An explanation for the enhanced activity for light alkane conversion in mildly steam dealuminated mordenite: The dominant role of adsorption

J. A. Van Bokhoven, M. Tromp, D. C. Koningsberger, J. T. Miller, J. A Z Pieterse, J. A. Lercher, B. A. Williams, Harold H Kung

Research output: Contribution to journalArticle

79 Citations (Scopus)

Abstract

This paper presents a catalytic, spectroscopic, calorimetric study of mildly steam-dealuminated mordenite (H-MOR). With increasing steam partial pressures at 673 K there is increasing dealumination, the loss of Brønsted acid sites and the presence of extra-framework Al (AlEF) likely in the zeolite pores. Additionally, the number of Lewis sites increases with increasing AlEF. As observed in previous studies, the catalytic activity for hydroisomerization of n-hexane and monomolecular cracking of n-butane and n-hexane initially increases and goes through a maximum with increasing steam severity. While the maximum increase in the rate of hexane cracking is about 5 times, the activation energies indicate that steaming does not significantly change the strength of the Brønsted acid sites. In the absence of AlEF, the enthalpy of alkane adsorption is constant at all surface coverages. However, in the presence of AlEF, the initial heat of alkane sorption increases by about 5-10 kJ/mol. After covering these adsorption sites, the heat of adsorption is identical to that on the unsteamed H-MOR until saturation coverage has been reached. The increased enthalpies of adsorption are suggested to occur on the AlEF, Lewis acid sites. Since the surface coverage of paraffins under typical reaction conditions is low, it is proposed that the enhanced rate is due to the increased initial heat of adsorption leading to a higher concentration of reactants in the steamed H-MOR zeolites.

Original languageEnglish
Pages (from-to)129-140
Number of pages12
JournalJournal of Catalysis
Volume202
Issue number1
DOIs
Publication statusPublished - 2001

Fingerprint

Alkanes
Steam
steam
Paraffins
alkanes
Adsorption
Hexane
adsorption
Zeolites
heat
acids
Acids
Enthalpy
enthalpy
Lewis Acids
Butane
Hexanes
paraffins
butanes
zeolites

Keywords

  • Adsorption
  • Cracking
  • Enhanced activity
  • Light alkane conversion
  • Zeolite mordenite
  • Zeolite steaming

ASJC Scopus subject areas

  • Catalysis
  • Process Chemistry and Technology

Cite this

An explanation for the enhanced activity for light alkane conversion in mildly steam dealuminated mordenite : The dominant role of adsorption. / Van Bokhoven, J. A.; Tromp, M.; Koningsberger, D. C.; Miller, J. T.; Pieterse, J. A Z; Lercher, J. A.; Williams, B. A.; Kung, Harold H.

In: Journal of Catalysis, Vol. 202, No. 1, 2001, p. 129-140.

Research output: Contribution to journalArticle

Van Bokhoven, JA, Tromp, M, Koningsberger, DC, Miller, JT, Pieterse, JAZ, Lercher, JA, Williams, BA & Kung, HH 2001, 'An explanation for the enhanced activity for light alkane conversion in mildly steam dealuminated mordenite: The dominant role of adsorption', Journal of Catalysis, vol. 202, no. 1, pp. 129-140. https://doi.org/10.1006/jcat.2001.3265
Van Bokhoven, J. A. ; Tromp, M. ; Koningsberger, D. C. ; Miller, J. T. ; Pieterse, J. A Z ; Lercher, J. A. ; Williams, B. A. ; Kung, Harold H. / An explanation for the enhanced activity for light alkane conversion in mildly steam dealuminated mordenite : The dominant role of adsorption. In: Journal of Catalysis. 2001 ; Vol. 202, No. 1. pp. 129-140.
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AB - This paper presents a catalytic, spectroscopic, calorimetric study of mildly steam-dealuminated mordenite (H-MOR). With increasing steam partial pressures at 673 K there is increasing dealumination, the loss of Brønsted acid sites and the presence of extra-framework Al (AlEF) likely in the zeolite pores. Additionally, the number of Lewis sites increases with increasing AlEF. As observed in previous studies, the catalytic activity for hydroisomerization of n-hexane and monomolecular cracking of n-butane and n-hexane initially increases and goes through a maximum with increasing steam severity. While the maximum increase in the rate of hexane cracking is about 5 times, the activation energies indicate that steaming does not significantly change the strength of the Brønsted acid sites. In the absence of AlEF, the enthalpy of alkane adsorption is constant at all surface coverages. However, in the presence of AlEF, the initial heat of alkane sorption increases by about 5-10 kJ/mol. After covering these adsorption sites, the heat of adsorption is identical to that on the unsteamed H-MOR until saturation coverage has been reached. The increased enthalpies of adsorption are suggested to occur on the AlEF, Lewis acid sites. Since the surface coverage of paraffins under typical reaction conditions is low, it is proposed that the enhanced rate is due to the increased initial heat of adsorption leading to a higher concentration of reactants in the steamed H-MOR zeolites.

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