ZSM‑35 is a crystalline microporous aluminosilicate zeolite with FER (ferrierite) topology, first hydrothermally synthesized by Plank and co-workers. It is isostructural with natural ferrierite and belongs to orthorhombic crystal system. Featuring a unique two-dimensional intersecting pore network, ZSM‑35 has become a highly valuable shape-selective catalyst in petrochemical and C4 hydrocarbon conversion processes, distinguished from ZSM‑5 (MFI) by its dual 10-membered and 8-membered ring pore architecture.

Crystal Structure

ZSM‑35 possesses a 2D interconnected pore system:

• 10-membered ring channels: 0.43 nm × 0.55 nm, running along the [001] crystallographic direction
• 8-membered ring channels: 0.34 nm × 0.48 nm, perpendicularly crossing the 10-MR channels along [010] direction

The framework is built from 5-membered ring secondary building units. Its unit cell formula can be written as \(\ce{|Mg_{2}Na_{2}(H_{2}O)_{18}|[Al_{6}Si_{30}O_{72}]-FER}\). The silica-to-alumina (Si/Al) ratio can be tuned in a moderate range, which directly modulates acid density, acid strength and hydrothermal stability. The combination of medium 10-ring pores and smaller 8-ring constrictions creates strong molecular confinement and outstanding shape selectivity, restricting bulky coke precursors inside the framework.

Key Physicochemical Properties

1. Moderate and tunable acidity After ammonium exchange and calcination, H-ZSM‑35 generates Brønsted acid sites. Compared with ZSM‑5, ZSM‑35 exhibits relatively moderate acid strength. Adjusting Si/Al ratio allows precise control over acid site distribution, which suppresses excessive cracking and heavy coke formation during hydrocarbon reactions.
2. Excellent thermal and hydrothermal stability FER framework maintains crystallinity at temperatures above 700 °C. It tolerates steam environments in industrial reaction-regeneration cycles and shows good structural resistance under high-temperature hydrocarbon conversion conditions.
3. Distinct shape selectivity The intersecting dual-pore system allows linear hydrocarbon molecules to diffuse in, while limiting the formation of large polycyclic aromatic coke species. This characteristic makes ZSM‑35 exceptionally suitable for skeletal isomerization of n-butene to isobutene.
4. Modification flexibility Post-treatment methods including desilication, dealumination, metal doping and hierarchical pore construction can be applied to optimize mass transfer, adjust acid distribution and extend catalyst lifetime. Hierarchical ZSM‑35 introduces mesopores to shorten diffusion path and reduce carbon deposition.

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