Acta Phys. -Chim. Sin. ›› 2013, Vol. 29 ›› Issue (04): 754-762.doi: 10.3866/PKU.WHXB201302063

• THEORETICAL AND COMPUTATIONAL CHEMISTRY • Previous Articles     Next Articles

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.

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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