ZSM‑22 (TON) consists of 5-membered ring secondary building units, assembling into non-intersecting one-dimensional 10-MR micropores with pore dimensions of 0.45 nm × 0.55 nm. Isomorphous substitution of tetrahedral Si⁴⁺ by Al³⁺ within the zeolite framework generates negative framework charges. After ammonium ion exchange and high-temperature calcination, protonated H-ZSM‑22 forms Brønsted acid sites (BAS), which are the main active centers for acid-catalyzed hydrocarbon reactions.
The Si/Al ratio of ZSM‑22 can be tailored during hydrothermal synthesis. Lower Si/Al provides higher density of Brønsted acid sites, while high-silica ZSM‑22 shows reduced acid density and weaker acid strength. Lewis acid sites (LAS) mainly originate from extra-framework aluminum species formed during dealumination or high-temperature steam treatment. Unlike MFI-type ZSM‑5 with crossed pore networks, the straight, isolated channels of TON ZSM‑22 restrict the formation of bulky carbonaceous intermediates, strongly affecting acid site accessibility and reaction pathways.
Common techniques including NH₃-TPD, pyridine FTIR, solid-state NMR and isopropylamine decomposition are used to quantify acid strength, acid type and acid site distribution of ZSM‑22. H-ZSM‑22 generally presents medium-strong Brønsted acidity. Excess strong acid sites will trigger undesired cracking and coke formation, while insufficient acid sites lead to low isomerization activity.
Multiple strategies are adopted to fine-tune acidity:
This is the most representative application of ZSM‑22. Pt/H-ZSM‑22 bifunctional catalyst is widely investigated for n-butane, n-pentane and n-hexane hydroisomerization to branched alkanes. The one-dimensional 10-ring channels exhibit excellent shape selectivity: monobranched isomers can diffuse out readily, whereas bulky dibranched products are kinetically restrained. Compared with FER-type ZSM‑35, ZSM‑22 delivers higher selectivity toward monobranched paraffins and suppresses cracking side reactions.
ZSM‑22 can serve as a catalyst or catalytic additive in light olefin conversion, alkene skeletal isomerization and selective hydrocarbon cracking. Its narrow linear pores limit the generation of large aromatic coke precursors, showing better coke resistance than many multidimensional zeolites under certain reaction conditions. Nevertheless, the 1D pore structure suffers from mass transfer limitation; coke deposition at pore mouths may block channels and cause rapid activity decay, which is the main bottleneck for industrial application.
Hierarchical ZSM‑22 with mesopores is developed to mitigate diffusion limitation. Metal loading optimization and surface passivation are used to weaken external surface acid sites, which are responsible for non-selective cracking. Template-free synthesis methods are also explored to lower manufacturing cost and reduce environmental impact.
TON-type ZSM‑22 zeolite possesses unique one-dimensional 10-membered ring channels and controllable medium Brønsted acidity. The matching between acid property and pore confinement endows it outstanding shape selectivity in alkane hydroisomerization. Rational acidity regulation and hierarchical pore construction are effective approaches to overcome the mass transfer drawback of 1D channels, further promoting the industrial application of ZSM‑22 in clean fuel production and low-carbon hydrocarbon upgrading.
