Mg-modified Pt/ZSM-48 for Efficient and Selective Hydroisomerization of Long-Chain Alkanes

Abstract

ZSM-48 zeolite, which features one-dimensional ten-membered-ring straight channels, is an ideal support for hydroisomerization catalysts to improve fuel low-temperature fluidity. To improve the poor low-temperature fluidity of Fischer–Tropsch synthesis products, this study employed Mg modification on Pt/ZSM-48 catalysts to optimize their hydroisomerization performance. A series of Mg-Pt/ZSM-48 catalysts with varying Mg loadings was prepared via incipient wetness impregnation. The effects of Mg modification on the pore structure, acidity, and metal–acid balance of the Pt/ZSM-48 catalyst system were systematically investigated. Catalytic performance was evaluated using n-hexadecane as a model compound. The Mg species selectively modified the micropore channels, decreasing the specific surface area and micropore volume. Moreover, Mg²⁺ selectively neutralized Brønsted acid sites, reducing the concentration of medium-strong acid sites and Brønsted acidity while introducing Lewis acid sites, thereby lowering the Brønsted/Lewis acid ratio. Appropriate Mg modification optimized the metal–acid balance, making acid-catalyzed skeletal isomerization the rate-determining step. Although the Mg-modified catalysts exhibited moderately reduced catalytic activity, their isomer selectivity and yield were significantly enhanced. The 0.5 Mg-Pt/Z48 catalyst achieved an isomer yield of 81.81% at 91.18% conversion, representing a 9% improvement over the unmodified catalyst. The micropore confinement effect weakened the interaction between alkane molecules and micropore channels, promoting the key–lock catalytic mechanism. This increased the formation of mono-branched isomers close to the center of the main carbon chain, significantly improving the low-temperature fluidity of the fuel. This study provides a simple and efficient strategy for designing high-performance hydroisomerization catalysts for long-chain n-alkanes.

Graphical Abstract

Keywords

Hydroisomerization / Mg promoter / ZSM-48 / n-Hexadecane

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