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

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 (H-1 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 C-H bond with the zeolite Bronsted 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.