【作者】 李娜；

【导师】 赵亮；

【作者基本信息】 中国石油大学(北京) ， 化学工程与技术， 2018， 硕士

【摘要】 异构化技术使得FCC汽油烯烃含量降低的同时辛烷值得到保持或提升。目前烯烃骨架异构机理的研究主要集中于C4烯上,戊烯研究较少,且大部分采用的分子筛簇模型,鲜有的采用周期性模型对戊烯骨架异构机理的研究中,也未涉及脱附过程的研究,比较片面。另外,目前烯烃骨架异构化的对象多集中于纯组分的研究,混合体系并不多见。FCC汽油轻馏分小于60℃馏分中烯烷含量相当的异构化原料使得烯烷共存体系反应历程的研究成为必然。为了探究反应内部的微观作用机制,从微观层面认识反应历程,本论文采用DFT方法对2-戊烯在H-ZSM-22分子筛上的骨架异构化机理及烯烷共存体系的骨架异构化反应历程进行量化研究。骨架异构过程中,2-戊基-氧化物和3-戊基-氧化物分别采用乙基转移机理和甲基转移机理,活化能分别为0.98 eV和0.73 eV,产物均为伯碳异戊基-氧化物;中间体和边缘质子化二甲基环丙烷机理速率控制步骤的活化能分别为0.74 eV和0.42 eV,产物均为仲碳异戊基-氧化物。脱附过程中,烷基转移机理活化能为1.12 eV,二甲基环丙烷机理活化能为0.30 eV。烷基转移机理中生成了伯碳分子,活化能相对较高,二甲基环丙烷机理生成了仲碳分子,活化能相对较低,但是前者基元反应步骤比较简单,计算易收敛,后者基元反应步骤较复杂,计算难收敛,综合比较,选择烷基转移机理对烯烷体系进行研究。烯烷共存体系下,烷烃的存在使得两种吸附构型稳定性增强。差分电荷图与bader电荷均显示烯烷体系中反应部位电荷转移量增大,相互作用增强,并且烷烃也参与了电荷转移。ELF的计算显示两种吸附质与分子筛的反应部位以及烷烃分子均处于一种电子高度局域化状态。PDOS表明烯烷共存体系下,两种吸附构型中个别原子峰的大小和位置发生了变化。烯烷共存体系骨架异构化过程中,当烷烃的存在使烯烃受空间位阻效应明显时,烯烃反应活化能增加,当烷烃的存在并未使烯烃受到明显空间位阻效应时,烯烃反应活化能降低,另外,烷烃的存在增强了烯烷体系的稳定性。经分析,计算中将烷烃作为一个单独的分子,并未涉及烯烃与烷烃之间的反应,是计算结果与实际存在差距的主要原因,这说明了烯烷体系研究中考虑烯烷之间反应的必要性,也为以后烯烷体系的进一步研究奠定了基础。烯烷共存体系的研究对FCC汽油异构化工艺的设计开发及汽油高辛烷值生产具有重要意义。更多还原

【Abstract】 Isomerization technology can reduce olefin content in FCC gasoline,at the same time,octane is maintained or increased.At present,the research on the mechanism of skeletal isomerization of olefin is mainly focused on C4,however,there is less research on pentene.Most researchers used the zeolite cluster models,and few of them used periodic models to study the mechanism of the pentene skeletal isomerization mechanism,and did not involve the study of the desorption process,it’s incomplete.In addition,most of the current isomerization of olefin is focused on the study of pure component,and mixed systems are rare.The isomerization of olefin in the light 60℃ fraction of FCC gasoline has made the study of the reaction process of the alkene-alkane coexistence system inevitable.In order to explore the internal microscopic mechanism of reaction and to understand the reaction process from the microscopic level,the skeletal isomerization mechanism of 2-pentene on H-ZSM-22 zeolite and the skeletal isomerization reaction process under the alkene-alkane coexistence system were studied using the DFT method.In the skeletal isomerization process,the 2-pentyl-oxide and 3-pentyl-oxide respectively adopt the ethyl transfer mechanism and the methyl transfer mechanism.The activation energy for ethyl transfer is 0.98 eV and methyl transfer is 0.73 eV and the products are primary carbon isoamyl-oxides.For the intermediate and edge protonated dimethylcyclopropane mechanism,the activation energy of the rate control step is 0.74 eV and 0.42 eV,respectively,and the products are all secondary carbon isoamyl-oxides.In the desorption process,the activation energy of the transalkylation mechanism is 1.12 eV,the activation energy of dimethylcyclopropane mechanism is 0.30 eV.In the transalkylation mechanism,primary carbon molecules are generated and the activation energy is relatively high.The dimethylcyclopropane mechanism generates secondary carbon molecules and the activation energy is relatively low.However,the former has relatively simple elementary reaction steps and the calculation is easy to converge.For the latter,the elementary reaction steps are more complex and the calculation is difficult to converge.After a comprehensive comparison,the transalkylation mechanism is chosen to study the alkene-alkane system.In alkene-alkane coexistence system,the presence of alkane enhances the stability of the two adsorption configurations.Both the charge density difference and the bader charge show that the amount of charge transfer at the reaction sites in the alkene-alkane system increases and the interaction increases,and the alkane also participate in the charge transfer.The calculation of ELF shows that the reaction sites between two adsorbates and the zeolite and the alkane molecules are all in an electronically highly localized state.PDOS shows that the size and position of the individual atom peaks in two adsorbates have changed under the coexistence system.In the skeletal isomerization process of alkene-alkane coexisting system,when the presence of alkane causes significant space steric effect to alkene,the reaction activation energy of alkene increases,when the presence of alkane does not cause significant space steric effect to alkene,the reaction activation energy of alkene decreases.In addition,the presence of alkane enhances the stability of the alkene-alkane system.After analysis,the alkane is calculated as a single molecule and does not involve the reaction between alkene and alkane,is the main reason for the difference between the calculated results and the actual.This illustrates the necessity of considering the reaction between alkene and alkane in the study of alkene-alkane system,and also lays the foundation for further research on alkene-alkane system.The research on the alkene-alkane coexistence system is of great significance for the design and development of the FCC gasoline isomerization process and the production of high octane gasoline.更多还原