1. Introduction
Mordenite (framework code MOR) is one of the earliest commercialized high-silica zeolites, first discovered as natural minerals in Morden town, Canada, and artificially synthesized by Barrer in 1948. The as-synthesized raw product is sodium-form mordenite, abbreviated Na-MOR, whose ideal anhydrous crystal formula is Na₈Al₈Si₄₀O₉₆, with adjustable Si/Al molar ratio ranging from 5 to 12 in conventional industrial products.
Different from H-MOR with strong Brønsted acidity, Na-MOR contains exchangeable Na⁺ cations instead of bridging hydroxyl protons, so it shows weak intrinsic acidity but outstanding cation exchange property. Its unique dual pore system consisting of straight 12-membered ring main channels and confined 8-membered ring side pockets brings remarkable shape-selective adsorption performance for small molecular gases, hydrocarbons and heavy metal ions.
In industrial production, Na-MOR acts as the universal precursor for all modified MOR catalysts: through ammonium exchange and high-temperature calcination, Na⁺ can be completely removed to obtain H-MOR solid acid catalyst; via transition metal (Cu, Fe, Ni, Pt) ion exchange, metal-modified MOR materials for SCR denitrification and alkane isomerization can be prepared. In addition, unmodified Na-MOR is directly applied in natural gas purification, VOCs adsorption, wastewater heavy metal removal and air drying fields due to low production cost and stable adsorption performance. In recent years, green template-free synthesis and hierarchical pore construction of Na-MOR have become hot research topics to reduce manufacturing cost and overcome mass transfer limitation of single micropores.
2. Crystal Structure and Physicochemical Properties of Na-MOR
2.1 Framework and Pore Structure
Na-MOR belongs to orthorhombic crystal system with Cmcm space group, unit cell parameters a≈18.3 Å, b≈20.5 Å, c≈7.5 Å. The framework is assembled by four-ring and five-ring secondary building units (SBUs) composed of corner-sharing TO₄ tetrahedra (T=Si, Al), forming two independent non-interconnected pore channels:
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12-membered ring (12-MR) main channels along the [001] axis: elliptical aperture of 6.5 Å × 7.0 Å, serving as the primary mass transfer channel for reactant molecules such as benzene, toluene and low-carbon alkanes;
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8-membered ring (8-MR) side pockets along the [010] axis: narrow aperture of 2.6 Å × 5.7 Å, which accommodate most Na⁺ exchangeable cations and produce size screening effect for small polar molecules.
The 8-MR side pockets are isolated from each other, leading to obvious intracrystalline diffusion resistance for macromolecules, which is the core structural drawback of conventional bulk Na-MOR crystals.
2.2 Core Physicochemical Characteristics of Na-MOR
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Cation exchange capacity (CEC)
Isomorphous substitution of Si⁴⁺ by Al³⁺ generates negative framework charges, which are fully balanced by Na⁺ in Na-MOR. The Na⁺ can be reversibly replaced by NH₄⁺, H⁺, Cu²⁺, Fe³⁺, La³⁺ and other cations under mild aqueous conditions, which is the foundation of subsequent modification and functionalization.
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Thermal and hydrothermal stability
Benefiting from high silica framework, Na-MOR maintains complete crystal structure after calcination at 650–720 °C. Under 400–550 °C steam atmosphere, its topological skeleton will not collapse, showing better hydrothermal resistance than FAU-type Y zeolite.
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Adsorption selectivity
Na⁺ cations in side pockets form polar adsorption sites, enabling Na-MOR to selectively adsorb polar molecules including H₂O, CO₂, H₂S and NH₃, while repelling non-polar long-chain hydrocarbons. This property makes it an ideal adsorbent for gas drying and impurity removal.
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Weak acidity property
Without proton sites, Na-MOR only exhibits negligible weak Lewis acidity derived from surface unsaturated Al species, so it cannot catalyze acid-catalyzed reactions such as alkylation and carbonylation directly, and must be converted to H-MOR via ion exchange for acid catalysis applications.
3. Synthetic Routes of Na-MOR Zeolite
3.1 Traditional Template-Assisted Hydrothermal Synthesis
This is the mature industrial mass-production method for high-crystallinity Na-MOR. Raw materials include silica sol/sodium silicate as silicon source, sodium aluminate as aluminum source, NaOH as mineralizer, and tetraethylammonium hydroxide (TEAOH/TEA⁺) as structure-directing agent (SDA).
Typical synthesis gel molar composition: SiO₂: Al₂O₃: Na₂O: TEA₂O: H₂O = 40: 1: 6–10: 0.2–0.6: 300–600. Crystallization proceeds at 160–190 °C autogenous pressure for 24–72 h. After crystallization, the solid product is filtered, washed with deionized water to neutral pH and dried at 120 °C to obtain powder Na-MOR. If molding is needed, binder (pseudo-boehmite) is added for extruding into strip or spherical particles.
Advantages: High crystallinity, pure MOR phase without impurity zeolites; Disadvantages: Expensive organic template, toxic flue gas released during template calcination, high COD wastewater, high overall production cost.
3.2 Seed-Induced Organic Template-Free Synthesis
To eliminate organic template pollution and cut cost, seed-assisted green hydrothermal synthesis has been developed for Na-MOR. No organic SDA is added; trace commercial Na-MOR seed crystals (3–8 wt% relative to SiO₂) are introduced to induce oriented crystallization and inhibit miscellaneous phases such as analcime.
Synthesis conditions: Gel pH controlled at 11.5–13.0, crystallization temperature reduced to 130–160 °C, time shortened by 30%–50%. Only inorganic sodium salt mineralizers are used, greatly reducing wastewater treatment pressure. At present, this route is widely applied to produce low-to-medium silica Na-MOR (Si/Al = 5–8) for adsorption purposes, while template-free synthesis of high-silica Na-MOR (Si/Al >10) still faces difficulty in phase purity control.
3.3 Solvent-Free Mechanochemical Synthesis
An emerging low-carbon preparation technology for Na-MOR. Solid silicon source, solid sodium aluminate and solid NaOH are fully ground uniformly with seed crystals, then crystallized in a sealed oven without liquid water solvent. This technology nearly eliminates wastewater discharge, improves single-kettle yield, and generates Na-MOR with abundant crystal surface defects and higher specific surface area (330–360 m²/g). The obtained solvent-free Na-MOR shows superior adsorption capacity for VOCs and heavy metal ions compared with traditional hydrothermal samples.
