Structural Characteristics of ZSM-12 Zeolite
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1. Crystal Topology and Framework Structure of ZSM-12
ZSM-12 belongs to the MTW topological type zeolite family, and its three-dimensional crystal framework is constructed by the regular connection of TO₄ tetrahedral units (T = Si, Al). The basic structural building unit (SBU) of MTW topology is composed of 5-membered rings, 6-membered rings and 12-membered rings. The primary structural units are linked by oxygen bridges to form layered structural sheets, and adjacent sheets are stacked along the c-axis to form a complete three-dimensional crystalline skeleton without interlayer dislocation or stacking disorder under ideal synthesis conditions.
In the crystal framework, silicon atoms and aluminum atoms occupy the tetrahedral T-sites randomly in accordance with Loewenstein’s rule, where Al-O-Al bonding is forbidden. The framework negative charge generated by trivalent Al substituting tetravalent Si is balanced by extra-framework cations such as Na⁺, NH₄⁺ or H⁺, which determine the ion exchange performance and Brønsted acid sites of ZSM-12. The unit cell of ZSM-12 zeolite contains a fixed number of 12-membered ring structural segments, and the crystal system is monoclinic with stable lattice parameters under normal hydrothermal synthesis conditions.
2. Pore Channel Structural Features
The most distinctive structural characteristic of ZSM-12 is its unidirectional straight large pore channel system formed by 12-membered oxygen rings:
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Pore size dimension: The free aperture of the 12-membered ring pore channel is approximately 0.56 nm × 0.60 nm, classified as large-pore zeolite, with a larger effective pore diameter than medium-pore ZSM-5 (10-membered ring, 0.51 nm × 0.55 nm).
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One-dimensional pore channel layout: The pore channels extend unilaterally along the crystal c-axis without intersecting cross channels. There are no secondary mesoporous interconnecting channels in the pure crystalline ZSM-12 framework, leading to relatively single mass transfer paths for reactant and product molecules.
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Absence of cage structure: Different from FAU (Y zeolite) and BEA (Beta zeolite) with supercages, ZSM-12 only has straight tubular pore channels without independent intracrystalline cages, which reduces the residence time of macromolecular intermediates and inhibits side reactions such as coking and polycyclic aromatic hydrocarbon generation to a certain extent.
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Pore channel smoothness: The inner wall of the 12-membered ring pore channel is relatively flat without narrow bottlenecks, which facilitates the diffusion of long-chain alkane and monocyclic aromatic molecules.
3. Adjustable Silicon-Aluminum Ratio and Skeleton Stability
ZSM-12 zeolite exhibits a wide adjustable range of SiO₂/Al₂O₃ molar ratios, which can be precisely controlled from low silicon-aluminum ratio (20) to high silicon-aluminum ratio (above 200) via regulating raw material feeding ratios, template agent dosage and hydrothermal crystallization parameters.
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Low Si/Al ZSM-12 contains abundant framework Al sites, generating dense Brønsted acid sites with high acid density, suitable for reactions requiring strong acid catalysis such as heavy aromatic transalkylation.
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High Si/Al ZSM-12 has fewer framework negative charges, weak acid strength and low acid density, with excellent hydrophobicity and thermal stability, applicable to alkane isomerization and selective adsorption separation.
The MTW skeleton of ZSM-12 possesses outstanding thermal and hydrothermal stability. The complete crystal structure can be maintained at a calcination temperature higher than 600 °C; under high-temperature steam aging conditions, the skeleton dealumination rate is significantly lower than that of Y-type zeolite, which is attributed to the dense connection mode of 5-membered ring structural units in the MTW topology that enhances the rigidity of the crystal framework.
4. Acid Site Structural Characteristics
The acid properties of ZSM-12 are directly derived from its skeleton Al distribution and pore channel confinement effect:
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Brønsted acid sites originate from the hydroxyl groups bridging Si and Al at the framework T-sites, and Lewis acid sites come from extra-framework aluminum species formed by skeleton dealumination after high-temperature treatment.
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Restricted by the one-dimensional 12-membered ring pore channel confinement effect, the acid sites distributed on the inner wall of the pore channels show shape-selective catalytic performance. Molecules with kinetic diameters matching the pore size can fully contact acid sites, while bulky polycyclic aromatics are hindered from entering the pore channels to avoid over-cracking.
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Compared with multidimensional pore Beta zeolite, the one-dimensional pore structure of ZSM-12 weakens the collision probability of intermediate molecules at acid sites, effectively suppressing bimolecular side reactions.
