Acid strength controlled reaction pathways of propylene conversion under olefin cracking conditions over ZSM-5

Olefins cracking to ethylene (C₂⁼) and propylene (C₃⁼) on Brønsted acid sites represents a critical method for converting low-value light olefins into value-added products. In this process, a competition between propylene formation and conversion directly determined the selectivity of C₂⁼ and C₃⁼, as well as the use of carbon resource. However, the conversion behaviour and specific reaction patterns of propylene have not been clarified. Here, we proposed a propylene conversion network under cracking conditions and demonstrated that the competition between dimerization-cracking reaction and hydrogen transfer reaction was pivotal for the use of olefin carbon resources. Hydrogen transfer index (HTI = S(C₃H₈) / S(C₂H₄ + C₄H₈)) was established as a metric to assess this competitive reaction. Based on the reaction mechanism of carbonium ions, the impact of acid strength of zeolite on this competitive reaction was predicted. Furthermore, an in-depth investigation of the total acid amount and acid strength of ZSM-5 on this competition was conducted. The results indicated that an increase in acid amount enhanced hydrogen transfer reactions of propylene, and strong acid sites remarkably promoted these reactions. Specifically, an increase in the acid strength of ZSM-5 led to a more pronounced decrease in the activation energy required for hydrogen transfer reaction compared to that for dimerization-cracking reactions. Consequently, an efficient catalyst for olefin catalytic cracking to C₂⁼ and C₃⁼ is characterized by minimized strong acid sites in ZSM-5, while ensuring the cracking reaction proceeds effectively. These insights are crucial for the design of high-performance catalysts for the production of C₂⁼ and C₃⁼ from olefins.