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Adsorption Device Based on a Langatate Crystal Microbalance for High Temperature High Pressure Gas Adsorption in Zeolite H-ZSM-5
Published on: August 25, 2016
How Do Zn2+ Ions Promote Methane Activation on Zinc-Modified Zeolites? A Hybrid Quantum-Chemical Study
Zoya N Lashchinskaya1, Anton A Gabrienko1, Alexander G Stepanov1
1Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk 630090, Russia.
None:
It was earlier established that Zn2+ species loaded in zeolites promoted alkane C-H bond activation by Brønsted acid sites (BAS). It was found that the rate of H/D hydrogen exchange between alkane and zeolite BAS increased dramatically, and the activation barrier decreased for Zn2+-modified zeolite compared to H-form zeolite. However, it still remains unclear what the nature of the promoting effect of Zn2+ species on C-H bond activation is. In this work, the possible mechanisms of methane hydrogen exchange on the Zn2+/H-ZSM-5 zeolite have been analyzed by means of hybrid quantum chemical calculations (PBEh-3c/DLPNO-CCSD(T)). Four models representing various locations of Zn2+ ions in H-ZSM-5 differing in Al distribution and local geometry have been investigated. Zn2+ sites are found to be rather diverse in their adsorption properties and reactivity. The earlier accepted hypothesis of H/D hydrogen exchange promotion via the formation of a transient molecular complex of methane with Zn2+ sites has been disproven by the PBEh-3c/DLPNO-CCSD(T) calculations. Conversely, the two-step mechanism of hydrogen exchange in methane on Zn2+/H-ZSM-5 involving zinc-methyl species formation is concluded to be the most probable. Thus, the rationalization of how Zn2+ sites affect alkane C-H bond activation by BAS has been provided.
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