Isomerization mechanism of xylene catalyzed by H-ZSM-5 molecular sieve

Ling Ling Li, Xiao Wa Nie, Chunshan Song, Xin Wen Guo

Research output: Contribution to journalArticle

8 Citations (Scopus)

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.

Original languageEnglish (US)
Pages (from-to)754-762
Number of pages9
JournalWuli Huaxue Xuebao/ Acta Physico - Chimica Sinica
Volume29
Issue number4
DOIs
StatePublished - May 22 2013

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Xylenes
Molecular sieves
xylene
absorbents
Isomerization
isomerization
Pore structure
Reaction rates
porosity
Desorption
selectivity
Molecular mechanics
Catalyst selectivity
Molecular orbitals
Density functional theory
Surface treatment
reaction kinetics
Rate constants
desorption
Activation energy

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry

Cite this

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title = "Isomerization mechanism of xylene catalyzed by H-ZSM-5 molecular sieve",
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.",
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Isomerization mechanism of xylene catalyzed by H-ZSM-5 molecular sieve. / Li, Ling Ling; Nie, Xiao Wa; Song, Chunshan; Guo, Xin Wen.

In: Wuli Huaxue Xuebao/ Acta Physico - Chimica Sinica, Vol. 29, No. 4, 22.05.2013, p. 754-762.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Isomerization mechanism of xylene catalyzed by H-ZSM-5 molecular sieve

AU - Li, Ling Ling

AU - Nie, Xiao Wa

AU - Song, Chunshan

AU - Guo, Xin Wen

PY - 2013/5/22

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N2 - 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.

AB - 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.

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