物理化学学报 >> 2013, Vol. 29 >> Issue (04): 754-762.doi: 10.3866/PKU.WHXB201302063

理论与计算化学 上一篇    下一篇

H-ZSM-5分子筛催化二甲苯异构化的反应机理

李玲玲1, 聂小娃1,2, 宋春山1,3, 郭新闻1   

  1. 1 大连理工大学化工学院精细化工国家重点实验室, PSU-DUT联合能源研究中心, 辽宁 大连116024;
    2 俄亥俄州立大学化工与生物分子工程系, 俄亥俄 43210, 美国;
    3 宾夕法尼亚州立大学能源与矿物工程系能源研究所, PSU-DUT联合能源研究中心, 宾夕法尼亚 16802, 美国
  • 收稿日期:2012-12-04 修回日期:2013-02-05 发布日期:2013-03-25
  • 通讯作者: 郭新闻 E-mail:guoxw@dlut.edu.cn
  • 基金资助:

    新世纪优秀人才项目(NCET-04-0268)资助和大连理工大学网络与信息化中心高性能计算部支持

Isomerization Mechanismof Xylene Catalyzed by H-ZSM-5 Molecular Sieve

LI Ling-Ling1, NIE Xiao-Wa1,2, SONG Chun-Shan1,3, GUO Xin-Wen1   

  1. 1 State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, China;
    2 Department of Chemical & Biomolecular Engineering, The Ohio State University, Columbus, OH 43210, USA;
    3 EMS Energy Institute, PSU-DUT Joint Center for Energy Research and Department of Energy & Mineral Engineering, Pennsylvania State University, University Park, PA 16802, USA
  • Received:2012-12-04 Revised:2013-02-05 Published:2013-03-25
  • Supported by:

    The project was supported by the Programfor New Century Excellent Talent in University, China (NCET-04-0268) and High Performance Computing Department of Network and Information Center, Dalian University of Technology, China.

摘要:

采用密度泛函理论(DFT)和ONIOM方法, 研究了H-ZSM-5分子筛上二甲苯异构化机理. 描述了中间体物种和过渡态的结构. 反应物吸附和产物脱附对二甲苯异构化的反应趋势有重要影响. 反应活化能的计算结果表明, 在H-ZSM-5分子筛延伸的孔道结构中, 异构化反应沿着生成间二甲苯的方向进行. 但是较高的脱附能使生成的间二甲苯滞留在分子筛孔道中,其进一步异构化生成对二甲苯具有动力学优势. 对二甲苯产物在分子筛孔道的酸中心上可选择性生成. 在H-ZSM-5分子筛外表面, 不受延伸孔道结构的静电限制时, 二甲苯异构化生成间二甲苯产物, 其可以很容易从活性位上脱附. 非选择性异构化降低了对二甲苯的选择性. 因此, 对H-ZSM-5分子筛外表面改性能够抑制二甲苯的非选择性异构化, 因此限制了反应在分子筛孔道中进行, 提高了对二甲苯的选择性. 二甲苯异构化相对反应速率常数的计算结果也表明, 在分子筛外表面上, 生成间二甲苯的异构化反应速率较快. 升高反应温度会降低对二甲苯的选择性.

关键词: 异构化机理, 二甲苯, 密度泛函理论, ONIOM, H-ZSM-5

Abstract:

The isomerization mechanism of xylene over H-ZSM-5 molecular sieve has been examined using the density functional theory (DFT) and our own-N-layered integrated molecular orbital+molecular mechanics (ONIOM) methods. The structures of intermediate species and transition states are described. The adsorption of reactant and desorption of product significantly affect the tendency of xylene to isomerize. Calculated activation energies suggest that isomerization occurs during the formation of meta-xylene within the extended pore structure of H-ZSM-5 molecular sieve. However, the produced meta-xylene is retained within the pore because of a high desorption energy, and further isomerization to form para-xylene is kinetically favorable. The acid sites within the pores of the molecular sieve allow selective formation of para-xylene. On the external surface of H-ZSM-5 molecular sieve, which lacks the steric constraints of the extended pore structure, xylene isomerizes to form meta-xylene, which can readily desorb from the active site. Such non-selective isomerization decreases the selectivity for para-xylene. Thus, external surface modification of H-ZSM-5 molecular sieve should suppress the non-selective isomerization of xylene, thereby increasing the selectivity for para-xylene by restricting isomerization to inside the pores of the molecular sieve. Calculated relative reaction rate constants for xylene isomerization also indicate that xylene isomerization occurring on the external surface of H-ZSM-5 with meta-xylene as the product has the highest reaction rate. The selectivity for para-xylene is decreased as the reaction temperature is increased.

Key words: Isomerization mechanism, Xylene, Density functional theory, ONIOM, H-ZSM-5

MSC2000: 

  • O641