Abstract
ZSM-35 zeolite possesses FER topological framework, also known as ferrierite molecular sieve, which is a medium-pore aluminosilicate zeolite with distinctive dual-pore channel system. Its unique intersecting 8-membered ring and 10-membered ring pore channels, controllable silica-alumina ratio, moderate acid strength and outstanding thermal-hydrothermal stability bring superior shape-selective catalytic performance. This paper systematically elaborates the crystal topology, framework construction, pore channel configuration, physicochemical features, acid site distribution and structural advantages of ZSM-35 zeolite. The inherent correlation between its structural characteristics and catalytic behavior is discussed, providing theoretical basis for its modification and industrial application in hydrocarbon conversion processes.
Keywords: ZSM-35 zeolite; FER topology; ferrierite; dual pore channels; medium-pore zeolite; framework structure; shape selectivity
1. Introduction
ZSM-35, ferrierite-type zeolite with FER topology, was first developed as a shape-selective catalytic material for hydrocarbon processing. Different from MFI-type ZSM-5 with three-dimensional 10-membered ring channels or MTW-type ZSM-12 with one-dimensional 12-membered ring pores, ZSM-35 features a special two-dimensional crossed pore system composed of 8-ring and 10-ring tunnels, forming a confined microenvironment with unique molecular screening capacity.
The framework of ZSM-35 can be synthesized under mild hydrothermal conditions with various organic templates, and its SiO₂/Al₂O₃ molar ratio can be regulated in a wide range. The rigid layered stacking structure endows ZSM-35 with excellent resistance to high-temperature steam aging, far exceeding many other medium-pore zeolites. Benefiting from its special pore geometry and tunable acidity, ZSM-35 has become an irreplaceable catalyst for light alkene isomerization, n-butene skeletal isomerization and hydrocarbon dehydration reactions. This work focuses on the full structural characteristics of ZSM-35 to reveal its structure-performance relationship.
2. Crystal Topology and Framework Construction
ZSM-35 belongs to orthorhombic crystal system with standard FER topological structure. The basic structural building units (SBUs) of FER framework are five-membered rings, which are connected via oxygen bridges to form layered sheets. Each sheet contains alternating 6-membered rings and 5-membered rings, and adjacent layers stack along the b-axis to form the complete three-dimensional crystal skeleton.
The TO₄ tetrahedra (T = Si⁴⁺, Al³⁺) constitute the whole framework. Al atoms randomly occupy tetrahedral T sites following Loewenstein’s rule, where Al-O-Al linkages are prohibited. Each framework Al generates a negative charge balanced by exchangeable extra-framework cations including Na⁺, K⁺, NH₄⁺ or H⁺. After ammonium exchange and high-temperature calcination, H-type ZSM-35 is obtained with abundant Brønsted acid sites originating from bridging Si-OH-Al groups.
The unit cell of FER-type ZSM-35 contains well-ordered layered stacks without serious stacking disorder under optimized crystallization parameters. The dense linkage of 5-ring units strengthens the overall framework rigidity, which is the fundamental reason for its excellent thermal and hydrothermal stability. Even after long-term aging at 600–700 °C with steam atmosphere, the crystal lattice of ZSM-35 maintains high integrity with limited dealumination.
3. Unique Two-Dimensional Dual Pore Channel System
The most representative structural feature of ZSM-35 is its intersecting two-dimensional pore network consisting of two sets of different-size tunnels:
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10-membered ring main channels (along [001] direction)
The straight 10-ring channels have a free aperture of 0.42 nm × 0.54 nm, classified as medium-pore channels. These tunnels allow the diffusion of linear and monomethyl-branched C4–C6 hydrocarbon molecules, serving as the primary mass transfer pathway for reactants and products.
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8-membered ring secondary channels (along [010] direction)
Narrower 8-ring channels with aperture 0.35 nm × 0.48 nm cross perpendicularly with 10-ring tunnels, forming interconnected two-dimensional pore space. The smaller 8-ring pores act as molecular sieving barriers, restricting bulky multi-branched isomers from passing through the framework.
Key structural distinctions from other common zeolites:
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No large supercages inside the ZSM-35 framework, unlike FAU, BEA or MOR zeolites;
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Dual-size crossed micropores create stronger steric confinement effect than single 10-ring ZSM-5;
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The layered stacking structure makes the pore channels present slab-like distribution, leading to anisotropic diffusion of molecules.
The confined dual-pore geometry realizes precise shape selectivity: linear n-butenes can easily enter the pore channels for skeletal isomerization to form isobutene, while bulky multi-branched by-products are blocked inside the pores and inhibited from generating, greatly improving target product selectivity.
