Abstract
ZSM-35, a high-silica zeolite with the FER (ferrierite) topology, exhibits exceptional shape-selective catalytic properties due to its unique two-dimensional pore system. Comprising intersecting 10-membered ring (10-MR) and 8-membered ring (8-MR) channels, ZSM-35 enables precise molecular sieving and transition-state selectivity in various petrochemical and fine chemical reactions. This article reviews the structural characteristics, acid site distribution, and shape-selective mechanisms of ZSM-35, highlighting its applications in isomerization, cracking, alkylation, and methanol-to-olefins (MTO) processes. Recent advances in modification strategies, including metal incorporation and hierarchical pore engineering, are also discussed to enhance catalytic efficiency and stability.
1. Introduction
Zeolites are crystalline aluminosilicates renowned for their uniform microporous structures, tunable acidity, and thermal stability. Among them, ZSM-35 (FER type) has garnered significant attention due to its distinctive pore architecture that facilitates shape-selective catalysis. First synthesized in the 1970s, ZSM-35 possesses an orthorhombic crystal system with a framework composed of interconnected 10-MR and 8-MR channels. These channels form a 2D network with pore dimensions ranging from 0.35 × 0.48 nm (8-MR) to 0.42 × 0.54 nm (10-MR), creating cage-like voids of approximately 0.6–0.7 nm. Such structural features allow ZSM-35 to discriminate molecules based on size, shape, and diffusion kinetics, making it ideal for reactions requiring high selectivity.
2. Structural Characteristics and Porosity
2.1 Framework Topology
ZSM-35 adopts the FER topology, characterized by:
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10-MR Channels: Running parallel to the [001] direction, these channels (0.42 × 0.54 nm) accommodate linear and slightly branched hydrocarbons.
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8-MR Channels: Oriented along the [010] direction, these narrower pores (0.35 × 0.48 nm) restrict access to bulky molecules, enhancing reactant selectivity.
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Intersection Cavities: The crossing of 10-MR and 8-MR channels generates spherical cages (~0.6–0.7 nm), which serve as active sites for transition-state stabilization.
2.2 Silicon-to-Aluminum Ratio (Si/Al)
The Si/Al ratio of ZSM-35 typically ranges from 10 to over 200, influencing:
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Acidity: Lower Si/Al ratios increase Brønsted acid site density, boosting activity but potentially reducing hydrothermal stability.
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Hydrophobicity: Higher Si/Al ratios enhance resistance to water poisoning, crucial for steam-rich environments.
2.3 Thermal and Hydrothermal Stability
ZSM-35 maintains structural integrity up to 800°C and exhibits robust performance under steaming conditions, attributed to its high silica content and rigid framework.
3. Mechanisms of Shape Selectivity
Shape selectivity in ZSM-35 arises from three primary mechanisms:
3.1 Reactant Selectivity
The narrow 8-MR channels exclude bulky molecules from entering the pore system. For instance, in xylene isomerization, only para-xylene (kinetic diameter ~0.58 nm) diffuses freely, while ortho- and meta-isomers are restricted.
3.2 Product Selectivity
Even if multiple products form within the cages, only those with compatible dimensions can exit. This phenomenon is exploited in methanol-to-propylene (MTP) processes, where ZSM-35 favors propylene over heavier olefins.
3.3 Transition-State Selectivity
The confined space within ZSM-35 cages stabilizes specific transition states while suppressing others. In n-butane isomerization, the formation of branched isomers is favored due to spatial constraints that inhibit cracking pathways.
