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Different Efficiency of Zn2+ and ZnO Species for Methane Activation on Zn-Modified Zeolite

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journal contribution
posted on 24.01.2017, 00:00 by Anton A. Gabrienko, Sergei S. Arzumanov, Alexander V. Toktarev, Irina G. Danilova, Igor P. Prosvirin, Vladimir V. Kriventsov, Vladimir I. Zaikovskii, Dieter Freude, Alexander G. Stepanov
Understanding methane activation pathways on Zn-modified high-silica zeolites (ZSM-5, BEA) is of particular importance because of the possibility of methane involvement in coaromatization with higher alkanes on this type of zeolites. Herein, two samples of Zn-modified zeolite BEA containing exclusively either small zinc oxide clusters or isolated Zn2+ cations have been synthesized and thoroughly characterized by a range of spectroscopic methods (1H MAS NMR, DRIFTS, XPS, EXAFS, HRTEM) to show that only one of the Zn-species, either Zn2+ cations or ZnO small clusters, exists in the void of zeolite pores. The ability of zinc sites of different nature to promote the activation of methane C–H bond with the zeolite Brønsted acid sites (BAS) has been examined in the reactions of methane H/D hydrogen exchange with BAS and the alkylation of benzene with methane. It has been found that both ZnO and Zn2+ species promote the reaction of H/D exchange of methane with BAS. The rate of H/D exchange is higher by 2 and 3 orders of magnitude for the zeolite loaded with ZnO or Zn2+ species, respectively, compared to pure acid-form zeolite H-BEA. So, the promoting effect of Zn2+ cations is more profound than that of ZnO species for H/D exchange reaction. This implies that the synergistic effect of Zn-sites and BAS for C–H bond activation in methane is significantly higher for Zn2+ cations compared to small ZnO clusters. It has been revealed, however, that only Zn2+ cations promote the alkylation of benzene with methane, whereas ZnO species do not. The isolated Zn2+ cations provide the formation of zinc-methyl species, which are further transformed to zinc-methoxy species. The latter is the key intermediate for the performance of the alkylation reaction. Hence, while both zinc oxide clusters and Zn2+ cationic species are able to provide a synergistic effect for the activation of C–H bonds of methane displayed by the dramatic acceleration of H/D exchange reaction, only the Zn2+ cationic species perform methane activation toward the alkylation of benzene with methane. This implies that only the Zn2+ cations in Zn-modified zeolite can activate methane for the reaction of methane coaromatization with higher alkanes.