| 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. |
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