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

An explanation for the enhanced activity for light alkane conversion in mildly steam dealuminated mordenite: the dominant role of adsorption
An explanation for the enhanced activity for light alkane conversion in mildly steam dealuminated mordenite: the dominant role of adsorption
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 extraframework 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 unsteamedH-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.
light alkane conversion, zeolite steaming, enhanced activity, adsorption, zeolite mordenite, cracking
0021-9517
129-140
van Bokhoven, J.A.
ed49a36e-7a29-4968-9e68-9739abdcc101
Tromp, M.
48c1ebbb-579c-42b6-83bb-7188c668b322
Koningsberger, D.C.
b2657d86-3d2b-4948-a46e-fbe6eaa3490f
Miller, J.T.
b4e7440d-707d-4be8-8eaf-6ea7f5e06e44
Pieterse, J.A.Z.
d59ea164-60e7-498d-8816-a716466ec379
Lercher, J.A.
72f525e3-adc1-470a-aacc-f4d75080d9ff
Williams, B.A.
ec2b4057-cc89-4a9c-bb08-d71ba4506eda
Kung, H.H.
ddd28eb7-722d-49e6-80d0-0047cca67d1a
van Bokhoven, J.A.
ed49a36e-7a29-4968-9e68-9739abdcc101
Tromp, M.
48c1ebbb-579c-42b6-83bb-7188c668b322
Koningsberger, D.C.
b2657d86-3d2b-4948-a46e-fbe6eaa3490f
Miller, J.T.
b4e7440d-707d-4be8-8eaf-6ea7f5e06e44
Pieterse, J.A.Z.
d59ea164-60e7-498d-8816-a716466ec379
Lercher, J.A.
72f525e3-adc1-470a-aacc-f4d75080d9ff
Williams, B.A.
ec2b4057-cc89-4a9c-bb08-d71ba4506eda
Kung, H.H.
ddd28eb7-722d-49e6-80d0-0047cca67d1a

van Bokhoven, J.A., Tromp, M., Koningsberger, D.C., Miller, J.T., Pieterse, J.A.Z., Lercher, J.A., Williams, B.A. and Kung, H.H. (2001) An explanation for the enhanced activity for light alkane conversion in mildly steam dealuminated mordenite: the dominant role of adsorption. Journal of Catalysis, 202 (1), 129-140. (doi:10.1006/jcat.2001.3265).

Record type: Article

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 extraframework 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 unsteamedH-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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More information

Published date: 15 August 2001
Keywords: light alkane conversion, zeolite steaming, enhanced activity, adsorption, zeolite mordenite, cracking

Identifiers

Local EPrints ID: 46479
URI: http://eprints.soton.ac.uk/id/eprint/46479
ISSN: 0021-9517
PURE UUID: 7e271c78-932a-4b99-b7ee-7cd583e924fa

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Date deposited: 02 Jul 2007
Last modified: 13 Mar 2019 21:02

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Contributors

Author: J.A. van Bokhoven
Author: M. Tromp
Author: D.C. Koningsberger
Author: J.T. Miller
Author: J.A.Z. Pieterse
Author: J.A. Lercher
Author: B.A. Williams
Author: H.H. Kung

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