1. Introduction
One‑dimensional medium‑pore zeolites occupy an irreplaceable position in modern petroleum‑refining catalysis, especially for alkane isomerization and selective dewaxing. ZSM‑48, first discovered in the 1980s, belongs to the *MRE topological family, built from stacked tetrahedral sheets to form continuous non‑intersected 10‑MR tubular channels. Different from three‑dimensional pentasil zeolites (ZSM‑5, ZSM‑11), ZSM‑48 only has unidirectional pore channels, so molecular diffusion strictly follows one‑way transport along the channel axis. Meanwhile, intrinsic stacking‑fault defects within the crystal bring both challenges and tunable opportunities for catalytic performance.
Traditional ZSM‑48 is normally synthesized with high silica‑alumina ratio, resulting in limited Brønsted‑acid density, which restricts its catalytic activity. In recent years, great progress has been achieved in preparing low‑Si/Al ZSM‑48, hollow and hierarchical‑pore ZSM‑48 via novel templates, seed‑assisted crystallization and post‑treatment methods, greatly expanding its application scopeACS Public.... When loaded with Pt or Pd active metals, ZSM‑48 forms classic bifunctional catalysts: metal sites undertake dehydrogenation‑hydrogenation cycles, while zeolite Brønsted‑acid sites promote carbocation‑mediated skeletal isomerization. Such bifunctional systems are widely applied for producing low‑freezing‑point diesel, jet‑fuel and high‑quality lubricant base‑oil through hydroisomerization dewaxing. This review focuses on structure‑activity relationships, synthesis‑modification progress and industrial application prospects of ZSM‑48 zeolite.
2. Crystal Structure and Physicochemical Properties
2.1 Topological Structure
ZSM‑48 (*MRE) consists of one‑dimensional non‑interconnected straight 10‑membered‑ring channels with pore opening of 0.53 × 0.56 nmResearchGa.... The framework is assembled by layered structural units, and stacking faults and polymorph intergrowth are frequently observed, which is an inherent structural feature of ZSM‑48 material family. These disordered structures do not destroy the integrity of main 10‑MR channels, but they change local pore‑wall environment, affect intracrystalline diffusion resistance and acid‑site accessibility, and further regulate product distribution of shape‑selective reactions.
Distinguished from ZSM‑22 (TON), ZSM‑48 has slightly larger effective pore size, which allows the generation of multi‑methyl‑branched isomer products, instead of only mono‑branched products formed over ZSM‑22 catalysts. This structural difference makes ZSM‑48 more suitable for converting long‑chain n‑alkanes into high‑value multi‑branched isoparaffins with improved cold‑flow properties中国化学会....
2.2 Key Physicochemical Properties
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Tunable Si/Al ratio: Conventional hydrothermal ZSM‑48 products are high‑silica zeolites (Si/Al > 80). Advanced synthesis technologies can lower Si/Al ratio down to ca. 35‑45, introducing more framework‑Al and abundant Brønsted acid sites, which significantly improves acid‑catalytic activity中国化学会....
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Acid‑site characteristics: Brønsted acid sites originate from framework aluminum atoms; Lewis acid sites derive from extra‑framework aluminum species. Moderate‑strength acid sites are favorable for hydroisomerization; excessive strong‑acid sites will aggravate alkane cracking and accelerate coke deposition.
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Thermal and hydrothermal stability: ZSM‑48 high‑silica framework possesses good thermal stability, tolerating temperatures above 750 °C. Severe hydrothermal treatment will trigger framework dealumination, loss of Brønsted‑acid sites and performance decay.
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Diffusion limitation risk: Owing to one‑dimensional channel architecture, once pore channels are blocked by coke or impurities, intracrystalline mass transfer will deteriorate sharply, resulting in rapid catalyst deactivation. Constructing hierarchical pores or hollow structures is an effective solution to alleviate this bottleneckACS Public....
3. Main Synthesis Approaches
3.1 Conventional Hydrothermal Synthesis
Hydrothermal crystallization is the mainstream preparation route for ZSM‑48. Silica sources, aluminum sources, alkali and organic structure‑directing agents (OSDAs) are mixed to form homogeneous precursor gel, followed by high‑temperature static or dynamic hydrothermal crystallization, filtration, washing, drying and calcination to obtain crystalline ZSM‑48 zeolite. Common organic templates include hexamethonium bromide, tetraalkylammonium salts and diamine compounds中国化学会.... Template selection critically determines whether pure‑phase *MRE‑type ZSM‑48 can be obtained and avoids miscellaneous crystal phases. Seed‑assisted hydrothermal synthesis shortens crystallization time, improves crystallinity and reduces impurity‑phase generation, which is favorable for industrial scale‑up. Nevertheless, direct synthesis of low‑aluminum ZSM‑48 remains challenging by traditional routes.
3.2 Seed‑Assisted and Template‑Optimized Synthesis
Novel OSDAs with molecular configuration simulating isomerization carbocation intermediates can realize direct synthesis of relatively Al‑rich ZSM‑48 with more accessible protonic sites, which provides new ideas for low‑Si/Al ZSM‑48 preparation without post‑modificationResearchGa.... Seed addition can reduce template dosage and accelerate nucleation rate, which is beneficial for cost‑saving industrial production.
3.3 Post‑Synthesis Construction of Hierarchical and Hollow ZSM‑48
Alkali desilication is widely adopted to introduce mesopores into ZSM‑48. Controlled NaOH treatment selectively removes partial framework‑Si species, generating abundant intracrystalline mesopores while preserving the *MRE crystalline framework, improving mass‑transfer efficiency and acid‑site accessibility. Sequential alkaline desilication‑alumination‑recrystallization strategy can fabricate hollow‑structured ZSM‑48 with adjustable shell thickness, simultaneously tuning Si/Al ratio and shortening molecular diffusion path, and thus enhance catalytic stability in acid‑catalyzed reactionsACS Public....
3.4 Microwave‑Assisted Hydrothermal Synthesis
Microwave heating achieves fast and uniform heating, speeds up nucleation and crystal growth. Microwave‑synthesized ZSM‑48 usually features smaller crystal size, larger specific surface area and shorter diffusion path, but suffers from high‑equipment‑cost restrictions for large‑scale manufacture.
