1. Introduction
Silicalite-1 (S-1) occupies a unique position among microporous materials. While it shares the identical MFI framework topology with ZSM-5—featuring a three-dimensional network of intersecting 10-membered-ring (10-MR) channels—it differs fundamentally in composition. S-1 contains virtually no aluminum in its framework, resulting in a purely siliceous (SiO₂) structure.
This all-silica composition imparts several distinctive properties:
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Hydrophobicity and oleophilicity: Without negatively charged AlO₄⁻ tetrahedra requiring charge-balancing cations, S-1 is highly hydrophobic and preferentially adsorbs organic molecules over water. This makes it ideal for recovering organics from aqueous streams.
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Absence of strong Brønsted acidity: S-1 lacks the strong protonic acid sites characteristic of aluminosilicate zeolites. Its catalytic activity stems instead from weakly acidic silanol groups, which can drive certain reactions (e.g., Beckmann rearrangement, alkylation) without promoting undesirable side reactions such as cracking, coking, or isomerization.
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Exceptional stability: S-1 retains its crystallinity and porosity after prolonged exposure to temperatures above 800 °C and to strongly acidic or basic environments, owing to the robustness of the pure Si–O–Si framework.
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Shape selectivity: The MFI pore system (straight channels ~0.53 × 0.56 nm; sinusoidal channels ~0.51 × 0.55 nm) imposes stringent size and shape constraints on molecular diffusion, enabling precise molecular sieving.
Since its discovery, S-1 has found applications in gas adsorption and separation, pervaporation membranes, catalysis (both as a standalone catalyst and as a support for metals), and as a shell material in core–shell composite zeolites. Understanding the relationship between its siliceous framework and functional performance remains an active area of research.
2. Structural Characteristics
2.1 Framework Topology
Silicalite-1 crystallizes in the orthorhombic crystal system with space group Pnma, identical to high-silica ZSM-5. Its framework is constructed entirely from corner-sharing SiO₄ tetrahedra, forming a three-dimensional network of intersecting 10-MR channels:
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Straight channels running parallel to the [010] direction, with elliptical cross-sections of approximately 0.53 nm × 0.56 nm.
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Sinusoidal ("zigzag") channels running parallel to the [100] direction, with near-circular cross-sections of approximately 0.51 nm × 0.55 nm.
These two channel systems intersect to create a complex three-dimensional pore network that imposes shape-selective constraints on molecular transport. The absence of aluminum eliminates framework charges, resulting in a neutral, purely covalent Si–O–Si network.
2.2 Silanol Groups and "Quasi-Acidity"
Although S-1 lacks conventional Brønsted acid sites (Si–O(H)–Al), it possesses silanol groups (Si–OH) that arise from:
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Terminal silanols on the external crystal surface.
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Silanol nests (geminal or vicinal Si–OH groups) associated with framework defects (missing Si atoms).
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Hydrogen-bonded silanol clusters within intracrystalline cavities or at grain boundaries.
These silanol groups exhibit weak acidity and can participate in hydrogen-bonding interactions with reactant molecules. Recent studies have demonstrated that densely packed, hydrogen-bonded silanol groups in hierarchical S-1 can generate enhanced "quasi-acidity" sufficient to catalyze reactions such as the vapor-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam, achieving caprolactam selectivity of 94.1% and catalyst lifetime of 126 h.
In 2026, researchers at Taiyuan University of Technology exploited the weak acidity of silanol-rich S-1 (AS-1) for CO₂ hydrogenation-coupled toluene alkylation to para-xylene (PX). Compared to conventional ZSM-5, AS-1 suppressed undesirable dealkylation and deep alkylation side reactions, reducing benzene selectivity from 6.04% to 0.5% and increasing PX selectivity from 23.6% to 33%.
2.3 Framework Defects and Flexibility
S-1 crystals invariably contain framework defects, including silanol nests and missing tetrahedral atoms. These defects influence hydrophobicity, mass transfer, and catalytic behavior. Interestingly, recent work from the Dalian Institute of Chemical Physics (DICP) and Xiamen University has revealed that S-1's sub-nanometer channels can accommodate fluorescent probe molecules (DB-BODIPY, minimum projection diameter ~11.08 Å) larger than the nominal pore opening. The molecule undergoes conformational adaptation under spatial confinement, shrinking its minimum projection to ~8.37 Å, enabling entry into slightly stretched channels. This finding challenges classical molecular sieving theory and highlights the dynamic nature of S-1's pore system.
3. Synthesis Methods
3.1 Conventional Hydrothermal Synthesis
The standard method for S-1 preparation involves hydrothermal crystallization of a gel containing a silicon source (e.g., tetraethyl orthosilicate, TEOS; silica sol; fumed silica) and an organic structure-directing agent (SDA), typically tetrapropylammonium hydroxide (TPAOH) or tetrapropylammonium bromide (TPABr). The mixture is heated in an autoclave at 130–180 °C for 1–5 days.
Limitations:
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High cost of organic templates (TPAOH).
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Generation of wastewater containing organic templates and ammonia-nitrogen.
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Long crystallization times.
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Difficulty in controlling crystal size and morphology.
3.2 Advanced Synthesis Strategies
To address these challenges, several innovative approaches have been developed:
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Seed-assisted synthesis: Adding pre-formed S-1 seeds dramatically accelerates nucleation, reduces template consumption, and shortens crystallization time. In 2024, researchers demonstrated that combining NH₄F (F/Si = 0.1) with S-1 seeds (10 wt%) in a low-template system (TPA⁺/Si = 0.007) enabled S-1 synthesis in just 8 hours, while suppressing SiO₂ impurity formation and allowing precise morphology control.
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Steam-assisted crystallization (SAC) and dry-gel conversion (DGC): These methods minimize water usage and enable the synthesis of nanosized S-1 with controlled particle sizes (100–800 nm). Using spherical mesoporous silica as a precursor, SAC produces S-1 nanocrystals without introducing additional templates, and the aqueous phase can be recycled.
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Solid-phase synthesis from solid waste: In 2024, researchers developed an alkali melting–pickling assisted solid-phase synthesis method using fly ash as the silicon source. SiO₂ with 97.84% purity was extracted from fly ash, and S-1 molecular sieve was prepared with a specific surface area of 623.30 m²/g and total pore volume of 0.31 cm³/g. The resulting S-1 exhibited a CO₂ adsorption capacity of 2.05 mmol/g at 298 K and 1 bar—four times higher than the extracted SiO₂—with high CO₂/N₂ selectivity and excellent regeneration stability over 10 adsorption–desorption cycles.
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Solvent-free synthesis: In 2024, researchers at Shenyang Normal University reported the successful solvent-free synthesis of both S-1 and TS-1, eliminating water from the synthesis system and significantly reducing environmental impact while maintaining good crystallinity and catalytic performance.
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Mother liquor recycling: A 2024 patent from Jilin University described a circular preparation method in which crystallization mother liquor is fully recycled for subsequent S-1 synthesis batches, greatly reducing industrial pollution and improving raw material utilization while maintaining stable product properties across multiple cycles.
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Nanocrystal synthesis: Various strategies have been developed to produce nanosized S-1 (10–800 nm), including microwave-assisted synthesis, changing silicon sources, adjusting aging time, and using different SDAs. Nanosized S-1 exhibits higher surface energy, shorter diffusion paths, and improved resistance to sulfur poisoning and coking deactivation.
