Zeolites are crystalline microporous aluminosilicates with well-defined pore structures, making them indispensable materials in the petrochemical industry, environmental protection, and energy sectors. Among the numerous zeolite frameworks developed over the past decades, ZSM-22 (also commercially known as Tonite™) stands out for its distinctive structural features.
ZSM-22 was first synthesized by Zones and Rollmann at Mobil Oil Corporation in 1982 and was assigned the code TON by the International Zeolite Association (IZA). Its framework consists of parallel, unidirectional channels formed by 10-membered rings, with elliptical pore apertures of approximately 4.4 × 5.4 Å. This unique one-dimensional channel architecture imparts ZSM-22 with pronounced shape-selectivity, distinguishing it from other medium-pore zeolites such as ZSM-5 (MFI-type) and ZSM-23 (MTT-type).
The general chemical composition of ZSM-22 can be expressed as:
where n represents the Si/Al ratio, which can be tuned during synthesis to modulate the acidity and hydrophobicity of the material.
| Property | Value |
|---|---|
| Framework type | TON |
| Channel system | 1D, parallel |
| Pore opening | 4.4 × 5.4 Å (10-MR) |
| Channel dimension | 4.6 × 5.7 Å |
| Space group | Immm |
| Si/Al ratio | 10–∞ (tunable) |
| Density | ~2.0 g/cm³ |
The one-dimensional channel structure of ZSM-22 is both its greatest strength and limitation. On one hand, it provides excellent shape-selectivity for reactions involving linear molecules. On the other hand, the lack of intersecting channels can lead to faster deactivation by coke deposition compared to three-dimensional zeolites like ZSM-5. However, this drawback can be mitigated through hierarchical structuring, post-synthetic modifications, and optimal reaction condition design.
The catalytic applications of ZSM-22 represent the most extensively studied and commercially significant area. Its medium-pore size and shape-selective properties make it particularly suited for several important industrial reactions.
One of the most prominent applications of ZSM-22 is in the hydrocracking and hydroisomerization of long-chain n-paraffins (C₁₀–C₂₀), which is critical for producing high-quality diesel fuel and lubricant base oils.
Compared to ZSM-5 and ZSM-11, ZSM-22 exhibits superior selectivity toward mono-branched isoparaffins while suppressing the formation of multi-branched and cracked products. This is attributed to the moderate pore size of ZSM-22, which restricts the formation and diffusion of bulky multi-branched isomers.
