In order to reveal the effects of molecular structure, the catalytic cracking of n-hexane, 1-hexene, cyclohexane and cyclohexene over HZSM-5 zeolites was carried out at 260–550 °C under an atmosphere. Particular attention was paid to the variation in the trends of the conversion and product distribution with the reaction temperature and time on stream (TOS). The fresh and spent HZSM-5 zeolites were studied by XRD, SEM, Py-IR, TPO, NH₃-TPD and N₂ physisorption. It was found that the catalytic activity was in descending order of 1-hexene > cyclohexene > n-hexane > cyclohexane, and the catalytic stability was in descending order of 1-hexene > n-hexane > cyclohexene > cyclohexane. n-Hexane benefited alkane formation, 1-hexene benefited alkene formation, while cyclohexane and cyclohexene benefited aromatic formation. Coke formation blocked the porous channels and reduced the acid sites of HZSM-5 zeolites, and C6 catalytic cracking exhibited a distinct response to increases in coke and TOS. Compared with n-hexane, the increase in coke formation significantly inhibited cyclohexane catalytic cracking. Although 1-hexene catalytic cracking achieved a similar amount of coke to cyclohexane, the conversion of 1-hexene remained unchanged with an increase in TOS, which was attributed to the high activity of the CC bond. Analogously, cyclohexene with high activity exhibited stable conversion with an increase in TOS. Interestingly, the coke was almost all generated at the beginning in cyclohexene catalytic cracking at 550 °C and selectivity for everything except benzene was close to zero after 3 h on stream. Considering the stable conversion and fluctuating product distribution with an increase in TOS, it was deduced that alkene was crucial to coke formation in cyclohexene catalytic cracking. The typical reaction pathways were summarized from the literature and mechanism indices were defined to reveal the role of molecular structure in C6 catalytic cracking. It was found that the CC bond and cyclic structure inhibited protolytic cracking, hydride transfer and isomerization, while they enhanced oligomerization and aromatization compared to a C–C bond and a linear structure, respectively.

中文翻译：

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HZSM-5分子筛正己烷、1-己烯、环己烷和环己烯催化裂化分析：分子结构的影响

为了揭示分子结构的影响，HZSM-5 沸石催化裂化正己烷、1-己烯、环己烷和环己烯在 260-550 ℃的气氛下进行。特别注意了转化率和产物分布趋势随反应温度和运行时间 (TOS) 的变化。通过XRD、SEM、Py-IR、TPO、NH ₃ -TPD和N ₂物理吸附研究了新鲜和废弃的HZSM-5沸石。结果表明，催化活性从大到小依次为1-己烯>环己烯>正己烷>环己烷，催化稳定性从大到小依次为1-己烯>正己烷>环己烯>环己烷。n-己烷有利于烷烃的形成，1-己烯有利于烯烃的形成，而环己烷和环己烯有利于芳烃的形成。焦炭的形成阻塞了多孔通道并减少了 HZSM-5 沸石的酸性位点，C6 催化裂化对焦炭和 TOS 的增加表现出明显的响应。与正己烷相比，结焦量的增加显着抑制了环己烷催化裂化。尽管 1-己烯催化裂化获得的焦炭量与环己烷相似，但随着 TOS 的增加，1-己烯的转化率保持不变，这归因于 C 的高活性C键。类似地，具有高活性的环己烯随着 TOS 的增加表现出稳定的转化率。有趣的是，焦炭几乎都是在开始时在 550 °C 的环己烯催化裂化中产生的，并且在投产 3 小时后对除苯以外的所有物质的选择性都接近于零。考虑到稳定的转化率和随着 TOS 的增加产物分布的波动，推断烯烃对环己烯催化裂化中的焦炭形成至关重要。从文献中总结了典型的反应途径，并定义了机理指标，以揭示分子结构在 C6 催化裂化中的作用。发现CC键和环状结构抑制了质子分解、氢化物转移和异构化，而与C-C键和线性结构相比，它们分别增强了低聚和芳构化。