Objective Isoparaffins are crucial clean blending components for enhancing the octane number of gasoline, but achieving optimal isomerization performance in the isomerization of n-heptane over HZSM-5 zeolite remains challenging. Therefore, it is necessary to introduce active metal Zn to modify HZSM-5 and study the effect of Zn loading on the intrinsic acidic structure of the catalyst, isomerization performance, and regulation of the octane number of the products.
Method Zn-modified HZSM-5 bifunctional catalysts with metal-acid centers were prepared by the equal-volume impregnation method. Characterization and investigations were performed using X-ray diffraction, field emission scanning electron microscopy, infrared rheometry, and ammonia programmed temperature desorption. The influence of Zn loading on catalyst structure and n-heptane isomerization performance was investigated, while the product octane number was calculated using the group contribution method.
Result The introduction of Zn improved the acidity and activity of HZSM-5, significantly enhancing the catalyst's isomerization performance. As the loading increased, the conversion rate and isomerization selectivity of n-heptane isomerization initially increased and then decreased.
Conclusion When the loading was 6% (mass fraction the same below), the support acidity was optimal, promoting the reaction toward maximum isomerization degree. Exceeding 6% loading led to excessive metal coverage of active sites, hindering the reaction. The 6% Zn/ZSM-5 catalyst achieved a n-heptane isomerization conversion rate of 60.04%, an isomerization selectivity of 44.11%, and a product octane number of 44.24, representing a significant improvement over n-heptane (octane number of 0). When applied to naphtha isomerization, the catalyst increased the octane number from 60.70 to 71.20.
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