Light olefins, including ethylene and propylene, are essential building blocks of the petrochemical industry, and the demand for them has been growing globally. Although HZSM-5 zeolite is widely used in catalytic cracking reactions, most relevant studies have not thoroughly examined the initial reaction behavior of n-hexane catalytic cracking, leading to an insufficient understanding of molecular-scale reaction mechanisms. In a study published in ENG. Chem. Eng., researchers at China University of Petroleum, Beijing and PetroChina conducted precise catalyst tests to establish a molecular-scale reaction kinetic model for n-hexane catalytic cracking over HZSM-5 with different SiO₂/Al₂O₃ ratios.
Experimental reactions were conducted between 450 and 650 °C. When the temperature increased from 450 to 650 °C, n-hexane conversion rose from 4.4 % to 43.8 %. Propylene selectivity remained highest across all temperatures, reaching 38.5 % at 650 °C. Selectivity to isohexane decreased from 6.7 % to 0.5 % as temperature increased, indicating that n-hexane preferentially isomerizes to isohexane at lower temperatures.
Analysis of the conversion-selectivity relationship revealed that isohexane is a primary product with non-zero initial selectivity, while ethylene and propylene are mainly secondary products with near-zero initial selectivity. Butenes act as intermediate products, with selectivity first increasing and then decreasing with conversion.
Two reaction networks were constructed and compared: one with n-hexane isomerization to isohexane involving 24 molecular species and 64 reactions and one without involving 23 molecular species and 58 reactions. The network with isomerization includes six reactions directly involving isohexane, which influence the formation pathways of ethylene and propylene. Isohexane also indirectly influences isobutane formation. Without isomerization, isobutane formation depends mainly on n-butane isomerization, whereas with isomerization, isobutane can form directly from isohexane cracking.
A kinetic model was developed based on each reaction network. In the model incorporating n-hexane isomerization, the mean absolute error between experimental and fitted data was 0.21 wt%. In the model excluding isomerization, the MAE increased to 1.13 wt%. Kinetic parameters revealed that the isomerization pathway has opposite effects on ethylene and propylene formation: it increases the propylene formation rate while decreasing the ethylene formation rate.
To experimentally verify these findings, co-feeding experiments were conducted with isohexane added to the n-hexane feed at 1, 2, 5, and 10 vol%. At similar conversion levels, the addition of isohexane increased ethylene selectivity and decreased propylene selectivity compared to pure n-hexane feed. This confirms that the isomerization pathway exerts opposing regulatory effects on ethylene and propylene production, providing an effective strategy for controlling the ethylene/propylene ratio.
This work deepens the understanding of the n-hexane cracking mechanism and provides a basis for the directional regulation of reaction pathways and the optimization of industrial process conditions.
DOI
10.1007/s11705-026-2702-4