ZSM-5 Properties
ZSM-5 zeolite catalyst properties stem from its MFI framework and tunable Si/Al ratio. Four properties define its industrial value for supplier evaluation: shape selectivity, controllable acidity, high thermal stability, and hydrophobicity at elevated Si/Al.
Shape Selectivity
The 10-MR channel system (5.1 to 5.6 A) excludes molecules larger than the pore diameter. Only linear and mono-branched hydrocarbons can enter. This reactant selectivity is the basis for ZSM-5’s use in FCC (cracking only low-octane linear olefins), MTO, and para-selective alkylation.
Acidity Control via Si/Al Ratio
| Si/Al Range | Acid Site Density | Typical Application |
|---|---|---|
| 20 to 50 | High | FCC additives, alkylation |
| 50 to 300 | Moderate | MTO/MTP, general acid catalysis |
| 300-1000 standard; 1500+ specialty | Low | VOC adsorption, hydrophobic catalysis |
Each framework Al creates one Brønsted acid site. Lower Si/Al equals more acid sites. Higher Si/Al equals fewer but more isolated, stronger sites, plus increased hydrophobicity.
Thermal and Hydrothermal Stability
ZSM-5 is stable to above 1000 C in dry conditions. In steam, high-Si/Al ZSM-5 retains crystallinity better than low-Si/Al grades. This stability is critical for FCC operation where the catalyst cycles between reactor (500 to 550 C) and regenerator (700 to 800 C in steam).
Hydrophobicity
At Si/Al above 300, ZSM-5 becomes increasingly hydrophobic. High-Si/Al ZSM-5 preferentially adsorbs non-polar VOCs over water, making it effective for VOC removal from humid gas streams.
See ZSM-5 Structure and the ZSM-5 product hub.
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