Pt/HZSM-5催化剂上甲基环戊烷的临氢转化 Ⅰ.反应条件对扩环反应的影响

Pt/HZSM-5催化剂上甲基环戊烷的临氢转化 Ⅰ.反应条件对扩环反应的影响

  • 摘要: 在连续流固定床反应装置上,考察了反应温度、压力及氢烃摩尔比等条件对甲基环戊烷(MCP)临氢转化反应活性及扩环反应选择性的影响;结合反应评价结果和热力学分析,探讨了Pt/HZSM-5催化作用下MCP的扩环反应规律。MCP临氢转化发生三类反应:扩环(RE)、开环(RO)和裂解(CR),其中扩环反应生成环己烷(CH)和苯(Bz)。结果表明,随温度的升高或氢烃比的降低,MCP的转化率增大,而提高反应压力不利于MCP的转化。扩环反应选择性呈现复杂的变化规律,随温度的升高,扩环反应选择性先增加而后逐渐降低,即存在极大值。氢烃比的影响与温度密切相关,在低温区时扩环反应的选择性随氢烃比的降低而降低,而在高温区时的影响很小。提高反应压力,对扩环反应不利。扩环产物Bz和CH的分布为MCP扩环反应的反应历程提供了信息,临氢反应的Bz/CH实验值大于理论平衡值的结果表明,与CH相同,Bz也是MCP临氢转化的一次产物。在此基础上提出了修正的MCP扩环反应历程,MCP在双功能催化剂作用下形成的中间体吸附物种,通过两条平行反应路径分别生成CH和Bz,即异构化反应和异构脱氢反应是平行反应。同时,在本催化反应体系下,Bz还可以由CH脱氢而来。

     

    Abstract: The effects of reaction parameters such as temperature, pressure, and hydrogen to hydrocarbon ratio (H2/HC) on the hydroconversion of methylcyclopentane (MCP) over Pt/HZSM-5 have been investigated in a fixedbed downflow reactor. The catalytic evaluation results were compared with the thermodynamic calculation data in order to investigate the behavior of MCP Ring enlargement (RE) reaction. Methylcyclopentane hydroconversion is described as a linear combination of three reactions: ring enlargement (RE), ring opening (RO) and cracking (CR), and RE reaction produces cyclohexane (CH) and benzene (Bz). It was shown that the conversion of MCP increased with the increase in temperature or with the decrease in H2/HC mol ratio, whereas decreased with the increase of pressure. The influence of the reaction parameters on the RE selectivity presented a complicated picture. With the increase in temperature, the RE selectivity first increased and then decreased, showing a maximum at a certain temperature. The effect of the H2/HC mol ratio was found to be relevant with temperature. At lower reaction temperatures, the RE selectivity decreased with decrease in the H2/HC mol ratio. However, at higher reaction temperatures, the H2/HC mol ratio had little influence on RE selectivity. The distribution of RE products between CH and Bz provides information on the reaction pathway. The experimental Bz/CH ratio was in excess of the equilibrium value, which strongly suggested that Bz, as the same as CH, could be a primary product from MCP. Thus, a modified reaction pathway of RE reaction could be proposed as follows: both CH and Bz were produced from the same intermediate, which was an adsorbed species on the dual function catalyst formed from MCP. In other words, the isomerization and dehydroisomerization are parallel reactions, producing CH and Bz, respectively. Meanwhile, Bz can come from CH through dehydrogenation.

     

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