正己烷热裂解中自由基竞争反应机理和选择性

Mechanisms and Selectivities of Radical Competition Reactions in n-Hexane Pyrolysis

在923～1 023K范围内,考察了正己烷热裂解反应,对各自由基上发生的竞争反应的选择性进行了定量分析,并讨论了反应温度对基元反应选择性的影响。实验结果表明,正己烷热裂解为链式反应机理,链反应由原料分子热均裂而引发,产生自由基;链反应通过自由基自身β-裂解和从反应物分子提取H反应而传播,同时生成烯烃和部分烷烃(包括氢气);链反应被双自由基重合反应而终止,同时生成另一部分烷烃产物。不同的自由基参与不同的基元反应,并具有不同反应途径或然率。升高反应温度有利于反应物分子的热均裂反应,促进自由基β-裂解反应,抑制自由基提取H反应,并提高焦炭和氢气收率。随反应温度从923K升至1 023K,动力学链长从7.33减小为5.73。

Pyrolysis of n-hexane was conducted in the temperature range from 923 K to 1 023 K.Selectivities to the free radical competition reactions were investigated quantitatively and effect of reaction temperature on selectivities to the elementary reactions were studied.The results showed that n-hexane pyrolysis undergone a chain reaction process.The reaction chain was initiated through homolysis of feed molecules,which led to formation of various radicals,propagated by β-decomposition of radicals and H-Abstraction from the feed,which produced olefins and one part of paraffins,and terminated by recombination between two radicals,which produced another part of paraffins.Various radicals participated in different elementary reactions and possessed varying reaction path probabilities.Raising temperature could promote the homolysis of feed molecules and β-decomposition of radicals,restrain radical H-Abstraction,and increase the yields of coke and hydrogen.The average reaction chain length decreased from 7.33 to 5.73 with rise of temperature from 923 K to 1 023 K.