ZSM-5 酸强度控制 1-庚烯催化裂解反应路径的研究

Heptene (C₇⁼) is an important chemical intermediate. The C₇⁼ catalytic cracking to ethylene/propylene (C₂⁼/C₃⁼) represents an effective approach for the high-value utilization of carbon resources. However, the reaction pathways involved in the catalytic cracking of C₇⁼ and the principles governing their modulation remain to be thoroughly analyzed and clarified. In this study, we established a reaction network for C₇⁼ catalytic cracking based on the carbenium ion mechanism and β-scission mechanism. We investigated the catalytic performance of various zeolite catalysts with different topological structures and found that ZSM-5 zeolite, characterized by its unique pore structure, served as an efficient catalyst for C₇⁼ catalytic cracking. On this basis, a series of ZSM-5 zeolites (ZSM-5(I) to ZSM-5(V)) with comparable acid density and varying acid strength were synthesized via hydrothermal processes using phosphorus-modified and ammonium fluorosilicate-modified methods. The effects of the acidity (acid amount and acid strength) of the ZSM-5 zeolites on the catalytic cracking reaction of C₇⁼ were systematically investigated under 550 ℃. The results indicated that the C₇⁼ catalytic cracking primarily involved unimolecular cracking, hydrogen transfer, dehydrogenative aromatization, and decarbonylation reactions. The reduction of both acid amount and acid strength in ZSM-5 suppressed the reaction pathways for generating non-olefinic products, such as hydrogen transfer and aromatization, while enhancing the cracking pathways that produce olefins, thereby increasing the carbon resource utilization efficiency. Specifically, the acid strength of ZSM-5 played a crucial role in controlling the unimolecular cracking reaction pathways: pathway I (C₇⁼→C₃⁼_C₄⁼) and pathway II (C₇⁼→C₂⁼_C₅⁼). An increase in strong acid sites within ZSM-5 enhanced pathway II, leading to increased ethylene yields. Conversely, a higher number of weak acid sites promoted pathway I, resulting in greater propylene production. This research provides new insights into the design of catalysts for the efficient utilization of carbon resources in the catalytic cracking of C₇⁼ to C₂⁼/C₃⁼.