Abstract
ZSM-5 is a typical medium-pore MFI-type zeolite with unique intersecting ten-membered-ring channel systems, adjustable acidity, and excellent thermal stability. Due to its superior shape selectivity and tunable Brønsted and Lewis acid sites, ZSM-5 has become one of the most widely used catalysts for hydrocarbon isomerization in petroleum refining. Compared with large-pore zeolites and amorphous acidic catalysts, ZSM-5 effectively promotes the skeletal isomerization of light alkanes and alkenes while suppressing excessive cracking and coking reactions. This article systematically introduces the structural advantages of ZSM-5 for isomerization, analyzes the effects of Si/Al ratio, acid distribution, metal modification, and pore structure on isomerization selectivity, and summarizes the reaction mechanism and recent progress of ZSM-5-based catalysts in alkane and alkene isomerization.
Keywords: ZSM-5 zeolite; MFI topology; skeletal isomerization; hydrocarbon; acidity modulation; shape selectivity
1. Introduction
With the increasing demand for high-quality clean fuels and high-value chemical raw materials, hydrocarbon isomerization has become an indispensable reaction in modern petrochemical industry. Linear alkanes and linear olefins usually have low octane numbers and poor reactivity, while branched isomers exhibit higher octane values, better combustion performance, and higher chemical activity. Catalytic isomerization can convert straight-chain hydrocarbons into branched isomers, which significantly improves fuel quality and industrial utilization efficiency.
ZSM-5 zeolite with MFI topology possesses a unique binary pore structure consisting of straight channels and sinusoidal channels, providing suitable spatial confinement for molecular isomerization. Different from small-pore SSZ-13 and large-pore Beta zeolites, the 10-membered-ring pore size of ZSM-5 matches well with the kinetic diameter of C4–C8 hydrocarbon molecules, allowing isomerization reactions to proceed smoothly while limiting over-cracking into small gas molecules. Therefore, ZSM-5-based catalysts have been widely applied in industrial isomerization processes, including n-butene isomerization, n-pentane/n-hexane isomerization, and light gasoline upgrading.
2. Structural Basis of ZSM-5 for Isomerization Catalysis
ZSM-5 has a rigid MFI framework composed of interconnected 10-membered ring (10-MR) channels, including straight channels (0.54 nm × 0.56 nm) and zigzag sinusoidal channels (0.51 nm × 0.55 nm). The medium pore size and three-dimensional channel structure endow ZSM-5 with moderate shape selectivity, which is the key structural foundation for efficient hydrocarbon isomerization.
First, the pore aperture of ZSM-5 allows the diffusion of linear and monobranched hydrocarbon molecules, but restricts the formation and diffusion of multi-branched products with larger molecular sizes. This feature effectively inhibits deep cracking and heavy coking, greatly improving the selectivity of target branched isomers. Second, ZSM-5 has adjustable Si/Al ratios in a wide range, which can precisely regulate the density and strength of Brønsted acid sites. Moderate acid strength is essential for isomerization: weak acid sites cannot activate C–C bonds effectively, while overly strong acid sites will cause severe cracking and hydrogen transfer side reactions.
In addition, ZSM-5 exhibits excellent thermal and hydrothermal stability, which enables long-term stable operation under high-temperature isomerization reaction conditions. Its high specific surface area can uniformly disperse active metal components, further improving isomerization activity and stability.
3. Acid-Catalyzed Isomerization Mechanism over ZSM-5
Hydrocarbon isomerization over ZSM-5 follows the classic carbonium ion reaction mechanism dominated by Brønsted acid sites. The entire process includes three main steps: reactant activation, carbon skeleton rearrangement, and product desorption.
Firstly, the Brønsted acid sites (Si–OH–Al) on ZSM-5 surface provide active protons to protonate linear alkene or alkane molecules, forming unstable carbonium ions. Subsequently, the high-energy carbonium ions undergo intramolecular hydrogen transfer and methyl shift rearrangement, converting straight-chain carbon skeletons into branched carbon skeletons. Finally, the branched carbonium ions lose protons to form stable branched isomer products and regenerate zeolite acid sites.
For n-alkane isomerization, the reaction requires mild cracking to generate alkene intermediates first, followed by skeletal isomerization and hydrogenation to produce branched alkanes. ZSM-5 with appropriate acidity can precisely control the degree of alkane activation, realizing efficient isomerization while minimizing dry gas generation. For linear olefin isomerization, ZSM-5 directly promotes rapid skeletal rearrangement of olefinic carbonium ions, showing high reaction efficiency under relatively low temperature conditions.
