On two alternative mechanisms of ethane activation over ZSM-5 zeolite modified by Zn2+ and Ga1+ cations

The activation of ethane over zinc- and gallium-modified HZSM-5 dehydrogenation catalysts was studied by diffuse reflectance infrared spectroscopy. Hydrocarbon activation on HZSM-5 modified by bivalent Zn and univalent Ga cations proceeds via two distinctly different mechanisms. The stronger molecul...

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Published in:Physical chemistry chemical physics : PCCP 2005-01, Vol.7 (16), p.3088-3092
Main Authors: Kazansky, V B, Subbotina, I R, Rane, N, van Santen, R A, Hensen, E J M
Format: Article
Language:English
Subjects:
Adsorption
Catalysis
Cations
Computer Simulation
Ethane - chemistry
Gallium - chemistry
Models, Chemical
Surface Properties
Zeolites - analysis
Zeolites - chemistry
Zinc - analysis
Zinc - chemistry
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Summary:The activation of ethane over zinc- and gallium-modified HZSM-5 dehydrogenation catalysts was studied by diffuse reflectance infrared spectroscopy. Hydrocarbon activation on HZSM-5 modified by bivalent Zn and univalent Ga cations proceeds via two distinctly different mechanisms. The stronger molecular adsorption of ethane by the acid-base pairs formed by distantly separated cationic Zn2+ and basic oxygen sites results already at room temperature in strong polarizability of adsorbed ethane and subsequent heterolytic dissociative adsorption at moderate temperatures. In contrast, molecular adsorption of ethane on Ga+ cations is weak. At high temperatures dissociative hydrocarbon adsorption takes place, resulting in the formation of ethyl and hydride fragments coordinating to the cationic gallium species. Whereas in the zinc case a Brønsted acid proton is formed upon ethane dissociation, decomposition of the ethyl fragment on gallium results in gallium dihydride species and does not lead to Brønsted acid protons. This difference in alkane activation has direct consequences for hydrocarbon conversions involving dehydrogenation.
ISSN:1463-9076
1463-9084
DOI:10.1039/b506782k