ZSM-5 (MFI): structure, channels and uses

ZSM-5 is one of the zeolites, the crystalline aluminosilicates built of corner-sharing SiO₄ and AlO₄ tetrahedra; its framework type code is MFI (Vogt and Weckhuysen 2015). Every pore of MFI is bounded by a ring of 10 T atoms, a 10-ring, which makes ZSM-5 a medium-pore zeolite (Vogt and Weckhuysen 2015; Shan et al. 2021).

The 10-ring channels run in two directions. Straight channels run along the b axis ([010]) and sinusoidal (zig-zag) channels wind along the a axis ([100]); the two sets cross, so a molecule of the right size can reach any point of the pore system from any other (Vogt and Weckhuysen 2015). The pores are slightly elliptical, 5.1–5.6 Å across, and the straight channels measure 5.3 × 5.6 Å (Vogt and Weckhuysen 2015; Feng Z. et al. 2021). Where the channels cross, the intersections are wider, with a radius of about 3.6 Å (Matam et al. 2023).

Al atoms, and so the acid sites, are not spread evenly over the framework. Resonant X-ray diffraction of a commercial H-ZSM-5 placed single Al at the T8 site in the straight channel and Al pairs (Al–O–Si–O–Al) at T6 and T4 at the straight–sinusoidal intersection (Li G. et al. 2025).

ZSM-5 can be made over an unusually wide range of composition. It is called high-silica above Si/Al = 20 and Al-rich below 15, and with tetrapropylammonium (TPA) as the organic structure-directing agent Si/Al from 20 to 220 is reached by changing the gel (Li J. et al. 2022). Up to a SiO₂/Al₂O₃ ratio of about 25 it crystallises without any organic template (Vogt and Weckhuysen 2015).

The essentially all-silica end member is silicalite, also written silicalite-1: pure-silica MFI (Dalena et al. 2026). It has a monoclinic cell, while low-silica ZSM-5 is orthorhombic, but the framework is the same (Vogt and Weckhuysen 2015). Because each Al brings one acid site, the Si/Al ratio controls catalysis directly: in methanol conversion, lower acid density (higher Si/Al) lengthens catalyst lifetime, and Al placed in the narrow channels rather than at intersections favours olefins such as propylene (Li J. et al. 2022). Neutron scattering shows methanol moves more freely in the pores as Si/Al rises from 11 to 140 (Matam et al. 2023).

1. Gel. A silica source, an aluminium source, sodium hydroxide and water, usually with a quaternary ammonium template such as tetrapropylammonium hydroxide (TPAOH) or tetraethylammonium hydroxide (Feng P. et al. 2024; Vogt and Weckhuysen 2015).
2. Hydrothermal crystallisation. The gel is heated in an autoclave; in one nano-ZSM-5 recipe, 170 °C for 36 h with silicalite-1 seeds gave crystals about 20 nm across (Feng P. et al. 2024).
3. Calcination. The template is burned out of the channels, for example at 550 °C for 6 h (Feng P. et al. 2024).
4. Acid form. Exchange with an ammonium salt and calcination converts Na-ZSM-5 to the Brønsted-acid form H-ZSM-5 (Feng P. et al. 2024).

• FCC additives for propylene. In fluid catalytic cracking the wide pores of zeolite Y allow bimolecular hydrogen-transfer reactions; the tighter channels of ZSM-5 cannot hold those large transition states, so it cracks gasoline-range molecules to light olefins. Additives with 25–50% ZSM-5 are used to raise the propylene yield (Vogt and Weckhuysen 2015). The market for propylene-selective FCC catalyst grew from about 10,000 t a year in 2005 to almost 90,000 t in 2014 (Vogt and Weckhuysen 2015).
• Methanol to hydrocarbons. ZSM-5 is a key catalyst for turning methanol into gasoline, olefins and aromatics (Matam et al. 2023); the methanol-to-hydrocarbons process can be tuned to gasoline (MTG) or olefins (MTO) by the choice of catalyst and conditions (Olsbye et al. 2012).
• Xylene isomerisation. ZSM-5 converts o- and m-xylene into p-xylene, the polyester raw material, a Brønsted-acid reaction limited to about 24% p-xylene at equilibrium (Feng P. et al. 2024).
• Other refinery and petrochemical uses. Hydroisomerisation, and light-alkane aromatisation over Ga-modified ZSM-5 (Feng P. et al. 2024; Feng Z. et al. 2021).

The intersecting channels also help the catalyst live longer: they keep acid sites from being blocked quickly by coke (Feng Z. et al. 2021). In FCC units steam slowly dealuminates ZSM-5, which is why it is often stabilised with phosphorus (Vogt and Weckhuysen 2015).

For a paper, slide or poster, show ZSM-5 looking down the straight channels, where the 10-rings appear as open pores, and add a side view with the zig-zag channels crossing them. The LabCanvas figure editor Library has the MFI crystal file: it draws the ZSM-5 framework as a high-resolution SVG, where you can show Si as tetrahedra or atoms, repeat the cell and turn the view down [010] or [100]. The Library's zeolite object draws a schematic ring framework for simpler diagrams, and the LabCanvas 3D Studio has zeolite models you can rotate and export as images.