Atomistic Insights into How Zeolite Acidity and Topology Control the Activity and Propylene Selectivity in Hexane Cracking

Catalytic cracking for propylene production is a key industrial process that addresses the growing demand for propylene in the polymer industry, where improving propylene selectivity remains a central challenge. In this study, ab initio molecular dynamics (AIMD) simulations combined with free energy sampling methods were employed to investigate the reaction network of hexane cracking, with a comparative analysis of the activity and propylene selectivity over three zeolite catalysts, HZSM-5, HSAPO-34, and HSAPO-41. The results reveal that the isomerization-cracking mechanism represents the most favorable pathway for propylene formation within the reaction network. The cracking activity of hexane is primarily governed by the acid strength of the zeolite, quantified by the reaction free energy of hexene protonation; stronger acidity corresponds to higher catalytic activity. In contrast, product selectivity is predominantly determined by the zeolite topology. A key geometric descriptor of the reactant, the Φαγ angle, defined between the α- and γ-bonds, exhibits a positive linear correlation with the effective free energy barrier of the side reaction pathway. Meanwhile, the effective free energy barrier of the main propylene-forming pathway is regulated by the maximum included sphere diameter (Di) of the zeolite, which is also positively correlated with the corresponding free energy barrier. These findings provide atomic-level insights into the structure–reactivity relationship underlying catalytic cracking and offer theoretical guidance for the rational design of zeolite catalysts with enhanced activity and propylene selectivity.