Abstract
ZSM-5 zeolite is a typical MFI-topology microporous aluminosilicate material developed by Mobil Corporation. Industrial high-silica ZSM-5 is defined as ZSM-5 zeolite with a molar silica-to-alumina ratio (SAR, SiO₂/Al₂O₃) higher than 50, which differs greatly from conventional low-silica ZSM-5 (SAR=20–50). Owning to its standard three-dimensional 10-membered ring intersecting pore system, low framework aluminum content, adjustable weak-medium acidity, excellent hydrophobicity and ultra-high hydrothermal stability, high-silica ZSM-5 becomes a long-life, anti-coking industrial catalyst. Compared with low-silica ZSM-5, Beta zeolite, SAPO-34 and Y-type zeolite, it features low acid density, weak overall acidity, strong water vapor resistance and slow carbon deposition rate. It is widely commercialized in fluid catalytic cracking (FCC) propylene enhancement, methanol-to-propylene (MTP), diesel hydrodewaxing, industrial VOCs catalytic purification and hydrocarbon isomerization. This paper elaborates the MFI framework structure, acid regulation rule, core physicochemical advantages, modification methods and mainstream industrial scenarios of industrial high-silica ZSM-5, and summarizes its industrial selection criteria versus ordinary ZSM-5 zeolite.
Keywords: High-silica ZSM-5; MFI topology; silica-alumina ratio; zeolite acidity; anti-coking; industrial catalysis; petrochemical industry
1. Introduction
With the upgrading of low-carbon petrochemical and coal chemical processes, industrial catalysts are required to possess long service life, high selectivity, low coking performance and high-temperature regeneration tolerance. As a classic pentasil-type zeolite, ZSM-5 has been applied in catalytic fields for decades. According to silica-alumina ratio, it is divided into low-silica, medium-silica and high-silica grades. Industrial high-silica ZSM-5 generally has SAR ranging from 50 to 200, and ultra-high-silica ZSM-5 can reach SAR above 200. Different from low-silica ZSM-5 which relies on abundant strong acid sites to drive cracking and aromatization reactions, high-silica ZSM-5 reduces total acid content by decreasing framework Al atoms, weakens Brønsted acid strength, and optimizes pore surface hydrophobicity. This structural characteristic solves the industrial pain points of rapid coking, hydrothermal dealumination and short cycle life of conventional zeolite catalysts, making it indispensable in modern refining, coal chemical and environmental catalytic industries.
2. MFI Framework Structure of Industrial High-Silica ZSM-5
2.1 Crystal and Pore Structure
High-silica ZSM-5 retains complete orthogonal MFI topological framework, assembled by basic 5-membered ring tetrahedral units (SiO₄ and AlO₄). It owns a set of interconnected three-dimensional 10-membered ring micropore channels with fixed aperture size:
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Sinusoidal zig-zag channels (parallel to a-axis): 0.51 nm × 0.55 nm
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Straight elliptical channels (parallel to b-axis): 0.53 nm × 0.56 nm
The intersection zone of two sets of channels forms a large cavity with diameter about 0.9 nm, providing active reaction space for small and medium hydrocarbon molecules (C3–C6). Benefiting from high silica composition, high-silica ZSM-5 has high framework integrity, with industrial relative crystallinity ≥95%, specific surface area 340–420 m²/g, and pore volume 0.18–0.25 cm³/g. Its Na₂O residual content is controlled below 0.1% for industrial finished products, avoiding sodium-induced framework destruction during high-temperature regeneration.
2.2 Acidity Rule Determined by Silica-Alumina Ratio
For all aluminosilicate zeolites including ZSM-5: the higher the silica-alumina ratio, the weaker the overall acidity.
Framework Brønsted (B) acid sites originate from Si-OH-Al bridging hydroxyl groups, and each framework Al atom corresponds to one independent B acid site. High-silica ZSM-5 has scarce distributed framework Al atoms, which reduces total acid quantity and strong acid proportion effectively:
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Low-silica ZSM-5 (SAR=20–50): high total acid, abundant strong B acid, high cracking activity, easy carbon deposition
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High-silica ZSM-5 (SAR=50–200): dominated by weak and medium acid sites, low acid density, inhibited hydrogen transfer and aromatization side reactions
Lewis (L) acid sites mainly come from extra-framework aluminum and structural defects, which are barely affected by bulk silica-alumina ratio. In addition, high-silica framework brings strong surface hydrophobicity, which repels water molecular adsorption and prevents hydrothermal dealumination under high-temperature water vapor conditions.
