ZSM-5 Structure: MFI Framework
ZSM-5 catalyst belongs to the MFI framework type, one of the most important zeolite topologies in industrial catalysis. Its defining feature is a two-dimensional 10-membered ring channel system that gives ZSM-5 its signature shape selectivity and helps buyers compare supplier TDS and XRD data.
Channel System
ZSM-5 has two intersecting channel types:
| Channel | Direction | Ring Size | Dimensions |
|---|---|---|---|
| Straight channels | [010] | 10-MR | 5.6 x 5.3 A |
| Sinusoidal channels | [100] | 10-MR | 5.5 x 5.1 A |
The intersection cavities (approximately 9 A diameter) are where most catalysis occurs. The 10-MR pore size allows linear and small branched molecules to enter while excluding larger, bulkier species. This molecular sieving is the structural basis for ZSM-5’s shape selectivity in FCC, MTO, and alkylation.
Framework Composition
MFI is an aluminosilicate with the general formula (Na, H) Al Si(96-x) Al(x) O192. The Si/Al ratio can range from about 10 to infinity (pure silica silicalite-1). Each framework Al creates a charge-balancing Brønsted acid site, making Si/Al the primary control for acidity.
Key Structural Features
- Orthorhombic crystal system (Pnma)
- 10-MR pore openings throughout (no cages or supercages)
- High thermal stability to above 1000 C
- Straight and sinusoidal channels create a tortuous diffusion path
See ZSM-5 Properties and the ZSM-5 product hub.
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