ZSM-23 zeolite, a microporous molecular sieve with a unique MTT topological structure, has demonstrated exceptional catalytic performance in isomerization reactions due to its one-dimensional pore channels and adjustable acidity. This article reviews the structural characteristics, modification strategies, and catalytic mechanisms of ZSM-23 zeolite in various isomerization processes, including hydrocarbon skeleton isomerization, alkane hydroisomerization, and aromatic isomerization. The impact of pore size, acid site distribution, and metal modification on reaction selectivity and catalyst stability is discussed, along with industrial applications and future research directions.
Isomerization reactions play a crucial role in petroleum refining and chemical synthesis, enabling the conversion of straight-chain hydrocarbons into branched isomers to improve fuel quality and produce high-value chemicals. Traditional catalysts such as zeolites and metal oxides often face challenges in selectivity and stability under harsh reaction conditions. ZSM-23 zeolite, with its unique pore structure and acid properties, has emerged as a promising catalyst for isomerization reactions, offering advantages in both activity and product distribution.
ZSM-23 zeolite belongs to the MTT topological framework, characterized by one-dimensional parallel channels composed of ten-membered rings (0.45 × 0.56 nm). The channels are non-intersecting and have a teardrop-shaped cross-section, which provides excellent shape-selective properties. The silicon-to-aluminum ratio (SiO₂/Al₂O₃) of ZSM-23 can be adjusted from 10 to 100, allowing for precise control over acid site density and strength. The high surface area (≈200 m²/g) and moderate thermal stability (up to 600°C) make it suitable for a wide range of catalytic applications.
ZSM-23 zeolite is highly effective in the skeleton isomerization of straight-chain hydrocarbons to branched isomers. For example, in the isomerization of n-hexane to 2,2-dimethylbutane, the one-dimensional channels of ZSM-23 restrict the diffusion of large molecules, promoting the formation of branched products. The acid sites on the zeolite surface facilitate the rearrangement of carbon skeletons through carbocation intermediates, while the pore size prevents the formation of undesired by-products. Studies have shown that ZSM-23 exhibits higher selectivity for branched isomers compared to other zeolites like ZSM-5, due to its more restrictive pore environment.
Alkane hydroisomerization is a key process for producing high-octane gasoline components and lubricant base oils. ZSM-23 zeolite, when modified with platinum (Pt) or other metals, demonstrates excellent activity and selectivity in the hydroisomerization of long-chain alkanes. The metal sites provide hydrogenation/dehydrogenation functions, while the acid sites on the zeolite facilitate skeletal rearrangement. For instance, in the hydroisomerization of n-hexadecane, Pt/ZSM-23 catalysts achieve high yields of branched isomers (up to 80 wt%) at moderate temperatures (340°C). The one-dimensional channels of ZSM-23 limit the diffusion of large molecules, reducing coke formation and extending catalyst lifetime.
ZSM-23 zeolite is also used in the isomerization of aromatic compounds, such as the conversion of ortho-xylene to para-xylene. The unique pore structure of ZSM-23 allows for selective adsorption and diffusion of aromatic molecules, promoting the formation of para-isomers. The acid sites on the zeolite surface facilitate the rearrangement of aromatic rings through carbocation intermediates, while the pore size restricts the formation of undesired by-products. Studies have reported para-xylene selectivities of over 97% using ZSM-23 catalysts in xylene isomerization reactions.
