrophilic than pure silica-aluminophosphate molecular sieves);

High adsorption selectivity for non-polar molecules (such as methane and olefins), and excellent thermal and hydrothermal stability. Typical Applications:

Natural gas purification (separation of methane and CO₂);

Catalysis (e.g., SAPO-34 for methanol to propylene);

Gas drying in low-humidity environments.

Key Differences

Pore Size: Determines the size of molecules that can be adsorbed (e.g., 3A only adsorbs water, 5A adsorbs propane, and 13X adsorbs larger aromatics);

Si/Al Ratio: A higher Si/Al ratio indicates greater thermal stability and acid resistance, but lower hydrophilicity (e.g., Y-type > X-type, ZSM-5 > A-type);

Cation Type: Pore size is adjusted through ion exchange (e.g., 4A → 5A: Na⁺ → Ca²⁺, pore size from 4 to 5 Å);

Application Focus: A/X types are primarily used for drying/separation, Y/ZSM-5 for catalysis, and SAPO is suitable for handling non-polar molecules.

Selection should be based on a comprehensive consideration of target molecule size, polarity, operating temperature, and stability requirements.